Fig. 1 World Health Organization (WHO) functional class in the patients before and after endothelin-1 receptor antagonist therapy. The mean functional class changed from class III to class II after therapy. Fourteen patients improved by 1 class, 7 patients improved by 2 classes, 1 patient improved by 3 classes, 1 patient experienced no change, and 1 patient's condition worsened by 1 class.