Positive P waves in leads II, III, and aVF suggest that the origin of AT is from the cranial portion of either atria. Shorter P-wave duration is correlated to AT arising from the paraseptal tissue versus the right or left atrial free wall.191 The precise location of the focal AT is ultimately confirmed by mapping during EP studies when successful ablation Focal AT has been localized to the crista terminalis, right or left atrial free wall or appendage, tricuspid or mitral annulus, paraseptal or paranodal areas, pulmonary veins, coronary sinus, and coronary cusps, but it originates more frequently from the right atrium than from the left atrium.197,198 The underlying mechanism of focal AT can be auto- matic, triggered activity, or microreentry, but methods to distinguish the mechanism through pharmacological testing or EP study are of modest value because of limited sensi- tivity and specificity.123,199,200 An automatic AT can be tran- siently suppressed by adenosine or by overdrive pacing and may be terminated by beta blockers, diltiazem, or verapamil. Whereas a triggered AT can be terminated by adenosine or overdrive pacing, its response to beta blockers, diltiazem, or verapamil may be variable. A microreentrant AT can be induced and terminated by programmed stimulation, but its response to adenosine, beta blockers, diltiazem, or vera- pamil may depend on the location of the microreentrant cir- cuit; the tachycardia can be terminated by these drugs if the microreentrant circuit involves tissue around the sinus node, Recommendations for Ongoing Management of IST COR LOE Recommendations I C-LD 1. Evaluation for and treatment of reversible causes are recommended in patients with suspected IST.160,185 See Online Data and 5. 