Physiologic and Monitoring Practice Test 2026 - Free Practice Questions and Study Guide

The key idea is that SvO2/ScvO2 tells you how much oxygen remains after the tissues have extracted what they need. Normal central venous saturation is roughly 65-75%; when delivery is adequate, there’s still a comfortable amount of O2 left in the venous blood returning to the heart.

If SvO2/ScvO2 falls into the 30-50% range, the extraction by tissues is already near its maximum. In other words, tissues are pulling as much O2 as they can, and there isn’t enough delivery to meet demand. When this happens and DO2 isn’t increased, cells switch to anaerobic metabolism, producing lactate and leading to lactic acidosis. That low range is a red flag that the balance has shifted from supply meeting demand to a state of oxygen debt.

Ranges higher than this suggest extraction is still adequate, while even lower values would indicate a more severe mismatch.