Indicator Dilution Method for Measuring Cardiac Output

To measure cardiac output by the so-called "indicator dilution method," a small amount of indicator, such as a dye, is injected into a large systemic vein or, preferably, into the right atrium. This passes rapidly through the right side of the heart, then through the blood vessels of the lungs, through the left side of the heart and, finally, into the systemic arterial system. The concentration of the dye is recorded as the dye passes through one of the peripheral arteries, giving a curve as shown in Figure 20-19. In each of these instances, 5 milligrams of Cardio-Green dye was injected at zero time. In the top recording, none of the dye passed into the arterial tree until about 3 seconds after the injection, but then the arterial concentration of the dye rose rapidly to a maximum in about 6 to 7 seconds. After that, the concentration fell rapidly, but before the concentration reached zero, some of the dye had already circulated all the way through some of the peripheral systemic vessels and returned through the heart for a second time. Consequently, the dye concentration in the artery began to rise again. For the purpose of calculation, it is necessary to extrapolate the early down-slope of the curve to the zero point, as shown by the dashed portion of each curve. In this way, the extrapolated time-concentration curve of the dye in the systemic artery without recirculation can be measured in its first portion and estimated reasonably accurately in its latter portion.

Once the extrapolated time-concentration curve has been determined, one then calculates the mean concentration of dye in the arterial blood for the duration of the curve. For instance, in the top example of Figure 20-19, this was done by measuring the area under the

Extrapolated dye concentration curves used to calculate two separate cardiac outputs by the dilution method. (The rectangular areas are the calculated average concentrations of dye in the arterial blood for the durations of the respective extrapolated curves.)

Figure 20-18
Figure 20-19

entire initial and extrapolated curve and then averaging the concentration of dye for the duration of the curve; one can see from the shaded rectangle straddling the curve in the upper figure that the average concentration of dye was 0.25 mg/dl of blood and that the duration of this average value was 12 seconds. A total of 5 milligrams of dye had been injected at the beginning of the experiment. For blood carrying only 0.25 milligram of dye in each deciliter to carry the entire 5 milligrams of dye through the heart and lungs in 12 seconds, a total of 20 1-deciliter portions of blood would have passed through the heart during the 12 seconds, which would be the same as a cardiac output of 2 L/12 sec, or 10 L/min. We leave it to the reader to calculate the cardiac output from the bottom extrapolated curve of Figure 20-19. To summarize, the cardiac output can be determined using the following formula:

Cardiac output (ml/min) =

Milligrams of dye injected x 60

r Average concentration of dye^

in each milliliter of blood v for the duration of the curve j

f Duration of^ x the curve ^ in seconds j

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  • Katariina
    What is the indicator dilution method of measuring cardiac output?
    11 months ago

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