Comparison of a two-compartment model and distributed models for indicator dilution studies

In nonstationary flow the estimation of the mean flow with an indicator dilution technique can be described by applying a two-compartment model for the system between the injection site and detection site. The aim of this study was to investigate whether the two-compartment approach could be used in experimental situations as encountered in nonsteady blood flow during artificial ventilation. For this reason a two-compartment function was fitted to probability density functions associated with distributed models. The choice of the distributed functions was based on their superiority over other approaches with regard to the representation of the dispersion process of the indicator. Moreover, reference functions enable the practicability of fitting the two-compartment model as a function of the skewness of the curve to be studied. The first passage times (f.p.t.) distribution and the local density random walk (l.d.r.w.) distribution were used as reference functions. Good similarity between the two-compartment curves and the distributed curves was established for the skewness range found in the indicator dilution curves (i.d.c.s.) obtained at the right and left sides of the heart. The closer similarity to the l.d.r.w distribution could be explained theoretically. Moreover, it is concluded that in this skewness range the ratio of the slopes of the ascending and descending limbs at the inflection points of a curve gives an estimate of the time-constant ratio of the compartments of the compartment model.