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Trifascicular heart block

See brief interpretation of 12 lead EKG above

Trifascicular block is seen on the EKG as a combination of a fascicular block (hemiblock), right bundle branch block (RBBB) and first degree av block.

It is a bit tricky, as this EKG pattern results (but not always) from a combination of right bundle branch block and block of the two fascicles of the left bundle branch (hence the term trifascicular), however with one of these three blocks being incomplete (see left illustration below). If all three fascicles are completely blocked a complete heart block equivalent to third degree AV block results (see right illustration below).

trifascicular block conduction Trifascicular heart block

As noted the EKG pattern consisting of fascicular block, right bundle branch block and first degree heart block is not always caused by trifascicular block. This pattern could potential be due to “true” first degree heart block combined with bifascicular block, thus with one of the intraventricular fascicles being fully intact. An absolute diagnosis of trifascicular block therefore require electrophysiological examination.

— Mathias

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