Introduction

During open abdominal surgery, the patient’s abdominal cavity is exposed to the operating theatre environmental air conditions which may disrupt tissue homeostasis and normal physiological functioning. The combination of anaesthetic-induced impaired thermoregulation and exposure of the warm humidified patient abdomen to the ambient environment which is often dry (0–5% relative humidity) and cold (20–22 °C)1 and the reported actual operation room ambient temperature which may vary from 15.6 to 25.6 °C can result in patient hypothermia and fluid loss: Drying and evaporative cooling of the abdominal cavity may also contribute to systemic dehydration, tissue desiccation, inflammation, reduced perfusion and oxygenation2,3,4,5,6,7. This can lead to impaired postoperative recovery and increase the risk of common colorectal surgery complications including surgical site infection (SSI), and anastomotic leak (AL)8. Although surgical standards are always improving, the cost of these postoperative complications on patients and the healthcare system remain high.

Intra-operative, operative field humidification, also known as surgical humidification is the conditioning of gas with warmth and humidity to mitigate the effects of evaporative cooling on patient tissue. Intraoperative insufflation of warm humidified CO2 into the open abdomen is an intervention designed to prevent the evaporative cooling and desiccation effects of laparotomy. In theory, warm humidified CO2 can raise local temperature, prevent evaporative heat loss, thereby increasing local tissue perfusion and oxygenation which may lead to a lower incidence of SSI and anastomotic leaks9,10. A targeted intraoperative intervention that increases local oxygenation may help to reduce postoperative complications in patients who undergo open abdominal surgeries, particularly major surgeries with long operating times.

The use of surgical humidification in laparotomy and its benefits in temperature maintenance are well established. However, the impact of surgical humidification on local tissue oxygenation has mainly been studies in small animal models. Four human randomized control trials (RCTs)11,12,13,14 and three preclinical studies2,5,7 have shown that surgical humidification in laparotomy significantly improved intraoperative core temperature compared to standard of care. Importantly, the RCT by Arachchi, et al.11 measured SSI incidence and reported that surgical humidification had a statistically non-significant trend towards reduced SSI incidence compared to control colorectal surgery patients (4.5% humidified vs. 13% controls, p = 0.092). However, it should be noted that the COVID-19 pandemic posed challenges to patient recruitment and follow-up, ultimately underpowering it11. Increased local wound and core temperature has previously been reported to reduce incidence and severity of SSI15,16,17and this is thought to be via improved tissue perfusion, oxygenation, immune cell function, and wound healing18. A rodent model of open abdominal surgery, demonstrated that surgical humidification with CO2 significantly increased mean sub-peritoneal tissue oxygen partial pressure (PtO2) by 96.6% from controls without warm humidified CO2 insufflation (p < 0.001)5. Further, their results suggested that this result was due to the additive effects of using warmth or humidity, and CO2 gas properties to reduce evaporative cooling at the surgical site5. The authors hypothesized that local insufflation of warm humidified CO2 increases micro-perfusion through vasodilation, and decreases oxygen haemoglobin affinity (Bohr effect), thereby increasing local tissue oxygenation5.

This study utilized a porcine model of laparotomy and a state-of-the-art intraoperative optical imaging technique, Hyperspectral Imaging (HSI) in combination with blood analyses, histology, and physiological biomarkers to measure tissue morphology, metabolism, and oxygenation. We hypothesized that insufflation of warm humidified CO2 into an open abdominal cavity would increase core and local intestinal temperature. Further, we hypothesized that surgical humidification would increase visceral perfusion and decrease local tissue lactate levels and markers of tissue damage compared with exposure to ambient air.

Methods

Animals

Sixteen adult female pigs (Sus scrofa domesticus, ssp. Large white, weight: 45–55 kg), were obtained from an authorized animal farm (Strasbourg, France) and were included in the present study, which is part of the VIDI VICI project (VIsualisation and quantification of the effects of surgical humiDIfication on intestinal perfusion and VIability in an acute porCIne model). This study was jointly approved by the local Ethical Committee on Animal Experimentation (approval number: ICOMETH registration number 38.2020.02.003) and the French Ministry of Superior Education and Research (MESR). This study was part of the ELIOS project (approval number: APAFIS #8721-2017013010316298 v2). All experiments were performed in accordance with the French laws for animal use and care, and with accordance to the directives of the European Community Council (2010/63/EU) and ARRIVE guidelines19.

