Class Ia drugs prolong the action potential and has an intermediate effect on the 0 phase of depolarization.

Class I agents are divided into three groups (Ia, Ib, and Ic) based upon their effect on the length of the action potential.

The prolongation of the action potential duration and refractory period, combined with the maintenance of normal conduction velocity, prevent re-entrant arrhythmias.

The class III agents exhibit reverse-use dependence (their potency increases with slower heart rates, and therefore improves maintenance of sinus rhythm).

Class III agents have the potential to prolong the QT interval of the EKG, and may be proarrhythmic (more associated with development of polymorphic VT).

Class III agents include: bretylium, amiodarone, ibutilide, sotalol, dofetilide, vernakalant, and dronedarone.

Since the development of the original Vaughan Williams classification system, additional agents have been used that do not fit cleanly into categories I through IV.

[24] This preserves the simplicity of the original Vaughan Williams framework while capturing subsequent discoveries of sarcolemmal, sarcoplasmic reticular and cytosolic biomolecules.

It retains but expands the original Vaughan Williams classes I to IV, respectively covering actions on Na+ current components, autonomic signalling, K+ channel subspecies, and molecular targets related to Ca2+ homeostasis.

It now introduces new classes incorporating additional targets, including: It also allows for multiple drug targets/actions and adverse pro-arrhythmic effects.