A young woman had what sounded like vasovagal syncope.  This ECG was recorded:

There is incomplete RBBB (QRS = 110 ms).  There is some ST depression and T-wave inversion in V1-V3, but remember this is the normal repolarization pattern for RBBB. What else?

It looks like a very long QT.

Is it?

You can clearly see the peak of the T-wave in lead II across the bottom.  So let’s find where the peak is on other leads:

There is sinus rhythm with inverted P-waves (low atrial pacemaker, of no clinical significance). The blue line shows the peak of the T-wave in II.  Drawn up to V1-V3, you can see that the large wave is AFTER the T-wave.  It is thus a U-wave, not a T-wave, and the QT is not long.  The arrows point to U-waves in other leads

The patient had a previous ECG:

It turns out she had had unspecified cardiac surgery in the past and had incomplete RBBB at baseline.  As you can see, there were no U-waves on that previous ECG.

Her K was 2.9 mEq/L.  This is not terribly low, but can certainly result in ECG abnormalities.

She had K replaced to a level of 3.9 mEq/L.  Then, 2 hours, 40 minutes later this ECG was recorded:

U-waves are still present but not as prominent. The treating physicians interpreted this as normalized back to baseline ECG; clearly this is not so.

Why are U-waves still present?

1. She may have some other unspecified reason for U-waves

2. Recurrent hypokalemia: Her K may have dropped back down after replacement.  For every 1 mEq/L of low potassium, there is a 200-400 mEq total body deficit.  So K may shift to the intracellular space soon after replenishment, and the serum K may drop within hours after administration.

The syncope and the ECG are probably completely unrelated, though it is possible that this resulted in ventricular dysrhythmias.

Here is another case of hypokalemia in RBBB.