Biphasic T waves can be either down-up or up-down in shape.
Some up-down biphasic T waves are known as Wellen’s waves.
Down-up biphasic T waves can be caused by infarction with some reperfusion / post-ischemia or hypokalemia.See also: T and U waves
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This ECG is from a middle aged man who presented with intermittent chest tightness and nausea on walking that resolved with rest. This had happened twice over the past two days. He was likely pain free at the time that this ECG was recorded.
This ECG shows terminal T wave inversion in V2-4, resembling Wellens' waves. There was no wall motion abnormality on echo and troponins were negative (so it was not a true Wellens' syndrome), but a stress echo was markedly positive. Angiogram showed a 95% LAD stenosis and 70% first diagonal stenosis. The cause of the T wave changes was likely ischemia without infarction, ie. unstable angina.