7.8.3 Atrial premature complexes
7.8.3 Atrial premature complexes

APCs (or premature atrial systoles, PASs) are a relatively common supravenarrhythmia. Occasional APCs are sometimes found in otherwise apparently normal horses and do not appear to limit athletic performance. Frequent APCs can be a sign of myocardial disease, electrolyte inbalance, toxsepticaemia, hypoxia or chronic valvular disease. They can be associated with previous episodes of respiratory disease and subsequent myocarditis. APCs may be a significant finding in animals with poor athletic performance. It is often unclear whether the effect on performance is directly due to the abnormal rhythm or is related to cardiac disease which is the underlying cause of the arrhythmia. When APCs occur, further investigation to establish the underlying cause is advisable. However, even after a thorough investigation it can be difficult to be certain of the effect of the arrhythmia on exercise tolerance.

Clinical examination

APCs occur due to abnormal impulse formation in the atrial myocardium. As the name implies, they occur earlier than a normal sinus impulse, causing a shortened P-P' and R-R interval. They originate from outside the SA node (ectopic), may be of a different configuration from the normal P wave (P') and may have a different P'-R interval. At high heart rates they may be lost in the preceding T wave. They often reset the SA node so that the subsequent P wave follows after a normal P-P interval (Figure 7.7). This is called a non-compenpause. It is a useful method of differentiating APCs from ventricular premature complexes (\,PCs), which are usually followed by a full compensatory pause (see below), although this distinction is not reliable because there are exceptions to the rule. On auscultation, therefore, most APCs can be recognised because they are preceded by a short diastolic interval and are followed by a normal diastolic interval. With a compensatory pause there is a long diastolic interval following the premature beat. The first and second heart sounds (S1 and S2) are often normal in intensity unless the APC is very early. However, very premature APCs may have a relatively loud S1 and quiet S2, and result in a marked reduction in stroke volume which may be palpated as a weak pulse or a pulse deficit.

Investigation of horses with APCs

When APCs are suspected on auscultation, it is helpful to document their preand frequency by recording an ECG. APCs may be more frequent during exercise than at rest, or may be detected during exercise in animals in which they were absent at rest. Radiotelemetric recording of the ECG at exercise or in the immediate post-exercise period is therefore invaluable. In horses with poor athletic performance associated with APCs, ECGs recorded during exercise may be the only way in which a diagnosis can be reached. Radiotelemetry is very helpful in determining whether APCs are occurring sufficiently frequently during exercise to limit performance. In some horses, APCs are present at rest or during the recovery period, but do not occur during exercise. In these cases, APCs are less likely to be associated with poor athletic performance.

APCs may be detected periodically during the period of heart rate slowing after exercise, particularly after sub-maximal exercise. In some cases it can be difficult to determine whether these early beats with a slightly different shape P wave are APCs or are a normal variation as part of sinus arrhythmia and a wandering pacemaker. Sometimes the atrial complexes are found to have been blocked at the AV node, resulting in an irregular rhythm. This may be as a result of the return of vagal tone during the period of autonomic imbalance and is a normal finding in some cases. As a general guide, if the APCs are single and occur in a cyclical fashion every four or five sinus beats for a limited period during shortened P-P' and R-R interval. They originate from outside the SA node (ectopic), may be of a different configuration from the normal P wave (P') and may have a different P'-R interval. At high heart rates they may be lost in the preceding T wave. They often reset the SA node so that the subsequent P wave follows after a normal P-P interval (Figure 7.7). This is called a non-compenpause. It is a useful method of differentiating APCs from ventricular premature complexes (VPCs), which are usually followed by a full compensatory pause (see below), although this distinction is not reliable because there are exceptions to the rule. On auscultation, therefore, most APCs can be recognised because they are preceded by a short diastolic interval and are followed by a normal diastolic interval. With a compensatory pause there is a long diastolic interval following the premature beat. The first and second heart sounds (S1 and S2) are often normal in intensity unless the APC is very early. However, very premature APCs may have a relatively loud S1 and quiet S2, and result in a marked reduction in stroke volume which may be palpated as a weak pulse or a pulse deficit.

