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A 46 year old African American woman with a history of hypertension and seizure disorder was hospitalised for seizures and aspiration pneumonia. On the second hospital day she suddenly became aphasic with loss of power in her left upper and lower extremities. Her ECG revealed sinus rhythm, prolongation of the QT interval, and beat to beat variation in the T wave amplitude consistent with T wave alternans (below). Computed tomography scan of the head showed a large right hemispheric stroke with mass effect, following which she suddenly collapsed and had a cardiac arrest. She had ventricular fibrillation, which successfully converted to sinus rhythm with defibrillation. An echocardiogram showed left ventricular hypertrophy with no other significant abnormalities. Her electrolyte profile was normal. T wave alternans (or repolarisation alternans) is a rare electrical phenomenon, seldom observed on electrocardiography, and is defined as beat to beat variation in the polarity or amplitude of T waves signifying an inhomogeneity in the refractoriness of the myocardium, setting the stage for re-entry and facilitating onset of malignant ventricular arrhythmias. In our patient, the massive stroke with mass effect (which is a well known cause of long QT), together with profound catecholamine release from such an event, probably set the stage for profound myocardial stimulation. Subsequently dispersion of refractoriness (compounded by left ventricular hypertrophy) may have created myocardial instability and precipitated a ventricular arrhythmic event leading to cardiac arrest. The presence of T wave alternans in the immediate pre-arrest setting was an important clue to the electrical instability of the myocardium, before ventricular fibrillation.