When my personal trainer said that a high level of potassium could be fatal for some distance runners, I had to research this. That what we take for granted in our training could have dire results was frightening.
I don't have a heart condition or heart disease in my family, that I know of. However, I have recently gone through various tests to try to determine the cause of my odd heart palpitations, and will be undergoing a M.A.P. test - maximum anaerobic power test for my anaerobic threshold. This is how the subject of potassium came up with regard to my training, as the question was raised about my hydration, nutrition and supplements.
What I found in my short research thus far was that electrolyte imbalances, such as too low or too high potassium level, can be symptomatic of a heart condition called bradycardia. Fortunately, it's rare among lupus patients (I'm a lupus patient). Unfortunately, Bradycardia symptoms also include some issues I've been dealing with since last summer, and mostly during or immediately following a distance event.
For more information on the connection between potassium and heart health, please see:
The Importance of Potassium for People with Heart Failure, American Heart Organization
See also:
Is potassium deficiency a common problem in athletes? and
Effects Of Too Much Potassium, BeWell at Stanford