This is sinus rhythm at about 88 bpm with a PR interval of 0.20 sec and a left bundle branch block (LBBB) configuration (QS in V1 and intrinsicoid delay in lead I).
At another time while this patient is being monitored on telemetry, the QRS is narrow with PRI of 0.20 sec at a rate of about 58 bpm.
Image 3 (histogram)
The histogram markers identified the beginning (Image 4)and termination (Image 5) of the QRS changes.
This revealed that the widening of the QRS is noted at a higher heart rate. This is also known as acceleration dependent LBBB (aberrancy). Recognition of this phenomenon will only be possible if observed over time. Random 12L lead or rhythm strips will be interpreted as LBBB.
This is different in many aspects from aberration recorded in normal hearts in response to premature stimulation. The latter often manifest as right bundle branch block (RBBB) because the right bundle branch has a longer refractory period. Acceleration dependent aberrancy usually displays LBBB; appears at relatively slow rates, often below 75 a minute; is independent of any change in the duration of the preceding cycle; is independent of abrupt change in cycle length, often appearing with gradual acceleration and frequently with a small, 0.5 ms or shorter, change of the cycle length; after an acceleration of the rate it may appear only after a number of cycles, all the cycles equal in duration; rarely disappears with acceleration of the heart rate; and is nearly always a marker of cardiac abnormality.
Fisch C. 1983. Aberration: seventy five years after Sir Thomas Lewis. Br Heart J; 50: 297-302 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC481414/