Page 5, O2 Uptake, Exercise & Cor. Circ., Dr. D. Penney

Cardiac Output Determination

The oxygen utilization equation can be re-written such that, knowing A-V02 diff. and O2 consumption, cardiac output can be calculated. The following are needed to do this: whole body O2 consumption, arterial O2 content, and mixed venous O2 content. This is the classical (Adolph) Fick method which has been in use for over 100 years. The infusion of small amounts of the dyes Cardiogreen or Evans blue, or radioactively-tagged molecules have also been used to determine cardiac output by this method. Cold saline is used clinically today for this purpose (a thermal-dilution method).

In a typical dye curve, a few seconds elapse before the dye appears at the sampling window (colorimetric or spectrophotometric) (Figure 3.03). Dye concentration rises to a peak then falls, and another smaller peak follows as the dye is recirculated through various vascular beds. The area under the curve allows one to compute the mean dye concentration.

Other "dye dilution" methods are commonly used to determine red cell, plasma, blood, and extracellular volume, total body water, circulation time, and the presence of abnormal shunts.