On each study day, pigs were randomly assigned to either the control group (n = 8 pigs) or surgical humidification group (SH, n = 8 pigs) which received warmed and humidified CO2 insufflation with the HumiGard* device (Fisher and Paykel Healthcare, New Zaeland). Pigs were fasted for 24 h before surgery with free access to water. Premedication was given 30 min before the procedure (zolazepam + tiletamine 10 mg/kg IM, and Azaperone 2 mg/kg IM). Anesthesia was induced with Propofol (3 mg/kg) intravenously and maintained with rocuronium 1–2 mg/kg and maintained after tracheal intubation with inhaled isoflurane 2 vol% in a mix of O2/NO2 50%/50% (fresh as flow of 2 L/min). All pigs were warmed using the default setting of an under-body forced air warming blanket. At the end of the procedure animals were euthanised with a lethal dose of pentobarbital (40 mg/kg).

Open surgery model

The operating room temperature was kept in the range of 17–20 °C at the start of the procedure and door movements throughout the experiment were limited to a minimum. Withing the used operation theatre, there was laminar flow standardized in the same settings for all surgical procedures. The experimental timeline is summarised in Fig. 1, once the animal arrived in the operating room the timing was started (T-2). Within the first 30 min, the animal was brought under general anaesthesia, endotracheal intubation was performed and all venous, arterial and endoesophageal catheters were inserted. In the remaining 30 min, the animal was covered with disposable sterile drapes, exposing only its abdomen. At T-1 a midline xiphoid-pubic laparotomy was performed by an experienced team of two colorectal surgeons. Two self-retaining retractors were inserted to maintain the abdominal exposure and ensure clear exposure of the small bowel (duodenum and jejunum) and colon. A segment of duodenum, jejunum and colon was loosely marked with a suture and the regions of interest were marked with a sterile marker. For standardization purposes throughout the VIDI VICI protocol, this step was planned to take a maximum of one hour. At timepoint T0 all animals in every intervention arm, were ready for the further analysis which lasted until T6. The total experimental time was 8 h. All other interventions and measurements as described in the following paragraphs, were done in a standardised fashion by the same operating team to reduce the influence of operator dependent variables.

Fig. 1
figure 1

Experimental Timeline. Difference between each timepoint is one hour. Animal anesthesia started at T-2. At T-1 the surgical incision was made, and the intestines exposed, surgical humidification (SH) was also turned on at T-1 if the animal was randomized to the intervention group. Arterial oxygen saturation (SaO2), respiratory rate (RR), operating room (OR), tissue oxygen saturation (StO2), tissue water index (TWI), local capillary lactate (LCL). Exposure of the open abdominal cavity was continued from T-1 to T6 for a total exposure time of 7 h. Total experiment time was 8 h.

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Intervention

The HumiGard™ device is a small surgical humidifier that delivers warm, humidified CO2 at a flow rate of 10 L per minute. For the surgical humidification group (n = 8 pigs), the Fisher & Paykel Healthcare Ltd. HumiGard™ system was prepared according to the user instructions by the investigators and the sterile patient interface was placed at the left upper quadrant of the abdomen by the primary surgeon at timepoint T-1, once the surgical incision had been made.

Temperature

Core body temperature was recorded using an oesophagus lumen thermometer from anaesthesia induction (T-2) every hour until end of surgery (T6). Local temperature of the duodenum and mid jejunum segment were recorded using a FLIR i5 thermographic camera (FLIR Systems, Wilsonville, Oregon, US), with measurements acquired from surgical incision and exposure of the intestines (T-1), every hour until the end of surgery (T6).

