CONTRIBUTIONS OF PULMONARY PERFUSION AND VENTILATION TO HETEROGENEITYIN VA Q MEASURED BY PET/

To estimate the contributions of the heterogeneity in regional perfusi on ((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 re fined positron emission tomography (PET) techniques to image local dis tributions 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 do gs. 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)), an d 2) the ratio of an apneic image after a bolus intravenous infusion o f (NN)-N-13-saline solution to an image collected during a steady-stat e 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 al l 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 wa s 0.69 prone and 0.81 supine) suggesting that ventilation to well-perf used units was higher than to those poorly perfused. In the prone posi tion, 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 significan t gravitational gradients; however, in the supine position, these vari ables were significantly more heterogeneous, mostly because of signifi cant gravitational gradients (15, 5.5, and -10%/cm, respectively) acco unting for 73, 33, and 66% of the corresponding coefficient of variati on (CV)(2) values. We conclude that, in the prone position, gravitatio nal forces in blood and lung tissues are largely balanced out by dorso ventral differences in lung structure. In the supine position, effects of gravity and structure become additive, resulting in substantial gr avitational gradients in s(Q) over dot and s(V) over dot A(p), with th e higher heterogeneity in (V) over dot A/(Q) over dot caused by a grav itational gradient in s(Q) over dot, only partially compensated by tha t in s(V) over dot A.