Case Studies In Small Animal

Cardiovascular Medicine

Case 14

Pacemaker Therapy Chapter from "Small Animal Cardiovascular Medicine" On-Line

Text from "Small Animal Cardiovascular Medicine"

CARDIAC PACEMAKER THERAPY

A pacemaker is a device that delivers battery-supplied electrical stimuli through electrodes in contact with the myocardium to produce an artificially triggered depolarization. Since the first implantable pacemakers were developed, the equipment and techniques available have improved dramatically. Recent advances in pacemaker technology include new lead designs, microprocessor-based circuitry, long-life batteries, telemetric manipulation, and newer pacing routines allowing rate-responsive adaptations and dual chamber pacing. Cardiac pacemakers are now much smaller and more durable than before and newer batteries can provide up to 12 years of continuous function. Probably the most important technologic advance has been the refinement of percutaneous transvenous pacing leads that can be introduced into the heart through a peripheral vein. Before this development, reliable pacemaker implantation required thoracotomy and surgical application of the lead directly to the epicardium.

Indications

The primary indications for artificial pacing in both humans and animals are symptomatic bradyarrhythmias and conduction disturbances which are unresponsive to medical control. In veterinary patients the most common arrhythmias treated with artificial pacing include sinus bradycardia/sinus arrest (usually due to sick sinus syndrome or high vagal tone), high grade second degree and complete atrioventricular blocks, and persistent atrial standstill.

The indications for cardiac pacing are outlined in Table 1. This table has been modified from the paper published in 1991 by the Joint Committee in the American College of Cardiology and the American Heart Association (ACC/AHA) outlining the indications for permanent pacing in humans. Although indications are not always clear-cut, we have modified the indications in humans to provide a classification system for use in dogs. This table is based on the reference cited and on our clinical experience. In class I are the conditions for which there is general agreement that a permanent pacemaker should be placed. Class II includes conditions for which pacemakers are used frequently but opinion diverges as to their necessity. Class III includes those conditions for which there is general agreement that a pacemaker is not necessary. Both arrhythmia definition and symptom correlation may be confusing and differences of opinion regarding which patients should receive artificial pacemakers exist. In most patients, the decision is based on the assessment of the resting ECG or a Holter monitor recording, clinical signs, the predicted response to artificial pacing, and the general health of the patient. It is important for the clinician to understand that the recognition of an abnormal electrocardiographic (ECG) rhythm alone is not an indication for artificial pacing. Clearly, in animals with documented ECG abnormalities and associated clinical signs (episodic weakness, syncope, etc.) artificial pacing is the preferred method of therapy when medical therapy is unrewarding. In patients with documented ECG abnormalities and clinical signs in which a definite correlation cannot be made, artificial pacing should be considered, but ultimately may not be pursued. However due to the cost, potential complications, and life-long special care inherent to permanent artificial pacing, it is not always rational therapy in asymptomatic animals, even when documented ECG abnormalities exist.

Contraindications

There are no absolute contraindications for pacemaker implantation, however, clinical and personal factors may preclude the decision to implant a pacemaker. Relative contraindications include debilitating generalized diseases and poor cardiac function secondary to a pre-existing cardiac disorder. Other serious medical problems may place the patient at an increased anesthetic risk and/or may significantly alter the patients prognosis even if pacemaker implantation is successful. Some patients have congestive heart failure or other cardiac manifestations of their underlying cardiac disorder that may not be controlled by artificial pacing (e.g., persistent atrial standstill). The cost of new pacemaker hardware is prohibitive in veterinary medicine. The use of previously used generators and donation of outdated components by pacemaker manufacturing companies provides free generators.¹⁴ Lead donations are more difficult to identify and may cost $700-800.

Dogs with hyperadrenocorticism (iatrogenic or naturally-occurring) have more complications than dogs without hyperadrenocorticism, in our experience. Dogs with hyperadrenocorticism have compromised immune function that makes the patient more susceptible to infection. They also often have very thin skin that predisposes the patient to having problems with wound healing of the area where the generator is implanted.

