What are the typical ECG (electrocardiogram) changes in pulmonary embolism?

ECG Changes in Pulmonary Embolism

ECG abnormalities in pulmonary embolism are common but usually non-specific, with the most frequent findings being sinus tachycardia and ST-segment/T-wave changes, while classic signs like S1Q3T3 occur infrequently but can help increase clinical suspicion when present. 1

Most Common ECG Findings

The typical ECG changes associated with PE include:

• Sinus tachycardia is the most common abnormality, occurring in approximately 28% of PE patients 2
• T-wave inversions in right precordial leads (V1-V4) suggest right ventricular overload and are more frequent in massive PE 1, 3
• Non-specific ST-segment and T-wave changes are frequently observed but lack diagnostic specificity 1
• Right ventricular strain pattern (T-wave inversions in ≥2 inferior leads plus ≥2 anterior leads) is highly specific when present, occurring in 11.1% of PE patients versus only 2.6% of controls (specificity 97.4%) 2

Classic but Infrequent Findings

Several "textbook" ECG patterns are associated with PE but occur rarely:

• S1Q3T3 pattern occurs in only 3.7-8.5% of PE cases, but when present increases likelihood of PE (positive likelihood ratio 2.07-3.7) 4, 2, 5
• Right bundle branch block (complete or incomplete) is seen in approximately 4.8-9% of cases 2, 5
• Right axis deviation occurs in only 4.2% of PE patients 2
• P pulmonale is extremely rare, found in only 0.5% of cases 2
• QR pattern in V1 indicates acute right ventricular overload but is uncommon 3

Correlation with PE Severity

ECG findings correlate with the extent of pulmonary vascular obstruction:

• Features of acute right heart strain are common with massive emboli and indicate hemodynamically significant PE 1
• Right ventricular strain pattern is significantly more common in patients with large clot burden (17.1%) compared to small emboli 2
• Normal ECG occurs in 20-25% of PE patients, including those with large clot load, so a normal ECG does not exclude PE 2
• ECG abnormalities typically only occur when there has been significant obstruction to the pulmonary circulation, particularly in patients with perfusion defects exceeding 30% 1

Clinical Utility and Limitations

The ECG should be used as a complementary tool rather than a standalone diagnostic test:

• ECG has modest diagnostic accuracy with sensitivity of 50-60% and specificity of 80-90% for PE 3
• Classical ECG findings modestly increase post-test probability but lack sufficient standalone accuracy to rule in or rule out PE 4
• The primary value of ECG is in excluding other diagnoses such as acute myocardial infarction and pericardial disease 1, 3
• ECG should be used in conjunction with clinical prediction scores (Wells score or revised Geneva score) for comprehensive assessment 3

Prognostic Significance

Beyond diagnosis, ECG findings provide prognostic information:

• Continuous ECG monitoring is highly recommended in patients with suspected PE during transport/transfer 3
• ECG findings correlate with severity of pulmonary hypertension and right ventricular dysfunction 3
• The presence of right ventricular strain patterns indicates more severe hemodynamic compromise and may guide treatment decisions 1, 2

Key Clinical Pitfalls

• Do not rely on ECG alone to diagnose or exclude PE—many patients with confirmed PE have normal or non-specific ECG findings 4, 2
• Beware of misdiagnosing PE as acute coronary syndrome when anterior T-wave inversions are present, as this is a common ECG manifestation of RV strain from PE 6
• ECG interpretation is particularly difficult in patients with underlying congestive heart failure or chronic obstructive pulmonary disease 1
• The absence of classic findings does not reduce the need for definitive imaging (CT pulmonary angiography) when clinical suspicion is present 4

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