Mr Jason Collinson, Chief Cardiac Physiologist
Essex Cardiothoracic Centre, Basildon and Thurrock University Hospital, UK
A 60-year-old male was implanted with a Sorin/MicroPort™ dual chamber pacemaker, following an emergent admission for syncope associated with intermittent atrioventricular block (AVB).
Three hours post implant, telemetry captured an episode of bradycardia as displayed in Figure 1.
An urgent pacemaker check was requested, with concern that there was device malfunction. The device settings programmed at implant are displayed below:
Mode: AAI-DDD (SafeR)
Lower rate: 60 BPM
Upper rate: 130 BPM
Paced AV delay: 220 ms
Sensed AV delay: 155 ms
Figure 1: Telemetry recording showing an episode of bradycardia a few hours after de-novo pacemaker implant. ECG leads displayed are lead I (top) and V1 (bottom).
Which answer bests explains the telemetry recording?
a. Loss of atrial capture.
b. Ventricular oversensing.
c. Normal pacemaker behaviour.
d. Loss of ventricular capture.
The telemetry recording displays normal pacemaker behaviour for the programmed settings and shows a mode switch episode, from atrial demand pacing (AAI) to dual chamber pacing (DDD), caused by an episode of AVB.
Figure 2: Telemetry recording showing the mode switch from AAI to DDD pacing with markers annotating events.
In the trace (Figure 2), we can see the first four atrial paced (AP) beats conduct to the ventricle, giving rise to an intrinsic QRS complex with right bundle branch block (rSR pattern in lead V1). There is subtle prolongation of the AV interval with each consecutive AP event: the 5th AP beat captures the atrium but fails to conduct to the ventricle, the 6th AP beat conducts to the ventricle, the 7th and 8th AP beat both fail to conduct to the ventricle, and this triggers a mode switch from AAI to DDD pacing. The rest of the ECG shows atrial paced (AP) and ventricular paced (VP) beats. Note, there is a clear change in QRS morphology with ventricular pacing.
The trace is an ECG example of Sorin/MicroPort’s™ SafeR algorithm in operation. SafeR is one of several manufacturer specific algorithms, designed to reduce unnecessary ventricular pacing, which can help reduce the detrimental effects of RV pacing in patients with sick sinus syndrome and intermittent AVB.
The algorithm works by providing AAI pacing, (this is actually ADI pacing with the ability to sense events in the atrium and ventricle), whilst monitoring AV conduction. If loss of AV conduction is detected by any of the four specific criteria below, the pacing mode switches from ADI pacing to DDD pacing, thus restoring AV synchrony.
The 4 criteria used to detect loss of AV conduction are:
• AVB I criteria – six consecutive long PR intervals;
• AVB II criteria – three non-conducted atrial events out of 12 consecutive cycles;
• AVB III criteria – two consecutive blocked atrial events;
• Pause criteria – V-V interval greater than the programmed pause duration (programmable at 2 or 3 seconds).
In this example, there were two consecutive blocked atrial events. Therefore, mode switch was triggered after AVB III criteria was met.
A device check confirmed normal pacemaker function, consistent with the programmed settings. Stored SafeR event electrograms, confirmed appropriate mode switch to DDD pacing, with AVB III criteria met. The patient was asymptomatic to the episode; however, the SafeR settings were reviewed. A minor change was made to the pause criteria, brought in to 2 seconds from the nominal 3 seconds. This adjustment allowed the device to react quicker to episodes of AVB, triggering mode switch to DDD pacing earlier and thus reducing the potential risk of bradycardia induced symptoms.
Microport™ CRM, Tech Corner: SafeR Pacing Mode: 2018. https://www.crm.microport.com/wp-content/uploads/2018/06/SafeR_revB_2018.pdf (accessed 08 June 2020).
Reproduced with thanks from Mr Jason Collinson and the British Heart Rhythm Society