Doppler echocardiographic measurement of cardiac output: Comparison with a non-golden standard

Abstract

Doppler echocardiography (echo) is an exciting and expanding area in diagnostic cardiac ultrasonography. It involves the use of sound wave signals to measure blood flow velocity. Doppler echo has been of clinical value in the noninvasive detection and quantification of cardiac valvular stenosis, valvular regurgitation and intracardiac shunts and in the quantification of cardiac output (CO).l Recent technologic advances have led to the commercial introduction of Doppler devices integrated into 2-dimensional(2-D) echocardiographs, and to availability of small, easily portable Doppler devices designed for bedside measurement of CO. The principle behind Doppler measurement of CO is as follows: If the mean velocity of flow in a fluid in a tube and the cross-sectional area of the tube are measured, the net rate of flow through the tube can be calculated. In the case of CO, blood (the “fluid”) flow velocity is measured ultrasonically in the aorta (the “tube”). This is done by looking for a change in frequency between the emitted ultrasonic signals and the signals reflected off moving red blood cells. Blood flow velocity is then calculated from the Doppler equation V = Af X c/2f X cos 8, where V = magnitude of blood flow velocity; Af = the measured Doppler frequency shift; f = the known frequency of the emitted ultrasonic signal; c = the speed of sound in tissue (1,540 m/s); and fl = the angle between the direction of blood flow and the ultrasonic signal. of each cardiac cycle, depending on intraluminal pressure. Thus, these changes in aortic cross section, should, at least theoretically, be taken into account. Second, the Doppler method neglects coronary blood flow (which originates proximal to the point of measurement). Third, the velocity of blood flow is not uniform throughout a cross section of the aorta. What we really wish to measure is a mean velocity of blood flow across a transverse plane through the aorta. What we actually measure are velocities within a sample volume which is more or less representative of transverse plane. Finally, the amplitude of measured velocity is dependent on the angle between the direction of blood flow and the direction of the Doppler signal. We can only estimate the angle.