Investigation of horses with APCs

When APCs are suspected on auscultation, it is helpful to document their preand frequency by recording an ECG. APCs may be more frequent during exercise than at rest, or may be detected during exercise in animals in which they were absent at rest. Radiotelemetric recording of the ECG at exercise or in the immediate post-exercise period is therefore invaluable. In horses with poor athletic performance associated with APCs, ECGs recorded during exercise may be the only way in which a diagnosis can be reached. Radiotelemetry is very helpful in determining whether APCs are occurring sufficiently frequently during exercise to limit performance. In some horses, APCs are present at rest or during the recovery period, but do not occur during exercise. In these cases, APCs are less likely to be associated with poor athletic performance.

APCs may be detected periodically during the period of heart rate slowing after exercise, particularly after sub-maximal exercise. In some cases it can be difficult to determine whether these early beats with a slightly different shape P wave are APCs or are a normal variation as part of sinus arrhythmia and a wandering pacemaker. Sometimes the atrial complexes are found to have been blocked at the AV node, resulting in an irregular rhythm. This may be as a result of the return of vagal tone during the period of autonomic imbalance and is a normal finding in some cases. As a general guide, if the APCs are single and occur in a cyclical fashion every four or five sinus beats for a limited period during heart rate slowing, they are unlikely to be associated with atrial disease or poor athletic performance. If, however, they occur in groups, at irregular intervals, or are present in animals with suspected atrial disease, they are more likely to be significant.

24-hour Holter monitoring can be a useful aid in horses with APCs because periods of paroxysmal atrial tachycardia or multiple APCs may occur at times other than those in which the animals is being examined or during a standard ECG recording.

When more frequent APCs occur (e.g. 1-5 per minute), it is more likely that underlying atrial disease is present. A thorough clinical examination should be performed, with particular note to evidence of AV regurgitation, even if no clinical signs appear to be associated with the APCs. Echocardiography should be used to assess the severity of valvular and myocardial disease. If significant mitral regurgitation (MR) is present, the prognosis is guarded. If tricuspid regurgitation (TR) is detected, it is possible that the valvular insufficiency is coincidental; echocardiography will help to indicate the severity of the condition. In a few cases, fractional shortening will be reduced suggesting poor myocardial function. In most cases, no obvious underlying pathology is found, and it is usually assumed that the arrhythmia is caused by atrial myocarditis. Often there is a history of a previous respiratory infection (see section 6.7.2). Haematology, and viral antibody serology may therefore be useful aids in some cases, and biochemistry should be performed to investigate the possibility of underlying systemic disease or electrolyte imbalance.

Management and treatment

When APCs are very infrequent and are asymptomatic, no treatment or change in management is warranted. When APCs are more frequent and are thought to be related to poor exercise tolerance, it is important that the horse is rested for approximately two to eight weeks to allow the problem to resolve. Pasture rest is usually sufficient. The animal should then be re-examined, preferably with the use of 24-hour ECG recordings and radiotelemetry, before being returned to training. A premature return to full work may result in further incapacity. However, on occasions rest alone may not be sufficient and other steps need to be taken. There may also be pressure from trainers to get the horse back to work in a shorter time period. Corticosteroids have been used for their anti-inflamaction in these circumstances, although their use is somewhat conDexamethasone can be used at a dose of 0.02~0. 15 mg/kg twice daily by mouth (see Appendix). The dose should be decreased over a 2-4 week period, initially reducing the frequency to daily and then every other day treatPrednisolone can also be used (0.02-1.0 mg/kg). Corticosteroids are contraindicated if an active viral infection is present. In addition, when these drugs are used, care should be taken to note any signs of laminitis. This is a recognised problem with the use of corticosteroids in horses, but is unlikely to occur with low doses in the short-term. Although there is a rationale for the use of steroids as soon as frequent APCs are detected, the author prefers a more conservative approach and uses them only if the horse fails to respond to rest, except in exceptional cases.

If the APCs are sufficiently frequent to cause obvious clinical signs at rest or at low levels of exercise, then treatment should include box rest. Usually an atrial tachycardia will be present; treatment is as described below.