Dr. Marek Jastrzebski has written extensively on the differential diagnosis of non selective His Bundle pacing (Ns HBP) versus myocardial only capture.
In Ns HBP not only is the His bundle captured but also is a segment of the myocardium in the IV septum.The ORS morphology is influenced by the signal from the captured segment of myocardium and the electricity traveling along the His Bundle resulting in a fusion beat.
His diagnostic algorithm is:
is there QRS notching in lead I v5 or v6 if yes then there is myocardial pacing
If no ,, then ask is the v6RWPT greater than 100 msec if yes then there is myocardial pacing.
However, there are false positives in the use of the notching rule wherein there may be QRS notching in cases of confirmed His Capture. particularly in cases of LBBB. A non corrected intra-hisian conduction defect resulting in a prolonged HV interval may occasionally result in Lateral lead QRS notching.
1)Jastrzebski M, ECG Pacing criteria for differentiating conduction system pacing from myocardial pacing.Arrhythmia and Electrophysiology Review June 2021 p 172