A NEW METHOD OF ANALYZING INDICATOR DILUTION CURVES

Objective: We are currently developing a new indicator dilution method of measuring cardiac output using lithium chloride as the indicator, The aim of the present study was to develop a simple and accurate meth od of deriving the area under the primary indicator dilution curve: th at is, the area which would have been inscribed had there been no reci rculation of the indicator. Method: A model based upon the representat ion of the mixing in the circulation as similar to that of the passage of an impulse through a series of filter elements was studied. This w as represented physically by a model which consisted of a series of mi xing chambers. The model was analysed theoretically using Laplace tran sforms and was used to test the new method of deriving the area of pri mary indicator dilution curves. Results: Theoretical analysis showed t hat such a filter model produces curves which closely approximate the shape of a lognormal distribution over a range of skewness greater tha n that of human indicator dilution curves. The single pass curves from the physical model were shown to be similar in shape to lognormal dis tributions, as were the curves obtained from patients to the point whe n recirculation occurred. A method of estimating the area under the pr imary curve based upon the lognormal distribution was developed and eq uations derived. The use of these equations to calculate flows from li thium dilution curves in the mixing chamber model was validated by com paring the results with simultaneous timed collection. Conclusions: Th eoretical justification for treating primary indicator dilution curves as lognormal is presented. A simple method of deriving the integral o f the primary indicator dilution curve is described. It uses the whole of the curve up to a point short of recirculation, avoiding the probl em which can occur with the classical Hamilton extrapolation method wh en the cardiac output is low and recirculation distorts the primary cu rve in the early part of the washout.