Agreement between intra-bladder and insufflation pressure readings during laparoscopic surgery.

In total, 202 measurements were done in 18 adult patients undergoing laparoscopy for hernia repair ( n  = 7), cholecystectomy ( n  = 6), groin exploration ( n  = 2), coagulation of endometriosis ( n  = 1), and general examination ( n  = 2). The average IAP 0 was 7.5 ± 1.9 mmHg and 15.4 ± 2.6 mmHg in patients without and with IAH. Table 1 summarizes the baseline characteristics of the patients. Table 1 Baseline characteristics of the sample population Total IAP 0  < 12 IAP 0  ≥ 12 p -value Patients ( n ) 18 11 7 - Measurements ( n ) 202 100 102 - Age [years] 47.9 ± 17.7 47.5 ± 20.2 49.5 ± 13.4 NS BMI [kg/m 2 ] 36.8 ± 10.7 30.5 ± 5.7 42.3 ± 11.2  < 0.001 Male/Female ( n ) 8/10 3/8 5/2 NS IAP 0 [mmHg] 10.7 ± 4.5 7.5 ± 1.9 15.4 ± 2.6  < 0.001 BMI: body mass index; IAP: intra-abdominal pressure; IAP 0 : baseline or opening pressure; NS: Not significant. Baseline characteristics of the sample population BMI: body mass index; IAP: intra-abdominal pressure; IAP 0 : baseline or opening pressure; NS: Not significant. The correlation analysis results are presented in Fig.  1 . Fig. 1 Two-way mixed-effects intraclass and Pearson’s correlation coefficients of 100 and 102 paired IAP insuf and IAP TG measurements in ( a ) patients without baseline IAH ( n  = 11) and ( b ) patients with baseline IAH ( n  = 7, p -value = 0.001) Two-way mixed-effects intraclass and Pearson’s correlation coefficients of 100 and 102 paired IAP insuf and IAP TG measurements in ( a ) patients without baseline IAH ( n  = 11) and ( b ) patients with baseline IAH ( n  = 7, p -value = 0.001) As illustrated in Fig.  1 , an intraclass and Pearson’s correlation coefficient of 0.7 and 0.9 (R 2  = 0.8) was found for patients without baseline IAH, compared to -0.3 and 0.7 (R 2  = 0.5) in patients with baseline IAH. The results of Bland and Altman’s analysis are presented in Fig.  2 . Fig. 2 Bland and Altman’s analysis for ( a ) patients without baseline IAH and ( b ) patients with baseline IAH. The dark blue line represents the ordinary least squares (OLS regression line, which indicates the variability in bias versus the average measured values. In this study, a p -value of 0.5 and 0.001 was observed for the OLS line in the patients without and with baseline IAH Bland and Altman’s analysis for ( a ) patients without baseline IAH and ( b ) patients with baseline IAH. The dark blue line represents the ordinary least squares (OLS regression line, which indicates the variability in bias versus the average measured values. In this study, a p -value of 0.5 and 0.001 was observed for the OLS line in the patients without and with baseline IAH The detailed results of Bland and Altman’s analysis with percentage error are presented in Table  2 . Table 2 Bland and Altman’s results for patients with an IAP 0 greater than or equal to 12 mmHg and patients with an IAP 0 smaller than 12 mmHg Study group IAP TG [mmHg] IAP insuf [mmHg] Bias [mmHg] Precision [mmHg] LLA [mmHg] ULA [mmHg] PE [%] IAP 0  < 12 mmHg ( n  = 100) 14.0 ± 4.6 11.2 ± 4.4  + 2.8  + 2.7 –2.5  + 8.1 38.6 IAP 0  ≥ 12 mmHg ( n  = 102) 24.1 ± 7.0 12.0 ± 5.1  + 10.6  + 5.3  + 0.1  + 21.1 44.0 Mixed study group ( n  = 202) 19.1 ± 7.8 11.6 ± 4.8  + 6.1  + 5.7 –5.2  + 17.3 59.7 IAP: intra-abdominal pressure; LLA: lower limit of agreement; n: number of measurements; ULA: upper limit of agreement; PE: percentage error. IAP TG and IAP insuf represent average values of the measurements obtained throughout the experiment. Bland and Altman’s results for patients with an IAP 0 greater than or equal to 12 mmHg and patients with an IAP 0 smaller than 12 mmHg IAP 0  < 12 mmHg ( n  = 100) IAP 0  ≥ 12 mmHg ( n  = 102) Mixed study group ( n  = 202) IAP: intra-abdominal pressure; LLA: lower limit of agreement; n: number of measurements; ULA: upper limit of agreement; PE: percentage error. IAP TG and IAP insuf represent average values of the measurements obtained throughout the experiment. The next evaluation criterion was the concordance analysis, which looked at changes in IAP TG and concomitant changes in IAP insuf (Fig.  3 ). Fig. 3 Concordance analysis for ( a ) patients without baseline IAH and ( b ) patients with baseline IAH Concordance analysis for ( a ) patients without baseline IAH and ( b ) patients with baseline IAH The concordance analysis revealed a concordance coefficient of 100.0% in patients without IAH, compared to 94.3% in those with IAH. Error-grid analysis revealed 94% no-low