What is meant by the term ‘global oxygen consumption’?

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 V̇O_2max and is closely related to maximal exercise performance (see Chapter 43). Training increases the V̇O_2max and therefore performance.

Global V̇O₂ can be calculated using the reverse Fick principle:

Key equation: reverse Fick principle

V̇O2=CO×CaO2–CvO2×10

where CO (L/min) is the cardiac output, C_aO₂ is the O₂ content of arterial blood (mLO₂/100 mL of blood), C_vO₂ is the O₂ content of venous blood (mLO₂/100 mL of blood), 10 is a unit conversion factor and

C_aO₂ = (1.34 × [Hb] × S_aO₂/100%) + 0.023 × P_aO₂

CaO2=1.34×Hb×SaO2/100%+0.023×PaO2

(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 C_vO₂ can be measured with the aid of a pulmonary artery catheter (as C_vO₂ should really be measured using mixed venous blood) and C_aO₂ can be measured by peripheral arterial blood gas analysis.

What is meant by the term ‘global oxygen delivery’?

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:

Key equation: oxygen flux equation

ḊO2=CO×CaO2×10

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, S_aO₂ = 98%, P_aO₂ = 13 kPa), first the C_aO₂ is calculated:

Related posts:

Chapter 9 – Carbon Dioxide Transport Chapter 16 – Ventilation–Perfusion Zones in the Lung Chapter 26 – Anaesthesia and the Lung Chapter 37 – Venous System Chapter 40 – Cardiovascular Reflexes Chapter 48 – Cerebral Blood Flow

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