Two pacing spikes on the T wave in a single-chamber pacemaker: What is the mechanism?

We report a case of a 54-year-old female, known symptomatic sick sinus syndrome, who had undergone a single-chamber pacemaker (VVI, St. Jude Medical, VERITY ADxXL SC SN 2413520) implantation in 2011. Electrocardiography (ECG) performed on a recent visit to the pacemaker clinic showed intermittent double pacing spikes on top of the T wave (Fig. 1A). What is the mechanism?

Commentary

The 1st, 4th, 6th, 8th, 10th, and 12th QRS complex represents captured beats (Fig. 1A) as evidenced by a pacing spike followed by myocardial depolarization. The 2nd QRS complex is an intrinsic beat which is followed by two pacing spikes, of which the second spike has captured the ventricle. Here, the 1st pacing spike following the 2nd QRS complex was delivered due to the under-sensing of the R wave. However, there was no evoked myocardial response due to physiological refractoriness of the ventricle as the spike fell on the apex of the T wave. The device interprets this as a loss of capture. This made the pacemaker deliver a back-up pulse of high output (AutoCapture phenomenon) which captured the ventricle as the pacing spike fell outside the refractory period. The 5th, 7th, 9th and 11th QRS complex (Fig. 1A) represents intrinsic beats followed by two pacing spikes, one on the ST segment and the other at the onset of the T wave, which has not captured the ventricle as both the spikes fell on the physiological refractory period. As the 1st pacing spike is delivered after the intrinsic QRS complex (in the ST segment), it denotes improper sensing rather than due to pseudo-fusion.

Fig. 1

(A) Initial surface ECG showing two pacing spikes after the 2nd, 5th, 7th, 9th and 11th QRS complex. (B) The final ECG after programming shows intrinsic rhythm at 45 bpm.

The AutoCapture algorithm of St Jude is a unique feature that verifies the capture of every ventricular paced complex and responds by giving a back-up pulse of high output when no myocardial depolarization is detected within the 15–62.5 ms detection window following the pacing spike, thereby increasing the patient safety [1,3]. The evoked response detection system in this pacemaker model is either the DMax algorithm in unipolar stimulation (as in this case) or the PDI algorithm in bipolar stimulation. If the local potential were sensed relatively early, there is a possibility that the back-up pulse would have been withheld. Autocapture function is activated when there is a true loss of capture, fusion/ pseudo-fusion beats, micro-dislodgement of the lead, or undersensing of the R wave [3]. In a dual-chamber pacemaker, this phenomenon is frequently observed in the presence of fusion beats. Programming helps to avoid the delivery of the unwanted backup pulse. In a dual-chamber pacemaker, turning on the ‘Fusion Avoidance” algorithm and the “Autointrinsic Conduction Search” option prolongs the AV interval by 100 ms in the next cycle after the back-up pulse, thereby avoids fusion and enables intrinsic conduction [4]. In patients with an implanted pacemaker, the R-on-T phenomenon occurs when there is under-sensing of an intrinsic R wave or asynchronous pacing [2]. There was a potential risk of the R-on-T phenomenon in this patient as the pacing spikes fell on the vulnerable period of repolarization, which could lead to fatal ventricular arrhythmia or fibrillation. We reprogrammed the pacemaker parameters (programming settings on presentation: Threshold-1.5V, output- 3.5V at 0.4 ms, sensitivity- 2 mV, R wave- 1.2 mV, lead impedance- 388 Ω, lower rate limit- 60 bpm, Hysteresis- Off) and increased the sensitivity to 1 mV, kept the basal rate at 50 bpm with a hysteresis of 40 bpm. The final ECG of the patient shows intrinsic rhythm at 45bpm (Fig. 1B).

The phenomenon of two spikes is specific to the AutoCapture algorithm of St Jude, which is not available with other manufacturers. This case highlights the importance of knowing the normal pacemaker algorithms with a demonstration of the AutoCapture and back-up pacing algorithm specific to St Jude.

Disclosures

The authors have no competing interests, funding or financial relationships to disclose.

Declaration of competing interest

This manuscript is not under simultaneous consideration elsewhere and has not been previously published in similar form.

There is no source of funding or conflicts of interest.