risk and 6% medium risk for the patients without baseline IAH, and 46% no-low, 15% medium, and 45% high risk for patients with IAH (see ESM Figure S1). The ANOVA results demonstrated a relatively high F -constant (221.47) with a small p -value of 0.001, indicating that the bias in patients with IAH at baseline is significantly higher than in patients without IAH at baseline. The post-hoc analysis showed that the bias between IAP TG and IAP insuf increases at higher BMIs (0.43 mmHg per kg/m 2 , p  < 0.001). After adjusting the insufflator data using the regression results, the mean bias was eliminated (0.0 mmHg), and the standard deviation of the bias decreased from 6.27 to 4.64 mmHg (see Fig.  4 ). Fig. 4 Study of how bias varies versus BMI. ( a ) Linear regression fit showing an increase in bias with increasing BMI ( p  < 0.001). ( b ) Boxplot of the bias for the unadjusted and BMI-adjusted samples Study of how bias varies versus BMI. ( a ) Linear regression fit showing an increase in bias with increasing BMI ( p  < 0.001). ( b ) Boxplot of the bias for the unadjusted and BMI-adjusted samples

The research was carried out in strict adherence to the established study protocol, aligning with the ethical guidelines outlined in the Declaration of Helsinki and relevant regulatory standards and requirements. The study protocol was approved by the local Institutional Review Board and the Ethics Committee at Hurley Medical Center (approval number 1201431–3). To ensure ethical compliance and respect for patient autonomy, written informed consent was diligently obtained from the patients or their legally authorized representatives before their inclusion in the study. This was a prospective observational study conducted over a four-month period in a single operating room in the Department of Surgery at a tertiary care hospital in Flint, Michigan, USA. All adult patients (older than 18 years) undergoing an elective laparoscopic intervention and needing a bladder catheter were eligible for inclusion in the study. Patients with pathologies related to the urinary tract and pelvic masses were excluded. After induction of general anesthesia, all patients were deeply sedated and received neuromuscular blocking agents according to standard anesthetic practice for laparoscopic surgery. All measurements were performed during a phase of stable anesthesia and effective neuromuscular blockade, before the start of any surgical manipulation. The level of muscle relaxation was routinely monitored using the train-of-four method to ensure that muscle tone did not influence the pressure measurements. During the entire IAP measurement protocol, patients were positioned in the supine position. A TraumaGuard Foley catheter (Sentinel Medical Technologies, Jacksonville, FL, USA) was lubricated and inserted into the urinary bladder under sterile conditions. This catheter has been previously evaluated in multiple in vitro and in vivo validation studies, including abdominal phantoms, cadaveric models, porcine experiments, and clinical studies, demonstrating good agreement with established reference methods [ 18 – 21 ]. Following insertion, the retention and protection balloons were inflated with 10 mL and 3 mL of sterile water, respectively, in accordance with the manufacturer’s instructions and prior validation studies. The smart cable incorporating the pressure sensor was connected to the standard anesthesia monitor and zeroed to atmospheric pressure prior to the procedure. Correct functioning was confirmed by the presence of respiratory pressure variations. The baseline opening pressure (IAP₀) was recorded immediately after peritoneal access and before the establishment of pneumoperitoneum, using the insufflator’s pressure transducer placed at the umbilicus level. This opening pressure reflects the IAP prior to achieving the preset insufflation pressure plateau. At this time point, a paired simultaneous measurement was obtained from the TraumaGuard catheter (IAP TG ) and the insufflator (IAP insuf ). Subsequently, IAP was increased per 5 mmHg in a stepwise manner, and paired measurements were obtained at predefined pressure levels. For the Stryker insufflator, target pressures were 5, 10, and 15 mmHg; for the ConMed insufflator, pressures of 5, 10, 15, and 20 mmHg were applied. After reaching the highest target pressure, insufflation pressure was reduced back to baseline in steps of 5 mmHg, followed by a second stepwise increase using the same pressure levels when feasible. Due to clinical and