Indicator-dilution dispersion models and cardiac output computing methods. Millard, Roger K. DEPARTMENT of ANAESTHETICS, UNIVERSITY of BRISTOL, BRISTOL, UK.
APStracts 3:0534H, 1996.
The general theory of indicator-dilution methods provides a basis for computing improved cardiac output estimates. Interpretation is via indicator dispersion modeling, with Brownian motion of drifting particles. Detected curves indicate the distribution of passage times from the injection site: the local density random walk (LDRW) function of a Wiener process. Fitting LDRW to 70 dye curves by nonlinear regression for examples, we show how all possible undistorted curves can, in principle, be simulated. We show via semilogarithmic plots that conventional exponential decay constructs systematically underestimate cardiac output by up to 8%. To help reconcile the predictions of LDRW-fitted dilution curves and contemporary practice, we show how curve shape asymmetry (skewness), dramatically affects the enclosed areas. Mean transit times may overestimate blood volumes by 15-100% in very skewed thermodilution curves if dispersion effects are overlooked. Triangle constructions, that accounted for hundreds of experimental findings, also have theoretical explanations. Curve-fitting methods reduce the extrapolation biases inherent in many computers, and any respiration -induced artefacts. Compatibility of cardiac output predictions from various dilution methods and modules becomes feasible. 

Received 20 November 1995; accepted in final form 22 November
1995.
APS Manuscript Number H1092-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996