Cardiac Enigma: Investigating the Causes of Unexpected Pacing Cessation

Abstract

A young adult in their 20s with recurrent presyncope, a diagnosed case of complete heart block, underwent a dual-chamber pacemaker implantation with left bundle branch area pacing (LBBAP). An immediate post-procedure electrocardiogram (ECG) showed atrial and ventricular paced complexes. However, an ECG two hours later showed no pacing pulses with an extremely prolonged PR interval of 400 ms. What is the probable mechanism?

Case Report

Case

A young adult in their 20s with recurrent presyncope was diagnosed with an intermittent congenital complete heart block (CHB). She underwent dual-chamber pacemaker implantation with left bundle branch area pacing (LBBAP) (Medtronic DDDR) (Table 1, Supplementary Figures 1,2). An immediate post-procedure ECG showed atrial and ventricular paced complexes. However, an electrocardiogram (ECG) two hours later showed no pacing pulses with an extremely prolonged PR interval of 400ms (Figure 1). Magnet ECG showed ApVp rhythm at a paced atrioventricular (AV) delay of 150 ms. What is the probable mechanism?

Table 1.

Parameters of the Pacemaker and Important Timing Intervals.

Parameters	Values
Paced AV delay	150 ms
Sensed AV delay	120 ms
Post-ventricular atrial blanking (PVAB)	180 ms
Post-ventricular atrial refractory period (PVARP)	Auto
Upper tracking rate	130 bpm
Search AV +	On, maximum additional increase of 170 ms
Mode switch	175 bpm

Figure 1.

12-lead Electrocardiogram of the Index Patient.

Discussion

Lead malfunction and pacemaker device failure were ruled out as magnet ECG showed normal atrial and ventricular pacing. The sensed and paced AV delay was 120 ms and 150 ms, respectively, and did not explain the extreme AV delay. The search AV+ was ruled out as the highest PR interval (maximum sensed AV delay) possible with the programmed parameters was 290 ms (120 + 170 ms). In AV hysteresis mode, if eight or more Vs events in the previous 16 ventricular events are within 15 ms of the scheduled Vp event (or if ≥8/16 Vp), the sensed and paced AV delay is extended by 62 ms to promote intrinsic rhythm up to a maximum value of 600 ms (Tables 2 and 3).

Table 2.

Ventricular Pacing Reduction Algorithm.

Manufacturer	Trade Name	Switch to DDD	Switch to Atrial-based	Default Settings
Abbott	Not available, only AV hysteresis possible
Biotronik	Vp suppression	No Vs for 2 s (only within 450 ms of the last As/Ap counted) 2 consecutive Ap/As without Vs ≥ 3/8 atrial cycles without Vs >15 switches to DDD/hour in the previous 24 h	Extends AV interval to 450 ms after a single Vs, or after a search interval (30 s to 20 h, doubling on each failed check)	Off
Boston scientific	Rhythmiq	≥ 3/11 cycles with RR >150 ms longer than PP [AV synchrony loss]	Intrinsic AV conduction for ≥ 25 cycles and last <2/10 are Vp	Off
Medtronic	MVP	≥ 2/4 atrial cycles without Vs Backup VP after the second Ap/As (of Ap/As pair without ventricular event in between)	After 1 min (doubling after each failed check, up to 16 h), when the check confirms Vs event	On (active 30 mins after implantation)

Table 3.

AV Hysteresis Algorithms.

Manufacturer	Trade Name	Algorithm	Evaluation Frequency	Default Settings
Abbott	VIP	Extends AVI in 100 ms for 1 cycle. If any Vs comes, extended AVI is maintained. Suspended if As/Ap rate is ≥ 110 bpm. Maximum extension 350 ms.	Every minute	Off
Biotronik	IRS plus	Extends AVI to 400 ms × 10 cycles. If any Vs comes, extended AVI is kept.	After any single Vs Every 180 Vp	Off
Boston Scientific	AV search +	Extends AVI to 300 ms for 8 cycles. If any Vs comes, extended AVI is kept. Reverts to programmed AVI when ≥ 2/10 are Vp	Every 32 cycles	Off
Medtronic	Search AV+	Extends AVI in 62 ms for 16 cycles, if ≥8/16 Vp, AVI is extended for another 62 ms up to a maximum of 250 ms more in addition to programmed AVI. If ≥ 8/16 Vs shorter than AVI minus 55 ms, AVI is shortened in 8-ms steps. Permanently suspended after 10 consecutive failed attempts at a 16-hour search	After 15 min (up to 16 h, doubling after each failed check)	Off

Note: AVI, atrioventricular interval; IRS, intrinsic rhythm support; VIP, ventricular intrinsic preference; Vs, Ventricular-sensed; Vp, ventricular-paced.

Managed ventricular pacing (MVP) mode is an algorithm to operate the pacemaker in AAI mode while having ventricular pacing as a backup with an immediate switch to DDDR mode once the AV conduction is absent (Table 1, Figure 2).^{1, 2} In MVP mode, the first check for AV conduction occurs after one minute. Subsequent checks occur at progressively longer intervals (2, 4, 8 … min) up to 16 hours and occur every 16 hours after that and the pacing mode is changed to AAI(R) from DDD(R) if AV conduction is detected (or vice versa when AV conduction is not detected). This mode is marked as AAI< = > DDD [AAIR < = > DDDR] in the device programmer, and when it is functioning in AAI(R) mode with backup ventricular pacing, an AAIR+ sign is mentioned in the top left corner of the programmer (Supplementary Figures 3, 4). ³ A look back at our case revealed a transition from DDD(R) mode [AsVp] mode to AAI(R) with a prolonged PR interval (Figure 3). This was probably due to intermittent congenital CHB with occasional presence of AV nodal conduction. In AAI(R) mode, when two out of four recent A-A intervals contain no intrinsic QRS, the pacemaker switches back to DDD(R) mode (Figure 4). If the conduction check fails and no intrinsic QRS is detected, backup ventricular pacing is given at 80 ms after the atrial sensed or paced events. In our patient, the likely mechanism for the extreme AV delay is that the conduction check detected intrinsic QRS complexes leading to a mode switch to AAI(R).

Figure 2.

Upper Panel. Mode Switch from AAI(R) to Temporary DDD(R). Two Out of Four Most Recent A-A Intervals with No Conducted Vs Event (Pacemaker Supplies a Ventricular Backup Pulse at 80 ms); Lower Panel. The Pacemaker Performs an AV Conduction Check, If Successful, It Changes from DDD(R) to AAI(R) Mode. If Unsuccessful, It Remains in DDD(R) Mode (Pacemaker Supplies a Ventricular Backup Pulse at 80 ms).

Figure 3.

An Electrocardiogram During the Procedure Shows That the Pacemaker Performs an AV Conduction Check, and It Changes from DDD(R) to AAI(R) Mode.

Figure 4.

Pacemaker in DDD(R) Mode with AsVp Rhythm When No Intrinsic Atrioventricular Conduction Exists.

The second iteration of MVP™ (MVP™ 2.0) incorporates a dynamic, adaptive A-A interval and a configurable maximum AV interval threshold, avoiding an erroneous detection of escape beat (in the presence of AV block) as AV conduction after a predefined AV interval. ² This case highlights the importance of knowledge of advance pacemaker algorithms to avoid unnecessary investigations.