Short RP tachycardias are characterized by an RP interval that’s shorter than the PR interval. Only 2 arrhythmias present as short RP tachycardias: AVNRT and AVRT.27 If the RP interval is <70 ms, AVNRT is the likely diagnosis.28
P wave position. A careful review of the position of the P wave with respect to the QRS complex can provide additional help in distinguishing between AVNRT and AVRT. In 66% of AVNRTs, the P wave is hidden within the QRS complex;29 in 30%, a retrograde P wave closely follows the QRS complex, creating a “pseudo-S” wave; and 4% of the time, the P wave precedes the QRS complex.
In AVRT, a retrograde P wave follows the QRS complex. This creates a potential dilemma in differentiating 30% of AVNRTs from AVRT. In AVNRT, the retrograde P wave typically appears very close to the QRS complex, creating a pseudo-S wave. In the orthodromic AVRT, there is usually a separation between the QRS and retrograde P wave. In general, if the RP interval is <70 ms, the arrhythmia is usually due to typical AVNRT.28
ST segment elevation in lead aVR on a 12-lead EKG in a supraventricular tachycardia is about 70% sensitive and 70% to 83% specific for a diagnosis of AVRT.30,31 ST depression of more than 2 mm or T wave inversion is more common in AVRT than in AVNRT.32 QRS alternans, which refers to variations in QRS amplitude or direction with every other beat, has been reported to be indicative of AVRT, 33,34 but may in fact be a rate-dependent phenomenon that has little to do with the mechanism of tachycardias.35
FAST TRACK
Multifocal atrial tachycardia is commonly—but not exclusively—associated with lung disorders.
Onset and termination and other indicators. Still uncertain? Patterns of arrhythmias and modes of onset and termination may provide additional help with the differential diagnosis.
Sinus tachycardias and atrial tachycardias frequently demonstrate a “warm up” in rate, for instance, while AVNRT and AVRT are often triggered by premature atrial contractions. A positive response to the Valsalva maneuver or to adenosine is typically characteristic of reentrant tachycardias using the AV node, such as AVNRT and AVRT.
Comparing a baseline 12-lead EKG with an EKG taken during an episode of tachycardia often provides further information about the mechanism of the arrhythmia. The presence of pre-excitation, the morphology of P waves, and the lack of retrograde P waves on a baseline EKG can be useful in narrowing the differential diagnosis.
Figures courtesy of: University of Buffalo and Buffalo General Hospital.
CORRESPONDENCE Vipul Gupta, MD, MPH, State University of New York at Buffalo, 131 Biomedical Education Building, Buffalo, NY 14214;
