EQUINE VETERINARY EDUCATION Equine vet. Educ. (2022) 34 (7) 347-350 doi: 10.1111/eve.13547


Clinical Commentary


Cardiac arrhythmias as a potential sign of systemic disease: Which laboratory tests are useful?


A. Decloedt Equine CardioTeam, Department of Large Animal Internal Medicine, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium Corresponding author email: annelies.decloedt@ugent.be


Keywords: horse; electrocardiography; bradycardia; ectopy; blood tests


Cardiac arrhythmias Cardiac arrhythmias can often be detected in horses, ranging from physiological arrhythmias driven by changes in vagal tone to potentially lethal arrhythmias such as ventricular tachycardia. Differentiating physiological from pathological arrhythmias is difficult based on auscultation alone, and electrocardiography (ECG) is usually needed for a definite diagnosis (Mitchell, 2019). The increased availability of telemetric ECG equipment and smartphone-based ECG recordings will undoubtedly lead to improved detection of cardiac arrhythmias in horses. However, the correct classification of arrhythmias can be complicated as standardized criteria for defining and reporting arrhythmias are lacking. For example, supraventricular and ventricular premature beats are traditionally differentiated based on the P wave presence and morphology, QRS morphology and the absence or presence of a compensatory pause (Reef et al., 2014). A supraventricular premature complex is typically described as an early P0 wave followed by a QRS complex with normal morphology and a so-called ‘non-compensatory pause’ caused by resetting of the sinus node. A ventricular premature complex typically has no associated P wave, an abnormal QRS morphology, and is followed by a ‘compensatory pause’ as the sinus rhythmis not interrupted. However, P waves are often hard to identify during tachycardia as they are buried in the preceding T wave or ST segment (Verheyen et al., 2010). In addition, the QRS morphology can also be altered in case of supraventricular premature complexes (Broux et al., 2016), while ventricular ectopy can sometimes result in QRS complexes which are very similar to sinus beat QRS morphology. Finally, the presence of a compensatory or non-compensatory pause is also not conclusive. In a study describing the arrhythmic events found in poorly performing trotters undergoing high-speed treadmill testing, the majority of isolated premature depolarizations showed compensatory or close to compensatory pauses while subjective QRS assessment revealed mainly sinus morphology (Slack et al., 2021). These ectopic beats would be classified either as ventricular premature beats based on the compensatory pause or as supraventricular based on the QRS morphology. Therefore, Slack and colleagues suggested that in future studies objective descriptions of cardiac arrhythmias should be preferred over classifying arrhythmias as supraventricular or ventricular. Labelling premature complexes as narrow or wide, and with or without a compensatory pause, may improve observer agreement. Once an arrhythmia is identified, the next challenge is to


distinguish what can be accepted as physiological and what is pathological. At rest, horses commonly show a large heart


Abbreviations


AV atrioventricular cTnI cardiac troponin I cTnT cardiac troponin T ECG electrocardiography


rate variability and presence of second-degree atrioventricular block due to high vagal tone. These physiological arrhythmias should be distinguished from pathological bradyarrhythmia, in which the rate and rhythm are inappropriate relative to the state of arousal and activity of the horse (Keen, 2020). Defining ‘pathological’ arrhythmias is particularly complicated for exercise-associated rhythm disturbances. While these have been linked to poor performance, collapse and sudden death (Lyle et al., 2010; Martin et al., 2000), the prevalence of arrhythmias during exercise and recovery is also high in healthy well-performing equine athletes (Buhl et al., 2010; Navas de Solis, 2016).


Cardiac or systemic underlying disease


When a pathological arrhythmia is detected, further examination should be performed to identify the potential presence of underlying disease. While arrhythmias may develop as isolated electrical disorders, other aetiological factors include primary cardiac disease, inadequate oxygenation of myocardial tissue, metabolic or endocrine disorders, systemic inflammation, autonomic imbalance, infection, intoxications or drugs (Reef et al., 2014). The history and clinical signs can be suggestive for underlying systemic disease, for example the presence of fever, anorexia and lethargy in case of viral or bacterial infection (Jesty & Reef, 2006). However, the clinical presentation is often unspecific and primary cardiac disease may also result in systemic clinical signs. For example, the case report by Conze et al. (2022) in this issue of Equine Veterinary Education describes a foal presented due to progressive inability to rise and nurse, which showed bradycardia caused by third-degree atrioventricular (AV) block at presentation. While the foal’s weakness and apathy could also be caused by other conditions, blood work confirmed underlying nutritional myodegeneration. The clinical signs could be attributed to massive rhabdomyolysis, although they could also be associated with the pathological bradycardia. The arrhythmia detected in this case was a rather unusual consequence of cardiac involvement with myocardial inflammation and


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