A large S wave in lead I, a Q wave in lead III, and an inverted T wave in lead III indicates acute right heart strain. This pattern only occurs in about 10% of people with pulmonary embolisms and is similar to the ECG findings in a left posterior fascicular block (LPFB).

The most common ECG finding in the setting of a pulmonary embolism is sinus tachycardia, however the "S1Q3T3" pattern of acute cor pulmonale is classic. This is termed the McGinn-White sign.