Plots of mean pulmonary artery pressure (PAP) and pulmonary vascular resistance (PVR) during a continuous infusion of U46619. Lambs breathed various levels of NO (5 to 80 ppm) at fraction of inspired oxygen 0.6 for 6 min, then breathed a gas mixture at fraction of inspired oxygen 0.6 for 6 min without NO (n = 8, mean±SEM). Reprinted with permission from Frostell C, Fratacci MD, Wain JC, Jones R, Zapol WM: Inhaled nitric oxide: A selective pulmonary vasodilator reversing hypoxic pulmonary vasoconstriction. Circulation 1991; 83:2038–47. Copyright © 1991 Wolters Kluwer Health.