RESULTS: Patients on prostacyclin therapy were younger than those without (49.5 (22 - 75) years vs. 65.5 (31 - 81) years, P < 0.0001. Despite no differences in right heart filling pressures, mPAP or CO between treatment groups, plasma BNP levels were significantly lower in patients on prostacyclin therapy than in those not on prostacyclin therapy(27.4 pg/mL [2.7 - 181] vs. 92.2 pg/mL [4.1 - 1117.2]). Use of prostacyclin was associated with a -204.5 pg/ml decrease in BNP (95 % CI -367.9 - -41.2, p = 0.015) without multivariate adjustment. After adjustment for age and sex, there was a trend towards a decrease in plasma BNP levels with use of prostacyclin (-138 pg/mL 95% CI -341.1 - 65.1, p = 0.178). In patients not on prostacyclin therapy, plasma BNP levels correlated directly with mPAP (r = 0.39, P=0.05) and tended to correlate inversely with CO (r = -0.41, p=0.07), whereas no correlation between plasma BNP and mPAP or CO was seen in patients on prostacyclin therapy (r = 0.11, p = 0.65 for mPAP, r = -0.04, p = 0.89 for CO).