"improving intensive care in Scotland"

Assessment and treatment of the acutely ill adult

The equations of life


Mean arterial pressure = cardiac output x systemic vascular resistance
Blood pressure is the product of flow and peripheral resistance

CO = HR x 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 contractilty.

DO2 = CO x 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 linked to it (Oxy-haemoglobin concentration). In shock and other acute conditions this system is disrupted eg in sepsis oxygenated blood is mal-distributed in the micro-circulation resulting in impaired oxygen consumption.

VO2 = CO x (CaO2 - CvO2)

VO2 is oxygen consumption which is flow weighted (related to cardiac output) and reflects the amount of oxygen utilised throughout the body (the oxygen content difference in arterial and mixed venous ie 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.

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