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

Abstract

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°C) and either HME (hydrophobic 35.3-35.4°C, hygroscopic 36.2-36.3°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.

Measurement of Tracheal Humidity and Temperature

“…We therefore conclude that the experimental setup for tracheal temperature and humidity detection used in our study proved to be suitable to be used in clinical trials in tracheotomized patients. Tracheal temperature and humidity obtained in mechanically ventilated patients are also similar to the values presented herein [10, 11]. However, due to the artificial condition of ventilation with our without humidified anesthetic gases in mechanically ventilated patients, values obtained in these patients were not further used for comparison with the results of our study.…”

Influence of passive humidification on respiratory heat loss in tracheotomized patients

“…The observation that humidity rather than temperature of the lower airways was increased after use of an HME 5,7 underlines the special relation of heat and moisture exchange on the mucosal surface of the lower respiratory tract. The total energy content of the tracheal air consists of sensible heat content (ie, the temperature of air) and latent or insensible heat content (ie, the absolute humidity).…”

“…[5][6][7] In a few investigations, mainly changes of airway humidity during and after use of HME have been reported, whereas significant changes of temperature have not been found 7 or have not been reported. 5,6,9 In this study, the significant decrease in W E also reflects a significant change in tracheal humidity during short-time use of an HME.…”

“…Thus, up to 70% of expired heat and humidity may be recovered. 3 Whereas numerous studies on heat and moisture exchange and thermoregulation in the lower airways during anesthesia have been performed, [4][5][6][7] only scanty data on clinical parameters of humidification in patients with tracheotomies is currently available. [8][9][10] This study was, therefore, done to examine the influence of HME on the convective heat exchange (W C ), the evaporative heat exchange (W E ), and the total respiratory heat loss (W T ) of the lower airways in patients after tracheotomy.…”

Respiratory heat and moisture loss is associated with eosinophilic inflammation in asthma

“…This is relevant because in resting conditions, the majority of heat exchange takes place in the upper airway and evaporative heat loss is a major component of total heat loss from the respiratory tract [10]. In intubated patients, measurements of tracheal temperature do not accurately predict total respiratory heat losses, but measurement of absolute humidity appears to be a better predictor [11].…”