Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study

pulmonary aspiration, incidence, pregnancy, surveillance, UK Obstetric Surveillance System ALL Metrics - Views Downloads How to cite this article Lucas N, Gooda A, Tunn R et al. Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.13797.1) NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article. Export Citation Sciwheel EndNote Ref. Manager Bibtex ProCite Sente Select a format first ▬ ✚ Research Article [version 1; peer review: 3 approved, 1 approved with reservations] Nuala Lucas1, Alison Gooda2, Ruth Tunn https://orcid.org/0000-0003-1187-1808 2, Marian Knight https://orcid.org/0000-0002-1984-4575 2, The UKOSS Aspiration in Pregnancy investigatorsNuala Lucas1, Alison Gooda2, [...] Ruth Tunn https://orcid.org/0000-0003-1187-1808 2, Marian Knight https://orcid.org/0000-0002-1984-4575 2, The UKOSS Aspiration in Pregnancy investigators PUBLISHED 08 Jan 2025 Author details Author details 1 London North West Healthcare NHS Trust, Watford Road, Harrow, Middlesex, HA1 3UJ, UK 2 National Perinatal Epidemiology Unit, University of Oxford, Old Road Campus, Headington, Oxford, OX3 7LF, UK 2 National Perinatal Epidemiology Unit, University of Oxford, Old Road Campus, Headington, Oxford, OX3 7LF, UK Nuala Lucas Roles: Conceptualization, Data Curation, Formal Analysis, Investigation, Project Administration, Writing – Original Draft Preparation Roles: Conceptualization, Data Curation, Formal Analysis, Investigation, Project Administration, Writing – Original Draft Preparation Alison Gooda Roles: Data Curation, Formal Analysis, Methodology, Writing – Original Draft Preparation Roles: Data Curation, Formal Analysis, Methodology, Writing – Original Draft Preparation Ruth Tunn Roles: Validation, Writing – Review & Editing Roles: Validation, Writing – Review & Editing Marian Knight Roles: Conceptualization, Data Curation, Formal Analysis, Funding Acquisition, Investigation, Methodology, Project Administration, Resources, Supervision, Writing – Review & Editing Roles: Conceptualization, Data Curation, Formal Analysis, Funding Acquisition, Investigation, Methodology, Project Administration, Resources, Supervision, Writing – Review & Editing OPEN PEER REVIEW REVIEWER STATUS Pulmonary aspiration of gastric contents is the most frequent cause of death associated with complications of airway management during general anaesthesia. Pregnancy increases aspiration risk owing to factors including delayed gastric emptying and increased intragastric pressure. We describe the incidence, risk factors, management, and outcomes of maternal pulmonary aspiration in pregnancy in the UK. We conducted a population-based surveillance and case-control study. Between September 2013 and August 2016, all UK consultant-led obstetric units prospectively identified cases of pulmonary aspiration among parturient women using a pre-defined case definition, and reported them via the UK Obstetric Surveillance System (UKOSS). Controls (n=1982) were obtained from four UKOSS studies conducted between 2005 and 2014. We calculated the incidence of pulmonary aspiration using 2013–2015 maternities as the denominator. We explored potential risk factors for aspiration using univariable logistic regression and described outcomes. We identified 12 cases of pulmonary aspiration, giving an incidence of 5.2 per 1,000,000 maternities (95% CI 2.69-9.09). Cases were significantly less likely than controls to be multiparous (unadjusted odds ratio [uOR] 0.255, 95% CI 0.069-0.946), and significantly more likely to undergo caesarean section (uOR 24.89, 95% CI 3.18-194.85) and to receive general anaesthetic for caesarean section (p<0.001). Gestation was significantly shorter in cases than controls (uOR 0.782, 95% CI 0.702-0.870). Women who aspirated were significantly more likely to be admitted to the intensive therapy unit than controls (p<0.001). Infants of women who aspirated had significantly lower Apgar scores and were more likely to be admitted to the neonatal intensive care unit or to be stillborn compared with infants of women in the control group. Pulmonary aspiration is rare in UK obstetric anaesthetic practice; however, it remains a risk of general anaesthesia. Despite a large study population, our analyses lacked power to evaluate many potential risk factors. Future research should focus on developing methods to accurately identify pregnant women at risk of aspiration. Pulmonary aspiration is a condition in which stomach contents are inhaled into the windpipe and lungs. This most commonly occurs in patients under general anaesthesia, and can be fatal. Pregnancy increases the risk of pulmonary aspiration because of physical and hormonal changes it causes in the digestive system. Therefore, pregnant women giving birth under general anaesthetic are at particularly high risk. We wanted to understand how common pulmonary aspiration is among pregnant women in the UK, and how women affected by this condition differ from other women. We also investigated how the condition was managed, and how affected women and their babies fared. The UK Obstetric Surveillance System (UKOSS) collects information from all UK maternity units about pregnant women with certain conditions. We used UKOSS to collect anonymous information about all women who experienced pulmonary aspiration during a three-year period. We compared these women with pregnant women who did not experience pulmonary aspiration. During the three years, 12 pregnant women in the UK experienced pulmonary aspiration when giving birth. Based on this, we estimated that approximately 5.5 women were affected per million women giving birth. As expected, women who aspirated were more likely than unaffected women to give birth by caesarean section under general anaesthesia. They were also more likely to be first-time mums, and to give birth earlier (at 37.6 weeks on average, compared with 39 weeks for unaffected women). The results of our study suggest that pulmonary aspiration is reassuringly uncommon among women giving birth in the UK, including among the higher-risk group of women giving birth under general anaesthesia. However, when it did occur, women were more likely to need to be admitted to intensive care; and the babies of affected women typically needed additional medical support and there was an increased risk of stillbirth. Better methods are needed to identify women who are at risk of pulmonary aspiration. pulmonary aspiration, incidence, pregnancy, surveillance, UK Obstetric Surveillance System Corresponding Author(s) Nuala Lucas ([email protected]) Grant information: This project is funded by the National Institute for Health and Care Research (NIHR) under its NIHR Policy Research Programme as part of the programme of work of the Policy Research Unit in Maternal Health and Care, National Perinatal Epidemiology Unit (Grant Reference Number 108/0001). MK is an NIHR Senior Investigator (Award Reference Number NIHR303806). The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Copyright: © 2025 Lucas N et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. How to cite: Lucas N, Gooda A, Tunn R et al. Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.13797.1) First published: 08 Jan 2025, 5:2 (https://doi.org/10.3310/nihropenres.13797.1) Latest published: 08 Jan 2025, 5:2 (https://doi.org/10.3310/nihropenres.13797.1) The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Pulmonary aspiration of gastric contents is a serious potential complication of anaesthesia and was the most frequent cause of death in cases reported to the Royal College of Anaesthetists 4th National Audit Project (NAP4)1. Pregnant women are considered at increased risk of aspiration as a result of the hormonal and physical changes of pregnancy and labour. Increased progesterone levels lead to a reduction in the lower oesophageal sphincter tone and delayed gastric emptying, and there is an increase in intragastric pressure as a result of the gravid uterus2. Besides pregnancy, identified risk factors for pulmonary aspiration of gastric contents include full stomach, hiatus hernia, dyspepsia, morbid obesity, receipt of opioids, and use of first-generation supraglottic airway devices3,4. Since Mendelson combined observational evidence with animal experiments to describe his eponymous syndrome of acid aspiration pneumonitis in 19465, measures to reduce morbidity and mortality from aspiration have been introduced to obstetric practice. These include restriction of oral intake in labour, use of H2-receptor drugs and sodium citrate to reduce gastric volume and acidity, airway protection with rapid sequence induction involving pre-oxygenation, cricoid pressure, use of succinylcholine, and avoidance of mask ventilation before tracheal intubation2,6. Despite these measures, aspiration remains a potential complication of obstetric anaesthesia. Although restriction of food intake during labour as a measure to prevent aspiration is common and is recommended in American7 and European8,9 national guidelines, available evidence does not support this practice10. Alongside these concerns is the desire to maximise a woman’s comfort in labour. In 2007, NICE guidance on intrapartum care was changed to advise a light diet in established labour, as long as women have not received opioids and are not at high risk of requiring an emergency caesarean birth11. The comparative rarity of obstetric pulmonary aspiration makes it challenging to estimate its incidence. Studies that have attempted to do so to date have mainly focused on the higher-risk population of women undergoing general anaesthesia. A prospective observational study in Australia confirmed one case of aspiration in 1095 women who underwent caesarean section under general anaesthesia, suggesting an incidence of 0.1% (95% CI 0.002%–0.5%) in this population12, while a German single-centre retrospective study