Consequence of varying input function on indicator dilution measurement of flow and average transit time

The authors investigated three input functions for indicator dilution measurement of blood flow and average transit time: impulse, constant infusion, and constant acceleration (ramp). As each input is the integral of the preceding, the output function should also be the integral of the preceding one, plus recirculation. The assumptions inherent in this interrelationship are that the system displays linearity, stationarity and constant mixing (Zierler). Measurements were made in patients using three input functions, two indicators, with simultaneous measurement of recirculation. Following correction for recirculation analysis of output curves indicate differences in measurements of flow and average transit time in the same patient. These observations raise doubt as to the validity of the assumptions in diseased vascular beds. The addition of a diffusion limited indicator, inulin, during a constant infusion revealed temporal oscillations, and the ramp marked delay. The impulse function tended to obscure these responses, particularly if the usual Hamilton method for correcting for recirculation was used. These data indicate that use of multiple inputs can reveal new information concerning vascular beds and that the assumptions inherent in the technique may not be valid.