6.2.5 Clinical guides to prognosis
Judgement of the significance of MR on clinical grounds alone can be difficult. The intensity of a murmur is not an accurate guide to the severity of MR. However, in general, loud murmurs are more often associated with significant disease than quiet murmurs. It can be helpful to note the radiation of the murmur; if the regurgitation is moderate or severe, the murmur is likely to radiate widely. In cases in which a thrill is palpated, significant regurgitation of blood is likely to be present. The position of the apex beat and the area over which heart sounds are heard may be more caudal than usual in cases with severe MR and cardiomegaly. Percussion may also reveal an enlarged cardiac silhouette in these cases. Pulse quality is seldom affected in animals with MR, except in those cases with CHF. The pulse rate is a more useful guide to the response of the heart in compensation for the haemodynamic effects of MR. In order to maintain cardiac output in the face of a significant regurgitant fraction, heart rate will be increased. It is very difficult to judge the appropriate heart rate for horses after exercise, particularly in animals other than racehorses. However, measurement of resting heart rate is very useful, as a persistently raised heart rate often accompanies severe MR. In the author's opinion, any heart rate above 45 bpm, when the animal is genuinely at rest, requires investigation to rule out non-cardiac causes. However, MR can be sigin horses with a resting heart rate below this level.
Arrhythmias may be found in animals with MR and should be characterised, using electrocardiography if necessary. Physiological arrhythmias (see section 7.7) are less likely to be found in animals with significant valvular disease than in normal horses because vagal tone is usually less marked. The presence of abnormal rhythms, particularly atrial and ventricular premature beats and atrial fibrillation, in animals with MR, is significant. These findings suggest that the valvular disease has caused, or is associated with, myocardial dysfunction or chamber dilation. Although the effects of exercise on the intensity of cardiac murmurs is an overvalued part of the clinical examination (see section 3.8.2), in some cases where valvular disease is present, arrhythmias may only be present during exercise. Auscultation after exercise and, ideally, radiotelemetry during exercise is helpful in order that these abnormalities are detected.
Sequential examinations can help to give a more accurate prognosis in horses with MR than a single examination because the rate of progression of the concan be monitored. It is seldom possible to know the rate at which MV disease will progress, but it must be assumed that the degenerative change will continue, resulting in an increased regurgitant fraction over a period of months or years. Sudden deterioration of the condition, which may precipitate more severe clinical signs, may be associated with rupture of chordae tendineae as a result of the degenerative change, the development of an arrhythmia such as atrial fibrillation, or non-cardiac disease which results in a further load on the heart.
Many horses can still perform useful work with mild or moderate MR; howfew horses will do well in athletic competition with moderate MR. If poor athletic performance develops, it may be appropriate to reduce the level of activity. The principle danger is that the animal will tire easily and then become more likely to fall. Owners should be made aware of this possibility and it may affect the suitability of the horse for different riders. Collapse is very uncommon except in animals which are showing signs of cardiovascular compromise (e.g. tachycardia, jugular distension) at rest. These individuals should not be ridden. As a rule, problems are most likely to be encountered when athletic activity is increased beyond the level at which it has been established that the horse can manage satisfactorily. The most difficult situation is in young animals with mild MR and little performance history. In some cases, after echocardiographic appraisal and a discussion of the signs of exercise intolerance with the owners, it may be appropriate to continue to increase the level of work in order to establish what sort of performance the horse is capable of providing. The progression of MR should be monitored regularly. A follow-up examination approximately three months after the problem is first identified, and then at six-month intervals, is justified in moderate cases of MR.