ECG Review

The Regular SVT Differential

By Ken Grauer, MD, Professor Emeritus in Family Medicine, College of Medicine, University of Florida. Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.

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Figure — 12-lead ECG and lead II rhythm strip from a patient with "rapid heart rate." What is your differential diagnosis?

Scenario: Interpret the rhythm for the 12-lead ECG and lead II rhythm strip shown above. The patient was aware of "rapid heart beat" — but was hemodynamically stable at the time the tracing was recorded. What is your differential diagnosis? What diagnostic maneuver might help to determine what the rhythm is?

Interpretation: The rhythm is regular at a ventricular rate that is close to 150/minute (the R-R interval is approximately two large boxes in duration). The QRS complex is narrow (not more than half a large box in duration in any of the 12 leads of the tracing). Normal atrial activity is absent, since upright P waves are not seen in lead II. Instead, there is suggestion of atrial activity having a negative deflection in lead II (as well as in other inferior leads). There also appears to be a negative notching in the ST segment in each of the inferior leads. Could this all represent atrial activity?

The best description of the tachycardia defined by the above ECG and rhythm strip is that this represents a regular SVT (supraventricular tachycardia) without sign of normal atrial activity. This description should prompt consideration of three entities as the most likely cause: 1) sinus tachycardia; 2) atrial flutter; and 3) PSVT (paroxysmal supraventricular tachycardia). Although many other entities might be included in the differential of a regular SVT, the overwhelming majority of cases encountered by primary care clinicians in or out of the hospital will be due to one of these three causes. In this particular case, sinus tachycardia is unlikely because there is no indication of an upright P wave in lead II. We suspect atrial flutter because this is the most common cause of a regular SVT at a rate between 140-160/minute when normal atrial activity is absent. This is especially true when there is hint of a "sawtooth" activity in one or more leads on the tracing (see leads II, III, aVF; and leads aVR, aVL, V1). Application of a vagal maneuver in this patient temporarily slowed the rate of AV conduction, allowing "telltale" flutter waves at ~300/minute to appear.