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PATIENT SAFETY

Resistive Polymer Versus Forced-Air Warming: Comparable Heat Transfer and Core Rewarming Rates in Volunteers

Oliver Kimberger, MD^*, Christine Held, MD^*, Karin Stadelmann, MD^*, Nikolaus Mayer, MD, Corinne Hunkeler, MD^*, Daniel I. Sessler, MD, and Andrea Kurz, MD

From the *Department of Anesthesiology and Pain Therapy, University Hospital Bern, Switzerland; Department of Anesthesiology, General Intensive Care and Pain Medicine, Medical University of Vienna, Austria; and Department of Outcomes Research, The Cleveland Clinic, Cleveland, Ohio.

Address correspondence and reprint requests to Dr. Oliver Kimberger, Department of Anesthesiology, General Intensive Care and Pain Medicine, Medical University of Vienna, Vienna, Austria. Address e-mail to study{at}kimberger.at.

Abstract

BACKGROUND: Mild perioperative hypothermia increases the risk of several severe complications. Perioperative patient warming to preserve normothermia has thus become routine, with forced-air warming being used most often. In previous studies, various resistive warming systems have shown mixed results in comparison with forced-air. Recently, a polymer-based resistive patient warming system has been developed. We compared the efficacy of a standard forced-air warming system with the resistive polymer system in volunteers.

METHODS: Eight healthy volunteers participated, each on two separate study days. Unanesthetized volunteers were cooled to a core temperature (tympanic membrane) of 34°C by application of forced-air at 10°C and a circulating-water mattress at 4°C. Meperidine and buspirone were administered to prevent shivering. In a randomly designated order, volunteers were then rewarmed (until their core temperatures reached 36°C) with one of the following active warming systems: (1) forced-air warming (Bair Hugger warming cover #300, blower #750, Arizant, Eden Prairie, MN); or (2) polymer fiber resistive warming (HotDog whole body blanket, HotDog standard controller, Augustine Biomedical, Eden Prairie, MN). The alternate system was used on the second study day. Metabolic heat production, cutaneous heat loss, and core temperature were measured.

RESULTS: Metabolic heat production and cutaneous heat loss were similar with each system. After a 30-min delay, core temperature increased nearly linearly by 0.98 (95% confidence interval 0.91–1.04)°C/h with forced-air and by 0.92 (0.85–1.00)°C/h with resistive heating (P = 0.4).

CONCLUSIONS: Heating efficacy and core rewarming rates were similar with full-body forced-air and full-body resistive polymer heating in healthy volunteers.

Even mild intraoperative hypothermia increases the risk of cardiac complications1 and wound infections,2 augments blood loss,3 increases thermal discomfort,4,5 augments nitrogen loss,6 and prolongs hospitalization.2 Perioperative warming to prevent hypothermia and maintain body temperature has become routine in recent years. Active warming is usually required, and forced-air warming is the most common approach.

Forced-air warming is highly effective,7,8 relatively inexpensive, comparably safe, and easy to use; forced-air warming has proved to be superior to several other warming systems.9–12 A recently developed warming system (HotDog, Augustine Biomedical, Eden Prairie, MN) uses a different technology; an electric current heats a reusable resistive polymer blanket covered by a polypropylene sheet. Potential advantages of the system over forced-air warming include moldable, thin, and reusable blankets, a cable connection from the blanket to the power unit that can be cleaned easily, and an almost silent operation.

From a physical perspective, ambient air is not the perfect medium for transferring heat because of its low heat capacity. To work efficiently, forced-air devices thus have to surround large areas of a patient's skin with redundant amounts of moving warm air; close contact between the inflated forced-air blanket and the patient is not necessary. In contrast, electric heating blankets rely primarily on conductive heating and therefore require direct skin contact for optimal function. Clinical efficacy of forced-air and the resistive heating systems might thus differ considerably, even if operated at similar temperatures. Previous studies with different resistive heating blankets, mostly carbon-fiber systems, have shown mixed results, performing adequately13–15 to poorly.16,17

We therefore evaluated metabolic heat production, cutaneous heat loss, and core rewarming rate, which we defined as efficacy (primary end-point of the study), with each system in a standardized volunteer setting. Specifically, we tested the hypothesis that the efficacy of forced-air and polymer resistive heating is similar.

