Alternating (2:1) Right Bundle Branch Block

Report:

Sinus tachycardia 126/min

Left anterior hemiblock

Frontal plane axis –60o

Alternating right bundle branch block

Poor R wave progression

Nonspecific ST/T changes

Comment:

The diagnosis depends, critically, on identical PR interval in all the beats; otherwise it could be ventricular bigeminy with fusion. The “complete” RBBB just makes it, at 0.12” duration.

This is a good illustration of the lack of effect of RBBB on frontal plane axis (in the initial 0.06”, where it counts). It’s also a good example of poor performance of Lead 2 in demonstrating intraventricular conduction.

The precordial leads show extremely poor R wave progression; this is often spurious, even in COAD or asthma. With either hemiblock, the QRS morphology in the left precordial leads is extremely sensitive to vertical electrode displacement. More so in female mammals. A repeat trace (Fig 70a) , taken three days later, shows where the V leads should have been positioned. There is now a lesser degree of incomplete RBBB in all the beats.