4.2.6 Interpretation of an echocardiogram
Measurement of cardiac dimensions
Echocardiography allows an objective quantitative assessment of the effects of cardiac disease, exercise training, physiological changes and drugs. It is most useful to measure the degree of volume overload associated with valvular heart disease, so that its significance can be assessed and an accurate prognosis for athletic use given. However, the value of quantitative methods depends on rigadherence to measurement guidelines, in particular obtaining the approimage plane for any one measurement. Measurements which are made without reference to these guidelines are positively unhelpful because they may lead to an incorrect judgement. The usefulness of measurements also depends on a suitable range of measurements from normal animals to compare with those made in the patient. Published normal ranges are now available for adult Thoroughbred horses, but are limited for other breeds. Cardiac dimensions tend to be greater in larger and more athletic breeds. However, as a general rule there is a weak relationship between heart size and body weight within each breed.
The most useful measurements are of LV and LA diameter, although LAD cannot be measured with as much accuracy as LVD. RV and RA size have to be assessed subjectively, with the aorta acting as a useful guide for comparison. Occasionally measurement of PA and aortic diameter can be helpful. Guidelines for measuring cardiac dimensions and a range of values obtained in normal adult Thoroughbred horses are shown in Table 4.7.
Measurement of cardiac dimensions allows calculation of figures which are a guide to cardiac function. The most commonly used echocardiographic paraof ventricular performance is fractional shortening (FS%); which is calfrom the formula shown in Figure 4.19. Fractional shortening is used as an estimate of myocardial contractility. However, this is only a guide and is very dependant on the loading factors which affect the contraction of the heart (see section 1.4.3). If the ventricle does not fill normally during diastole the FS% will be reduced. FS% is particularly sensitive to changes in afterload. An increase in systemic blood pressure or an increase in myocardial stiffness will therefore reduce FS%. FS% can also be influenced by the heart rate. Excitement may result in an increased FS% as a result of catecholamine release. Valvular heart disease will affect ventricular function before any change in myocardial contractility occurs. For example, mitral regurgitation will result in a decreased afterload because it acts as a let-off valve during systole. In addition, if the valvular disease is severe enough to have resulted in volume overload, preload may be increased. These factors increase FS% by decreasing systolic dimensions and increasing diastolic dimensions respectively. Once myocardial failure develops, FS% will fall.
Some ultrasound machines have software which can calculate stroke volume and ejection fraction from echocardiographic measurements. Unfortunately, these estimates are only as accurate as the original measurements, and depend on the use of appropriate formulae. They should not be regarded as accurate measurements, but may be a helpful guide in some circumstances.
Evaluation of the cardiac chambers
The overall shape of the ventricles and atria should be examined. The chambers become more globular in animals with marked volume overload. The septum may bow away from the enlarged chamber. The apex of the LV has a more rounded appearance in volume overload, although this can be created artefacif the beam does not transect the true long axis of the ventricle.
The structure of the cardiac chambers is examined to ensure that no defects are present. By far the most common defect is a VSD. This is most often recognised as a gap between the base of the IVS and the base of the aorta. It is usually seen in the long-axis view but may be more easily seen in the short-axis view or oblique views in some animals. Atrial septal defects (ASDs) are much less commonly recognised. Drop-out of the interatrial septum is a common artefact and must be distinguished from true defects. ASDs may occur at different levels within the septum. If they are real, the edges of the septum either side of the defect usually appear rather echogenic and thickened.
Analysis of myocardial texture is seldom helpful. The echogenicity of the myocardium is very dependant on the gain and post-processing settings of the machine, the angle of the beam, the depth of the tissue and the acoustic impedance of structures between it and the transducer. For example, in the right parastemal reference view, the middle of the IVS may appear more echogenic than other areas because it is perpendicular to the plane of the beam. The papillary muscles normally have a rather heterogeneous appearance and are more echogenic in comparison with the rest of the myocardium. Hypoechoic areas may be seen if there is dissection of blood into the myocardium, for example in the case of a rupture of the aortic root.
Masses within the chambers are unusual. Vegetations on the endocardium as well as valves may be seen in animals with endocarditis. However, unlike some other species, thrombi within the atria are seldom seen in the horse.
Where precise measurement of the size of chambers is not possible, a subassessment of their relative size is helpful. The aortic root only changes in size slightly during systole, and acts as a baseline for comparison with other structures. However, the root can be dilated by an aneurysm or the presence of a VSD and may be smaller than usual in low-output cardiac failure. Subjective comparison of RA and RV diameter with the size of the aorta and LV is partiuseful in assessing volume overload of the right side of the heart.
An important consideration in animals with severe left-sided heart disease resulting in pulmonary hypertension is measurement of pulmonary arterial size. Dilation of this vessel can terminate in rupture, resulting in sudden death. Animals with a dilated pulmonary artery are unsafe to ride.
The most significant echocardiographic feature of valvular lesions is their effect on cardiac dimensions and performance. However, lesions can be seen and may be significant findings. The most striking are the large vegetations found in most animals with endocarditis (Figure 4.20). The aortic valve is most commonly affected, although the MV is also affected in a significant number of cases.
Vegetative lesions are seldom seen because endocarditis is very uncommon. More often, a slight increase in the thickness and echogenicity of the AV valves is seen, or small nodular thickenings on the aortic valve may be present. The apparent thickness and echogenicity of valves is markedly affected by the seton the machine, in the same way as was described earlier for changes in myocardial texture.
