Antiarrhythmic agents of the formula:-
and their pharmaceutically acceptable salts,
wherein R is a C₁-C₄ alkyl group;
R¹ is a C₁-C₄ alkyl group;
X is -CH₂-, -CO- or -CH(OH)-;
n is 2, 3 or 4;
and "Het" is a 5- or 6-membered nitrogen-containing heterocyclic group which is attached to the adjacent carbon atom by a carbon or nitrogen atom and optionally contains a further heteroatom selected from 0 and N, said 5- or 6-membered nitrogen-containing heterocyclic group being either (i) substituted by a phenyl or benzyl group or (ii) fused at two adjacent carbon atoms to a benzene ring, said benzene ring being optionally substituted by 1 or 2 substituents each independently selected from halo and C₁-C₄ alkyl, and said 5- or 6-membered nitrogen-containing heterocyclic group being optionally substituted by up to 2 substituents each independently selected from oxo, halo and C₁-C₄ alkyl with the proviso that only one oxo substituent can be present.