Antiarrhythmic drugs increase pacing threshold, but this is rarely of clini
cal significance. Administration of flecainide acetate in a 75-year-old wom
an with an implanted AAIR pacemaker because of sick sinus syndrome caused a
n abrupt rise of pacing threshold and failure of pacing. Pacing threshold r
eturned to the normal value a few days after flecainide treatment was stopp
ed.