IS THE INDICATOR DILUTION THEORY REALLY THE ADEQUATE BASE OF MANY BLOOD-FLOW MEASUREMENT TECHNIQUES

The indicator dilution theory is the underlying model of many blood fl ow measurement techniques used daily in hospitals, for instance in car diac catheterization laboratories. The basic version of this theory ap plies to a ''stationary'' flow system with one inlet and one outlet, i nto which a small amount M of indicator is injected ''suddenly'' at ti me t=0 at the inlet. The quintessence of the theory consists in three equations, which themselves result from some apparently simple assumpt ions about the considered flow systems. The first equation states that the (constant) flow Q through the system can be calculated by use of the known amount of indicator, M, and of the indicator concentration-t ime curve c(t) recorded at the outlet. The second one allows the calcu lation of the ''mean transit time'' t of fluid and indicator particle s through the system from the curve c(t). The third equation, V=Qt, y ields the system volume V. It is generally believed that these three e quations would be absolutely valid if the assumptions of the theory co uld be perfectly fulfilled. We show, by considering a simple model, th at all three equations are actually incorrect for most flow systems wh en the detector used to record the curve c(t) is of the ''trans-illumi nation'' type, as is the case for instance in dye dilution methods and in many angiographic or CT techniques. A further consequence is that t, which is truly the ''center of mass'' of the concentration-time cu rve c(t), does not have the well known property of being the adequate parameter for flow determinations. Many flow measurement techniques th us appear to have no theoretical base. (C) 1997 American Association o f Physicists in Medicine.