Surgical humidification (HumiGard™) improves tissue temperature during open spinal surgery: a first in-human randomised controlled trial

Introduction

Surgical theatres are relatively cold (18–20 ℃) and dry (~50% relative humidity) compared to human body conditions (1). During open surgery the warm, hydrated internal tissues are exposed to this environment, leading to heat and moisture loss from the wound through evaporative cooling (2). Consequently, the open surgical wound is susceptible to cooling and tissue desiccation. Studies have shown that during open abdominal surgery, wound temperatures can drop below 30 ℃ even when the core body temperature is within the normothermic range (3,4).

There is a well-documented correlation between surgical site infections (SSIs) and perioperative hypothermia (5,6). Lower temperatures adversely affect tissue perfusion, oxygenation, and the immune response. Therefore, guidelines from agencies such as The National Institute for Health and Care Excellence (NICE), The Centre for Disease Control (CDC) and The Association of periOperative Registered Nurses (AORN) emphasise the importance of maintaining intraoperative normothermia. However, the effects of intraoperative cooling and drying of the local surgical wound and the impact on postoperative outcomes have been overlooked. An open surgical wound that cools develop physiological responses comparable to the hypothermic state: vasoconstriction, reduced blood supply, and decreased tissue oxygen tension (7-11).

Surgical humidification provides a warmed and hydrated environment to the wound, counteracting the detrimental effects of a cold, dry surgical wound. In a human randomised controlled trial (RCT), the use of surgical humidification during open abdominal surgery significantly preserved peritoneal tissue viability compared to wounds that did not receive surgical humidification (4). Similar findings were observed in open cardiac surgery, where surgical humidification conserved microvilli and reduced pericardial damage (12). Animal studies have also provided evidence in support of tissue preservation and improvements in tissue perfusion and oxygenation with surgical humidification during open surgery (10,11,13). By preserving tissue integrity, a warmed and hydrated surgical site may enhance resistance to SSI (14). In open abdominal surgeries, the incidence of SSI decreased from 14% to 5% with the use of surgical humidification (15). A similar reduction was shown in laparoscopic surgery where SSI decreased from 13% to 5.7% (16).

Despite improvements in infection prevention protocols, SSI remain a significant challenge in orthopaedic surgery, with an increasing number of revision surgeries attributed to infections. Factors such as an aging population and higher body mass index (BMI) are thought to contribute to this trend (17). SSI result in longer hospital stays, increased readmissions, and higher healthcare costs, and are associated with functional loss and higher revision rates (18). Based on evidence from cardiac and open abdominal surgeries, we propose using surgical humidification in open orthopaedic surgeries to mitigate the cooling and drying of surgical wounds. This approach has the potential to enhance resistance to SSI and improve the wound healing process.

This study aimed to investigate whether surgical wound temperature decreases during open spinal surgeries and to assess the effectiveness of delivering warm (37 ℃) and humidified (100% relative humidity) air over the surgical site as a potential method for mitigating local tissue cooling. This study seeks to replicate the benefits observed in open abdominal and cardiac surgeries and is the first application of surgical humidification in orthopaedic surgery. We present this article in accordance with the CONSORT reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-67/rc).

Methods

Study design and data collection

This parallel-group RCT was conducted at Middlemore Hospital in Auckland, New Zealand, from February 2021 to January 2023. This study was approved by the Health and Disability Ethics Committee (HDEC) of New Zealand and was registered in the Australian New Zealand Clinical Trials Registry (ANZCTR) [No. ACTRN12620001321932; Universal Trial Number (UTN): U1111-1257-0011]. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments.

Participants were consented and randomised 1:1 to either the Control group (no intervention to mitigate evaporative cooling of the surgical wound) or the Surgical Humidification group based on a computer-generated randomisation list. Blinding was maintained for participants and the statistician; however, it was not feasible to blind the operating surgeon due to the presence of the Surgical Humidification system.

The primary outcome was local surgical wound temperature determined by thermal imagery captured at 15-minute intervals by a FLIR E8 Thermal Camera (FLIR Systems, Oregon, USA). Temperature values were obtained from a region of interest drawn around the wound edge that excluded instruments and tissue retractors. Digital images of the surgical wounds were also taken at these intervals. Additional intraoperative data including patient core temperature, blood loss, duration of surgery and patient demographics were collected. Adverse events were monitored from the time of surgery through to the routine 6-week post-surgery follow-up. Operating surgeons provided usability feedback on the Surgical Humidification system.

