Warmed intravenous fluid administration attenuates heat loss in horses under general anaesthesia

Warmed intravenous fluid administration attenuates heat loss in horses under general anaesthesia | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 23 February 2026 V1 Latest version Share on Warmed intravenous fluid administration attenuates heat loss in horses under general anaesthesia Authors : Miranda Starr 0000-0003-0105-4863 [email protected] , Alycia Frampton , Allison Mika 0009-0001-8564-9428 , Klaus Hopster , and Hope Douglas 0000-0002-5667-6283 Authors Info & Affiliations https://doi.org/10.22541/au.177180675.54955840/v1 137 views 89 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Background : Peri-anaesthetic hypothermia is a well-documented complication of general anaesthesia and is associated with prolonged recovery, impaired wound healing, and delayed coagulation. Fluid warmers can be used to mitigate heat loss, but there is little evidence supporting their effectiveness in equine anaesthesia. Objectives : To determine whether administration of warmed intravenous fluids attenuates heat loss during general anaesthesia in horses. Study design : Prospective, randomized, crossover study. Methods : Six healthy adult horses were anesthetized twice and administered intravenous fluids with the HOTLINE (R) Blood and Fluid Warmer either activated or inactivated. Core body temperature was recorded at four anatomic sites (rectum, nasopharynx, pulmonary artery, and urinary bladder), and fluid temperature was measured in the fluid bag, proximal to the warmer, and distal to the warmer. Linear mixed-effects models evaluated the effects of warmer status, time, measurement site, and ambient temperature, including relevant interactions, with random intercepts for horse. Results : Overall core body temperature differed statistically but not clinically meaningfully between warmer conditions; however, warmed fluid administration significantly reduced the rate of temperature decline (time × warmer interaction, β = 0.0033°C/min, SE = 0.001, p = 0.0004). Fluid temperature distal to the warmer increased substantially with warmer activation, and ambient temperature strongly influenced fluid temperature (χ 2 = 634.2, p < 0.0001). Main limitations : Small sample size; environmental variability; no surgical procedure. Conclusions : Although warmed intravenous fluids did not increase core temperature, they significantly attenuated intra-anaesthetic heat loss. The HOTLINE (R) Blood and Fluid Warmer increased delivered fluid temperature and provided a modest but measurable protective effect against hypothermia. These findings are the first to demonstrate that warmed fluid administration can slow heat loss in horses under general anaesthesia. Warmed intravenous fluid administration attenuates heat loss in horses under general anaesthesia Summary Background : Peri-anaesthetic hypothermia is a well-documented complication of general anaesthesia and is associated with prolonged recovery, impaired wound healing, and delayed coagulation. Fluid warmers can be used to mitigate heat loss, but there is little evidence supporting their effectiveness in equine anaesthesia. Objectives : To determine whether administration of warmed intravenous fluids attenuates heat loss during general anaesthesia in horses. Study design : Prospective, randomized, crossover study. Methods : Six healthy adult horses were anesthetized twice and administered intravenous fluids with the HOTLINE (R) Blood and Fluid Warmer either activated or inactivated. Core body temperature was recorded at four anatomic sites (rectum, nasopharynx, pulmonary artery, and urinary bladder), and fluid temperature was measured in the fluid bag, proximal to the warmer, and distal to the warmer. Linear mixed-effects models evaluated the effects of warmer status, time, measurement site, and ambient temperature, including relevant interactions, with random intercepts for horse. Results : Overall core body temperature differed statistically but not clinically meaningfully between warmer conditions; however, warmed fluid administration significantly reduced the rate of temperature decline (time × warmer interaction, β = 0.0033°C/min, SE = 0.001, p = 0.0004). Fluid temperature distal to the warmer increased substantially with warmer activation, and ambient temperature strongly influenced fluid temperature (χ² = 634.2, p < 0.0001). Main limitations : Small sample size; environmental variability; no surgical procedure. Conclusions : Although warmed intravenous fluids did not increase core temperature, they significantly attenuated intra-anaesthetic heat loss. The HOTLINE (R) Blood and Fluid Warmer increased delivered fluid temperature and provided a modest but measurable protective effect against hypothermia. These findings are the first to demonstrate that warmed fluid administration can slow heat loss in horses under general anaesthesia. Introduction Peri-anaesthetic hypothermia is a common, well-documented effect of general anaesthesia, and contributes to prolonged recovery, delayed coagulation, and impaired wound healing in both people and in veterinary species. 