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necrosis (Conze et al., 2022). Third-degree AV block in itself can cause lethargy, syncope and episodic collapse (Keen, 2020). Similarly, any primary cardiac disease with decreased cardiac output may result in various unspecific clinical signs such as depression, lethargy, poor performance, exercise intolerance, weakness or ataxia (Keen, 2019). In horses with arrhythmias, echocardiography is indicated to evaluate the presence of structural heart disease. This includes the assessment of cardiac size and function, presence of valvular regurgitation, as well as screening for endo-, myo- or pericardial abnormalities. In the case report by Conze et al. (2022), echocardiography revealed a hyperechoic zone in the atrioventricular node region which was indicative for myocardial damage. This should be differentiated from the normal echogenic appearance of the ventral part of the interatrial septum which can often be seen on a four- chamber view in healthy individuals (Figure 1).
Laboratory testing
Laboratory tests are important to identify cardiac arrhythmias which develop as a result of systemic or metabolic diseases. However, it should be kept in mind that primary cardiac disease may also alter routine laboratory tests; for example, horses with a low cardiac output may present prerenal azotaemia (Schwarzwald, 2018). Table 1 contains a list of laboratory tests that can be valuable in horses with cardiac arrhythmias to identify underlying cardiac or systemic disease. A complete blood count and serum protein test is useful to identify anaemia or inflammation, for example, acute haemorrhage has been shown to result in myocardial injury and ventricular ectopy (Navas de Solis et al., 2015). Markers of inflammation such as serum amyloid A and fibrinogen can be indicators for viral or bacterial myocarditis (Decloedt, 2019). Serum biochemistry to assess muscle enzymes, renal function and hepatic function is helpful to identify the presence of systemic disease or to assess the consequences of primary cardiac disease. Nutritional myodegeneration was suspected in the case report by
Conze et al. (2022) based on the markedly increased muscle enzymes. The diagnosis was confirmed by the analysis of vitamin E and selenium concentrations in blood samples of the foal and dam, which revealed remarkably low selenium levels. While nutritional myodegeneration typically occurs in foals, adult horses may also be affected. The typical presentation in adult horses is masseter myodegeneration resulting in dysphagia and mastication problems. However, cardiac muscle involvement may result in cardiac arrhythmias and even sudden death (Polledo et al., 2013; Schefer et al., 2011). Atypical myopathy should also be considered as a differential diagnosis in case of massive rhabdomyolysis with presence of cardiac arrhythmias (Verheyen et al., 2012). Serum electrolyte levels should always be evaluated in
case of cardiac arrhythmias, as electrolyte abnormalities may cause alterations of the cardiac rhythm. Potassium is particularly important, as hyperkalaemia results in increased excitability of the myocardial cells and slowed conduction, which may lead to ventricular fibrillation duetore-entry. Severe hyperkalaemia, for example in case of uroperitoneum in foals, can result in bradycardia due to impaired sinus and AV node function and depression of intra-atrial conduction (Marolf et al., 2018). A rare finding in horses with hyperkalaemia is the presence of atrial standstill, with absence of atrial myocardial depolarization and therefore absence of P waves (Figure 2). Hypokalaemia may result in enhanced automaticity of pacemaker cells, with supraventricular or ventricular ectopy. Hypercalcaemia may also cause arrhythmias, although the effects depend on the potassium levels. Hypomagnesaemia can cause ventricular arrhythmias, and magnesium sulphate may be administered intravenously as a physiological calcium channel blocker in case of ventricular tachycardia (Schwarzwald, 2018). Elevated levels of serum cardiac biomarkers such as cardiac troponin I (cTnI) or cardiac troponin T (cTnT) indicate
TABLE 1: List of laboratory tests that can be useful in case of cardiac arrhythmias to identify underlying cardiac or systemic disease
Complete blood cell count
Fig 1: Four-chamber view demonstrating the echogenic appearance of the ventral part of the interatrial septum detected as a coincidental finding in a 3-year-old Warmblood horse with mild mitral valve regurgitation. This can often be seen on a four- chamber view in healthy individuals.
Markers of inflammation, for example serum amyloid A and fibrinogen Serum biochemical tests: Electrolytes, particularly potassium, calcium and magnesium Renal function tests Muscle enzymes Serum protein and electrophoresis Liver enzymes Vitamin E/selenium Cardiac troponin I or T (cTnI, cTnT) Arterial and venous blood gas analysis Blood lactate Blood culture Serum/plasma assays for digoxin, digitoxin and other cardio- active drugs Gastro-intestinal and feed sample analysis for toxicology screening, for example, ionophores Urinalysis Cytology and culture of pericardial effusion Myocardial biopsies
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