Local capillary lactate, blood gas analysis, and respiratory parameters

Local capillary lactate was measured on blood samples obtained by puncturing the bowel serosa at the jejunum and duodenum, and by placing the sample on test strips connected to the EDGE™ Blood Lactate Analyzer (Apex Biotechnology Corp. Hsinchu, Taiwan, People’s Republic of China). Systemic lactate was measured on blood samples obtained by standard blood gas analysis performed on venous blood obtained from the central venous catheter, along with prothrombin time (PT) at every hour until the end of surgery via the EPOC® Blood Analysis System (Siemens Healthineers, Erlangen, Germany), a portable blood analyser. Heart rate, blood oxygen saturation (SaO2), and respiratory rate (RR) parameters were also continuously monitored and noted at each time point.

Hyperspectral optical imaging (HSI)

The HSI camera (TIVITA®, Diaspective Vision GmbH, Germany) was a push-broom scanning device with a complementary metal oxide semiconductor (CMOS) image sensor with a spatial resolution of 640 × 476 pixels and a spectral range from 500 to 1000 nm (5 nm spectral resolution increments, totaling 100 bins)20. We imaged the jejunum and duodenum from exposure (T-1) every hour until the end of surgery (T6) to assess superficial oxygen tension (StO2%) and tissue water index (TWI).

Histopathological assessment and scanning electron microscopy (SEM)

Surgical biopsies were taken from the parietal peritoneum for histopathological and SEM assessment at T0 and at the clinically relevant timepoint of 2 h (T2), which is within the usual range of duration of common abdominal surgical procedures. Another reason to limit the biopsies to only two timepoints were the relatively high costs of the scanning electron microscopy (SEM) analysis. Pig peritoneal biopsies were fixed by immersion in 2.5% glutaraldehyde and 2.5% PFA in cacodylate buffer (0.1 M, pH 7.4), and post fixed in 1% osmium tetroxide in 0.1 M cacodylate buffer for 1 h at 4 °C. Dehydration was then performed with increasing ethanol concentrations up to the critical point-dried (Bal-Tec CPD 030). Dried samples were sputtered with palladium (Bal-Tec SCD 005) and observed by SEM (XL FEG SIRION, FEI, USA).

Biopsies for histopathology assessment were also taken from the duodenum and mid jejunum segment at T0, T2, T4, and T6 timepoints. Specimens were fixed in Paraformaldehyde (PFA) 4%. Sections (5 μm thick) were cut from paraffin-embedded tissues and stained with Haematoxylin and Eosin. A semi-quantitative blinded analysis was performed by a pathologist who assigned a binary score for the presence/absence of mesothelium cell damage, inflammatory elements, and the integrity of the connective tissue.

Statistical analysis

All data are presented as means with standard error of the mean unless stated otherwise. Statistical analyses were conducted using GraphPad 10.1.1 software (GraphPad Software, San Diego, CA, USA). For parametric tests comparing differences in continuous variable over time between the groups a two-way repeated measures ANOVA or mixed effects analysis with Šídák’s multiple comparisons tests comparisons were utilized. For parametric tests comparing between the groups at one timepoint an unpaired t-test was used and for non-parametric tests, the Mann Whitney test was used. Statistical significance was defined as a two-tailed p-value < 0.05.

Results

All the 16 animals survived the procedures and no unexpected complication occurred during the protocol.

There were no differences in the baseline measurements in both groups; the average weight of the 16 pigs was 40.49 ± 2.92 Kg. The mean ambient temperature and humidity in the operating room was 20.03 ± 0.68 ˚C, and 60.23 ± 12.36% respectively, and neither was significantly different between the groups at any timepoint (p = 0.7171; and p = 0.9137 respectively). There was no difference in heart rate or blood oxygen saturation (SaO2) at any timepoint between the groups (p = 0.1550, and p = 0.5103 respectively).

Surgical humidification maintains intraoperative core and local intestinal temperatures

Mean core temperature before start of surgery was similar between the groups with 38.09 °C in the control group and 37.25 °C in the surgical humidification group (p = 0.1157). During surgery both groups showed a decrease in core temperature (~ 10%). However, the control group showed a significantly steeper slope than pigs in the humidified group (p = 0.0007, Fig. 2A). Further, the average intraoperative core temperature decrease from the start to end of surgery was significantly less in the surgical humidification group (− 3.85 ± 0.98 °C) compared with the control (-5.63 ± 0.87 °C; p = 0.0016; Fig. 2B).

