Echocardiography uses echoes of ultrasound waves to map the heart and study its function. To provide detailed images, ultrasound wavelengths of 1 mm or less are used, which correspond to frequencies of 2 MHz (1 MHz = 1 000000 cycles s’) or more. At such high frequencies, the ultrasound waves behave more like light and can be focused into a ‘beam’ and aimed at a particular region of the heart. The waves are generated in very short bursts or pulses a few microseconds long by a crystal transducer, which also detects returning echoes and converts them into electrical signals.
When the crystal transducer is placed on the body surface, the ultrasound pulses emitted encounter interfaces between various body tissues as they pass through the body. In crossing each interface, some of the wave energy is reflected, and if the beam path is approximately at right angles to the plane of the interface, the reflected waves return to the transducer as an echo. Since the velocity of sound in body tissues is almost constant (1550 m S-I), the time delay for the echo to return measures the distance of the reflecting interface. Thus, if a single ultrasound pulse is transmitted, a series of echoes return, the first from the closest interface, and so on, until the distance becomes too great for further echoes to be detected. To document in detail the motion patterns of individual structures, a technique called M-mode is used. The echo signals from a particular beam direction are recorded as a column of dots on a roll of photosensitive paper which is pulled past the cathode-ray tube display at constant speed. Stationary structures thus generate straight lines, the distances of which from the top of the paper indicate their depths, and movements, such as those of heart valves, are indicated by zig-zag lines.
Calibration markers indicate depth at 1 cm intervals and lines along the edges of the paper show time intervals of 0.04 s. It is customary to add an ECG trace as an aid to identifying the phases of the heart cycle. Alternatively a series of views from different positions can be obtained in the form of a two-dimensional image (cross-sectional 2-D echo cardiography) (Fig. 11.22a,b,d). This method is useful for delineating anatomical structures.
This technique is being increasingly used. The ultrasound probe in the oesophagus which is nearer the heart. It is particularly useful for detecting dissecting aneurysm. Doppler echocardiography Echocardiography imaging utilizes echoes from tissue interfaces. Using high amplification, it is also possible to detect weak echoes scattered by small targets, including those from red blood cells. If the blood is moving relative to the direction of the ultrasound beam, the frequency of the returning echoes will be changed according to the Doppler phenomenon. The Doppler shift frequency is directly proportional to the blood velocity. Blood velocity data can be acquired and displayed in several ways. Continuous-wave (cw) Doppler collects all the velocity data from the path of the beam and analyses it to generate a spectral display. The outline of the envelope of the spectral display shows the value of peak velocity throughout the cardiac cycle. Normal velocities are of the order of 1 m s-I, but if there is an obstructive lesion, such as a stenotic valve, velocities of 5 m S-1 or more can occur. These velocities are generated by the pressure gradient that exists across the lesion. According to the Bernoulli equation:
Pressure gradient = 4 x (velocity)”, This equation has been validated in a wide variety of clinical situations, including valve stenoses, and ventricular septal defects, and makes it unnecessary to resort to invasive methods to measure intracardiac pressure gradients in many cases.
CW Doppler does not provide any depth information. Pulsed-wave (Pw) Doppler extracts velocity data from the pulse echoes used to form a two-dimensional image and gives useful qualitative information. It can be thought of as a small intracardiac ‘stethoscope’ the location of which can be determined precisely. Doppler colour flow imaging uses one colour for blood flowing towards the transducer and another colour for blood flowing away. This technique allows the direction, velocity and timing of the flow to be measured with a simultaneous view of cardiac structure and function.
The echocardiographic examination
Echocardiography is a ‘non-invasive’ procedure that causes the patient no discomfort and is harmless. Studies are performed by a physician or technician and a comprehensive examination takes 15-30 min. The major problem of echocardiography is that access to the heart is restricted by the lungs and rib cage, both of which form impenetrable barriers to ultrasound in the adult subject. Small ‘windows’ can usually be found in the third and fourth left intercostal spaces (termed left parasternal); just below the xiphoid process of the sternum (subcostal); and, with the subject turned to the left and exhaling, from the point where the apical beat is alpated (apical). By positioning the transducer successively over these sites and angling and rotating it to align the scan plane, a series of standard sectional views is obtained. In children, and some adults, the aortic arch can be visualized from a suprasternal position. The standard nomenclature for two-dimensional echocardiographic images is shown The left parasternal position gives access to the long-axis and short-axis planes. The apical approach gives a second view of the long-axis plane, but with the apex in the foreground, and also shows the four-chamber plane. Note that the convention of showing the transducer position at the top of the image results in these views being ‘upside-down’.
