Background:We investigated the acute vasodilator effects of i.v. fasudil, a specific Rho-kinase inhibitor, on pulmonary circulation in patients with congenital heart defects (CHD) and severe pulmonary arterial hypertension (PAH).Methods and Results:Thirty-five patients (34.23±12.10 years old) with CHD and severe PAH were consecutively enrolled. All patients underwent heart catheterization. At baseline and 30 min after initiation of i.v. fasudil, the following hemodynamic parameters were measured and calculated: right atrial pressure, pulmonary and systemic artery pressure (PAP and SAP), pulmonary and systemic vascular resistance, pulmonary-to-systemic blood pressure ratio (Pp/Ps), pulmonary-to-systemic blood flow ratio (Qp/Qs), cardiac index (CI) and artery oxygen saturation (SaO₂). After fasudil treatment, marked decrease in mean PAP (mPAP), pulmonary vascular resistance (PVR), total pulmonary resistance, pulmonary-to-systemic vascular resistance ratio (Rp/Rs) and mean Pp/Ps (mPp/Ps) was found, while Qp/Qs increased significantly without affecting CI and SAP. mPAP, PVR, Rp/Rs and Qp/Qs tended to be improved more significantly in the post-tricuspid shunt group compared with the pre-tricuspid shunt group.Conclusions:Fasudil was well tolerated in patients with CHD and severe PAH, and significantly reduced PAP and PVR without affecting CI, SAP or SaO₂. (Circ J 2015; 79: 1342–1348)