1–5 Several factors influence the loss of body heat during general anaesthesia. In horses, major contributors of heat loss include evaporative losses (e.g., high fresh gas flows, open abdominal cavity), decreased metabolic rate from immobility, and drug-induced vasodilation. 6 While several active warming techniques exist for small animal species under general anaesthesia, fewer options are available for equine patients, and evidence supporting their efficacy remains limited. Fluid warmers are one measure used to maintain body temperature during general anaesthesia. Studies have demonstrated the ability of in-line fluid warmers to increase the temperature of fluids delivered to patients, 7,8 and a study in people has demonstrated that patients receiving fluids warmed using the HOTLINE (R) Blood and Fluid Warmer device maintained higher core body temperatures. 9 The HOTLINE (R) Blood and Fluid Warmer and Fluid Warming Set are a distance-independent fluid warming system: triple-lumen tubing allows circulation of heated (41-42ºC) water around a sterile centre lumen, warming intravenous fluids by counter-current heat exchange along the entire length of tubing, minimising cool-down between the warmer and the patient. 10 The warmer is reportedly effective at delivering normothermic fluids at flow rates up to 5 L/h. 10 Despite widespread clinical use, no studies have demonstrated whether the administration of warmed intravenous fluids meaningfully affects core body temperature in large animals. The primary objective of this study was to evaluate the impact of delivering warmed intravenous fluids on the core body temperature of horses under general anaesthesia. We hypothesised that horses receiving warmed intravenous fluids would maintain higher core temperatures and exhibit a slower rate of temperature decline than horses receiving room-temperature (unwarmed) fluids. Materials and methods Ethical approval This study protocol was approved by XXXX. Research animals and facilities Six, university-owned, healthy adult horses were included in the study. All horses were deemed healthy based on physical examination by a veterinarian the day before and morning of general anaesthesia and were assigned an American Society of Anesthesiologists (ASA) Physical Status of I. Horses were housed in shared paddocks and brought into an individual stall the day prior to anaesthesia and returned to the paddock the morning following anaesthesia. Horses maintained free access to water, and hay was withheld the morning of anaesthesia. Study design This was a prospective, crossover, randomized, experimental study. Each horse was anesthetized twice with a washout period of two weeks between anaesthetic events. Horses were administered both warmed and room-temperature fluids, the order of which was randomly assigned using a standard randomization generator (https://www.randomizer.org/). All data were collected in September 2025. Atmospheric temperature and humidity were measured using a digital hygrometer/thermometer (Pro Digital Mini Hygrometer with Built-In Indoor Thermometer and Humidity Meter with Comfort Scale, ACURITE). Anaesthetic protocol and instrumentation Before each experiment, the skin over the left and right jugular veins (LJV and RJV, respectively) was clipped and aseptically prepared for intravenous catheter placement. A 14-gauge intravenous catheter (1412; MILA International, Inc.) was placed in the left jugular vein. After infiltration of the skin over the right jugular vein with 2% lidocaine, a 9FR catheter introducer was placed aseptically, and a 7Fr x 110cm Swan-Ganz pulmonary artery catheter (Criticath (R) Merit Medical Systems, Inc.) was advanced to 110cm. Horses were sedated with xylazine (0.5-1mg/kg; Rompun 100 mg/mL; Dechra Pharmaceuticals) IV to effect in the induction stall and anaesthesia was induced with ketamine (2.5 mg/kg IV; Zetamine TM 100 mg/mL; VetOne/MWI) and midazolam (0.05 mg/kg IV; Midazolam HCl 5 mg/mL; West-Ward, Inc.). Horses were nasotracheally intubated with a 20mm endotracheal tube (LAET20; MILA International, Inc.), placed in dorsal recumbency on