COMPARATIVE EFFECTS OF A HYGROPHOBIC FILTER AND A HEATED HUMIDIFIER ON INTRAOPERATIVE HYPOTHERMIA

A study was carried out to find out whether the use of a hygrophobic filter (Pall, Ultipor(R)) or of a heated humidifier (Drager, Aquapor(R)) during surgery had any effect on a patient's intraoperative core temperature and thermal balance. Seventy-five ASA I or II patients scheduled for gynaecological surgery were randomly assigned to three groups: group A (n = 25), where no warming device was used; and two groups (n = 25 for each) where inhaled gases were humidified and heated with either a hygrophobic filter set up between the endotracheal tube and the Y-piece (group B) or a heated humidifier set to 100 % saturation at a temperature of 41.5-degrees-C (group C). The patients were all anaesthetised with the same technique (thiopentone 5 mg . kg-1, dextromoramide 0.03 mg . kg-1 and 0.1 mg . kg-1 of either pancuronium or vecuronium, followed by enflurane with nitrous oxide in oxygen); the perfused fluids were not heated. Room, tympanic, rectal, oesophageal and four skin (thorax, arm, leg, thigh) temperatures were measured with calibrated Exacon(R) thermistances, on arrival in the operating theatre, during induction, every ten minutes for two hours, and then every twenty minutes for two hours more. Ramanathan's and Burton's formulae were used to calculate mean skin temperature and heat loss respectively. In the recovery room, patients were warmed up with an electric blanket. Shivering was ranked from << 0 >> to << + + >>. There were no differences between groups as far as age, drug doses, perfusion volumes and room temperature were concerned. The temperature time course was similar in all groups, with normothermia on arrival in theatre, followed by a statistically significant decrease in tympanic and oesophageal temperatures during induction, and thereafter. Rectal temperatures declined more slowly. The drop in oesophageal temperature was well fitted by an exponential model (T(o)) = Ae(-at) + Be(-bt)). There were no differences between groups, except that coefficient b in group B was lower than that in group A. Heat loss and shivering was similar in the three groups. The passive and active heat exchangers used did not prevent intraoperative hypothermia, probably because respiratory heat loss accounts for less than 30 % of the total body heat loss. Nonetheless, as the second part of the slope in group B was significantly reduced, the use of passive heat exchangers would seem to be advantageous for long procedures. However, other warming devices must be used to really prevent intraoperative hypothermia.