A mathematical derivation is presented of an equation that allows quantitation, in the cardiac catheterization laboratory, of the effective output of the right ventricle (QRV) in patients with pulmonary atresia and intact ventricular septum (PA/IVS) after palliation with a right ventricular outflow patch and an aorticopulmonary shunt. This derivation is based on the Fick principle and accounts for that portion of the right-to-left atrial shunt that flows through the aorticopulmonary shunt to the lungs and thus contributes to the effective pulmonary blood flow (Qep). This equation relates QRV to pulmonary blood flow (Qp), systemic cardiac output (Qs), and Qep by QRV = Qp + Qs - (Qp)(Qs)/Qep. By providing an assessment of right ventricular function, this equation may be helpful in determining the suitability of the right ventricle to support biventricular repair.