Mechanics of pulmonary circulation in isolated rabbit lungs

A new method has been devised for measuring the compliance (volume change ÷ pressure change) of the pulmonary arterial tree (down to the capillaries), in isolated, Krebs-Ringer-perfused rabbit lungs. The lungs were suspended in a plethysmograph and inflated with N 2 O. Fluid was introduced into the pulmonary artery by a motor driven syringe and the volume injected was recorded. The capillary flow was deduced from the plethysmograph pressure change which resulted from N 2 O uptake. Vascular volume change equaled arterial inflow minus outflow into capillaries. The compliance of the venous tree was similarly measured by retrograde perfusion. Mean compliance of the pulmonary arterial tree was 0.17 (S.E. 0.02) ml/cm H 2 O in six rabbits, mean weight, 4.4 kg. Mean compliance of the pulmonary venous tree was 0.41 (S.E. 0.03) ml/cm H 2 O in five rabbits, mean weight, 4.3 kg. Compliance of the whole bed (not including the left atrium) was 0.69 (S.E. 0.06) ml/ cm H 2 O in six rabbits, mean weight, 4.4 kg. Equations were derived relating the length, diameter and number of vessels in the vascular bed. From these the inertance of the blood in the pulmonary arterial tree was estimated to be 8 gm cm –4 in rabbit lungs and 1 gm cm –4 in human lungs. The flow response to sinusoidal pressure waves of different frequencies was calculated, assuming a system with lumped compliance, inertance and resistance. The input impedance of the pulmonary vascular bed, in the rabbit and in man, was minimum for a frequency midway between the fundamental frequency and the second harmonic of the resting heart rate.