procedural constraints, not all pressure levels could be obtained in every patient. The IAP measured by the TraumaGuard catheter was recorded continuously (continuous intra-abdominal pressure monitoring, CIAP), allowing real-time assessment of pressure changes. Therefore, no predefined waiting time was required at each pressure level. Pressure values were recorded once a stable pressure plateau was observed after 1 min on the continuous pressure tracing. Both TraumaGuard and insufflator pressures were captured in real time during the procedure and were not retrieved retrospectively from stored monitoring data. Patients were categorized into two groups based on their opening pressure IAP 0 : patients with baseline IAH (IAP 0  ≥ 12 mmHg) and patients without baseline IAH (IAP 0  < 12 mmHg). Demographic variables, including age, body mass index (BMI), and sex, were recorded for all participants. Continuous variables were tested for normality via the Shapiro–Wilk test. Normally distributed variables (IAP and demographic data) were expressed as mean ± standard deviation (SD), while those characterized by a non-normal distribution were presented as median (with interquartile ranges). Statistical differences between patients with and without baseline IAH were determined by the two-sided unpaired student’s t -test for continuous quantitative variables and the Chi-square test for categorical variables. A p -value smaller than 0.05 was considered statistically significant. Intraclass and Pearson’s correlation analysis were conducted to determine the absolute agreement and linear relationship by exploring the intraclass correlation coefficient (ICC) and correlation coefficient (R) between IAP TG and IAP insuf , respectively. For the ICC analysis, a two-way mixed-effects model with single measures (ICC[ 1 , 3 ]) was used, as the subjects were randomly selected and the measurement methods (IAP TG and IAP insuf ) were fixed. The bias was defined and calculated as the mean difference between IAP TG and IAP insuf . Subsequently, the precision and limits of agreement were defined as the standard deviation of the bias and the bias ± 1.96 times the precision, respectively, as explained by Bland–Altman analysis. However, since there were multiple measurements per subject, the assumption of independence between observations was violated. To account for this, a linear mixed-effects model was employed, in which the difference between IAP TG and IAP insuf was modeled using a fixed intercept (representing the bias) and a random intercept for each subject to capture intra-subject variability. This correction prevents subjects with more measurements from disproportionately influencing the results and provides more accurate estimates of precision. Subsequently, the percentage error was calculated by dividing twice the precision by the mean IAP. Percentage error, in fact, is a measure of how wide the agreement between two measurement methods is, compared to the magnitude of the measurements themselves (mean IAP value). To assess proportional bias, we also regressed (ordinary least squares) the differences between methods against their means. The agreement between the IAP TG and IAP insuf for tracking changes in IAP over time (ΔIAP) was evaluated by the concordance analysis, where the ΔIAP TG was measured and plotted against the ΔIAP insuf during the same time interval. The concordance coefficient was defined as the percentage of pairs with the same direction of change after excluding pairs with both ΔIAP TG and ΔIAP insuf  ≤ 2.5 mmHg and excluding pairs with either ΔIAP TG or ΔIAP insuf equal to zero. Error grid analysis was carried out to assess the risk level for a wrong treatment strategy due to erroneous IAP measurements in each study group, as described previously [ 18 ]. Analysis of variance (ANOVA) was also used to assess the significance of the difference between the studied groups (with and without IAH). A post-hoc analysis was done to determine how the measurement bias varies with BMI. This analysis was used to determine whether increasing BMI was associated with a systematic shift between the two measurement methods rather than increased random variability. Using a linear regression model, a BMI-dependent adjustment of IAP insuf was calculated, and the agreement between methods was reassessed by comparing the bias and precision before and after adjustment. The statistical analyses were conducted using Excel (Microsoft Corporation, WA, USA), MATLAB (MATLAB, NA, USA), and SPSS (SPSS, IL, USA).

