DIFFERENTIATING TACHYCARDIAS

Is there a simple way to differentiate a supraventricular tachycardia from a ventricular, wide QRS tachycardia on a rhythm strip?
—Jorge Lizquiano, MD, Milwaukee

The simplest initial approach is to look at the lead with the most discernible P waves, then try to identify atrioventricular dissociation and screen for fusion beats seen with ventricular tachycardia (VT) and ectopy, respectively. The pattern of the QRS can also be helpful, depending on the leads available. Left ventricular ectopy is often detectable in V1 as a dominant R wave that has a taller early peak with notching on downstroke; note, however, that this can also be seen in Wolff-Parkinson-White syndrome. Other features of ventricular tachycardias include little R and deeper, wider S in V6; negative deflection >15 mm in V6; and frontal plane axis in the right upper quadrant. Patients with right bundle branch block who develop VT will exhibit a different pattern featuring a monophasic R with taller left rabbit ear in V1 and an absent Q, small R, and deeper wider S in V6. In atrial fibrillation, it may be difficult to differentiate aberration from ectopy. Look for fixed coupling, which strongly favors ventricular ectopy, and Ashman phenomenon (relatively long cycle followed by short cycle that ends with aberrancy) for supraventricular origin. When identified, concordance in the V leads can also help to indicate a ventricular source, but multiple precordial leads are often not readily available in rhythm strips from telemetry.
—Norma M. Keller, MD (109-23)

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