Intraoperative local intestinal temperature was significantly higher with surgical humidification in both the jejunum and duodenum as shown in Fig. 2C and D. When the humidifier was turned on at T-1 the surgical humidification group trended towards warmer local intestinal temperatures and this difference became significant from T0 in the jejunum (Fig. 2C; at T0 control was 31.313 ± 0.849 °C l vs. surgical humidification 35.613 ± 0.549 °C, p = 0.0090), and T1 in the duodenum (Fig. 2D; at T1 controls were 30.029 ± 1.112 °C vs. surgical humidification 35.925 ± 0.921 °C, p = 0.0117) until end of surgery.

Fig. 2
figure 2

Core and intestinal temperature (°C) with and without surgical humidification (SH) over time. (A) Core temperature changes over time are significantly different between the groups. Statistical analyses performed via linear regression and slope comparison p = 0.0007, mean and standard error of the mean at each timepoint displayed. (B) Core temperature difference from the start to the end (T0–T6) of surgery, differences between the groups assessed via an independent t-test. Individual pig data is represented by dots with the mean and standard error of the mean are displayed. Local (C) jejunum and (D) duodenum temperature both show significant differences between groups (p < 0.0001 and p = 0.0005 respectively). Statistical analyses for C, and D performed via Two-way repeated measures ANOVA followed by Šídák’s multiple comparisons tests, *p < 0.05, **p < 0.01, and ***p < 0.001. Note that core temperature has been added as a X symbol at timepoint T-2 for graphs C and D to show the instantaneous effect of surgical humidification on local temperature when turned on at T-1. For all graphs blue: control group (no insufflation); red: surgical humidification group (SH), *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001.

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Surgical humidification reduces intraoperative systemic and intestinal lactate levels

Both systemic and intestinal LCL were examined in this study and presented in Fig. 3. Intestinal LCL levels for the jejunum and duodenum over time are presented in Fig. 3A and B respectively. As per machine settings, LCL levels below 0.6 mmol/L were expressed as ‘low’ using the lactate analyser. For the statistical analysis, all ‘low’ LCL levels were regarded as equal to 0.5 mmol/L in order to standardize and prevent underestimation. When comparing intestinal LCL levels between the groups, there was a significant decrease in jejunum LCL levels with surgical humidification (Fig. 3A; average cumulative (all timepoints) reduction of ≈ 41% (LCL 1.475 ± 0.1094 mmol/L controls vs. 0.8656 ± 0.06523 humidified, p = 0.0273). Although similar trends were seen in the duodenum (≈ 38% reduction), this decrease in LCL with surgical humidification was not statistically significant (Fig. 3B; average cumulative (all timepoints) LCL 1.431 ± 0.1319 mmol/L controls vs. 0.8929 ± 0.07011 humidified, p = 0.0867).

Systemic lactate levels over time are presented in Fig. 3C. There was no significant difference in systemic lactate levels between the groups (p = 0.1002), although surgical humidification trended towards lower systemic lactate at all time points post incision with an overall average reduction of ≈ 20% (Fig. 3C, average cumulative (all timepoints) systemic lactate 1.728 ± 0.08739 mmol/L controls vs. 1.39 ± 0.1571 humidified).

Fig. 3
figure 3

Local capillary lactate (intestinal) and systemic lactates (mmol/L) with and without surgical humidification over time. Local capillary lactate (intestinal) and systemic lactates were lower in the surgical humidification group however mixed effects statistical analysis (mean ± standard error) showed that only the (A) jejunum showed a statistically significant difference between the groups p = 0.0273, whereas the (B) duodenum p = 0.0867, and (C) systemic p = 0.1002 did not (Mixed Effect analysis). For all graphs blue: control group (no insufflation); red: surgical humidification group (SH, insufflation of warm humidified CO2), *p ≤ 0.05.

