avO2

avO2, short for arteriovenous oxygen difference, is a physiological parameter that quantifies the amount of oxygen extracted by tissues from circulating blood. It is defined as the difference between the oxygen content of arterial blood (CaO2) and mixed venous blood (CvO2) and is usually expressed in milliliters of O2 per deciliter of blood (mL O2/dL).

Calculation of avO2 relies on oxygen content values. CaO2 ≈ Hb × 1.34 × SaO2 + 0.003 × PaO2 and CvO2 ≈ Hb × 1.34 × SvO2 + 0.003 × PvO2, where Hb is hemoglobin concentration, SaO2 and SvO2 are arterial and mixed venous oxygen saturations, and PaO2 and PvO2 are their respective partial pressures. Therefore avO2 = CaO2 − CvO2. In clinical practice, arterial blood and mixed venous blood (typically from the pulmonary artery) are sampled to determine these values.

The avO2 difference is linked to oxygen delivery and consumption through the Fick principle: VO2 = CO × avO2, where VO2 is oxygen consumption and CO is cardiac output. Thus avO2 reflects tissue oxygen extraction and will rise when tissues demand more oxygen or when delivery is impaired. At rest, avO2 is typically around 4–5 mL O2 per 100 mL of blood; it can increase substantially during exercise or in states of increased metabolic demand.

Clinical significance and limitations: avO2 helps assess the balance between oxygen delivery and utilization. An increasing avO2 can indicate higher extraction due to reduced delivery or increased demand, while a low avO2 may suggest impaired extraction or adequate delivery with low demand. Limitations include its systemic nature, dependence on Hb and saturations, potential sampling errors, and regional variations in tissue oxygenation that it may not capture.