Written by Dr. Karen Shaw Becker
Certain dog breeds, including brachycephalic (flat-faced) breeds, golden retrievers and especially Labrador retrievers, are genetically predisposed to a rare, but life-threatening condition in which an extra electrical connection known as an accessory pathway (AP) exists between the upper and lower (pumping) chambers of the heart.
APs are abnormal electrical circuits in the heart that can become activated and disrupt the normal pathways of electrical current, severely impairing the heart’s ability to pump.
When APs cause electrical impulses to travel in the reverse direction (lower to upper chambers instead of the normal pathway of upper to lower), it sets up a large electrical circuit within the heart that causes it to beat much too quickly. Tachycardia (the medical term for rapid heart rate) can ultimately result in congestive heart failure or sudden death.
Technique Borrowed From Human Medicine Provides Cure
Now for the good news. A team of researchers led by Dr. Kathy Wright, a board-certified veterinary cardiologist, have developed a minimally invasive, catheter-based procedure called radiofrequency catheter ablation (RFCA) that cures the abnormal rhythm. RFCA uses radiofrequencies to destroy those rogue circuits and allow the heart’s normal function to resume.
The research was funded by the Morris Animal Foundation1 and published in the Journal of Veterinary Internal Medicine.2 The technique is modified from a human cardiology procedure and has a greater than 95 percent success rate in treating dogs with this type of arrhythmia.
“Accessory atrioventricular pathways are one of the more common causes of rapid heart rhythms in young dogs,” Dr. Wright told Science Daily, “and we were pleased to prove they are curable with radiofrequency catheter ablation. The dogs can then go on to have their hearts recover and be off all heart drugs within a period of three months, and then go on to live normal lives.”3
For the study, Dr. Wright and her colleagues treated 89 dogs with AP-related arrhythmia. Although there were 23 different breeds among the 89, more than half were Labrador retrievers due to their genetic predisposition to this particular type of arrhythmia.
The researchers fed a catheter into each dog’s heart through which radio waves were delivered to the accessory pathways. After the procedure, the dogs were monitored for a minimum of 16 hours before they were allowed to go home. At two months post-procedure, the dogs’ conditions were re-evaluated, and 86 of the 89 were cured. The remaining three dogs were cured with a second RFCA treatment.
Signs of AP-Related Arrhythmia
The condition is most often seen in young to middle-aged dogs, although it has also occurred in seniors (over 8 years of age). Any breed can be affected, but nearly half the cases reported so far have involved Labrador retrievers.
Symptoms of AP-related arrhythmia are nonspecific and occur in many other disorders, which makes the condition harder to spot. Signs include lethargy that comes and goes, and gastrointestinal (GI) signs such as decreased appetite and intermittent vomiting during episodes of tachycardia.
These symptoms are also common in dogs who are indiscriminate eaters, a group that includes young dogs and particularly, Labradors. The symptoms can persist or clear up in a matter of hours, and then reappear days or months later.
Diagnostic challenges include the fact that this type of arrhythmia isn’t well-known among veterinarians. In addition, the dog’s heart rhythm has often returned to normal by the time he or she arrives at the veterinary clinic. If the heart rate does happen to be elevated during the exam, it is often attributed to pain or dehydration.
According to Dr. Wright, the heart rate of a dog with AP-related tachycardia is typically 240 to 410 bpm, which is much higher than the sinus tachycardia caused by pain or dehydration, which is typically 200 bpm or less.
She also points out that electrocardiograms (ECGs) of dogs with the condition are typically normal when they are in sinus rhythm. The type of accessory pathways most of these dogs have is concealed, meaning it is hidden on the ECG unless they are actually in tachycardia during the procedure.
“Dogs with frequent episodes of tachycardia can develop weakening of their heart muscle and congestive heart failure,” Dr. Wright explains in an article for MedVet. “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.”4
The idea that AP-related arrhythmia, according to Dr. Wright, is often misdiagnosed as dilated cardiomyopathy in young and middle-aged dogs is quite a revelation, given all the news over the past several months of an increase in cases of what we assume is diet-related DCM in dogs.
Recommendations for Veterinarians on How to Diagnose Accessory Pathways
In her MedVet article, Dr. Wright offers suggestions to veterinarians on how to diagnose AP-related arrhythmia. She states that while an electrophysiologic study (which involves a catheter and requires sedation) must be performed for a definitive diagnosis, it’s possible to get important clues from a surface ECG.
She recommends running as many leads as possible, ideally six to 12, and since the tachycardia is intermittent, it’s also important to record the activity of the heart for as long as possible. She suggests moving to a Holter monitor, or 24-hour ECG, if the baseline ECG is normal and no other cause for the dog’s symptoms can be identified.
Dr. Wright states that after diagnostic testing is complete, it’s very important to consult with a veterinary cardiologist. She has had several vets send ECGs to her and her team for analysis, and they are happy to do it.
If a review of the ECG shows signs of dysfunction in the heart’s pumping action or congestive heart failure in a young or middle-aged patient, the dog should be carefully screened for AP-related arrhythmia and also heart muscle inflammation before arriving at a diagnosis of idiopathic (cause unknown) dilated cardiomyopathy.
“The good news in all of this,” Dr. Wright writes, “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.”
Dr. Wright and her colleagues place catheters through peripheral vessels into the dog’s heart at specific locations, map the accessory pathway and deliver radiofrequency energy to the abnormal electrical connection, which destroys it. To date, the team has performed RFCA on over 100 dogs.
After the procedure, symptoms such as vomiting, lack of appetite and lethargy rapidly improve. The function of the heart muscle improves over the first month and can continue to progress for up to six months. Heart medications are typically discontinued a month after the procedure.
As of this writing, there are only two locations in the U.S. performing the procedure. One is the MedVet Cincinnati facility where Dr. Wright practices. You and your veterinarian can learn more about the procedure and other cardiology services for both dogs and cats by visiting MedVet Cardiology.
Laurelwood Animal Hospital,located near Jesuit High School on Beaverton-Hillsdale Highway offers a full range of companion animal services, including surgery, nutrition and behavior counseling, parasite control and preventative medicine. The hospital also offers advanced imaging through an all-digital spiral CT scanner, a comprehensive dental program and laser treatment.
If you’re looking for quality, compassionate veterinary care in Beaverton, Oregon, come visit us at Laurelwood Animal Hospital.
Laurelwood Animal Hospital
9315 SW Beaverton-Hillsdale Highway
Beaverton, Oregon 97005
Phone: (971) 244-4230
Fax: (503) 292-6808
E-mail: [email protected]