a padded mattress, and were mechanically ventilated with an electronically driven piston ventilator (Tafonius Large Animal Anesthesia Workstation; Hallowell EMC) with a tidal volume (V T ) of 13 mL/kg, an inspiratory-to-expiratory ratio of 1:3, and zero end-expiratory pressure. Respiratory rate (RR) was adjusted to maintain an end-tidal CO 2 (ETCO 2 ) of 35-45 mmHg. Anaesthesia was maintained with isoflurane (Isospire; Dechra Pharmaceuticals) in oxygen, targeting an end-tidal isoflurane concentration (ETiso) of 1.4%. Following 120 minutes of anaesthesia, recovery was assisted using head and tail ropes. Physiologic parameters were monitored continuously throughout general anaesthesia. Heart rate (HR), invasive blood pressure (IBP), oxygen saturation (SpO 2 ), RR, and ETCO 2 were recorded every five minutes. A 20-gauge arterial catheter (SURFLO; Terumo Medical) was placed in the transverse facial or facial artery for IBP measurement. After aseptic preparation, a 137cm urinary catheter (Stallion Urinary Catheter, JorVet TM ) was passed through the urethra into the urinary bladder. Once urine was obtained to confirm bladder placement, a gas-sterilized temperature probe (YSI Compatible Reusable Temperature Probe, Cables and Sensors) was passed through the lumen of the urinary catheter into the urinary bladder for temperature monitoring. Temperature probes (YSI Compatible Reusable Temperature Probe, Cables and Sensors) were also placed approximately 30cm into the nasopharynx and 15cm into the rectum as described by Tomasic and Nann. 11 Additional ventilatory and maintenance parameters including V T , peak airway pressures, inspired oxygen concentration, ETiso, fresh gas flow (L/min), and any continuous rate infusions (CRIs) were recorded every 15 minutes. Dobutamine (DOBUTamine HCl, Hospira, Inc.) was administered as a CRI between 0.25-1 mcg/kg/min as needed to maintain a mean arterial pressure (MAP) > 70 mmHg. Horses received room-temperature isotonic fluids (PlasmalyteA; Baxter International, Inc.) via the 14-g LJV catheter at an approximately 7-8 mL/kg/h (approximately 4 L/h). Fluids were delivered through a gravity IV administration set (SafeDAY TM IV Administration Set, B. Braun Medical, Inc. connected to a HOTLINE (R) Fluid Warming Set (Smiths Medical ASD, Inc.) and then connected to the jugular intravenous catheter extension set (Extension Set, 30 inch, ICU Medical, Inc.). The Fluid Warming Set was placed inside a HOTLINE (R) Blood and Fluid Warmer (Smiths Medical ASD, Inc) as per device manufacturer instructions. The fluid warmer remained “off” until connected to the intravenous catheter and was only then turned “on” if indicated by earlier-determined randomization. Data collection Following induction and instrumentation, body and fluid temperatures were recorded every five minutes for the remaining 90-95 minutes of the 120-minute anaesthetic period. Fluid bags were at room temperature and temperature was measured in the bag (BAG), 15 cm proximal to the warmer (PROX), and 15cm distal to the warmer (DIST) using a non-contact infrared thermometer 12 (Fisherbrand TM Traceable TM Noncontact Infrared Thermometer). Fluid temperature measurements were obtained in triplicate and averaged for analysis. For the purposes of this study, ‘core body temperature’ refers to temperatures measured at rectal, pulmonary artery, urinary bladder, and nasopharyngeal sites, recognizing that these sites differ in physiologic responsiveness and absolute values. Pulmonary artery (PA) temperature was measured using the thermistor probe of the Swan-Ganz catheter connected to a multiparameter monitor (CARESCAPE TM B650, GE Healthcare Technologies, Inc.). The nasopharyngeal (NP) temperature probe was also connected to the CARESCAPE TM monitor. Urinary bladder (UB) temperature and rectal (RE) temperature were measured using the temperature probes connected to a YSI 400, Dual Female Mono Plug Connector (Marquette Compatible Temperature Adapter, GE Healthcare) connected to a second multiparameter monitor (GE Datex-Ohmeda S/5 Patient Monitor, GE Healthcare Technologies, Inc.). Due to patient size and logistical limitations (e.g. cable length and monitor display options), two multiparameter monitors had to be used. Inter-monitor agreement in temperature readings was confirmed by simultaneously submerging probes in water and confirming temperature measurements read within 0.1ºC of each other. Data analysis Analyses and data presentation were performed in R (version 4.5.2) 13 using the following packages: dplyr, 14 tidyr, 15 lme4 , 16 lmerTest, 17 emmeans , 18 ggplot2, 19 sjplot, 20 simr, 21 flextable , 22 broom.mixed , 23 and officer . 