Consultation with the Specialist

Abstract

What is the likely pathophysiology of this event? What are the more common complications of hypoxemia in the older infant and young child? This clinical scenario is uncommon, but it represents one of the two feared central nervous system complications of cyanotic congenital heart disease, (ie, cerebrovascular accident and brain abscess). A uniform response to hypoxemia of cardiac etiology is the production of erythropoietin to produce more red blood cells. This is a compensatory mechanism to maintain oxygen delivery to the peripheral tissues. Normally, hemoglobin is about 96% saturated with oxygen. Therefore, the oxygen-carrying capacity of blood with a normal hemoglobin concentration of 15 g/dL is approximately 20.3 mL of oxygen per 100 mL of blood (ie, 15 g of hemoglobin x 1.35 mL of O2 per g of hemoglobin = 20.3). The oxygen content of blood equals the oxygen-carrying capacity multiplied by the oxygen saturation. At a normal oxygen saturation of 96%, the O2 content of arterial blood (Hgb 15 g/dL) equals 19.5 mL/dL (96% x 20.3 mm3/dL) or 195 mL per liter of cardiac output. The arterial O2 content of this child, assuming an average arterial saturation of 85%, will be 11.1 mL/dL. Therefore, every liter (10 dL) of cardiac output will carry 111 mL of O2 or 84 mL of O2 less than the child with a 15 g/dL hemoglobin level.