Ectopic left atrial rhythm that produces QRS changes in absence of Wolff-Parkinson-White syndrome.

In an 18-year-old patient without manifest or concealed Wolff-Parkinson-White syndrome, spontaneous and paced left atrial impulses penetrated a left-sided AV nodal input and thereafter activated the ventricles in a normal fashion exclusively through the His-Purkinje system. On the other hand, sinus and paced right atrial impulses entered a right-sided atrioventricular nodal input that was completely dissociated from the left-sided input to subsequently activate the ventricles partly through Mahaim fibers and partly through the His-Purkinje system. The Mahaim fibers, which acted as "bystanders" during episodes of atrioventricular nodal reciprocating tachycardia, seemed to have extended from a "distal," common (right-sided) intranodal pathway (or "proximal" His bundle) to the right ventricle or, although this is less likely, to the right bundle branch. More studies are necessary to determine whether the association on the surface electrocardiogram of an ectopic slow left atrial rhythm with changes in QRS morphology (but not in QRS duration) always reflects the existence of Mahaim fibers.