Rate-dependent left bundle branch block
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.