**7.2 Cardiac catheterization**

Although a mainstay in diagnosis of all congenital heart defects in the past, cardiac catheterization is now reserved for cases requiring measurements of PVR. In the patients with PAH, cardiac catheterization may be undertaken to determine operability. Catheterization data indicating operability suggests the likelihood of a favorable versus an unfavorable outcome [17]. But, there is no validated consensus data accurate enough to define which patients will be free of major postoperative complications related to pulmonary vascular disease. However, a baseline ratio between indexed pulmonary vascular resistance (PVRi) and indexed systemic vascular resistance (SVRi) of <0.3 and PVRi of <6 indexed Woods units/m2 (iWU. m2) is indicative of favorable outcome. Pulmonary vasoreactivity study with O2 and iNO has been used to determine operability in subjects with PVRi of 6–9 iWU m2 or resistance ratio (PVRi/SVRi) of 0.3–0.5. A positive test is defined as decrease of both PVRi and resistance ratio by 1/5th of initial value as well as final PVRi of 6 iWUm2 and resistance ratio of <0.3. All patients should meet all of these criteria before being considered operable with decreased risk of serious postoperative complications [26, 27]. In patients with PVRi >10 iWUm2 and resistance ratio of >0.7, surgical repair is not beneficial [28].

Other protocols have been proposed for the vasoreactivity studies to assess operability as well as to assess prognosis and indication for specific anti PAH therapies. Measurements of resistance and flow in systemic and pulmonary vascular beds are carried out in several conditions such as room air, nitric oxide, IV epoprostenol or inhaled iloprost and in some cases oral phosphodiesterase 5 inhibitors. Some recommend to avoid use of high oxygen concentrations in these patients if other agents are available due to high amounts of dissolved oxygen as a potential source of error causing overestimation of Qp [29].
