A wide-complex tachycardia (QRS duration >120 ms) may represent either VT or a supraventricular rhythm with abnormal conduction. Conduction abnormalities may be due to rate-related aberrant conduction, pre-existing bundle- branch block seen in sinus rhythm, or an accessory pathway that results in pre-excitation (Table 4). The presence of atrio- ventricular (AV) dissociation (with ventricular rate faster than atrial rate) or fusion complexes representing dissociation of supraventricular impulses from a ventricular rhythm provides the diagnosis of VT (Figure 1). Other criteria are useful but not diagnostic. Concordance of the precordial QRS com- plexes such that all are positive or negative suggests VT or pre-excitation, whereas QRS complexes in tachycardia that are identical to those seen in sinus rhythm are consistent with SVT. Other, more complicated ECG algorithms have been developed to distinguish VT from SVT, such as the Brugada criteria, which rely on an examination of the QRS morphol- ogy in the precordial leads,73 and the Vereckei algorithm, which is based on an examination of the QRS complex in lead (Table 5). The failure to correctly identify VT can be potentially life threatening, particularly if misdiagnosis results in VT being treated with verapamil or diltiazem. Adenosine is suggested in the 2010 AHA Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Part 8: Adult Advanced Cardiovascular Life Support (2010 Adult if a wide-complex tachycardia is mono- morphic, regular, and hemodynamically tolerated, because adenosine may help convert the rhythm to sinus and may help in the diagnosis. When doubt exists, it is safest to assume any wide-complex tachycardia is VT, particularly in patients with known cardiovascular disease, such as prior myocardial infarction. For a patient presenting in SVT, the 12-lead ECG can potentially identify the arrhythmia mechanism (Figure 7). 