No apparent effect of nitric oxide on the local matching of pulmonary perfusion and ventilation in awake sheep

The respiratory tissue in the lung receives nitric oxide (NO) from two sour ces; NO produced in upper airways, and NO produced in lung parenchyma. It h as been hypothesized that optimal local matching of ventilation and perfusi on (which is necessary for effective gas exchange) is ensured because well- ventilated lung tissue has a higher concentration of NO and thereby higher blood flow owing to the vasodilatory effect of NO. To test this hypothesis, we simultaneously measured the distributions of local (regions of approxim ate to 1.5 cm(3)) blood flow (radioactive microspheres) and local ventilati on (fluorescent aerosol) in five tracheostomized, awake and standing sheep. Tracers for perfusion and ventilation were administered (1) at baseline, ( 2) during endogenous NO production blockage (L-NAME 25 mg kg(-1)) and admin istration of NO free air, and (3) when the sheep received exogenous NO (app roximate to 30 p.p.m.), but having its endogenous NO production blocked. Th e intrapulmonary distribution of ventilation was similar in ail three situa tions. Within horizontal levels of the lung, distribution of perfusion was not affected by variable access to NO, but along the gravitational axis per fusion was more evenly distributed when the sheep had no access to NO. Exog enous NO tended to restore the baseline vertical profile. These changes in vertical distribution of perfusion can be explained by the effect of variab le NO concentrations on pulmonary arterial pressure acid cardiac output. Va riable access to NO had no effect on arterial blood gases. We conclude that NO is important for the vertical distribution of pulmonary perfusion, but has no apparent effect on the local matching of ventilation and perfusion w ithin horizontal layers of the lung.