Anesth Analg 1999;89:1541
© 1999 International Anesthesia Research Society
GENERAL ARTICLES
The Failure of Negative Pressure Rewarming (ThermostatTM) to Accelerate Recovery from Mild Hypothermia in Postoperative Surgical Patients
Charles E. Smith, MD FRCPC,
Ali Parand,
Alfred C. Pinchak, PE, PhD, MD,
Joan F. Hagen, BA, and
Don E. Hancock, AEET
Department of Anesthesiology, MetroHealth Medical Center, Case Western Reserve University, Cleveland, Ohio
Address correspondence to Dr. Charles E. Smith, Department of Anesthesiology, MetroHealth Medical Center, 2500 MetroHealth Dr., Cleveland, OH, 44109. Address e-mail to ces4{at}po.cwru.edu
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Abstract
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The ThermostatTM device (Aquarius Medical Corp., Phoenix, AZ) is used in a new technique to accelerate recovery from hypothermia by mechanically distending blood vessels in the hand, thereby increasing transfer of exogenous heat to the body core. We evaluated the use of the ThermostatTM device in patients with mild postoperative hypothermia (<36°C). We studied adult patients undergoing elective surgery, general anesthesia, and neuromuscular blockade. Patients with an initial postoperative tympanic membrane temperature < 36°C were randomized into two groups: 1) ThermostatTM, which consisted of a hypothermia warming mitt/seal and thermal exchange chamber for 60 min, and 2) conventional treatment, which consisted of warm blankets and/or radiant heat. Of the 191 patients enrolled, 60 (31%) developed hypothermia and were randomized to receive the ThermostatTM (n = 30) or conventional methods (n = 30). Fourteen patients in the ThermostatTM group and 17 patients in the conventional group rewarmed to 36°C before discharge from the recovery room (P is not significant). There were no differences in vital signs, rewarming time, time to discharge from the recovery room, or postoperative temperature between groups. We conclude that patients with mild postoperative hypothermia rewarmed in a similar fashion, regardless of whether the ThermostatTM or conventional methods were used.
Implications: We found that a commercially available negative pressure rewarming device (ThermostatTM; Aquarius Medical Corp., Phoenix, AZ) was not effective in accelerating rewarming in postoperative hypothermic surgical patients after general anesthesia.
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Introduction
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Mild hypothermia (core temperature < 36°C) may occur after general anesthesia because of thermal redistribution of body heat, anesthesia-induced depression of the core temperature threshold for vasoconstriction, exposure to a cold operating room (OR), infusion of room temperature IV fluids, and a variety of other factors (1–3). Perioperative hypothermia can lead to an increased incidence of surgical wound infections, prolonged hospitalization, impaired coagulation and immune response, myocardial ischemia, cardiac arrhythmias, increased norepinephrine concentrations, peripheral vasoconstriction, prolonged discharge from the postanesthesia care unit (PACU), and compensatory increases in oxygen consumption during rewarming (4–11).
A variety of methods, such as warm blankets, radiant heat, and convective warming, are available to rewarm hypothermic patients in the PACU (12–14). These methods may be less effective in hypothermic patients recovering from general anesthesia because thermoregulatory vasoconstriction limits heat transfer from the peripheral to central thermal compartments and impedes skin surface rewarming (15). Recently, Grahn et al. (16) have shown that the combined application of subatmospheric pressure, -30 to -40 mm Hg, and heat, 44°–46°C, to the hand of hypothermic patients recovering from general anesthesia was associated with a very rapid rate of rewarming, from 34.3° to 36.0°C in 8.1 min (negative pressure rewarming). These authors speculated that mechanical distension of subcutaneous venous plexuses and arteriovenous anastamoses of the hand permitted free exchange of heat between the body core and the exogenously provided heat source (16)
The purpose of this study was to evaluate the use of a commercially available negative pressure rewarming device (ThermostatTM; Aquarius Medical Corp., Phoenix, AZ) in postoperative surgical patients recovering from general anesthesia. The ThermostatTM was compared with our current method of rewarming in the PACU, warm blankets and radiant heat.
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Methods
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The protocol was approved by our institutional review board, and written, informed consent was obtained from the patients. We studied adult patients, ASA physical status I–III, undergoing elective gynecologic, orthopedic, general, and thoracic surgery scheduled to last 
90 min. Preoperative exclusion criteria were emergency surgery, intensive care unit admission postoperatively, use of calcium channel blockers, head injury, core temperature 38°C or <35.5°C, otitis, and history of malignant hyperthermia.