Patient population

A sample size of 28 participants (including contingency for 20% drop out) was determined using a bootstrap strategy based on previous studies with Surgical Humidification in open abdominal surgery (19-21). Inclusion criteria included patients aged 22 years and older who underwent elective one or two-level spinal decompression (laminectomy or laminectomy and discectomy) between the L1–S1 vertebrae. Exclusion criteria included patients with incision lengths foreseeably longer than 160 mm, skin sensitivity to 3M Ioban™, undergoing concomitant procedures, active medical or psychiatric conditions, or participation in another clinical trial.

Intervention devices

The study used two iterations of the F&P HumiGard™ Surgical Humidification system (Fisher and Paykel Healthcare Ltd., Auckland, New Zealand). Initial observations of the HumiGard System Version 1 (V1) indicated that the system could be further optimised to achieve the target temperature of 37 ℃ within the open wound environment. Following an approved protocol amendment, HumiGard Version 2 (V2) was used for the remainder of the study and is the current iteration of the HumiGard System for Orthopaedic Surgery.

Surgical procedure

All patients were prepared for surgery following the hospital’s standard of care irrespective of randomisation. When HumiGard was used, the non-sterile components of the device (flow source, unheated circuit, humidification chamber and humidifier) were set up by the circulating nurse according to the device user instructions while the patient was being prepared for surgery (see Figure 1). Once the aseptic field was established, the surgeon placed the patient interface centrally along the marked incision, connected it to the heated circuit, and established gas flow. The patient interface remained on the patient from incision to wound closure. Figure 2 shows a representative image of the on-patient use of the HumiGard system.

Figure 1 Diagrammatic representation of the surgical humidification system for orthopaedic surgery. The system is composed of non-sterile and sterile components. Air travels to the heated humidification chamber that is on top of a humidifier where it is saturated with sterile water. Water vapour is carried via the tubing kit and delivered to the open surgical wound via the patient interface.

Figure 2 On-patient use of the surgical humidification system. The patient interface adheres around the surgical site, is flexible and seamlessly integrates with the procedure (including use of retractors).

Statistical analysis

Data were analysed using independent, two-tailed t-tests to compare V1 and V2 with the Control group. Results are presented as mean ± standard deviation (SD). Discrete variables were analysed using a Chi-squared test. Statistical significance was defined as P<0.05.

Results

Participant demographics and surgery data

Twenty-eight participants completed the study: 14 in the Control group, 8 in V1 and 6 in V2 (Figure 3). When used, HumiGard was delivered throughout the procedure from the first incision to wound closure. The average operating theatre temperature was 20 ℃ with a relative humidity of 50%. There were no significant differences in participant characteristics between the study groups including age, gender, BMI, duration of surgery and the incision length of the surgical wound (Table 1).

Figure 3 CONSORT diagram of patient randomisation in this study. Fourteen participants were randomised to the Control group, eight to V1 and six to V2. One patient was consented and randomised but did not participate due to the cancellation of surgery. V1, HumiGard Version 1; V2, HumiGard Version 2.

Local surgical wound temperature

Figure 4 shows the representative thermal images captured during surgery, comparing wound temperatures with and without the use of HumiGard. Under standard operating conditions, open spinal wound temperature decreased significantly over the course of 90-min in surgery. At 15-min post-incision, the wound temperature was 32.7±4.0 ℃, which decreased to 28.5±0.9 ℃ by 90-min (P=0.007) (Figure 5). The drop in Control wound temperature occurred independently of incision length, as evidenced by the low regression coefficient (R²=0.0578), indicating weak correlation between incision length and wound temperature at closure (Figure 6). Wounds treated with the HumiGard humidification system maintained stable wound temperatures throughout the same intra-operative period, with no statistically significant decline observed (P=0.95 for V2 and P=0.76 for V1; comparison between 15 and 90 min for each device). Wound temperatures in the HumiGard V1 group were 35.4±3.9 ℃ at 15-min and 33.5±0.9 ℃ at 90-min, and the HumiGard V2 group were 38.4±1.7 ℃ at 15-min and 36.8±1.5 ℃ at 90-min (Figure 5).

