Rate-dependent left bundle branch block

ECG: AF (disorganized, rapid atrial activity) with alternating narrow and wide QRS complexes: when the ventricular rate is relatively slow, narrow QRS pattern; when ventricular conduction improves and the ventricles are faster, rate-dependent complete left bundle branch block pattern;

Comments: This tracing shows a functional rate-dependent left bundle branch block (which appears beyond a given heart rate). This conduction abnormality appears when a supraventricular impulse occurs while the left branch is still in its refractory period.

The functional block called phase 3 block (phase 3 of the action potential) is therefore tachycardia-dependent and makes the electrocardiographic interpretation of a tachycardia more difficult (differentiation between ventricular tachycardia and supraventricular tachycardia with conduction abnormality).

When the activity is blocked in one of the branches (long refractory period), there is a hidden retrograde conduction on the blocked branch which maintains the likelihood of a “functional” block on the ensuing cycle.

Take-home message: The refractory periods of the two branches may differ; a branch block can thus preferentially appear when the RR cycles are short