The first clinical, fully implantable pacemaker in a human was attached to the epicardium of a patient’s heart via thoracotomy in 1958 in Sweden. This device failed after three hours, and a second device was then implanted which lasted for two days. The patient, Arne Larsson, went on to receive 26 different pacemakers during his lifetime, before dying at the age of eighty-six in 2001. One of the first clinical implantations of a pacemaker into a canine patient occurred here at Angell Animal Medical Center, by Dr. Neil Harpster with assistance by human cardiologists from Beth Israel Deaconess Medical Center. The first published report of such an epicardial pacemaker implantation into a canine patient came out in 1968, followed less than a decade later, in 1976, by the first published transvenous placement of a pacing system in a dog. Since then, veterinary cardiologists have been able to use this technology to provide improved quality and quantity of life to countless patients.
Insertion of a pacemaker
Pacemakers can be used to treat a variety of bradyarrhythmias in veterinary patients, most commonly third degree AV block, followed by sick sinus syndrome, high grade second degree AV block, and atrial standstill. Pacemakers are more commonly placed in canine patients, but have been placed in cats and ferrets. The canine breeds that are most often reported to undergo pacemaker surgery include Miniature Schnauzers and West Highland White Terriers (typically for sick sinus syndrome), German Shepherds and Labrador Retrievers (typically for third degree AV block), Cocker Spaniels, and mixed breed dogs.
The pacing system consists of the pulse generator, which provides the electric impulse, and a pacing lead, which transmits the electrical activity to the myocardium and enables the generator to detect spontaneous cardiac depolarizations (sensing function). The lead can be placed either epicardially, via thoracotomy, or via a less-invasive, transvenous approach, commonly to the right ventricular endocardium. The epicardial approach may be preferred in very active dogs where transvenous lead dislodgement has already occurred or seems highly likely, or in very small patients (such as cats), where the physical size of the lead within the right atrium may cause detrimental increases in pressure and obstruction to blood flow.
The pacemaker is about the size of a silver dollar.
Transvenous pacemaker placement is performed using fluoroscopy to guide lead placement. After placing a temporary pacing lead down the left jugular vein and ensuring an adequate heart rate, our patient is placed under general anesthesia. (Approximately 5% of canine patients have the anatomic anomaly of a persistent left cranial vena cava, taking a circuitous route to the right atrium, which is why we preserve the use of the right jugular vein for the permanent pacing lead.) A surgical approach to the right jugular vein allows for a small venotomy incision as the access point for the pacing lead. The permanent pacing lead is then passed down into the heart, guided via fluoroscopy, until the tip is in contact with the right ventricular apical endocardium. The pacing lead may either have a “passive” tip, with small tines resisting backward motion via interference with the trabeculations in the right ventricular surface, or it may have an “active” tip, with a small corkscrew that is screwed into the endocardium. After positioning, the lead is tested to determine how much electrical activity is required to achieve “capture” or successful pacing of the myocardium. If the spot requires too high of a voltage (> 1 mV), repositioning is attempted. Next, a subcutaneous pocket for placement of the pacing generator is made, typically on the right side of the neck. The pacing lead is passed subcutaneously to the pocket, the generator is attached, and both incisions (venotomy site and generator site) are closed.
Although reported minor complication rates following pacemaker implantation are fairly high, we find serious complications relatively uncommon. Minor complications most commonly include seroma formation at the generator site, and inconsequential arrhythmias. The most common major complication is lead dislodgement, followed less frequently by infection. Strict exercise restriction is critical for the 4-6 week period following pacemaker implantation, to try to decrease risk of lead dislodgement. The tip of the pacing lead becomes more securely adhered to the endocardium after this time due to adjacent fibrosis. Depletion of the pacing generator battery may also be considered a complication, albeit a predictable one. As new generators have become more widely available (as opposed to using explanted generators from human patients), the expected battery life of the generators has improved. Depending on how often the generator is required to provide impulse (which will be higher for a third degree AV block patient who is paced 100% of the time vs. a sick sinus syndrome patient where the pacemaker just “fills in the blanks”), a new generator may last 3-5 years or more. After that time, if the battery is depleted, generator replacement using the same pacing lead may be pursued.
Clients with a pacemaker candidate may inquire about cost, post-operative considerations, and prognosis. At Angell, a transvenous pacemaker implantation and hospitalization for a typical canine patient costs approximately $4,000-4,500. Individualized detailed estimates are provided for each patient prior to the procedure. As previously discussed, the patient must be kept strictly exercise-restricted for 4-6 weeks after the surgery, to decrease risk of lead dislodgement, and the patient should always avoid very strenuous running and jumping activities. Pacemaker patients undergo periodic reevaluations, including echocardiograms and “interrogation” of the pacemaker, where an external computer can tell us how much voltage is being required to maintain pacing, how much battery life is left in the generator, and other information about programmable pacing functions. Finally, prognosis will largely depend on any concurrent structural cardiac disease which may be present, but can be excellent for patients with primary bradyarrhythmias without underlying cardiomyopathy.
If you have a patient who you suspect may benefit from pacemaker implantation, please do not hesitate to contact the Angell Cardiology service to discuss how urgent the surgery may be, individual considerations for your patient, and to make a referral. You may contact Angell Cardiology at firstname.lastname@example.org, or call the Cardiology service at 617-541-5038.
- Oyama MA, Sisson DD, Lehmkuhl LB. Practices and outcomes of artificial cardiac pacing in 154 dogs. J Vet Intern Med 2001;15:229-239.
- Wess G, Thomas WP, Berger DM et al. Applications, complications, and outcomes of transvenous pacemaker implantation in 105 dogs (1997-2002). J Vet Intern Med 2006;20:877-884.
- Johnson MS, Martin MWS, and Henley W. Results of pacemaker implantation in 104 dogs. J Sm Anim Pract 2007;48:4-11.