Electrical Alternans: More Than Just a Pericardial Effusion

Electrical alternans is a fascinating ECG phenomenon typically associated with large pericardial effusion—but it is not pathognomonic for it. This beat-to-beat variation in the amplitude or axis of the QRS complex (and sometimes P or T waves) can also be seen in supraventricular tachyarrhythmias, especially when there is rapid atrial or AV nodal conduction.

In this article, we explore the two major mechanisms behind electrical alternans—mechanical swinging of the heart in tamponade and alternating conduction dynamics in rapid supraventricular rhythms—and how to distinguish between them clinically and electrocardiographically.

What Is Electrical Alternans?

Electrical alternans is defined as a repetitive, beat-to-beat alternation in the morphology, amplitude, or axis of ECG waveforms, most commonly seen in the QRS complexes.

• Classic presentation: Alternating tall and short QRS complexes

• May involve P waves (atrial alternans) or T waves

• Often most visible in the limb leads, particularly lead II or V2

It can be global (in all leads) or lead-specific, depending on the underlying mechanism.

Mechanism 1: Pericardial Effusion and Cardiac Tamponade

In the setting of a large pericardial effusion, especially with tamponade physiology, the heart is suspended in a fluid-filled pericardial sac. This leads to:

Mechanical "Swinging" of the Heart

• With each beat, the heart physically shifts within the pericardial fluid.

https://en.m.wikipedia.org/wiki/File:Pericardial_effusion_with_tamponade.gif

• This back-and-forth movement alters the vector of depolarization as seen by surface ECG electrodes.

• The result is an alternating amplitude and/or axis of the QRS complex, reflecting changes in the heart’s orientation.

Other Clues on ECG

• Sinus tachycardia

• Low voltage QRS complexes

• Electrical alternans across multiple leads

• Possible pulsus alternans (mechanical counterpart)

Clinical Implication

Electrical alternans in this context is a warning sign of tamponade, particularly when paired with hypotension, jugular venous distension, and muffled heart sounds. Emergent bedside ultrasound and pericardiocentesis are often required.

Mechanism 2: Supraventricular Tachyarrhythmias

Electrical alternans can also occur in rapid supraventricular rhythms—including:

• Atrial fibrillation with rapid ventricular response

• Atrioventricular nodal reentry tachycardia (AVNRT)

• Atrial flutter with 1:1 or 2:1 conduction

• AVRT (e.g., WPW syndrome)

The Mechanism Here Is Electrical, Not Mechanical

This form of alternans arises from alternating conduction properties, especially when conduction velocity or repolarization varies from beat to beat.

Key contributors include:

1. Rate-dependent bundle branch block or aberrancy

   • Rapid conduction may lead to alternating degrees of bundle branch delay

   • One beat conducts with a slightly different QRS morphology or axis than the next

2. Alternating action potential durations

   • In very fast rhythms, alternating refractory periods can cause variable conduction through the AV node or intraventricular tissue

ECG Characteristics

• Seen during narrow complex tachycardias (but may also appear during aberrancy)

• Alternans may be subtle and transient

• No effusion or tamponade clinically or on imaging

Mechanism 3: Alternating Pre-Excitation in Wolff-Parkinson-White (WPW) Syndrome

Electrical alternans can also occur in the setting of Wolff-Parkinson-White (WPW) syndrome, where it results from beat-to-beat variation in pre-excitation through an accessory pathway.

How It Happens:

• In WPW, impulses from the atria may conduct to the ventricles via both the AV node and an accessory pathway (e.g., the Bundle of Kent).

• In some cases, some beats are conducted with pre-excitation (i.e., via the accessory pathway), while others use the AV node exclusively.

• This produces an alternating pattern of QRS morphology:

  • Pre-excited beats: Wide QRS with delta wave and abnormal axis

  • Non-pre-excited beats: Narrow QRS via the normal His-Purkinje system

The result is a beat-to-beat alternation in QRS width and shape, a form of electrical alternans caused by conduction variability, not mechanical factors or AV nodal reentry.

ECG Characteristics:

• Alternating wide and narrow QRS complexes

• May see intermittent delta waves on pre-excited beats

https://www.sciencedirect.com/science/article/abs/pii/S0022073606005267

Clinical Implications

• Electrical alternans is not diagnostic of tamponade. It must be interpreted in clinical context.

• In a stable patient with a narrow complex tachycardia, alternans is most likely related to rate-dependent conduction phenomena.

• In an unstable patient with low voltage, sinus tachycardia, and alternans, tamponade should be suspected, and bedside ultrasound is mandatory.

Clinical Takeaway

Electrical alternans is not exclusive to pericardial tamponade. While it remains an important diagnostic clue in pericardial effusion, it can also emerge from electrophysiological instability in rapid supraventricular rhythms. Understanding the mechanism behind the pattern—mechanical vs. electrical—is critical for accurate diagnosis and timely management.