PROCAINAMIDE (PA) INCREASES THE EFFECTIVE REFRACTORY PERIOD OF THE ATRIA, AND TO A LESSER EXTENT THE BUNDLE OF HIS-PURKINJE SYSTEM AND VENTRICLES OF THE HEART. IT REDUCES IMPULSE CONDUCTION VELOCITY IN THE
ATRIA, HIS- PURKINJE FIBERS, AND VENTRICULAR MUSCLE, BUT HAS VARIABLE EFFECTS ON THE ATRIOVENTRICULAR (A-V) NODE, A DIRECT SLOWING ACTION AND A WEAKER VAGOLYTIC EFFECT THAT MAY SPEED A-V CONDUCTION SLIGHTLY. MYOCARDIAL EXCITABILITY IS REDUCED IN THE ATRIA, PURKINJE FIBERS, PAPILLARY MUSCLES, AND VENTRICLES BY AN
INCREASE IN THE THRESHOLD FOR EXCITATION, COMBINED WITH INHIBITION OF ECTOPIC PACEMAKER ACTIVITY BY RETARDATION OF THE SLOW PHASE OF DIASTOLIC DEPOLARIZATION, THUS DECREASING AUTOMATICALLY ESPECIALLY IN ECTOPIC SITES.