This study showed that IAP measurements via the bladder seem to be higher than the IAP values measured via the insufflator in both patient groups. In the group without baseline IAH, IAP TG exceeded IAP insuf in 82% of measurements, while IAP insuf was greater than IAP TG in 5% of measurements. Conversely, in the patient group with baseline IAH, IAP TG was consistently higher than IAP insuf in all measurements. Considering the correlation results, although a strong linear relationship was observed between measurement methods in both study groups (0.9 (R 2  = 0.8) and 0.7 (R 2  = 0.5)), the ICC values of 0.7 and –0.3 suggest that there is a reasonable absolute agreement between IAP TG and IAP insuf only in the patient group without baseline IAH. Bias and precision were 2.8 ± 2.5 and 12.1 ± 5.4 mmHg for patients without and with baseline IAH. The percentage error was 36.8% and 44%, while concordance analysis revealed concordance coefficients of 100% and 94.3%, respectively. This study suggests that the bladder and insufflator pressure readings are closely correlated. However, the absolute agreement between the measurements is not sufficiently strong. Comparison of the Bland–Altman results with the reference threshold of the WSACS shows that insufflator and bladder pressure measurements can not be used interchangeably since bias, precision, limits of agreement, and percentage error were all beyond the safety margins. The error-grid analysis demonstrated that the associated risk is mainly within the low to medium risk categories in patients without IAH; however, in the group with baseline IAH, almost 50% of the measurements resulted in a high associated risk level. An overview of the previously done studies on comparing the bladder pressure with the insufflation pressure is presented in Table  3 . In agreement with the findings of this study, some of the previous investigations [ 6 – 9 , 11 ] observed a bias higher than two mmHg between the bladder and insufflation pressure. Although the obtained bias in the rest of the studies is smaller than two mmHg, further review of these studies shows that the precision and, therefore, the limits of agreement were higher than the acceptable thresholds (± 4 mmHg) suggested by the WSACS ( www.wsacs.org ). Table 3 An overview of the previous studies that have compared bladder pressure with insufflation pressure Study Sample size Bias [mmHg] Limits of agreement [mmHg] Maximal insufflation pressure [mmHg] Baseline bladder pressure [mmHg] Obeid et al. 1995 [ 6 ] 26 8.1 ± 0.2 [7.70, 8.50] 95% CI: [7.57, 7.84] to [8.36, 8.63] 16 12.5 ± 8.9 Yol et al 1998 [ 7 ] 40 3.1 ± 1.1 [0.90, 5.30] 95% CI: [0.35, 1.53] to [4.67, 5.85] 15 Not reported Johna et al. 1999 [ 8 ] 21 6.9 ± 4.3 [–1.50, 15.30] 95% CI: [–4.71, 1.66] to [12.14, 18.51] 15 Not reported Fusco et al. 2001 [ 9 ] 37 3.8 ± 0.3 [3.20, 4.40] 95% CI: [3.04, 3.38] to [4.22, 4.56] 25 Not reported Lee et al 2002 [ 10 ] 20 –0.6 ± 3.1 [–6.70, 5.50] 95% CI: [–9.03, –4.32] to [3.12, 7.83] 15 Not reported Schachtrupp et al. 2003 [ 11 ] 6 4.5 ± 3.3 [–2.00, 11.00] 95% CI: [–6.54, 2.61] to [6.39, 15.54] 30 2.4 ± 1.2 Schachtrupp et al. 2006 [ 12 ] 11 1.4 ± 4.6 [–7.60, 10.40] 95% CI: [–12.32, –2.91] to [5.71, 15.12] 30 Not reported Jacob et al. 2010 [ 13 ] 12 1.0 ± 0.7 [–0.40, 2.40] 95% CI: [–1.06, 0.31] to [1.69, 3.06] 9 6.0 ± 1.0 Cresswell et al. 2012 [ 14 ] 20 –1.3 ± 3.6 [–8.40, 5.80] 95% CI: [–11.09, –5.62] to [3.02, 8.49] 20 Not reported Abbasi et al. 2018 [ 15 ] 21 0.2 ± 6.4 [–12.30, 12.70] 95% CI: [–17.09, –7.60] to [8.00, 17.49] 22 Not reported Hamoud et al. 2022 [ 16 ] 12 0.2 ± 0.5 [–0.80, 1.20] 95% CI: [–1.27, –0.29] to [0.69, 1.67] 15 Not reported An overview of the previous studies that have compared bladder pressure with insufflation pressure [7.70, 8.50] 95% CI: [7.57, 7.84] to [8.36, 8.63] Yol et al 1998 [ 7 ] [0.90, 5.30] 95% CI: [0.35, 1.53] to [4.67, 5.85] [–1.50, 15.30] 95% CI: [–4.71, 1.66] to [12.14, 18.51] [3.20, 4.40] 95% CI: [3.04, 3.38] to [4.22, 4.56] Lee et al 2002 [ 10 ] [–6.70, 5.50] 95% CI: [–9.03, –4.32] to [3.12, 7.83] [–2.00, 11.00] 95% CI: [–6.54, 2.61] to [6.39, 15.54] [–7.60, 10.40] 95% CI: [–12.32, –2.91] to [5.71, 15.12] [–0.40, 2.40] 95% CI: [–1.06, 0.31] to [1.69, 3.06] [–8.40, 5.80] 95% CI: [–11.09, –5.62] to [3.02, 8.49] [–12.30, 12.70] 95% CI: [–17.09, –7.60] to [8.00, 17.49] [–0.80, 1.20] 95% CI: [–1.27, –0.29] to [0.69, 1.67] In general, central obesity (rather than just a high BMI) can alter the distribution of abdominal fat and decrease abdominal wall compliance, leading