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Cardiac Output

Dye Dilution Method

The dye dilution method is done by rapidly injecting a dye, indocyanine green, into the right atrium of the heart. The dye flows with the blood into the aorta. A probe is inserted into the aorta to measure the concentration of the dye leaving the heart at equal time intervals [0, T] until the dye has cleared. Let c(t) be the concentration of the dye at time t. By dividing the time intervals from [0, T] into subintervals Δt, the amount of dye that flows past the measuring point during the subinterval from $$t=t_{i-1}$$ to $$t=t_i$$ is:

$$(concentration)(volume)=c(t_i)(F\Delta t)$$

where $$F$$ is the rate of flow that is being calculated. The total amount of dye is:

$$\sum_{i=1}^nc(t_i)(F\Delta t)=F \sum_{i=1}^nc(t_i)(\Delta t)$$

and, letting $$n\rightarrow\infty$$, the amount of dye is:

$$A=F\int_{0}^{T} c(t)dt$$

Thus, the cardiac output is given by:

$$F=\frac{A}{\int_{0}^{T} c(t)dt}

$$

where the amount of dye $$A$$ is known, and the integral can be determined using the concentration readings.

The dye dilution method is one of the most accurate methods of determining cardiac output during exercise. The error of a single calculation of cardiac output values at rest and during exercise is less than 5%. This method does not allow measurement of 'beat to beat' changes, and requires a cardiac output that is stable for approximately 10 s during exercise and 30 s at rest.