Delayed afterdepolarization implies-

Ans. is 'd' i.e., All of the above Triggered Activity* Triggered activity occurs when abnormal action potentials are triggered by a preceding action potential, and can result in either atrial or ventricular tachycardia. The abnormal impulses are seen as spontaneous (yet triggered) depolarizations that occur during either phase 3 or 4 of an action potential (called "afterdepolarizations"). Triggered activity is more likely to occur when the action prolonged action potential manifested as a long Q-T interval on the electrocardiogram (long Q-T syndrome). Drugs that prolong the action potential duration, such as potassium-channel blockers (class III antiarrhythmics) can sometimes precipitate triggered activity.* After depolarizations can be either early or delayed. Early afterdepolarizations occur during late phase 2 or phase 3 and can lead to a salvo of several rapid action potentials or a prolonged series of action potentials. This form of triggered activity is more likely to occur when the action potential duration is increased. Therefore, drugs that decrease action potential duration (e.g., lidocaine) are often effective against tachyarrhythmias generated by this mechanism. Calcium channel blockers, by decreasing gCa++ can also be effective in their treatment.* Delayed afterdepolarizations occur in late phase 3 or early phase 4 when the action potential is nearly of fully repolarized. The mechanism is poorly understood; however, this type of arthythmia is found to be associated with high intracellular Ca++ concentrations as occurs with digitalis toxicity or excessive catecholamine stimulation. The triggered impulse can lead to a series of rapid depolarizations (i.e. a tachyarrhythmia).