Effect of increase of pulmonary blood flow on segmental pulmonary vascular resistance--with special emphasis on the pulmonary capillary bed

Abstract

The decrease of pulmonary vascular resistance with increase of pulmonary blood flow has been attributed to the function of the pulmonary vascular bed, consisting of capillary recruitment or dilatation. However, experimental evidence of a decrease of capillary resistance is lacking. To clarify this phenomenon, we perfused isolated cat lungs with diluted autologous blood and measured pulmonary microvascular pressure while changing pulmonary blood flow rate. We set the initial flow rate to achieve Ppa of 20 cmH2O and changed the flow rate while keeping left atrial pressure (Pla) and alveolar pressure (Palv) constant at 10 cmH2O and 9 cmH2O, respectively. Thereafter, pulmonary flow rate was increased 2 to 3 fold, and pulmonary arterial pressure (Ppa) and venular pressure (Pvo) were measured by venous occlusion method. The initial flow rate was 150 to 270 ml/min and the achieved maximum flow rate was from 300 to 543 ml/min. There was a linear correlation between Ppa, Pvo, and flow rate, in which the slope (rate of increase of pressure with flow) of Ppa was slightly steeper than that of Pvo. We calculated total and segmental pulmonary vascular resistance from the mean pressure-flow relationship. The total and upstream resistance involving arterial and capillary segments decreased hyperbolically with increase in flow rate. The mean upstream resistance decreased about 54%, from 35.20 x 10(-3) (cmH2O/ml/min) at a flow rate of 150 ml/min to 16.27 x 10(-3) (cmH2O/ml/min) at a flow rate of 450 ml/min. The degree of this decrease was similar to the result observed by micropuncture method (Nagasaka, 1990). Downstream resistance, which represents venous resistance, did not change significantly with the flow rate.(ABSTRACT TRUNCATED AT 250 WORDS)