**3. Oxygen delivery**

For proper oxygenation of tissues, oxygen delivery is essential and is achieved by circulation of oxygenated blood. To understand the mechanisms and factors that influence delivery one must first understand certain terms and their interconnected influence on oxygen delivery.

Firstly, the term **oxygen delivery (DO2)** refers to the rate at which oxygen is delivered per unit time to cells, tissues and organs. **Oxygen consumption (VO2)** subsequently is the rate at which oxygen is consumed per unit time by a cell, tissue or organ [1]. We'll begin by discussing oxygen delivery and the essential factors that influence delivery.

Oxygen delivery is dependent on two factors, **Cardiac output (CO)** and **arterial oxygen content (CaO2)**. A change in one of these can decrease or increase the amount of global oxygen delivery. Physiologically, these factors are not independent and changes in one will be compensated with changes in the other to maintain adequate DO2. These components of DO2 can be expressed (Eq. 1) as:

$$\text{DO}\_2 = \text{CO} \times \text{CaO}\_2 \tag{1}$$

#### **3.1 Arterial oxygen content**

At any given time, blood leaving the left ventricle will be oxygenated to a certain degree expressed as the **arterial oxygen content (CaO2)**. This term refers both to the amount of **hemoglobin saturated with oxygen (SaO2)** and the amount of **oxygen dissolved in blood (PaO2)**. Furthermore, since hemoglobin accounts for most of arterial oxygenation, the **concentration of hemoglobin in blood (Hgb)** is also an important determinant of CaO2 (**Figure 1**).

These three factors determining arterial oxygen content can be mathematically represented by the following formula (Eq. 2):

$$\mathbf{CaO\_2} = (\mathbf{Hbg} \times \mathbf{1.34} \times \mathbf{SaO\_2}) + (\mathbf{PaO\_2} \times \mathbf{0.003}) \tag{2}$$

As can be appreciated from this equation (Eq. 2) hemoglobin concentration and saturation account for a vast majority of arterial oxygenation with dissolved oxygen only making a fraction of the total oxygen level.

*Shock Pathophysiology: Classifications and Management DOI: http://dx.doi.org/10.5772/intechopen.105506*

**Figure 1.** *Determinants of CaO2*.

#### **3.2 Cardiac Output**

Circulation of oxygenated blood allows for oxygen to reach the most distal parts of the body. Circulation is determined by the heart's functionality which is represented by cardiac output (CO). Factors that influence cardiac output are the **Stroke Volume (SV)** and the **Heart Rate (HR)**. The relationship of these measurements to cardiac output is expressed in the equation below (Eq. 3):

$$\text{CO} = \text{SV} \times \text{HR} \tag{3}$$

Stroke volume is defined as the amount of blood pumped from the left ventricle into the aorta within a single contraction. Three factors determine stroke volume: **Preload, Contractility,** and **Afterload**. These components are often difficult to directly assess clinically and are often estimated using indirect methods and assumptions. Preload refers to the amount of end-diastolic stress or pressure exerted on the walls of the left ventricle influencing myocardial sarcomere length. The major determinant of preload is venous pressure and subsequent venous return. Other factors influencing preload include ventricular wall compliance, atrial contractility and valvular resistance. End-diastolic volume is usually used as an estimate for preload. Contractility refers to the rate of sarcomere shortening during contraction. It represents the functionality of cardiac muscle and is influenced by stimulation via catecholamine and concentration of electrolytes such as calcium, magnesium and potassium. Afterload refers to the force against which the left ventricle contracts and is defined as the left ventricular wall stress.
