Number of cardiac cycles required to accurately determine mean velocity of blood flow in the ascending aorta and pulmonary trunk

Abstract

Instantaneous blood flow for any site measured by Doppler echocardiography is calculated as the instantaneous product of flow area (cm 2) and mean velocity (cm/s). Flow area is calculated from diameter measurements (d 2/4 × π) in great vessels. Blood flow per unit time is computed by summing the instantaneous products over time. A high correlation of diameter measurements compared with angiographic measurement in the aorta has been reported (r = 0.97; standard error of the estimate = 0.75 mm). Accordingly, the expected error (±2 standard deviations) induced by inaccurate aortic measurement should be about 7% for a 20-mm aorta. 1 Mean velocity measurements are determined in a variety of ways, but the probability of measurement error has not been evaluated in a systematic manner. Off-line methods require hand digitization of the modal velocity of a number of heartbeats recorded on a strip-chart recording. Although numerous beats can be digitized with relative ease, this method requires a computer separate from the ultrasonograph, a digitizing pad and software program. Another method for determining mean velocity requires that velocity traces be digitized on the ultrasonoscope by establishing a number of points with a joystick or attempting to trace over them with a trackball. Both of these input mechanisms are tedious and few echocardiographers digitize more than 1 or 2 cardiac beats with this method. Finally, approximation formulas using ejection time and amplitude have been suggested. 2 However, the number of cardiac beats that should be included for any of these methods to achieve the lowest error has not been established. We have digitized 5 or more beats to keep variation in flow due to respiration and other physiologic factors to a minimum. 3 In this study, we estimated the magnitude of error introduced if fewer than 6 beats are used in determination of mean velocity in the ascending aorta and pulmonary artery of normal persons.