Subpleural pulmonary microvascular pressures in the dog lung.

The reported values for the pressure difference between lobar artery and subpleural arteriole and between subpleural venule and lobar vein as a fraction of the total arterial-to-venous pressure drop across the dog lung have varied considerably. We carried out the present study to provide an additional set of measurements and to determine whether it is likely that differences in venous pressure or transpulmonary pressure between studies might make a substantial contribution to variations between studies. We measured the lobar arterial pressure (Pa) to subpleural arteriole (22-60 microns diam) pressure (Pma) to subpleural venule (30-80 microns diam) pressure (Pmv) to lobar venous pressure (Pv) distribution over a range of alveolar pressures (PA; 2.5-13.1 mmHg) and venous pressures (0-24.2 mmHg) in isolated dog lung lobes using the micropuncture servo-null technique. On average, near functional residual capacity (PA = 3 mmHg) and venous pressure equal to PA, (Pa-Pma)/(Pma-Pmv)/(Pmv-Pv) was 37:30:33%. Under zone 3 conditions, there was a small positive correlation between the fractional Pa-to-Pma pressure difference and PA and Pv, but dependence of the Pmv-to-Pv fraction on PA and Pv was not consistent. The overall effects of PA and Pv on the fractional pressure drops were not sufficient to account for differences between previous studies. Under zone 2 conditions as the venous pressure was varied, the changes in Pmv were nearly equal to the changes in Pv, whereas Pma was relatively insensitive to Pv. Thus, the zone 2 results were consistent with a capillary location for the flow-limiting segment under zone 2 conditions.