In patients with shock, how may ScvO2 readings fluctuate compared to SVO2?

In patients experiencing shock, ScvO2 (central venous oxygen saturation) readings tend to be approximately 5-7% higher than SvO2 (mixed venous oxygen saturation). This difference often arises due to the location from which the samples are taken. ScvO2 is measured from the superior vena cava, reflecting the oxygen saturation of blood returning from the upper body, while SvO2 is derived from the pulmonary artery, representing oxygen saturation from the entire body.

In conditions like shock, where the distribution of blood flow may be altered, blood returning from the upper body (reflected in ScvO2) can have a higher saturation level due to relatively better perfusion in that region compared to the lower body, which may be less perfused and thus have a lower oxygen saturation reflected in SvO2. This physiological distinction is crucial for monitoring and managing patients in critical care settings effectively. The 5-7% higher reading in ScvO2 can sometimes give clinicians detailed insights into patient status, helping in guiding treatment decisions based on the adequacy of oxygen delivery and utilization.