The pharmacological action represents a mechanism by means of a specific effect can be obtained. Depending on the class and type of the drug, the pharmacological action may be different.
In the case of electrophysiology, the drug directly acts at the level of the cells, affecting the mechanism of opening/closing of the ionic channels, as it happens with the anti-arrhythmic drugs. Due to the ionic permeability properties of the cardiac cells membrane, during the action potential, the opening of the ion channels generates ion currents that flow in/out of the lipophilic cell membrane.
The anti-arrhythmic drugs action is that of modifying such ion currents, acting on the structure of the ion channel, and trying to restore the physiological opening/closing mechanism of the ion channels. It may be that, instead of providing a benefit to the heart, such as the aforementioned desired effect, a new drug can negatively affect the ion currents, ending up to excessively modifying the amount of ion currents flowing throughout the cell membrane, thus increasing the risk of inducing a potentially fatal arrhythmias.