The approximate frequency of the various ECG findings in patients with an acute pulmonary embolus are shown here. The table is derived from four major studies performed in the last fifty years. Usually, the larger the embolus, the more likely the occurrence of ECG changes. It is estimated that approximately 70-75% of patients with an acute pulmonary embolus will demonstrate at least three of the findings shown in the table. Recognize, however, that the ECG changes are usually transient and that more than 20% of patients with a pulmonary embolus will have none of these ECG findings.