Calculation of intracardiac shunts by cardiac cath

Calculation of intracardiac shunts by cardiac cath

Pulmonary blood flow is designated as Q_P and systemic blood flow as Q_S in hemodynamic calculations. When there is a left to right shunt, pulmonary blood flow is more than the systemic blood flow. The systemic blood flow is more than the pulmonary blood flow when there is a right to left shunt. Calculation of shunt ratio is useful in deciding the operability of shunt lesions as well as their hemodynamic significance. In the absence of significant pulmonary hypertension, a left to right shunt of less than 1.5 : 1 may be left alone if at the atrial or ventricular level. But even a small left to right shunt due to patent ductus arteriosus is closed by a device or coil now a days.

Left to right shunt (Q_L-R) = Q_P – Q_S

Q_P/Q_S = (SA O₂ – MV O₂)/(PV O₂ – PA O₂)

Q_P/Q_S is the ratio of pulmonary to systemic blood flow. It will be more than one in left to right shunts and less than one in right to left shunts.

SA O₂: systemic arterial oxygen saturation; MV O₂: mixed venous oxygen saturation; PV O₂: pulmonary venous oxygen saturation; PA O₂: pulmonary arterial oxygen saturation.

MV O₂ is calculated using Flamm formula: {3(SVC O₂ content) + 1(IVC O₂ content)}/4  [1].

Effective flow Q_EFF is calculated when there is a bidirectional shunt. Effective pulmonary blood flow is the volume of unoxygenated blood flowing through the lungs, which is the volume which actually participates in the respiratory gas exchange.

Effective pulmonary blood flow will be Q_P – Q_L-R. In pure left to right shunt, this value will be equal to the systemic blood flow.

Reference

1. Testuz A, Roffi M, Bonvini RF. Left-to-right shunt reduction with intra-aortic balloon pump in postmyocardial infarction ventricular septal defect. Catheter Cardiovasc Interv. 2013 Mar;81(4):727-31.