Hyperkalemia is dangerous because it changes cardiac electrophysiology.
But the ECG is an imperfect warning system.
A patient can have severe hyperkalemia with subtle ECG findings. Another patient can deteriorate quickly once conduction becomes unstable.
Potassium helps determine the resting membrane potential of cardiac myocytes.
As extracellular potassium rises, the resting membrane potential becomes less negative. Initially, this may increase excitability. As hyperkalemia worsens, sodium channels become inactivated, depolarization slows, and conduction becomes dangerous.