On-line cardiac output by digital computer

Abstract

Many devices for fast cardiac output calculations employ special assumptions and departures from the Stewart-Hamilton method. These assumptions and departures can cause substantial errors. This publication describes an on-line computer system for evaluating dye-dilution curves according to the classical Stewart-Hamilton method. Dilution curves are smoothed, using a digital filter to eliminate heartbeat noise. Recirculation is removed using the constant ratio property of exponentials. The quality of the curves is evaluated by determining the length of the base line, the extent and the fit to an exponential downslope, and a figure of merit based on the five tanks-in-series model. Dilution curves, the logarithmic form, and the exponential ratios are displayed on a CRT for visual checks. Comparison of cardiac outputs obtained by this on-line system with results obtained by hand calculations showed a mean difference of 150 ml. Repeated evaluation of one electronically simulated dye curve resulted in SD = 10 ml for computer results and SD = 200 ml for hand calculations.