What does an electrocardiogram (ECG) look like in a patient with a large pericardial effusion?

EKG Findings in Large Pericardial Effusion

An EKG in a patient with large pericardial effusion characteristically shows low QRS voltage and electrical alternans, both resulting from the damping effect of pericardial fluid and the "swinging heart" phenomenon. 1

Primary EKG Features

Low QRS Voltage

• Most common finding in large pericardial effusions, particularly when progressing toward tamponade
• Defined as QRS amplitude <0.5 mV in limb leads
• Results from the insulating effect of fluid between the heart and chest wall electrodes
• More strongly associated with moderate-to-large effusions and cardiac tamponade (odds ratio 4.7 for tamponade) 2
• Important caveat: Low QRS voltage is highly specific (89-100%) but poorly sensitive (1-17% for effusion, 0-42% for tamponade) 2

Electrical Alternans

• Nearly pathognomonic when present but occurs infrequently 3, 4
• Beat-to-beat variation in QRS amplitude (and sometimes P wave and T wave)
• Caused by the heart's pendular swinging motion within the fluid-filled pericardial sac
• Highly specific (99%) but very insensitive (occurs in only a small minority of cases) 2
• When present, strongly suggests hemodynamically significant effusion approaching tamponade 1
• Resolves immediately after pericardiocentesis 3, 4

Additional EKG Changes

P Wave Abnormalities

• Low P wave voltage is actually more frequent than low QRS voltage in large effusions 5
• Often overlooked but clinically significant finding
• Results from the same damping mechanism affecting atrial depolarization

PR Segment Depression

• May be present, particularly when pericarditis accompanies the effusion
• More specifically associated with cardiac tamponade (odds ratio 2.0) than with effusion alone 2
• Reflects atrial injury pattern

Other Findings

• Sinus tachycardia (common, reflecting compensatory response)
• T wave changes/inversions (nonspecific)
• Signs of underlying pericarditis if inflammatory etiology

Critical Clinical Context

The absence of classic EKG findings does NOT exclude large pericardial effusion or tamponade. 2, 6 Studies demonstrate that:

• No EKG variable is sensitive for detecting pericardial effusion 6
• ECG findings are "too few, subtle, insensitive, and nonspecific" to reliably diagnose effusion 6
• Echocardiography remains the definitive diagnostic tool and should never be delayed based on normal EKG findings 1

Size and Hemodynamic Correlation

• Low voltage shows weak correlation with effusion size (r = 0.296) 6
• No consistent correlation exists between low QRS voltage and tamponade, effusion size, etiology, or pericardial thickness 5
• Electrical alternans occurred in only 1 of 5 patients with large effusions in one study 6

Clinical Algorithm

When evaluating suspected large pericardial effusion:

1. Do not rely on EKG alone - proceed directly to echocardiography regardless of EKG findings
2. If low voltage or electrical alternans present: High specificity warrants urgent echocardiography
3. If EKG is normal: Still perform echocardiography if clinical suspicion exists (dyspnea, chest pain, hemodynamic instability)
4. Electrical alternans with hemodynamic compromise: Assume tamponade until proven otherwise and prepare for urgent pericardiocentesis 3, 4

Common Pitfall

The most dangerous error is excluding pericardial effusion based on a normal EKG. The sensitivity of EKG findings is so low that most patients with large effusions will have unremarkable EKGs 2, 6. Always maintain clinical suspicion and proceed to imaging when the clinical picture suggests effusion.