Diagnostic Imaging, Echocardiography, and Catheterization
Imaging and hemodynamics
Echocardiography is the first-line tool for anatomy, shunts, ventricular function, and gradients. Cardiac MRI quantifies ventricular volumes, flow, and fibrosis; CT is useful for complex vascular anatomy and airways. Cardiac catheterization provides hemodynamic data, oxygen saturations, shunt calculations, and opportunities for intervention. The Fick principle relates oxygen consumption to arteriovenous oxygen differences. Qp/Qs = (systemic arterial saturation − mixed venous saturation)/(pulmonary venous saturation − pulmonary arterial saturation). A Qp/Qs above 1.5 suggests a significant shunt; above 2.0 generally favors closure if pulmonary vascular resistance is acceptable.
Traps
Echo-estimated right ventricular systolic pressure is not a catheterization measurement. Do not calculate shunts from peripheral oxygen saturations alone; mixed venous saturation matters. A normal echo does not exclude anomalous coronary arteries or a small fistula. Catheterization risks include arrhythmia, bleeding, contrast reactions, and vascular injury.
Worked example
A child with an ASD has arterial saturation 98%, mixed venous saturation 70%, pulmonary venous saturation 99%, and pulmonary arterial saturation 85%. Qp/Qs = (98 − 70)/(99 − 85) = 28/14 = 2.0. This is a significant left-to-right shunt; assess pulmonary vascular resistance and defect anatomy before closure. Rule of thumb: a Qp/Qs near 2.0 means pulmonary blood flow is about twice systemic flow.