Since the dawn of clinical electrophysiology, attempts have been made to pace the ventricles via the AV conducting system. It wasn’t until it was recognized that right ventricular apical pacing could result in left ventricular dysfunction, that interest was once again rekindled, although early attempts at His bundle pacing were limited by the available technology. Today, His bundle pacing, is used to recruit the intrinsic AV conducting system, thereby establishing “ventricular pacing” with a narrow QRS.

Pre-pacing - atrial flutter with high degree AV block and a narrow QRS.His bundle pacing - small bipolar stimulus artefacts (red vertical arrows) and a latency period (red highlight) indicating His-ventricular conduction. The subsequent QRS is identical, confirming normal ventricular depolarization.It is not surprising that the resting 12-lead ECG footprints of a patient with His bundle pacing can be easily missed or misinterpreted. The appearances are dependent on where capture occurs, whether it be “selective” or within the bundle or “non-selective” which also involves adjacent myocardium. Other factors that determine the pacing appearance include the pathophysiology of the conducting system, the pacing thresholds, and the programmed energy.Recognition of selective His bundle pacing on the 12-lead ECG is a

• bipolar or unipolar stimulus artefact
• short latency period – conduction tissue transmission
• narrow QRS.
• both QRS and T waves are identical to the pre-paced ECG

The 12-lead ECGs show identical narrow QRS pre-pacing and following His bundle pacing.Right: Small stimulus artefact (red vertical arrow) and latency period (red highlight).With non-selective His bundle pacing, the adjacent myocardium is also depolarized resulting in characteristic ECG changes.Recognition of non-selective His bundle pacing on the 12-lead ECG is a

• bipolar or unipolar stimulus artefact
• usually absent latency period
• a pseudo-delta wave
• T wave changes compared to pre-pacing

12-lead ECG showing non-selective His bundle pacing with a narrow QRS, but prominent pseudo-delta waves (red highlight) following the stimulus artefacts (red vertical arrows). The amplitude and duration of these pseudo-delta waves depend on the mass of tissue depolarized as well as the level at which the His bundle is depolarized. Selective His bundle pacing may become non-selective, if the voltage is increased thus depolarizing adjacent myocardium.T wave changes resultant from adjacent myocardial depolarization, compared to the pre-paced ECG are also a feature of non-selective His bundle pacing.

Non-selective His bundle pacing in a patient with atrial fibrillation.Left: Pre-pacing with upright T waves in leads I and V6.Right: His bundle pacing with subtle pseudo-delta waves and T wave inversion in leads I and V6.His bundle pacing may also completely or partially correct a right or left bundle branch block, as longitudinal dissociation, or the level of differentiation into the bundle branches, may occur high in the His bundle with the pacing distal to this.Pre-pacing: Sinus rhythm with 2:1 AV block and right bundle branch block.

Following His bundle pacing, the bundle branch block has resolved.

Non-selective His bundle pacing; stimulus artefact (red vertical arrow) followed by a pseudo-delta wave. There is now no bundle branch block.Although His bundle pacing is theoretically the ultimate site for physiologic ventricular pacing, it has inherent limitations such as high pacing thresholds, poor sensing, procedural issues, and the possibility of direct atrial capture.An alternate method is left bundle branch pacing also known as deep septal pacing using a bipolar screw-in lead anchored deep within the ventricular septum, so that it encroaches on the fan-like left bundle branch. It is particularly useful to correct left bundle branch block in patients otherwise requiring pacing for bradyarrhythmias. Unlike the His bundle, there is no fibrous capsule surrounding the bundle branches and hence the initial QRS is mostly fusion between left bundle and surrounding myocardium and therefore there is usually no latency. Depending on the exact location within the conducting system, different appearances may result from varying voltages. Also, of importance is the depth of the lead tip in the septum. The anode may lie to the right of the septum and thus, the right bundle or just the adjacent right septal myocardium can be paced at the same time as the left bundle.Here is an example of a pre-pacing ECG; sinus rhythm, right axis, first degree AV block and a left bundle branch block. Seep septal pacing was performed, and different voltages used to pace the heart.

Deep septal pacing: Low left bundle branch at 0.75 volts. (VVI, red vertical arrow).

Left anterior fascicular block, a partial right bundle branch block and no latency.Deep septal pacing: Low left bundle branch at 8 volts. (VVI, red vertical arrows).

With high voltage, we would expect anodal contribution and thus non-selective capture. In lead I, there is a suggestion of a pseudo-delta wave (red highlight). There is also a suggestion of trivial latency (yellow highlight). However, this is limited to only a few leads and one must take care not to over-diagnose this, particularly with the lead embedded low in the septum. The loss of the partial right bundle branch block is possibly due to anodal pacing.Deep septal pacing may be an alternative to cardiac vein lead placement for cardiac resynchronization therapy, when left ventricular lead placement fails.The interpretation of ECGs with conduction system pacing is very new and we are learning something every day. In the future, we may interpret these ECGs differently.I had a lot of help with this one. Thanks Irene Stevenson.Harry Mond