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Cardiac resynchronization and the implantable cardioverter–defibrillator
Cardiac resynchronization is a recently developed technique in which biventricular pacing is used to improve ventricular function. In patients with de-pressed ejection fractions, intraventricular conduc-tion delay, and advanced heart failure (New York Heart Association [NYHA] functional class III or IV), cardiac resynchronization may improve hemodynamic function, increase exercise tolerance, and lower the NYHA functional class. Preliminary re-ports from two randomized trials indicate that combining cardiac resynchronization with defibrillator therapy may improve functional status and lower mortality.
Complications
The evolution of the implantable defibrillator from a large device that required an abdominal pocket and insertion of an epicardial lead system by thoracotomy to the present generation of smaller trans-venous pectoral devices has markedly decreased the complications related to implantation. The surgical complications are similar in type and frequency to those seen with routine pacemaker implantation. Infection occurs in 1 to 2 percent of cases after implantation and requires further surgery to remove the device. Malfunctions in a lead after implantation continue to be a problem. Fractures in a lead or failure in the insulation can cause false signals, which, when detected, prompt delivery of inappropriate shocks. Changes in the patient’s condition, the addition of drug therapy, or abnormalities in the levels of electrolytes may increase the defibrillation threshold. The unnecessary use of ventricular pacing may have led to an increased number of hospitalizations among the subjects in the second MADIT and those in the Dual Chamber and VVI Implantable Defibrillator (DAVID) trial,59 in which defibrillators that provide dual-chamber pac-ing were compared with defibrillators that provide ventricular backup pacing. Frequent shocks, whether appropriately delivered during a ventricular arrhythmia or inappropriately delivered in the absence of an arrhythmia, are the most common complications encountered after implantation of a defibrillator. When the shocks are appropriate, antitachycardia pacing can be re-programmed to improve its effectiveness, antiarrhythmic-drug therapy can be instituted or changed, or catheter ablation can be performed.60 When shocks are inappropriately delivered because of supraventricular arrhythmias in the detection zone, reprogramming of the defibrillator to include an arrhythmia-discrimination algorithm, drug therapy, or an unexplained syncope with inducible sustained VT or VF or with advanced structural heart disease and no other identifiable cause.
Primary prevention
Coronary disease, LV dysfunction, inducible VT
Chronic coronary disease, LVEF ≤30 percent
High-risk, inherited or acquired conditions (e.g., long-QT syndrome, Brugada’s syndrome, hypertrophic cardiomyopathy)
ICD therapy plus biventricular pacing Above indications with QRS ≥130 msec,
LV dilatation, LVEF ≤35 percent, and advanced heart failure
Contraindications
Unexplained syncope in the absence of structural heart disease or inducible VT or VF
Incessant VT or VF
VT or VF due to completely correctable cause
Psychiatric illness potentially aggravated by ICD therapy