to higher baseline IAP values. These alterations in the abdominal wall may cause a greater height difference between the bladder catheter tip and the pressure gauge of the insufflator (sagittal abdominal diameter). This results in a hydrostatic pressure discrepancy in IAP measurements obtained from these two methods. A post-hoc analysis demonstrated that the increase in bias with higher BMI reflects a systematic, BMI-dependent shift rather than increased random measurement error. This finding persisted after adjustment for BMI and re-evaluation of the agreement between bladder and insufflation pressures. Although the bias was eliminated after this adjustment, the precision was improved by 1.63 mmHg. Potential leakage via the insufflator can also be a significant factor contributing to discrepancies between different IAP measurements. Any leakage in the insufflator system can lead to a loss of gas and pressure, affecting the accuracy and consistency of the IAP insuf measurements. First, the study’s sample population (albeit in accordance with the WSACS guidelines for research) was small and could be higher in future follow-up studies. The small sample population limits the statistical power of the research findings, while the cohort nature of the study limits the ability to generalize the results to a broader population. Second, in line with the above, another limitation could be the small number of patients with a BMI smaller than 25 kg/m2 (5 out of 18). Third, the insufflators used had a pressure limit of 15 to 20 mmHg, thereby preventing the evaluation of higher IAP levels. Finally, incorporating an additional reference method via the bladder (e.g., FoleyManometer) would further strengthen the scientific validity of the results. Another improvement for follow−up studies could be to examine the agreement between bladder and insufflation pressures at different body positions. Altering the body position from supine to head−of−bed elevation or reverse Trendelenburg might shed light on an optimal position where bladder and insufflation pressure readings align more closely.

The present findings discourage the use of insufflation pressure as the gold standard for IAP validation studies.

Intra-abdominal pressure (IAP), the steady-state pressure within the abdominal compartment, has been recognized as an important physiological parameter and vital sign that should ideally be monitored continuously in critically ill patients with two or more risk factors [ 1 , 2 ]. Intra-abdominal hypertension (IAH), defined as sustained increased IAP values equal to or higher than 12 mmHg, reduces abdominal perfusion pressure and results in insufficient blood perfusion to vital organs such as the kidneys [ 2 , 3 ]. If not detected promptly, IAH may progress to abdominal compartment syndrome (ACS), which is defined as a sustained IAP above 20 mmHg accompanied by new-onset organ failure and is associated with relatively high morbidity and mortality [ 2 ]. IAP measurement via the bladder is considered the gold standard technique advocated by the Abdominal Compartment Society (WSACS, www.wsacs.org , https://wsacs.mn.co ) [ 2 – 4 ]. Following the research guidelines of the WSACS, evaluation and validation of a novel IAP measurement technique against an established standardized technique for intermittent IAP measurement is vital [ 5 ]. Although the best approach to validate a novel method for IAP measurement is to compare it with an intra-bladder measurement technique, reviewing relevant literature shows that insufflators during laparoscopic surgeries have also been one of the commonly used reference methods to investigate and validate a novel IAP measurement technology [ 6 – 17 ]. The results of these studies, however, indicate that using an insufflator can be challenging and requires several precautions. Accordingly, this research aims to investigate the agreement between IAP measurements via the bladder and insufflator during laparoscopic surgeries. In addition to ensuring higher scientific quality and reliability in future IAP validation studies, the outcome of this research will also enhance our understanding of potential discrepancies between bladder and insufflation pressure readings during laparoscopic surgeries.

Below is the link to the electronic supplementary material. Supplementary file 1: Error-grid analysis for ( a ) patients without baseline IAH, ( b ) with baseline IAH. Supplementary file 1: Error-grid analysis for ( a ) patients without baseline IAH, ( b ) with baseline IAH.