The basic physics

If you inject a known amount of a substance upstream, the change in its concentration downstream is related to the rate of the flow. The flow, or volume over time, in this case is the cardiac output.

dye dilution measurement of cardiac output

The "known substance" is usually indocyanine green, a benign dye which is strongly protein bound and has a very rapid (~ 150sec) hepatic clearance.

Dye dilution concentration over time curve

dye dilution concentration over time curve

Data from this measured concentration curve can then be used to plug into the Stewart-Hamilton equation. George Neil Stewart first described it in 1897, after some animal experiments. His subjects were dogs, and his indicator was sodium chloride.

the Stewart-Hamilton formula


The Stewart paper on dye dilution from 1897:

Stewart GN. Researches on the Circulation Time and on the Influences which affect it. J Physiol. 1897 Nov 20;22(3):159-83.