Helium inhalation enhances vasodilator effect of inhaled nitric oxide on pulmonary vessels in hypoxic dogs

There are theoretical and experimental indications that the presence of He as a balance gas markedly increase the diffusion velocity of other gases contained in a gas mixture. We allowed dogs with pulmonary vasoconstriction induced by hypoxia to inhale a mixture of 5 parts per million (ppm) of nitric oxide (NO) and O-2 balanced with He (NO in He) instead of N-2 (NO in N-2). The dilating effect of NO in He and NO in N-2 on the pulmonary artery was evaluated by determining conventional pulmonary hemodynamic parameters, mean pulmonary artery (PA) pressure (MPAP), and pulmonary vascular resistance indexed to body surface area (PVRI), pulmonary impedance (Z), and the recently developed hemodynamic index, time-corrected wave intensity (WI). The main findings in this study were as follows: 1) hypoxia increased MPAP, PVRI, Z at 0 Hz (Z(o)), Z at the first harmonics, characteristic impedance (Z(c)), the reflection coefficient (Gamma), and the first peak of WI; 2) NO in N-2 reduced Z(o) and Gamma; and 3) NO in He reduced the first peak of WI and reduced Z(o) and Gamma more than NO in N-2. The enhanced vasodilatory effect of NO in He might be associated with facilitated diffusion of NO diluted in the gas mixture with He. In conclusion, increased efficacy of NO in He offers the possibility to reduce the inhaled NO concentration.