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Contempo Updates | Clinician's Corner

Device Therapy for Cardiac Arrhythmias

Fred M. Kusumoto, MD; Nora Goldschlager, MD
JAMA. 2002;287(14):1848-1852. doi:10.1001/jama.287.14.1848.
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Since the introduction of the implantable pacemaker in 1958 and the implantable cardioverter defibrillator (ICD) in 1980, implantable devices for rhythm control are now commonly used for treating bradycardia and certain types of ventricular arrhythmias. The first pacemakers and ICDs were large devices (40-200 cm3) that required a prolonged hospitalization for implantation and postoperative recovery, and had few programmable features. In contrast, the current devices are significantly smaller (9-45 cm3), can be implanted on an outpatient basis, and provide a myriad of programming options to optimize therapy. During the last several years, the actual and potential indications for pacemaker and ICD implantation have expanded significantly as results from several large clinical trials have become available. These advances have led to increased patient and physician acceptance and a steady increase in implantation rates. In 1997, 153 000 new pacemakers and 29 000 ICDs were implanted in the United States.1 We summarize the function of and current indications for pacemakers and ICDs. More comprehensive discussions can be found elsewhere.25

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Figure 1. Dual-Chamber Pacing
Graphic Jump Location
AV indicates atrioventricular; ECG, electrocardiogram. A, Schematic of a dual-chamber pacemaker. Leads are usually placed in the right atrium and right ventricle. The leads are connected to a pulse generator usually located in the shoulder area. The leads are used to transmit current from the pulse generator (pacing) and also to transmit intrinsic electrical activity from the cardiac tissue (sensing). B, In sinus node dysfunction, loss of atrial activity leads to a prolonged pause. When a pacemaker is implanted, atrial (pink asterisk) and ventricular (blue circle) activity are sensed. If the low-rate timer expires without sensed atrial activity, an atrial pacing stimulus is provided to maintain the heart rate. C, In complete AV block there is no relation between atrial and ventricular activity. With dual-chamber pacing, atrial activity is sensed and the AV interval is initiated. When the interval expires, a ventricular pacing stimulus is provided. In patients with sinus node dysfunction sensed ventricular activity due to normal AV conduction inhibits the pacemaker. Using this set of timers (low-rate and AV interval), dual-chamber pacemakers maintain AV synchrony regardless of the cause of bradycardia.
Figure 2. Implantable Cardiac Defibrillator (ICD) Function
Graphic Jump Location
AV indicates atrioventricular; ECG, electrocardiogram. A, A schematic of an ICD. A lead is placed in the right ventricular apex. The ICD monitors the heart rate via a pacing and sensing electrode. If a ventricular arrhythmia is detected, a shock (usually 10-30 J) is delivered between a large proximal, high-voltage electrode and the ICD pulse generator. B, If the patient develops a ventricular tachyarrhythmia, the rapid ventricular activity is sensed by the ICD and a shock is delivered, which returns the patient to sinus rhythm.

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