CHAPTER 89: PACEMAKERS AND DEFIBRILLATORS

INTRODUCTION

Cardiac implantable electronic devices (CIEDs), notably pacemakers and implantable cardioverter-defibrillators (ICDs), represent dramatic technological advances that have become essential in addressing the problems of arrhythmia and heart failure (HF) in modern cardiovascular patients. Although similar in design, these technologies represent two distinct paradigms of treatment. Where conventional single or dual-chamber pacemakers are used to address the fundamental problem of bradyarrythmia, ICDs are designed to automatically detect and treat tachyarrhythmias, specifically hemodynamically unstable ventricular tachycardia (VT) or ventricular fibrillation (VF). In the past two decades, a third paradigm has also emerged—cardiac resynchronization therapy (CRT)—in which biventricular (BiV) pacing is used to improve left ventricular (LV) pump function in patients with intraventricular conduction delay and systolic HF. Along with developments in fundamental approaches to pacing and defibrillation, recent years have also brought about exciting advances in design, with the development of leadless pacing and subcutaneous defibrillators. The purpose of this chapter is to provide an overview on pacing, defibrillation, and the indications for specific types of devices, and to review seminal clinical trials supporting their use in clinical practice.

INITIAL WORK

Although experimental use of short-term direct myocardial pacing for cardiac arrest had been described in 1932 by Dr. Albert Hyman,¹ Dr. Paul Zoll is credited with spurring widespread interest in cardiac pacing after his description of successful resuscitation through external subcutaneous electrical stimulation in 1952.² Pacing soon became indispensable to the nascent field of cardiac surgery when, in 1957, Dr. C. Walton Lillehei applied a direct myocardial electrode to the heart of a child undergoing a ventricular septal defect repair who developed complete atrioventricular (AV) block intraoperatively. He attached the electrode to an externalized transistor pacemaker developed by Earl Bakken.³ The first description of a technique for transvenous pacing in humans came only 1 year later.⁴ Subsequently, the field has undergone an explosion of development and miniaturization, although has remained astonishingly faithful in concept to these initial pioneering descriptions (Fig. 89–1). It was not until close to three decades of the seminal work of Dr. Michel Mirowski and Dr. Morton Mower that an “automatic” implantable defibrillator was used in humans for the termination of malignant ventricular arrhythmias.⁵ Fast forward more than a decade later to the first case report of BiV pacing in the treatment of dilated cardiomyopathy.⁶ Both pacing and defibrillation CIED systems share similar components, consisting of an impulse generator—the “battery” that stores electrical charge—and a lead or leads that are used to deliver electrical energy to either pace or defibrillate myocardial tissue. Although with similarities in overall design, it has been the process of developing and refining indications in large trials that has led to the adaptation of these devices for the use of specific clinical needs and that informs their continuing evolution today. An overview of key indications and seminal trials will now be given prior to discussion of the mechanics of ...