Figure 4 Representative thermal images of the surgical wounds at 45 minutes from the first incision. Control wounds were colder (A) compared to wounds with surgical humidification (B). Scale bar indicates temperature in ℃.

Figure 5 Average skin and surgical wound temperature from time of incision to 90 min during open spinal surgery. Control wounds (blue) showed a significant temperature decline from 32.7±4.0 ℃ at 15 min to 28.5±0.9 ℃ at 90 min (P=0.007). HumiGard-treated wounds remained thermally stable: V1 (black) from 35.4±3.9 ℃ (15-min) to 33.5±0.9 ℃ (90-min), and V2 (red) from 38.4±1.7 ℃ (15-min) to 36.8±1.5 ℃ (90-min). Wound temperatures were significantly higher in the V2 group compared to the Control at all timepoints from 15 to 90 min (*P<0.001). V1-treated wounds (black) showed significantly elevated temperatures at 60 min (⁺P=0.03) and 90 min (⁺P=0.03). Data are presented as mean ± standard deviation. V1, HumiGard Version 1; V2, HumiGard Version 2.

Figure 6 Scatter plot showing the relationship between wound temperature at closure and incision length. Each point represents an individual measurement. A linear regression line is fitted to the data, indicating a weak correlation between the two variables (R²=0.0578).

At 15-min post-incision, wounds treated with HumiGard V2 exhibited temperatures that were, on average, 5.7 ℃ higher than those in the Control group (P<0.001). This thermal advantage was sustained throughout the procedure, with V2-treated wounds maintaining significantly higher temperatures at all measured timepoints from 15 to 90 min (P<0.001). At the 90-min mark, the average temperature difference between Control and V2-treated wounds was 8.3 ℃. Wounds treated with HumiGard V1 also demonstrated significantly elevated temperatures compared to Control at 60 min (P=0.03) and 90 minutes (P=0.03)

Skin temperatures recorded immediately prior to incision (0 min) were comparable across all groups. The mean skin temperature was 32.4±2.4 ℃ for the Control group, 34.1±1.6 ℃ for the V1 group, and 31.7±2.6 ℃ for the V2 group.

Core temperature and adverse events

The core temperatures between study groups did not differ significantly. The average core temperatures were 36.0±0.7 ℃ for the Control group, 35.8±0.5 ℃ for V1, and 35.9±0.9 ℃ for V2. No SSI, device-related serious adverse events, or other adverse events were reported during the 6 weeks from surgery to patients’ post-op follow-up.

Operating surgeon/user feedback

Operating surgeons provided usability feedback on the HumiGard System. Surgeons consistently reported that applying the patient interface was “easy” and that performing the procedure with the interface in place felt the “same as normal”, indicating that HumiGard integrates seamlessly into the surgical workflow without introducing operational complexity or disruption. Intraoperative visual and tactile evaluations performed by operating surgeons provided supplementary qualitative insights into tissue and musculature characteristics associated with the use of HumiGard:

• Muscle wall and superficial fat appeared healthier than usual;
• Tissue appeared moister and healthier;
• Muscle was redder, dura not as dry.

Discussion

Open surgical wounds become vulnerable to cooling and desiccation as soon as they are opened and exposed to the cold (18–20 ℃) and dry (~50% relative humidity) theatre environment (1). In this study, the average theatre temperature was 20 ℃ with a relative humidity of 50%, which is equivalent to the current standard of care during open surgeries. While irrigation is often used to keep the wound hydrated (22,23), it does not maintain the warmth of the surgical wound and can perpetuate heat loss through evaporative cooling. Moreover, it has limited efficacy in preventing SSIs. A recent study by Groenen et al. found no statistically significant difference in SSI rates between intraoperative saline irrigation and no irrigation (24). These findings highlight the need for alternative strategies to maintain wound warmth and potentially enhance resistance to SSIs.

