What are the signs of pulmonary embolism (PE) on a 12-lead electrocardiogram (ECG)?

ECG Signs of Pulmonary Embolism

The 12-lead ECG in pulmonary embolism is typically abnormal but non-specific, with the most common findings being sinus tachycardia and T-wave changes in the precordial leads, though these have limited diagnostic accuracy and cannot reliably rule in or rule out PE. 1, 2

Most Common ECG Findings

High-Frequency but Non-Specific Signs

• Sinus tachycardia is the most frequent finding, present in the majority of PE cases, though it lacks specificity 1, 2
• T-wave inversions in the right precordial leads (V1-V4) and inferior leads (III, aVF) are among the most common ECG manifestations, with inverted T waves showing the best sensitivity and specificity among traditional PE signs 3, 4
• Non-specific ST-segment and T-wave changes occur frequently but are of limited diagnostic value alone 1

Classic but Less Common Patterns

• S1Q3T3 pattern (S wave in lead I, Q wave and inverted T wave in lead III) is the traditional "textbook" sign but has modest diagnostic accuracy with a positive likelihood ratio of only 2.07 2
• Right bundle branch block (complete or incomplete) may be present, particularly in more severe cases 1, 5
• Right axis deviation can occur with significant right ventricular strain 5

Signs of Severe/Massive PE

Indicators of Hemodynamic Compromise

• ST-segment elevation in lead aVR with concomitant ST-segment depression in leads I and V4-V6 is a specific pattern associated with hemodynamic instability 4
• ST-segment elevation in leads V1-V3/V4 may occur during clinical deterioration, mimicking anterior myocardial infarction 6, 4
• Combination of S1Q3 pattern with abnormal QRS morphology in V1 is significantly more common (90%) during hemodynamic instability compared to baseline (5%) 4

Right Ventricular Strain Patterns

• T-wave inversions in ≥2 inferior leads plus ≥2 anterior leads (liberal RV strain criteria) yields the highest positive likelihood ratio of 4.75 among traditional ECG signs 2
• Features of acute right heart strain including right axis deviation and right bundle branch block are more common with massive emboli 1

Uncommon but Reported Findings

• QTc prolongation (>480 ms) with new T-wave inversions may occur during symptomatic episodes 5
• S-wave notch in lead V1 with clockwise rotation can indicate acute right ventricular overload 5
• Transient arrhythmias including atrioventricular junctional rhythm or sinus arrest may occur, possibly related to vagal reflex 5
• Atrial tachyarrhythmias are possible but less common 6

Critical Clinical Context

Diagnostic Limitations

ECG findings have minimal standalone diagnostic accuracy and should never be used in isolation to rule in or rule out PE. 2 The overall predictive value of any single ECG feature is less than 80%, and most changes have low sensitivity and specificity 1, 6

Primary Role of ECG

• Exclude alternative diagnoses such as acute myocardial infarction, pericarditis, left heart failure, and other cardiac conditions that may present similarly 1
• Support clinical suspicion when combined with other clinical features, but cannot confirm or exclude PE 1
• Identify patients at higher risk when signs of right ventricular strain are present, though echocardiography is superior for this purpose 1

Common Pitfalls to Avoid

• Do not mistake ST-segment elevation in anterolateral leads for acute coronary syndrome without considering PE in the differential, especially if accompanied by dyspnea and signs of right heart strain 6
• A normal ECG does not exclude PE - in fact, a normal ECG in an acutely breathless, hypoxic patient increases the likelihood of PE 1
• Do not rely on the S1Q3T3 pattern alone as it has poor sensitivity and modest specificity 2
• Recognize that ECG changes may be transient and can normalize within days to a week with appropriate anticoagulation 3

When ECG is Most Useful

The ECG is most valuable in suspected high-risk PE with hemodynamic instability, where it may show ischemic patterns combined with right ventricular strain, prompting urgent echocardiography to assess for RV dysfunction 1, 4. In this setting, echocardiography plays the critical diagnostic role, with ECG serving as a supportive tool 6.