The total amount of oxygen flowing to the tissues is termed the oxygen delivery ( Do2) and is calculated (in ml/min/m2) as follows:
where CI is the cardiac index and CaO2 is the arterial oxygen content. CI is the ratio of CO to the patient's body surface area which is derived from standard nomograms based on measurements of the patient's height and weight. CaO2 is given (in ml/dl) by
where [Hb] is the hemoglobin concentration (in g/dl) and SaO2 is the percentage arterial oxyhemoglobin saturation. The dissolved oxygen is calculated by multiplying the arterial partial pressure of oxygen (Pa02) by 0.023 (for kPa) or 0.003 (for mmHg). The mixed venous oxygen content (C E^"02) is given (in ml/dl) by a similar equation:
where Sf^02 is the percentage mixed venous oxyhemoglobin saturation which is measured in blood taken from the pulmonary artery. Subtracting C f^02from Ca02 gives the difference Ca02- CV02 between the arterial and mixed venous oxygen contents, often inappropriately abbreviated to D(a - V)02.
The relationship between supply and consumption of oxygen can be calculated from the oxygen extraction ratio (OER) given (as a percentage) by
was developed in order to calculate CO from measured volumes and concentrations of oxygen in respired gases and from Ca02 and C V02. Now that CO can be measured routinely at the bedside by the thermodilution technique, an alternative form of the Fick equation (often inaccurately termed the reverse or indirect Fick equation) is used to calculate oxygen consumption (in ml/min/m2):
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