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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 dis-charges. 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 sensing.
Medical progress detection of arrhythmia
The original implantable cardioverter–defibrillator was designed to detect only ventricular fibrillation, by means of a wave-form analysis termed a probability-density function. Use of this device indicated that therapy for organized ventricular tachycardia was also important. Subsequently, the rate of R waves detected by the defibrillator’s ventricular-sensing circuit became the standard measurement used to identify cardiac rhythm. In the present generation of defibrillators, the ventricular bipolar sensing circuit filters the incoming signal to eliminate unwanted low-frequency components (e.g., T waves and base-line drift) and high-frequency components (e.g., skeletal–muscle electrical activity). One or more tachycardia-detection zones may be programmed. The fastest rate, or ventricular-fibrillation zone, is treated by delivery of a shock. Zones with lower rate boundaries may be treated with antitachycardia pacing or low-energy synchronized shocks or, in some cases, just observed. Because the amplitude of the bipolar electrogram may be low or unstable during ventricular fibrillation, all implantable cardioverter–defibrillators allow sensitivity-gain adjustment. Pacing electrodes during intervals when an R wave is not sensed, in or-der to detect low-amplitude signals when ventricular fibrillation does occur. In many cases, the rates of sinus tachycardia or of other supraventricular arrhythmias may be within the zones set for detection of ventricular tachycardia or ventricular fibrillation, which may result in inappropriate delivery of the therapy. Therefore, most implantable defibrillators can be programmed to enhance the discrimination between supraventricular and ventricular arrhythmias. Single-chamber devices most commonly can distinguish the sudden onset of sinus tachycardia from ventricular tachycardia. They can also identify the stability of cardiac-cycle lengths in order to detect atrial fibrillation and can characterize morphology and width in electrograms. In dual-chamber devices, information from the atrial electrogram may be included in the algorithm used to perform the analysis. Features that enhance detection are primarily used in ventricular-tachycardia zones, where even a transient inhibition of the delivery of the appropriate therapy is undesirable. Early models delivered therapy after the criteria for detecting arrhythmia had been met, which could lead to the delivery of unnecessary shocks when the arrhythmia was spontaneously terminated.
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