M-mode recordings are obtained from the parasternal position to document motion patterns of the aorta, aortic valve and left atrium, the mitral valve, and the left and right ventricles .
The 1 cm calibration markers on M-mode recordings permit measurement of cardiac dimensions at any point in the cardiac cycle with an accuracy typically of ± 2- 3mm.
Comparison of end-diastolic and end-systolic values allows some parameters of cardiac function to be derived: for example, the percentage reduction in the left ventricular cavity size (,shortening fraction’ (SF)) is given by:
where LVDD is left ventricular diastolic diameter and LVSD is left ventricular systolic diameter. The echo cardiographic findings in particular conditions are discussed in relevant sections, but a brief overview is given below.
VALVE STENOSIS. Congenitally abnormal aortic or pulmonary valves show a characteristic ‘dome’ shape in systole because the cusps cannot separate fully and a bicuspid configuration may be demonstrated. The presence of calcium in a valve gives rise to intense echoes that generate multiple, parallel lines on M-mode recordings. CW Doppler directed from the apex measures velocity of the jet crossing the diseased valve, from which the pressure gradient can be calculated. In mitral stenosis, the M-mode shows restriction and reversal of direction of the posterior leaflet motion . A short-axis view shows the shape of the mitral0 orifice in diastole and its area can be measured directly from the image. Peak, mean and end diastolic pressure gradients can be obtained from CW Doppler. Additional imaging views indicate the size of the left atrium, and may show the presence of left atrial thrombus.
VALVE REGURGITATION. Doppler is extremely sensitive for detecting valve regurgitation and, indeed, demonstrates mild physiological regurgitation through the tricuspid and pulmonary valves in the majority of normal subjects. It is hard to quantify the amount of regurgitation with echo Doppler techniques but echocardiography is an excellent way to determine the underlying cause of valve regurgitation, e.g. rheumatic disease or mitral valve prolapse.
AORTIC ANEURYSMS AND DISSECTIONS. Dilatation of the aortic root can be measured accurately and the presence of a reflecting structure within the lumen of the aorta is strongly suggestive of an intimal flap associated with dissection.
PROSTHETIC HEART VALVES. Each type of heart valve prosthesis has characteristic echocardiographic features. Irregularity or restriction of movement can be shown on M-mode recordings. The presence of stenosis or regurgitation may be documented by Doppler.
INFECTIVE ENDOCARDITIS. Vegetations >2 mm can be detected.
CARDIAC FAILURE. Left ventricular function and response to treatment is readily assessed and should be performed in all patients with heart failure. CARDIOMYOPATHIES. Dilated cardiomyopathy is characterized by an enlarged, globular-shaped, thinwalled left ventricle with poor function and low stroke output shown by reduced movements of the valves.
In hypertrophic cardiomyopathy, the left ventricle is small, with a grossly thickened, immobile interventricular septum (asymmetric septal hypertrophy (ASH)). There is a characteristic, though poorly understood, displacement of the mitral valve apparatus towards the septum in systole (systolic anterior motion (SAM)). PERICARDIAL EFFUSION. Fluid in the pericardial cavity shows as an echo-free region between the myocardium and the intense echo of the parietal pericardium.
MASSES WITHIN THE HEART. Echocardiography is a sensitive method for detecting masses within the heart .
ISCHAEMIC DISEASE. Coronary arteries cannot be imaged adequately using echo techniques but images may be useful for the diagnosis of complications related to myocardial infarction, such as mitral papillary muscle rupture, tamponade or ventricular septal rupture. In the postinfarction period, echocardiography and Doppler are used to diagnose left ventricular aneurysm, left ventricular thrombus, mitral regurgitation and pericardial effusion as well as to assess left ventricular function (ejection fraction).
CONGENITAL HEART DISEASE. Echocardiography has largely replaced cardiac catheterization and angiography. The aim of the examination is first to establish the sequence of blood flow through the heart, and to define anatomical abnormalities.