4-Oxospiro[benzopyran-2,4'-piperidines] as class III antiarrhythmic agents. Pharmacological studies on 3,4-dihydro-1'-[2-(benzofurazan-5-yl)- ethyl]-6-methanesulfonamidospiro[(2H)-1-benzopyran-2,4'-piperidin]-4-on e (L-691,121).