A simple algorithm for defining the mean cardiac cycle of aortic flow and pressure during steady state.

A fast procedure for defining a cardiac cycle using simultaneously recorded and digitized aortic flow and pressure is presented. A simple algorithm, based on a double-threshold method, initially involves singling the dicrotic notch of flow in order to separate contiguous cardiac cycles during a given steady state. The individual cycles are carried back to a common origin of time, then they are normalized to the mean length and averaged. As a result of an averaging operation the algorithm gives a "mean cycle" of both pulsatile aortic pressure and flow. An "a posteriori" analysis of the noise components in the data has been carried out in order to justify the averaging operation. The "mean cycle" of aortic flow and pressure are suitable to be used as the input quantities of the automatic identification procedures recently assessed to estimate the parameters of simple models of the arterial input impedance. Our algorithm was defined and implemented as a FORTRAN program for a digital PDP 11/24 computer. This algorithm was tested by using pressure and flow data measured in the ascending aorta of dogs. About 26 sec were necessary to select 10 cardiac cycles (each one being about 200 samples long) of both flow and pressure in sequences of 2500 samples per signal and to compute the respective "mean cycles." Total peripheral resistance, total arterial compliance, and aortic characteristic impedance were estimated by aid of the simple three-element windkessel model. The results obtained by our method of determining parameters on the "mean cycle" of aortic pressure and flow were compared to the results obtained by averaging the parameters determined on each heart cycle.