Abstract: | Treppo, Steven, Srboljub M. Mijailovich, and José G. Venegas. Contributions of pulmonary perfusion and ventilation toheterogeneity in A/ measured by PET. J. Appl. Physiol. 82(4): 1163-1176, 1997. To estimate the contributions of the heterogeneity in regionalperfusion ( ) and alveolar ventilation( A) to that of ventilation-perfusionratio ( A/ ), we haverefined positron emission tomography (PET) techniques to image localdistributions of and A per unit of gas volume content(s and s A,respectively) and VA/ indogs. s A was assessed in two ways:1) the washout of 13NN tracer after equilibrationby rebreathing (s Ai), and2) the ratio of an apneic image after a bolus intravenousinfusion of 13NN-saline solution to an image collectedduring a steady-state intravenous infusion of the same solution(s Ap).s Ap was systematically higher than s Ai in allanimals, and there was a high spatial correlation betweens ands Ap in both body positions(mean correlation was 0.69 prone and 0.81 supine) suggesting thatventilation to well-perfused units was higher than to those poorlyperfused. In the prone position, the spatial distributions ofs , s Ap, and A/ were fairlyuniform with no significant gravitational gradients; however, in thesupine 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 thecorresponding coefficient of variation (CV)2 values. Weconclude that, in the prone position, gravitational forces in blood andlung tissues are largely balanced out by dorsoventral differences inlung structure. In the supine position, effects of gravity andstructure become additive, resulting in substantial gravitationalgradients in s ands Ap, with the higherheterogeneity in A/ caused by agravitational gradient in s , only partially compensated by that in s A. |