METHODS

With approval from the Institutional Review Board of the University Hospital of Bern and written informed consent, we studied eight young, healthy volunteers. In female volunteers, a negative pregnancy test was confirmed. Volunteers were included if they had a body mass index <30 kg/m² and did not take any medications regularly. Volunteers were excluded if they had any chronic disease or reported problems with anesthesia.

Each volunteer participated on two study days, separated by no less than 2 days. The volunteers fasted 8 h before the experiment. Throughout each study day, the volunteers were minimally clothed (men: only briefs or boxers, women: two-part swimsuit) and rested supine on a standard patient bed equipped with a water mattress.

Treatment Protocol
All experiments started between 8:30 and 9:00 am to avoid the potential confounding effect of thermoregulatory circadian fluctuations. Immediately after arrival in the laboratory, volunteers were given 60 mg oral buspirone to reduce the shivering threshold18 and ondansetron 8 mg prophylactically to treat possible meperidine-induced nausea and vomiting. An IV catheter was inserted into an antecubital vein for meperidine administration. Meperidine was infused via a computer-controlled syringe pump to a target plasma concentration of 1.2 µg/mL using previously published pharmacokinetic data.19,20

The volunteers' core temperatures were decreased with a forced-air cover (PolarAir, Arizant, Eden Prairie, MN) set to 10°C, and a circulating-water mattress (Medi-therm, Gaymar, Orchard Park, NY) set to 4°C. Cooling was continued until core temperature reached 34°C; severe vasoconstriction in all volunteers was confirmed by a forearm-fingertip temperature-gradient exceeding 4°C. This was followed by a 30–45 min hypothermic equilibration period.

On the first study day, volunteers were randomly assigned to one of two rewarming methods using computer-generated assignments that were kept in sequentially numbered opaque envelopes:

1. Forced-air warming with a Bair Hugger whole-body warming cover model # 300, inflated with a model # 750 blower unit set to "high" (approximately 43°C) (Arizant, Eden Prairie, MN);
   
2. Resistive polymer warming with a HotDog whole-body electric warming blanket and the HotDog standard controller unit, set to "high" (approximately 43°C) (Augustine Biomedical, Eden Prairie, MN).
   

Rewarming continued until tympanic membrane temperature reached 36°C. On each volunteer's second study day, the alternate rewarming method was used.

Measurements
Demographic and morphometric characteristics were recorded. Heart rate was measured continuously using an electrocardiogram; oscillometric blood pressure was determined at 10-min intervals at the left arm. Body surface area was calculated using Mosteller's formula;21 Body Mass Index was calculated as weight (kg)/height (m²).

Oxygen consumption and carbon dioxide production were measured by a Vmax 29n metabolic monitor (SensorMedics Corp., Yorba Linda, CA). The system was calibrated daily before the experiment with two known gas mixtures. Measurements were recorded every minute. A sustained increase in oxygen consumption to at least 20% above baseline values identified shivering. Incidence of shivering was determined post hoc by an investigator blinded to treatment and core temperature.

Heat flux (W/m²) was measured at 1-min intervals from 15 area-weighted sites using thermal flux transducers (Concept Engineering, Old Saybrook, CT).22

Core temperature was recorded from the tympanic membrane using Mon-a-therm thermocouples (Mallinckrodt Anesthesiology Products, St. Louis, MO). The volunteers were asked to insert the tympanic probe until they felt the thermocouple touch the tympanic membrane; appropriate placement was confirmed when volunteers detected gentle rubbing of probe on the tympanic membrane. To ensure correct placement, tympanic probes were inserted into both ears; a difference of 0.2°C was considered acceptable. After insertion, the aural canal was occluded with cotton, the probe securely taped in place, and a gauze bandage positioned over the external ear.