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Intraoperative hyperspectral assessment of intestinal tissues

Intestinal hyperspectral parameters in all regions of interest (jejunum and duodenum) over time are presented in Fig. 4. When comparing intestinal StO2 levels between the groups, there were no statistically significant differences with surgical humidification in the jejunum (Fig. 4A, average cumulative (all timepoints) StO2 66.44 ± 2.661% controls vs. 61.79 ± 1.209% humidified, p = 0.2127), or duodenum (Fig. 5B, average cumulative (all timepoints) StO2 66.68 ± 2.283% controls vs. 65.92 ± 4.558 humidified, p = 0.8306). Similarly, when comparing intestinal tissue water index (TWI) levels between the groups, there were no statistically differences with surgical humidification in the jejunum (Fig. 4C, average cumulative (all timepoints) StO2 62.46 ± 1.263% controls vs. 64.34 ± 2.207 humidified, p = 0.4447), or duodenum (Fig. 4D, average cumulative (all timepoints) TWI 63.99 ± 0.6153% controls vs. 65.83 ± 0.7369 humidified, p = 0.4863).

Fig. 4
figure 4

Over time evolution of superficial oxygen tension (StO2%) and tissue water index (TWI) in the intestine with and without surgical humidification. StO2 imaging provides a quantification of the oxygenated hemoglobin within the visible light wavelength at the different ROIs. The TWI computes the tissue water content. Both hyperspectral imaging parameters showed a significant increase with time (p < 0.05 for all parameters) but mixed effects statistical analysis did not show an effect of surgical humidification on local StO2 or TWI. Data presented are mean ± standard error of the mean.

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Surgical humidification reduces peritoneal and intestinal tissue damage

Figure 5A and B display the results of histopathology and SEM from peritoneal biopsies taken at the start and 2 h of surgery for both groups. A semi-quantitative blinded analysis was performed by a pathologist who assigned a binary score for the mesothelium cell damage, inflammatory elements, and the integrity of the connective tissue. Where a score of 0 indicates no mesothelium cell damage, no inflammatory elements, and intact connective tissue, and a score of 1 indicated mesothelium cell damage, presence of inflammatory elements, and damaged connective tissue (Fig. 5C).

Both groups show similar histology scores at the start of surgery however two hours later, at timepoint T2, the control group had significantly increased mesothelium cell damage, inflammatory elements, and reduced connective tissue integrity compared to the group with surgical humidification. In the controls (histological sections) the epithelium was present with a very fine structure and connective tissue packed with collagen fibres and capillaries and in some area’s lipid droplets. At T2, there was an increase in inflammatory cells just below the epithelium which shows significant degradation.

Figure 5D and E depict histology images of the duodenal and jejunal tissues at T0, T2, T4 and T6. Both groups showed similar intestinal histology scores at the start of surgery. However, at later timepoints the surgical humidification group showed less inflammatory elements than controls. Average histology scores were significantly lower in the surgical humidification group, indicating less tissue damage, at T6 in the jejunum and T4 and T6 in the duodenum.s

Fig. 5
figure 5

Peritoneal histological analysis. (A) H&E staining showed increasing tissue damage with time in surgery and a protective effect of humidifier use. (B) SEM pictures of control and humidifier groups of the peritoneum. (C) Semi-quantification of the histological analysis, data are presented as mean ± standard error. and compared with unpaired t-tests to the controls at each timepoint. (A) The histology scores showed a significant increase in peritoneal damage with time. (D) Duodenum histology and the relative semi-quantification of the histological analysis. (D) Jejunum histology and the relative semi-quantification of the histological analysis. Both duodenum and jejunum show an improved quality and integrity of the tissue with humidifier use, marked by a reduced presence of inflammatory elements. Statistical analyses for D and E were Two-Way ANOVA with multiple comparison via Šídák test. *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001.

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Discussion

In this preclinical large animal model, surgical humidification was shown to improve core and local intestinal temperature maintenance, reduce intestinal local capillary lactate (LCL), and protect against peritoneal and intestinal damage. These findings may potentially lead to improved postoperative recovery and mitigate the risk of common colorectal surgery complications including surgical site infection (SSI), and anastomotic leak (AL)8.

