6.2.6 Diagnostic and prognostic aids
Echocardiography can be helpful to identify gross valvular lesions, generalised valve thickening, valve prolapse, or abnormal motion such as flail leaflet. However, in most horses with MR, little gross valvular change is visible on echocardiography. The main purpose of the examination is to identify any volume overload of the LA and LV in an animal in which MR has already been diagnosed from auscultation. In animals with no volume overload, it is likely that the MR is of limited clinical significance at the time of examination (although it does not mean that the condition will not deteriorate to this point at some time in the future). Animals with marked volume overload are likely to have clinical signs of decreased cardiac reserve or even CHF. Echocardiography is most helpful in enabling the clinician to make an objective assessment of the significance of MR in those cases where mild volume overload may have resulted in clinical signs such as decreased athletic performance.
Echocardiographic evaluation of MR combines examination of the structure and motion of the MV, and subjective and quantitative assessment of the conof the regurgitation in terms of the degree of volume overload and the effect on cardiac function. In order to evaluate MV structure fully, it is essential that long- and short-axis views are obtained from the right and left parasternal positions. The long-axis views are particularly helpful and may allow detection of localised or nodular thickening of the valve, or abnormal motion of the valve such as prolapse or flail of a leaflet. Both the right and left parasternal views are required because they transect the valve along different axes. For example, the right commissural leaflet is best seen from the left parasternal view because this view tends to be along a left cranial to right caudal axis, which takes it through this leaflet.
Quantitative measurements can also be made from both sides of the chest. Usually, LV diameter is measured from the right parasternal short-axis view at the level of the chordae tendineae, using 2DE to guide the position of the Mcursor. Measurement of LAD is made from a left parasternal long-axis view with the dimension of the atrium maximised (see section 4.2.4). Although LAD cannot be measured as accurately as LVD, it is a very useful measurement because it is a good guide to the significance of MR. A range of measurements from normal Thoroughbred horses was given in Table 4.7. As a very rough guide, adult Thoroughbred horses with MR resulting in LVD measurements at enddiastole (LVDd)> 14.5cm and LAD> 15 cm are unlikely to perform well in competition, those with LVDd> 15.5 cm, LAD > 16 cm should not be ridden at all. However, these figures are only approximate and should be considered in combination with other clinical information, and after due consideration of the circumstances of each individual owner and horse. Measurements for other breeds are not available. Generally speaking, large horses have larger cardiac dimensions than smaller animals, and fit, athletic horses have larger chambers than those which have little exercise training. Severe MR may result in pulhypertension, leading to dilation of the pulmonary artery (PA). Since sudden death due to rupture of the PA has been reported, these horses should not be ridden (see below).
In addition to assessment with two-dimensional echocardiography and measurement with 2DE and M-mode echocardiography, DE can be used to show the extent of the jet of MR within the LA. This is performed by mapping out the area of the LA over which the regurgitant jet can be detected (see section 4.2.7).
Quantitative echocardiography is the most objective method of assessing the severity of MR in clinical practice and is an exceptionally helpful technique. The echocardiographic features of MR were summarised in Table 4.9.
Electrocardiography is a useful technique in cases in which arrhythmias are detected at rest, during or after exercise. These may be caused by atrial dilation and may complicate the prognosis in horses with MR. However, ECGs are not useful in a clinical setting for the detection of either atrial or ventricular enlar
Clinical pathology is rarely of any value in the evaluation of MV disease except in animals in which endocarditis is suspected. There is no clear evidence of any association between MV disease and raised cardiac isoenzymes. Clinical pathology may be helpful in order to evaluate the presence of other diseases which may be related to MR, or which may result in similar clinical signs. The evaluation of horses with MR is summarised in Table 6.2.