The equations of life are basic and vital equations which help us to understand physiology and to look after sick patients. Take your time to make sure that you understand them. Click on each box below to find out more.  

MAP = CO × SVR
Mean arterial pressure = cardiac output × systemic vascular resistance.

Blood pressure is the product of flow and peripheral resistance.

Cardiac output (CO)
CO = HR × SV
Cardiac output is the product of heart rate and stroke volume. Stroke volume relies on preload (mainly influenced by venous return and circulating blood volume), afterload (SVR) and cardiac muscle contractility.

Oxygen content of the blood (CaO₂)
The CaO₂ means the oxygen content of the blood. The amount of oxygen present in the blood is the sum of the:
– (O₂ carried by Hb) + (O₂ in solution)
= (1.34 × Hb × SpO₂ × 0.01) + (0.023 × PaO₂)
If Hb is stable, oxygen saturation is the most important variable. PaO₂ has only a minor effect in most circumstances.

Oxygen delivery (DO₂)
DO₂ = CO × CaO₂
DO₂ is oxygen delivery—the amount of oxygen leaving the left ventricle (and delivered to the respiring tissues in health), which is the product of blood flow and the amount of oxygen in the blood. In most situations this depends on the amount of haemoglobin and the level of oxygen bound to it (oxy-haemoglobin concentration). In shock and other acute conditions this system is disrupted, e.g., in sepsis oxygenated blood is mal-distributed in the micro-circulation resulting in impaired oxygen consumption.

Oxygen consumption (VO₂)
VO₂ = CO × (CaO₂ – CvO₂)
VO₂ is oxygen consumption. It reflects the amount of oxygen utilised throughout the body (the oxygen content difference in arterial and mixed venous, i.e. pulmonary arterial blood). In the acutely ill patient we should be aiming to reduce oxygen consumption as well as optimising oxygen delivery and this will be covered in detail in the appropriate modules.