Skin temperatures were measured at 15 sites.22 Ambient temperature was recorded at the height of the volunteer. Arteriovenous shunt vasoconstriction was evaluated by forearm minus fingertip gradient.23 Temperatures were recorded at 1-min intervals from thermocouples connected to calibrated Iso-Thermex 16 channel electronic thermometers (Columbus Instruments International, Corp., Columbus, OH) having an accuracy of approximately 0.1°C and a precision of approximately 0.01°C.

Thermal comfort was evaluated at 10-min intervals with a 100-mm long visual analog scale. Zero millimeter was defined as the worst imaginable cold; 50 mm as thermal comfort, and 100 mm as the worst imaginable heat. Both scores were obtained at 10-min intervals throughout cooling.

Data Analysis
Oxygen consumption in mL/min was converted to metabolic heat production using a ratio of 4.82 kcal/L oxygen at a respiratory quotient of 0.82 as in a previous study;24 for conversion from W to kcal/h a factor of 1 W = 0.86 kcal/h was used.

Ambient temperature and physiological responses on each study day were first averaged for each volunteer. The resulting values were then averaged among volunteers.

To calculate the sample size, a power analysis for equivalence (paired test) was performed. Lower and upper equivalence bounds were ±0.2°C core temperature, with a coefficient of variation of 0.11, calculated from previous data. A sample size of eight volunteers was estimated to achieve a power of 85% to detect equivalence within the specified equivalence bounds.

Data were tested for normal distribution using QQ-plot and Kolmogorov-Smirnov test. Potential confounding factors were compared with a paired t- or Wilcoxon's signed rank test, as appropriate. The rates of core temperature increase (primary end-point) and systemic heat balance change (secondary end-point) were calculated by linear regression (30–120 min, linear portion of the curves) and compared with paired t-tests. Heat production was averaged over time for each volunteer and compared with a paired t-test.

Results are presented as means (95% confidence intervals). Differences were considered statistically significant when P values were <0.05. SPSS 11.0 (SPSS, Chicago, IL) and SAS 8.0 (SAS Institute, Cary, NC) were used for statistical analysis, Graphpad Prism 4.0 (Graphpad Software, San Diego, CA) was used for figures.

RESULTS

The volunteers were 22 ± 3 yrs old, weighed 66 ± 11 kg, were 172 ± 12 cm tall and had a body mass index of 23 ± 3 kg/m². Three volunteers were women and five were men.

During the study there were no major adverse events; four volunteers complained of nausea and vomiting several hours after meperidine administration.

Physiological or ambient variables were similar on the two study days (Table 1). All volunteers were vasoconstricted before warming was started and vasodilated by the time they reached a core temperature of 36°C. At no point during the experiments did oxygen consumption exceed normothermic baseline values by more than 5% and there was no visible shivering at any time. All volunteers remained hemodynamically stable and no respiratory toxicity was noted despite meperidine administration. Thermal comfort scores were similar at the beginning and at the end of each study day, typically ranging from 40 to 60 mm on a 100-mm Visual Analog Scale.

Heat production, as measured by oxygen consumption, was similar on both study days (P = 0.3, Fig. 1). Cutaneous heat loss was also similar on each study day (P = 0.8, Fig. 2). Consequently, systemic heat balance (the difference between heat production and loss) was similar.

View larger version (10K): [in this window] [in a new window]   Figure 1. Metabolic heat production. Time-weighted individual average values on each study day shown with connected squares. Means ± 95% confidence intervals for each study day shown with circles and whiskers.

View larger version (15K): [in this window] [in a new window]   Figure 2. Cutaneous heat loss, means ± sds. Negative values indicate heat transfer across the skin surface into the volunteers.

