Introduction

Cardiac anesthesia and surgery can generally be described as high risk, high reward. Pacemaker implantation exemplifies this since no other group of patients has such a high risk of sudden asystole and death, but yet can have a dramatic improvement in quality of life with successful implantation. The anesthetist can have a significant impact on the management of perioperative risk through understanding the surgical procedure and the needs of the cardiologist, selecting drugs and techniques that minimize the effects on heart rhythm and function, and having an emergency plan should severe bradycardia or asystole occur. Since pacemaker implantation can extend the lifespan of many animals, it is becoming more common to perform non‐cardiac elective and emergency procedures on animals with pacemakers.

Since the first report of pacemaker implantation in a dog with complete heart block [1], artificial pacemakers have become a mainstay for dogs, and less commonly cats, for bradyarrhythmia management. Because of the increasing number of pacemaker implantations, there are two common scenarios that will be encountered by the anesthetist: (1) anesthesia for pacemaker implantation or pacemaker system modification in a patient with a bradyarrhythmia or (2) anesthesia of a patient with a previously implanted and functional artificial pacing system undergoing an unrelated medical or surgical procedure [2]. While anesthesia delivery to these patients can be challenging, by better understanding implantation procedures, the function of pacemakers, and interactions of anesthetic and hemodynamic supportive drugs, overall anesthetic risk can be reduced.

Artificial pacemakers

Permanent transvenous artificial pacemakers are comprised of one or more pacing leads and a pulse generator (Fig. 35.1). The pacing lead wire delivers the electrical impulses to the heart and serves as the sensing electrode to detect native electrical activity. The lead consists of three parts: (1) the conductor (a coil of wire that conducts the electrical current), (2) lead insulation, and (3) a lead connector. Attachment of a transvenous pacemaker lead to the endocardial surface is most often accomplished using either passive (e.g., tined) or active fixation (e.g., fixed or retractable helical screw) leads (Fig. 35.2). Many leads incorporate a steroid eluting reservoir in an attempt to reduce scar tissue formation, which, if it were to occur, could contribute to unacceptably high pacing thresholds.

The pacemaker lead is the weakest link of the implantable artificial pacemaker system. It serves a passive function to deliver current developed from the pulse generator to the heart and to relay signals from the heart to the pulse generator. At a stimulus rate of 70 beats/min, the heart contracts 36.8 million times per year. Respiration, along with motion induced by ventricular and atrial contraction and closure of the tricuspid valve, exposes transvenous leads to profound mechanical stresses of flexion, torsion, and elongation that must be combated. Despite ever‐improving lead technology, failure related to conductor fracture, insulation breakage, or lead dislodgement and failure at the header remain recognized complications (Fig. 35.3) [3].

The pulse generator contains the battery and computer circuitry controlling the timing of electrical impulses sent to the heart, the sensing threshold of the pacing lead, and the response to sensed electrical activity. Pulse generator technology has progressed with the incorporation of advanced circuitry and microprocessors providing telemetrically programmable pacing parameters and a wealth of diagnostic data to help monitor and manage arrhythmias [4]. These advancements have allowed the development of more sophisticated and physiologic pacing modalities.

Pacemaker modalities

Pacemakers implanted in dogs most often employ a single lead that paces the apex of the ventricle (e.g., right ventricle for a single transvenous lead) irrespective of atrial activity. More advanced pacing systems may employ a single‐lead physiologic pacemaker (e.g., coordinates native atrial depolarization with ventricular stimulation) [5], a dual‐lead system to pace the atria and ventricles, or a three‐lead system to pace the atria along with simultaneous activation of the right and left ventricles [6]. Programmed pacemaker modalities are most often described using a five‐letter code (Table 35.1). The NBG coding system is a joint project between the North American Society of Pacing and Electrophysiology (NASPE) and the British Pacing and Electrophysiology Group (BPEG). Although some newer pulse generators defy description by the NBG code, most pacemakers implanted in veterinary patients employ only the first three or four letters. However, with a potential increase in implantation of cardiac defibrillators in dogs [7,8], veterinary medicine may have need of the complete five‐letter NBG code.

The first position in the code indicates the chamber paced, the second indicates the chamber sensed, the third indicates the response to spontaneous depolarizations, and the fourth commonly describes rate modulation. Hence a VVIR pacemaker paces and senses only the ventricle. In response to a sensed impulse, a VVIR pacemaker is inhibited until there is another period of quiescence in the sensed chamber. If no impulse is detected in a VVIR pacemaker at the end of a programmable lower rate interval, a ventricular stimulus will be delivered. The fourth position, most commonly an R, represents rate modulation wherein an activity sensor (e.g., accelerometer, respiratory rate, and QT interval) is able to increase the programmed heart rate during activity. A VDD pacemaker paces only the ventricle, but it senses both the atrium and ventricle. Delivery of a stimulating electrical impulse to the ventricle is triggered by a sensed atrial impulse and it is inhibited by a sensed ventricular impulse. A DDD pacemaker is able to sense and pace both the atrium and the ventricle. Biventricular (BiV) pacing implies simultaneous pacing of the right ventricular apex and left ventricular free wall.

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