Effects of minute ventilation and cardiac output on lung heat exchange in intubated humans

We investigated the influence of volume ventilation (Ve) and cardiac output (Q) on lung heat exchange in intubated humans In 50 anesthetized, ventilated humans we used a step decrease in the humidity of inspired gas. The temperature of the expired gas at the distal end of the endotracheal tube decreased monoexponentially We evaluated the relationship between the inverse time constant of the temperature fall (1/τ) and either Ve or Q. In 8 subjects Ve was changed during the study and in 11 subjects Q varied. When Ve was increased from (MEAN±SD) 5.67 ± 1.28 L/min to 7.14 ± 1.60 L/min (p=0.02), 1/τ did not change significantly (initial 1.25 ± 0.38 min^-1, final 1.21 ± 0.51 min^-1) When Q changed from 5.07 ± 1.81 L/min to 7.38 ± 2.45 L/min (p = 0.02), 1/τ increased correspondingly from 0.89 ± 0.22 min^-1 to 1.52 ± 0.44 min^-1 (p= 0.003). When all 50 patients were combined, 1/τ was linearly proportional to cardiac output (r = 0.89). The relationship between 1/τ and Ve was weak (r=0.64). Airway Thermal Volume (ATV) was defined as the ratio of total lung heat capacity to the coefficient of thermal conductivity The ATV can be estimated by the ratio of Q and 1/τ Estimated ATV related to the Body Height (BH) was ATV(L)=0.086 BH(cm)-9.55.(r = 0.90.