Direct Fick

The amount of substance passing into a flowing system is equal to the difference in concentration of the substance on each side of the system multiplied by the flow within the system. Cardiac output is thus usually calculated by dividing total body oxygen consumption by the difference in oxygen content between arterial and mixed venous blood. Alternatively, CO2 production can be used instead of VO2 as the indicator. Arterial CO2 can be derived non-invasively from end-tidal CO2 while mixed venous CO2 can be determined by rapid rebreathing into a bag until CO2 levels have equilibrated.

Advantages

'Gold standard' for cardiac output estimation.

Disadvantages

For VO2: Invasive (requires measurement of mixed venous blood), requires leak-free open circuit or an unwieldy closed circuit technique. Oxygen consumption measurements via metabolic cart unreliable if FIO2 is high. Lung oxygen consumption not measured by pulmonary artery catheter technique (may be high in ARDS, pneumonia.).

For CO2: Non-invasive but requires normal lung function and is thus not generally applicable in ICU patients.

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