Arrhythmia, both atrial (supraventricular) and ventricular are a common, and sometimes normal, activity of the canine heart.
Concerns arise when the severity of arrythmias leads to life-threatening atrial of ventricular fibrillation.
Types of arrhythmias include bradyarrhythmias (slow) and tachyarrhythmias (fast).
These heart beat irregularities may be observed during routine electrocardiogram studies and are commonly observed during postural changes associated with routine anesthesia and many are thought to be non-pathological.
A number of arrhythmias are due to heart rate turbulence, defined as a return to normal beat after a ventricular premature complex which results from an abnormal baroreceptor response, and is commonly observed in the Boxer with mild and moderate subaortic stenosis and dilated cardiomyopathy in the Boxer, Doberman Pinscher and Great Dane.
However, many canine arrhythmias are the result of underlying cardiac conduction disorders (e.g. atrial fibrillation). These conduction disorders, which may be present without any evidence of valvular disease, are thought to arise as a result of decreased calcium channeling across the myocytes of the heart and defective cytosolic calcium removal following myocontractility, leading to increased refractory myocytes that become dyssynchronous with the majority of remaining cardiac myocytes. Certain breeds, such as the German Shepherd have such defective calcium channeling due to a genetic dysfunction of myocardial tissues, and in these cases, may lead to ventricular fibrillation, syncope or sudden death.
Common causes of ventricular arrhythmias include:
- Atrial fibrillation
- Atrial standstill
- Atrial tachycardia
- Bundle branch block
- Congestive heart failure
- Gastric dilatation-volvulus
- Isorhythmic atrioventricular dissociation - predisposition in the Labrador Retrievers
- Mitral valve dysplasia
- Myocarditis - e.g. Trypanosoma spp, Bartonella berkhoffii
- Sick sinus syndrome
- Supraventricular tachycardia
- Vasovagal syncope
- Ventricular tachycardia
- Ventricular fibrillation
- Wolff-Parkinson-White syndrome
Routine ECGs, chest radiographs, echocardiography and serological testing of the cardiac biomarker NT-proBNP may help determine the underlying cause of the arrhythmia. A 24-hour Holter electrocardiography may be required in difficult cases.
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