24 Linear mixed-effects models were fitted using lme4 , with Satterthwaite-adjusted degrees of freedom provided by lmerTest . Model assumptions were visually evaluated using Q–Q and residual-versus-fitted plots, which supported approximate normality and homoscedasticity. Plots of temperature over time did not indicate non-linearity, and time was modelled as a linear fixed effect. A priori power calculations were not performed specifically for this study, as sample size (n=6) was dictated by a concurrent protocol. A post-hoc assessment using simr indicated adequate power to detect the observed effects; however, the sample size should be interpreted as a convenience sample. Body temperature models (primary outcome) Core body temperature was analysed using a linear mixed-effects model with fixed effects for time, warmer status (on/off), anatomical site (RE [intercept], NP, PA, UB), and the time x warmer interaction. Horse was included as a random effect to account for repeated measurements within individuals. Ambient temperature was excluded as it did not improve model fit and did not alter estimates of the time × warmer interaction. Model: temperature ~ time * warmer + site + (1 | horse) Post-hoc estimated marginal means (EMMs) and Tukey-adjusted pairwise comparisons were used to compare mean temperatures between warmer conditions and anatomical sites, and to contrast slopes associated with the time x warmer interaction. Fluid temperature models (secondary outcome) Fluid temperatures (BAG [intercept], PROX, DIST) were analysed with a linear mixed-effects model including fixed effects for warmer status, location, ambient temperature, and the warmer x location interaction, with horse as a random intercept. Inclusion of ambient temperature significantly improved model fit based on AIC and likelihood ratio testing (ΔAIC = 632; χ² (1) = 634.2, p < 0.0001). Ambient temperature was included as an uncentered covariate; therefore, the model intercept reflects predicted temperature at 0 °C ambient temperature and is not intended to represent a physiologic baseline. Time was non-significant (p = 0.81) and removed from the final model. Model: temperature ~ warmer * location + atmtemp + (1 | horse) EMMs were generated for each warmer x location combination. Tukey-adjusted post-hoc contrasts compared warmed versus unwarmed conditions at each location and evaluated differences among locations within each condition. Results The six horses in this study included five Thoroughbreds and one Quarter Horse (three geldings, three mares) with a median age of 9 years (range 3-14 years) and mean body weight of 530kg (range 473-585kg). Environmental conditions in the study area remained between 22.2-27.2ºC with 44-63% humidity. Body temperature model A total of 928 temperature measurements were collected from six horses receiving warmed and unwarmed fluids during general anaesthesia across four sites (RE, NP, PA, and UB; Figure S1). Temperature measurements differed significantly between sites. NP (β = –0.80, p < 0.0001), PA (β = –0.58, p < 0.0001), and UB (β = –0.20, p < 0.0001) were all consistently lower than RE temperatures (Table 1). Temperature decreased steadily over time in both treatment conditions. Time was a strong negative predictor of temperature (β = –0.017°C/min, SE = 0.00065, p < 0.0001). The main effect of warmer status did not significantly impact baseline temperature at time zero (β = –0.07°C, p = 0.34). A significant interaction between time and warmer (β = 0.0033°C/min, SE = 0.001, p = 0.0004) indicated that horses’ temperatures declined more slowly when the fluid warmer was active (Figure 1). This effect was confirmed by a significant difference estimated temperature slopes (0.003 °C/min difference, p = 0.0004) between warmer conditions: warmer off: –0.017 °C/min (95% CI: –0.019 to –0.016) warmer on: –0.014 °C/min (95% CI: –0.015 to –0.013) This corresponds to an approximate 19% reduction in the rate of heat loss with warmed fluid administration. Estimated marginal means controlling for time and site demonstrated a mild but significant difference in overall temperature between warmer conditions (warmer off: 35.7 °C; warmer on: 35.9 °C; difference: 0.17 °C, SE = 0.03, p < 0.0001). There was modest variability in baseline temperatures (SD = 0.23°C), suggesting consistent treatment effects across horses. Fluid temperature model A total of 696 fluid temperature measurements were recorded at three locations along the fluid line (BAG, PROX, and DIST) during warmed and unwarmed fluid administration in six horses under general anaesthesia. Fluid bags were at