Breithardt's research focuses on clinical electrophysiology, arrhythmia diagnostics and therapy, molecular genetics of arrhythmias, mechanisms of heart failure and of arrhythmias and acute cardiac death, mechanisms of re-stenosis as well as arteriosclerosis.

Initially, he conducted electron microscopic and histoautoradiographic studies on organ preservation at the Institute of Pathology of the University of Düsseldorf in collaboration with the Department of Applied and Clinical Pharmacology (Prof. M. Siess) of the Philipps University of Marburg.