6.5.3 Further diagnostic aids
Echocardiography can be very helpful in the confirmation of a presumptive diagnosis of AR and in evaluating the significance of the condition, particularly because the intensity of the murmur is often of little value in determining the severity of the disease. A number of characteristic echocardiographic features may be seen in horses with AR. These were summarised in Table 4.11. Abnormal valve motion which results from regurgitant flow of blood is sufficiently common for the examination to be a useful method of confirming the diagnosis; however, the most important consideration is to evaluate the effect of the conin terms of the volume overload which may accompany it. This can be very substantial. It is the best guide to the severity of the condition and is very useful in making a suitable judgement regarding the future athletic use of the animal.
Examination of the aortic valve is best performed from the right parasternal long- and short-axis views at the level of the valve. In the long-axis view the dilation of the aorta at the level of the sinuses of Valsalva is easily seen. The left coronary cusp and sinus are most clearly seen because the beam bisects these structures. The right coronary cusp or non-coronary cusp are seen in the near field. In the short-axis view the valve is said to have a 'Mercedes-Benz appearance, with the coronary arteries clearly seen (Figure 4.15). Echogenic lesions may be seen on the valve cusps in either of these views (Figure 4.21), and these may be the cause of AR or may be incidental findings in animals with no murmur. The valve structure appears more echogenic if it lies perpendicular to the line of the beam; this may artefactually give the appearance of thickening of the valve. For example, the tip of the left coronary cusp always appears to be thickened in the long-axis view. Real lesions may be missed in the long-axis view but are more easily identified in the short-axis view. In some animals with AR detected on auscultation, no clear echogenic lesion will be seen.
High-frequency vibration of the aortic valve cusps may be identified during diastole and is best seen using M-mode echocardiography. Flailing of the tip of a valve leaflet is found occasionally. In this situation, the murmur is often very loud and musical. A small piece of valve will be seen fluttering on the ventricular side of the valve like a flag in the wind. On rare occasions, the valve ruptures and a whole cusp can flail into the LV. Often the septal mitral leaflet is seen to vibrate; it may even fail to open fully as a result of the action of the jet. This has been termed 'relative mitral stenosis'. However, the flow of blood through the valve is more likely to be affected by changes in diastolic LV pressure than a jet impinging on the valve. In some animals, the IVS will vibrate if the jet is directed towards it.
In addition to confirming a diagnosis of AR and assessing its effects on cardiac function, it is also important to investigate the presence and the significance of other conditions which may have a bearing on treatment and prognosis. The most important complicating conditions are endocarditis and MR. Large valvular vegetations alert the clinician to the presence of endocarditis (Figure 4.20). If MR is present in addition to AR, the prognosis for future athletic use and eventually for life are diminished. It is therefore important to evaluate aortic and MV structure and to assess LAD, in addition to measuring LVD. AR may be detected in some cases of congenital heart disease. It may complicate a ventricular septal defect if the aortic root prolapses into the defect resulting in distortion of the valve. This would reduce the prognosis for athletic use in a horse with a VSD.
Having confirmed the diagnosis of AR, the most important part of the examination is to assess the severity of the condition. This can be judged on a number of features. Measurement of LVD is the most helpful guide to the severity of the disease. However, in horses with AR, the same amount of volume overload is less likely to be clinically significant than in animals with MR. Opinions differ as to the dimensions at which volume overload is sufficient to warrant advising against continued use of the animal. This advice, as always, is a matter of judgement, depending on the athletic demands and individual cirAs a rough guide, the author would not recommend the use of animals with a LVDd> 15cm for hard athletic use and one with a LVDd> 16cm under any circumstances (in an adult Thoroughbred horse). However, these are only arbitrary guidelines; there are anecdotal reports of Thoroughbred horses with LVDd measurements in excess of 16cm competing successfully, but others have poor exercise tolerance with LVDd measurements of <15cm. Other crisuch as the presence of myocardial disease or arrhythmias also have to be taken into account.
In animals with severe AR, there will be exaggerated motion of the IVS, which may give the appearance of continuous motion towards the RV throughout diastole. If the LV diastolic pressure rises to very high levels as a result of the volume of blood regurgitating through the aortic valve, the MV may close in mid-late diastole instead of remaining open throughout the conduit phase of diastasis. In these situations the prognosis is likely to be poor.
Doppler echocardiography is helpful for identifying the retrograde flow of blood through the aortic valve during diastole, and for assessing the severity of the disease. The extent of the jet of AR, mapped using pulsed-wave DE, is a semi-quantitative measure of severity. Very extensive jets suggest a significant degree of regurgitant flow. However, this guideline is not as useful with AR as with MR. The area of the origin of the jet of blood is another indication of the severity, with a large area of origin at the valve being associated with a larger quantity of regurgitant blood flow. A small amount of regurgitant flow is found in many apparently normal animals. The velocity of the regurgitant jet is related to the pressure gradient across the valve during diastole.
Blood pressure measurement
Arterial blood pressure measured either directly, or more commonly indirectly using a pressure-cuff system such as a 'Dynamap', allows the pulse profile to be quantified (see section 4.5). In severe AR, there is an increase in the difference between the systolic and diastolic pressures, which is why the arterial pulse feels strong. This is largely due to the increase in the systolic pressure in main arteries, which is a consequence of the large stroke volume which results from severe regurgitation, and run-off of pressure as blood regurgitates back into the LV. If myocardial failure occurs, the systolic pressures may drop and the arterial pulses may feel weaker; however, this is usually an end-stage event. Blood pressure measurement is thus a relatively simple way of assessing the severity of AR.
Clinical evaluation of horses with AR is summarised in Table 6.4.