title: Action potentials, afterpotentials, and arrhythmias.
abstract: Triggered activity must be added to spontaneous activity and to circus movement as a cause for extrasystoles and tachycardias of either atrial or ventricular origin. The activity of a triggerable focus requires phase 4 depolarization caused by an afterpotential; this distinguishes it from the activity seen in circus movement. A triggerable focus becomes rhythmically active only if driven at a critical rate or by a critically timed premature impulse; this distinguishes it from a focus of spontaneous or automatic activity. The ease of triggering a triggerable focus increases in the presence of catecholamines; triggerable foci in the atrium become quiescent when exposed to acetylcholine. At the present time, fibers within the coronary sinus provide the most persuasive example of triggered activity as a possible cause of arrhythmias of clinical significance. It is possible that the coupled extrasystoles of digitalis toxicity may be triggered; there is every reason to believe that further examples of triggered arrhythmias of possible clinical significance will be discovered.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000139:chemically induced / Q000503:physiopathology*; D002395:Catecholamines / Q000494:pharmacology; D003331:Coronary Vessels / Q000502:physiology; D004071:Digitalis Glycosides / Q000494:pharmacology; D004285:Dogs; D000882:Haplorhini; D006329:Heart Conduction System / Q000502:physiology* / Q000503:physiopathology; D011690:Purkinje Fibers / Q000502:physiology / Q000503:physiopathology; D013997:Time Factors
chemical_list: D002395:Catecholamines; D004071:Digitalis Glycosides