Oximetry is the most convenient and commonly used method for detecting and calculating left-to-right shunts in the cardiac catheterization laboratory. To document the presence of the shunt and calculate its size, venous and arterial access should be obtained. Oxygen saturation is measured in SVC, IVC, RA, RV, and PA and compared with normal oxygen saturation values (see Chapter 19).¹ If femoral vein access is used for right heart catheterization, the small-diameter, 4-Fr short entry sheath is placed in the common femoral artery. The peripheral arterial line can be utilized for systemic arterial oxygen saturation measurement when jugular or subclavian veins are used as access sites for right heart catheterization. A “step-up” of mean O₂ blood saturation from SVC to PA > 7% suggests presence of an intracardiac shunt. If a steady and consistent rise > 5% in mean oxygen saturation is noted at any point while moving the PA catheter from one cardiac chamber to another, the presence of a shunt is highly suspected.

In general, when consequently performing blood oximetry from the caval veins towards the pulmonary artery, the chamber where such a “step-up” is first noted is usually the one where the shunt exists. On some occasions, a further rise in O₂

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