Effect on Pulmonary Vascular Resistance of Inflation of the Rabbit Lungs

Abstract

The old and continuing controversy as to the effect of inflation of the lungs on pulmonary vascular resistance is shown to depend upon erroneous use of ‘effective arterial pressure’ in the calculations and not on conflict of experimental results. Reinterpretation of old data, and new experiments on the rabbit lungs indicate: a) with ‘negative pressure inflation,’ and in normal respiration, the resistance falls as the lung is distended; b) with ‘positive pressure inflation’ the resistance rises from moderate inflation to full inflation; c) however, with positive pressure inflation, between a collapsed and a moderately inflated lung, the resistance falls, i.e. the curve of resistance vs. inflation pressure is U-shaped with a minimum for pressures of 8–15 cm H₂O. A hemodynamic analysis based on ‘transmural pressure’ shows that these results are predicted, except for (c) above. This is thought to be due to ‘kinking’ and to change of shape of the lumen of small vessels as the lung collapses. There is, however, some evidence of participation of a veno-vasomotor reflex. A similar U-shaped curve was found in a rubber model of the lungs. Submitted on September 16, 1957