Note: Always check with your physician or alternative health care provider before using any herbal medical preparations!
From the University of Eastern Carolina health library:
An animal study showed that an extract of hawthorn significantly reduced the number of experimentally induced arrhythmias.  Although the use of hawthorn for arrhythmia in humans has not been studied scientifically, it traditionally has been used for this purpose. 
An active constituent in corydalis, dl-tetrahydropalmatine (dl-THP), may exert an anti-arrhythmic action on the heart. This action was observed in a preliminary trial with 33 patients suffering from a specific type of arrhythmia called supraventricular premature beat or SVPB.  Each patient took 300 to 600 mg of dl-THP per day in tablet form, and the dl-THP was found to be significantly more effective than placebo in reducing arrhythmia.
Are there any side effects or interactions?
Refer to the individual herb for information about any side effects or interactions.
 Al Makdessi S, Sweidan H, Dietz K, Jacob R. Protective effect of Crataegus oxycantha against reperfusion arrhythmias after global no-flow ischemia in the rat heart. Basic Res Cardiol 1999;94:71-7.
 Ellingwood F. American Materia Medica, Therapeutics and Pharmacognosy. Sandy, OR: Eclectic, 1919, 217-20.
 Xiaolin N, Zhenhua H, Xin M, et al. Clinical and experimental study of dl-tetrahydropalmatine effect in the treatment of supraventricular arrhythmia. J Xi'An Med Univ 1998;10:150-3.