[Postoperative warming therapy in the recovery room. A comparison of radiative and convective warmers]

Hypothermia (Tcore < 36 degrees C) can be observed in 60%-80% of all admissions to the post-anaesthetic recovery unit. Effective warming devices may accelerate rewarming, improve patient comfort, and suppress shivering thermogenesis. This study was designed to compare the efficiency of warming devices in extubated postoperative patients and their effect on postoperative oxygen uptake (VO2). METHODS. Thirty-five ASA I and II patients after laparoscopic hernioplastic repair with core temperatures < 36 degrees C were randomly assigned to either postoperative nursing under a radiant heater (group R, n = 11, Aragona Thermal Ceilings CTC X, Aragona Medical AB, Täby, Sweden), a forced air system (group L, n = 12, Bair Hugger, Augustine Medical Inc., Eden Prairie, Minnesota, USA), or a normal cotton hospital blanket (group K, n = 12). Anaesthesia was conducted totally intravenously with propofol, alfentanil, and vecuronium. Mean body temperature and total body heat were calculated from urinary bladder temperature and four subcutaneous temperature measurements. The rate of thermogenesis was calculated from continuous measurement of VO2 (Datex Deltatrac Metabolic Monitor, Datex Instrumentarium Corp., Helsinki, Finland). Heat balance was derived from the increase in total body heat minus body heat production. Heart rate and noninvasive blood pressure were measured by the Cardiocap (Datex Instrumentarium Corp., Helsinki, Finland). All data were transferred to an IBM-compatible computer at 60-s intervals. Measurements were stopped when core temperature reached 37 degrees C. The rate of change was calculated for each variable for the period 15 min after the beginning of rewarming to attainment of 37 degrees C. Data are presented as median, minima, and maxima (min<==>max); the Mann-Whitney U test was used to test for significance of group differences. RESULTS. All groups were comparable for body weight, height, age, and amount of postoperative infusions. Temperatures at admission were 35.2 (33.4<==>35.9), 34.7 (34.3<==>35.8), and 35.4 (34.3<==>35.9) degrees C for groups R, B, and K, respectively. No significant differences in the rate of central rewarming could be found for these groups with 0.81 (0.41<==>1.32), 0.76 (0.40<==>1.07), and 0.70 (0.37<==>1.13) degrees C/h (Fig. 1). The mean VO2 of 3.41 (3.07<==>3.73), 3.55 (2.78<==>4.06), and 3.79 (2.51<==>7.00) ml/kg/min also did not differ significantly (Fig. 3). Significant differences between groups R and B [4.39 (3.74<==>6.19) and 4.30 (3.46<==>6.67) ml/kg/min] and K [5.92 (3.79<==>10.64) ml/kg/min] were found for VO2 maxima during the course of investigation (Fig. 4). The heat balance revealed significant differences among treatment and control groups with -88 (-226<==>+30), -41 (-212<==>+12), and -191 (-265<==>-86) kJ/h for groups R, B, and K. We additionally calculated the heat balance as a quotient, which showed 0.70 (0.22<==>1.07), 0.86 (0.44<==>1.04), and 0.49 (0.31<==>0.79) for groups R, B, and K (Fig. 4). The mean rate-pressure product of all groups did not differ significantly during the period of investigation. CONCLUSIONS. Neither external heat supply by radiant heat nor by a forced warm air system significantly reduced rewarming time in extubated, awake patients. As measured by heat balance, both active treatments saved about 20% more body heat production than in the control group. Continuing peripheral vasoconstriction may be the reason for the low efficiency of heat transfer. Thermal treatment did reduce the peak load (max. VO2) on the oxygen transport systems, though shivering was treated by pethidine if it occurred. External rewarming did not reduce the average load (mean VO2). Thus, concerning the goal of accelerating rewarming, it appears more rational to prevent intraoperative heat loss. For a comparison of efficiency of different warming devices, postoperative extubated patients do not appear to be an ideal model for study.