This study is the first clinical investigation to monitor surgical wound temperature during open spinal surgery. In the Control group, wound temperatures began to decline from 15 min post-incision and reached approximately 29 ℃ by 45 min, despite core body temperature being maintained at 36 ℃. This pattern of localised cooling is consistent with previous findings in the open abdominal surgeries (3,4), and is attributed to evaporative heat loss from exposed tissues. Notably, the decline in wound temperature occurred irrespective of incision length (Figure 6), suggesting that even small exposures can result in significant thermal loss. Cold wounds undergo vasoconstriction, reducing perfusion and tissue oxygenation (25). This reduction can impair immune and bactericidal activity in the wound, leading to poor wound healing and increasing the risk of developing SSI (26-28). In contrast, the application of warm, humidified air via HumiGard (V1 and V2) significantly mitigated this temperature drop. Wound temperatures in the HumiGard-treated groups remained significantly higher than in Controls across all timepoints, with V2 maintaining near-physiological tissue temperatures throughout the procedure. Maintaining local tissue normothermia may be critical for optimal surgical outcomes. Warmth enhances tissue perfusion, ensuring continuous delivery of oxygen and nutrients while facilitating the removal of metabolic waste. Tissue oxygenation is also essential for immune defence, particularly for neutrophil-mediated microbial killing, and supports key phases of wound healing including epithelisation, angiogenesis, and collagen synthesis. These findings underscore the potential physiological importance of preserving wound temperature during surgery and support the use of HumiGard as a non-invasive strategy to reduce intraoperative heat loss from surgical wounds.

Tissue exposed during open surgery is vulnerable to evaporative cooling, which can result in desiccation and potential tissue damage. Operating surgeons noted that, anecdotally, muscle tissue appeared moister and redder when HumiGard was used. These visual assessments are inherently subjective, but provide a preliminary insight into tissue appearance, hydration, perceived integrity, and thereby the potential effectiveness of HumiGard in reducing tissue desiccation. However, given the inherently observational nature, these findings should be interpreted with caution and considered as the subject of future studies. Continued research should quantitatively evaluate the effects of intraoperative desiccation and cooling on tissue integrity, as well as assess the impact of maintaining physiological wound temperature on perfusion, oxygenation levels, and wound healing outcomes following open orthopaedic surgery. The long-term outcomes related to wound healing quality, scarring, and functional recovery should ideally be assessed over extended follow-up periods of several months.

A limitation of this study was the inability to directly measure tissue moisture, as practical tools for quantifying hydration in surgical wounds are currently unavailable. Nevertheless, the HumiGard system delivers warmed, fully saturated gas to the wound site, creating a thermally stable and hydrated microenvironment that minimises evaporative loss and helps preserve tissue moisture. Additionally, the authors acknowledge the small sample size of this study. Despite this, the cohort was sufficient to demonstrate significant improvements in the wound temperature and the usability of the system.

Conclusions

Open surgical sites are vulnerable to evaporative heat loss, resulting in a progressive decline in local tissue temperature. This study demonstrated that similar thermal loss occurs in open spinal surgical wounds during procedures lasting up to 90 min. The HumiGard humidification system effectively mitigated this effect by maintaining wound temperatures near physiological levels throughout the 90-min period. Application of the device was straightforward and did not disrupt surgical workflow. These findings support the feasibility of using HumiGard to preserve the temperature of the surgical wound environment during open spinal procedures.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the CONSORT reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-67/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-67/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-67/prf

Funding: The study was sponsored by Fisher & Paykel Healthcare Ltd. who manufacture and market HumiGard.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-67/coif). S.M. and J.F.B. reports consulting fees and support for attending meetings and/or travel received from Fisher and Paykel Healthcare. S.M., A.P. and J.F.B. report that the study was sponsored by Fisher & Paykel Healthcare Ltd. who manufacture and market HumiGard. D.S. was an employee of Fisher and Paykel Healthcare at the time this manuscript was written. C.J.T.S. is an employee of Fisher and Paykel Healthcare Ltd. who manufacture and market the studies intervention device (HumiGard) and hold shares in Fisher and Paykel Healthcare Ltd. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was approved by the Health and Disability Ethics Committee of New Zealand and was registered in the Australian New Zealand Clinical Trials Registry (ANZCTR) (No. ACTRN12620001321932; Universal Trial Number (UTN): U1111-1257-0011) and informed consent was obtained from all individual participants. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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