Energy balance in the intubated human airway is an indicator of optimal gas conditioning.
OBJECTIVE
The optimal level of inspired heat and humidity for patients receiving long-term mechanical ventilation is still the subject of debate. Many laboratory studies examining surrogate markers for optimal humidity suggest that inspired gas should be at body temperature and fully saturated. The aim of this study was to determine the inspired gas condition that was thermodynamically neutral to the airway of intubated patients, and also examine the contribution of the endotracheal tube to airway heat and water balance.


METHODS
Prospective, block-randomized, observational study.


METHODS
General adult intensive care unit of a metropolitan teaching hospital.


METHODS
Ten adult patients requiring intermittent positive pressure ventilation for nonpulmonary reasons.


METHODS
Each patient was given four different gas conditions--30 degrees C, 30 mg/L; 34 degrees C, 38 mg/L; 37 degrees C, 44 mg/L; and 40 degrees C, 50 mg/L--to breathe in random order.


RESULTS
Inspired and expired gas temperature and humidity, and the temperature gradient down the endotracheal tube, were measured and the inspired gas condition that gave thermodynamic neutrality was determined. This was found to be gas at body temperature, saturated. Airway workload and airway water loss increased linearly as the inspired gas departed from this condition, at approximately 1.4 kJ/hr/ degrees C and 0.5 mL/hr/ degrees C, respectively. The endotracheal tube contributed very little to heat and water exchange.


CONCLUSIONS
Inspired gas at body temperature and saturated is thermodynamically neutral to the intubated airway, and thus may be considered the optimal condition for ventilation lasting more than a few hours.
Ryan
Stuart N
SN
Fisher & Paykel Healthcare, Panmure, Auckland, New Zealand.
Rankin
Nigel
N
Meyer
Erwin
E
Williams
Robin
R
D000293:Adolescent; D000328:Adult; D000368:Aged; D000704:Analysis of Variance; D005260:Female; D006801:Humans; D006813:Humidity; D007385:Intermittent Positive-Pressure Ventilation / Q000379:methods*; D007442:Intubation, Intratracheal / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D015656:Respiratory Mechanics; D013696:Temperature; D013816:Thermodynamics