Implantable Device Functionality Flashcards

A patient with a dual-chamber pacemaker programmed in DDD mode reports palpitations. The device interrogation reveals a tachycardia with a ventricular rate locked at the upper tracking rate. The intracardiac electrogram shows that each ventricular paced event is followed by a retrograde P-wave, which is then sensed by the atrial lead, triggering the next ventricular paced event. What is this rhythm called?

Answer: Pacemaker-mediated tachycardia (PMT)

Pacemaker-mediated tachycardia (PMT) is a re-entrant tachycardia in which the pacemaker acts as the antegrade limb (atrial sensing to ventricular pacing) and the patient's own AV node acts as the retrograde limb (ventricle to atrium conduction). This creates an endless loop where a ventricular paced event causes a retrograde P-wave, which is then tracked by the pacemaker, leading to another ventricular paced event, perpetuating the tachycardia at the upper tracking rate.

An ICD is programmed with SVT discriminators to prevent inappropriate shocks. Which of the following findings during a high-rate episode would most strongly suggest a diagnosis of VT, causing the device to override SVT criteria and deliver therapy?

Answer: AV dissociation

AV dissociation, where the atria and ventricles beat independently (often with the ventricular rate being faster than the atrial rate), is a highly specific marker for ventricular tachycardia (VT). ICDs use this as a powerful discriminator. If AV dissociation is detected, it strongly indicates a ventricular origin, prompting the device to classify the rhythm as VT and deliver therapy, often overriding other criteria that might suggest SVT.

A patient with a VVI pacemaker presents for follow-up. The device log shows multiple episodes of pacing inhibition despite the patient's intrinsic rate being below the programmed lower rate limit. The intracardiac electrogram shows that after each intrinsic QRS complex, a second, smaller signal is consistently sensed on the ventricular channel, which inappropriately resets the pacing timer. What is the most likely cause of this pacing inhibition?

Answer: T-wave oversensing

T-wave oversensing occurs when the device's sensing circuit incorrectly interprets the T-wave (cardiac repolarization) as an intrinsic R-wave. This extra 'sensed' event resets the pacemaker's timing cycle, leading to a pause and inhibition of pacing when it should occur. The description of a second, smaller signal consistently following each intrinsic QRS is the classic presentation of T-wave oversensing.

A modern pacemaker's ventricular automatic capture feature confirms capture after each pacing stimulus by looking for an evoked response. To avoid misinterpreting the pacing artifact's afterpotential as a captured beat, the sensing amplifier is blanked for a short period. What is this afterpotential?

Answer: A residual electrical charge at the electrode-myocardium interface following the pacing stimulus

Following the delivery of a pacing stimulus, a residual electrical charge, known as the polarization artifact or afterpotential, develops at the electrode-tissue interface. This signal can be large enough to be sensed by the pacemaker's own amplifier. Automatic capture algorithms must differentiate this artifact from the true evoked response of depolarized myocardial tissue by using a short blanking period to allow the polarization artifact to dissipate.

What is the primary purpose of the Mode Switch feature in a dual-chamber implantable device?

Answer: To prevent rapid ventricular pacing in response to a detected atrial tachyarrhythmia

The primary function of Mode Switch is to protect the patient from inappropriately rapid ventricular pacing. In a tracking mode like DDD, if the patient develops an atrial tachyarrhythmia (e.g., atrial fibrillation), the device would try to pace the ventricle at that same high rate. Mode Switch detects the high atrial rate and automatically changes the pacing mode to a non-tracking mode (e.g., DDI, VVIR), decoupling the ventricular pacing from the chaotic atrial rhythm.

An EGM from a dual-chamber pacemaker shows an atrial paced (AP) event followed 110 ms later by a ventricular paced (VP) event. This AP-VP sequence is intermittent and significantly shorter than the programmed AV delay of 200 ms. This behavior is most characteristic of which device feature?

Answer: Ventricular Safety Pacing (VSP)

Ventricular Safety Pacing (VSP) is a protective algorithm designed to prevent ventricular asystole that could result from inappropriate inhibition of ventricular pacing due to crosstalk (when the atrial pacing stimulus is sensed on the ventricular lead). If the device senses a non-physiologic event in the ventricle within a specific window (typically ~110 ms) following an atrial paced event, it will deliver a ventricular pace at that very short AV delay to ensure a ventricular contraction occurs. This EGM pattern is the classic signature of a VSP event.