ECG criteria
- Sinus tachycardia is a frequent, but nonspecific and optional, compensatory response to a decreased stroke volume; it may be absent in the elderly, in hypothyroidism, during beta-blocker therapy, or in conduction disorders
- Low QRS voltage: amplitude of the complex <5 mm (0.5 mV) in all limb leads and/changes or <10 mm (1 mV) in chest leads
- Electrical alternans of QRS: alternation of the amplitude (and sometimes the axis) of the complex from beat to beat, usually with a relatively regular rhythm — evaluated in all leads, dynamically, and in relation to respiration, rhythm, and clinical data
- Alternation of P, QRS, and T (total) has been described in large effusions and swinging heart, but is rare and is not an obligatory or sufficient criterion for tamponade
- Diffuse ST-segment elevation (usually upwardly concave) and PR depression in concomitant pericarditis (see "Pericarditis on ECG")
- Electrical axis of the heart without a consistent deviation — not characteristic of tamponade on its own
- In acute hemopericardium (e.g., wall rupture), alternans may be absent — the effusion is still small, but hemodynamics are already critical
What it looks like on ECG
The ECG in cardiac tamponade has low sensitivity: the classic signs — sinus tachycardia, low QRS voltage, and electrical alternans — occur with varying frequency, and none of them is obligatory for the diagnosis.
Sinus tachycardia is a frequent finding in significant stroke volume reduction, but is nonspecific and may be absent (elderly patients, hypothyroidism, beta-blocker therapy, conduction disorders, terminal or neurogenic shock).
Low QRS voltage is associated with an increased distance between the myocardium and electrodes and attenuation of the recorded signal by fluid; changes in heart position and concomitant disease may also contribute. Criterion: QRS amplitude <5 mm in all limb leads (I, II, III, aVR, aVL, aVF) or <10 mm in chest leads. This sign is nonspecific — a similar pattern is caused by COPD, emphysema, obesity, myxedema, and amyloidosis.
Electrical alternans is the alternation of the amplitude (less commonly, the axis) of the QRS complex from beat to beat. It is usually easier to detect with a relatively regular rhythm and tachycardia, unlike respiratory alternans, which is synchronous with respiratory phases, although amplitude can also vary due to respiration and body position — changes should be evaluated in all leads, dynamically, and taking the clinical picture into account. Mechanically, alternans is associated with the rhythmic positional changes of the "swinging" heart within a large effusion relative to the electrodes. It is most often visible in chest leads.
In concomitant acute pericarditis, diffuse ST-segment elevation, usually upwardly concave, is added along with PR-segment depression in almost all leads except aVR and often V1 (see "Pericarditis on ECG"). Convex or monophasic ST elevation is atypical for pericarditis and requires ruling out ACS.
Reference table:
| Sign | What to look for | Specificity |
|---|---|---|
| Sinus tachycardia | HR >100 bpm | low |
| Low voltage | QRS <5 mm (limbs) | low |
| Electrical alternans | alternation of QRS amplitude from beat to beat | higher, but the sign is rare and optional |
Mechanism
The pericardium is a poorly distensible fibrous sac. With slow fluid accumulation, the pericardium manages to stretch, and pressure in the cavity remains low for a long time — the effusion can be large without causing tamponade. With rapid accumulation (rupture, trauma, aortic dissection), even a small volume sharply increases intrapericardial pressure.
Tamponade develops when the pericardial pressure exceeds the filling pressure of the heart chambers, especially the right-sided chambers with low diastolic pressure. This limits diastolic ventricular filling, reduces stroke volume and cardiac output — leading to obstructive shock. Compensatory increases in heart rate and systemic vascular resistance occur, although this compensation is not always pronounced.
Electrical alternans is associated with the mechanical "swinging heart" within a large volume of fluid: with each contraction, the heart rhythmically changes position relative to the chest wall, which periodically alters the amplitude and/or axis of the electrical signal recorded by surface electrodes.
Causes
- Pericarditis of any etiology (viral, bacterial, tuberculous, autoimmune, uremic)
- Malignant tumors (metastatic pericardial involvement, primary mesothelioma)
- Chest trauma, penetrating heart injury
- Iatrogenic causes: perforation during catheterization, pacemaker lead placement, coronary intervention
- Free wall rupture of the left ventricle in myocardial infarction
- Type A aortic dissection with rupture into the pericardial cavity
- Postpericardiotomy syndrome after cardiac surgery
- Hypothyroidism (myxedematous effusion — usually large, slowly progressive, without tamponade)
- Uremia in chronic renal failure
- Mediastinal radiation therapy
Differential diagnosis
- Low voltage without effusion (COPD, obesity, amyloidosis, myxedema) — differentiated by echocardiography; cannot be distinguished on an isolated ECG.
- Respiratory voltage alternans — amplitude changes synchronously with respiratory phases, unlike true electrical alternans, which alternates from beat to beat.
- Alternans in tachyarrhythmias (e.g., VT or antidromic tachycardia in WPW) — QRS alternans is possible even without effusion, in the setting of very high heart rate; evaluated in combination with clinical data and echocardiography.
