8.2.4 Cardiovascular disease
The effects of different cardiovascular diseases on athletic performance were discussed in the appropriate chapter describing each condition. It is difficult to quantify the cardiovascular effects of heart disease exactly; evaluation of each case is a matter of judgement. As a rule, valvular heart disease is not as coma cause of poor athletic performance as is widely thought. However, arrhythmias which occur during exercise, and which may affect performance, may not be detected without the aid of exercise ECGs and may be under-diagnosed. Echocardiography allows an accurate estimate of the significance of valvular regurgitation in horses which are presented with a history of poor performance.
MR is the most common valvular condition resulting in poor athletic perforLocalised grade 2/6 murmurs are unlikely to be associated with poor athletic performance, but more widespread and louder murmurs should be regarded with suspicion. Animals with a grade 5/6 murmur of MR are unlikely to perform athletic work normally. Tachycardia and pathological arrhythmias, particularly AF, may be present if marked volume overload results from the valvular incompetence. Animals with moderate or severe MR are likely to start a race normally but tire quickly. Epistaxis is observed in some cases during exerAfter pulling up there is likely to be marked tachycardia and tachypnoea. The recovery period is usually prolonged. Echocardiography is the best method of assessing the severity of grade 3 and 4 murmurs (see section 6.2.6).
Many horses with AR perform normally. This may be partly accounted for by the fact that it usually affects older horses which often perform lower intensity work than young animals. However, in some circumstances, it may be responsible for poor athletic function. Where a tachycardia or abnormal pulse quality is detected, or when the condition is found in association with significant arrhythmias or MR, it is more likely that performance will be restricted. The murmur associated with AR can be extremely loud (even grade 5 or 6) in horses with normal exercise tolerance. When AR appears to be sudden in onset it is more likely to affect race times. Other causes of poor performance should also be investigated. However,where other causes do not appear to account for the signs seen, echocardioand possibly blood pressure measurement are useful guides to severity (see section 6.5.3).
TR is a common finding in National Hunt racehorses which have a normal performance history, or even an excellent racing record. It may therefore be an incidental finding when it is detected in a horse which is presented for poor athletic performance. Other causes of poor performance should be investigated, but when no other significant findings are made, an echocardiographic assessexamination should be performed (see section 6.3.3).
Congenital heart disease
Complex congenital heart disease is incompatible with athletic performance. Small restrictive ventricular septal defects (VSDs) are found in some animals which are able to record useful racing performances, but seldom in the very best athletes. Affected animals may be able to perform less demanding work satisNormal right ventricular size, normal interventricular septal motion and a defect of less than 2.0-2.5 cm are good prognostic signs. Doppler echo-cardiography is very useful; a velocity of greater than 4.0-4.5 m/sec indicates that the defect is likely to be restrictive (see section 5.5.5).
AF is the most common arrhythmia to cause poor athletic performance. Few horses will perform well in competitive events while in AF. AF should therefore be considered as the likely cause of poor performance in animals presented for this reason. Treatment is usually required for them to recover normal perforlevels. If treatment is successful and there is no significant underlying heart disease, it is likely that the horse will return to previous levels of perforAnimals which need repeated treatment may perform normally between bouts of AF. Some horses with AF have significant underlying heart disease and may not respond to treatment or recover normal performance even if they can be treated successfully.
Although animals have been reported to win races while in paroxysmal AF it usually results in poor performance. It can be very difficult to diagnose. ParoxAF usually only occurs during the stress of a race rather than at home on the gallops or on a treadmill, although there are exceptions. Unless the horse is examined before normal sinus rhythm returns it may be difficult to prove that paroxysmal AF is the cause of fading. It may be worthwhile visiting the raceto record an ECG if the episode of poor performance is repeated. Exercising and 24-hour ECGs may be helpful in establishing that an arrhythmia is the cause of the problem. Although the recordings may not show AF, an increased number of APCs may be detected and this would make a horse more likely to develop paroxysmal AF during a race. Fractional excretion tests may be useful because electrolyte disturbances predispose the horse to AF (see section 2.3.6). Echocardiography may be worthwhile to ensure that there is no evidence of underlying heart disease.
Atrial premature complexes, atrial tachycardia and ventricular arrhythmias
APCs and/or VPCs may be detected during a routine clinical examination or during heart rate slowing after exercise. It is frequently difficult to determine the significance of these abnormalities with regard to athletic performance. ExerECGs are invaluable in determining whether APCs occur more frequently or less frequently during exercise and therefore whether they are likely to affect performance. Similar tests to those described above for paroxysmal AF should be performed to determine whether there is any predisposing factors. Treatment and management of these conditions were discussed in section 7.8.3 and section 7.8.8.
Myocarditis may be suspected if arrhythmias are detected. Investigation should include echocardiography to assess myocardial function and identification of any other conditions which predispose to arrhythmias. 'Heart strain' has been a popular concept in some areas of the world; however, there is little evidence that training can have any deleterious effects on the heart, except in exacerbating the effects of a virally mediated myocarditis T wave abnormalities have been said to indicate the presence of heart strain, but there is no clear evidence that they are reliable indicators of cardiac disease. They may be found in animals with poor athletic performance, however, they are also found in horses with good racing records (see section 4.1.4).
A summary of causes of poor athletic performance is shown in Table 8.3. Useful diagnostic aids are summarised in Tables 8.4 and 8.5.