Maximum skin temperature recorded at any point during the entire study was 37.4°C during a forced-air treatment (probe located at the medial thigh) and 38.9°C (probe located at the abdomen) during a resistive heating treatment.

There was no difference in core temperatures before warming started on each experimental day. Before starting, patients had 34.0°C (33.9–34.0) core temperature on the resistive warming day, 34.0°C (33.9–34.1) core temperature on the forced air warming day [means, 95% confidence interval (95% CI); P = 0.3]. After rewarming, core temperatures did not increase for 32 (26–39) min with forced-air and for 35 (26–44) min with resistive heating (means, 95% CI; P = 0.8). Core temperatures increased between 30 and 120 min by 0.98 (0.91–1.04)°C/h during forced-air treatments and by 0.92 (0.85–1.00)°C/h during resistive warming treatments (means, 95% CI; P = 0.4) (Fig. 3). Similarly, areas under the core-temperature curves did not differ significantly.

View larger version (10K): [in this window] [in a new window]   Figure 3. Core temperature, means ± sds.

DISCUSSION

Forced-air warming has become the standard for perioperative thermoregulatory management because of its excellent safety record and its high efficacy.7–12 The efficacy of forced-air systems is relatively independent of the blanket design and relies mostly on blower strength, air temperature and area of skin covered.8,25 Blower strength is limited by energy consumption and fan noise, and varies among devices8; in contrast, the amount of skin covered is dictated by the extent of the surgical field, while the upper temperature limit is set by the amount of heat human skin tissue tolerates. Consequently, the high setting in most forced-air devices heats air to no more than approximately 43°C.

Forced warm air is distributed to regions of the skin surface via an inflatable blanket and is encapsulated under the patient's sheets and drapes. Forced-air primarily warms the uncompressed, anterior part of the skin and thus avoids the danger of thermal injury caused by warming of compressed, poorly perfused posterior skin (like e.g., circulating-water mattresses26,27). Consequently, there are only few reports of thermal injury caused by forced-air warming,28,29 almost all related to improper use (i.e., using the warming hose without the blanket, i.e., "hosing").30

Forced-air warming also has some intrinsic disadvantages. Disposable covers must be purchased for each case and the fan in forced-air blowers generates some noise. Furthermore, some authors have suggested that the nondisposable parts of forced-air devices, i.e., blower and hose, may be locations of bacterial colonization,31 and that forced-air may deliver these bacterial pathogens to the patient.32 Despite these theoretical concerns, two separate studies have evaluated bacterial dispersion in operating room environments, with and without forced-air, and found no clinically relevant difference.33,34

A recently developed warming system (HotDog) uses resistive heating of a polymer blanket covered in a polypropylene sheet. There are three major potential advantages of the resistive warming device compared with forced-air warming: (1) the HotDog system does not contain moving parts, with the exception of a small fan ventilating the main circuit board, and is therefore relatively quiet; (2) the blankets are reusable, thus potentially reducing the per patient cost of warming; and, (3) all parts of the resistive warming device can be easily cleaned to minimize the risk of bacterial colonization.

The potential advantages of resistive warming are obviously only important if the warming efficacy of both devices are comparable. Our results indicate that a conventional, full-body forced-air cover, and a new polymer resistive heating blanket were comparably effective in our volunteers. Specifically, metabolic heat production, cutaneous heat loss, and core temperature were comparable with each treatment. There was also no significant difference between the two devices in the elapsed time from the start of warming until core temperature increased. As initially only the cold periphery is warmed, core temperature usually increases after approximately 30 min.

Obviously, the resistive heating system also has some disadvantages. It has to be reused and cleaned, which requires manpower, labor time, and cleaning equipment. If not properly cleaned the HotDog system could become a possible source of bacteria. The HotDog system is less flexible than warm air blankets, which can be easily compressed and molded. In contrast to warm air blankets, creases and wrinkles in the resistive blanket because of patient positioning may impair performance and, probably, safety. Unsurprisingly, sweating occurred more often under the synthetic fabric of the resistive warming blanket than under the warm air blanket. Further studies are needed to determine whether patients feel more comfortable during treatment with a resistive heating or a forced-air device.

