Pre-hospital torso-warming modalities for severe hypothermia: a comparative study using a human model

EM Advances

CJEM 2005;7(6):378-386

Abstract

Résumé

Introduction

It is now generally accepted that active warming of severely hypothermic patients is both safe and beneficial.^1–3 External heat application is particularly important for severely hypothermic patients (defined as a core temperature <28°C), in whom endogenous warming capability is minimal due to absence of shivering heat production. Although active warming of severely hypothermic patients is routinely applied in the emergency department, it remains uncommon in the pre-hospital setting. Since there are approximately 100 hypothermia deaths annually in Canada,^4,5 active pre-hospital warming of hypothermic patients could be beneficial, particularly in situations involving lengthy scene or transport times.

Several torso-warming modalities are currently available that are potentially appropriate for pre-hospital use. These include heating pads,⁶ a charcoal heater,⁷ body-to-body contact with a normothermic heat donor,^8,9 and forced-air warming with either a commercial soft blanket¹⁰ or a rigid cover.¹¹ However, comparatively evaluating such modalities is difficult; pre-hospital studies pose both methodologic and logistic challenges, and laboratory studies are complicated by ethical and safety considerations that preclude rendering volunteers severely hypothermic. Because of these challenges, a human model, using meperidine in mildly hypothermic volunteers to inhibit shivering and simulate the thermal responses of severe hypothermia, has been developed and applied successfully.^12,13

The purpose of this study was to comparatively evaluate the following 5 torso-warming modalities using our human model of severe hypothermia: 1) forced-air warming with a 600-W heater and commercial soft warming blanket; or 2) a 600-W heater and rigid cover; or 3) an 850-W heater and rigid cover; or 4) a charcoal heater on the chest; or 5) direct body-to-body contact with a normothermic partner.

These specific modalities were chosen for study because they are either commonly advised (body warming), or commercially available and of proven effectiveness (forced-air warming and charcoal heater). A baseline of spontaneous warming served as a comparative control. After considering relative heat production and surface area for heat transfer, we hypothesized that forced-air warming would be most effective, followed by the charcoal heater, body-to-body contact and spontaneous warming.

Methods

Design, setting and subjects

This prospective study was carried out in the Laboratory for Exercise and Environmental Medicine at the University of Manitoba between Januray and April 1999. University students without history of narcotic allergy or current use were recruited to volunteer for participation. Six subjects (5 male, 1 female) with a mean age of 22.8 years (range 21–26 yr) were studied after providing written informed consent (Table 114). The study was approved by the Faculty Human Ethics Committee at the University of Manitoba.

Monitoring

Esophageal temperature (Tes),^15,16 electrocardiogram, heart rate, blood pressure and arterial oxygen saturation were continuously monitored during the experimental procedures as described previously.^12,13 Intravenous (IV) access was obtained in the right forearm or hand for the purpose of drug and/or saline administration. Oxygen consumption was determined, and skin heat transfer (Qskin; W·m–2) and skin temperature (°C) were measured from 12 sites using thermal flux transducers (Concept Engineering, Old Saybrook, Conn.).

Protocol

Pre-hospital active warming, if applied, is typically delayed in real-world resuscitations from hypothermia and follows patient rescue, removal from cold stress, insulative packaging, and often emergency medical services (EMS) arrival. Because of this, a protocol intended to mimic such a scenario was applied. Each subject served as their own control for comparative evaluation of each of the warming modalities, and was cooled at the same time of day on 6 occasions, separated by at least 3 days for each occasion. The order of the experiments followed a modified random balanced design, with spontaneous warming the first modality to be tested for every subject. The results from spontaneous warming served both as a comparative control and for determination of the meperidine dose for subsequent trials. Subjects dressed in a bathing suit and sat quietly at an ambient temperature of approximately 22°C for 10 minutes of baseline data collection after monitors were applied. The subjects were then immersed to the level of the sternal notch in a stirred water bath, the temperature of which was lowered by the addition of ice from 21ºC to 8 ºC over a period of 10 minutes. Subjects were immersed for 30 minutes or until they reached a core temperature of 35°C. Ten minutes before the end of immersion, subjects were administered 1.5 mg/kg of IV meperidine (diluted in five 2-mL aliquots and injected over successive 2-minute intervals). Subjects were then hoisted out of the water, towel dried and placed in an emergency transport blanket for 30 minutes before exogenous heating was initiated. Emergency transport blankets were modified for use with a transport stretcher. Velcro-secured slits were placed in the blanket to allow the stretcher straps to pass through and be secured around the patient. Shivering metabolism was indicated by increased oxygen consumption, and post-immersion supplemental injections of meperidine (to a maximum cumulative dose of 3.2 mg/kg) were administered to maintain shivering suppression.

