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The canine pacemaker: these tiny devices can extend the lives of most dogs that have abnormally slow pulses. Here's what you should know.

At some point during the course of a routine canine physical examination, your veterinarian will use a stethoscope to listen carefully to the sound that your dog's heart produces as it beats. What the veterinarian wants to hear is a gentle, rhythmic thump-thump sound. This clearly indicates that oxygen-depleted blood is moving fluently through the heart to the lungs, where it is refreshed with new oxygen, and is then returning to the heart and being pumped back into your animal's circulatory system. In a healthy dog, this rhythmic pumping activity will occur anywhere from 60 to 160 times per minute, depending on an animal s activity level.

Unfortunately, a veterinarian will sometimes detect an unacceptably slow heart rate, less than 30 or 40 beats per minute, say--a condition called bradycardia (from bradys, the Greek word for slow). Another condition can result in alarmingly intermittent pauses between each beat--as long as six or seven seconds, according to Marc Kraus, DVM, a senior lecturer in the Department of Clinical Sciences at Cornell University's College of Veterinary Medicine who is board-certified in internal medicine and in cardiology by the American College of Veterinary Internal Medicine. During such pauses, a dog may faint--or suddenly die. Until the late 1960s, a diagnosis of bradycardia, alternatively known as bradyarrhythmia, was tantamount to a death warrant for any dog affected by the condition, which stems from a defect in the heart's ability to allow the proper generation or conduction of electrical signals to regulate heartbeat.

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Today, however, this once irreparable cardiac disorder is treatable, thanks to the implantation in dogs of pacemakers. The first such procedure was performed in 1968 by Dr. James Buchanan, a veterinary cardiologist at the University of Pennsylvania's School of Veterinary Medicine. Hundreds of these procedures are now performed annually at veterinary medical centers throughout the United States. According to Dr. Kraus, cardiologists at the Cornell University Hospital for Animals (CUHA) implant about 20 pacemakers per year.

Corrective Measure

Pacemakers are most often implanted to correct either of two types of brady-cardiac arrhythmias, explains Dr. Kraus. One of the disorders is an impairment of the sinus node referred to as "sick sinus syndrome." This condition, which is most often diagnosed in geriatric dogs, involves a cluster of cells in the heart that normally generate the electrical signals governing the rhythm of the heartbeat. The other major pacing defect--called "AV block"--occurs when the electrical impulses that originate in the upper chambers (atria) of the heart are unable to pass to the lower chambers (ventricles). An AV block can afflict animals of all ages. Bradycardia can also occur as a secondary consequence of another systemic condition, such as hormonal imbalance, a central nervous disorder or chronic gastrointestinal disease, says Dr. Kraus.

An implanted pacemaker is a small, computerized device that senses when the spontaneous electrical signals in the heart have slowed to an abnormal level. When the device, called a pulse generator, detects this decline in activity, it transmits an electrical stimulus to correct and normalize the heart rate.

Brief Procedure

A pacemaker is implanted during a procedure that usually lasts about an hour and a half, says Dr. Kraus. "The pulse generator is very small, about the size and shape of a silver dollar, and is made of titanium," he notes. "It's basically a power pack that contains all of the necessary electronics for pacing. It is placed in the anesthetized patient's neck, just under the skin." Then, with the aid of a fluoroscopy unit (a type of real-time X-ray imaging device), a wire lead from the power pack is threaded carefully through the jugular vein and into the heart and eventually seated in the organs right atrium or ventricle.

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"There is no major incision," says Dr. Kraus, "and in most cases, the patient can go home within 24 hours." Noting that, prior to surgery, an affected dog is likely to be very lethargic--sometimes nearly comatose--due to its bradycardia, he says: "Most dogs will be acting normally again the next day. In fact, I've had owners tell me that a dog has suddenly become so lively that they wonder whether the pacemaker should be turned down a bit."

At CUHA, he says, a pacemaker implant procedure will cost an owner about $2,500. The cost will increase, he points out, if the patient must remain under observation at the hospital for a day or two following the implant.

The Collapsing Dog

In most cases, Dr. Marc Kraus points out, a dog that eventually undergoes a pacemaker implant will have experienced a condition such as sick sinus syndrome or atrioventricular block for many months or even longer prior to diagnosis of the condition. "The owner may have noticed that the animal has slowed down," he says, "that it tires easily, doesn't want to go on walks and so forth. But these conditions tend to affect all older dogs, so the owner may attribute the increased lethargy to aging and the normal slowing down that you often see in geriatric animals."

Indeed, he says, owners may not show concern until a dog has actually collapsed. "That's what we see most commonly," he says. "We can try medical therapy to stimulate the heart rate, but that usually is not an effective treatment for a dog that has reached that stage.

"If we have a collapsing dog on our hands and we diagnose bradycardia, then pacemaker therapy is definitely the treatment of choice."
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Title Annotation:Health
Author:Ewing, Tom
Publication:Dog Watch
Geographic Code:1USA
Date:Jun 1, 2008
Words:910
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