of 2,390 obstetric surgery patients at considered at high risk of pulmonary aspiration did not detect any aspiration events13. A UKOSS study of failed intubation during obstetric anaesthesia estimated a higher incidence of pulmonary aspiration of 8% in cases of failed intubation and 1% in matched controls (pregnant women from the same hospitals who underwent general anaesthesia and successful intubation)14. The aims of this study were to use the UK Obstetric Surveillance System (UKOSS)15 to calculate the national incidence of pulmonary aspiration in obstetrics, to identify the risk factors for pulmonary aspiration in obstetrics in the UK, and to describe how pulmonary aspiration is currently managed in the UK. Patients were not directly involved in the design of the study. Two members of the public were indirectly involved in the design of the study via representatives on the UKOSS Steering Committee, which reviews, comments on, and approves all studies to be run through UKOSS. A national, population-based prospective surveillance and case-control study was undertaken using UKOSS. UKOSS is designed to study rare and severe complications of pregnancy and childbirth in the UK and is now well established, having started data collection in 200515. Nominated clinicians at each obstetrician-led maternity unit in the UK completed a monthly return to UKOSS. This return notified UKOSS of any cases of pulmonary aspiration that occurred between September 1, 2013 and August 31, 2016. A data collection form was then sent to any unit reporting a case to collect more detailed information. The data collection form sought confirmation of the appropriate case definition and additional information on risk factors, management, and outcomes. The full form is available at https://www.npeu.ox.ac.uk/assets/downloads/ukoss/forms/UKOSS-Aspiration-in-Pregnancy.pdf. It was developed specifically for reporting cases of pulmonary aspiration and designed to be short and easily completed from a woman's case notes without requiring reference to any other sources of information. The co-investigator group developed the form and crosschecked with anaesthetists in a series of revisions to ensure that the questions thoroughly covered the details required for the study. The case definition was based on the findings of NAP41 and included women at 20 weeks’ gestation or greater who had an unprotected airway while unconscious, semiconscious or paralysed as well as a clinical history consistent with regurgitation of stomach contents and pulmonary aspiration, such as vomiting or gastric contents seen in the oropharynx. This must have been accompanied by signs of respiratory compromise requiring supplementary oxygen and antibiotics or level 2/3 respiratory support without any other apparent cause. To minimise missed cases, the Mothers and Babies: Reducing Risk through Audits and Confidential Enquiries across the UK (MBRRACE-UK) maternal mortality reports for the surveillance period of this study were crosschecked for additional aspiration cases. Unmatched controls were obtained from four UKOSS studies conducted between 2005 and 201416–19, defined as the two women giving birth before the notified case in these studies. This gave a total of 1982 women without severe life-threatening complications throughout pregnancy or childbirth. Incidence of pulmonary aspiration with 95% confidence interval (CI) was calculated using the national surveillance data. Denominators were total maternities in the UK between 1 January, 2013 and 31 December, 2015, obtained from the Office for National Statistics (ONS) for England and Wales20, National Records of Scotland (NRS)21, and Northern Ireland Statistics and Research Agency (NISRA)22. The characteristics of cases and controls were compared using Chi-Square test and Fisher's Exact test for difference in proportions for categorical variables. Medians and interquartile ranges were calculated for continuous variables (maternal age, BMI, and gestational weeks at delivery) for cases and controls, then compared using Wilcox rank-sum test, as they were found to be non-normally distributed. Because it is thought to increase the risk of difficult tracheal intubation23, obesity was examined as a categorical variable (obese, BMI ≥30 vs non-obese, BMI <30) in addition to BMI being evaluated as a continuous variable. Univariable logistic regression analysis was conducted to examine the association of potential risk factors individually with pulmonary aspiration. Owing to the small number of events, multivariable logistic regression analysis was not undertaken to prevent bias due to multiple testing. Missing values were excluded from these calculations owing to their small number. Case characteristics for which control data were not available (medical history specifically relevant to pulmonary aspiration, observed clinical signs of aspiration, setting of aspiration, X-ray consolidation, X-ray abnormalities, presence of consultant anaesthetist, oral intake, antacid prophylaxis, use of opioid, use of airway device and mechanisms to reduce risk) are described. Maternal and infant outcomes (maternal ITU admission, stillbirth, infant death, infant neonatal intensive care admission and infant Apgar score) were compared between cases and controls using Chi-Square/Fisher's Exact tests. Associations were considered significant at a p-value of <0.05 (two-sided). All analyses were undertaken using STATA version 13.1, SE24. Over the three years of surveillance, there were 12 confirmed cases of pulmonary aspiration out of a total of 2,305,920 maternities20–22. Data (including zero-value notifications) were received from all UK consultant-led maternity units and no missed cases were identified via crosschecking with the MBRRACE-UK reports. This gives an estimated incidence of 5.2 cases per 1,000,000 maternities (95% CI 2.69–9.09), equating to 1 in 192,309 maternities or 0.00052%. There were no deaths resulting from pulmonary aspiration; one woman in the case group died from other causes. There were four cases in year one, five in year two and three in year three, with no statistically significant trend within this time period (p=0.72). Of the 12 cases, aspiration in nine was associated with general anaesthesia: seven for caesarean section and two for surgical management of miscarriage and cardiac arrest respectively. The three non-general anaesthesia-associated cases of aspiration occurred in association with 1) recurrent epileptic fits, 2) cardiac problems during pregnancy and 3) in a semiconscious patient with obtunded airway reflexes. The characteristics of cases and controls are shown in Table 1. The odds of aspiration were higher in obese women (uOR 1.59, 95% CI 0.410–6.182), but this difference was not statistically significant (p=0.502). The odds of aspiration were also higher in women who had caesarean sections (uOR 24.89, 95% CI 3.18–194.85, p=0.002). Of those patients who underwent a caesarean section, cases were statistically significantly more likely have undergone general rather than regional anaesthesia (p<0.001). Multiparous women were less likely to aspirate (uOR 0.255, 95% CI 0.069–0.946). Median duration of gestation was significantly shorter in cases (37.6 weeks, interquartile range (IQR) 33–40.4 weeks) than in controls (39 weeks, IQR 38–40 weeks; uOR 0.782, 95% CI 0.702–0.870). | Category | Risk Factor | Cases (N = 12 unless otherwise stated) | Controls (N = 1982 unless otherwise stated) | p-value* | uOR | uOR p-value | 95% CI | |---|---|---|---|---|---|---|---| | Sociodemographic factors | Ethnicity, n (%) | 1.000 | ||||| | White | 10 (83) | 1540 (78) | 1 | |||| | Non-white | 2 (16) | 424 (21) | 0.726 | 0.681 | 0.159–3.328 | || | Unknown | 0 (0) | 18 (0.9) | ||||| | Age, median (IQR) | 30.5 (27–33.5) | 29 (25–34) | 0.565 | 1.03 | 0.518 | 0.940–1.131 | | | Number of fetuses, n (%) | 0.177 | |||||| | 1 | 11 (92) | 1951 (98) | 1 | |||| | 2 | 1 (8) | 31 (2) | 5.721 | 0.100 | 0.716–45.689 | || | Employment, n (%) | 0.304 | |||||| | Employed | 9 (75) | 1168 (59) | 1 | |||| | Unemployed | 1 (8) | 434 (22) | 0.299 | 0.253 | 0.038–2.367 | || | Unknown | 2 (17) | 380 (19) | ||||| | Medical and obstetric factors | BMI (kg/m2), median (IQR) | 27.95 (24.5–35.4) | 24.6 (21.9–29) | 0.070 | 1.08 | 0.091 | 0.988–1.181 | | Obesity, n (%) | 0.451 | |||||| | Obese (BMI ≥30) | 3 (25) | 409 (21) | 1.59 | 0.502 | 0.410–6.182 | || | Non-obese (BMI <30) | 7 (58) | 1519 (77) | 1 | |||| | Unknown | 2 (17) | 54 (3) | ||||| | Gestation (weeks), median (IQR) | 37.6 (33–40.4) | 39 (38–40) | 0.044 | 0.782 | <0.001 | 0.702–0.870 | | | Parity, n (%) | 0.038 | |||||| | Multiparous | 3 (25) | 1119 (56) | 0.255 | 0.041 | 0.069–0.946 | || | Nulliparous | 9 (75) | 857 (43) | 1 | |||| | Unknown | 0 (0) | 6 (0.3) | ||||| | Parturition | Labour induction, n (%) | 0.497 | ||||| | Yes | 4 (33) | 525 (26) | 1.58 | 0.467 | 0.461–5.421 | || | No | 7 (58) | 1452 (73) | 1 | |||| | N/A | 1 (8) | 5 (0.3) | ||||| | Mode of delivery, n (%) | ||||||| | Spontaneous vaginal | 0 (0) | 1138 (57) | ||||| | Operative vaginal | 1 (8) | 266 (13) | ||||| | Breech | 0 (0) | 3 (0.2) | ||||| | Pre-labour C-section | 4 (33) | 339 (17) | ||||| | C-section after labour onset | 6 (50) | 228 (12) | ||||| | N/A (case) / missing (control) | 1 (8) | 8 (0.4) | ||||| | C-section, n (%) | <0.001 | |||||| | Yes | 10 (83) | 567 (29) | 24.89 | 0.002 | 3.18–194.85 | || | No | 1 (8) | 1411 (71) | 1 | |||| | N/A (case) / missing (control) | 1 (8) | 4 (0.2) | ||||| | Anaesthesia method for C-section, n (%) | N=10 | N=567 | <0.001 | |||| | Regional | 0 (0) | 527 (93) | ||||| | General | 9 (90) | 36 (6) | ||||| | Unknown | 1 (10) | 4 (1) | *Calculated without missing values, using Fisher’s exact test for categorical variables and Wilcox rank-sum test for continuous variables IQR, interquartile range; BMI, body mass index; uOR, unadjusted odds ratio; CI, confidence