Species

Dog and cat

Breed Predilections

• Sick sinus syndrome-cocker spaniel, miniature schnauzer, pug, dachshund
• Ventricular arrhythmias-boxer, German shepherd

Mean Age and Range

More common in old animals

Cardiac Causes

• Bradyarrhythmias-sinus bradycardia, sinus arrest, second-degree atrioventricular block, complete AV block, atrial standstill.
• Tachyarrhythmias-ventricular tachycardia, supraventricular tachycardia, atrial fibrillation.
• Low cardiac output (non-arrhythmic)-cardiomyopathy, AV valve endocardiosis, subaortic stenosis, pulmonic stenosis, pulmonary hypertension, heartworm disease, pulmonary embolism, cardiac tumor, cardiac tamponade.

Neurologic and Vasomotor Instability

• Vasovagal syncope-emotional stress and excitement may cause heightened sympathetic stimulation, leading to transient tachycardia and hypertension, which is followed by a compensatory rise in vagal tone, leading to excessive vasodilation without a compensatory rise in heart rate and cardiac output; bradycardia often occurs.
• Situational syncope refers to syncope associated with coughing, defecation, urination, and swallowing.
• Carotid sinus hyperactivity may cause hypotension and bradycardia-often the cause of syncope when one pulls on a dog's collar.

Miscellaneous Causes

• Drugs that affect blood pressure and regulation of autonomic tone.
• Hypoglycemia, hypocalcemia, and hyponatremia (rare).
• Hyperviscosity syndromes (e.g., polycythemia and paraproteinemia) cause sludging of blood and impaired cerebral perfusion (rare).

Differential Signs

• Must differentiate from other altered states of consciousness, including seizures and narcolepsy (a sleep disorder).
• Seizures are often associated with a prodromal and postictal period; syncope occurs without warning, and animal usually has rapid, spontaneous recovery. Unlike syncope, seizure activity is usually associated with tonic clonic muscle activity rather than flaccidity.
• Like syncope, narcolepsy occurs suddenly, results in muscle flaccidity, and resolves spontaneously. Unlike syncope, narcolepsy can last for minutes and can be terminated by loud noises or harsh external stimuli.
• Must differentiate from other causes of collapse such as musculoskeletal disease and neuromuscular disease (e.g., myasthenia gravis), which are not associated with loss of consciousness.

Differential Causes

• Syncope with excitement or stress suggests vasovagal syncope.
• Syncope with coughing, urination, or defecation suggests situational syncope.
• Syncope with exercise suggests low-output states associated with arrhythmias or structural heart disease.
• A murmur supports heart disease but does not confirm cardiac cause for syncope.

Echocardiography

• May detect structural heart disease or pericardial disease that could lower cardiac output.
• Doppler echocardiography may aide in the diagnosis of pulmonary hypertension.
• Computed tomography pulmonary angiography may detect pulmonary embolism.
• Ventilation perfusion scintigraphy may detect pulmonary embolism.

Electrocardiographic Findings

• Post-exercise ECG may reveal intermittent arrhythmia.
• Holter monitoring (24-hour ECG recording) or use of an ECG event (loop) recorder-useful for evaluating arrhythmic causes.
• Carotid sinus massage with ECG and blood pressure monitoring useful in evaluating carotid sensitivity.

Bradyarrhythmias

• Correct metabolic causes.
• Anticholinergics (e.g., atropine, propantheline bromide, hyocyamine sulfate).
• Sympathomimetics (e.g., isoproterenol, bronchodilators).
• Pacemaker implantation in some patients.

Tachyarrhythmias

• Atrial arrhythmias-administer digoxin, beta-blocker, or diltiazem.
• Ventricular arrhythmias-administer lidocaine, mexiletine, sotalol, or beta-blocker.

Low Cardiac Output

• Institute treatment to improve cardiac output, which varies according to specific cardiac disease.

Vasovagal

• Theophylline or aminophylline-sometimes helpful; mechanism of action in this setting is unclear.
• Beta-blockers (e.g., atenolol, propranolol, and metoprolol) may indirectly prevent vagal stimulation by blocking the initial sympathetic response.
• Anticholinergics may blunt the vagal response.