The present paper reviews classification and mode of action of agents that 
suppress extrasystoles and tachyarrhythmias. These are classified according to 
their electrophysiological effects observed in isolated cardiac tissues in vitro 
(Vaughan Williams, 1989). Fast sodium channel blockers (class I) which reduce 
the upstroke velocity of the action potential are usually subclassified into 
three groups, class I A-C, according to their effect on the action potential 
duration. Beta-adrenergic antagonists (class II) exert their effects by 
antagonizing the electrophysiological effects of beta-adrenergic catecholamines. 
Class III antiarrhythmic agents (eg amiodarone) prolong the action potential and 
slow calcium channel blockers (class IV) suppress the calcium inward current and 
calcium-dependent action potentials. The classification of antiarrhythmic drugs 
is still under debate. This particularly applies to agents of class I and III. 
The effect of class I agents is frequency-dependent because the binding affinity 
of these drugs to the sodium channel is modulated by the state of the channel 
(modulated receptor hypothesis). Class I agents bind to the channel in the 
activated and inactivated state and dissociate from the channel in the rested 
state. This occurs at a drug-specific rate so that class I agents can be 
subclassified into only two groups, namely in those of the slow- and 
fast-recovery type respectively (time constant of reactivation greater or 
smaller than 1 s). Slow-recovery class I agents affect regular action potentials 
at normal heart rates which can more easily lead to a lengthening of the QRS 
duration in the ECG, to conduction disturbances and hence to pro-arrhythmic 
effects.(ABSTRACT TRUNCATED AT 250 WORDS)