Where should blood be extracted for analysis of mixed venous oxygen saturation?

Blood for analysis of mixed venous oxygen saturation is best extracted from the pulmonary artery. This is because mixed venous blood is defined as the aggregate of blood returning from the systemic circulation to the heart, which has already delivered oxygen to tissues. The pulmonary artery carries this mixed venous blood away from the right ventricle to the lungs for oxygenation.

When blood is drawn from the pulmonary artery, it reflects the overall oxygen content of blood returning to the heart from the systemic circulation, providing accurate measurements of mixed venous oxygen saturation (SvO2). This measurement is crucial for assessing a patient's oxygen delivery and utilization, especially in critical care settings.

The other options provide blood samples that do not represent mixed venous blood. For example, extracting blood from the pulmonary vein would yield oxygenated blood returning from the lungs, while blood from the vena cava will have mixed venous characteristics but may not accurately reflect the overall saturation due to regional variances. Blood drawn from the aorta represents fully oxygenated blood after it has passed through the lungs, not mixed venous blood. Therefore, choosing the pulmonary artery for this analysis is essential for obtaining the most relevant data regarding a patient's respiratory and circulatory status.