['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.', 'Prospective, block-randomized, observational study.', 'General adult intensive care unit of a metropolitan teaching hospital.', 'Ten adult patients requiring intermittent positive pressure ventilation for nonpulmonary reasons.', '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.', '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.'] Is energy balance in the intubated human airway an indicator of optimal gas conditioning?