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Blood is nonhomogeneous; hence, the relationship between light transmission and increasing concentration of dye in whole blood is never the perfect exponential curve predicted by Beer's law. Instead, as the concentration of indocyanine green is increased to high levels (40 mg/liter) the light transmission decreases exponentially toward an asymptote at 6–8% transmission for nearly monochromatic densitometers (half-band width: 13–20 mμ), but at 30–40% for densitometers using light of wide-band width. Consequently, following recording of a dilution curve, circulating background dye reduces the change in transmission per unit increase in dye concentration in subsequent curves. This decrease in sensitivity cannot be compensated for by a simple increase in the sensitivity of the densitometer or in the intensity of its light source. Compensation can be attained, however, if increasing densitometer sensitivity is associated with the automatic scale expansion provided when a suppressed zero point is used. At correct zero suppression, the deflection for zero output of the densitometer coincides with the asymptotic transmission value mentioned above. indicator-dilution in circulation; blood flow measurement; blood optical density; dye dilution technic; cardiac output measurement Submitted on April 15, 1963

In studies on dogs, blood was sampled as for the recording of dye-dilution curves through monochromatic densitometers. Injection of hypertonic media into the superior vena cava caused transient increases in the optical density of blood in the pulmonary and femoral arteries so as to produce apparent “dye” curves. The injection of hypotonic and hemolyzing media produced negative deflections, representing decreases in the optical density of blood, as did brief ventilation with 5% and 50% carbon dioxide in oxygen. Similar changes were demonstrated in vitro. They were compensated for in vivo by the use of a dichromatic densitometer. The changes observed are attributed to altered physical properties of the erythrocytes, which consequently change their light-transmitting and light-reflecting characteristics. Such nonspecific changes in the optical density of blood may constitute an important source of error in monochromatic densitometry done on whole blood. Submitted on January 11, 1961

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