[The effect of convection warming during abdominal surgery on the early postoperative heat balance]

Abstract

Hypothermia (core temperature < 36 degrees C) is common after longer-lasting surgical procedures. Heat loss mainly occurs during anaesthesia and surgery and leads to increased risk, especially in the early recovery period of elderly patients. In the present study we investigated the effects of intraoperative forced-air warming, administered via an upper-body blanket ("Warm Touch", Mallinckrodt, USA), with the specific aims of: (1) drawing up heat balances; and (2) analysing postoperative thermoregulation, oxygen consumption (VO2) and cardiovascular reactions of mechanically ventilated patients. The general aim of our study was to compare intraoperative forced-air-warming and conventional patient-insulation with cotton blankets.

Methods: Twenty four ASA II and III patients scheduled for elective colon surgery were randomly assigned to a control group (n = 12, no warming therapy, upper body covered with a cotton hospital blanket) or a convective warming group (n = 12). Anaesthesia was administered with etomidate (0.2 mg/kg), fentanyl (approximately 10 micrograms/kg) and vecuronium bromide (0.1 mg/kg). During surgery the lungs were mechanically ventilated with 70% nitrous oxide in oxygen and enflurane (end-tidal-concentration max. 0.7%) using a semiclosed circuit with a fresh gas flow of 3 l/min. A hygrophobe heat and moisture exchanger ("Sterivent," Darex Corp., Italy) was used. At the end of surgery patients were transferred to the ICU, covered with a hospital cotton-quilt and normo-ventilated using a Bennett 7200 a. Patients were sedated/kept free of pain by administering titrated doses of midazolam and/or piritramide. Postoperative oxygen consumption (VO2) was recorded continuously with a Deltatrac Metabolic Monitor (Datex Corp., Finland). Pre-, intra- and postoperative measurements included heart rate, invasive blood pressure, core-temperature (before and after operation: urinary bladder-temperature, during surgery: oesophageal temperature) and mean-skin-temperature (according to Ramanathan) up to 180 min from the end of surgery. Shivering, pharmacological interventions (e.g. pethidine) and time of extubation were noted. Data are presented as median, minima and maxima. The results were analysed using the Mann-Whitney U test or Chi-Square test (shivering). Statistical significance was assumed when P < 0.05.

Results: Both groups were comparable for gender, body weight, height, age, duration of their operations and amount of intraoperative fluids, narcotics and muscle relaxants. Room temperatures in the control group were significantly higher than in the forced air group (24 vs 22 degrees C). Initial setting of the forced-air blower was "high" (42-46 degrees high air flow). When the oesophageal-temperature reached 36.5 degrees C, the blower temperature was reduced to 36-40 degrees C. Reduction was necessary approximately 60 min from start in the operation. At the end of surgery/administration to the ICU core-temperatures of both groups differed significantly (35.2/ 35.4 degrees C vs 36.3/36.2 degrees C). Mean-skin temperatures were higher, too, but no statistical analysis was carried out for the intraoperative period, because warm air influenced skin thermometers located on the upper body. At admission to the ICU patients in the control group had a heat loss of 4.4 kJ/kg; those in the convective warming group had a heat-gain of 0.8 kJ/kg. Further measurements of postoperative core temperatures did not differ significantly, but the skin-temperatures of patients who received forced-air warming in the theatre remained higher (P < 0.05) until 120 min from the end of surgery. Shivering was more frequent and lasted longer in the control group (8 patients, 20 min vs 4 patients, 9 min; P < 0.05). Patients in the control group needed more drugs to stop increased cardiovascular reactions (hypertension, tachycardia) or shivering.(ABSTRACT TRUNCATED)