Cardiac Shunt Management

Definition and Clinical Significance

The American Heart Association notes that a Qp/Qs ratio > 1.5 is generally considered hemodynamically significant for left-to-right shunts and may warrant intervention, with higher values indicating larger shunts 1
A left-to-right shunt results in greater blood flow to the pulmonary vasculature compared to systemic circulation (Qp/Qs > 1), with the American College of Cardiology recommending intervention for ratios ≥ 1.5:1 1
Quantifying Qp/Qs is essential for clinical decision-making regarding intervention in patients with shunt lesions, according to the American Heart Association 1

The reference standard method is invasive oximetry during cardiac catheterization, measuring oxygen saturations in pulmonary and systemic arterial and venous systems, as recommended by the American College of Cardiology 1
Cardiac Magnetic Resonance (CMR) has become the non-invasive reference standard using phase-contrast CMR (PC-CMR), allowing highly accurate and reproducible quantification without assumptions, with a strength of evidence rated as high by the American Heart Association 3, 4
CMR offers several advantages over invasive oximetry, including direct measurement of flow without assumptions, multiple measurement methods providing internal quality assurance, and ability to measure flow in alternative vessels when standard measurements are not possible, as noted by the American College of Cardiology 4

The American College of Cardiology recommends surgical or device closure for atrial septal defects when Qp/Qs ≥ 1.5:1 with right atrial and RV enlargement, provided pulmonary vascular resistance is less than one-third of systemic resistance, with a strength of evidence rated as moderate 2
Closure should not be performed when pulmonary arterial systolic pressure is greater than two-thirds systemic, or pulmonary vascular resistance is greater than two-thirds systemic, according to the American Heart Association 2, 6
In Eisenmenger syndrome, the Qp/Qs ratio becomes < 1 as blood flows from right-to-left, contraindicating closure, as recommended by the American College of Cardiology 2, 6

CMR can measure Qp/Qs using multiple methods, including direct flow measurement in the pulmonary trunk and aorta, flow measurement in pulmonary veins and vena cavae, and ventricular stroke volumes, with a strength of evidence rated as high by the American Heart Association 4
When using ventricular stroke volumes to calculate Qp/Qs, shunt location must be considered, with the American College of Cardiology noting that shunt location is crucial for accurate calculation 4

CMR can quantify collateral flow in complex congenital heart disease, including systemic-to-pulmonary collaterals in single ventricle patients, veno-venous collaterals, and aortic collaterals in coarctation, as recommended by the American Heart Association 3
CMR provides comprehensive evaluation of complex shunts like sinus venosus defects and anomalous pulmonary venous connections that are difficult to image with echocardiography, with a strength of evidence rated as high by the American College of Cardiology 5