103 Oxygen extraction index (O2ER)
The oxygen extraction ratio (O₂ER) is the fraction of the oxygen delivered to the tissues that they actually take up and use: oxygen consumption divided by oxygen delivery. At rest the body extracts only about a quarter of the oxygen delivered, leaving a large reserve. When delivery falls (low cardiac output, anaemia, hypoxaemia) or demand rises (exercise, fever), the tissues extract more, and venous oxygen saturation falls. Extraction depends on capillary blood flow, the diffusion distance and the permeability of cell membranes, in which cholesterol and phospholipids play an important role.
Clinical significance
Low extraction
Sepsis (microcirculatory shunting and mitochondrial dysfunction: tissues cannot use oxygen despite adequate delivery), high cardiac output states, cyanide or carbon monoxide poisoning, hypothermia. A high venous saturation with raised lactate is a warning sign.
High extraction
Low oxygen delivery (heart failure, bleeding, anaemia, hypoxaemia) or high demand (exercise, fever, shivering, seizures). A low central venous saturation (ScvO₂) indicates that delivery is barely meeting needs.
Organs differ widely: the heart extracts most of the oxygen it receives even at rest, so it must increase flow when demand rises; the kidneys, which receive far more blood than they need for their own metabolism, extract little.
for signs of poor circulation: cold, mottled skin, confusion, rapid breathing and low urine output.
References
- Dunn JO, Mythen MG, Grocott MP. Physiology of oxygen transport. BJA Educ. 2016;16(10):341–348.
- Duncker DJ, Bache RJ. Regulation of coronary blood flow during exercise. Physiol Rev. 2008;88(3):1009–1086. doi:10.1152/physrev.00045.2006
This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.
Published on 6 May 2024