interval; C-section, caesarean section. N/A, not applicable (pregnancy ended in miscarriage) Additional characteristics of cases are presented in Table 2. Two of the twelve women had epilepsy, one had swallowing problems resulting from tracheoesophageal surgery, and one had a respiratory illness. Prior to the aspiration event, low oxygen saturation was observed in seven of the women. Gastric contents were detected in the oropharynx of six of the women, and four women vomited prior to aspiration. Pre-aspiration management of the cases is shown in Table 3. Seven of the 12 cases (58%) were known to have had oral intake during the 6 hours preceding the aspiration event: three food and fluid, one fluid, and three clear fluid only. Seven of the 12 cases (58%) had received antacid prophylaxis in the 6 hours preceding the aspiration event: six women had received ranitidine, three sodium citrate, and two metoclopramide. Six of the 12 women (50%) had received opioids. Airway devices were in place in six patients at the time of aspiration; these included one oropharyngeal airway, two supraglottic airway devices and two cuffed tracheal tubes. One woman aspirated following oesophageal intubation. Of the nine cases in whom aspiration was thought to be associated with general anaesthesia, seven (78%) received protective cricoid pressure. In women in whom aspiration occurred at the end of surgery and anaesthesia, none was extubated in the left lateral position. Details of case management following the aspiration event are presented in Table 4. Six cases (50%) underwent tracheal intubation, four (33%) received cricoid pressure, and seven (58%) had their oropharynx suctioned to prevent further aspiration. Four patients (33%) were placed in the left lateral, head down position, and ten (83%) received supplementary oxygen. Bronchoscopy was performed on one woman. All of the cases received antibiotics, with co-amoxiclav (n=5), metronidazole (n=5), and tazocin (n=4) being the most common. Only two women (17%) received steroids (budesonide/formoterol and hydrocortisone). Maternal and infant outcome measures for cases and controls are presented in Table 5. Women who aspirated were statistically more likely to be admitted to the ITU (p<0.001). Infants of women who aspirated had significantly lower Apgar scores (p=0.003) and were more likely to be admitted to the NICU (p<0.001) or to be stillborn (p=0.048) compared with infants born to women in the control group. | Category | Outcome | Cases | Controls | p-value* | |---|---|---|---|---| | Maternal | N = 12 | N= 1982 | || | Maternal ITU admission, n (%) | <0.001 | ||| | Yes | 9 (75) | 23 (1) | || | No | 3 (25) | 1955 (99) | || | Unknown | 0 (0) | 4 (0.2) | || | Length of ITU stay (days), median (IQR) | 2 (1–4) | ||| | Maternal death, n (%) | 1 (8) | 0 (0) | || | HDU admission, n (%) | |||| | Yes | 5 (42) | ||| | No | |||| | Length of HDU stay (days), median (IQR) | 2 (1–3) | ||| | Miscarriage, n (%) | |||| | Yes | 1 (8) | ||| | No | 11 (92) | ||| | Unknown | 0 (0) | ||| | Morbidity, n (%) | |||| | Mechanical ventilation | 6 (50) | ||| | ECMO | 1 (8) | ||| | Cardiorespiratory arrest | 2 (17) | ||| | Resuscitation required | 6 (50) | ||| | Length hospital stay post aspiration (days), median (IQR) | 8 (5–18) | ||| | Infant | Stillbirth, n (%) | N = 12** | N = 2013 | 0.048 | | Yes | 1 (8) | 8 (0.4) | || | No | 11 (92) | 1996 (99) | || | Unknown | 0 (0) | 9 (0.4) | || | Infant death, n (% of live births) | N = 11 | N = 2005 | 1.000 | | | Yes | 0 (0) | 4 (0.2) | || | No | 11 (100) | 1989 (99) | || | Unknown | 0 (0) | 12 (0.6) | || | NICU admission, n (%) | N = 11 | N = 2005 | <0.001 | | | Yes | 6 (55) | 147 (7) | || | No | 4 (36) | 1846 (92) | || | Unknown | 1 (9) | 12 (0.6) | || | Apgar score, median (IQR) | 8 (5–9) | 9 (9–10) | 0.003 | *Calculated without missing values, using Fisher’s exact test for categorical variables and Wilcox rank-sum test for continuous variables **12 babies born to 11 mothers ITU, intensive therapy unit; HDU, high-dependency unit; IQR, interquartile range; ECMO, extracorporeal membrane oxygenation; NICU, neonatal intensive care unit The incidence of pulmonary aspiration in obstetric patients in the UK is very low, with an incidence estimated by this study of 5.2 per 1,000,000 maternities (one in 192,309 maternities or 0.00052%). No significant temporal trend was found in the study's timeframe. Duration of gestation, parity, caesarean delivery, and general anaesthesia for caesarean section were all statistically significantly associated with the occurrence of pulmonary aspiration. Pulmonary aspiration of gastric contents was a significant cause of anaesthesia-related mortality during the early years of the UK Maternal Confidential Death Enquiries, which began in the early 1950s. In the 1950s and 1960s, aspiration accounted for a substantial proportion of maternal deaths, highlighting the need for better airway management and preoperative fasting guidelines25,26. Improved training, advancements in anaesthetic techniques such as endotracheal intubation, and the introduction of cricoid pressure in the 1970s led to a gradual decline in these deaths, although the role of cricoid pressure remains controversial. The 1980s and beyond saw further reductions due to the implementation of rigorous preoperative fasting protocols and antacid prophylaxis. Reports from the Confidential Enquiries into Maternal Deaths during these decades reflect a marked improvement in maternal safety, with pulmonary aspiration becoming a less common cause of anaesthesia-related mortality27,28. There have been only two maternal deaths from anaesthesia related aspiration in the UK since 200029,30. This low incidence has been attributed to factors including improvements in training, the success of strategies such as the use of antacids, fasting protocols and finally, the increased use of neuraxial techniques31. While the use of neuraxial anaesthesia reduces the risk of pulmonary aspiration compared with general anaesthesia, its precise role in the reduction in obstetric airway deaths in the United Kingdom is unclear, although it would seem self-evident that an increase in the use of neuraxial anaesthesia will lead to a reduction in deaths relating almost entirely to a complication linked to loss of consciousness. A survey of UK obstetric anaesthetic practice in the 1980s and 1990s demonstrated that although the use of neuraxial anaesthesia increased, so too did the number of emergency caesarean sections32. Thus, although the proportion of emergency caesarean sections performed under general anaesthesia – the group identified as most at risk from pulmonary aspiration – decreased, the absolute number remained constant. However, fewer of these women died as a result of aspiration. Therefore, although the increased use of neuraxial anaesthesia may have prevented an increase in the number of maternal deaths from aspiration, it does not fully explain the dramatic decline of aspiration deaths in the UK. However, it is clear that avoiding general anaesthesia almost eliminates the risk of aspiration. All the patients in our study who aspirated in association with anaesthesia had some oral intake in the 6 hours before aspirating. The practice of restricting oral intake during labour was initially introduced in the 1940s in the USA and UK in case of general anaesthesia being required for caesarean section. This was intended to prevent gastric aspiration pneumonitis, as described by Mendelson5. Despite the widespread practice of restricting oral intake, debate about this topic has continued, particularly since the publication of the NICE Intrapartum Care guideline in 200711, which recommended a less restrictive attitude to oral intake in labour in women who had not received opioids. The inherent problem is that epidural analgesia in labour predominantly employs the use of low dose local anaesthetic solutions combined with fentanyl. Opioids administered via any route can delay gastric emptying33,34. However, an effective epidural in situ also reduces the chance of general anaesthesia being needed should a caesarean section be required7. Alongside concerns about the risks of oral intake are concomitant concerns about potential detrimental effects of limiting calorific intake in labour. A randomised trial examining the effect of a light, low-fat diet versus water alone on labouring women’s metabolic profile, labour outcomes, and aspiration risk found that women who received the light diet were less likely to become ketotic compared with women who received water only. There were no differences in the duration or outcome of labour. However, mothers in the eating group had significantly larger residual gastric volumes at the time of delivery suggesting a potential higher risk of aspiration if general anaesthesia was required35. Conversely, a meta-analysis that compared a strategy of less-restrictive food intake in labour with a more restrictive food intake strategy on maternal and neonatal outcomes36 found that across ten studies including almost 4000 low-risk labouring women, a less-restrictive food intake policy was associated with a significantly shorter labour duration compared with a more restrictive policy and identified no cases of aspiration in either group. Aspiration prophylaxis was not consistently used in the women who aspirated in this study. A UK survey of obstetric anaesthesia lead consultants between 1994 and 2004 found a decrease in the proportion of units using routine acid aspiration prophylaxis in all patients and an increase in the proportion using acid aspiration prophylaxis in at-risk groups only37. Only 7% of respondents never used antacid prophylaxis. A large Australian study found that only 64% of women who underwent emergency caesarean section had received antacid medication12. A Cochrane review examined the effectiveness of interventions