Warming modalities

No exogenous heat source was used in the spontaneous warming trials. Each warming experiment was terminated after 120 minutes, a time duration sufficient to establish a steady rate of core warming. Subjects were then immersed in 42°C water until their Tes rose to a normothermic level. The materials and protocols for the warming modalities were are follows.

Forced-air warming

Three forced-air warming modalities were evaluated.

1. A conventional blanket forced-air warming system (Bair Hugger 505 Heater/Blower, Augustine Med. Inc., Minn.) with a soft cover (Model 300 Full Body Blanket, Augustine Med. Inc.). This heater/blower produces 600 W and is small enough for use in pre-hospital situations (size 30 × 30 × 30 cm, weight 4.5 kg).
2. A rigid forced-air warming system, using the same heater/blower with a specially designed rigid cover.¹¹ The rigid cover is a folding unit (93 × 62 × 3 cm when folded) made out of corrugated plastic and neoprene that fits over the subject's torso and upper legs (Fig. 1). This cover configuration results in a higher heat delivery to the chest, compared with the soft blanket,¹¹ and is portable when folded.
3. A larger (60 × 40 × 30 cm), more powerful heater/blower that produces 850 W (Bair Hugger 250 Heater/Blower, Augustine Med. Inc.), combined with the rigid cover.

Fig. 1. Top: Forced-air warming system with Bair Hugger 505 Heater/Blower (Augustine Med. Inc., Minn.) and folding rigid torso cover. Bottom: Forced-air warming system in use in a United States Coast Guard rescue helicopter (can be used in any transport vehicle equipped with an inverter that produces 120 VAC and 10 A).

Fig. 2. Top: Charcoal heater (HEATPAC Personal Heater, Emergco Tech. Solutions, Vancouver). Charcoal fuel (in silver canister) is ignited and placed in combustion chamber (centre of photo). Heated air is blown through impermeable flexible ducts. Bottom: Charcoal heater in use. Combustion chamber is placed on the chest with flexible ducts applied to other areas of high heat transfer (i.e., neck and axillae). Initial ignition produces smoke but subsequent operation is safe in any ventilated area

During the forced-air warming trials, the heater/blower was set to the “high” heat setting. When the rigid cover was used, it was placed over the subject and an emergency transport blanket was closed over the cover. When the regular soft blanket cover was used, a thin cotton blanket was placed over the cover as the heavier emergency blanket would have deflated the cover and prevented air movement.

Charcoal heater

The heater studied consists of a combustion chamber, charcoal fuel and a branched heating duct that produces 250 W of heat (Emergco, Vancouver). The combustion chamber is placed on a towel on the subject's anterior chest and the heating ducts are applied dorsally over the shoulders, and then anterior under the axillae to cross over the lower anterior chest (Fig. 2). This heater is small and light (23 × 12 × 6 cm, 500 g) and is safe provided the exhaust hose is routed properly. After the heater is ignited, carbon monoxide production is minimal and does not pose a risk. The heater was ignited and set to the “high” setting 15 minutes before being applied to the subject.

Body-to-body warming

Each subject recruited their own partner for the body-to-body warming evaluation. As done in previous work, heat-donor subjects lay on their sides and made direct skin-to-skin contact with their back against the recipient's anterior torso.⁹ Midway through the study, one subject's warming partner withdrew from the study for medical reasons. This subject was thus warmed in the body-to-body experiment with a constant-heat source manikin.⁹

Data analysis

The following variables were calculated for each trial: initial cooling rate during the final 10 minutes of cold-water immersion; post-exit cooling rate following exit from cold water until the nadir in Tes before commencement of active warming; initial afterdrop (difference between Tes on exit from cold water and its nadir before active warming); post-warming afterdrop (the difference between Tes at the start of active warming and its subsequent nadir); maximum core rewarming rate during the linear increase following the Tes nadir; and the absolute change in Tes (from the start of active warming to the conclusion of the trial; also known as amount of rewarming). The oxygen uptake (VO2 l.min–1) and respiratory quotient (RQ) were used to calculate metabolic heat production (M).¹⁷

Data were compared using a repeated-measures ANOVA with Post Hoc Analysis using Fisher's protected least significant difference (PLSD) test to identify significant differences. Results are reported as means ± standard deviation (SD), and p < 0.05 was the threshold defined for statistically significant differences.

