The implantable cardioverter defibrillator (ICD), an electric device which includes a pacemaker, has undergone revolutionary changes. Although the device initially was developed only to defibrillate patients with ventricular tachycardia or ventricular fibrillation, who did not respond to antiarrhythmic drug therapy, all current models are able to provide defibrillation, low-energy cardioversion, antitachycardia pacing for ventricular tachycardia termination, and bradycardia pacing backup. | Low-level cardioversion is occasionally the first-line treatment for VT with rates greater than 150 to 180 and less than 240 bpm not responsive to tachypacing (Fig. 9-7B). The tiered ICD delivers the cardioversion synchronized with the QRS complex, thus avoiding the vulnerable period of the T wave and risking initiation of VF. The amount of energy may be programmed to a minimum of J. Significant reduction in pain perception may be noted below 2 J, whereas no perceptible differences may be observed between 5 and 34 J in many patients. The current required for conversion will vary with electrode location and the shape and surface area of the lead system being used. Another factor in conversion success may be the direction in which electricity travels across the heart. More efficient cardioversion has been affected by improvements in lead design, lead arrangement, and the shape of the pulse waveform (all current ICDs use a biphasic waveform which can be modified). If antitachycardia pacing or cardioversion fails to terminate the tachycardia after a programmed time interval or if the tachycardia accelerates, defibrillation therapy will be initiated (Fig. 9-7C).
