Accurate estimation of cardiac output is essential in the management of patients with congenital and acquired heart disease and is particularly important in patients with left to right shunt lesions and pulmonary hypertension. Patients with pulmonary arterial hypertension and closed shunt lesions have significantly worse survival compared to patients with uncorrected defects, suggesting that in patients with pulmonary vascular disease defect closure can be detrimental.1 To diagnose pulmonary vascular disease in the presence of significant left to right shunting, documentation of increased pulmonary pressures is insufficient and pulmonary vascular resistance (PVR) needs to be calculated.

PVR is calculated as the ratio of the mean transpulmonary gradient (TPG) to transpulmonary blood flow.

Equation 1: Calculation of PVR on cardiac catheterisation. Usually mean pulmonary artery wedge pressure is used a surrogate for mean left atrial pressure. If pressure is expressed in mm Hg and flow in L/min, the equation results in ‘Wood units’, to be multiplied by 80 for dyn·s·cm⁻⁵.

The TPG can be accurately measured during cardiac catheterisation, while assessment of transpulmonary flow (Q_p) is more challenging. In clinical practice Q_p is usually assessed by the Fick method (equation 2). It has been demonstrated that a small and relatively …