Doppler Echocardiography [antikvár]

Echocardiography has enabled us to visualize the intracardiac structures noninvasively. M-mode and two-dimensional echocardiography together have contributed substantially to our understanding of cardiac physiology in normal and diseased states. The limitation of M-mode technique is mainly the "ice pick" sampling of the heart by the narrow beam of ultrasound. However, when used along with two-dimensional echocardiography, it is possible to make accurate measurements of cardiac chamber dimensions and of indices of contractility and relaxation, as well as estimates of the ejection fraction. However, these techniques, have limited usefulness in diagnosing the presence and severity of valvar regurgitation and the severity of aortic valve stenosis. Movement of a stenosed valve orifice is a function not only of the anatomic severity of narrowing but also of the myocardial function and consequent cardiac output. One can overcome these limitations of echocardiography by measuring the velocity of blood flow and detecting the abnormal direction of blood flow, as well as by identifying areas of turbulent blood flow, by Doppler echocardiography. Continuous-wave Doppler echocardiography began in 1961 (Franklin et al), when it was used to measure blood flow in the peripheral arteries. It was found to be useful in the diagnosis of peripheral vascular disease, and has subsequendy been used for automatic devices to measure blood pressure. Introduction of pulsed Doppler echocardiography facilitated the measurement of blood flow velocity at preselected depths. This capability, when incorporated into a standard M-mode echocardiographic system, could identify patterns of smooth or turbulent blood flow in various cardiac chambers or vessels. However unless the ultrasonic beam was directed parallel to the blood flow, velocity of the blood could not be measured. This problem was overcome by incorporating Doppler echocardiography into the two-dimensional echo imaging systems, which permitted more precise spatial orientation of the Doppler beam and Doppler sample volume. Pulsed Doppler echocardiography was limited in that it could measure blood velocities of only up to 1.7 meters per second (m/sec). This problem has been pardy overcome by newer equipment, in which extended pulsed Doppler technique or continuous-wave Doppler facilities M 1 ¦ '!.« A