Our results are consistent with several previous studies: In a clinical study of 24 patients undergoing laparoscopic cholecystectomy, Matsuzaki et al.13 tested a carbon fiber resistive warming blanket; the electric blanket performed better than a posterior circulating water mattress and comparable to forced-air. Similar results were reported in a study by Negishi et al.14 in 24 patients undergoing open abdominal surgery. In 60 patients with minor orthopedic surgeries, Ng et al.15 again found forced-air warming and an electric pad warming system comparably effective. However, the study by Ng et al. contains a major methodological flaw: Core temperature was measured only rectally and with an infrared tympanic thermometer, both inadequate methods for accurate core temperature assessment. In contrast, Russell and Freeman16 reported a better performance of forced-air in comparison with a warm-air under-body cover and an electric silicone-rubber heating pad in 60 patients undergoing orthotopic liver transplantation. The results of this study were not entirely surprising, as the authors accepted differences for the setpoints of the warming devices: the electric warming pad had the lowest setpoint at 39°C, whereas forced-air warming allowed temperatures up to 48°C.

In a more recent study, Leung et al.17 compared an electric heating pad with forced-air warming in 60 patients undergoing open abdominal surgery and found a significant difference in temperatures at the end of surgery, with many patients in the electric pad group remaining hypothermic at the end of surgery. However, the electric system used by Leung et al. was a posterior electric heating pad system. As mentioned, the posterior surface of supine patients is a suboptimal location for active patient heating because of low regional perfusion and the risk of pressure-heat necrosis.35 In contrast, the highest skin temperature we recorded at any site with the anterior-surface resistive polymer system was 38.9°C at the abdomen of one volunteer, which should be safe in the absence of pressure or ischemia.36

An obvious limitation of our study is that we evaluated volunteers rather than surgical patients. Extrapolation to surgical patients must be made cautiously, especially as it is not often possible to use a full-body cover during surgery. In surgical patients actual heat transfer and rewarming rates will surely be lower than what we observed if only upper- or lower-body blankets are used. Furthermore, we were unable to fully mimic a surgical situation during which patients receive different anesthetics, vasoactive medication, IV fluid, blood transfusions, and lose heat via surgical incisions. These factors can have a major effect on core temperature during surgery; thus, patient studies are needed to determine long-term safety and clinical efficacy of the new resistive polymer warming system.

On the other hand, a volunteer setting gives us the opportunity to directly compare each device in each volunteer, thus eliminating person-to-person variability. Only the volunteer setting allows us to determine the exact systemic heat balance, which is difficult at best during surgery because evaporative loss from within incisions can be substantial and impossible to quantify.37

In summary, metabolic heat production and cutaneous heat loss were similar with forced-air and resistive polymer heating. Core rewarming rates were thus comparable. The resistive polymer heating system appears suitable for maintaining perioperative normothermia, but patient-based studies are required to confirm the new system's efficacy in clinical settings.

Footnotes

Accepted for publication May 7, 2008.

Supported by the Research Fund of the Department of Anesthesia, University of Bern, Switzerland. Dr. Oliver Kimberger was funded by a Scholarship of the Swiss Confederation for University Studies. Thermocouples were donated by Mallinckrodt, Hazelwood, MO; the Gaymar water mattress was donated by Nufer Medical AG, Guemligen, Switzerland; the HotDog system was donated by Augustine Biomedical, Eden Prairie, MN.

The sponsors were not involved in data analysis or manuscript preparation.

None of the authors has a personal financial interest related to this research.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kimberger, O. Articles by Kurz, A. Search for Related Content PubMed PubMed Citation Articles by Kimberger, O. Articles by Kurz, A. Related Collections Patient Safety Technology