['In idiopathic pulmonary arterial hypertension (IPAH), increased right ventricular (RV) power is required to maintain cardiac output. For this, RV O2 consumption (MVO2) must increase by augmentation of O2 supply and/or improvement of mechanical efficiency-ratio of power output to MVO2. In IPAH with overt RV failure, however, there is evidence that O2 supply (perfusion) reserve is reduced, leaving only increase in either O2 extraction or mechanical efficiency as compensatory mechanisms. We related RV mechanical efficiency to clinical and hemodynamic parameters of RV function in patients with IPAH and associated it with glucose metabolism.', 'The patients included were in New York Heart Association (NYHA) class II (n=8) and class III (n=8). They underwent right heart catheterization, MRI, and H2(15)O-, (15)O2-, C(15)O-, and 18FDG-PET. RV power and O2 supply were similar in both groups (NYHA class II versus class III: 0.54±0.14 versus 0.47±0.12 J/s and 0.109±0.022 versus 0.128±0.026 mL O2/min per gram, respectively). RV O2 extraction was near-significantly lower in NYHA class II compared with NYHA class III (63±17% versus 75±16%, respectively, P=0.10). As a result, MVO2 was significantly lower (0.066±0.012 versus 0.092±0.010 mL O2/min per gram, respectively, P=0.006). RV efficiency was reduced in NYHA class III (13.9±3.8%) compared with NYHA class II (27.8±7.6%, P=0.001). Septal bowing, measured by MRI, correlated with RV efficiency (r = -0.59, P=0.020). No relation was found between RV efficiency and glucose uptake rate. RV mechanical efficiency and ejection fraction were closely related (r=0.81, P<0.001).'] Is right ventricular failure in idiopathic pulmonary arterial hypertension associated with inefficient myocardial oxygen utilization?