at caesarean section to reduce the risk of aspiration pneumonitis found that antacids in combination with H2 antagonists were more effective than either no intervention or antacids alone in preventing low gastric pH38. However, none of the studies included in that review evaluated potential harms or substantive clinical outcomes, and the quality of studies was poor. The continued use of these agents in women identified as being at higher risk of operative intervention would seem prudent and is recommended by the 2017 UK Confidential Maternal Death Enquiries Report30. A large proportion of cases had relevant medical problems including epilepsy, tracheoesophageal surgery causing swallowing problems and gastric bypass. It is unclear whether these conditions could be predisposing, or due to chance, but conditions such as these should be considered when identifying high-risk women to ensure the presence of a senior anaesthetist, giving birth in a consultant-led facility, fasting during labour and antacid prophylaxis. Of the nine women who aspirated in association with general anaesthesia, seven received cricoid pressure. Cricoid pressure is still used routinely during obstetric general anaesthesia in the UK39 and recommended for obstetric general anaesthesia40. However, there is no high-quality evidence demonstrating its effect in reducing aspiration and concerns exist about its effect on visualisation of the glottis during intubation40. The women who aspirated had significantly worse outcomes, showing that pulmonary aspiration has serious sequelae. It is more difficult to draw conclusions about the fetal outcomes. Aspiration of gastric contents can increase the difficulties associated with airway management and potentially lead to hypoxia, which can affect fetal outcomes. However, major confounding factors exist for all studies involving general anaesthesia for caesarean section and neonatal outcomes and it is therefore impossible to imply causation. This study was prospective and population-based, and did not rely on retrospective hospital records, which often do not contain the necessary information. This approach also standardised and simplified data collection. Inclusion of all consultant-led maternity units in the UK meant that no extrapolation was required to gain national-level data to estimate incidence, eliminating selection bias. However, despite this study being national and conducted over three years, we still had very limited power to identify statistically significant risk factors and were not able to investigate their relationship to the severity of the outcome. This study has shown that the incidence of pulmonary aspiration in obstetrics is very low, despite probable increases in food and fluid intake during labour following NICE policy changes11. This suggests that current anaesthetic techniques are effective in preventing aspiration events, although the high morbidity among cases emphasises the importance of prevention. The recognised risk factors of caesarean section and general anaesthesia were statistically significantly associated with aspiration, but the low power of the study meant no definite conclusions could be drawn about other probable risk factors including BMI, and lack of data for controls meant that effects of antacid prophylaxis, receipt of opioids, and anaesthetic technique could not be evaluated. Ideally, further studies would look to combine national data from several countries to gain sufficient power to draw statistically significant conclusions about risk factors and ideal management. The results of this study suggest a reassuringly low incidence of pulmonary aspiration in obstetric anaesthetic practice; however, the complication remains a risk of general anaesthesia. Future research should focus on developing methods to accurately identify women at risk of aspiration. Gastric ultrasound has emerged as a valuable tool in obstetric anaesthetic practice, offering real-time visualisation of gastric contents and volume41. This non-invasive technique allows anaesthetists to assess the risk of pulmonary aspiration by identifying patients with full stomachs, which is particularly crucial in emergency situations or when adherence to fasting guidelines is uncertain. By providing immediate and accurate information, gastric ultrasound may help stratify patients' risk and tailor anaesthetic management to enhance safety. Its use can lead to more informed decisions regarding the need for rapid sequence induction, the application of cricoid pressure, or alternative airway management strategies, ultimately reducing the incidence of aspiration-related complications in obstetric patients. The existing UKOSS programme has been approved by the North London REC1 (ref. 10/HO717/20) and a substantive amendment was approved on 30/04/2012 to include pulmonary aspiration in pregnancy. Consent was not required for the collection of anonymous routine data. Data cannot be shared because of confidentiality issues and potential identifiability of sensitive data as identified in the Research Ethics Committee approval. Requests to access the data can be made by contacting the National Perinatal Epidemiology Unit data access committee via [email protected]. The Research Ethics Committee approved the study on the basis that access will only be allowed after review of the request by the UK Obstetric Surveillance System Steering Committee. The estimated response time for requests is 4 weeks. Data sharing outside the UK or the European Union may require consultation with the UK Health Research Authority. For more information, please refer to the National Perinatal Epidemiology Unit Data Sharing Policy available at: https://www.npeu.ox.ac.uk/assets/downloads/npeu/policies/Data_Sharing_Policy.pdf Preliminary incidence data from the first two years of surveillance were presented at the annual meeting of the Obstetric Anaesthetists' Association in May 2016 in Manchester, UK, and published in abstract form as: Knight M, Bogod D, Lucas DN, Quinn A and K. JJ. (2016). Pulmonary aspiration during pregnancy or immediately postpartum in the UK: a two-year national descriptive study. International Journal of Obstetric Anesthesia 26(Supplement 1): S6-S54 (Available from: https://www.obstetanesthesia.com/article/S0959-289X(16)00034-0/pdf). All statistical analyses were performed using STATA v13.1, a commercial software. All analyses in this study can be replicated using freely available statistical software such as R. The authors report this study on behalf of the UKOSS Aspiration in Pregnancy investigators: Vikash Mistry, National Perinatal Epidemiology Unit; David Bogod, Nottingham City Hospital (retired); Audrey Quinn, James Cook University Hospital (retired); and Kim Hinshaw, Sunderland Royal Hospital. We would like to thank the UK Obstetric Surveillance System (UKOSS) reporting clinicians who notified cases and completed the data-collection forms, and the UKOSS staff who enable these studies to be completed. Faculty Opinions recommendedReferences - 1. Cook TM, Woodall N, Frerk C, et al.: Major complications of airway management in the UK: results of the Fourth National Audit Project of the Royal College of Anaesthetists and the difficult airway society. Part 1: anaesthesia. Br J Anaesth. 2011; 106(5): 617–31. PubMed Abstract | Publisher Full Text - 2. Pinder A: Complications of obstetric anaesthesia. Curr Anaesth Crit Care. 2006; 17(3–4): 151–62. Publisher Full Text - 3. 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Cantwell R, Clutton-Brock T, Cooper G, et al.: Saving mothers' lives: reviewing maternal deaths to make motherhood safer: 2006-2008. The eighth report of the confidential enquiries into maternal deaths in the United Kingdom. BJOG. 2011; 118(Suppl 1): 1–203. PubMed Abstract | Publisher Full Text - 30. Knight M, Nair M, Tuffnell D, et al.: Saving lives, improving mothers’ care – lessons learned to inform maternity care from the UK and Ireland confidential enquiries into maternal deaths and morbidity 2013– 2015. Oxford: National Perinatal Epidemiology Unit, University of Oxford, 2017. Reference Source - 31. Lucas DN, Bamber JH: UK confidential enquiry into maternal deaths - still learning to save mothers' lives. Anaesthesia. 2018; 73(4): 416–20. PubMed Abstract | Publisher Full Text - 32. Brown GW, Russell IF: A survey of anaesthesia for caesarean section. Int J Obstet Anesth. 1995; 4(4): 214–8. Reference Source - 33. 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PubMed Abstract | Publisher Full Text | Free Full Text Author details Author details 1 London North West Healthcare NHS Trust, Watford Road, Harrow, Middlesex, HA1 3UJ, UK 2 National Perinatal Epidemiology Unit, University of Oxford, Old Road Campus, Headington, Oxford, OX3 7LF, UK 2 National Perinatal Epidemiology Unit, University of Oxford, Old Road Campus, Headington, Oxford, OX3 7LF, UK Nuala Lucas Roles: Conceptualization, Data Curation, Formal Analysis, Investigation, Project Administration, Writing – Original Draft Preparation Roles: Conceptualization, Data Curation, Formal Analysis, Investigation, Project Administration, Writing – Original Draft Preparation Alison Gooda Roles: Data Curation, Formal Analysis, Methodology, Writing – Original Draft Preparation Roles: Data Curation, Formal Analysis, Methodology, Writing – Original Draft Preparation Ruth Tunn Roles: Validation, Writing – Review & Editing Roles: Validation, Writing – Review & Editing Marian Knight Roles: Conceptualization, Data Curation, Formal Analysis, Funding Acquisition, Investigation, Methodology, Project Administration, Resources, Supervision, Writing – Review & Editing Roles: Conceptualization, Data Curation, Formal Analysis, Funding Acquisition, Investigation, Methodology, Project Administration, Resources, Supervision, Writing – Review & Editing Competing interests Nuala Lucas president of the Obstetric Anaesthetists' Association. Marian Knight is an NIHR Senior Investigator; award reference NIHR303806. All other authors: no competing interests were disclosed. Grant information This project is funded by the National Institute for Health and Care Research (NIHR) under its NIHR Policy Research Programme as part of the programme of work of the Policy Research Unit in Maternal Health and Care, National Perinatal Epidemiology Unit (Grant Reference Number 108/0001). MK is an NIHR Senior Investigator (Award Reference Number NIHR303806). The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Copyright © 2025 Lucas N et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. metrics VIEWS $counts.viewCount downloads Citations CITE how to cite this article Lucas N, Gooda A, Tunn R et al. Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.13797.1) NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article. track receive updates on this article Track an article to receive email alerts on any updates to this article. Current Reviewer Status: ? Key to Reviewer Statuses VIEW HIDE ApprovedThe paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approvedFundamental flaws in the paper seriously undermine the findings and conclusions Version 1 VERSION 1 PUBLISHED 08 Jan 2025 Views 0 How to cite this report: Rosseland LA and Heitmann G. Reviewer Report For: Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.14988.r34474) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34474 https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34474 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Reviewer Report 08 Feb 2025 Leiv Arne Rosseland, University of Oslo, Oslo, Norway; Research & Development, Division of Emergencies and Critical Care, Oslo University Hospital, Oslo, Oslo, Norway Approved with Reservations VIEWS 0 Thank you for the opportunity to assess this manuscript which presents an important presentation of the risks for pulmonary aspiration in obstetric patients and how this serious anaesthetic complication has been managed in UK. The high-quality of ... Continue reading 2. Bouvet L, Schulz T, Piana F, Desgranges FP, et al.: Pregnancy and Labor Epidural Effects on Gastric Emptying: A Prospective Comparative Study.Anesthesiology. 2022; 136 (4): 542-550 PubMed Abstract | Publisher Full Text 3. Sakai T, Planinsic RM, Quinlan JJ, Handley LJ, et al.: The incidence and outcome of perioperative pulmonary aspiration in a university hospital: a 4-year retrospective analysis.Anesth Analg. 2006; 103 (4): 941-7 PubMed Abstract | Publisher Full Text We confirm that we have read this submission and believe that we have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however we have significant reservations, as outlined above. Close The high-quality of ... Continue reading Thank you for the opportunity to assess this manuscript which presents an important presentation of the risks for pulmonary aspiration in obstetric patients and how this serious anaesthetic complication has been managed in UK. The high-quality of the UK surveillance system for reporting and evaluating mishaps in obstetrics is a source of important knowledge relevant to the international community of obstetric anaesthetists. The objectives of this study are well presented and the methods used are appropriate and easy to understand. The main finding of this study was that the risk of pulmonary aspiration was low in pregnant women. However, as stated by the authors, this was very different a few decades ago. Owing to modern anaesthesia practices, we have succeeded in making obstetric anaesthesia safer. As the data was collected a decade ago it may limit generalisability as this development has continued. Reflexions on if, and how the practice has changed since the data was collected would help the reader evaluate the representativeness of the findings a decade after the data was collected. In the introduction, the authors describe the risk factors and present an overview of the topic. However, some statements and conclusions remain debatable. For example, pain may be a risk factor for delayed gastric emptying. Even if this is not evidenced by high-quality studies, it is regarded as common-sense knowledge based on the experience of generations of anaesthesiologists. However, a statement for which the evidence is stronger is regarding gastric emptying during pregnancy. Delayed gastric emptying during the first trimester is well documented. However, gastric emptying is not delayed in the second and third trimesters. The authors referred to Ref. #2 to support the statement that progesterone causes delayed gastric emptying. However, this is not in line with ref. #2. We question this statement and urge the authors to modify it accordingly, as updated information on the topic questions this, e.g., the recent review by Lawson et al (2024[Ref-1]) (https://doi.org/10.1016/j.bja.2024.09.005). In the plain language summary, they conclude that “during the three years, 12 pregnant women in the UK experienced pulmonary aspiration when giving birth.” When reading the entire manuscript, we were unsure if all cases occurred when giving birth. In Table 1, there was one case of missing information regarding the mode of delivery and anaesthesia, and 9 of 10 women had general anaesthesia. Please clarify if all cases occurred during delivery. It might seem like there is one contradiction between the results and statement in the discussion and that there is a need for clarification. It is stated: “Seven of the 12 cases (58%) were known to have had oral intake during the 6 hours preceding the aspiration event” it is also stated that 9 patients received GA. However, in the discussion part it is stated: “All the patients in our study who aspirated in association with anesthesia had some oral intake in the 6 hours before aspirating.” It is therefore unclear if all the 9 patients who received GA, and by this aspirated in association with anesthesia, had some oral intake. Another statement that might be debatable is: “Opioids administered via any route can delay gastric emptying. However, an effective epidural in situ also reduces the chance of general anaesthesia being needed should a caesarean section be required”. One of the references is from 1975, and there might be more recent publications on this topic. A publication from 2022 found that EDA (with opioids) increased gastric emptying during labour compared to the control group. Bouvet L et al (2022 [Ref-2]) (DOI: 10.1097/ALN.0000000000004133) The results indicate a low risk of pulmonary aspiration when general anaesthesia is given during delivery, whereas the risk during pregnancy, in general, is zero. From a practical clinical perspective, general anaesthesia during pregnancy is safe, but general anaesthesia during delivery carries a small but significant risk of pulmonary aspiration. This also leads us to the first question: delayed gastric emptying is not a significant problem in the second and third trimesters. Women in labour experience pain and most often do not fast, both of which are reasons to expect a high risk of aspiration. As shown in Table 1, only four cases were pre-labour caesarean deliveries. As most planned C-sections are performed under regional anaesthesia, the population of planned C-sections performed under general anaesthesia is highly selected and should be interpreted with caution. If available, more detailed information on these four cases would be interesting, such as individual risk factors, amount and type of food intake during the previous hours, indication for caesarean section, and general anaesthesia. Presumably, these four cases were emergency deliveries, but since the cases are summarised in different tables, it is difficult to get an understanding of the individual clinical situation. Presenting the data as a case series might have made this easier. The information that all patients who aspirated in association with anaesthesia had some food intake during the last 6 hours may be one of the most important messages: anaesthesiologists should follow general rules for fasting for all planned C-sections, even when regional anaesthesia is the chosen anaesthesia method as we never know who will need general anaesthesia due to incomplete regional anaesthesia, post-partum haemorrhage, or other unforeseen complications. As these cases can be interpreted as pulmonary aspiration in non-fasting patients undergoing urgent surgery, it would be interesting to compare the risk with data from a non-pregnant population. Tetsuro et al (2006 [Ref-3]) reported an incidence of 1 in 7103 GAs (DOI: 10.1213/01.ane.0000237296.57941.e7). The high-quality of the UK surveillance system for reporting and evaluating mishaps in obstetrics is a source of important knowledge relevant to the international community of obstetric anaesthetists. The objectives of this study are well presented and the methods used are appropriate and easy to understand. The main finding of this study was that the risk of pulmonary aspiration was low in pregnant women. However, as stated by the authors, this was very different a few decades ago. Owing to modern anaesthesia practices, we have succeeded in making obstetric anaesthesia safer. As the data was collected a decade ago it may limit generalisability as this development has continued. Reflexions on if, and how the practice has changed since the data was collected would help the reader evaluate the representativeness of the findings a decade after the data was collected. In the introduction, the authors describe the risk factors and present an overview of the topic. However, some statements and conclusions remain debatable. For example, pain may be a risk factor for delayed gastric emptying. Even if this is not evidenced by high-quality studies, it is regarded as common-sense knowledge based on the experience of generations of anaesthesiologists. However, a