**4. Oxygen consumption and oxygen extraction**

**Oxygen consumption (VO2)** by cells and tissues is dependent on their energetic requirements and expenditure. It is defined as the amount of oxygen consumed per minute. The more metabolically active a tissue is the more oxygen it will require to maintain that activity. Therefore, oxygen consumption is a measure of energy expenditure. It can be expressed as the equation (Eq. 4) given below where CmvO2 is the mixed venous oxygen content:

$$\text{VO}\_2 = (\text{CaO}\_2 - \text{CmvO}\_2) \times \text{CO} \tag{4}$$

From equation 4 it can be appreciated that consumption is an estimate of global oxygen usage calculated using the difference in oxygen content in arterial and venous blood standardized to CO or the cardiac index (cardiac output in relation to body surface area).

At rest, given normal hemoglobin concentration and adequate cardiac output, oxygen delivery exceeds oxygen consumption levels with only 25% consumed of the total oxygen delivered. This number represents an average consumption of all tissues across the body. With adequate oxygen delivery, the central venous oxygen content (ScvO2) is greater than 70%. When metabolic demand increases (e.g., fever, shivering), oxygen extraction increases, and central venous oxygen content may fall.

**Oxygen extraction ratio (O2ER)** represents the amount or fraction of arterial oxygen content that is consumed as blood crosses across a tissue bed [2]. The equation for the oxygen extraction ratio is given below (Eq. 5) and is the difference between arterial O2 content and venous O2 content divided by arterial O2 content

$$\text{CO}\_2\text{ER} = (\text{CaO}\_2 - \text{CvO}\_2) / \text{CaO}\_2 \tag{5}$$

Normally, this ratio is around 0.2–0.3 which indicates the abundance of delivered oxygen [1]. The actual extraction ratio varies with different tissues depending on the basal metabolic rates. For instance, the brain and myocardial tissue extract the most oxygen when compared to other organs. Conversely, the kidney and liver extract the least oxygen. Tissues such as myocardial tissues are therefore more dependent on oxygen delivery and are more susceptible to ischemia as they are unable to extract more oxygen. Additionally, certain tissues can increase their oxygen extraction (e.g., skeletal muscles during heavy exercise) depending on changes in their metabolic demands.

**Figure 2** summarizes the relationships between oxygen delivery consumption and extraction. As oxygen delivery falls, extraction rises in order to maintain a fixed consumption level required by tissues. With increased extraction, ScvO2 begins to fall. Consumption is maintained by increasing oxygen extraction levels up until the critical point. This phase is termed the supply independent phase as a decrease in delivery will not

**Figure 2.** *Oxygen delivery and consumption relationship.*

reduce consumption. Beyond the critical point, extraction can no longer maintain the cells metabolic demands and consumption begins to fall linearly with decreasing oxygen delivery. This phase is termed the supply dependent phase. In this phase ischemia sets in and lactic acid begins to accumulate due to oxygen deprivation and anaerobic respiration.
