Anesthesia Management |
|Chapter 30:||Cardiovascular Monitoring|
Fick Cardiac Output Measurement
The Fick method for cardiac output determination is a form of indicator dilution 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, 591 who described a means to determine blood flow by measuring overall oxygen uptake and content in the blood. 592 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:
And 10 converts deciliters to liters (dL = 100 mL)
The denominator of the Fick equation is the arteriovenous oxygen content difference.
This difference is calculated from the arterial and mixed venous oxygen contents.
In general, the contribution of dissolved oxygen is ignored in these calculations because it is small (0.003 × 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 required collecting the patients exhaled air over 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. 593, 594, 595 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. 559 Normal values for all of these variables are given in Table 30–2.
TABLE 30–2. Normal Hemodynamic Values
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