Bradycardia Detection And Therapy

All currently available ICDs provide basic single-chamber ventricular (VVI) pacing with separate programmable post-shock lower rate limit and output. Recently, dual-chamber devices have been introduced with use of an atrial lead for diagnostic use only or for atrioventricular (AV) synchronized pacing. These devices allow multiple programmable pacing modes including DDDR, DDD, DDIR, DDI, AAIR, AAI, VVIR, and VVI. These expanded pacing modes have obviated the need for a separate dual-chamber pacemaker that is sometimes necessary in the chronically ill patients who typically receive ICDs. Additionally, they will likely be shown to reduce the incidence of inappropriate shocks attributed to SVT.

In AV-synchronized devices, the ICD continues to sense in both chambers for tachyarrhythmias regardless of the programmed bradycardia pacing mode. To maintain proper sensing, both atrial and ventricular sensing thresholds are adjusted with autogain. The ICD has multiple blanking periods to avoid post-pacing depolarization, T-wave oversensing, and cross-communication between chambers. To avoid undersensing

Atrial Fibrillation With

Fig. 2. ICD delivery of defibrillation shock for termination of VF. From top to bottom, intracardiac ventricular electrogram, intracardiac atrial electrogram, and ventricular shock electrocardiogram. Note that in addition to VF there is atrial fibrillation (AF) that was also terminated with the defibrillation shock.

Fig. 2. ICD delivery of defibrillation shock for termination of VF. From top to bottom, intracardiac ventricular electrogram, intracardiac atrial electrogram, and ventricular shock electrocardiogram. Note that in addition to VF there is atrial fibrillation (AF) that was also terminated with the defibrillation shock.

Atp Terminating
Fig. 3. ICD electrogram showing ATP terminating VT. From top to bottom, intracardiac ventricular electrogram and ICD marker channel with annotations and intervals in ms. VT Rx = VT therapy, TS = VT sense, TD = VT detection, TP = VT pace (ATP), VS = ventricular sense.

of tachyarrhythmias, short cross-chamber blanking periods after paced-events and no cross-chamber blanking after sensed events are necessary. AV synchronous devices have programmable refractory periods available for bradycardia functions, but these refractory periods do not affect tachyarrhythmia detection.

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