Indicator Recirculation as a Limiting Factor of Indicator Dilution Techniques

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Abstract
Short injections of Evans blue dye were made into the aortic root, pulmonary artery and femoral vein of dogs under normotensive and hypotensive conditions. The time intervals between the initial appearance of the indicator at the arterial sampling site and its first reappearance at the site (‘reappearance time’) were determined. Indicator concentration levels were measured simultaneously at the aortic root and femoral artery during the period of each injection. After the aortic root injections the indicator was rapidly cleared at the sampling sites so that the onset of recirculation was directly detectable from the time-concentration curve. Following pulmonary artery and femoral vein injections reappearance time was derived by the Hamilton semilogarithmic extrapolation procedure and the mean values were compared for reappearance time determined directly and indirectly. Under normotensive conditions it was found that the onset of recirculation can be adequately approximated by the Hamilton procedure when short injections are made into the pulmonary artery, but this approximation becomes less reliable in hypotension. When indicator is injected into a peripheral vein, the arterial time-concentration curve becomes so distorted that the true onset of recirculation cannot be detected by the Hamilton method. For the same injection, the mean values for reappearance times obtained at the femoral artery sampling site were greater than those obtained at the aortic root sampling site.

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