statement for which the evidence is stronger is regarding gastric emptying during pregnancy. Delayed gastric emptying during the first trimester is well documented. However, gastric emptying is not delayed in the second and third trimesters. The authors referred to Ref. #2 to support the statement that progesterone causes delayed gastric emptying. However, this is not in line with ref. #2. We question this statement and urge the authors to modify it accordingly, as updated information on the topic questions this, e.g., the recent review by Lawson et al (2024[Ref-1]) (https://doi.org/10.1016/j.bja.2024.09.005). In the plain language summary, they conclude that “during the three years, 12 pregnant women in the UK experienced pulmonary aspiration when giving birth.” When reading the entire manuscript, we were unsure if all cases occurred when giving birth. In Table 1, there was one case of missing information regarding the mode of delivery and anaesthesia, and 9 of 10 women had general anaesthesia. Please clarify if all cases occurred during delivery. It might seem like there is one contradiction between the results and statement in the discussion and that there is a need for clarification. It is stated: “Seven of the 12 cases (58%) were known to have had oral intake during the 6 hours preceding the aspiration event” it is also stated that 9 patients received GA. However, in the discussion part it is stated: “All the patients in our study who aspirated in association with anesthesia had some oral intake in the 6 hours before aspirating.” It is therefore unclear if all the 9 patients who received GA, and by this aspirated in association with anesthesia, had some oral intake. Another statement that might be debatable is: “Opioids administered via any route can delay gastric emptying. However, an effective epidural in situ also reduces the chance of general anaesthesia being needed should a caesarean section be required”. One of the references is from 1975, and there might be more recent publications on this topic. A publication from 2022 found that EDA (with opioids) increased gastric emptying during labour compared to the control group. Bouvet L et al (2022 [Ref-2]) (DOI: 10.1097/ALN.0000000000004133) The results indicate a low risk of pulmonary aspiration when general anaesthesia is given during delivery, whereas the risk during pregnancy, in general, is zero. From a practical clinical perspective, general anaesthesia during pregnancy is safe, but general anaesthesia during delivery carries a small but significant risk of pulmonary aspiration. This also leads us to the first question: delayed gastric emptying is not a significant problem in the second and third trimesters. Women in labour experience pain and most often do not fast, both of which are reasons to expect a high risk of aspiration. As shown in Table 1, only four cases were pre-labour caesarean deliveries. As most planned C-sections are performed under regional anaesthesia, the population of planned C-sections performed under general anaesthesia is highly selected and should be interpreted with caution. If available, more detailed information on these four cases would be interesting, such as individual risk factors, amount and type of food intake during the previous hours, indication for caesarean section, and general anaesthesia. Presumably, these four cases were emergency deliveries, but since the cases are summarised in different tables, it is difficult to get an understanding of the individual clinical situation. Presenting the data as a case series might have made this easier. The information that all patients who aspirated in association with anaesthesia had some food intake during the last 6 hours may be one of the most important messages: anaesthesiologists should follow general rules for fasting for all planned C-sections, even when regional anaesthesia is the chosen anaesthesia method as we never know who will need general anaesthesia due to incomplete regional anaesthesia, post-partum haemorrhage, or other unforeseen complications. As these cases can be interpreted as pulmonary aspiration in non-fasting patients undergoing urgent surgery, it would be interesting to compare the risk with data from a non-pregnant population. Tetsuro et al (2006 [Ref-3]) reported an incidence of 1 in 7103 GAs (DOI: 10.1213/01.ane.0000237296.57941.e7). - Is the work clearly and accurately presented and does it cite the current literature? Partly - Is the study design appropriate and is the work technically sound? Yes - Are sufficient details of methods and analysis provided to allow replication by others? Yes - If applicable, is the statistical analysis and its interpretation appropriate? Yes - Are all the source data underlying the results available to ensure full reproducibility? Partly - Are the conclusions drawn adequately supported by the results? Partly

1. Lawson J, Howle R, Popivanov P, Sidhu J, et al.: Gastric emptying in pregnancy and its clinical implications: a narrative review.Br J Anaesth. 2025; 134 (1): 124-167 PubMed Abstract | Publisher Full Text2. Bouvet L, Schulz T, Piana F, Desgranges FP, et al.: Pregnancy and Labor Epidural Effects on Gastric Emptying: A Prospective Comparative Study.Anesthesiology. 2022; 136 (4): 542-550 PubMed Abstract | Publisher Full Text 3. Sakai T, Planinsic RM, Quinlan JJ, Handley LJ, et al.: The incidence and outcome of perioperative pulmonary aspiration in a university hospital: a 4-year retrospective analysis.Anesth Analg. 2006; 103 (4): 941-7 PubMed Abstract | Publisher Full Text Competing Interests: No competing interests were disclosed. Reviewer Expertise: Obstetric anaesthesia CITE HOW TO CITE THIS REPORT Rosseland LA and Heitmann G. Reviewer Report For: Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.14988.r34474) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34474 https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34474 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. Views 0 How to cite this report: Leffert LR. Reviewer Report For: Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.14988.r34471) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34471 https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34471 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Reviewer Report 05 Feb 2025 Lisa R Leffert, Yale Medicine/Yale New Haven Health System, Yale, USA Approved VIEWS 0 The authors present a population-based surveillance and case-controlled study utilizing the UKOSS dataset (2013-2016) to ascertain the incidence of clinically significant pulmonary aspiration during pregnancy or immediately postpartum. This is a highly worthwhile endeavor, as concern for aspiration continues to ... Continue reading I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard. Close The authors present a population-based surveillance and case-controlled study utilizing the UKOSS dataset (2013-2016) to ascertain the incidence of clinically significant pulmonary aspiration during pregnancy or immediately postpartum. This is a highly worthwhile endeavor, as concern for aspiration continues to be a major consideration driving obstetric anesthesia practice. Commenced in 2005, UKOSS has been extraordinarily well positioned to study rare and severe complications of pregnancy such as this, due to its high degree of data completion and ability to be modified to address specific research questions. Unlike in many others, in this study aspiration is clearly defined. Patients of interest are those that have clinically significant aspiration defined as: “women at 20 weeks’ gestation or greater who had an unprotected airway while unconscious, semiconscious or paralyzed as well as a clinical history consistent with the regurgitation of stomach contents and pulmonary aspiration, such as vomiting or gastric contents seen in the oropharynx that is accompanied by signs of respiratory compromise requiring supplementary oxygen and antibiotics or levels 2/3 respiratory support without any other apparent cause. “ These patients have been compared to UKOSS controls. The results indicate that aspiration is 1) a rare event (1: 192,309 maternities) with no more than five cases in each of the three consecutive years, and 2) not fatal in this sample population. Not surprisingly, 9/12 cases were associated with general anesthesia. All the non-general anesthesia patients had comorbidities that likely put them at risk for aspiration, such as recurrent seizures or depressed consciousness. Detailed clinical data are provided as to the affected patients’ po status, premedication, and airway management. Whereas these data are interesting to consider considering our current, highly variable, premedication and liberalizing po practices, they are inconclusive due to the very small sample size. What is not provided, but would be of interest, is the incidence of aspiration in the pregnant women who received general anesthesia for cesarean delivery. In addition, the readers would benefit greatly from a more detailed discussion of the rationale for choosing unmatched controls from UKOSS datasets from two decades ago, rather than from the same dataset as the cases of aspiration. In the decade since this aspiration data was collected, intubation practices, including the more widespread use of videolaryngoscopy, have evolved, raising the possibility that the contemporary aspiration incidence has changed. Perhaps it is even lower. Finally, univariate analysis of these data showed that duration of gestation, parity, cesarean delivery, and general anesthesia for cesarean delivery were all statistically significantly associated with the occurrence of pulmonary aspiration. As noted by the authors, the total number of cases was insufficient to permit the use of a multivariable model and establishment of independent associations. Notably, both earlier gestation and nulliparity are known to