RATIONALE - Patients with pulmonary hypertension (PH) suffer from inspiratory muscle weakness. However, the pathophysiology of inspiratory muscle dysfunction in PH is unknown. We hypothesized that weakness of the diaphragm, the main inspiratory muscle, is an important contributor to inspiratory muscle dysfunction in PH-patients. OBJECTIVES - To combine ex vivo diaphragm muscle fiber contractility measurements with measures of in vivo inspiratory muscle function in chronic thromboembolic pulmonary hypertension (CTEPH) patients. METHODS - To assess diaphragm muscle contractility, function was studied in vivo by maximum inspiratory pressure (MIP), and ex vivo in diaphragm biopsies of the same CTEPH-patients (N=13) obtained during pulmonary endarterectomy. Patients undergoing elective lung surgery served as controls (N=15). Muscle fiber cross sectional area (CSA) was determined in cryosections and contractility in permeabilized muscle fibers. RESULTS - Diaphragm muscle fiber CSA was not significantly different between control and CTEPH-patients in both slow-twitch and fast-twitch fibers. Maximal force generating capacity was significantly lower in slow-twitch muscle fibers of CTEPH-patients, while no difference was observed in fast-twitch muscle fibers. The maximal force of diaphragm muscle fibers correlated significantly with MIP. The calcium sensitivity of force generation was significantly reduced in fast-twitch muscle fibers of CTEPH-patients, resulting in a ~40% reduction of submaximal force generation. The fast skeletal troponin activator CK-2066260 (5µM) restored submaximal force generation to levels exceeding those observed in control subjects. CONCLUSIONS - Diaphragm muscle fiber contractility is hampered in CTEPH-patients and contributes to the reduced function of the inspiratory muscles in CTEPH-patients.