Although vasomotor activity in small pulmonary vessels has been studied extensively in the past, using the concept of resistance to flow, information on the distensibility of these vessels is very sparse. In an attempt to reduce this deficit, we adapted a theoretical method developed for small systemic vessels, to estimate distensibility of pulmonary resistance vessels in experimental animals and man. Pressure-flow data from 11 dogs and 10 human subjects (5 control subjects and 5 patients with long-standing left heart failure) were used to calculate distensibility of small pulmonary vessels. The conductance, G, was calculated from these data as the ratio of blood flow to driving pressure. The slope of the relationship between the logarithm of G1/4 and the average distending pressure (ADP) provides a graphic picture of circumferential extensibility, E, defined as percent change in radius for an infinitesimal change in ADP. Results indicate that: (1) the value of E in dogs was 1.85 ± .40 mmHg−1 for the control state, which decreased to 1.45 ± .43 mmHg−1 during norepinephrine administration; however, the decrease in the value was not statistically significant (p = 0.53); (2) the value of E in control human subjects was 3.38 ± .47 mmHg−1 and the value of E in patients with left heart failure was −0.64 ± 0.39 mmHg−1; the difference was significant (P = .0001). Moreover, at a given ADP in the range of overlapping pressures, the “average” radius of small pulmonary vessels in patients with left heart failure was smaller than that in the control subjects; and (3) small pulmonary vessels were more distensible than both the small systemic vessels and the large pulmonary arteries.

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