Background

Nitric oxide (NO), an endothelium-derived relaxing factor, acts as a local vasodilator. The authors examined the effects of NO on pulmonary and systemic circulation in human volunteers.

Methods

Nine healthy adults were studied awake while breathing 1) air, 2) 12% O2 in N2, 3) followed by the same mixture of O2 and N2 containing 40 ppm of NO. Pulmonary artery and radial artery pressures were monitored.

Results

The PaO2 decreased from 106+/-4 (mean+/-standard error of the mean) while breathing air (21% O2) to 47+/-2 mmHg after 6 min of breathing 12% O2. Concomitantly, the pulmonary artery mean pressure (PAP) increased from 14.7+/-0.8 mmHg to 19.8+/-0.9 mmHg, and the cardiac output (CO) increased from 6.1+/-0.4 to 7.7+/-0.6 L/min. After adding 40 ppm NO to the inspired gas while maintaining the FIO2 at 0.12, the PAP decreased (P< 0.01, by analysis of variance) to the level when breathing air while the PaO2 and PaCO2 were unchanged. The dilation (or recruitment) of pulmonary vessels produced by inhaling NO during hypoxia was not accompanied by any alteration in the systemic vascular resistance or mean arterial pressure (MAP). The authors also examined the effects of inhaling NO while breathing air. Breathing 40 ppm NO in 21% O2 for 6 min produced no significant changes of PAP, CO, PaO2, MAP, or central venous pressure. Plasma endothelinlike immunoreactivity concentrations did not change either during hypoxia or hypoxia with NO inhalation.

Conclusions

Inhalation of 40 ppm NO selectively induced pulmonary vasodilation and reversed hypoxic pulmonary vasoconstriction in healthy humans without causing systemic vasodilation.