What is the role of ECG (electrocardiogram) in diagnosing pulmonary embolism?

ECG in Pulmonary Embolism

Primary Role of ECG

The ECG cannot diagnose or exclude pulmonary embolism but serves primarily to exclude alternative diagnoses (such as acute myocardial infarction and pericardial disease) and to assess disease severity through detection of right ventricular strain patterns. 1, 2

Diagnostic Limitations

• 20-25% of patients with confirmed PE have completely normal ECGs, including those with large clot burden. 3 This fundamentally limits the ECG's utility as a diagnostic tool for ruling out PE.

• The ECG has modest diagnostic performance with sensitivity of only 50-60% and specificity of 80-90% for PE diagnosis. 2 These characteristics make it unsuitable as a standalone diagnostic test.

• ECG abnormalities in PE are typically non-specific, with most changes representing ST segment and/or T wave alterations that can occur in numerous other conditions. 1

Common ECG Findings and Their Frequency

The most frequent ECG abnormality in PE is sinus tachycardia (28%), though this finding lacks specificity. 3, 1

Classic signs of right ventricular strain include:

• T wave inversions in precordial leads V1-V4 (most common specific finding in submassive/massive PE) 4, 2
• S1Q3T3 pattern (occurs in only 3.7-8.5% of PE cases) 3, 5
• Right bundle branch block (complete or incomplete, seen in 4.8-9% of cases) 1, 3
• QR pattern in lead V1 (indicates acute RV overload) 1, 2

Less common findings:

• Atrial dysrhythmias including atrial fibrillation (10-23.5%) 1, 3
• Right axis deviation (4.2%) 3
• P pulmonale (0.5%) 3
• Clockwise rotation (20.1%, but also common in controls) 3

Prognostic Value and Risk Stratification

The presence of ≥1 classic RV strain sign (S1Q3T3, right bundle branch block, or T wave inversions V1-V4) is associated with higher adverse event rates (13% vs 6% in those without these signs, adjusted OR 2.11). 6

• ECG signs of right ventricular overload are typically seen in more severe cases and correlate with the degree of pulmonary hypertension and RV dysfunction. 1, 2

• The number of RV strain signs present correlates with adverse outcomes (OR 1.35 per additional sign). 6

• RV strain pattern has high specificity (97.4%) but low sensitivity (11.1%) for PE, making it useful when present but not helpful when absent. 3 This specificity increases to 17.1% sensitivity in patients with large clot burden. 3

Clinical Application Algorithm

For suspected PE patients:

1. Obtain ECG immediately to exclude acute myocardial infarction and pericardial disease as alternative diagnoses. 1, 2

2. Use ECG in conjunction with clinical prediction scores (Wells' rule or revised Geneva score) rather than in isolation. 1, 2

3. Implement continuous ECG monitoring during patient transport/transfer for suspected PE. 1

4. If RV strain pattern is present in a breathless patient, this is highly suggestive of PE (specificity 97.4%) and should prompt urgent definitive imaging. 3

5. In hemodynamically unstable patients (shock or hypotension), ECG findings of RV strain support the diagnosis but echocardiography is the most useful initial test to detect RV overload. 7, 1

6. Perform serial ECG evaluations in confirmed PE cases, as changes and resolution of abnormalities may have prognostic implications. 8

Critical Pitfalls to Avoid

• Never rely on a normal ECG to exclude PE - up to 25% of patients with large clot burden have normal ECGs. 3

• Do not mistake anterior T wave inversions for acute coronary syndrome - this is the most frequent ECG finding in submassive/massive PE and can lead to inappropriate treatment. 4, 8

• Avoid overinterpreting non-specific findings like sinus tachycardia or atrial dysrhythmias, as these occur commonly in controls without PE. 3

• Recognize that many "classic" PE findings are actually rare - S1Q3T3 occurs in <4% of PE cases and P pulmonale in <1%. 3

• The variable interrater reliability of individual ECG signs (κ 0.40-0.95) limits their practical utility in risk stratification. 6

Integration with Diagnostic Workup

The ECG should be viewed as one component of a structured diagnostic approach that includes clinical probability assessment (Wells score or Geneva score), D-dimer testing in appropriate patients, and definitive imaging with CT pulmonary angiography or V/Q scanning. 1, 7 The ECG's primary value lies in excluding competing diagnoses and, when RV strain is present, supporting the clinical suspicion and severity assessment of PE rather than establishing the diagnosis itself. 1, 2