Reproduced with permis- sion from Blomstr m-Lundqvist et al.11 Page et al 2015 ACC/AHA/HRS SVT Guideline e515 tachycardia, a form of focal AT, the P-wave morphology is identical to the P wave in sinus rhythm. 2.4. Principles of Medical Therapy See Figure 8 for the algorithm for acute treatment of tachy- cardia of unknown mechanism; Figure 9 for the algorithm for ongoing management of tachycardia of unknown mechanism; Table 6 for acute drug therapy for SVT (intravenous adminis- tration); and Table 7 for ongoing drug therapy for SVT (oral administration). 2.4.1. Acute Treatment: Recommendations Because patients with SVT account for approximately emer- gency physicians may be the first to evaluate patients whose tachycardia mechanism is unknown and to have the opportunity to diagnose the mechanism of arrhythmia. It is important to record a 12-lead ECG to differentiate tachy- cardia mechanisms according to whether the AV node is an obligate component (Section 2.3.2), because treatment that targets the AV node will not reliably terminate tachycardias Figure 2. Typical AVNRT and normal sinus rhythm after conversion. Upper panel: The arrow points to the P waves, which are inscribed at the end of the QRS complex, seen best in the inferior leads and as a slightly positive R (pseudo r prime) in lead V1. The reentrant circuit involves anterograde conduction over a slow atrioventricular node pathway, followed by retrograde conduction in a fast atrioventricular node pathway. Typical AVNRT is a type of short RP tachycardia. Middle panel: When the patient is in sinus rhythm, the arrow indicates where the R is absent in V1. Bottom panels: Magnified portions of lead V1 in AVNRT (left) and sinus rhythm (right) are shown. AVNRT indicates atrioventricular nodal reentrant tachycardia. that are not AV node dependent. 