Pacemaker Therapy

Overview

Pacemakers are small devices that are implanted in the patient’s heart that can modify the patient’s heart rate. The pacemakers that are used in veterinary patients are used to prevent the heart rate from getting too low, unlike some used in human patients that can slow the heart rate down as well. Pacemakers function by delivering small shocks to the heart muscle at a predetermined rate. This causes the heart to contract and pump blood.

The critical components of pacemakers are a pulse generator and a pacing lead. The pacemaker functions as an electrical circuit whereby the generator (battery) provides electrical stimulation that travels through the pacing lead to the heart. This stimulation is what causes the heart to contract. Bipolar pacemaker leads are most commonly used and create a smaller circuit with the electrodes located near the end of the pacemaker lead. (Figure 1) Unipolar leads have one pole located near the end of the lead with the other pole encompassing the metal of the generator. Modern pacemakers are fairly small with most being about the diameter of a half dollar and the thickness of a pencil.

Modern pacemakers have many options available to make the pacing as close to the dog’s natural rhythm as possible. They sense the heart’s normal activity and do not pace when the heart is beating on its own. Additionally, if the impulses are being blocked (atrioventricular block), newer leads can sense the blocked impulses and customize the patient’s heart rate. If there is a lack of impulses being formed (sinus node dysfunction), modern pacemakers can sense the patient’s movements and increase the heart rate to match the activity level.

Signs & Symptoms

Pacemakers are needed when the heart rate becomes very slow and the patient either becomes very lethargic or faints frequently or both. Some patients can die if their heart rates gets too low. The heart rhythms that can cause these conditions are most frequently:

Atrio-ventricular block (AV block) in which the heart is activated normally but the activation impulses are blocked in the heart
Sinus node dysfunction (SND) where the heart is not activated frequently enough.

Medical therapy can be tried to correct these rhythms, but it is rarely successful. Many patients are somewhat elderly when they need a pacemaker, yet pacemakers can be placed successfully in these patients as well. Dog breeds that are prone to needing pacemakers include West Highland White Terriers, Miniature Schnauzers and Cocker Spaniels, but any breed of dog or cat can require a pacemaker.

Treatment & Aftercare

Great care is taken anesthetizing patients that are having a pacemaker placed. A temporary packing unit is placed to make sure that the heart rate does not go too low during anesthesia. These are placed either externally on the chest or through a blood vessel. The most common method of pacemaker placement is through a relatively small surgical incision in the neck, and the lead is threaded through blood vessels into the right side of the heart. (Figure 2) The lead is guided with a special kind of moving x-rays called fluoroscopy. In these cases, the generator is usually placed in the neck. The tip of the lead is either attached with a grappling hook type mechanism or a screw in tip. Another method of placing the lead on the heart is to make an incision in the abdomen and the diaphragm and attach the electrodes directly to the heart. This is generally done in very small dogs and cats because the leads can cause fluid to build up in the chest cavity in those patients. In these cases, the generator is placed in the abdomen.

Pacemaker surgery is generally successful but significant complications may occur in up to 33% of cases. However, most complications can be managed without long term harm to the patient. The most common serious complication is lead dislodgement where the tip of the lead is floating in the heart rather than securely attached. Lead dislodgement necessitates placing a new lead as the dislodged lead will not pace if it is not attached. Other complications may include the lead breaking or the pacemaker becoming infected.

Most patients have a recheck examination 2-3 weeks after the placement of the pacemaker. The patient needs to be kept quiet throughout this time period. At the recheck, the pacemakers can be reprogrammed to best maintain battery life and to suit the patient’s activity levels. Periodic recheck examinations will check battery life and pacemaker function. In general, the batteries last 5-7 years. The generators can be replaced when the battery runs out. When the battery runs low, the pacemaker defaults to an energy saving heart rate so it does not stop suddenly. Patients need to avoid strong magnets near their necks but being near microwaves is OK for modern pacemakers.

References

Moise NS: Pacemaker Therapy. In Fox PR, Sisson D, Moise NS (eds): Textbook of Canine and Feline Cardiology. Philidelphia, Saunders, 1999, pp 400-425.

Bright JM. Pacemaker Therapy. In Smith FWK JR, Tilley LP, Oyama MP, Sleeper MM eds: Manual of Canine and Feline Cardiology, 5^th ed. St Louis, MO, 2016.Elsevier, pp 382-393.

Figure 1. Pacemaker generator (GEN), lead and active fixation tip (AF) with bipolar electrodes (arrowheads).

Figure 2. X-ray of a small dog showing the pacemaker generator in the neck (Gen) and the pacemaker lead (Lead) extending from the generator through the neck and into the heart. For orientation purposes, the spine runs along the top of the picture the breast bone runs along the bottom of the picture and the front legs are under the lead legend. The lead is the white structure running from the generator into the heart.