Gas Exchange in the Normal Lung : Experimental studies on the effects of positive end-expiratory pressure and body position

Abstract

BackgroundLocal formation of nitric oxide (NO) in the lung in proportion to ventilation, leading to vasodilation, is a putative mechanism behind ventilation- perfusion matching. We examined the role of local constitutive NO formation on regional distributions of ventilation (V) and perfusion (Q) and ventilation-perfusion matching (V/Q) in mechanically ventilated adult sheep with normal gas exchange.MethodsV and Q were analyzed in lung regions (≈1.5 cm3) before and after inhibition of constitutive nitric oxide synthase (cNOS) with Nω-nitro-L-arginine methyl ester (L-NAME) (25 mg/kg) in seven prone sheep ventilated with PEEP. V and Q were measured using aerosolized fluorescent and infused radiolabeled microspheres, respectively. The animals were exsanguinated while deeply anaesthetized; lungs were excised, dried at total lung capacity and divided into cube units. The spatial location for each cube was tracked and fluorescence and radioactivity per unit weight determined.ResultsPulmonary artery pressure increased significantly after L-NAME (from mean 16.6 to 23.6 mmHg, P<0.01) while there were no significant changes in PaO2, PaCO2 or SD log(V/Q). Distribution of V was not influenced by L-NAME but a small redistribution of Q from ventral to dorsal lung regions resulting in less heterogeneity in Q along the gravitational axis was seen (p<0.01). Perfusion to regions with the highest ventilation (5th quintile of the V distribution) remained unchanged with L-NAME.ConclusionsThere was minimal or no influence of cNOS inhibition by L-NAME on the distributions of V and Q, and V/Q in prone anesthetized and ventilated adult sheep with normal gas exchange.