What is the role of an electrocardiogram (ECG) in the diagnosis and management of a patient suspected of having a pulmonary embolism (PE)?

ECG in Pulmonary Embolism: Diagnostic Role and Clinical Utility

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

Primary Role of ECG in PE Diagnosis

The ECG should be obtained immediately in all patients with suspected PE, but it functions as an adjunctive tool rather than a definitive diagnostic test 1, 2. The key roles are:

• Excluding alternative diagnoses: The ECG helps rule out acute coronary syndrome, pericardial disease, and other cardiac causes of chest pain or dyspnea 1, 2
• Risk stratification: ECG findings correlate with severity of pulmonary hypertension and right ventricular dysfunction, helping identify high-risk patients 2
• Guiding management in hemodynamically unstable patients: In suspected high-risk PE with shock or hypotension, ECG findings of RV strain support urgent echocardiography and potential reperfusion therapy 1

Common ECG Findings and Their Diagnostic Accuracy

Most patients with PE have abnormal ECGs, but 20-25% may have completely normal ECGs, even with large clot burden. 3 The most common findings include:

High-Specificity Findings (Most Clinically Useful)

• T-wave inversions in right precordial leads (V1-V4): This is the most clinically significant finding with the highest specificity (97.4%) for RV strain, suggesting more severe PE 2, 3. Present in 11.1% of PE patients versus 2.6% of controls (OR 4.58, p=0.002) 3
• Right ventricular strain pattern: Sensitivity 11.1%, specificity 97.4%; when present, strongly suggests PE, particularly in patients with large clot load (17.1% vs 2.6% in controls, OR 7.55) 3

Common but Non-Specific Findings

• Sinus tachycardia: Present in approximately 28-40% of PE cases, but lacks specificity and indicates only mild disease 2, 3
• S1Q3T3 pattern: Classic but insensitive finding (present in only 3.7% of cases), with limited standalone diagnostic value (LR+ 2.07,95% CI 1.27-3.39) 2, 4, 3
• Right bundle branch block: Occurs in 4.8-9% of cases, associated with RV strain but also common in controls 2, 3
• Atrial dysrhythmias: Including atrial fibrillation, present in 10-23.5% of patients 2

Overall Diagnostic Performance

The ECG has limited diagnostic accuracy with sensitivity of 50-60% and specificity of 80-90% for PE 2. No single ECG finding has sufficient accuracy to rule in or rule out PE when used in isolation. 4

Clinical Algorithm for ECG Integration

In Hemodynamically Unstable Patients (Shock or Hypotension)

1. Obtain ECG immediately to assess for RV strain patterns 1
2. Perform bedside transthoracic echocardiography as the most useful initial test—this is more important than ECG in this setting 1
3. If echocardiography shows RV dysfunction in a highly unstable patient, this is sufficient to prompt immediate reperfusion therapy without further testing 1
4. Absence of echocardiographic RV overload practically excludes PE as the cause of hemodynamic instability 1

In Hemodynamically Stable Patients (Non-High-Risk PE)

1. ECG should NOT be used as part of the diagnostic work-up for confirming or excluding PE in stable patients 1
2. Use ECG in conjunction with clinical prediction scores (Wells or revised Geneva score) to determine pre-test probability, never in isolation 2
3. Follow structured diagnostic algorithm: Clinical probability assessment → D-dimer (if low/intermediate probability) → CT pulmonary angiography (if indicated) 1
4. Implement continuous ECG monitoring during transport/transfer of patients with suspected PE, especially those with high-risk features 2

Critical Pitfalls to Avoid

• Do not rely on ECG to exclude PE: A normal ECG does not rule out PE, as 20-25% of patients with confirmed PE (including large clot burden) have normal ECGs 3
• Do not use S1Q3T3 as a standalone diagnostic criterion: This classic finding is present in less than 4% of PE cases and has insufficient sensitivity 4, 3
• Do not delay definitive imaging based on ECG findings: The ECG cannot replace CT pulmonary angiography or other definitive diagnostic tests 1, 2
• Recognize that ECG abnormalities are non-specific: Most ECG changes (ST segment and T wave alterations) can occur in numerous other conditions 2

Special Considerations

In patients with RV strain on ECG and hemodynamic instability, urgent echocardiography should be obtained to assess RV function and guide reperfusion therapy decisions 2. The presence of ≥1 classic RV strain sign is associated with higher adverse event rates and worse short-term prognosis 2.

Right-sided ECG derivations can increase diagnostic sensitivity, with 88.1% of PE patients showing at least one abnormality on right-sided ECG, though this is not routinely performed 2.