What is measured in the numerator of the modified Fick equation?

The modified Fick equation is utilized in the context of assessing cardiac output, particularly in patients requiring advanced support like ECMO. In this equation, the numerator is focused on the difference in oxygen saturation between the arterial and venous blood. This difference is critical as it reflects the amount of oxygen that the tissues have extracted from the blood. When measuring cardiac output using the modified Fick equation, knowing how much oxygen is taken up (which is represented by the saturation difference) is essential for understanding the effectiveness of the heart's pumping action and the overall oxygen delivery to tissues.

In this manner, the numerator effectively captures the essential aspect of oxygen consumption in the body, which is vitally important for those using ECMO and assessing the underlying circulatory and respiratory function. This allows clinicians to evaluate how well the ECMO is performing in terms of oxygenating the blood and, ultimately, ensuring adequate tissue perfusion.