Tachycardia Detection And Therapy

The ICD senses the intracardiac electrogram signal via the implanted ventricular sensing electrodes. The determination of a ventricular arrhythmia is recognition of a specified number of V-V intervals that fall within the programmed detection rate and duration criteria. Each V-V interval that falls inside a therapy zone increments the ICD event counter. When the device reaches a specified number of intervals for detection, the ICD will deliver the prescribed therapy. After delivery of therapy, the device either confirms termination of the episode or meets criteria for redetection, and the next programmed therapy is delivered. Some devices are committed to deliver secondary therapy if redetection is met. This is an important aspect of multiple therapies and is designed to assure tachycardia termination. However, it may lead to multiple ICD therapies for NSVT following initial ICD therapy. The ICD automatically adjusts its sensitivity thresholds following sensed and paced events through an auto-gain mechanism. This allows the device to automatically adjust its sensitivity during the tachycardia episode in response to the amplitude of the ventricular signal. For example, the autogain may rapidly increase sensitivity to detect small amplitude V-signals during VF rather than interpreting no signal and begin pacing. This auto-gain feature also allows the device to reduce the incidence of T-wave oversensing as well as cross-chamber sensing, particularly with paced events.

Atrioventricular-sequential devices incorporate supraventricular criteria that consider the atrial rhythm and exclude supraventricular tachyarrhythmias when detection of a high ventricular rate is observed. Some single-chamber ventricular devices have morphology discrimination algorithms, which assess the morphology of the ventricular sensed electrogram in order to withhold therapy for supraventricular tachyarrhythmias. Rate stability, sudden-onset, V>A event counters are all helpful in discrimination of SVT from VT.

Most devices allow programming of multiple VT zones. The VT zone is programmed with a lower detection cutoff, which would include any clinical VT events. Ideally, the cutoff rate for detection of tachycardia should be above the patient's maximal heart rate to avoid therapy for sinus tachycardia. Anti-tachycardia pacing schemes with burst pacing, ramp pacing, and inter-burst decrement are all currently available features. Burst pacing sequences consist of a set of ventricular pulses delivered at equal intervals to treat VT. Ramp pacing consists of a set of ventricular pulses delivered at decreasing intervals to treat VT. Following a failed anti-tachycardia pacing (ATP) attempt, inter-burst decrement allows a more aggresssive shortening of the intervals during either a burst or ramp attempt. The first pulse of a burst or ramp sequence (S1) is delivered at a calculated percentage of the tachycardia cycle length. The S1 percentage cycle lengths, number of pulses, interburst decrement, and number of ATP attempts are all programmable features. Additionally, cardioversion therapy (1-38 J) can be programmed in a VT zone. All VT zones have a programmable time limit on episode duration, when the device defaults to the next zone. Also, if a tachycardia is accelerated to a faster arrhythmia, then the ICD will deliver the therapy appropriate for the rate of the accelerated tachycardia.

Successful ICD treatment of VF occurs with defibrillation therapy. All devices are programmed with a VF zone because of the risk of acceleration with anti-tachycardia pacing or cardioversion. Because of the hemodynamic instability observed with fast VT or VF, the device is typically programmed to treat any sustained episode with intervals <300 ms (heart rate >200 beats per minute (BPM)) with defibrillation therapy. The device should be programmed with at a least 10-J safety margin over the defibrillation threshold observed either at implant or during follow-up testing. Up to six additional shocks may be programmed with maximal outputs programmed at the second or third

Icd Burst Therapy

Fig. 1. Various ICD therapies for VT and VF demonstrated during device-based ICD testing. All panels show, from top to bottom, surface electrocardiographic leads I, II, and V1 and ICD marker channel. CL = cycle length. (A) ICD delivery of antitachycardia pacing (ATP) for termination of VT. (B) ICD delivery of cardioversion for termination of VT. (C) ICD delivery of defibrillation shock for termination of VF.

Fig. 1. Various ICD therapies for VT and VF demonstrated during device-based ICD testing. All panels show, from top to bottom, surface electrocardiographic leads I, II, and V1 and ICD marker channel. CL = cycle length. (A) ICD delivery of antitachycardia pacing (ATP) for termination of VT. (B) ICD delivery of cardioversion for termination of VT. (C) ICD delivery of defibrillation shock for termination of VF.

shock and onward. Once a device exhausts its programmed therapies for a single episode, it quits (12).

Fig. 1, 2, and 3 illustrate the various ICD therapies available for VT and VF.

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Responses

  • WILLIAM BERNHARDT
    HOW IS TACCACARDIA DETECTED?
    8 years ago
  • Stanley
    How is tachycardia detected?
    7 years ago

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