room temperature when hung (mean 23.6ºC, range 20.6ºC - 26.7ºC). Ambient temperatures varied between anaesthetic events and was included as a covariate. Fluid temperatures at PROX and DIST were significantly warmer than BAG when the warmer was off (PROX: +0.80 °C, p < 0.0001; DIST: +0.91 °C, p < 0.0001; Table 2). Ambient temperature strongly and positively influenced fluid temperature (β = 0.71°C per 1°C increase in ambient temperature, p < 0.0001). There were slight differences in baseline temperature between horses (SD = 0.19°C). A significant warmer x location interaction demonstrated a substantial increase in temperatures distal to the warming unit. When the warmer was active: • DIST temperatures increased by 6.86 ºC (p < 0.0001) relative to BAG • PROX temperatures increased by 0.86°C (p < 0.001) • BAG temperature did not change (β = 0.05 °C, p = 0.68) EMMs confirmed that DIST temperatures increased substantially with warmer activation (+6.91°C, p < 0.0001; Figure 2). PROX temperatures experienced a modest but significant increase (+0.92°C, p < 0.0001), and BAG temperatures remained unchanged (+0.05°C, p = 0.68). Pairwise comparisons showed no difference between PROX and DIST temperatures with the warmer off (estimate = 0.11, p = 0.95), but there was a clear gradient when the warmer was active (DIST > PROX > BAG; all p < 0.0001). These findings confirm that the counter-current warming unit markedly increased delivered fluid temperature at the patient end of the line, while pre-warming components of the system were minimally affected. Discussion These results demonstrate that administration of warmed intravenous fluids significantly slowed temperature loss over time in anaesthetized horses. This effect represents a meaningful reduction of intra-anaesthetic heat loss, particularly given the limited availability of evidence-backed warming strategies during equine anaesthesia. The 19% decrease in the rate of temperature decline observed in this study suggests that warmed fluid administration provides a modest but clinically relevant adjunctive source of heat support to anaesthetized horses during prolonged procedures. The finding that overall core temperatures dropped significantly regardless of warmer conditions is not surprising, considering previous studies in people 25–27 and in veterinary species 28–30 demonstrating that warmed fluids alone are insufficient to maintain normothermia in anaesthetized patients. Nevertheless, the reduced rate of heat loss observed in this study indicates that warmed fluid administration can partially mitigate anaesthetic-related thermal losses. In an average person, one litre of room-temperature intravenous fluids results in approximately 0.25ºC of cooling due to convective heat transfer from blood vessels to the infused fluid. 1 It is therefore reasonable that warming intravenous fluids closer to body temperature could reduce such convective losses and slow overall decline in core temperature. The absence of a significant effect of warmer status on baseline temperature in our study further indicates that the benefit of warmed fluid administration arises not from increasing initial temperature, but from moderating the progressive cooling that occurs during prolonged general anaesthesia. NP temperatures were consistently the lowest among all measurement sites, a pattern that aligns with findings of previous studies demonstrating lower nasopharyngeal temperatures compared with rectal or pulmonary artery measurements in horses under general anaesthesia. 11,31 Nasopharyngeal measurements approximate core temperature in humans when probes are positioned near the carotid artery. 32,33 However, in horses, the guttural pouch separates the internal carotid artery from the nasopharynx, 34 limiting the accuracy of NP temperature as an indicator of true core temperature. This anatomic separation likely explains the consistently lower NP temperatures observed in this study. Nasoesophageal temperature probe placement may offer more accurate core temperature monitoring. However, future studies examining this location are warranted. Additionally, although the temperature probes were passed a consistent distance into the nasopharyngeal cavity in all trials, precise replication of site placement could not be guaranteed. UB temperatures were slightly lower than RE values in this study, a difference that is physiologically plausible. In human critical-care research, bladder temperature generally tracks central blood temperature but can underestimate core temperature during periods of rapid heat loss or reduced urine flow, likely reflecting delayed thermal equilibration within the bladder. 