A compound of the following general formula or a salt thereof ##STR1## (where R.sub.1 and R.sub.2 which may be the same or different are each an alkyl group having 1 to 3 carbon atoms; R.sub.3 is a hydrogen atom or an alkyl group having 1 to 3 carbon atoms; R.sub.4 and R.sub.5 which may be the same or different are each a hydrogen atom or an alkyl group having 1 to 8 carbon atoms; m and n which may be the same or different are each an integer of 1 to 7; provided that R.sub.4 and R.sub.5 are not a hydrogen atom at the same time), and a process of preparing the same are disclosed. The compound is useful in the treatment of arrythmias because it exhibits desirably strong effects for treating arrythmias and minimum undesirable side effects.