UC Davis veterinary cardiologists diagnose third-degree AV block.
University of California, Davis, veterinary cardiologists placed a pacemaker in Rocket to normalize his dangerously low heart rate. (Photos courtesy of UC Davis)Rocket, a 10-year-old Boston terrier, was taken to his veterinarian after he appeared to hurt himself jumping on the bed, according to a recent case report from the veterinaty school at University of California, Davis. Rocket's veterinarian did notice some tightness in his neck but, more importantly, noticed something else that was much more serious.
A dog's normal heart rate is generally around 100, but Rocket's had dipped to less than half of that. He was eventually referred to the heart specialists in the Cardiology Service at the UC Davis Veterinary Medical Teaching Hospital for further evaluation of his extremely slow heart rate.
Josh Stern, DVM, PhD, DACVIM (cardiology), chief of the cardiology service at the hospital, diagnosed Rocket with a condition known as a complete or third-degree atrioventricular (AV) block, which stops the “pace-making” area of the heart from communicating appropriately to the rest of the heart for a unified contraction. This “block” in communication causes the heart muscle to find alternate ways of beating on its own, but at a much slower rate. There are a few causes of third-degree AV block, but it is most commonly caused by an idiopathic fibrosis of the conduction area of the heart.
The slower beats sustaining Rocket are known as ventricular escape complexes. This, however, is not a long-term way for the heart to function, and studies have shown that dogs with third-degree AV block are at high risk of sudden death. Therefore, it was recommended that Rocket undergo the standard treatment for this condition-the placement of a permanent pacemaker to provide a normal heart rate again.
To ensure that Rocket was a good candidate for the pacemaker, Stern and his team performed a cardiac evaluation, consisting of multiple diagnostic tests. Radiographs of the chest were taken and showed enlargement of the heart. Abdominal ultrasound showed a slightly enlarged liver and a small accumulation of fluid in the abdomen. These findings were consistent with a very mild right-sided congestive heart failure. When the heart is weaker, its ability to properly drain blood from the body is impaired, causing backup of pressure in the vessel that travels to the heart from the abdomen. As this vessel distends, the increased pressure causes fluid to leak into the abdomen. This condition was likely secondary to Rocket's third-degree AV block and would resolve with the increased heart rate that a pacemaker generates.
Rocket at UC Davis, ready to go home after pacemaker surgery.Also performed were an electrocardiogram (which tested the electrical activity in Rocket's heart) and an echocardiogram (an ultrasound of the heart that allowed for size and pressure measurements of Rocket's heart chambers). Additionally, complete blood count, chemistry panel, heartworm test, tick-borne disease assessment and urinalysis were performed to ensure that Rocket did not have any concurrent illness that would contraindicate the interventional surgery procedure.
All of Rocket's tests were unremarkable, and Stern proceeded with pacemaker placement the following day. Rocket was placed under general anesthesia, and a transvenous, permanent pacemaker was placed without complication. An external battery attached to the pacemaker was secured under the skin and muscle of the right side of his neck. Rocket recovered for a night in the ICU where he was continually monitored and kept quiet with sedation to minimize movement.
UC Davis offers state-of-the-art pacemakers made especially for dogs. The battery lifespan for a brand new device is more than 10 years. Rocket's pacemaker is set to deliver 80 beats per minute, with an upper limit of 160 beats per minute. The pacemaker will respond to Rocket's activity level and increase accordingly depending on how much he moves in conjunction with the range that was set. After a month of strict rest, Rocket will be able to slowly resume a normal activity level and return to playing with his two young human siblings.