Valves normally have areas which are more echogenic than others because they are more perpendicular to the line of the beam. For example, the tip of the left coronary cusp of the aortic valve often looks thicker and more echogenic than the rest of the valve. Absence of obvious thickening or nodules on a valve does not eliminate it as a site of disease and regurgitation and the AV valves quite frequently appear normal even when they leak. Nodular thickening of the valve, particularly the aortic valve (Figure 4.21), does not always result in valvular insufficiency.
Evaluation of the motion of valves and chamber walls
Abnormalities of the valves may be apparent only because of changes in their motion. Prolapse of the valve is defined as billowing of a leaflet into the chamber which it guards. It can be created artefactually if the plane of the beam does not truly bisect the valve; however, it can be real (Figure 4.22). Prolapse of the aortic valve is considered by some cardiologists to be the cause of high-pitched early diastolic murmurs. Prolapse of the MV is associated with late systolic murmurs, and may be the cause of systolic clicks. It may occur in the same individual on one occasion and not on the next. In the author's experience it is seldom associated with severe mitral regurgitation and volume overload, although moderate levels of regurgitation can occur. Prolapse of the TV is quite common and may not be clinically significant if it does not result in volume overload.
Occasionally a portion of a valve can be seen curling back into the preceding chamber. This is known as flail and results from ruptured chordae tendineae to the AV valves and torn semi-lunar valves. Most commonly these are seen on the left side of the heart. Ruptured chordae tendineae most often affect the right commissural cusp of the MV. Flail of this leaflet is best seen from a left paralong-axis view, when a portion of valve will be seen curling into the LA against the far wall (Figure 4.23). If the flail is not entirely in the line of the beam only an echogenic dot is seen in the LA during systole. A cine-loop facility or good quality video for slow replay are very helpful in identifying prolapse and flail because they are best appreciated on 2DE.
Flail leaflets frequently vibrate, resulting in very loud and often musical murThis high-frequency vibration is best seen on M-mode echocardiography. Vibration of a valve, IVS or ventricular free-wall may occur when it is struck by a high velocity jet of blood. This is most commonly found in animals with aortic regurgitation (AR), when the regurgitant jet strikes the septal mitral leaflet (Figure 4.24). It may even result in an apparent reduction in the extent to which the MV opens. Vibration of the aortic valve is also seen in many animals with AR (Figure 4.25).
Abnormal motion of the valves may be seen with some arrhythmias. For example, the A wave of the MV will be absent in animals with atrial fibrillation. Undulations of the valves and myocardium occur normally in animals with sinus or atrioventricular block. In horses with myocardial failure, abnormal motion of the walls may be seen. The IVS and LV free-wall may fail to contract synchroa feature known as dyskinesis. Septal motion is often exaggerated in animals with severe volume overload (Figure 4.26). The IVS may give the impression of bowing towards the RV with severe AR or mitral regurgitation (MR) (Figure 4.27). In RV volume or pressure overload, the IVS will appear flat or may bow into the LV (Figure 4.28). In severe cases, paradoxical septal motion may become apparent, with the IVS moving towards the LV free-wall during diastole and away during systole.
In many animals, a grey haze will appear in the cardiac chambers when the heart rate is slow. This is known as 'smoke'. It is usually most marked in the RA and RV. It may occur in normal horses and it is particularly obvious in sedated anibut also occurs in horses with valvular or myocardial disease. However, horses with severe disease will have more marked spontaneous contrast because they have slow intra-cardiac blood flow. There is no clear cut-off between normal and abnormal. Spontaneous contrast is unusual in other species and it is regarded as significant in horses by some veterinarians. However, in the author's opinion, it is a common finding in fit horses and by itself should not be taken to be a sign of abnormality. Its intensity is also very dependent on the settings of the ultrasound machine and monitor and the quality of the image. Larger 'particles' of echogenic material have been reported to be more common in animals which suffer from EIPH than in normal horses. These particles can also be seen in some normal animals, particularly immediately after deep inspiration.
The pericardium is a fibrous structure and, although it is very thin, it appears as a bright white line surrounding the heart on echocardiography. Usually it is in contact with air-filled lung, so no structures will be seen beyond this line. Occasionally it acts as a mirror to echoes, with reverberation artefact producing a mirror image of the heart in the lung beyond the pericardium.
A pericardial effusion appears as an anechoic band surrounding the heart, between the moderately echogenic myocardium and the echogenic pericardium (Figure 4.29). A pericardial effusion becomes most significant when it results in a sufficiently high pericardial pressure to restrict venous return, a situation called tamponade (see section 6.8.1). The first echocardiographic sign of tamponade is collapse of the RA. When it is more severe it may cause partial collapse of the RV during diastole. IVS motion may appear abnormal, partly because the heart can swing from side to side within the pericardium. Strands of fibrin may appear as echogenic fronds within the pericardial space in bacterial pericarditis. Very echogenic particles may result from gas bubbles indicating the presence of an anaerobic infection. These findings indicate the need for aggressive treatment even if tamponade has not occurred. Ultrasound may be helpful to guide drainage of the effusion to relieve the tamponade and analyse the fluid.
Constrictive pericarditis is much more difficult to diagnose echocardioThe pericardium may appear thickened. If there is sufficient fibrous tissue, restriction of diastolic flow may occur, with flow checked midway through early diastolic filling resulting in a step in the early diastolic motion of the MV on the M-mode trace.
The pleural space
Accumulation of fluid within the pleural space may be seen in animals with right-sided heart failure, or pleural disease. An anechoic or hypoechoic space can be seen between the pericardium and the chest wall. Further investigation of pleural fluid accumulations is required because it usually indicates severe disease.
A protocol for 2DE and M-mode echocardiography is shown in Table 4.8.