Inhibition of Constitutive Nitric Oxide Synthase Does Not Influence Ventilation-Perfusion Matching in Normal Prone Adult Sheep With Mechanical Ventilation.

BACKGROUND: Local formation of nitric oxide in the lung induces vasodilation in proportion to ventilation and is a putative mechanism behind ventilation-perfusion matching. We hypothesized that regional ventilation-perfusion matching occurs in part due to local constitutive nitric oxide formation. METHODS: Ventilation and perfusion were analyzed in lung regions (≈1.5 cm) before and after inhibition of constitutive nitric oxide synthase with N-nitro-L-arginine methyl ester (L-NAME) (25 mg/kg) in 7 prone sheep ventilated with 10 cm H2O positive end-expiratory pressure. Ventilation and perfusion were measured by the use of aerosolized fluorescent and infused radiolabeled microspheres, respectively. The animals were exsanguinated while deeply anesthetized; then, 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. RESULTS: After administration of L-NAME, pulmonary artery pressure increased from a mean of 16.6-23.6 mm Hg, P = .007 but PaO2, PaCO2, and SD log(V/Q) did not change. Distribution of ventilation was not influenced by L-NAME, but a small redistribution of perfusion from ventral to dorsal lung regions was observed. Perfusion to regions with the highest ventilation (fifth quintile of the ventilation distribution) remained unchanged after L-NAME. CONCLUSIONS: We found minimal or no influence of constitutive nitric oxide synthase inhibition by L-NAME on the distributions of ventilation and perfusion, and ventilation-perfusion in prone, anesthetized, ventilated, and healthy adult sheep with normal gas exchange.

Marked differences between prone and supine sheep in effect of PEEP on perfusion distribution in zone II lung.

The classic four-zone model of lung blood flow distribution has been questioned. We asked whether the effect of positive end-expiratory pressure (PEEP) is different between the prone and supine position for lung tissue in the same zonal condition. Anesthetized and mechanically ventilated prone (n = 6) and supine (n = 5) sheep were studied at 0, 10, and 20 cm H2O PEEP. Perfusion was measured with intravenous infusion of radiolabeled 15-microm microspheres. The right lung was dried at total lung capacity and diced into pieces (approximately 1.5 cm3), keeping track of the spatial location of each piece. Radioactivity per unit weight was determined and normalized to the mean value for each condition and animal. In the supine posture, perfusion to nondependent lung regions decreased with little relative perfusion in nondependent horizontal lung planes at 10 and 20 cm H2O PEEP. In the prone position, the effect of PEEP was markedly different with substantial perfusion remaining in nondependent lung regions and even increasing in these regions with 20 cm H2O PEEP. Vertical blood flow gradients in zone II lung were large in supine, but surprisingly absent in prone, animals. Isogravitational perfusion heterogeneity was smaller in prone than in supine animals at all PEEP levels. Redistribution of pulmonary perfusion by PEEP ventilation in supine was largely as predicted by the zonal model in marked contrast to the findings in prone. The differences between postures in blood flow distribution within zone II strongly indicate that factors in addition to pulmonary arterial, venous, and alveolar pressure play important roles in determining perfusion distribution in the in situ lung. We suggest that regional variation in lung volume through the effect on vascular resistance is one such factor and that chest wall conformation and thoracic contents determine regional lung volume.

Positive end-expiratory pressure affects regional redistribution of ventilation differently in prone and supine sheep.

OBJECTIVE: To examine interactions between positive end-expiratory pressure (PEEP) and posture on regional distribution of ventilation and to compare measurements of regional ventilation with two aerosols: a wet fluorescent microsphere aerosol (FMS, median mass aerodynamic diameter 1.1 microm) and a dry Tc-labeled carbon particle aerosol (Technegas, TG, median mass aerodynamic diameter approximately 0.1 microm). DESIGN: Experimental study. SETTING: Academic laboratory. SUBJECTS: : Anesthetized and mechanically ventilated sheep (n = 16). INTERVENTIONS: Four conditions were studied: prone or supine posture with or without 10 cm H2O PEEP. MEASUREMENTS AND MAIN RESULTS: Comparisons of FMS and TG were made in five animals. The median correlation coefficient of the two ventilation tracers was .95 (range, .91-.96). The mean ventilation per unit weight of dry lung for horizontal planes was almost identical whether measured with TG or FMS. The distribution of ventilation was assessed by analyzing deposition of aerosol in about 1,000 lung regions per animal. Distribution of ventilation down the vertical axis was linear in prone (the slope indicated a dorsal-to-ventral three-fold difference in ventilation) but unimodal in supine animals with the mode in the center of the lung. Redistribution of ventilation with 10 PEEP differed between posture, shifting the mode in supine toward dependent lung regions while eliminating the dorsal-to-ventral gradient in prone. The regional heterogeneity in ventilation was greater in supine sheep at both levels of PEEP, and this was due mostly to greater isogravitational heterogeneity in supine than in prone position. CONCLUSIONS: The wet fluorescent microsphere aerosol was as reliable as Technegas for high-resolution measurements of regional ventilation. The markedly different effects of 10 PEEP in supine and prone sheep may have important implications for gas exchange both in noninjured and injured lungs.