Fick Cardiac Output Measurement

The Fick method for determination of cardiac output is a form of the indicator dilution technique in which exogenous indicators are not required, but instead, transported oxygen serves this purpose. This method was first proposed in 1870 by the German physiologist Adolph Fick, who described a means to determine blood flow by measuring overall oxygen uptake and content in blood.^{[680] [681]} The Fick equation relates cardiac output to oxygen consumption and blood oxygen content and is recognizable as a special form of the generalized indicator dilution equation.





where = cardiac output (L/min)
V̇O₂ = oxygen consumption (mL O₂ /min)
CaO₂ = oxygen content of arterial blood (mL O₂ /100 mL blood)
Cv̄O₂ = oxygen content of mixed venous blood (mL O₂ /100 mL blood)

Multiplying the denominator by 10 converts deciliters to liters (10 dL = 1L).

The denominator of the Fick equation is the arteriovenous oxygen content difference. This difference is calculated from the arterial and mixed venous oxygen contents.

CaO₂ = hemoglobin concentration (g/dL) × 1.36 mL O₂ /g hemoglobin × arterial hemoglobin saturation (%)

CO₂ = hemoglobin concentration (g/dL) × 1.36 mL O₂ /g hemoglobin × venous hemoglobin saturation (%)

In general, the contribution of dissolved oxygen is ignored in these calculations because it is small (0.003 × the arterial or mixed venous partial pressure of oxygen).

To calculate cardiac output by the Fick method, arterial and mixed venous blood must be drawn for blood gas analysis. The latter requires pulmonary artery catheterization to collect a true mixed venous sample. In addition, oxygen consumption must be measured; traditionally, this measurement required collection of the patient's exhaled air over a period of several minutes with cumbersome equipment. In an attempt to simplify and automate the Fick technique, newer approaches have used pulse oximetry, pulmonary artery oximetry, and on-line respiratory gas analysis or indirect calorimetry to measure oxygen consumption.^{[682] [683] [684]} Although clinicians sometimes estimate the Fick cardiac output by assuming a constant basal value for oxygen consumption of approximately 200 to 250 mL/min, this approach is likely to be inaccurate in critically ill patients.^[643] Normal values for all these variables are given in Table 32-14 .