Her ECG is shown again here. There is sinus tachycardia with a rate of 140, a frontal plane axis of +90 degrees, and incomplete right bundle branch block. Note the S wave in lead I, the Q wave in lead III and the inverted T wave in lead III. This is referred to as the “S1- Q3- T3 pattern". It is quite characteristic of the ECG changes that accompany acute cor pulmonale and is sometimes seen in patients with an acute pulmonary embolus. The sinus tachycardia and incomplete right bundle branch block also occur in this setting. Her further evaluation revealed a large pulmonary embolus lodged in the right main pulmonary artery.