How does a decrease in cardiac output affect mixed venous oxygen saturation (SvO2) in a patient with stable oxygen consumption?

A decrease in cardiac output, with stable oxygen consumption, leads to a decrease in mixed venous oxygen saturation (SvO2). Cardiac output is the amount of blood pumped by the heart per minute. SvO2 represents the percentage of oxygen bound to hemoglobin in blood returning to the right side of the heart. It reflects the balance between oxygen delivery to tissues and oxygen consumption by tissues. When cardiac output decreases, less blood is delivered to the tissues per unit of time. If oxygen consumption remains stable (meaning the tissues are using the same amount of oxygen), then the tissues must extract a greater percentage of oxygen from each unit of blood that is delivered. This increased oxygen extraction results in a lower oxygen saturation in the blood returning to the heart, leading to a decrease in SvO2. For example, if cardiac output is normal, tissues might extract 25% of the oxygen from the blood, leaving an SvO2 of 75%. If cardiac output decreases by half, and oxygen consumption remains the same, tissues must extract 50% of the oxygen from the blood, resulting in a lower SvO2 of 50%. Therefore, a decreasing SvO2 in the setting of stable oxygen consumption indicates that the tissues are extracting more oxygen than usual due to reduced blood flow, which can be a sign of inadequate oxygen delivery or circulatory compromise.