Segmental vascular resistance and compliance from vascular occlusion

Abstract

When the pulmonary arterial inflow, venous outflow, or both arterial inflow and venous outflow, are rapidly occluded, the subsequent time varying arterial, P_a(t) and venous, P_v(t) pressures contain information regarding the longitudinal (arterial to venous) distribution of pulmonary vascular resistance, R, with respect to vascular compliance, C. The longitudinal distributions of geometric and mechanical properties of the pulmonary vascular bed are such that the longitudinal R and C distribution can be correlated with anatomical divisions (e.g., arteries, capillaries, and veins) to the extent that anatomical sites of changes in R and C can be inferred from the occlusion data. This concept was originally introduced by Hakim et al. [16] who observed that when the venous outflow from a pumpperfused, dog lung lobe was suddenly occluded, while the flow into the lobar artery continued at a constant rate, P_a(t) and P_v(t) followed a characteristic pattern (Fig. 7.1). The venous pressure jumped rapidly to a pressure near midway between the preocclusion arterial and venous pressures and then P_a(t) and P_v(t) increased more slowly with time almost linearly. They interpreted the data using the simple “two R, one C, T-section” electrical analog depicted in Figure 7.2. In this simple model interpretation, the rapid rise in the venous pressure was the result of the elimination of the pressure drop across a resistance, R₂, downstream from a region of high compliance, C_T. Then the difference between P_a(t) and P_v(t) after the rapid jump in P_v(t) would be the preocclusion pressure drop across a resistance, R₁, upstream from the high compliance region, and the slopes of the slowly rising arterial and venous pressures would be the flow rate divided by C_T.