espite advances in emergency medical systems and in techniques of resuscitation, sudden death from cardiac arrest remains a major public health problem. Most persons who have an out-of-hospital cardiac arrest do not survive. 1,2 Those who are resuscitated may have severe, long-term cognitive impairment and motor impairment due to delays before a stable rhythm could be restored. In the 1970s, motivated by the death of a colleague, Drs. Michel Mirowski and Morton Mower, and their colleagues, developed the concept of an implantable device that could automatically monitor and analyze cardiac rhythm and deliver defibrillating shocks when it detected ventricular fibrillation. 3,4 After years of testing, in 1980 the first clinical implantation was performed in a young woman with recurrent ventricular fibrillation. 5 Subsequently, the implantable cardioverter-defibrillator evolved from a therapy of last resort for patients with recurrent cardiac arrest to a management standard for use in primary prevention (the prevention of a first life-threatening event) and secondary prevention (prevention of a recurrence of a potentially fatal arrhythmia or cardiac arrest) in patients with coronary heart disease. An implantable cardioverter-defibrillator system comprises a pulse generator and one or more leads for pacing and defibrillation electrodes (Fig. 1). The pulse generator has a number of components (Table 1). 6 A sealed titanium can encloses a lithium-silver vanadium oxide battery, voltage converters and resistors, capacitors to store charges, microprocessors and integrated circuits to control the analysis of the rhythm and the delivery of the therapy, memory chips to store electrographic and other data, and a telemetry module. Technological advances have made possible a gradual reduction in the size of the pulse generator, permitting subcutaneous implantation of the defibrillator on the anterior chest wall in most patients. The top of the pulse generator contains an epoxy resin header for connecting the pacing and defibrillation leads. The defibrillation leads must be capable of delivering high-energy shocks to the heart without damaging the myocardium. In the earliest defibrillators, epicardial patches were used, but transvenous leads are now standard. Each defibrillation lead contains one or two coils that dissipate heat during high-voltage discharges. In most systems, the pulse generator can serve as a part of the defibrillation pathway. The defibrillation lead also contains bipolar electrodes, which are used for ventricular pacing and sensing. If both pacing electrodes are independent of the defibrillation coils, they form what is called a dedicated bipole. If a defibrillation coil is linked to the ring electrode for sensing, it forms what is called an integrated bipole. Both systems are effective in most patients. Active-fixation (screw-in) and passive-fixation lead systems are in clinical use. Dual-chamber and biventricular devices also have ports for atrial or left ventricular electrodes, which are used for pacing and...
