Discussion

Both physiological and pathological cardiac dysrhythmias are common during prepurchase examination in horses and can occur at rest and/or during exercise. Careful auscultation, which means sufficiently long at both sides of the thorax, will often allow to make a diagnosis or at least have a strong suspicion about the origin of the arrhythmia and whether it is physiological or pathological. Attention should also be paid to presence of murmurs as these might be associated with arrhythmias (e.g. mitral regurgitation and atrial fibrillation or aortic regurgitation and ventricular ectopy). In certain cases the diagnosis or confirmation of the type of dysrhythmia can only be made by electrocardiography (ECG). Nowadays, ECG equipment is easy to use and affordable to private practice which allows recording at rest, during exercise or during 24 h on site. If one is not familiar with interpretation of the data, digital files can be easily sent to more experienced colleagues or online services.
During the PPE it is important for the practitioner to be able to distinguish between innocent, physiological dysrhythmias that do not require further examination, and pathological dysrhythmias that result in 'failure to pass PPE', and finally to point out dysrhythmias that require additional examinations. The first step for an additional exam is obviously to take an ECG, but in some cases, to get a full picture, one might request ultrasound and blood examination, depending on the expectations of the owner. Some cases might show subtle abnormalities such as occasional premature beats that need to be discussed with the clients. Although such dysrhythmias also occur in normally performing horses, the owner should be accurately informed about possible consequences or even risks for horse and rider. Naturally, all decisions and interpretations must always be done in the light of type of sport, level of sport, owner expectations, insurance, future re-sale, ...

Most important: differentiation between second degree atrioventricular bloc (2degree AVB) and atrial fibrillation (AF) 2degree AVB is the most common physiological dysrhythmia in horses appearing at rest when vagal tone is high. It is completely innocent as long as it disappears (often temporarily) with slight excitation of the horse. In 2degree AVB the underlying rhythm is regular but at regular intervals one beat (or sometimes 2) is blocked producing a pause that is exactly double (or sometimes triple) the normal interbeat interval. Careful auscultation will reveal an atrial sound (fourth heart sound) during the pause. 2degree AVB should be clearly differentiated from the most important pathological dysrhythmia in horses, atrial fibrillation (AF). Although AF might occasionally present paroxysmally, e.g. during and immediately after racing, in a PPE setting it will generally be permanent which means it sustains for ever unless treated. On auscultation AF appears as an irregularly irregular rhythm with a loud first heart sound, but be careful: in some horses the rhythm may mimic 2 degree AVB (although the atrial sound is absent during a pause). Therefore one needs to listen long enough because during AF a too early beat will always be found. The combination of loud first heart sounds, pauses without fourth heart sound, AND too early beats is by far most likely to be atrial fibrillation. Differential diagnosis could be an ectopic atrial and/or ventricular rhythm but is much less likely. The only way to get certainty is to obtain an ECG. Horses with AF, intended to be used for exercise, should fail the PPE. When auscultating an AF horse after exercise, increased heart rates are found and due to the high rate, the irregularity of the rhythm becomes less pronounced. These findings however have no clinical value. On rare occasions people may want to buy a (cheap) horse and treat the AF (quinidine sulphate or electrical cardioversion). If so, they should be aware that recurrence rate on long term is 20 - 35%. Increased resting heart rates in AF horses might be found when significant underlying disease is present and represent a grave sign but are unlikely to be found in a PPE setting.

Auscultation after exercise:
Remember that in the AF horse the high rate might attenuate the irregularity (more difficult to detect the dysrhythmia on auscultation). In normal horses, sinus arrhythmia may appear during recovery from exercise, typically when heart rate drops from 110 toward 50 beats/min. A waxing and waning of heart rate is found ('accordion' effect) Intensity of the heart sound remains more or less similar in sinus arrhythmia.

Bradydysrhythmias:
Bradydysrhythmias, such as 2 degree AVB, sinus arrest, sinus block or sinus bradycardia are usually vagally driven and should disappear immediately with exercise or excitation. If not one should consider advanced 2 degree AVB, or rarely, 3 degree AVB, and an ECG is required.

Other dysrhythmias:
On auscultation rapid tachydysrhythmias or occasional premature beats within an otherwise regular rhythm may be found whereby the intensity of the first heart sound often changes (a beat with a markedly louder first heart sound is likely to be a ventricular premature contraction). Frequent ectopy in the resting horses is not normal and should fail the horse at PPE although they sometimes completely disappear during exercise. In case of (sub)maximal exercise ECGs, occasional, isolated ventricular ectopy in the immediate post exercise period is regarded as normal. Additional examinations, such as blood exam (electrolytes, cardiac troponin) and ultrasound, are required to provide an advice on importance, safety or reversibility. The exact origin of ectopy, however, often remains unknown. The owner should be aware that a high burden of atrial premature beats might trigger atrial fibrillation. Ventricular ectopy on the other hand represents a risk factor for ventricular tachycardia, ventricular flutter or even ventricular fibrillation. The latter 2 can result in sudden collapse or even death: the owner needs to be informed about this risk.

Conclusion:
Dysrhythmias are commonly found at PPE. Often one can pass or fail the horse based upon auscultation alone. However, in case of doubt taking an ECG is easy to perform in private practice. With these 2 techniques one can decide whether to pass or fail the horse. A full cardiac work-up, including blood examination and cardiac ultrasound might be necessary to get a full picture of the disease.