Continuous Positive Airway Pressure Modulates Effect of Inhaled Nitric Oxide on the Ventilation-Perfusion Distributions in Canine Lung Injury

¹ From the Department of Anesthesiology, University of South Florida, College of Medicine, Tampa

Objectives: The present study was designed to evaluate if continuous positive airway pressure (CPAP) augments the effect of nitric oxide (NO) inhalation on matching between ventilation and perfusion (VA/Q) during acute lung injury.

Design: Prospective, randomized study.

Setting: A research laboratory at a university medical center.

Subjects: Ten anesthetized mongrel dogs with oleic acid-induced lung injury.

Interventions: Zero or 40 parts per million of NO in the inspiratory gas, with and without 10 cm H₂O CPAP in random order.

Measurements and main results: Gas exchange was assessed by estimating the VA/Q distributions using the multiple inert gas elimination technique. Application of CPAP decreased blood flow to shunt units by 26±2 percent (mean±SD) and increased the fraction of cardiac output to normal VA/Q units (VA/Q ratio of 0.1 to 10) by 26±2 percent (p<0.05). Inhalation of NO during CPAP accounted for a further 10±2 percent decrease in the blood flow to shunt units and an 8±2 percent increase in the fraction of the cardiac output to normal VA/Q units (p<0.05). Inhalation of NO alone had no significant effect on the VA/Q distributions. Inhalation of NO decreased mean transmural pulmonary artery pressure (Ppa_tm) both without (Ppa_tm from 30±2 to 23 ±2 mm Hg; PVR from 323±44 to 228±43 dynes·s ·cm⁵; p<0.05) and with CPAP (Ppa_tm from 25±2 to 20±2 mm Hg; PVR from 255±30 to 173±31 dynes·s·cm⁵; p<0.05).

Conclusions: Although pulmonary vascular resistance can be lowered with NO inhalation alone, recruitment of gas exchange units with CPAP is necessary to produce a beneficial effect of NO inhalation on VA/Q matching and oxygenation. When recruitment of gas exchange units with CPAP brings gaseous NO in contact with enough pulmonary blood vessels, NO-induced vasodilation will augment VA/Q matching by a steal mechanism.

Key Words: acute lung injury • continuous positive airway pressure • nitric oxide • pulmonary gas exchange • ventilation-perfusion distribution

Submitted on November 17, 1993
Accepted on January 31, 1994

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