Two children suffering from leukemia developed cardiac arrhythmias severe enough to produce symptoms. Extensive leukemic infiltration led to heart block in one patient and palliative radiotherapy was beneficial. Episodes of severe bradycardia, due to increased vagal tone, occurred in the other child. Drug interaction was the likely cause.
Habel A. Symptomatic Disorders of Cardiac Conduction in Childhood Leukemia. Am J Dis Child. 1974;128(6):841-843. doi:10.1001/archpedi.1974.02110310089018