Incidence and significance of a "step-down" in oxygen saturation from superior vena cava to pulmonary artery

Several studies^1-4 have assessed the normal variability of oxygen saturations in blood from the right-sided heart chambers. An increase in saturation from the peripheral to the central chambers that exceeds this variability suggests left-to-right intracardiac shunting. In some subjects, a decrease in saturation from the superior vena cava (SVC) to the central right-sided chambers occurs. In previous studies,^5,6 such a "step-down" was often observed in patients with shock. This study was performed to assess the incidence of a step-down in oxygen saturation of ≥5% from the SVC to the pulmonary artery (PA) and to determine if such a step-down is associated with cardiac or renal dysfunction.