Narrow QRS complexes within left bundle branche block in a patient with chest pain.

We present the case of a patient with chest pain, palpitations, atrial fibrillation, left bundle branch block (LBBB) and occasional early narrow QRS complexes. An intracardiac study revealed rate-dependent LBBB with additional hidden rate-dependent slowing in the contralateral bundle branch, resulting in normalization of the early conducted QRS complexes.