What are the primary causes of electrical alternans?

Causes of Electrical Alternans

Cardiac tamponade is the most clinically significant cause of electrical alternans, representing a life-threatening emergency requiring immediate recognition and intervention. 1, 2, 3

Primary Causes

Cardiac Tamponade (Most Important)

• Electrical alternans with pericardial effusion strongly suggests impending or established cardiac tamponade and mandates urgent intervention. 1, 2, 3, 4
• The mechanism involves the heart swinging or oscillating within a fluid-filled pericardial sac, producing alternating QRS amplitude at a frequency equal to one-half the heart rate (exact alternans), one-third (2:1 alternans), or one-quarter (3:1 alternans) of the heart rate. 4, 5
• This "swinging heart" motion creates beat-to-beat variation in the electrical axis as the heart physically rotates within pericardial fluid. 3, 5
• Electrical alternans in tamponade can occur even with heart rates below 100 bpm, contrary to common teaching. 4
• The European Society of Cardiology identifies electrical alternans as a key diagnostic finding requiring immediate echocardiography and urgent pericardiocentesis or surgical drainage. 1, 2, 3

Critical Pitfall: In pacemaker-dependent patients, electrical alternans with hypotension indicates tamponade even without tachycardia, as the fixed pacing rate masks this typical diagnostic sign. 2

T-Wave Alternans (Distinct Pathophysiology)

• T-wave alternans represents true electrical alternans—a cellular-level phenomenon distinct from the mechanical alternans of tamponade. 1, 6
• This arises when heart rate exceeds the capacity of cardiac cells to cycle intracellular calcium, creating repolarization alternans that amplifies electrical heterogeneities between neighboring myocytes. 1
• T-wave alternans is a rate-dependent phenomenon occurring at relatively lower heart rates in patients susceptible to life-threatening ventricular arrhythmias. 1
• Associated conditions include:
  • Ischemic and nonischemic heart failure (relative risk 2.42 and 3.67 for arrhythmic events, respectively) 1
  • Coronary artery disease with reduced ejection fraction 1, 2
  • Dilated cardiomyopathy 1
  • Long QT syndrome (particularly during emotional stress) 6

Other Causes (Less Common)

• Acute myocardial ischemia and Prinzmetal's angina 6
• Electrolyte abnormalities 6
• Various supraventricular and ventricular tachyarrhythmias 7, 6

Diagnostic Algorithm

When Electrical Alternans is Detected:

1. Immediate assessment for tamponade physiology: 1, 2, 3

   • Check for hypotension, tachycardia (unless paced), jugular venous distension
   • Assess for quiet/muffled heart sounds
   • Measure pulsus paradoxus (>10 mmHg inspiratory drop in systolic BP)
   • Look for low voltage QRS complexes accompanying the alternans

2. Urgent echocardiography is mandatory to visualize pericardial effusion and assess for: 1, 2, 3

   • Right ventricular diastolic collapse
   • Right atrial late diastolic collapse
   • Swinging heart motion
   • Inferior vena cava plethora without respiratory variation

3. If tamponade is confirmed, proceed immediately to pericardiocentesis or surgical drainage without delay in unstable patients. 1, 2, 3

4. If no pericardial effusion is present, consider T-wave alternans testing (requires special electrodes and processing to detect microvolt-level alternations) for risk stratification of sudden cardiac death. 1, 2

Clinical Significance by Etiology

For tamponade-related alternans: This is a medical emergency with 62% historical mortality when untreated, requiring immediate intervention to prevent cardiovascular collapse and death. 3, 6

For T-wave alternans: Abnormal findings (>1.9 μV) predict a hazard ratio of 4.8 for 2-year mortality and identify patients who benefit most from ICD implantation, while negative results confer excellent prognosis (<1% annual mortality). 2