After this diagnostic hemodynamic baseline examination, patients with PAH were treated with regular iILO (n = 18), bosentan (n = 16), and/or sildenafil (n = 2). The initial treatment of patients with CTEPH was with iILO (n = 20) and/or bosentan (n = 3). Five patients with CTEPH underwent successful pulmonary thrombendarterectomy after the initial vasoreactivity testing, nine patients declined the operation, major surgery was contraindicated in seven patients, and one patient underwent successful lung transplantation 2 years later. Therefore, the initial acute vasodilator response to iNO and/or iILO at the diagnostic baseline evaluation was not related to an improvement in WHO functional class or 6MWD. Again, there was no difference between the PAH and CTEPH patients. We found that the inhalation of iNO and/or iILO during the initial diagnostic right heart catheterization decreased MPAP and increased CI in patients with CTEPH and PAH who were in WHO functional classes III to IV, with the magnitude ofthe response as well as the number of responders being not different between CTEPH and PAH patients. Here
Patients with PAH and CTEPH also had similar indexes of proximal pulmonary arterial compliance. These results suggest that CTEPH and PAH may share some pathophysiologic characteristics, which probably involve both the proximal and the distal precapillary pulmonary arteries. Previous studies in patients with PAH have shown that iNO and iILO decrease MPAP by 9% on average, increase CO by 5 to 8%, and decrease PVR by 18%. We found a similar extent of vasodilator effect of these agents in our patients. However, despite a similar effect on MPAP and CI, in our study more patients could be identified as responders to iILO compared with iNO, independently of the criterion that we used to distinguish between vasoreactivity testing results in responders and nonresponders. The inhalation of iILO predicted a positive response to iNO in 75% of the patients; conversely, the inhalation of iNO identified only 25% of iILO responders. Whether this difference between iNO and iILO is of clinical relevance for the small number of responders in our study and other studies remains to be established. Until then, vasoreactivity testing with either agent singly is considered to be sufficient in the baseline evaluation of patients with PH. It is remarkable that in our cohort the acute vasodilator response between patients with major-vessel CTEPH and PAH was not different. The acute responses to various vasodilators have been widely investigated in patients with PAH,’’’’’ but not in patients with CTEPH. Comparable acute hemodynamic responses have been reported for different PAH subclasses and were attributed to qualitatively similar histopathologic vascular characteristics between sub-classes, which were mainly located in the distal pulmonary vascular bed, with a higher capacity to vasodilate. In recent years, a better understanding of the mechanisms responsible for elevated pulmonary arterial pressure and vascular resistance in the different types of PH has led to the suggestion that the thromboembolic and nonthromboembolic types of PH may share common pathophysiologic features.