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Since pulmonary artery (PA) catheters were introduced in the early 1970s, cardiac output (CO) measurement has become a readily available clinical tool for the diagnosis of hemodynamic disturbances in critically ill patients. Randomized studies have not proven that PA catheters yield improved outcomes; therefore, caution should be used prior to clinical application. As an alternative, less invasive technologies to provide CO monitoring are now available.


In 1870, Adolf Fick observed that CO could be calculated from whole-body oxygen uptake and the difference in the amount of oxygen between arterial and venous blood. The Fick equation is

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where CO is the cardiac output (often denoted as “Q”) (L/min), MVO2 is the minute oxygen consumption (mL/min), CaO2 is the arterial oxygen content (mLO2/100 mL blood), and CvO2 is the mixed venous oxygen content (mLO2/100 mL blood).

The content of oxygen in blood is quantified as

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where CaO2 is the content of oxygen in blood (mL O2/100mL), Hb is the hemoglobin concentration (gm/100 mL), SatO2 is the oxygen saturation (%), PaO2 is the partial pressure of oxygen (mmHg), and the dissolved (unbound) oxygen = PaO2 × 0.003, approximately 0.

This arterial–venous content difference can be rewritten within the Fick equation as

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The Fick principle is simple, but the need for the actual measurement of oxygen consumption and sampling arterial and mixed venous blood limit clinical utility. An arterial line and PA catheter are required to measure the difference in whole-body blood oxygen content. Spirometric, calorimetric, or other measures of oxygen uptake are required for an accurate MVO2. Attempts to estimate MVO2 introduce error as the demographic calculation of MVO2 to roughly 250–300 mL/min produces inaccurate estimates in perioperative or critically ill patients. Multiplying inspired minus expired oxygen (FIO2 – FEO2) by MVO2 and other indirect measurements of MVO2 have had mixed success. Fiber-optic PA catheters (“oximetric”), which obtain continuous colorimetric PA saturation, track relative changes in CO, but still require oxygen consumption and arterial data to be fully quantitative.

The Fick principle can be practically applied by measuring short-term uptake of gases other than oxygen. These marker gases include xenon, nitrous oxide, anesthetic vapors, helium, carbon dioxide, and others. By using uptake during short time periods, venous concentration approximates zero, the arterial concentration is close to end-tidal, and the equation avoids arterial–venous data. A transient carbon dioxide rebreathing method has been commercially developed for clinical use.

Thermodilution and Dye Dilution

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