Objective To test the vasodilatory response of the pulmonary vascular bed in children with pulmonary hypertension.

Design Prospective dose response study in which the effects of inhaled nitric oxide (NO) are compared with those of oxygen and intravenous prostacyclin.

Patients and interventions The vasodilator test was performed in 20 patients in whom mean pulmonary artery pressure (PAPm) was ⩾ 40 mm Hg and/or pulmonary vascular resistance index was ⩾ 4 Um². Haemodynamic effects of inhaled NO (20, 40, and 80 ppm) at a fractional inspired oxygen (FiO₂) value of 0.3, pure oxygen, oxygen at FiO₂ 0.9–1.0 combined with NO as above or with intravenous prostacyclin at 10 and 20 ng/kg/min were measured.

Result NO decreased PAPm with a dose response from 20 to 40 ppm (mean change at 40 ppm −5.50, 95% confidence interval (CI) −7.98 to −3.02 mm Hg). Maximal decrease in the ratio of pulmonary to systemic vascular resistance was achieved with a combination of NO 80 ppm and oxygen (−0.18, 95% CI −0.26 to −0.10). Increase in the pulmonary flow index was greatest with pure oxygen in those with an intracardiac shunt (8.52, 95% CI −0.15 to 17.20 l/min/m²). Neither NO nor oxygen altered systemic arterial pressure but intravenous prostacyclin lowered systemic arterial pressure and resistance.

Conclusions NO selectively reduces pulmonary vascular resistance and pressure maximally at 40 ppm. Oxygen reduces pulmonary vascular resistance and NO potentiates this reduction without affecting the systemic circulation. Prostacyclin vasodilates the pulmonary and the systemic circulations.