A 62-year-old woman presents with acute dyspnea and chest discomfort 5 days post-operatively after hip replacement. CTPA confirms PE. Which electrocardiographic finding best distinguishes acute PE from acute myocardial infarction in this clinical context?

## ECG Findings: PE vs Acute MI **Key Point:** The S₁Q₃T₃ pattern (S wave in lead I, Q wave in lead III, T-wave inversion in lead III) is a classic but relatively insensitive ECG finding that, when present, strongly suggests acute PE and helps distinguish it from acute MI. ### ECG Pattern Comparison | ECG Finding | PE | Acute MI | Discriminatory Value | |-------------|----|-----------|-----------------------| | **S₁Q₃T₃ pattern** | Classic (if present) | Absent | **Highly specific for PE** | | **ST elevation** | Rare, non-territorial | Territorial (STEMI) | Favors MI if present | | **Diffuse ST depression** | Common | Posterior MI or NSTEMI | Non-specific | | **Pathological Q waves** | Absent | Present (old MI) | Favors MI | | **T-wave inversion** | Widespread (V1–V4) | Regional (territory-specific) | Context-dependent | | **Sinus tachycardia** | Very common | Variable | Non-specific | **High-Yield:** The S₁Q₃T₃ pattern reflects acute right ventricular strain in PE: - **S in lead I** = rightward axis deviation (RV strain shifts axis rightward) - **Q in lead III** = right atrial enlargement and RV dilation - **T inversion in III** = RV ischemia from increased afterload **Mnemonic:** **S₁Q₃T₃ = PE's ECG signature** — when you see this triad, think PE first, especially in the post-operative or immobilized patient. ### Clinical Context In a **post-operative patient with acute dyspnea**, the differential is: 1. **PE** (most likely given immobility, surgery, hypercoagulability) 2. **Acute MI** (less likely but possible, especially if cardiac risk factors) The S₁Q₃T₃ pattern, if present, strongly favors PE over MI. However: - **Sensitivity of S₁Q₃T₃ in PE:** ~20–30% (not all PE patients have it) - **Specificity of S₁Q₃T₃ in PE:** ~99% (when present, PE is highly likely) **Clinical Pearl:** Most PE patients have **non-specific ECG changes** (sinus tachycardia, T-wave inversion in V1–V4, ST depression). The S₁Q₃T₃ pattern is a minority finding but is pathognomonic when it appears. **Warning:** Do NOT rely on ECG alone to diagnose or exclude PE. CTPA is the gold standard. ECG is used to rule out MI and assess RV strain.