In supraventricular tachycardia, the QRS is narrow because the ventricles are depolarized over the normal specialized conduction tissues (light blue region). Five possible arrhythmias are commonly encountered. First, in atrial fibrillation, multiple microreentrant circuits can lead to chaotic activation of the atrium. Because impulses are reaching the AV node at irregular intervals, ventricular depolarization is irregular. Second, in atrial flutter, a macroreentrant circuit, traveling up the interatrial septum and down the lateral walls, can activate the atria in a regular fashion at approximately 300 bpm. The AV node can conduct only every other or every third beat, so that the ventricles are depolarized at 150 or 100 bpm. In AV nodal reentrant tachycardia, slow and fast pathways exist in the region of the AV node and a microreentrant circuit can be formed. Fourth, in atrioventricular reentry, an abnormal connection between the atrium and ventricle exists so that a macroreentrant circuit can be formed with the AV node forming the slow pathway, and the abnormal atrioventricular connection, the fast pathway. Finally, in atrial tachycardia an abnormal focus of atrial activity as a result of either reentry, triggered activity, or abnormal automaticity can activate the atria in a regular fashion. Source: Cardiovascular Disorders: Heart Disease, Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e Citation: Hammer GD, McPhee SJ. Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e; 2013 Available at: http://accesspharmacy.mhmedical.com/DownloadImage.aspx?image=/data/books/961/ham007_fig_10-15.gif&sec=53629997&BookID=961&ChapterSecID=53555691&imagename= Accessed: November 01, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved