Left ventricular output derived from the time-derivative and phase velocities of the aortic pressure wave

A method of deriving aortic flow from pressures measured in the proximal aorta has been tested. The Womersley equation was used with the pressure gradient substituted by the time derivative of the pressure and the measured phase velocity c′ of each harmonic component. Reflected waves alter the value of c′ and are a source of error when a constant value of c is used, as in previous methods in which dP/dt has been used. The predicted mean flow was compared with that measured with a flowmeter. The results were N=720, r=0·97, y=1·00x+0·19 ml/s, s.e.e. ±12%. This was not significantly altered by a series of interventions which produced a wide range in stroke outputs. The prediction of a pulsatile flow wave was less accurate than that for mean flow. Analysis of the results shows that this is due to the fact that any method using the time derivative of the pressure cannot predict the phase of the input impedance; this leads to a distortion of the waveform on resynthesis. The amplitude of the impedance is accurately measured by the present method even when reflections markedly alter the wave velocity, as at slow heartrates. The results for predicting mean flow are shown to be better than those reported by other methods.