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 expi red gas at the distal end of the endotracheal tube in anesthetized hum ans. In 63 mechanically ventilated adults, we used a step decrease in the humidity of inspired gas to cool the lungs. After change from humi d to dry gas ventilation, the temperature of the expired gas decreased . We evaluated the relationship between the inverse monoexponential ti me constant of the temperature fall (1/tau) and either VE or Q. When V E 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 bod y height (BH): ATV(liters) = 0.086 BH (cm) -9.55 (r = 0.90).