CONTINUOUS POSITIVE AIRWAY PRESSURE MODULATES EFFECT OF INHALED NITRIC-OXIDE ON THE VENTILATION-PERFUSION DISTRIBUTIONS IN CANINE LUNG INJURY

Objectives: The present study was designed to evaluate if continuous p ositive airway pressure (CPAP) augments the effect of nitric oxide (NO ) inhalation on matching between ventilation and perfusion (VA/Q) duri ng acute lung injury. Design: Prospective, randomized study. Setting: A research laboratory at a university medical center. Subjects: Ten an esthetized mongrel dogs with oleic acid-induced lung injury. Intervent ions: 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 resu lts: Gas exchange was assessed by estimating the VA/Q distributions us ing the multiple inert gas elimination technique. Application of CPAP decreased blood now to shunt units by 26+/-2 percent (mean+/-SD) and i ncreased the fraction of cardiac output to normal VA/Q units (VA/Q rat io of 0.1 to 10) by 26+/-2 percent (p<0.05). Inhalation of NO during C PAP accounted for a further 10+/-2 percent decrease in the blood now t o shunt units and an 8+/-2 percent increase in the fraction of the car diac 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 decr eased 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 d ynes s cm(-5); 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.cm(-5); p<0.05). Conclusion s: Although pulmonary vascular resistance can be lowered with NO inhal ation alone, recruitment of gas exchange units with CPAP is necessary to produce a beneficial effect of NO inhalation on VA/Q matching and o xygenation. When recruitment of gas exchange units with CPAP brings ga seous NO in contact with enough pulmonary blood vessels, NO-induced va sodilation will augment VA/Q matching by a steal mechanism.