Measurement of Cardiac Output

The most commonly used methods to measure cardiac output are the Fick and indicator-dilution methods. Both of these methods are invasive, and as such require cardiac catheterization for placement of a catheter into vessels close to the heart.

The Fick Method

The Fick principle is based on the measurement of the concentration of a substance entering and leaving an organ and on the total amount of the substance consumed, produced, or added. Since the total amount consumed or added will determine the concentration of the substance leaving the organ, flow may be calulate as the amount added per unit time divided by the concentration difference.

The physical law that underlies Fick’s principle is that matter can neither be created nor destroyed. Thus, the volume of oxygen taken up by the lungs must equal the amount of oxygen used by the tissues. Fick’s principle is expressed by the following equation:

Cardiac Output = Oxygen Consumption / Arteriovenous Oxygen Difference

The Indicator-Dilution Methods

These methods require the injection of a known amount of dye or other indicator substance into the circulation and the measurement of the dilution of this material during a known period of time. In practice, a known amount of the dye is injected into the right heart via a cardiac catheter. A few seconds later the dye concentration is detected in arterial blood by withdrawing small samples of blood continuously from an arterial catheter and measuring the dye concentration with a photo-sensitive device. The dye concentration gradually increases until it reaches a maximum, and then it begins to decline until a second rise in concentration occurs as a result of re-circulation.

“Thermodilution” is a further application of this principle, which is based on the injection of a bolus of ice-cold dextrose solution into the right atrium. A second catheter in the pulmonary artery with a thermistor tip provides a continuous temperature record. The degree of temperature dilution depends on the flow, and similar to dye dilution, a temperature-time cure permits estimation of the cardiac output.