Reduced patient body temperature, surgical site perfusion and oxygenation are known to significantly increase the risk of SSI and AL, impair wound healing, and prolong hospital stay9,10,16,21. SSI’s affect around 8% of open surgery patients, with rates approximating 20% in colorectal surgery22,23. Further, AL occur in approximately 2–30% of patients undergoing colorectal surgery, and significantly impair patient survival rates24. Therefore, a targeted intraoperative intervention which increases patient temperature, local wound perfusion and oxygenation may be a valued adjunct in reducing these postoperative complication rates.

This study showed that insufflation of warm humidified CO2 enabled significantly better core and local intestinal temperature maintenance compared to controls exposed to ambient air. This improvement in core temperature with surgical humidification has been corroborated in clinical RCTs11,12,13,14 and other animal studies2,5,7. The warmer core and local intestinal temperatures can be attributed to the density of CO2, which sinks into and fills the wound space providing effective thermal insulation to reduce surgical site evaporative and convective cooling12,25,26. By contrast, the control group without surgical humidification, had a continuous decrease in core and intestinal temperature likely due to a loss of heat from the wound space throughout surgery and evaporative cooling. Although insufflation of warm humidified CO2 has previously been shown to improve average laparotomy wound temperature5,7,12,27the specific increase in intestinal temperature with surgical humidification is novel to this study. Previous work suggests that intraoperative hypothermia increases the risk of colonic AL by suppressing the inflammatory response and collagen formation, which is vital for effective anastomotic healing28,29,30. Therefore, the maintenance of local intestinal temperature may be particularly important to reduce the risk of AL. In support of this, previous studies have shown that surgical humidification significantly increased average wound temperature, and the authors noted that wound dehiscence needing reoperation only occurred in two control group patients, with none in the surgical humidification group12.

We also measured LCL levels in two intestinal ROIs as a surrogate for local tissue oxygenation. Systemic lactate is an indicator of whole-body metabolic homeostasis. However, the local metabolic status at the surgical site might go undetected at the systemic level due to homeostatic mechanisms, and can instead be measured using LCL. We demonstrated a significant improvement with surgical humidification in the jejunum, and similar but non-significant trends in the duodenum LCL and systemic lactate levels. LCL is a robust marker of local tissue perfusion and oxygenation31,32. Lactate is the product of glycolysis and its accumulation reflects a lowered mitochondrial activity in the presence of reduced O2 concentration. In this study, surgical humidification significantly reduced jejunum LCL over time with an approximate average reduction of ≈ 41% compared to controls, suggesting that tissue perfusion-oxygenation was improved in the jejunum with surgical humidification. Although surgical humidification reduced duodenum LCL to a similar extent (average ≈ 38% reduction), these effects were not statistically significant. The difference in effect size between the jejunum and duodenum for LCL is likely due to differences in metabolic activity levels and their regulation between these two intestinal locations33. In vitro studies have shown that jejunal cells have a higher metabolic capacity and trend more towards oxidative metabolism than duodenal cells, further the metabolism rate of jejunal cells adapts faster than duodenal cells34. As a result, the slower adaption of the duodenum may increase the variability of LCL in this location reducing the likelihood of statistically significant difference between the groups. Overall, the lactate data provides support towards surgical humidification improving intestinal perfusion and oxygenation. This may be due to either increased perfusion or off-loading of oxygen from blood into the tissues via the Bohr effect as both increased temperature and CO2 are known to promote this35,36,37. A prospective trial of 22 patients assessing metabolic markers of perfusion during colorectal resections, demonstrated that the LCL levels at the clinically selected proximal resection site were higher in cases with AL38. Additionally, in porcine survival experimental models of bowel anastomosis with pre-set levels of perfusion determined by FLER (25 vs. 75% of perfusion), demonstrated that LCL levels were directly correlated with anastomotic inflammation score of the mucosa/submucosa (2.1(0.4) vs. 1.2(0.4); P = 0.003) and serosa (1.8(0.4) vs. 0.8(0.8); P = 0.014)39 and potentially leaks (only the 25% pre-set perfusion group had 2 instances of leak, whereas the 75% had none).

