What are the ECG findings in a patient suspected of having cardiac tamponade?

ECG Findings in Cardiac Tamponade

The two key ECG findings in cardiac tamponade are low voltage QRS complexes and electrical alternans, though these have limited sensitivity and should not be used to exclude the diagnosis. 1, 2

Primary ECG Abnormalities

Low Voltage QRS Complexes

• Low voltage QRS complexes occur in approximately 56% of cardiac tamponade cases and result from the dampening effect of pericardial fluid on electrical signals 2, 3
• This finding has moderate specificity (74%) but limited sensitivity, meaning its absence does not exclude tamponade 3
• The odds ratio for cardiac tamponade when low voltage is present is 3.7 (95% CI: 1.65-8.30) 3

Electrical Alternans

• Electrical alternans represents alternating QRS amplitude caused by the "swinging heart" motion within pericardial fluid 2, 4
• This is the most specific ECG finding (98% specificity) but has very low sensitivity (23%), making it uncommon even when tamponade is present 3
• When electrical alternans is present, it carries an odds ratio of 12.3 (95% CI: 1.58-95.17) for cardiac tamponade 3
• The combination of electrical alternans with pulsus paradoxus and pericardial friction rub is diagnostic of cardiac tamponade requiring urgent intervention 2

Sinus Tachycardia

• Tachycardia is present in 76% of cardiac tamponade cases as a compensatory mechanism to maintain cardiac output 2, 3
• This finding has the highest sensitivity (76%) but lowest specificity (60%) among the three ECG abnormalities 3
• The odds ratio for tamponade with sinus tachycardia is 4.9 (95% CI: 2.22-10.80) 3

Combined ECG Findings

The presence of any one of the three ECG abnormalities (low voltage, electrical alternans, or sinus tachycardia) has a sensitivity of 89% but specificity of only 47% 3

When all three ECG abnormalities are present simultaneously, specificity reaches 100% with positive predictive value of 100%, but this occurs in only 8% of tamponade cases 3

Critical Clinical Caveat

The 12-lead ECG cannot be used as a screening tool to exclude cardiac tamponade due to its low negative predictive value (36-69% depending on findings) 3

• The European Society of Cardiology emphasizes that pre-hospital risk assessment must be based on clinical signs including shock, hemodynamic instability, respiratory distress, jugular vein distension, quiet heart sounds, AND the ECG findings of low voltage and/or electrical alternans 1
• ECG abnormalities should supplement, not replace, clinical examination 3

Diagnostic Algorithm

Cardiac tamponade remains fundamentally a clinical diagnosis that must be confirmed by echocardiography, not ECG alone 1, 2, 5

1. Suspect tamponade clinically based on Beck's triad (hypotension, jugular venous distension, muffled heart sounds) and pulsus paradoxus 2, 6, 7
2. Check ECG for supportive findings (low voltage, electrical alternans, tachycardia) but do not rely on their absence to exclude diagnosis 1, 3
3. Perform immediate echocardiography to confirm pericardial effusion and assess hemodynamic impact—this is the single most important diagnostic test 2, 5
4. Proceed to urgent pericardiocentesis or cardiac surgery without delay in unstable patients 2, 8

Additional ECG Context

In patients with acute pericarditis progressing to tamponade, you may also see diffuse ST segment elevation without reciprocal ST depression and PR segment depression, though these reflect the underlying pericarditis rather than the tamponade itself 1