What is the cause of T wave inversion in bundle branch blocks?

What is the cause of T wave inversion in bundle branch blocks?

In the normal heart, depolarization is from subendocardial region to epicardial region (inner to outer) and repolarization is from epicardium to subendocardial region. That is why both depolarization wave (QRS) and repolarisation wave (T wave) are both upright. Even though subendocardium is the first region to depolarised, the high intracavitary pressure of the left ventricle prevents repolarisation of the subendocardium from starting earlier.

When depolarization sequence is abnormal as in bundle branch block or WPW syndrome, repolarization is from subendocardial region to epicardial region. That is why T waves get inverted.