Chapter 21 Pacemaker Therapy

Janice McIntosh Bright

“It struck me how easily excitable the myocardium is. You just touch it and it gives you a run of extra beats—so why should the heart that is so sensitive…die because there’s nothing there to stimulate the chest?”

Paul M. Zoll, inventor of the first clinically successful external pacemaker (Jeffrey K: Pace 22:1713;1999)

INTRODUCTION

Pacemaker therapy has become a common method of treating symptomatic bradycardia in dogs and cats. Pharmacologic therapy may provide temporary chronotropic support for patients with bradycardia, but successful long-term management usually requires implantation of a permanent pacemaker. The ultimate objective of cardiac pacing is to normalize cardiac function by providing optimal heart rate and rhythm chronically.

INDICATIONS FOR PACING

• Permanent pacemaker implantation is indicated for treatment of patients with chronic symptomatic bradyarrhythmias. In dogs the most frequent clinical manifestations of bradycardia are syncope, collapse, exercise intolerance, and/or lethargy. Less commonly, bradyarrhythmia results in congestive heart failure or seizures. In cats the most frequent complaint associated with bradycardia is syncope.

• The most common bradyarrhythmia necessitating pacemaker therapy in dogs and cats is advanced (high-grade second- or third-degree) atrioventricular (AV) block. Antibradycardia pacing is also indicated in dogs with sinus node dysfunction (sick sinus syndrome) and permanent atrial standstill. Less often permanent pacing is used to prevent vasovagal syncope.

• Whereas most dogs needing pacemaker therapy have little to no underlying structural heart disease, many cats have AV conduction block associated with underlying myocardial disease.

PRE-IMPLANTATION EVALUATION

• A standard electrocardiogram (ECG) should be obtained for definitive diagnosis of the arrhythmia. Occasionally, serial ECGs are needed to confirm an intermittent bradycardia. If it is unclear whether a patient’s clinical signs are due to bradyarrhythmia, correlation of the clinical signs with arrhythmia should be obtained using some form of ambulatory ECG monitoring (Holter monitoring or event recording). Ambulatory ECG monitoring allows the ECG to be recorded over a longer period of time, either continuously for 24 to 48 hours (Holter monitoring) or intermittently during weeks to months (event recording).

• After confirming that an indication for permanent cardiac pacing exists, the most appropriate pacing system and pacing mode for the patient should be determined. Factors to be considered for this determination include: (1) specific underlying rhythm disturbance, (2) overall physical condition, (3) nature of any associated medical problems, and (4) exercise requirements of the patient.

• Therefore, all patients needing implantation of a permanent pacemaker should receive a thorough medical and cardiovascular evaluation prior to implantation to identify presence and severity of structural heart diseases or noncardiac diseases. Certain co-existing conditions may require additional medical treatment. Furthermore, these conditions often affect the type and programming of the pacing system used.

COMPONENTS AND TYPES OF PACING SYSTEMS

• A permanent artificial cardiac pacing system consists of a pulse generator (pacemaker) and a pacing lead. The pulse generator contains electronic circuitry and a lithium-iodide power cell (battery) sealed within a metal case with a connector block into which the lead is inserted. The pacing lead consists of an insulated wire or set of wires to conduct electrical impulses from the generator to the myocardium, but the lead also enables the generator to detect or sense native (endogenous) cardiac electrical activity.

• Modern cardiac pacemakers have sophisticated electronic circuitry capable of discharging pacing impulses of varying duration and voltage, sensing intracardiac signals, filtering signals, providing rate response functions, and storing rhythm data. Data retrieval and generator programming are done via telemetry using a pacemaker programmer. The programmer is also used to display real-time ECG and intracardiac electrograms and to test battery life, lead impedance, retrograde ventriculoatrial conduction, and pacing thresholds.

• The heart can be paced by an epicardial lead placed surgically (or thoracoscopically) or by a permanent endocardial pacing lead placed transvenously. Permanent transvenous pacing has largely replaced epicardial pacing because of its ease and safety; however, epicardial leads remain available and may be preferred when venous access is limited or when there is an associated condition that would increase likelihood of bacteremia or embolism with a permanent transvenous lead (e.g., a focus of sepsis or a hypercoaguable state). Epicardial leads are often used in feline patients.

• Permanent transvenous pacing leads use either active or passive fixation for attachment of the lead tip to the endomyocardium. A passive fixation transvenous lead has a “collar” of tines encircling the distal tip which anchor the lead by becoming enmeshed in the right ventricular trabeculae (Figure 21-1). An active fixation lead has a small metal helix that exits the tip of the lead to penetrate the endomyocardium (see Figure 21-1). Although the type of fixation of a transvenous lead does not appear to affect the incidence of lead dislodgement, it is wise to avoid use of passive fixation in animals with significant right ventricular dilation.

• Another consideration for selection of the pacing system is whether a unipolar or bipolar system is desired. A unipolar pacing system uses the lead tip as the cathode (negative pole) of the electrical circuit and the metal case of the pulse generator as the anode (positive pole). The impulse travels from generator to myocardium via the lead and returns to the generator via the soft tissues. A major disadvantage of unipolar pacing is the proximity of the electrical circuit to skeletal muscle which may result in skeletal muscle twitching. Advantages of unipolar pacing include smaller diameter of the pacing lead, a single attachment site of the lead to the epicardium when epicardial pacing is used, and superior sensing of endogenous cardiac potentials. Bipolar pacing systems have two closely spaced electrodes located distally on a transvenous lead or closely adjacent at two ends of an epicardial lead (Figure 21-2). One electrode (usually the distal electrode on a transvenous bipolar lead) is the cathode, and the other is the anode. Electrical impulses travel to the cathode from the pulse generator and return to the anode to complete the circuit. In the majority of cases a bipolar pacing system is preferred because there is less potential for electromagnetic interference (EMI) with bipolar pacing and because of absence of skeletal muscle stimulation.

• Although lead length should be given consideration prior to pacemaker implantation, length is often determined by availability. Most leads are longer than necessary, and excess length can be accommodated within the generator pocket; however, in large or giant breed dogs, adequate lead length may become an important factor.

Key Point

When selecting a permanent pacing lead, it is imperative that the lead be compatible with the pacing site selected (epicardial vs. endocardial, atrial vs. ventricular), that the lead and generator be of compatible size (typically IS-1), that the polarity of the lead matches polarity of the generator, and that the lead is of sufficient length.

Figure 21-1 The distal ends of several transvenous (endocardial) pacing leads are shown. The lead at the top is a passive fixation lead. Note the collar of tines (arrow) which become trapped within trabeculae to anchor the tip of the lead to the right ventricular endocardium. At the bottom are two active fixation leads, one showing the helix retracted into the lead for placement and the other showing the helix extruded for as it would be for attachment into the endomyocardium (arrowhead).

Figure 21-2 Shown are distal tips of two epicardial pacing leads. The lead at the bottom of the photo is a unipolar lead with a single electrode (cathode) which has an epicardial stab-in fixation mechanism. The lead at the top of the photo is a bipolar lead with two suture-on electrodes (cathode and anode).