Airway thermal volume in humans and its relation to body size.

The objective of this study was to investigate the influence of volume ventilation (VE) and cardiac output (Q) on the temperature of the expired gas at the distal end of the endotracheal tube in anesthetized humans. In 63 mechanically ventilated adults, we used a step decrease in the humidity of inspired gas to cool the lungs. After change from humid to dry gas ventilation, the temperature of the expired gas decreased. We evaluated the relationship between the inverse monoexponential time constant of the temperature fall (1/tau) and either VE or Q. When VE was increased from 5.67 +/- 1.28 to 7.14 +/- 1.60 (SD) l/min (P = 0. 02), 1/tau did not change significantly [from 1.25 +/- 0.38 to 1.21 +/- 0.51 min-1, P = 0.81]. In the 11 patients in whom Q changed during the study period (from 5.07 +/- 1.81 to 7.38 +/- 2.45 l/min, P = 0.02), 1/tau increased correspondingly from 0.89 +/- 0.22 to 1. 52 +/- 0.44 min-1 (P = 0.003). We calculated the airway thermal volume (ATV) as the ratio of the measured values Q to 1/tau and related it to the body height (BH): ATV (liters) = 0.086 BH (cm) - 9. 55 (r = 0.90).