- Pericarditis without effusion — diffuse ST elevation and PR depression are present, but there is no low voltage or alternans.
- Pulseless electrical activity — a possible terminal event in severe tamponade, cardiac arrest with an organized ECG rhythm without effective output; requires immediate CPR and correction of the reversible cause.
Clinical significance and management
The ECG in tamponade has low sensitivity and is nonspecific — it is an auxiliary, rather than a primary, diagnostic method. The diagnosis of tamponade is established integrally: clinically (Beck's triad — arterial hypotension, distended neck veins, muffled heart sounds; pulsus paradoxus), hemodynamically, and echocardiographically. Echocardiography reveals signs of the hemodynamic impact of the effusion — collapse of the right chambers (right ventricle in early diastole, right atrium in systole), dilation of the inferior vena cava with reduced respiratory variability, pronounced respiratory variations in transmitral and transtricuspid flows, and other signs; none of them is obligatory or sufficient on its own, and interpretation takes into account the clinical context and ventilation mode.
The physician's goal when tamponade is suspected:
- Obtain an ECG as part of the workup, but do not wait for its results to begin diagnostics
- Urgent bedside echocardiography
- In unstable tamponade, immediate intervention is required: for non-traumatic effusions, urgent drainage (pericardiocentesis) is usually performed, while trauma, myocardial rupture, or aortic dissection warrant immediate surgical repair; pericardiocentesis in these cases may be considered only as a temporary measure per the team's decision
- Search for and treat the cause of the effusion (pericarditis, ventricular wall rupture in myocardial infarction, aortic dissection, etc.)
- New or worsening ECG changes (increasing alternans, decreasing voltage) require immediate clinical reassessment and repeat echocardiography, but are not quantitative markers of effusion volume or tamponade severity
Common interpretation errors
- Consider a normal ECG as a reason to rule out tamponade — the ECG can be nearly normal in clinically significant tamponade
- Confuse respiratory voltage alternans with true electrical alternans of tamponade
- Interpret low voltage as a specific sign of effusion without accounting for COPD, obesity, and other causes
- Fail to correlate diffuse ST elevation with the clinical presentation and mistake pericarditis for STEMI
- Wait for classic ECG signs to appear while delaying echocardiography and clinical evaluation
- Assess effusion volume or tamponade severity solely based on voltage dynamics and alternans on the ECG
1. Which ECG sign is associated with a swinging heart in a large pericardial effusion?
Electrical alternans is explained by the rhythmic shifting of the heart's position within a fluid-filled cavity; this sign is not always present, but is considered more specific than tachycardia or low voltage.
2. How does electrical alternans in cardiac tamponade differ from respiratory voltage alternans?
In tamponade, alternans typically manifests as beat-to-beat variation; it is not strictly synchronized with respiratory phases, although respiration and body position can affect amplitude, so the sign must be evaluated in conjunction with the clinical picture and echocardiography.
3. What is the primary method to confirm the diagnosis of cardiac tamponade?
ECG has low sensitivity and specificity in tamponade; the diagnosis is confirmed by echocardiography combined with the clinical presentation.
Frequently asked questions
What ECG signs are sought when tamponade is suspected?
Sinus tachycardia, low QRS voltage, and electrical alternans. All three signs are nonspecific and may be absent individually or together; electrical alternans is less common but considered more specific for a large effusion with cardiac swinging.
Can the ECG be normal in cardiac tamponade?
Yes, it can. ECG is an auxiliary method with low sensitivity; the absence of changes does not rule out tamponade. The diagnosis is confirmed clinically and echocardiographically.
How does respiratory alternans differ from alternans in tamponade?
Respiratory alternans is synchronous with respiratory phases and diminishes during breath-holding. Alternans in tamponade alternates beat-to-beat and is typically observed with a relatively regular rhythm, but differentiation based solely on the relationship to respiration is unreliable—evaluation must be integrated with the clinical picture.
Is a pericardial effusion dangerous by itself?
Not always. A slowly accumulating effusion can reach a large volume without causing tamponade because the pericardium has time to stretch. The danger is determined by the combination of effusion volume and accumulation rate, pericardial compliance, and intrapericardial pressure relative to chamber filling pressures; the rate is especially critical in acute hemopericardium.
Is an ECG necessary for diagnosing tamponade in an emergency?
An ECG is recorded as part of the standard evaluation, but it should not delay echocardiography and emergency interventions. The decision for urgent drainage or surgery is made based on clinical findings and echocardiography, not the ECG.
ECG Course
Interpretation from basics to myocardial infarction — learn to read any ECG using a systematic algorithm.
- Video lessons and theory for every topic
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- Study guide: "All ECGs in 25 minutes"
ECG Strip Simulator
ECG, radiology, histology, pathomorphology, pharmacology — thousands of flashcards with detailed explanations.
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Related terms
More in the section "Pericardium, Lungs, Brain and Others"
Sources
- Goldberger A.L. Clinical Electrocardiography: A Simplified Approach
- Hampton J.R. The ECG Made Easy
- Surawicz B., Knilans T.K. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.