Global O2 consumption V̇O2 (mL/min) is the volume of O2 that is consumed by the body per minute.
During aerobic metabolism, V̇O2 is closely matched to the body’s metabolic rate. Resting V̇O2 for a 70‑kg man is typically 250 mL/min (at sea level and standard temperature). During exercise, V̇O2 increases as the body consumes additional O2 to power muscle contraction. As exercise intensity increases, V̇O2 increases. There comes a point where V̇O2 is limited by the rate of O2 delivery to the tissues, which is likely due to diffusion limitation being reached in the muscle microcirculation. When this happens, the muscles switch from aerobic metabolism, where O2 is consumed, to anaerobic metabolism, where energy is obtained from glycolysis. The O2 consumption at this transition point is termed V̇O2max and is closely related to maximal exercise performance (see Chapter 43). Training increases the V̇O2max and therefore performance.
Global V̇O2 can be calculated using the reverse Fick principle:
where CO (L/min) is the cardiac output, CaO2 is the O2 content of arterial blood (mLO2/100 mL of blood), CvO2 is the O2 content of venous blood (mLO2/100 mL of blood), 10 is a unit conversion factor and
(see Chapter 8), where [Hb] (g/dL) is the Hb concentration.
The equation above essentially states that the V̇O2 is the same as the O2 ‘taken out of the arterial blood’, as reflected in the difference in arterial and venous O2 contents. CO and CvO2 can be measured with the aid of a pulmonary artery catheter (as CvO2 should really be measured using mixed venous blood) and CaO2 can be measured by peripheral arterial blood gas analysis.
Global O2 delivery ḊO2 (mL/min) is the volume of O2 delivered to the tissues from the lungs per minute. ḊO2 can be calculated using the O2 flux equation:
where 10 is a unit conversion factor.
What is a typical resting global oxygen delivery?
Using the O2 flux equation with typical values for a resting patient breathing room air (CO = 5 L/min, [Hb] = 15 g/dL, SaO2 = 98%, PaO2 = 13 kPa), first the CaO2 is calculated: