The S1Q3T3 pattern on an EKG in the setting of pulmonary embolism indicates which finding?

• A. Atrial enlargement
• B. Pericardial effusion
• C. Left ventricular hypertrophy
• D. S1Q3T3 pattern indicating right heart strain

Answer: (D)

When a pulmonary embolism abruptly blocks blood flow, the right ventricle faces a sudden increase in afterload and can become strained and dilated. This right-sided strain can produce a characteristic ECG change known as S1Q3T3: a deep S wave in the first (limb) lead, a Q wave in the third lead, and inverted T waves in the third lead. The pattern reflects the heart axis shifting and abnormal repolarization caused by the stressed right ventricle.

This finding points to acute right heart involvement rather than problems on the left side. It helps differentiate from atrial enlargement (which would show different P-wave changes), pericardial effusion (which often causes low QRS voltage or electrical alternans), or left ventricular hypertrophy (which shows high voltages and LVH criteria). Remember, though, that S1Q3T3 is not highly sensitive and may be absent in many PE cases; its presence supports—though it does not prove—acute PE with right ventricular strain.