Wellen's phenomenon occurs when biphasic T waves are seen in leads V1-V3, or deep symmetric inverted T waves are seen in the precordial leads. Both of these ECG findings are indicative of a severe proximal left anterior descending stenosis (acute or chronic).
Note that there is no ST segment elevation that occurs with Wellen's ECG changes.
1. Surawicz B et al. Circulation. 2009;119:e235-240.
2. Chou's Electrocardiography in Clinical Practice: Adult and Pediatric, 6e