EFFICIENCY OF WARMING DEVICES IN EXTUBATE D POSTOPERATIVE-PATIENTS - COMPARISON OF RADIANT AND CONVECTION HEATING-SYSTEMS

Hypothermia (T(core) < 36-degrees-C) can be observed in 60%-80% of all admissions to the post-anaesthetic recovery unit. Effective warming d evices may accelerate rewarming, improve patient comfort, and suppress shivering thermogenesis. This study was designed to compare the effic iency 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 t emperatures < 36-degrees-C were randomly assigned to either postoperat ive nursing under a radiant heater (group R, n = 11, Aragona Thermal C eilings CTC X, Aragona Medical AB, Taby, Sweden), a forced air system (group L, n = 12, Bair Hugger, Augustine Medical Inc., Eden Prairie, M innesota, USA), or a normal cotton hospital blanket (group K, n = 12). Anaesthesia was conducted totally intravenously with propofol, alfent anil, and vecuronium. Mean body temperature and total body heat were c alculated from urinary bladder temperature and four subcutaneous tempe rature measurements. The rate of thermogenesis was calculated from con tinuous measurement of VO2 (Datex Deltatrac Metabolic Monitor, Datex I nstrumentarium Corp., Helsinki, Finland). Heat balance was derived fro m the increase in total body heat minus body heat production. Heart ra te and noninvasive blood pressure were measured by the Cardiocap (Date x Instrumentarium Corp., Helsinki, Finland). All data were transferred to an IBM-compatible computer at 60-s intervals. Measurements were st opped when core temperature reached 37-degrees-C. The rate of change w as calculated for each variable for the period 15 min after the beginn ing 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 g roups were comparable for body weight, height, age, and amount of post operative infusions. Temperatures at admission were 35.2 (33.4<-- -->3 5.9), 34.7 (34.3<-- -->35.8), and 35.4 (34.3<-- --> 35.9)-degrees-C fo r 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)deg rees-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 significa nt 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 q uotient, 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 durin g the period of investigation. Conclusions. Neither external heat supp ly by radiant heat nor by a forced warm air system significantly reduc ed rewarming time in extubated, awake patients. As measured by heat ba lance, both active treatments saved about 20% more body heat productio n than in the control group. Continuing peripheral vasoconstriction ma y be the reason for the low efficiency of heat transfer. Thermal treat ment did reduce the peak load (max. VO2) on the oxygen transport syste ms, though shivering was treated by pethidine if it occurred. External rewarming did not reduce the average load (mean VO2). Thus, concernin g the goal of accelerating rewarming, it appears more rational to prev ent intraoperative heat loss. For a comparison of efficiency of differ ent warming devices, postoperative extubated patients do not appear to be an ideal model for study.