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.

figure 1

Figure: 3-lead rhythm strip from a patient in a narrow-complex tachycardia.

The simultaneously recorded 3-lead rhythm strip in the Figure was obtained from a patient in a supraventricular (narrow-complex) tachycardia. What is the reason for alternating morphology of the QRS complex (and T wave) with every-other-beat? Does this finding providing useful clinical information?

Interpretation: The underlying rhythm in this simultaneously recorded 3-lead tracing is a narrow-complex tachycardia. The interesting feature is the variation in morphology with alternate beats. The reason for this variation is electrical alternans. There are actually three components of this rhythm that are changing with alternate beats: 1) QRS morphology, 2) T wave morphology, and 3) the R-R interval.

  • Although all QRS complexes are narrow on the 3 leads shown in this tracing — the red and blue double arrows highlight unmistakable variation in QRS amplitude from beat-to-beat (albeit the difference is no more than slight variation in R wave height and S wave depth).
  • Red and blue ovals show that there is also subtle-but-real variation in T wave amplitude from beat-to-beat. The consistency of this finding in the absence of baseline artifact indicates that this variation is a real phenomenon.
  • Finally, there is slight variation in the R-R interval.

This difference in R-R interval from one beat to the next is so small that it could be easily missed unless measured with calipers. That said, the result is that this is not a “regular” SVT (supraventricular tachycardia) — but rather an irregular SVT rhythm with a repetitive pattern of alternate cycle length variation.

Electrical alternans is a fascinating clinical entity that is frequently misunderstood. Because this ECG finding is so often overlooked, the true incidence of electrical alternans is much higher than is generally appreciated. The term “electrical alternans” encompasses phasic fluctuation in one or more cardiac signals from one beat to the next within the cardiac cycle. This phasic variation may affect complex morphology (of the P wave, QRS complex and/or T wave), as well as interval duration (of the PR interval, QT interval, or R-R interval). Although most commonly associated with pericardial tamponade, there are many other clinical conditions that may produce this phenomenon.

For the SVT rhythm in this case, the presence of electrical alternans is highly suggestive of re-entry as the mechanism of the arrhythmia (not necessarily due to an accessory pathway). Other conditions that have been associated with electrical alternans include acute pulmonary embolus, cerebral hemorrhage, recent cardiac arrest, cardiomyopathy, ventricular dysfunction, pericardial effusion without tamponade, and others. Therefore, recognition of electrical alternans is indication for the clinician to look for an underlying cause.

For additional information on alternating morphology, go to