The amount of oxygen in venous blood is the difference between the amount supplied by arterial blood and the amount extracted by the tissue. In the absence of significant anemia, hypoxemia, or increased/ decreased metabolic rate, venous oxygen (specifically, venous saturation – SvO2) is directly proportional to the cardiac output (CO). Higher cardiac output leads to blood moving fast by the tissue, so less oxygen is extracted, leading to higher SvO2. Lower cardiac output results in lower SvO2.
Ideally, the sample should be obtained from the distal port of the pulmonary artery catheter so the blood represents the mix of IVC, SVC, and coronary sinus content. In this case, the value is called mixed venous saturation. In practice, the sample can be obtained from SVC via a central line, and the value will be called ScvO2.
Remember that the most accurate result comes from Cooximeter, which provides direct measurement. A blood gases analyzer calculates the value using partial oxygen pressure, which makes it less accurate.