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Normally, the electrical impulse causing the heart to beat begins in the sinoatrial node -- the pacemaker of the heart located in the right atrium (one of the heart's top two chambers) -- is transmitted to the ventricles (the heart's bottom two chambers) and then passes through the atrioventricular (AV) node into the AV bundle. Wolff-Parkinson-White Syndrome (WPW) is when ventricular pre-excitation occurs as impulses originating in the sinoatrial node or atrium activate a portion of the ventricles prematurely through an accessory pathway without going through the AV node, causing, among other things, an abnormally fast heart beat rhythm (supraventricular tachycardia). (The remainder of the ventricles is activated normally through the usual conduction system.) 

Symptoms and Types

• Fainting (syncope)
• Extremely rapid heart rate (greater 300 beats per minute)

Causes

WPW syndrome can be associated with congenital or acquired heart defects.

Congenital Heart Disease

• Congenital defect limited to the heart beat’s conduction system
• Hole in between the two atria (atrial septal defect)
• An improperly developed valve separating the right atrial chamber from the left ventricular chamber (tricuspid valvular dysplasia in dogs)

Acquired Heart Disease

• Hypertrophic cardiomyopathy (though more often seen in cats)

Diagnosis

You will need to give a thorough history of your dog’s health to your veterinarian, including the onset and nature of the symptoms. He or she will then perform a complete physical examination as well as a complete blood count, biochemistry profile, urinalysis, and electrolyte panel -- the results of which are typically normal. Echocardiography, meanwhile, may show structural heart disease often associated WPW syndrome.