The key message
The ECG in pulmonary embolism is supportive, never diagnostic, and is often normal or shows only sinus tachycardia. Its main value in the exam is (a) recognising signs of right heart strain that point to a larger clot, and (b) excluding mimics such as MI.
ECG features (most to least common)
- Sinus tachycardia — the commonest finding
- T-wave inversion in the right precordial (V1–V3) and inferior leads — right heart strain
- Right axis deviation
- New right bundle branch block (complete or incomplete)
- S1Q3T3 — a deep S wave in I, a Q wave and T inversion in III; famous but present in a minority and not specific
- Atrial fibrillation or other atrial arrhythmia
Signs of right heart strain (RBBB, right axis, anterior T inversion) suggest a more significant clot burden and correlate with RV dysfunction.
Putting it in context
- The ECG cannot rule PE in or out — use a validated pre-test probability score (e.g. Wells), D-dimer in low-probability patients, and CTPA (or V/Q) to confirm
- Look for RV dysfunction (echo, troponin, BNP) to risk-stratify — this guides whether thrombolysis is considered
See a real 12-lead example
For an annotated tracing of this pattern, see the Pulmonary Embolism page in the LITFL ECG Library:
ECG image examples are hosted by Life in the Fast Lane (LITFL), whose ECG library is published under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 licence. We link to LITFL rather than reproducing their tracings. The teaching notes above are original and written independently for MedNotes Hub.