Determination of Relative Changes in Cardiac Output from Noncalibrated Earpiece Dye-Dilution Curves

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Abstract
Many investigations require information on percentage changes in cardiac output rather than absolute values. This study investigates a simple method for measuring such relative changes, using noncalibrated earpiece dye-dilution curves. The basic principle involved is that, in an ideal system, the areas beneath successive dilution curves produced by equal injections of indicator are inversely proportional to the cardiac outputs. Applied to the earpiece dye-dilution curves, the restrictive assumptions relate mainly to linearity of response to blood concentration of dye and to the constancy of the blood volume of the ear. The method has been tested by recording simultaneous earpiece and arterial cuvette dye curves produced by injections of Coomassie Blue during changes in posture, exercise, and intravenous administration of vasoactive drugs. The relative changes in cardiac output predicted from the earpiece curves compared well with those from the arterial cuvette.