Left Axis Deviation in LBBB Conduction

Report:

Sinus rhythm 72/min

Borderline left atrial abnormality (LAA)

Left bundle branch block

Comment:

In LBBB conduction the axis is derived from the entire QRS complex (unlike the situation with normal or RBBB conduction, where only the initial 0.06” are considered). Approximately 50% of LBBBs have LAD. The axis is not normally reported with LBBB – hence the omission above.

The frontal plane axis here is about –40o. This may imply extra pathology in the anterior superior fascicle of the left bundle branch and some authorities call this type of LBBB “divisional” and the one with normal axis “predivisional”. There is some evidence that LAD in LBBB may imply more severe myocardial disease and, perhaps, worse outcomes12.

Failure to attain monophasic R morphology in V6 may be due to either cardiomegaly or LAD itself; the latter makes the anterolateral leads exquisitely sensitive to even minor vertical lead displacement.

Below (Fig 22a) is another example, from a different patient, in AF.