Page 3
The pulse generator is usually placed in a subcutaneous pocket in the pectoral region. It contains a header with ports for leads, the battery and capacitors, memory chips, integrated circuits and microprocessors, and the telemetry module. The transvenous right ventricular lead contains the shock coils and pacing electrodes. Additional leads may be connected for right atrial or left ventricular pacing.
Now reanalyze the rhythm before delivering shocks and painlessly dump the stored charge when the criteria for detection are no longer met. In an implanted cardioverter–defibrillator, two basic methods are used to terminate arrhythmias: antitachycardia pacing and direct-current shocks. Physicians select the method to be used first to de-liver therapy in each tachycardia-detection zone. Antitachycardia pacing is a standard electrophysiological technique that is useful for terminating monomorphic tachycardias. The electrophysiologist can program the device to deliver one or more bursts of pacing in an attempt to terminate the tachycardia. The characteristics of the bursts can be programmed and may vary, depending on the detection zone. Antitachycardia pacing is painless for the pa-tient and, because the capacitor does not need to be Battery Lithium–silver vanadium oxide.
Systems that can be used for defibrillation and resynchronization are more ex-pensive. The costs of the implantation procedure include only payments for the hospitalization and physicians’ services. Defibrillator charged, can be delivered rapidly. However, anti-tachycardia pacing is not always effective, and it can accelerate ventricular tachycardia or, if applied during a supraventricular rhythm, induce a ventricular arrhythmia. Thus, delivery of a shock is always included in the prescription for therapy when anti-tachycardia pacing is ineffective. All implantable cardioverter–defibrillators can be programmed to deliver either synchronized, usu-ally low-energy shocks (less than 5 J) or unsynchronized high-energy shocks. Low-energy shocks may have very short charge times, but they may accelerate ventricular tachycardia and, in spite of the low energy, are uncomfortable for the patient. High-energy shocks are used in the zone with the highest rate and in zones with lower rates, if antitachycardia pacing or low-energy shocks are either unsuccessful or not programmed. Traditionally, the energy of the first shock is set at least 10 J above the threshold of the last defibrillation measured. Early models used monophasic wave forms, but the use of biphasic wave forms improved defibrillation thresholds. Defibrillation administered by transvenous systems that deliver up to about 30 J can be successful in most patients, but in rare cases, alternative lead configurations or high-energy devices may be necessary to deliver the therapy.