8.1.5 Arrhythmias
8.1.5 Arrhythmias

The same principles apply to arrhythmias detected at a pre-purchase examas to murmurs, namely that a specific diagnosis of the type of arrhythmia should be made. Physiological arrhythmias include second-degree atriovenblock (20AVB) and sinus block. 20AVB is the most common vagally mediated arrhythmia and is usually characteristic at rest with predictable periods of sinus beats followed by a blocked beat, during which an atrial contraction sound can usually be heard in the absence of S1 and S2. The arrhythmia diswhen the heart rate is raised by exercise or excitement (see section 7.7.2). Sinus block may also be identified at rest and is abolished by increased sympathetic tone. These vagally mediated arrhythmias are of no significance for the purposes of purchase.

If abnormal arrhythmias are noted, the most significant considerations are their rate and any evidence of underlying cardiac disease.

Atrial fibrillation

Atrial fibrillation (AF) is a relatively common cause of poor athletic performance. Animals with AF are not suitable for sale as performance animals. Some animals can perform less arduous work normally while in AF, particularly if there is no underlying heart disease. These animals may be suitable for some forms of activity. The resting heart rate, heart rate during exercise, and the significance of any underlying valvular and myocardial disease should all be investigated if animals with atrial fibrillation are to be ridden (see section 7.8.5). However, purchase of any animal with AF is a risk, even though they may perform normally in the short-term for relatively undemanding events such as dressage, show-jumping and hacking.

Atrial premature complexes

Occasional premature beats are a relatively common clinical finding and present a difficult problem at a pre-purchase examination because occasional atrial premature beats (APCs) can occur in normal horses. APCs result in the first heart sound (S1) being detected earlier than expected. It may also be louder than with sinus beats. APC are not usually followed by a compensatory pause (see section 7.8.3).

APCs are usually more frequent after exercise and if they are numerous during this period horses may not be suitable for athletic use until the arrhythmia has resolved. An ECG may be required to distinguish APCs from vagally mediated arrhythmias during the period of autonomic imbalance as it may not be possible to distinguish them on auscultation alone. Runs of APCs are more likely to be clinically significant than those which occur intermittently or cyclically. As a rough guide, 1-3 APCs per minute would prompt further investigation, without which purchase should not be recommended. If more than 3-10 APCs per minute are detected, the horse should not be recommended for purchase until the arrhythmia has resolved. This may need to be supported by radiotelemetricand 24-hour Holter recordings in order to ensure that the arrhythmia is not occurring intermittently (see section 7.8.3).

Premature ventricular complexes

Premature ventricular complexes (VPCs) are most commonly found in animals with myocardial disease, severe systemic disease or marked electrolyte imbalAnimals in the second of these categories are unlikely to be subject to a pre-purchase examination. Occasional VPCs may be detected in normal animals. However, since the period of auscultation during a pre-purchase examination is relatively short, if a premature beat is detected, purchase should not be recommended without further investigation.

VPCs are recognised by auscultation of an early beat, in which S1 is often relatively loud, and are usually followed by a full compensatory pause (see section 7.8.8). An ECG, echocardiogram and laboratory investigations may be justified if more than the occasional beat is detected, should the prospective purchasers wish to pursue the case. If these tests show that there is no evidence of underlying heart disease or systemic disease, re-examination in a few months time might be worthwhile. Otherwise, purchase should not be recommended until the condihas resolved, if ft does so. If VPCs are frequent or multiform, the animal is unlikely to be suitable for purchase. If exercise results in an increased frequency athletic use should be curtailed and purchase should not be recommended. Some VPCs will be absent once the sinus node rate increases above a certain level. These are less likely to be significant, although they may be sufficient cause for concern to prevent purchase because the arrhythmia could deteriorate. Exercise ECGs and 24-hour recordings are useful for assessing the significance of VPCs if further investigations are considered (see section 7.8.8).

Post-exercise arrhythmias

Arrhythmias commonly occur in the immediate post-exercise period, while the heart rate slows, and can be a cause for concern. If exercise has been sub-maximal, there may still be sufficient vagal tone present for vagally mediated arrhythmias to be detected. The period is therefore known as a phase of 'autonomic imbalance'. Post-exercise transient sinus arrhythmia is a well-recognised rhythm; sometimes 20AVB also is present during this period (see section 7.7.2). A wandering pacemaker may be detected during the sinus arrhythmia and it may be difficult to determine whether this indicates the preof an APC, or is due to a change in the position of origin of the impulse within the sinus node. If these APCs/wandering sinus beats occur periodically, in a cyclical manner, they are unlikely to be clinically significant. Occasionally, blocked APCs will be seen. Runs of atrial tachycardia, or frequent APCs would be a cause for concern, and these animals may not be suitable for athletic use until the arrhythmia has resolved (see above).

If arrhythmias are heard during the period of autonomic imbalance during heart rate slowing, they may be abolished by an increased level of exercise. Therefore, if an arrhythmia is detected and an ECG or radiotelemetry cannot be performed, it is sensible to exercise the animal further to see if the arrhythmia resolves. Exercise ECGs and 24-hour recordings may be useful for assessing the significance of arrhythmias detected during the post-exercise section of a preexamination if further investigation is considered necessary (see sections 4.1.6 and section 4.1.8).