The 12-lead ECG in the figure below was obtained from a hemodynamically stable older adult with new-onset palpitations. Is this ventricular tachycardia (VT)? How certain are you?

Although we were told this patient was hemodynamically stable at the time this ECG was recorded, knowing this does not help determine the etiology of the arrhythmia. Why? Some patients presenting with VT may remain alert and hemodynamically stable for a surprisingly long time (hours or even days). However, knowing this patient is stable provides an extra moment to contemplate your differential diagnosis.

Unfortunately, there is no simultaneously obtained long lead rhythm strip. Nevertheless, it should be clear that the rhythm is a regular wide complex tachycardia (WCT) at a rate of ~110/minute without clear sign of sinus-conducting P waves (i.e., there is no consistent upright P wave with constant PR interval in lead II). Thus, a ventricular etiology should be assumed until proven otherwise. Statistically, more than 80-90% of all regular WCT rhythms that lack sinus P waves will turn out to be VT.

Features that further increase the likelihood of a ventricular etiology for this tracing include: extreme axis deviation (i.e., all negative QRS in each of the inferior leads); marked QRS widening (to at least 0.14 seconds); all positive QRS in lead aVR; all negative QRS in lead V6; and QRS morphology not resembling any known form of bundle branch block or hemiblock. Based on these ECG features, the predicted likelihood of a ventricular etiology increases to more than 95%.

There is one additional clue that allows 100% certainty of a ventricular etiology in this case. Although subtle, note the presence of unmistakable P waves periodically punctuating the baseline. These are best seen in lead III (before the second QRS complex in this lead, and then notching the end of the QRS of the third beat) and in lead aVF (notching the ST segment of the first beat in aVF, as well as appearing before and after the last beat in this lead’s recording). These P waves are regular and unrelated to neighboring QRS complexes, which defines this as AV dissociation.

The rate of the ventricular rhythm in the figure above is relatively slow for VT (i.e., well under 130/minute). Therefore, this rhythm is best classified as accelerated idioventricular rhythm, which is seen most commonly as an escape rhythm that occurs in association with acute or recent infarction — or following reperfusion of a major coronary artery. The need for an immediate cardioversion is rare, and the rhythm often resolves spontaneously. Clinical correlation is essential.

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ECG Review