35,36 Although veterinary- and equine-specific validation studies are limited, similar mechanisms likely apply under inhalant general anaesthesia, during which renal perfusion and urine production may vary. Thus, the mild discrepancy observed here is consistent with expected physiologic lag rather than measurement inaccuracy. Unfortunately, bladder size was not measured in this study, and the effect of bladder fill and urine volume on temperature could not be assessed. PA temperature was unexpectedly lower than RE and UB temperatures, even though these sites are typically considered interchangeable indicators of core body temperature in both people 35,37 and in veterinary 6,11,38,39 species. In the present study, one possible explanation is transient cooling of venous return from the left jugular infusion site; even at modest infusion rates, room-temperature fluid could reach the right heart before fully equilibrating with central circulation. This localized cooling effect may selectively lower PA temperature measurements without altering slower-responding compartments such as RE or UB. Additionally, catheter position was not confirmed, raising the possibility of right atrial or ventricular placement, which would also produce slightly lower readings. Collectively, these technical factors likely account for the lower-than-expected PA temperature values. Activation of the fluid warmer produced a substantial increase in delivered fluid temperature, consistent with previous studies showing that in-line warmers effectively heat fluids immediately before administration. 7,8,40 In the present study, fluid temperature distal to the warmer increased by nearly 7 °C with warmer activation, whereas fluids in the bag and proximal to the warmer were minimally affected. Despite this substantial warming, the overall thermal contribution to the horse was limited. As discussed previously, warmed fluids reduce heat loss but are insufficient alone to maintain body temperature. 25–30 Ambient temperature also strongly influenced fluid temperature, emphasising the susceptibility of crystalloid solutions to environmental thermal conditions. There are several limitations that should be considered when interpreting these findings. The small sample size (n = 6) was dictated by a concurrent protocol rather than an a priori power calculation, and although post-hoc assessment suggested adequate power for the primary outcomes, the results should still be interpreted in light of this constraint. The study was not blinded, which may introduce subtle measurement bias, although objective temperature endpoints reduce the likelihood of meaningful observer influence. Temperature probe placement was not confirmed using pressure waveform analysis or imaging, and variable probe placement could result in inaccurate temperature measurements between horses. Catheter placement for PA temperature monitoring was not confirmed with wedge pressures, leaving open the possibility that some measurements originated from the right atrium or ventricle rather than the main pulmonary artery. Differences among temperature measurements may therefore partly reflect technical, rather than physiologic, variation. Environmental conditions were not fully controlled, and ambient temperature varied between anaesthetic events. Although this variability was incorporated into the fluid-temperature model, it may still have contributed to between-day differences. Finally, the anaesthetic procedures were performed in healthy horses and did not include surgery or associated evaporative heat losses. Consequently, the generalisability of these findings to critically ill horses or those undergoing prolonged, invasive procedures may be limited. The Swan-Ganz catheters used in this study had undergone prior use and gas re-sterilisation, which may have affected thermistor accuracy. Information regarding accuracy of re-sterilised thermistor probes in the Swan-Ganz catheters could not be obtained from the manufacturers nor the literature. However, ethylene oxide sterilization does not affect the accuracy of YSI temperature probes 41 and is widely used for sterilisation of electrical and plastic medical devices. In conclusion, warmed intravenous fluid administration did not increase core body temperature in horses under general anaesthesia but did significantly reduce the rate of heat loss compared with room-temperature fluids. The HOTLINE (R) Blood and Fluid warmer reliably raised delivered fluid temperature at the patient end of the administration set at clinically relevant infusion rates. These results indicate that warmed