L. Thomachot et al., Measurement of tracheal temperature is not a reliable index of total respiratory heat loss in mechanically ventilated patients, CRIT CARE, 5(1), 2001, pp. 24-30
Background: Minimizing total respiratory heat loss is an important goal dur
ing mechanical ventilation. The aim of the present study was to evaluate wh
ether changes in tracheal temperature (a clinical parameter that is easy to
measure) are reliable indices of total respiratory heat loss in mechanical
ly ventilated patients.
Method: Total respiratory heat loss was measured, with three different meth
ods of inspired gas conditioning, in 10 sedated patients. The study was ran
domized and of a crossover design. Each patient was ventilated for three co
nsecutive 24-h periods with a heated humidifier (HH), a hydrophobic heat-mo
isture exchanger (HME) and a hygroscopic HME. Total respiratory heat loss a
nd tracheal temperature were simultaneously obtained in each patient. Measu
rements 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 ca
l/min with the hygroscopic HME (P< 0.01). Simultaneous measurements of maxi
mal tracheal temperatures revealed no significant differences between the H
H (35.7-35.9 <degrees>C) and either HME (hydrophobic 35.3-35.4 degreesC, hy
groscopic 36.2-36.3 degreesC).
Conclusion: In intensive care unit (ICU) mechanically ventilated patients,
total respiratory heat loss was twice as much with either hydrophobic or hy
droscopic 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.