Although the cause of an arrhythmia is often of more prognostic value than the arrhythmia rate or complexity, ventricular tachycardia with a rate exceeding 250 beats/min is always of concern. Lethal ventricular tachycardia occurs at a rate of > 300 beats/min. At this rate, ventricular tachycardia can degenerate into ventricular fibrillation.^7-9,11-13 Dogs with subaortic stenosis are an exception because ventricular tachycardia can degenerate into ventricular fibrillation at rates as slow as 170 beats/min. So all Doberman pinschers, boxers, and other dogs with subaortic stenosis should be evaluated by Holter recordings to determine the severity of the arrhythmia.^{2,3,7-11,13,14,16}

Assessing antiarrhythmic treatment efficacy

The increased sensitivity of the ambulatory ECG in detecting spontaneous arrhythmias has been proven.^2-10 These arrhythmias usually have great inherent variability, which often necessitates a pretreatment and post-treatment Holter recording to determine whether medical therapy is proarrhythmic or antiarrhythmic or whether it lacks efficacy.^3,12,18 If a post-treatment static ECG reveals numerous VPCs or ventricular tachycardia, then Holter recording is probably not indicated at that time and treatment can be adjusted. However, if a post-treatment static ECG reveals no or only occasional VPCs, it is inappropriate to assume that the arrhythmia is well-controlled.

Diagnosing the cause of syncope when heart rhythm disturbances are not evident

Syncope due to cardiac arrhythmia is common in dogs. Syncope can be secondary to bradycardia, rapid ventricular tachycardia, or, less often, rapid supraventricular tachycardia. Holter recording may be a useful diagnostic test in patients with syncope and episodic weakness when the cause of syncope is not readily apparent on a static ECG.^3,11

Neurally mediated bradycardia. Neurally mediated bradycardia and syncope are common in elderly small-breed dogs.¹⁹ Situational syncopes result from vagal triggers caused by coughing, gagging, retching, vomiting, urinating, or defecating.¹⁹ Neurocardiogenic syncope is an adrenergic-triggered, vagal-reflex bradycardic syndrome associated with excitement or exertion in elderly small-breed dogs with advanced mitral valve disease, severe left heart enlargement, and high preload.¹⁹ Other than the bradycardia during the syncopal episodes, the heart rates and rhythms of dogs with neurally mediated bradycardia are usually normal.¹⁹ Holter recording documents the heart rhythm, as would event recording during syncope.

Neurally mediated bradycardia is also a cause of syncope in some large-breed dogs, notably boxers and golden retrievers.²⁰ Some normal dogs experiencing severe fight, fright, or flight stimuli demonstrate an adrenergic-triggered, vagal-reflex bradycardia.²⁰ When cardiomyopathy is absent, these dogs have normal echocardiogram results and no or few VPCs.²⁰ Cardiomyopathic boxers also experience an excitement-exertion-startle-triggered neurocardiogenic bradycardia but also have ventricular tachyarrhythmias of variable severity, with or without echocardiographic abnormalities.²⁰ Cardiomyopathic Doberman pinschers can faint because of transient neurocardiogenic bradycardia in response to exertion-excitement or just because of a sudden change from less activity to more activity.¹¹ In Doberman pinschers, neurocardiogenic bradycardia along with syncope is a marker of cardiomyopathy.¹¹ Holter recording helps differentiate neurocardiogenic bradycardia from ventricular tachycardia when no or few VPCs occur in a dog with recent syncope.²⁰

Bradycardia due to sick sinus syndrome. Not all bradyarrhythmias are neurally mediated. Sick sinus syndrome is usually associated with American cocker spaniels, miniature schnauzers, and West Highland white terriers. When the bradycardia of this syndrome causes syncope, it is usually apparent on a static ECG. Advanced atrioventricular (AV) conduction blocks are usually due to degeneration of conduction pathways. AV conduction blocks severe enough to cause syncope are usually evident on a static ECG.