ECG in Pulmonary Embolism

The ECG is of limited diagnostic value in patients with suspected Pulmonary Embolism (PE).Many of the classically described ECG changes in patients with suspected PE are equally common in patients suspected of having PE but in whom the diagnosis is ultimately excluded.Even the two ECG changes noted commonly in pulmonary embolism, namely tachycardia and incomplete right bundle branch block, are infrequently observed and are only slightly more frequent in patients with PE. The ECG is thus a poor diagnostic test for pulmonary embolism. Changes have low sensitivity and low specificity. The greatest utility of the ECG in the patient with suspected PE is ruling out other potential life threatening diagnoses such as myocardial infarction.

Findings

a)ECG shows non-specific changes in 80 per cent

b) Classic Findings − S1 Q3 T3 (seen in under 20 per cent of cases)

i) S-Wave in Lead I

ii) Q-Wave in Lead III

iii) T-wave inversion in Lead III

c) Common Findings

i) Sinus tachycardia

ii) Right sided strain pattern

⎯ Right bundle branch block

⎯ Right axis deviation

iii) Findings that mimic myocardial infarction

⎯ ST segment changes

⎯ T-wave changes

iv) Atrial fibrillation(new onset)

The S1Q3T3 was first described by McGinn and White in JAMA in 1935.The S1Q3T3 sign is a manifestation of combined right heart pressure and volume overload of acute onset leading to repolarization abnormalities. In other words, this is a reflection of acute cor pulmonale. An S-wave in lead I signifies a complete or more often incomplete RBBB. In lead III,look for a Q-wave, slight ST elevation, and an inverted T-wave. Any cause of acute cor pulmonale can cause the S1Q3T3 finding on the ECG. This includes PE, acute bronchospasm,  pneumothorax, and other acute lung disorders. In addition, transient LPFB may cause this finding as well.