A 58-year-old man presents to the emergency department with acute-onset dyspnea and pleuritic chest pain for 2 hours. He returned from a 12-hour flight 3 days ago and has been immobilized with a leg fracture. His vital signs show HR 118/min, RR 24/min, BP 128/82 mmHg, SpO₂ 91% on room air. An ECG is obtained and shows the pattern marked **A** in the diagram. Which of the following best describes the pathophysiological mechanism underlying this ECG finding in acute pulmonary embolism?
A. Chronic left ventricular hypertrophy from sustained systemic hypertension causing concentric remodeling
B. Acute pericardial inflammation with fibrinous exudate causing global ST elevation and PR depression
C. Acute left ventricular ischemia secondary to decreased coronary perfusion pressure from systemic hypotension
D. Acute right ventricular pressure overload due to thromboembolic obstruction of pulmonary arteries, causing RV dilatation and septal bowing into the left ventricle
Explanation
Why Option 1 is correct
The pattern marked A (S1Q3T3 + RBBB + V1-V4 TWI) is the classic ECG signature of acute pulmonary embolism, first described by McGinn and White in 1935. This pattern reflects acute right ventricular pressure overload. When thromboemboli obstruct pulmonary arteries, pulmonary vascular resistance rises acutely. The thin-walled RV, normally a low-pressure chamber, cannot acutely generate >40–50 mmHg of pulmonary pressure and dilates. This RV dilatation causes septal bowing into the LV, which explains the S wave in lead I (septal forces directed away from I) and the Q wave and T-wave inversion in lead III (inferior wall RV strain). The precordial T-wave inversions (V1–V4, Dagnini-Meneghini sign) reflect RV strain in the anterior wall. Although highly specific when present, the S1Q3T3 pattern has only 20–30% sensitivity; most PE patients have only sinus tachycardia or nonspecific changes. (ESC PE Guidelines 2019; AHA/ACC)
Why each distractor is wrong
Option 2: Acute LV ischemia would produce ST elevation or depression in the distribution of a coronary artery (LAD, LCx, RCA), not the classic S1Q3T3 pattern. Moreover, this patient is hemodynamically stable (BP 128/82), making acute coronary syndrome less likely. The ECG pattern is specific to RV strain, not LV ischemia.
Option 3: Chronic LVH with strain develops over years of hypertension and produces a characteristic pattern of LV voltage criteria, ST-segment depression, and T-wave inversion in lateral leads (I, aVL, V5–V6). This is a chronic process and does not explain the acute presentation with S1Q3T3 or the acute clinical context of immobility and recent travel.
Option 4: Pericarditis produces diffuse ST elevation (except aVR and V1) with PR depression and evolves over days with a typical viral prodrome. It does not produce S1Q3T3, RBBB, or the specific precordial T-wave inversion pattern seen in PE. The clinical context (immobility, recent travel) strongly favors PE over pericarditis.
High-YieldNEET PG
S1Q3T3 is highly specific but only 20–30% sensitive for PE; it reflects acute RV pressure overload from pulmonary vascular obstruction. Most PE patients have only sinus tachycardia or nonspecific changes; CTPA is the gold standard for diagnosis.
ESC PE Guidelines 2019; AHA/ACC
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