Applications of ultrasound in cardiology and cardiovascular physiology.

Summary Ultrasound, in contrast to lower frequency audible sound, may be transmitted into tissue as a straight beam with relatively little divergence. Since these high frequency sound waves travel through a medium in a predictable, characteristic manner, it has been possible to develop instruments which employ ultrasound to delineate structures or measure changing parameters within the conducting medium. Changes in cardiac dimensions may be measured by recording the time required for pulses, traveling at a known velocity, to pass between transducers mounted on opposite sides of the heart chambers. The consistency of the propagation of ultrasound has also permitted the development of the ultrasonic flow meter. Each of two transducers, which are mounted diagnonally across a vessel, functions alternately as a sender or as a receiver of short pulses of ultrasound. Since the speed of pulses going upstream is retarded by the flowing fluid while the downstream pulses are speeded, the difference between the upstream and downstream transit times reflects the velocity of flow of the fluid. The heart chamber or vessel which is to be studied must be exposed for application of the gauges. However, externally applied ultrasound has been used to record cardiac motion from the intact animal and man. This is possible since some portion of an ultrasound beam traversing the chest will be reflected at each interface of tissue of different acoustical impedance. In contrast to the transmission principal of the flowmeter, the two technics now employed to study cardiac motion are based upon the detection of the echoes returned from moving acoustical interfaces. In one method, a continuous beam of ultrasound is directed toward the heart. The Doppler frequency shift of the returning echoes indicates the motion of structures moving in the plane of transmission of the beam. Although the time of motion of the heart valves and chambers may be determined in this manner, it does not give a curve of continual motion. The second method employs pulsed ultrasound and is based on transit time of pulses and returning echoes rather than Doppler shift of frequency. Since the speed of transmission of ultrasound in tissue is known, the position of the reflecting surface relative to the transducer can be determined by the time interval between the transmission of a pulse and the return of the echo. With rapid pulsing, an essentially continual curve of motion can be obtained. With the transducer applied in the third or fourth left interspace, a prominent curve of motion can be obtained which appears to reflect movement of the mitral valve. This has a distinctive configuration with two peaks of anterior motion during each cardiac cycle. One peak is related to atrial activity and is not seen in the absence of effective atrial contraction. The other peak is quite prominent and occurs in early ventricular diastole. In the normal individual, this peak is followed by rapid movement posteriorly during the period of ventricular filling. In persons with mitral stenosis, however, the ultrasound pattern is distorted in a distinctive manner due to a delay of this posterior movement. Following successful mitral valvulotomy, the ultrasound pattern is changed toward normal but none of our postoperative patients have had an entirely normal pattern. In the patient with predominant mitral insufficiency, the velocity of motion of the curve during ventricular diastole is normal or exceeds the normal range. In some individuals, records may be obtained which appear to represent tricuspid valve motion. Some workers have also reported echoes which seem to originate from the aortic valve. However, these echoes are obtained with considerable difficulty. Only that curve which has been analyzed in relation to mitral valve function has been adequately assessed to indicate some practical clinical usefulness in cardiac diagnosis.