fluids provide a measurable but modest protective effect against intra-anaesthetic hypothermia and should be considered an adjunct to, rather than a replacement for, other active warming strategies in equine patients. Predictor Estimate Std. Error 95% CI p-value df Intercept (Rectal; warmer off; time = 0) 37.16* 0.11 36.91 – 37.40 <0.001 8.55 Time (min) -0.02* 0.00 -0.02 – -0.02 <0.001 916.12 Warmer on -0.07 0.07 -0.20 – 0.07 0.344 916.09 Site: Urinary Bladder 0.20* 0.04 0.13 – 0.27 <0.001 916.00 Site: Nasopharynx -0.60* 0.04 -0.67 – -0.53 <0.001 916.00 Site: Pulmonary artery -0.38* 0.04 -0.45 – -0.31 <0.001 916.00 Time × Warmer on 0.00* 0.00 0.00 – 0.01 <0.001 916.09 Random Effects σ 2 0.15 Horse (intercept) 0.05 ICC 0.26 Model Fit N horse 6 Observations 928 Marginal R 2 / Conditional R 2 0.59 / 0.70 AIC 980.39 Model: temperature ~ time * warmer + site + (1 | horse). Note: Reference levels were rectal temperature, warmer off, and time = 0 minutes. Horse was included as a random intercept to account for repeated measures within animals. Degrees of freedom were estimated using the Satterthwaite approximation. Statistically significant differences (p < 0.05) are denoted with an asterisk. Predictor Estimate Std. Error 95% CI p-value df Intercept (Bag; warmer off) 6.38* 0.55 5.29 – 7.47 <0.001 585.64 Warmer on 0.05 0.12 -0.19 – 0.29 0.677 684.00 Location: Distal 0.91* 0.12 0.67 – 1.15 <0.001 683.89 Location: Proximal 0.80* 0.12 0.56 – 1.04 <0.001 683.89 Ambient temperature (ºC) 0.71* 0.02 0.66 – 0.75 <0.001 674.30 Warmer x Distal 6.86* 0.17 6.52 – 7.20 <0.001 683.89 Warmer × Proximal 0.86* 0.17 0.52 – 1.21 <0.001 683.89 Random Effects σ 2 0.87 Horse (intercept) 0.04 ICC 0.04 Model Fit N horse 6 Observations 696 Marginal R 2 / Conditional R 2 0.90 / 0.91 AIC 1924.30 Model: temperature ~ warmer * location + atmtemp + (1 | horse). Note: The model intercept represents the predicted fluid bag temperature with the warmer off at an ambient temperature of 0°C; this value reflects model scaling rather than observed experimental conditions. All estimated marginal means and contrasts were calculated at observed ambient temperatures. Horse was included as a random intercept. Degrees of freedom were estimated using the Satterthwaite approximation. Statistically significant differences (p < 0.05) are denoted with an asterisk. Figure legends Figure 1. Model-estimated core body temperature (°C) over time in horses receiving warmed or unwarmed intravenous fluids during general anaesthesia. Shaded areas represent 95% confidence intervals. The time × warmer interaction demonstrates a slower rate of temperature decline when warmed fluids were administered. Estimates are marginal means averaged across all anatomical measurement sites included in the model. Figure 2. Estimated marginal means (±95% CI) of fluid temperature (°C) measured in the fluid bag (BAG), proximal to the warmer (PROX), and distal to the warmer (DIST) during warmed and unwarmed conditions. The warmer produced a substantial increase in distal fluid temperature, while bag temperature remained unchanged. List of legends for Supplementary items Figure S1. Mean body temperature (ºC) over time at each anatomic measurement site (rectal, nasopharyngeal, pulmonary artery, and urinary bladder) in horses receiving warmed or unwarmed intravenous fluids. Shaded areas represent 95% confidence intervals. References 1. Sessler DI. Mild perioperative hypothermia. N Engl J Med. 1997;336(24):1730–7. https://doi.org/10.1056/NEJM199706123362407. 2. Kurz A, Sessler DI, Lenhardt R. Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. N Engl J Med. 1996;334(19):1209–16. https://doi.org/10.1056/NEJM199605093341901. 3. 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Authors Affiliations Miranda Starr 0000-0003-0105-4863 [email protected] University of Pennsylvania New Bolton Center View all articles by this author Alycia Frampton University of Pennsylvania New Bolton Center View all articles by this author Allison Mika 0009-0001-8564-9428 University of Pennsylvania New Bolton Center View all articles by this author Klaus Hopster University of Pennsylvania New Bolton Center View all articles by this author Hope Douglas 0000-0002-5667-6283 University of Pennsylvania New Bolton Center View all articles by this author Metrics & Citations Metrics Article Usage 137 views 89 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Miranda Starr, Alycia Frampton, Allison Mika, et al. Warmed intravenous fluid administration attenuates heat loss in horses under general anaesthesia. Authorea . 23 February 2026. 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