be associated with cesarean delivery, which was associated with aspiration in this study. In summary, these data support a very low risk of clinically significant aspiration in pregnant women, especially in those who have not received general anesthesia nor have relevant comorbidities. As the authors state, what remains is to identify those minority of pregnant patients at higher risk, so that more stringent harm reduction strategies can be focused on them. Respectfully Submitted, Lisa Leffert, MD Unlike in many others, in this study aspiration is clearly defined. Patients of interest are those that have clinically significant aspiration defined as: “women at 20 weeks’ gestation or greater who had an unprotected airway while unconscious, semiconscious or paralyzed as well as a clinical history consistent with the regurgitation of stomach contents and pulmonary aspiration, such as vomiting or gastric contents seen in the oropharynx that is accompanied by signs of respiratory compromise requiring supplementary oxygen and antibiotics or levels 2/3 respiratory support without any other apparent cause. “ These patients have been compared to UKOSS controls. The results indicate that aspiration is 1) a rare event (1: 192,309 maternities) with no more than five cases in each of the three consecutive years, and 2) not fatal in this sample population. Not surprisingly, 9/12 cases were associated with general anesthesia. All the non-general anesthesia patients had comorbidities that likely put them at risk for aspiration, such as recurrent seizures or depressed consciousness. Detailed clinical data are provided as to the affected patients’ po status, premedication, and airway management. Whereas these data are interesting to consider considering our current, highly variable, premedication and liberalizing po practices, they are inconclusive due to the very small sample size. What is not provided, but would be of interest, is the incidence of aspiration in the pregnant women who received general anesthesia for cesarean delivery. In addition, the readers would benefit greatly from a more detailed discussion of the rationale for choosing unmatched controls from UKOSS datasets from two decades ago, rather than from the same dataset as the cases of aspiration. In the decade since this aspiration data was collected, intubation practices, including the more widespread use of videolaryngoscopy, have evolved, raising the possibility that the contemporary aspiration incidence has changed. Perhaps it is even lower. Finally, univariate analysis of these data showed that duration of gestation, parity, cesarean delivery, and general anesthesia for cesarean delivery were all statistically significantly associated with the occurrence of pulmonary aspiration. As noted by the authors, the total number of cases was insufficient to permit the use of a multivariable model and establishment of independent associations. Notably, both earlier gestation and nulliparity are known to be associated with cesarean delivery, which was associated with aspiration in this study. In summary, these data support a very low risk of clinically significant aspiration in pregnant women, especially in those who have not received general anesthesia nor have relevant comorbidities. As the authors state, what remains is to identify those minority of pregnant patients at higher risk, so that more stringent harm reduction strategies can be focused on them. Respectfully Submitted, Lisa Leffert, MD - Is the work clearly and accurately presented and does it cite the current literature? Yes - Is the study design appropriate and is the work technically sound? Partly - Are sufficient details of methods and analysis provided to allow replication by others? Partly - If applicable, is the statistical analysis and its interpretation appropriate? Yes - Are all the source data underlying the results available to ensure full reproducibility? Yes - Are the conclusions drawn adequately supported by the results? Partly Competing Interests: No competing interests were disclosed. CITE HOW TO CITE THIS REPORT Leffert LR. Reviewer Report For: Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.14988.r34471) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34471 https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34471 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. Views 0 How to cite this report: van Dyk APD. Reviewer Report For: Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.14988.r34404) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34404 https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34404 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Reviewer Report 04 Feb 2025 Associate Professor Dominique van Dyk, University of Cape Town, Cape Town, Western Cape, South Africa Approved VIEWS 0 Congratulations to the authors on attempting to answer lingering questions concerning the recent prevalence of, and risk factors for, an obstetric complication which has become rare, but which remains potentially very serious when it occurs. The introduction ... Continue reading I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard. Close The introduction ... Continue reading Congratulations to the authors on attempting to answer lingering questions concerning the recent prevalence of, and risk factors for, an obstetric complication which has become rare, but which remains potentially very serious when it occurs. The introduction contextualises the historical background of aspiration in obstetric anaesthesia practice and outlines the traditional thinking surrounding pregnancy physiology and risk factors for aspiration. However, newer physiological work suggesting that stomach emptying is chiefly slowed once the pregnant person goes into labour was not mentioned. The authors evidently put much thought and consultation into the study design, and I agree with the prospective case-control approach. There are generally some trade-offs when developing a pragmatic case reference form (CRF) - some individual case detail is invariably lost. The CRF employed sought to overcome this by asking for many data points (the relevance of some not being obvious) and was quite lengthy, but given the low incidence of aspiration, it seems to have been completed adequately when aspiration was identified, as per case definition. One addition I might have suggested to section 5b (Aspiration associated with general anaesthesia) would be laryngoscopic technique in cases where general anaesthesia was utilised- direct vs video laryngoscopy, as inadvertent oesophageal intubation may be reduced by video laryngoscopy. There was no record of difficulties associated with airway management and the need to release cricoid pressure or institute facemask ventilation. Anaesthesia technique for caesarean was not recorded by intended method- there is a difference between a "controlled" induction of general anaesthesia and emergent intubation due to cardiac arrest, on-table convulsions, or total spinal anaesthesia. A strength of the methodology was the comprehensive coverage of all consultant-led obstetric facilities, and the cross-check with MBRRACE-UK mortality reports over the same period. The statistical analysis was appropriate; it was unfortunate that the small event numbers precluded multivariable logistical regression, but the authors were cognisant of this limitation. The authors explained why not all data could be shared and provided a pathway for accessing data upon request to the UKOSS steering committee. This was contained in the Data availability statement. Ultimately, the chief finding is that obstetric aspiration has become rarer in recent years. No new but plausible risk factors could be confirmed given the small case numbers. For the same reason, the description of prophylactic measures and treatment of aspiration may not reflect standard practice in the UK currently. It might have been interesting to discuss the much lower incidence of obstetric aspiration in this prospective study compared with a contemporary multi-centre retrospective study conducted in Israel which investigated the incidence of aspiration during caesarean delivery (see following citation). The introduction contextualises the historical background of aspiration in obstetric anaesthesia practice and outlines the traditional thinking surrounding pregnancy physiology and risk factors for aspiration. However, newer physiological work suggesting that stomach emptying is chiefly slowed once the pregnant person goes into labour was not mentioned. The authors evidently put much thought and consultation into the study design, and I agree with the prospective case-control approach. There are generally some trade-offs when developing a pragmatic case reference form (CRF) - some individual case detail is invariably lost. The CRF employed sought to overcome this by asking for many data points (the relevance of some not being obvious) and was quite lengthy, but given the low incidence of aspiration, it seems to have been completed adequately when aspiration was identified, as per case definition. One addition I might have suggested to section 5b (Aspiration associated with general anaesthesia) would be laryngoscopic technique in cases where general anaesthesia was utilised- direct vs video laryngoscopy, as inadvertent oesophageal intubation may be reduced by video laryngoscopy. There was no record of difficulties associated with airway management and the need to release cricoid pressure or institute facemask ventilation. Anaesthesia technique for caesarean was not recorded by intended method- there is a difference between a "controlled" induction of general anaesthesia and emergent intubation due to cardiac arrest, on-table convulsions, or total spinal anaesthesia. A strength of the methodology was the comprehensive coverage of all consultant-led obstetric facilities, and