A middle-age alcoholic presents with weakness and vague abdomimal discomfort.  The patient’s previous ECG was from 4 years prior and was completely normal.  Here is the presenting ECG:

There is sinus rhythm and Right Bundle Branch block.  There is a tall R-wave in V2 and V3 and slightly excessive ST depression in lead V2, with a biphasic wave.  What is it?  Is this ischemia?  Is it posterior STEMI?  Answer below.

Answer:

Here I have annotated the ECG:

The apex of the T-wave is seen in V1 (inverted).  If you draw a line down to V2, you see that the nadir of the negative wave is at the same time as the peak of the T-wave in V1.  The positive wave comes later.  So this positive wave is a U-wave, not a biphasic T-wave.  The arrow  in V5 shows a small hump that is also U-wave.

If the U-wave were really a T-wave, then the QT would be 440ms and the QTc would be 568 ms, which is very long.  Whenever the QT interval seems too long, suspect a U-wave.

The patient had no myocardial ischemia.  The potassium returned at 2.2 mEq/L.

Diagnosis: U-wave from hypokalemia, in the presence of right bundle branch block.