Results

Heat loss

Total surface heat loss during cooling was similar in all study groups, increasing from a baseline level of 107.5 ± 5.1 W to a maximum of 638.1 ± 78.9 W before meperidine injection, and decreasing to 503.4 ± 45.4 W by the end of immersion. The cooling rates and the initial afterdrop values after exiting the cold water were similar with all modalities studied (Table 2 and Fig. 3). Following commencement of active warming, the afterdrop was greatest in spontaneous and body-to-body warming, and least in the 850-W rigid heater/blower modality (p &lg; 0.05, Table 3 and Fig. 3). Rewarming rates were greatest using the 850-W rigid heater/blower, with the charcoal heater and 600-W rigid heater/blower being more efficient than body-to-body and spontaneous warming (p &lg; 0.05). Spontaneous warming was minimal (0.36°C/hr) and significantly slower than all active warming approaches (Table 3 and Fig. 3).

Endogenous heat production

Metabolic heat production increased from approximately 115 W at baseline to 250 W during the first 20 minutes of immersion. Meperidine suppressed shivering, with heat production returning to approximately 100 W during the first 30 minutes post-exit and subsequently falling to between 80 and 100 W throughout active warming. Subjects' heart rates increased from a baseline value of approximately 76 beats/min to a maximum of 95 beats/min just before meperidine administration. Post-immersion, heart rates declined and stabilized below baseline values (50 ± 6 beats/min), with no significant differences across the 6 modalities.

Fig. 3. (enlarge) Change in core temperature during 6 warming protocols (mean, n = 6). Change in temperature normalized to exit from 8°C water (time 0), active warming started at 30 minutes. FAW = forced-air warming.
**Significantly higher than all other conditions;
*Significantly higher than Body-to-Body and Spontaneous conditions (p < 0.05).

Exogenous heat delivery

Fig. 4 compares total cutaneous heat gain following cold-water exit. During active warming, average total surface heat gain was greatest with the 850 W rigid heater/blower modality (99 ± 6 W), followed by the 600-W blanket heater/blower (59 ± 9 W) and 600-W rigid heater/blower (45 ± 5 W) modalities, which functioned identically. Total heat gain was near 0 W with the charcoal heater and body-to-body warming, and –21 ± 2 W during spontaneous warming.

Although sedated, all subjects recovered sufficiently to leave the laboratory with an escort within 2 hours of the completion of each experiment.

Discussion

Overview

Although experimental warmers have been studied previously,¹³ this study is unique in that we used a human model of severe non-shivering hypothermia to evaluate several commercially-available or practical torso-warming modalities appropriate for use in the pre-hospital setting. We found forced-air warming and the charcoal heater were most effective in minimizing the post-cooling afterdrop and facilitating core rewarming. Forced-air warming with a rigid cover was more effective than a soft blanket, and the 850-W forced-air heater/blower led to core rewarming rates 4 times greater than spontaneous warming and more than double those of the other modalities evaluated.

Several studies on mildly hypothermic shivering subjects have found that exogenous skin heating attenuates shivering heat production by an amount equivalent to the amount of heat donation.^7,9,10,18 Under such conditions external warming does not increase the core warming rate, but it does increase comfort, decrease cardiac work and preserve substrate availability. In contrast, a previous study using a human model of severe hypothermia found that external warming with a prototype forced-air warming device increased the core rewarming rate 5-fold over spontaneous warming.¹³

