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Patient
74-year-old man with chronic AF;
Trace
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;
Exergue
The refractory periods of the two branches may differ; a branch block can thus preferentially appear when the RR cycles are short.
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.