Diagnostic Imaging, Catheterization, and Physiology
Hemodynamics that decide operability
Catheterization answers questions imaging cannot: pressures, resistances, shunt fraction, and reactivity. The Fick principle underlies all of it; oxygen consumption is assumed or measured, and small errors in assumed VO2 change calculated output substantially. Pulmonary vascular resistance index is (mean pulmonary artery pressure − wedge pressure)/cardiac index, reported in Wood units per square meter. Transpulmonary gradient is mean pulmonary artery pressure minus wedge pressure. For a Fontan candidate, typical thresholds are a transpulmonary gradient under 10 mmHg, pulmonary vascular resistance index under 3 Wood units·m², and mean pulmonary artery pressure of 15 mmHg or less; a positive vasoreactivity test with oxygen or nitric oxide does not by itself make an inoperable patient operable.
Interventions performed in the same session include balloon atrial septostomy for transposition, balloon valvuloplasty for critical pulmonary or aortic stenosis, angioplasty and stenting of coarctation or conduit stenosis, and device closure of septal defects and the patent ductus.
Traps
Superior vena cava saturation is often used as mixed venous blood, but it overestimates true mixed venous saturation when there is streaming. Sedation, acidosis, and high FiO2 alter resistance measurements, so a single set of numbers taken under general anesthesia must be interpreted cautiously. A wedge pressure may not reflect left atrial pressure when the catheter is poorly positioned, and echocardiographic gradients alone should not decide operability in an adult with a shunt.
Rule of thumb
With mean pulmonary artery pressure 45 mmHg, wedge 10 mmHg, and cardiac index 3 L/min/m², PVR index is 35/3 ≈ 11.7 Wood units·m² - far above Fontan criteria.