electrical alternans

A beat to beat alteration in the electrocardiogram. Total electrical alternans involving the p-wave, the QRS complex, the ST segment & the T-wave with sinus tachycardia is a relatively specific sign of pericardial effusion, often with pericardial tamponade. This occurs from a periodic swing of the heart in the effusion at a frequency of 1/2 the heart rate. ST-T electrical alternans is a sign of electrical instability & may signal impending ventricular fibrillation.

Related

electrocardiogram (ECG, EKG) pericardial effusion ventricular fibrillation (V Fib)

References

Harrison's Principles of Internal Medicine, 14th ed. Fauci et al (eds), McGraw-Hill Inc. NY, 1998, pg 1246