Characteristics and natural history of abnormal atrial rhythms in left isomerism.

The object of this study is to document variation in atrial pacemaker rate and P-wave axis, and to clarify the natural history of abnormally slow atrial rhythms in left isomerism. Standard 15-lead electrocardiograms of 50 patients with left isomerism were retrospectively studied. On the average, 9 electrocardiograms covering 7 years for each patient were available. For comparison, atrial rhythms in patients with right isomerism and in those with situs solitus and 5 representative congenital cardiac conditions were studied with the same study protocol. In left isomerism, atrial rhythm with constantly normal P-wave axis was exceptional and the presence of atrial rhythm with abnormal P-wave axis was the rule. The most frequent abnormal mean frontal P-wave axis was between -30 and -90 degrees, recorded in 70%. Slow atrial rates below the second percentile were observed in 50%, either transiently and recurrently (in 40%), or persistently (in 10%), and were associated with junctional escape in 42%. The number of patients with slow atrial rate increased with age. In right isomerism, multiple atrial rhythms were also frequent, but slow atrial rhythms was present in only 4%. Multiple atrial rhythms or bradycardia associated with junctional escape were rare in the group of patients with congenital heart disease and situs solitus. It is concluded that multiplicity and progressive slowing of the atrial rhythm are characteristic in patients with left isomerism.