A 58-year-old man presents to the emergency department with acute onset dyspnea and pleuritic chest pain. He had undergone knee arthroscopy 10 days ago. Vital signs show heart rate 112/min, BP 128/82 mmHg, RR 24/min, SpO₂ 92% on room air. His ECG is shown in the diagram. The pattern marked **B** (S1Q3T3 with RV strain pattern) is noted. Which of the following best describes the pathophysiological mechanism underlying this ECG finding in this clinical context?
A. Left ventricular hypertrophy secondary to chronic systemic hypertension causing leftward axis deviation
B. Acute myocardial infarction of the inferior wall with secondary right ventricular involvement
C. Atrial fibrillation with rapid ventricular response causing diffuse ST-T wave changes
D. Acute right ventricular pressure overload due to pulmonary arterial obstruction by thrombus, causing clockwise rotation of the heart and acute RV strain
Explanation
Why "Acute right ventricular pressure overload due to pulmonary arterial obstruction by thrombus, causing clockwise rotation of the heart and acute RV strain" is right
The S1Q3T3 pattern (McGinn-White sign) with RV strain pattern (T-wave inversions in V1–V4) is a classic but uncommon ECG finding in pulmonary embolism, occurring in only 10–20% of PE cases. This pattern reflects acute right ventricular pressure overload when a thrombus (typically from DVT of the lower extremities) obstructs the pulmonary arterial circulation. The prominent S wave in lead I, Q wave in lead III, and inverted T wave in lead III result from clockwise rotation of the heart due to RV dilation and acute strain. In this patient with recent immobilization (knee arthroscopy 10 days prior), acute dyspnea, tachycardia, and hypoxemia, PE is the most likely diagnosis. The ECG pattern marked B directly demonstrates acute RV strain, which is the pathophysiological hallmark of PE-induced right heart dysfunction (Harrison's 21e; 2019 ESC Guidelines on PE).
Why each distractor is wrong
Left ventricular hypertrophy secondary to chronic systemic hypertension causing leftward axis deviation: LVH causes leftward axis deviation and LV strain pattern (lateral T-wave inversions in V5–V6), not the S1Q3T3 pattern or anterior precordial RV strain. The acute presentation and normal BP make chronic hypertension unlikely.
Acute myocardial infarction of the inferior wall with secondary right ventricular involvement: Inferior STEMI (marked as A in the diagram) would show ST elevation in leads II, III, and aVF with reciprocal changes, not the S1Q3T3 pattern. The clinical context (recent surgery, acute dyspnea without chest pain typical of ACS) and hemodynamic stability favor PE over MI.
Atrial fibrillation with rapid ventricular response causing diffuse ST-T wave changes: While atrial fibrillation can occur in PE (5–10% of cases), it is not the primary mechanism of the S1Q3T3 pattern. AF causes irregular rhythm and may produce ST-T changes, but does not produce the specific clockwise rotation pattern seen in acute RV strain.
High-YieldNEET PG
S1Q3T3 + V1–V4 T inversions in acute dyspnea + risk factor (immobility, surgery) = PE with acute RV strain → CTPA → anticoagulate.
