Measurement of tracheal temperature is not a reliable index of total respiratory heat loss in mechanically ventilated patients

BACKGROUND Minimizing total respiratory heat loss is an important goal during mechanical ventilation. The aim of the present study was to evaluate whether changes in tracheal temperature (a clinical parameter that is easy to measure) are reliable indices of total respiratory heat loss in mechanically ventilated patients. METHOD Total respiratory heat loss was measured, with three different methods of inspired gas conditioning, in 10 sedated patients. The study was randomized and of a crossover design. Each patient was ventilated for three consecutive 24-h periods with a heated humidifier (HH), a hydrophobic heat-moisture exchanger (HME) and a hygroscopic HME. Total respiratory heat loss and tracheal temperature were simultaneously obtained in each patient. Measurements were obtained during each 24-h study period after 45 min, and 6 and 24 h. RESULTS Total respiratory heat loss varied from 51 to 52 cal/min with the HH, from 100 to 108 cal/min with the hydrophobic HME, and from 92 to 102 cal/min with the hygroscopic HME (P < 0.01). Simultaneous measurements of maximal tracheal temperatures revealed no significant differences between the HH (35.7-35.9 degrees C) and either HME (hydrophobic 35.3-35.4 degrees C, hygroscopic 36.2-36.3 degrees C). CONCLUSION In intensive care unit (ICU) mechanically ventilated patients, total respiratory heat loss was twice as much with either hydrophobic or hydroscopic HME than with the HH. This suggests that a much greater amount of heat was extracted from the respiratory tract by the HMEs than by the HH. Tracheal temperature, although simple to measure in ICU patients, does not appear to be a reliable estimate of total respiratory heat loss.