the cross-check with MBRRACE-UK mortality reports over the same period. The statistical analysis was appropriate; it was unfortunate that the small event numbers precluded multivariable logistical regression, but the authors were cognisant of this limitation. The authors explained why not all data could be shared and provided a pathway for accessing data upon request to the UKOSS steering committee. This was contained in the Data availability statement. Ultimately, the chief finding is that obstetric aspiration has become rarer in recent years. No new but plausible risk factors could be confirmed given the small case numbers. For the same reason, the description of prophylactic measures and treatment of aspiration may not reflect standard practice in the UK currently. It might have been interesting to discuss the much lower incidence of obstetric aspiration in this prospective study compared with a contemporary multi-centre retrospective study conducted in Israel which investigated the incidence of aspiration during caesarean delivery (see following citation). - Is the work clearly and accurately presented and does it cite the current literature? Partly - Is the study design appropriate and is the work technically sound? Yes - Are sufficient details of methods and analysis provided to allow replication by others? Yes - If applicable, is the statistical analysis and its interpretation appropriate? Yes - Are all the source data underlying the results available to ensure full reproducibility? Partly - Are the conclusions drawn adequately supported by the results? Yes

1. Binyamin Y, Orbach-Zinger S, Ioscovich A, Reina YY, et al.: Incidence and clinical impact of aspiration during cesarean delivery: A multi-center retrospective study.Anaesth Crit Care Pain Med. 2024; 43 (2): 101347 PubMed Abstract | Publisher Full TextCompeting Interests: No competing interests were disclosed. Reviewer Expertise: Obstetric anaesthesia CITE HOW TO CITE THIS REPORT van Dyk APD. Reviewer Report For: Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.14988.r34404) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34404 https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34404 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. Views 0 How to cite this report: Smiley RM. Reviewer Report For: Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.14988.r34469) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34469 https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34469 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Reviewer Report 03 Feb 2025 Approved VIEWS 0 The authors report on 12 cases of pulmonary aspiration in obstetric patients, drawn from the UK Obstetric Surveillance System (UKOSS) form 2005-2014. They estimated this corresponds to approximately 5.2 cases per million deliveries (based on ~ 2.3 million deliveries). They ... Continue reading 2. D'Angelo R, Smiley RM, Riley ET, Segal S: Serious complications related to obstetric anesthesia: the serious complication repository project of the Society for Obstetric Anesthesia and Perinatology.Anesthesiology. 2014; 120 (6): 1505-12 PubMed Abstract | Publisher Full Text 3. Lucas DN, Bamber JH: UK Confidential Enquiry into Maternal Deaths - still learning to save mothers' lives.Anaesthesia. 2018; 73 (4): 416-420 PubMed Abstract | Publisher Full Text I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard. Close The authors report on 12 cases of pulmonary aspiration in obstetric patients, drawn from the UK Obstetric Surveillance System (UKOSS) form 2005-2014. They estimated this corresponds to approximately 5.2 cases per million deliveries (based on ~ 2.3 million deliveries). They compared the 12 cases to a cohort of 1982 controls form other UKOSS studies in an attempt to define risk factors for and associations with aspiration. The results of this study demonstrate the very low incidence of aspiration in modern obstetric practice. In addition, there was only one death among the 12 patients who aspirated, and it was not attributed to the aspiration, so the number of deaths form aspiration was zero. As might be expected, 9 of the cases were associated with general anesthesia, while the other three were associated with a seizure, a semi-conscious patient, and an unspecified cardiac issue. The association with general anesthesia, or altered consciousness is of course expected. There were no other significant associations found with aspiration events, as might be expected with such a small number of events. This report is a very interesting and valuable addition to our knowledge about the true risk of aspiration in pregnancy, which has been a major concerns since Mendelson's seminal report in 1946.(1) This study confirms the very low incidence of the event in actual modern practice, which is consistent with the zero incidence in 250,000 obstetric anesthetics in the SOAP Severe Complication Registry (SCORE) (2), and the very low number reported in recent UK triennial reports of maternal deaths. (3) The low incidence and even lower mortality of aspiration in the UK, a nation where “NPO” rules in labor suites are not nearly as restrictive as in the US does suggest that our restrictions on oral intake in labor may well be too strict, with fears of aspiration being based on historical practices from decades or more than half a century ago rather than reflecting the risks in contemporary practice. My comments in the checkboxes about data availability and the inability to reproduce the authors results are due to the fact that this UKOSS data is not publicly available. The results of this study demonstrate the very low incidence of aspiration in modern obstetric practice. In addition, there was only one death among the 12 patients who aspirated, and it was not attributed to the aspiration, so the number of deaths form aspiration was zero. As might be expected, 9 of the cases were associated with general anesthesia, while the other three were associated with a seizure, a semi-conscious patient, and an unspecified cardiac issue. The association with general anesthesia, or altered consciousness is of course expected. There were no other significant associations found with aspiration events, as might be expected with such a small number of events. This report is a very interesting and valuable addition to our knowledge about the true risk of aspiration in pregnancy, which has been a major concerns since Mendelson's seminal report in 1946.(1) This study confirms the very low incidence of the event in actual modern practice, which is consistent with the zero incidence in 250,000 obstetric anesthetics in the SOAP Severe Complication Registry (SCORE) (2), and the very low number reported in recent UK triennial reports of maternal deaths. (3) The low incidence and even lower mortality of aspiration in the UK, a nation where “NPO” rules in labor suites are not nearly as restrictive as in the US does suggest that our restrictions on oral intake in labor may well be too strict, with fears of aspiration being based on historical practices from decades or more than half a century ago rather than reflecting the risks in contemporary practice. My comments in the checkboxes about data availability and the inability to reproduce the authors results are due to the fact that this UKOSS data is not publicly available. - Is the work clearly and accurately presented and does it cite the current literature? Yes - Is the study design appropriate and is the work technically sound? Yes - Are sufficient details of methods and analysis provided to allow replication by others? Partly - If applicable, is the statistical analysis and its interpretation appropriate? Yes - Are all the source data underlying the results available to ensure full reproducibility? Partly - Are the conclusions drawn adequately supported by the results? Yes

1. MENDELSON CL: The aspiration of stomach contents into the lungs during obstetric anesthesia.Am J Obstet Gynecol. 1946; 52: 191-205 PubMed Abstract | Publisher Full Text2. D'Angelo R, Smiley RM, Riley ET, Segal S: Serious complications related to obstetric anesthesia: the serious complication repository project of the Society for Obstetric Anesthesia and Perinatology.Anesthesiology. 2014; 120 (6): 1505-12 PubMed Abstract | Publisher Full Text 3. Lucas DN, Bamber JH: UK Confidential Enquiry into Maternal Deaths - still learning to save mothers' lives.Anaesthesia. 2018; 73 (4): 416-420 PubMed Abstract | Publisher Full Text Competing Interests: No competing interests were disclosed. Reviewer Expertise: Obstetric anesthesia CITE HOW TO CITE THIS REPORT Smiley RM. Reviewer Report For: Pulmonary aspiration during pregnancy or immediately postpartum in the UK: A population-based case-control study [version 1; peer review: 3 approved, 1 approved with reservations]. NIHR Open Res 2025, 5:2 (https://doi.org/10.3310/nihropenres.14988.r34469) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34469 https://openresearch.nihr.ac.uk/articles/5-2/v1#referee-response-34469 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. Alongside their report, reviewers assign a status to the article: - Approved - Approved with reservations - Not approved | Invited Reviewers | |||| |---|---|---|---|---| | 1 | 2 | 3 | 4 | | | Version 1 08 Jan 25 | read | read | read | read | - Associate Professor Dominique van Dyk, University of Cape Town, Cape Town, South Africa - Lisa R Leffert, Yale Medicine/Yale New Haven Health System, Yale, USA - Leiv Arne Rosseland, University of Oslo, Oslo, Norway; Oslo University Hospital, Oslo, Norway Sign up for content alerts You are now signed up to receive this alert Alongside their report, reviewers assign a status to the article: Approved - the paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations - A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. 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