Nitric oxide and prostacyclin as test agents of vasoreactivity in severe precapillary pulmonary hypertension: Predictive ability and consequences on haemodynamics and gas exchange

Background - In patients with primary pulmonary hypertension who respond to vasodilators acutely, survival can be improved by the long term use of calcium blockers. However, testing for response with calcium channel blockers or prostacyclin (PGI(2)) may cause hypotension and adversely affect gas exchange. Nitric oxide (NO), which does not have these effects, could be a better test agent. Methods - NO (10, 20, and 40 ppm for 15 minutes), PGI(2) (1- > 10 ng/kg/min), and oral nifedipine (10 mg, then 20 mg/h) were administered sequentially to 10 patients after determination of the 24 hour spontaneous variability of their pulmonary and systemic mean arterial pressures. Patients were considered responders if the mean pulmonary artery pressure or pulmonary vascular resistance decreased by 20% or more. Results - Six patients (60%) responded to all three agents, and three to none of the agents. One patient responded to PGI(2) only. In those who responded to vasodilators, NO had no major effect on gas exchange or systemic haemodynamics, while PGI(2) and nifedipine both induced systemic hypotension (mean (SD) systemic arterial pressure 72 (14) versus 89 (19) mm Hg with PGI(2) and 72 (15) versus 86 (17) mm Hg with nifedipine, p < 0.05) and hypoxaemia (PaO2 8.7 (1.4) versus 10.8 (1.0) kPa with PGI(2) and 8.6 (1.4) versus 10.2 (1.5) kPa with nifedipine, p < 0.05) and increased venous admixture (28 (9) versus 14 (4)% with PGI(2) and 22 (9) versus 13 (5)% with nifedipine, p < 0.05). Conclusions - NO inhalation can accurately predict a vasodilator response to nifedipine in patients with severe pulmonary hypertension without adverse effects on systemic haemodynamics and gas exchange. This absence of side effects may make it a more appropriate agent for testing the vasodilator response.