For a patient presenting in SVT, the 12-lead ECG can potentially identify the arrhythmia mechanism (Figure 7). The tachycardia should first be classified according to whether there is a regular or irregular ventricular rate.An irregular ven- tricular rate suggests AF, MAT, or atrial flutter with variable AV conduction. When AF is associated with a rapid ventricu- lar response, the irregularity of the ventricular response is less easily detected and can be misdiagnosed as a regular SVT.76 If the atrial rate exceeds the ventricular rate, then atrial flutter or AT (focal or multifocal) is usually present (rare cases of AVNRT with 2:1 conduction have been described77 ). If the SVT is regular, this may represent AT with 1:1 con- duction or an SVT that involves theAV node. Junctional tachy- cardias, which originate in the AV junction (including the His bundle), can be regular or irregular, with variable conduction to the atria. SVTs that involve the AV node as a required com- ponent of the tachycardia reentrant circuit include AVNRT and 6). In these reentrant tachycardias, the retrogradely con- ducted P wave may be difficult to discern, especially if bundle- branch block is present. In typical AVNRT, atrial activation is nearly simultaneous with the QRS, so the terminal portion of the P wave is usually located at the end of the QRS complex, appearing as a narrow and negative deflection in the inferior leads (a pseudo S wave) and a slightly positive deflection at the end of the QRS complex in lead V1 (pseudo R ). In ortho- dromic AVRT (with anterograde conduction down the AV node), the P wave can usually be seen in the early part of the ST-T segment. 