Anesthesia was induced with thiopental 3–5 mg/kg, midazolam 1–2 mg, and fentanyl 1–3 µg/kg. Anesthesia was maintained with isoflurane, supplemented with fentanyl and N2O. All patients received nondepolarizing neuromuscular relaxants, and anesthetic gases were delivered via a tracheal tube using a circle system, heat and moisture exchanger, and CO2 soda lime absorber. Ventilation was controlled to maintain normocarbia (PETCO2 30–35 mm Hg), by using a fresh gas flow of 2 L/min. The ambient room temperature was set at 21°C, and unwarmed IV fluids were infused as clinically indicated to maintain normovolemia. After surgery, neuromuscular blockade was reversed, and the trachea was extubated in the OR. All patients were covered with warm blankets after surgery before leaving the OR. During recovery from anesthesia, the patients breathed oxygen 2–4 L/min via nasal cannula.
Core temperature was measured perioperatively using tympanic membrane thermocouples. The tym- panic membrane thermocouple (Mon-a-therm® probes; Mallinkrodt Medical, Inc., St Louis, MO; Catalogue No. 503-0013 and 503-0102) was placed in close proximity to the tympanic membrane, and the external auditory meatus was covered with gauze and a piece of tape. The temperature probes were attached to an electronic readout unit (Mon-a-therm® Model 6510; Mallinkrodt Medical, Inc.). Patients with a core temperature <36°C on admission to the PACU were randomized into two groups using a table of random numbers: treatment and controls.
Patients in the treatment group were rewarmed using the ThermostatTM device, which was operated in accordance with the manufacturer’s instructions. The device consisted of a warming mitt and a thermal exchange chamber (Figures 1 and 2). The warming mitt had a neoprene collar that sealed around the forearm at least 3 in. distal to the elbow. A glycerol solution was applied to the patient’s forearm under the neoprene collar to enhance the seal. The warming mitt contained a supersaturated solution of sodium acetate that became warm on initiation of exothermic crystallization. The thermal exchange chamber enveloped and sealed to the warming mitt. A manual pump was attached to the chamber for the operator to create and maintain a vacuum of -40 mm Hg. After the crystallization reaction was activated, it was applied to one of the patient’s forearms such that it covered the fingers, hand, and lower forearm. The thermal exchange chamber was then attached to the mitt and pumped until a green light illuminated, indicating the desired vacuum level was reached.
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Figure 1. A, Before applying the mitt, a black disk was depressed (arrow) to initiate exothermic crystallization. B, The mitt was then applied to the hand and forearm and the neoprene collar (arrow) sealed around the forearm.
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Figure 2. Commercially available negative pressure rewarming device (ThermostatTM; Aquarius Medical Corp., Phoenix, AZ) showing the seal (1), exchange chamber (2), and manual vacuum pump. A green light (arrow) illuminated to indicate that a vacuum of -40 mm Hg had been achieved.
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ThermostatTM rewarming was initiated within 10 min after the patient had been admitted to the PACU, after the PACU nurse had completed the initial assessment. The device was removed after 60 min of rewarming, per the manufacturer’s instructions.
Patients in the control group were rewarmed according to the standing PACU protocol, which consisted of warm blankets if the temperature was 35.5°C and warm blankets and a radiant heat lamp if temperature was <35.5°C.
Postoperative temperature and vital signs were measured on admission to the PACU, every 5 min for the first 30 min after initiation of rewarming, and every 15 min thereafter until discharge from the PACU. Presence or absence of shivering and time to discharge were recorded by the PACU nurse who was not blinded as to treatment group Standardized clinical discharge criteria were used to determine recovery room stay. After removal of the ThermostatTM device, the fingers, hand, and forearm were examined for redness, swelling, or blisters. Patients were contacted the following day regarding any problems with the assigned rewarming technique.
Parametric data were compared between groups with Student’s t-tests. Nonparametric data were compared between groups with Fischer’s exact test. A P value < 0.05 was considered significant.
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Results
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Of the 191 patients enrolled, 60 (31%) developed hypothermia and were randomized to the ThermostatTM (n = 30) or conventional group (n = 30). There were no significant differences between the two groups with respect to age, gender, weight, height, type of surgery, duration of anesthesia, and fluid requirements (Table 1). Two patients, both in the ThermostatTM group, did not complete the study, one because of an emergency return to the OR for bleeding, and one because of an inability to maintain the vacuum in a patient with implanted prostate seeds. Data from these patients are reported up to and including PACU admission. Sixteen patients (53%) in the control group received radiant heat in addition to warm blankets.
Perioperative temperatures were similar between groups (Table 1, Figure 3). Fourteen patients (50%) in the ThermostatTM group and 17 patients (57%) in the conventional group rewarmed to 36°C before discharge from the PACU (Table 2). The rate of rewarming during the first hour was 0.7° ± 0.4°C in the ThermostatTM group, and 0.6° ± 0.5°C in the control group (P = NS). There were no differences in vital signs, incidence of shivering, rewarming time, or time to discharge from the PACU between groups (Tables 2 and 3).
