Contributions of pulmonary perfusion and ventilation to heterogeneity in VA/Q measured by PET

To estimate the contributions of the heterogeneity in regional perfusion ((Q) over dot) and alveolar ventilation ((V) over dot A) to that of ventilation-perfusion ratio ((V) over dot A/(Q) over dot), we have refined positron emission tomography (PET) techniques to image local distributions of (Q) over dot and VA per unit of gas volume content (s(Q) over dot and s(V) over dot A, respectively) and VA/(Q) over dot in dogs. s(V) over dot A was assessed in two ways: 1) the washout of (NN)-N-13 tracer after equilibration by rebreathing (s(V) over dot A(i)), and 2) the ratio of an apneic image after a bolus intravenous infusion of (NN)-N-13-saline solution to an image collected during a steady-state intravenous infusion of the same solution (s(V) over dot A(p)). s(V) over dot A(p) was systematically higher than s(V) over dot A(i) in all animals, and there was a high spatial correlation between s(Q) over dot and s(V) over dot A(p) in both body positions (mean correlation was 0.69 prone and 0.81 supine) suggesting that ventilation to well-perfused units was higher than to those poorly perfused. In the prone position, the spatial distributions of s(Q) over dot, s(V) over dot A(p), and (V) over dot A/(Q) over dot were fairly uniform with no significant gravitational gradients; however, in the supine position, these variables were significantly more heterogeneous, mostly because of significant gravitational gradients (15, 5.5, and -10%/cm, respectively) accounting for 73, 33, and 66% of the corresponding coefficient of variation (CV)(2) values. We conclude that, in the prone position, gravitational forces in blood and lung tissues are largely balanced out by dorsoventral differences in lung structure. In the supine position, effects of gravity and structure become additive, resulting in substantial gravitational gradients in s(Q) over dot and s(V) over dot A(p), with the higher heterogeneity in (V) over dot A/(Q) over dot caused by a gravitational gradient in s(Q) over dot, only partially compensated by that in s(V) over dot A.