The rectilinear pressure-flow relationship in the pulmonary vasculature: zones 2 and 3.

To elucidate the significance of the rectilinear pressure-flow relationship of the pulmonary vasculature under zone 2 and 3 conditions, isolated left lower lobes of dog lungs were perfused at different flow rates under the two zone conditions. The lobar arterial pressure (Pa) was measured directly, whereas the pressures in the peripheral end of the arteries (Pa') and veins (Pv') were measured with the arterial and venous occlusion (AVO). We thus obtained three pressure-flow relationships: Pa-Q, Pa'-Q and Pv'-Q. From these relationships the contribution of each of the three segments (defined by the arterial and venous occlusion) to the slope and intercept of the Pa-Q relationship was determined. To a first approximation, in zone 3 the ohmic resistance was mainly in the arterial and venous segments, whereas the Starling resistance, although small, was equal in the three segments. In zone 2 (Palv = 15 mm Hg) the Starling resistance increased markedly in the middle segment. Because zone 3 and 2 conditions can co-exist within the lung, a simple three-segment model to represent the pulmonary vasculature was inadequate to explain all the pressure-flow data and therefore was modified to include two parallel channels one of which contains a critical closing pressure (Pc). Such a model implies that two different driving pressures determine the total flow: in vascular channels where Pc greater than Pv (outflow venous pressure) flow is determined by Pa-Pc gradient, and in channels were Pc less than Pv flow would be determined by the Pa-Pv gradient.(ABSTRACT TRUNCATED AT 250 WORDS)