Case Selection

The primary clinical goal for patients with symptomatic bradyarrhythmias is to prevent clinical signs related to bradycardia and/or episodic cessation of cardiac activity. Complete resolution of episodic weakness and/or syncope is a reasonable goal in properly selected candidates. A secondary goal may be to increase the heart rate such that a normal cardiac output can be maintained at rest or over a wide range of activity levels. In human patients, some asymptomatic patients with cardiac arrhythmias are predisposed to sudden death and receive pacemakers to prevent this occurrence.

All patients should receive a complete medical screening and a complete cardiovascular evaluation prior to implantation of a pacemaker. At minimum, a biochemical panel, a complete blood count, an ECG before and after atropine administration, and thoracic radiographs should be performed. Patients with pre-existing cardiac abnormalities or abnormal findings on thoracic radiographs should be evaluated echocardiographically. The main rationale behind a complete patient evaluation is to identify problems which may affect anesthesia and/or significantly alter the long-term prognosis of the patient. If other significant medical problems exist, artificial pacing may not be pursued.

Pacemaker Equipment

The basic pacing system consists of an implantable pulse generator (IPG) that houses the circuitry and power supply and a pacing lead or electrode that transmits the electrical information between the IPG and the myocardium. Other ancillary equipment is necessary for proper implantation and maintenance of the pacing system.

Pulse Generator

The IPG is a sophisticated power pack that carries all the necessary electronics for pacing, hermetically sealed in a titanium or stainless steel casing. Modern pulse generators are compact, reliable and long-lived. The lithium anode battery (usually lithium iodide) has become the industry standard power supply. The major advantages are small size, long life, reliability, and a satisfactory voltage output for up to 90% of the battery’s life.

Three basic electronic circuits are housed within the IPG. The timing circuit controls the pacing interval, the output circuit controls the charging and discharging of the electrical impulse, and the sensing circuit is responsible for the recognition of spontaneous intracardiac signals. Most modern pacing circuits are further modified by a variety of other electrical accessories which include filters, protective devices, and the complex circuitry required for programmability, telemetry, memory, and rate-adaptive functions. Pacemaker pulse generators also contain a special electromagnetic switch that allows pacemaker function to be temporarily converted to asynchronous mode by placing it in a strong magnetic field.

Pacing Lead

The pacing lead is an insulated wire or set of wires that delivers the pulse stimuli from the generator to the myocardium and conducts intracardiac potentials to the sensing circuit. Originally, epicardial or epimyocardial leads were surgically affixed to the external surface of the heart. To eliminate the need for thoracotomy, transvenous leads are now preferred. There are two major types of lead systems. Unipolar leads provide one electrode (cathode) within the heart. Impulses travel through the lead to the myocardium and return to the IPG casing (anode) to complete the circuit. The potential advantages of unipolar leads are smaller design and theoretical sensing superiority. Because of the larger distance between the electrodes, the unipolar system uses a large sensing area. The major disadvantage of unipolar systems is the proximity of skeletal muscle to the circuit. Skeletal muscle may twitch as it is stimulated by the IPG discharge. This can be diminished by insulating a portion of the IPG and in many situations is only a temporary inconvenience that resolves when the IPG is walled off by fibrosis. Bipolar leads provide two closely spaced electrodes, both of which lie within the heart. The distal electrode is usually the cathode and the proximal electrode is a ring anode. Although many bipolar pacemakers require two connections at the generator causing slightly larger generator size, coaxial bipolar leads (one connection at the generator) on newer systems have overcome this problem. Bipolar leads are generally preferred due to a greater signal-to-noise ratio, less sensitivity to extraneous interference, and avoidance of skeletal muscle stimulation.

Lead fixation to the myocardium may be active (invading the myocardium) or passive (promote fixation by indirect means). Active leads provide myocardial penetration by grasping screws or small retractable prongs and are used for both transthoracic and transvenous implantation. Passive leads use tines or fins to enhance entanglement within the trabeculae of the right ventricle during transvenous placement. The choice of fixation is largely personal as equally good performance is obtained with both active and passive fixation when the leads are implanted properly. Several enhancements in lead design, including smaller surface area tips, porous electrodes, and steroid-eluding leads, have been developed to reduce stimulation threshold and increase pacemaker efficiency.

Table 1. Indications For Permanent Pacing in Dogs

Class I

Class II

Class III

©Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.