Possible mechanisms for findings

The torso is considered the most efficient area to deliver exogenous heat due to the proximity of the heart and the fact that, unlike the arms and legs, heat transfer is relatively independent of changes in peripheral blood flow. Although we found forced-air warming provided more total surface heat delivery when delivered through a soft blanket, this heat was distributed over more of the body, including legs and arms. As the rigid cover concentrated heat on the torso, it is likely that this accounts for its greater efficiency at core rewarming.¹¹ We found the charcoal heater produced a total heat delivery similar to body-to-body warming, but this heat was focused on the upper chest and axillae, resulting in core rewarming rates higher than all other modalities except forced-air warming with the 850-W heater. The 250-W charcoal heater produced rewarming rates similar to forced-air warming with a more powerful 600-W heater, likely because much of the heat from a heater/blower is lost through the heating hose and top of the covers, and virtually all of the 250 W of the charcoal heater is delivered directly to the torso and transferred conductively from the combustion chamber itself rather than through convective air currents.

Fig. 4. (enlarge) Cutaneous heat gain during 6 warming protocols (n = 6). Time 0 refers to exit from cold water.
FAW = forced-air warming.
*Separates mean values, during active warming, which are significantly different (p < 0.05).

Practical implications

Clinical and experimental evidence indicates that exogenously warming hypothermic patients during transport is beneficial, particularly for those with severe hypothermia.^1,3 Since core temperature can continue to drop, initially stable patients have the potential to become unstable. Severely hypothermic patients are disadvantaged because their shivering heat production defence is abolished and their basal metabolic rate is lower than normal.¹⁹ As a result, post-cooling afterdrop can be significant and continue for a long as several hours in such patients,¹² potentially leading to ventricular fibrillation. Reversing this afterdrop in an efficient safe manner is desirable.

We found spontaneous warming resulted in very slow warming consistent with a positive heat balance of only approximately 60 W (heat production 80 W, surface heat loss 20 W). Moreover, body-to-body warming provided little benefit over spontaneous warming, although since a truly severely hypothermic patient would have an even lower basal metabolic heat production than subjects in our model, body contact may still have some clinical advantage by at least preventing continued cooling of the heart. Forced-air warming with a 850-W heater/blower doubled the core warming rates seen with a 600-W heater. This difference is clinically significant, thus development of a more compact and practical 850-W heater for pre-hospital use may be warranted.

It is evident that none of the 5 active warming modalities studied provides warming rates sufficiently rapid to promote cardiovascular or thermal instability. As a result, we feel any of these modalities would be safe for use pre-hospital use by search and rescue or EMS personnel. Although individual rewarming responses to torso warming vary with such factors as body size, age and comorbid diseases, we expect the relative heat delivery and efficacy of each rewarming method would be similar for any patient. We feel emergency physicians involved in EMS administration, and search and rescue and EMS personnel, particularly those in rural or remote locations, should be made aware of the potential benefits and equipment for active prehospital warming for hypothermic patients. External warming is likely to provide clinical benefit for any patient when more than 30 minutes will pass before reaching advanced medical care,³ and should be considered in such situations.

Limitations

Our study has certain limitations. The order of experiments did not follow a strictly balanced design as the control modailities were conducted first according to previous work.¹³ Since the shivering stimulus is maximal with this modality, the maximum dose of meperidine, and its dosing schedule can be established to ensure shivering supression in the active warming conditions. The forced-air warming (850 W) trial was completed last for each subject. This allowed the use of the more powerful heater with the most efficient cover (i.e., rigid cover) without requiring a seventh trial. This modified balanced design is unlikely to provide any bias in the results. The cooling process was similar for all modalities, and previous studies indicate that inter-treatment differences in surface-heat transfer and core temperature are dependent on heat delivery and exposed surface area; with these effects unlikely to be affected by order of treatment. Unavoidably, one subject required a constant-heat source manikin in the body-to-body trial instead of a human donor. Previous work has shown this manikin to produce the same warming results as human donors in shivering subjects, thus we feel the results for this modality were not adversely affected.⁹

Conclusions

Each of the warming modalities evaluated in this study would appear to be practical for use in the pre-hospital setting. Body-to-body warming is only minimally effective, has large resource implications, and may prevent or complicate patient transport. The charcoal heater is very effective and has the advantages of a small size and weight. Forced-air warming provides the highest core warming rates, particularly with a rigid cover, but requires appropriate electrical power. Finally, the exceptional results seen with the 850-W heater/blower warrant further consideration, and the development of a more compact and practical unit of this configuration for pre-hospital use may be warranted. 

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