Some dog breeds, such as Labrador Retrievers, Golden Retrievers, and brachycephalic breeds, are predisposed to a condition where an extra electrical connection between the upper and the pumping chambers in the heart persists during development. This extra electrical connection is known as an accessory pathway. In this article, we will review how to alleviate accessory pathway with minimally-invasive radiofrequency catheter ablation (cardiac ablation) in dogs.

An accessory pathway can conduct electrical impulses from the upper to the lower heart chambers in parallel with the normal conduction system, or more commonly, will conduct electrical impulses intermittently in the reverse direction (from the pumping chambers to upper chambers). Particularly when conducting in the reverse direction, the accessory pathway sets up a large electrical circuit within the heart that can drive the heart rate much too quickly (240-410 bpm in most dogs).

A rapid heart rate, known as tachycardia, can cause many problems and eventually lead to congestive heart failure or sudden death. Thankfully, this abnormal rhythm is curable with a minimally-invasive, catheter-based procedure that we can perform called radiofrequency catheter ablation or cardiac ablation.

6 Tips to Recognize Accessory Pathway Arrhythmias in Dogs 

It can be challenging to diagnose an accessory pathway. Below are some tips to help you in your practice:

  1. Signalment: This most commonly is recognized in young to middle-aged dogs, although we have had some dogs over 8 years of age. All breeds can be affected, but nearly half of the dogs thus far have been Labrador Retrievers.
  2. Clinical signs recognized by clients are nonspecific and occur with other common issues. Most commonly, intermittent lethargy and gastrointestinal signs such as decreased appetite and intermittent vomiting occur when the dog is experiencing tachycardia. These are the same signs shown by dogs with dietary indiscretion, which is also commonly seen in young dogs (particularly Labradors). These signs may persist or clear up within hours, only to recur days to months later.
  3. Tachycardia secondary to an accessory pathway is not yet well known in the veterinary community. Adding to the diagnostic challenge, a dog’s abnormal heart rhythm may have converted to normal by the time of their examination. Additionally, one may assume the dog’s rapid heart rate is due to dehydration or pain. In either scenario, the accessory pathway mediated tachycardia can be easily overlooked.
  4. The heart rate of a dog with accessory pathway mediated tachycardia is typically 240-410 bpm, which is significantly faster than the sinus tachycardia caused by pain or dehydration, which is typically 200 bpm or less.
  5. The electrocardiograms (ECG) of affected dogs are typically normal when they are in sinus rhythm. The type of accessory pathways most dogs commonly have are known as “concealed,” because they are hidden on the surface ECG unless the dog is actually in its tachycardia.
  6. Dogs with frequent episodes of tachycardia can develop weakening of their heart muscle and congestive heart failure. This is frequently misdiagnosed as dilated cardiomyopathy (DCM) in a young to middle-aged dog, when in fact, it is tachycardia-induced cardiomyopathy (TICM) that is completely reversible with rhythm control. The breeds predisposed to accessory pathways and resultant tachycardia-induced cardiomyopathy are the same breeds that can develop idiopathic DCM, so it becomes important to distinguish these two conditions.

How to Diagnose Accessory Pathways in Dogs 

To diagnose an accessory pathway in a dog, an electrophysiologic study must be performed. We can, however, get, strong clues from the surface electrocardiogram (ECG). It is preferable to run as many leads as possible (6-12 leads is best)As tachycardia or preexcitation (the latter refers to conduction from upper to lower chambers during sinus rhythm) are generally intermittent, it is important to record as long of an ECG as possible. This means that a Holter monitor, or 24-hour ECG, will be used if the baseline ECG is normal and no other cause for the clinical signs, for example a foreign body, is found.

Once the diagnostic testing is complete, consulting with a veterinary cardiologist to examine the ECG is similarly crucial. We have had several practitioners send us ECGs for interpretation, and we are happy to do so. If signs of pumping dysfunction or congestive heart failure are found in a young to middle-aged dog, then the pet needs to be carefully screened for this condition, as well as heart muscle inflammation, before diagnosing idiopathic DCM.

Treatment for Accessory Pathway in Dogs 

The good news in all of this is that we can alleviate accessory pathway mediated tachycardia and its resulting complications through a minimally-invasive, catheter-based procedure called radiofrequency catheter ablation or cardiac ablation. We are one of only two centers in the United States performing these procedures, and have performed over 100 to date. Using catheters placed through peripheral vessels into the heart at strategic locations, the accessory pathway is mapped and radiofrequency energy is delivered to this abnormal electrical connection to destroy it.

Post-procedure, signs of poor appetite, vomiting, and lethargy improve very rapidly. The heart muscle function improves over the first month and can continue to improve for six months. Additionally, heart failure medications can typically be discontinued after the first month.

Certain other heart rhythms are also amenable to ablation, such as focal atrial tachycardias. We are happy to discuss individual cases, review EKG’s, or see your patients for a consultation. Learn more about Dr. Kathy Wright and her work at MedVet Cincinnati.