What are the ECG (electrocardiogram) signs of a pulmonary embolism?

ECG Signs of Pulmonary Embolism

The ECG in pulmonary embolism is typically abnormal but highly non-specific, with sinus tachycardia being the most common finding, while classic signs of right ventricular strain (T-wave inversions in V1-V4, S1Q3T3 pattern, and right bundle branch block) are more specific but occur primarily in severe cases. 1, 2

Most Common ECG Findings

Sinus Tachycardia

• Sinus tachycardia (heart rate >100 bpm) is the single most frequent abnormality, present in approximately 40% of PE cases, though it is entirely non-specific 3, 4
• In milder cases, sinus tachycardia may be the only detectable ECG abnormality 3

Right Ventricular Strain Pattern (Most Clinically Significant)

• T-wave inversions in the right precordial leads (V1-V4) are the most clinically significant finding, indicating RV strain and typically found in more severe cases 1, 3
• These inversions have the highest specificity (97.4%) among ECG findings for PE 3
• The pattern is usually seen in more severe cases and correlates with the severity of pulmonary hypertension and RV dysfunction 1, 2

Classic Patterns

• S1Q3T3 pattern (S wave in lead I, Q wave and inverted T wave in lead III) occurs in 8.5% of PE patients versus 3.3% without PE, with a positive likelihood ratio of 3.7 4
• This pattern was observed in 34 of 51 patients with critical PE in one series 5
• QR pattern in lead V1 suggests acute RV overload but is uncommon 1, 3

Bundle Branch Block

• Right bundle branch block (complete or incomplete) occurs in 4.8-9% of PE cases and is associated with RV strain 1, 2, 3
• Incomplete right bundle branch block has a positive likelihood ratio of 1.7 4

Atrial Arrhythmias

• Atrial fibrillation and other atrial dysrhythmias occur in 10-23.5% of PE patients 2, 3
• Non-sinus rhythm is present in 23.5% of PE patients versus 16.6% without PE 4

Diagnostic Limitations and Clinical Context

Poor Standalone Accuracy

• The ECG cannot diagnose or exclude PE when used in isolation 2, 6
• Individual ECG findings have limited diagnostic accuracy, with sensitivity of 50-60% and specificity of 80-90% 2
• A recent 2025 study confirmed that classical ECG findings have minimal standalone diagnostic accuracy and should not be used alone to rule in or rule out PE 6

Primary Role: Exclude Alternative Diagnoses

• The primary role of ECG is to exclude alternative diagnoses such as acute myocardial infarction, pericarditis, and other cardiac conditions that may present similarly 2, 3
• ECG should be obtained immediately in suspected PE to rule out acute MI and pericardial disease 2, 3

Clinical Algorithm for ECG Interpretation in Suspected PE

Step 1: Immediate ECG Acquisition

• Obtain 12-lead ECG immediately upon presentation with suspected PE 2, 3
• First priority: exclude acute MI, pericarditis, and other cardiac emergencies 2, 3

Step 2: Look for RV Strain Pattern

• Specifically assess for T-wave inversions in V1-V4, as this has the highest specificity and suggests more severe PE 3
• Check for S1Q3T3 pattern, right bundle branch block, and QR pattern in V1 3

Step 3: Integration with Clinical Assessment

• ECG must be used in conjunction with clinical prediction scores (Wells or revised Geneva) rather than in isolation 1, 2, 3
• The combination of clinical variables with ECG findings allows accurate categorization into probability categories 1

Step 4: Risk Stratification

• The presence of ≥1 classic RV strain sign is associated with higher adverse event rates 2
• ECG findings correlate with severity of pulmonary hypertension and RV dysfunction 2

Step 5: Hemodynamic Assessment

• If hemodynamic instability is present with RV strain on ECG, obtain urgent echocardiography to assess RV function 3
• In patients with shock or cardiac arrest, echocardiographic findings combined with ECG may be sufficient to guide reperfusion therapy 1

Step 6: Continuous Monitoring

• Implement continuous ECG monitoring during transport/transfer for suspected PE, especially in high-risk patients 1, 2, 3
• Major ventricular arrhythmias are uncommon even in critical PE, but monitoring remains essential 5

Important Caveats

• Up to 20% of patients with PE have completely normal ECG findings 1
• ECG abnormalities are generally non-specific, with most changes representing ST segment and/or T-wave alterations that can occur in numerous other conditions 2
• The "septal embolic pattern" was noted in 27 of 51 patients with critical PE, showing a trend toward regression at 48 hours 5
• Right-sided ECG derivations can increase sensitivity, with 88.1% of PE patients showing at least one abnormality on right-sided ECG 2
• ECG should be viewed as one component of a structured diagnostic approach that includes clinical probability assessment, D-dimer testing, and definitive imaging 2