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Figure 3. Postoperative core temperature (mean ± SD) in control and ThermostatTM (Aquarius Medical Corp., Phoenix, AZ) patients. Admit = initial temperature on admission to postanesthesia care unit. Time refers to time after postanesthesia care unit admission (Control) or after application of the ThermostatTM device. There were no significant differences between groups.
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Problems with the ThermostatTM device included patient discomfort, requiring removal of the device (two patients), increased diastolic blood pressure when the blood pressure cuff was applied to the same arm as the ThermostatTM device (two patients), and difficulty maintaining vacuum seal (seven patients). No burns, erythema, or blisters were noted or reported.
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Discussion
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The present study showed that there was no advantage of applying the commercially available ThermostatTM device to hypothermic postoperative patients recovering from general anesthesia. Rewarming rates were approximately 0.8°C over 90–120 minutes in both groups, and a similar number of patients in both groups failed to rewarm to 36°C during their PACU stay. Rewarming rates in our study were similar to those reported by Ereth et al. (15) in 12 hypothermic total hip arthroplasty patients after isoflurane anesthesia. In their study, rewarming rates were from approximately 34.9° to 35.4°C over 90 minutes in the warm blanket group and from approximately 34.8° to 35.7°C over 90 minutes in the convective warming group (15). Similarly, Lennon et al. (13) showed that hypothermic postsurgical patients receiving warmed cotton blankets rewarmed approximately 0.7°C over 90 minutes in the PACU, although those patients treated with convective warming rewarmed at a much greater rate (approximately 1.7°C/90 minutes).
Core temperature on admission to the PACU is an important factor affecting length of PACU stay (17). Hypothermic (34.8°C) postsurgical adults undergoing elective major abdominal surgery with isoflurane required approximately 94 minutes to reach fitness for discharge compared with 53 minutes in normothermic (36.7°C) patients (10). Actual discharge times, however, were not reported in the study (10). In our study, PACU admission temperature was 35.1°-35.2°C in the two groups, and the average stay in the PACU was nearly identical in the groups, approximately two hours. This length of stay is similar to that reported by Hines et al. (17) in patients with a PACU admission temperature between 35° and 36°C (129 ± 60 minutes). The power of detecting a 30-minute difference in discharge time between groups was 0.7 at an 
of 0.05.
It has previously been shown that the application of subatmospheric pressure enhanced heat transfer and sped recovery from anesthesia-induced hypothermia (16). The maximal rate of rewarming with this technique was as high as 13.6°C/hour, although examination of the rewarming curves indicated that increases in core temperature were complete within 15 minutes of application of negative pressure and heat (16). Given the number of patients and scatter of the temperature measurements in our study, the chance of missing a clinically significant difference in temperature between groups (e.g., 0.5°C) at 30 and 60 minutes was very low (power = 0.97 and 0.98 at an of 0.05).
One possible reason for the failure of the present study to demonstrate enhanced rewarming using the ThermostatTM device is the use of a sodium acetate chemical heat source. In the initial study, the heat source was a water-perfusion blanket connected to a circulating water bath (16). The temperature of the water bath was adjusted according to a constant digital display in order to maintain a temperature of 44°-46°C in the water perfusion blanket. The chemical heat source in our study may have been inadequate to transfer enough heat through the heat exchange vasculature compared with a constant water bath heat source. As well, the temperature of the chemical heat source may have dropped below a critical temperature necessary to ensure rapid heat exchange between the skin surface and the body core. Currently, the optimal temperature at which the heat source needs to be maintained is unknown. Another reason for possible failure of the ThermostatTM device in our study was an inadequate seal. Seven patients, particularly those with small, hairy arms required almost constant pumping to maintain adequate vacuum. Analysis of the data in the ThermostatTM group excluding those patients with difficulty maintaining vacuum seal, however, did not show any improved rates of rewarming.
In summary, the ThermostatTM device did not accelerate rewarming in postsurgical adults with mild hypothermia.
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Acknowledgments
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Supported by the MetroHealth Foundation, Chester Summer Scholar Program.
The authors are grateful to the Post-Anesthesia Care Unit Staff at MetroHealth Medical Center for their support, and to Dennis Grahn, PhD, for his useful comments. The authors also acknowledge the insightful comments and suggestions of Ron Isaac, MD, whose recent death has been a great loss to us all.
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Footnotes
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Hypothermia warming mitts/seals and thermal exchange chambers provided by Aquarius Medical Corp., Phoenix, AZ. Tympanic membrane thermocouples provided by Mallinckrodt Medical, Inc., St. Louis, MO.
Presented in part at the 12th Annual Trauma Anesthesia and Critical Care Symposium, Chicago, IL, May 13–15, 1999, and at the annual meeting of the American Society of Anesthesiologist, Dallas, TX, October 11, 1999.
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Accepted for publication August 13, 1999.
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Abstract
Introduction
