Cardiac Output Procedures

Cardiac output denotes the volume of blood pumped by the heart in one minute. Cardiac output increases as the demands the body puts on the cardiovascular system increase. Thus, changes cardiac output is greater during exercise than when at rest, notes "Exercise Physiology." Reduced cardiac output can be caused by various medical conditions, medications or drugs and even an electrolyte imbalance, notes Nursing Care Plans. There are three main ways of measuring cardiac output.

Direct Fick Method

The Fick method uses an equation developed by the notable German physiologist Adolph Fick in 1870. According to Fick's equation, cardiac output can be measured with the knowledge of two factors: the average difference between arterial blood and venous blood and the amount of oxygen consumed in one minute. In the equation, the amount of oxygen consumed in a minute is divided by the average difference in arterial and venous blood, which is then multiplied by 100, yielding cardiac output, according to "Exercise Physiology." Unfortunately, the Fick method it quite invasive, as it involves arterial blood sampling, as well as inserting a cardiac catheter and because of this it is limited to a clinical setting. However, despite its invasiveness, the Fick method is widely considered the standard, and is often used to validate other methods of measuring cardiac output, notes "Exercise Physiology."

Indicator Dilution Method

The indicator dilution method is similar to Fick's in that it also requires samples of venous and arterial blood; however, cardiac catheterization is not required. In this method, a large vein is injected with a specific amount of inert dye, like indocyanine green, which has been the most successful, or radioactive substance and a radioactive counter or photosensitive device continually assesses arterial blood samples. The equation used to calculate cardiac output in this method divides the quantity of dye or radioactive substance injected by the average dye concentration in blood samples multiplied by the duration, notes "Exercise Physiology." The indicator method is accurate, but like the Fick method, it is invasive and difficult to carry out. Additionally, the equipment and the dye are expensive.

CO2 Rebreathing Method

The CO2 rebreathing method uses a rapid CO2 gas analyzer and makes some reasonable assumptions regarding gas exchange to arrive at valid estimates of venous and arterial blood CO2 levels. This method is non-invasive and 'bloodless,' not requiring any blood sampling, notes "Exercise Physiology." What is required in the CO2 rebreathing method is a breath-by-breath measurement of CO2 levels. This method modifies the equation used in the Fick methods so that cardiac output equals the volume of CO2 divided by the difference in CO2 between veins and arteries multiplied by 100. This method has some obvious advantages when compared to the previous two. It does not require blood sampling or close medical supervision, and because it is so non-invasive, it may provide a more accurate estimate of the real cardiac output, particularly during exercise, than the other two strategies.