Further to these local effects, in this study surgical humidification trended towards lower systemic lactate levels at all time points with an approximate average reduction of ≈ 20% compared to controls. Both systemic serum lactate and C-reactive protein have been used as biomarkers for laparotomy patient postoperative complications including sepsis, AL, and SSI40,41,42,43. Surgical humidification has previously been shown to significantly reduce serum C-reactive protein levels from post-operative days 1 to 4 in both colorectal laparoscopic (P = 0.0041) and open surgery patients (P = 0.0001)44. This same group also showed non-significant trends for reduced AL (0% vs. 44% of cases, P = 0.05), and length of hospital stay (15 vs. 17.5 days) with surgical humidification compared with controls, however the small open surgery cohort (Total n = 19) was not powered to detect differences in these clinical outcomes44 .

We hypothesize that our results may also be relevant for the laparoscopic setting. During minimally invasive surgery internal tissues are exposed to cold, dry CO2 insufflation, which has been shown to cause tissue desiccation, peritoneal damage, and reductions in core body temperature, in a similar manner to ambient air exposure during open surgery45,46. Surgical humidification mitigates these effects and has been shown to improve core temperature in laparoscopic patients and reduce the incidence of perioperative hypothermia (≤ 36 °C) and surgical site infection after laparoscopic colorectal surgery47.

Further, in the present study pathological analysis of the peritoneum and intestinal regions of interest showed significantly higher histological damage scores at T2 in the control group, suggesting that surgical humidification can protect against peritoneal, jejunal, and duodenal tissue damage during laparotomy. The specific improvement in intestinal tissue quality and integrity, with surgical humidification is novel to this study. The peritoneal results are corroborated by clinical data that compared peritoneal samples from open colorectal surgery patients and found that at the end of surgery peritoneum was visible in only 11 of 19 samples from the control group versus 19 of 20 samples in the surgical humidification group (P = 0.006)3. In addition, these authors evaluated oxidative stress on the peritoneum by measuring chlorotyrosine (Cl-Tyr) as a fraction of native tyrosine, and found that oxidative stress was significantly increased in the control group compared to the surgical humidification group3. Together these data support the findings from the current study that surgical humidification can protect against tissue damage and improve local tissue oxygenation.

In this study, despite the jejunal LCL determined improvement in intestinal tissue oxygenation levels, hyperspectral assessment of the same jejunal location did not show a difference in StO2 or TWI between the groups. Our group have previously found that HSI parameters correlate well with LCL levels and histopathology damage scores in a porcine open surgery model of bowel ischemia48. However, the change of LCL seen with an ischemic insult is much greater than the observed subtle difference between the two groups in this study. Therefore, although there was a reduction in intestinal LCL with surgical humidification, the absolute change in oxygenation may be beyond the resolution of HSI and therefore not detected in the StO2 or TWI results. Previous animal studies using other methods to assess perfusion and oxygenation provide support for surgical humidification improving local tissue perfusion and oxygenation2,5,7. Further, a rodent model of open abdominal surgery also reported that surgical humidification increased perfusion within the lateral viscera edge of the surgical site when compared to no surgical humidification (5.2 units/min decrease (95% CI − 6.83 ± 3.58; p < 0.0001)7. Another rodent model study also showed that sub-peritoneal tissue oxygen partial pressure (PtO2) was significantly increased with use of warm humidified CO2 (by 29.8 mmHg or 96.6% increase, P < 0.001) from ambient air controls5. These findings are supported by the results from this study that surgical humidification increased core temperature and local intestinal temperature and LCL levels.

Carbon dioxide (CO2) is denser than air and naturally settles downward into the surgical site and disperses toward the floor, away from the breathing zone of the surgical team. To date, there have been no reported cases of surgical team CO2 intoxication from use of surgical humidification during open surgery including laparotomy.

In summary, surgical humidification was shown to significantly improve core and local intestinal temperature, LCL levels and protect against histological peritoneal and intestinal damage in a porcine model of laparotomy. Together, these results suggest that surgical humidification may be a valuable local intervention for improved tissue temperature and oxygenation in laparotomy to protect against intestinal hypoxia, tissue damage and possibly surgical complications, further confirmatory studies are needed.