You might find this interesting:

I've occassionally experienced an arrhythmia that's never been diagnosed as
being anything serious, and was just put down to anxiety -- it hasn't occurred
for about 3 years. My heart would beat out of time, but not fast, and I would
feel weak, feeble and a bit tired.

The last few times it happened, it seemed to be triggered by drinking something
very cold on a hot day after a long bike ride. So, i put two and two together, and
considered the effects of dehydration and low potassium levels. I quized one of
my physiology lectureres, and he more or less confirmed that, when low on
potassium, the electrical activity of the heart can be disturbed by the "shock"
of the gut being suddenly subjected to ice-cold liquid.