We as veterinary cardiologists have the ability to perform several types of interventional procedures. The more commonly performed procedures include closure of a left-to-right patent ductus arterious with an Amplatz Canine Ductal Occulder (ACDO), balloon valvuloplasty for a pulmonic stenosis and temporary and permanent transvenous pacemaker implantation.
This article will discuss the benefits of pacemaker implantation. Permanent transvenous pacemaker implantation is indicated in patients with clinical signs related to their arrhythmia. The most common arrhythmias seen are sick sinus syndrome, high-grade 2nd degree AV block and 3rd degree AV block and less commonly, atrial standstill. Common clinical signs are lethargy, exercise intolerance, decreased appetite or inappetence and collapsing or syncope. Most dogs improve significantly after pacemaker implantation and their clinical signs resolve.
Signalment: “Sammie” is a 12-year-old, female spayed, mixed breed dog
Sammie presented to the Emergency Service for bradycardia which was diagnosed by the referring veterinarian during an examination for a routine ear infection. She had no history of collapse but had been slowing down for several months. She was more tired and very slow on walks. Her heart rate at home was 52bpm which decreased to 36bpm several months later.
On presentation to Hope, Sammie had a heart rate of 64 bpm. The remainder of her exam was non-remarkable. Her echocardiogram showed diastolic mitral regurgitation present due to the arrhythmia and very mild systolic mitral regurgitation which was likely secondary to mild chronic valvular disease.
Figure 1 is a lateral chest radiograph of Sammie’s pacemaker.
Sammie’s electrocardiogram (ECG) showed an atrial rate of 150bpm and a junctional rate of 60bpm. The rhythm diagnosis was 3rd degree AV block with a slow junctional escape rate. There were also infrequent ventricular escape beats. An atropine response test was performed by the referring veterinarian and showed an increase in the atrial rate with no significant increase in the heart rate.
Sammie had mild clinical signs thought to be attributed to the bradycardia although she was not collapsing. Given these findings, it was recommended that a permanent transvenous pacemaker be placed to prevent further weakness, lethargy, and collapse. Without the pacemaker, Sammie would be at risk of progressive left ventricular dilation, heart failure, lethargy, weakness, syncope, and sudden death. Some clients may elect to try medical therapy with theophylline, terbutaline and/or propantheline bromide. These medications typically do not improve the heart rate and clinical signs. If an improvement is seen, it tends to be limited as the arrhythmia will worsen and the patient will become less responsive to medical management.
Sammie’s pacemaker was implanted without complications (see Figure 1). If the animal’s heart rate is too low and the patient is unstable for anesthesia, a temporary pacemaker can be placed prior to general anesthesia. This can be placed under sedation using fluoroscopic guidance. This allows us to set the heart rate to a fixed number until the permanent pacemaker can be implanted.
After the pacemaker implantation, the pacemaker is most often set to a fixed heart rate of 100bpm. In the future, the pacemaker will be adjusted to allow for variation in the heart rate. This is called a rate-responsive setting which is more physiologic. The heart rate can then be faster when the dog is active or slower when the dog is sleeping. Another type of pacemaker that can be implanted is a called a VDD pacemaker. The lead for this type of pacemaker has a sensor that is placed within or close to the right atrium (see Figure 2). The sensor picks up atrial depolarization and will then wait a preprogrammed amount of time (the PR interval) before depolarizing the ventricle. This type of pacemaker is generally implanted in patients with AV block and is considered more physiologic as it allows for normal atrial filling and ventricular depolarization.
Figure 2 is a lateral chest radiograph from a dog with a VDD pacemaker. Note the atrial sensor on the lead that is located within the right atrium. This senses atrial depolarization.
Sammie did well for 2 years after pacemaker implantation but eventually developed renal failure. Many dogs that receive pacemakers typically do not die from heart-related conditions. The reported complications include lead dislodgement, lead fracture, infection at pacemaker site, seroma formation, myocardial dysfunction (pacemaker syndrome), and sudden death.
Pacemaker implantation is a viable option for many patients that develop bradyarrhythmias.