Potassium is 1 of 3 critical electrolytes involved in the function of the action potential of cardiac muscle cells (the others are calcium and sodium). The most significant role for potassium occurs in phase 3 of the action potential, when potassium moves out of the cell to return the resting membrane potential (RMP) to a negative state (repolarization). Serum potassium (measured in mEq/L) is normal when the serum level is in equilibrium with intracellular levels. High serum potassium can lead to alterations in the waveforms of the surface electrocardiogram (ECG) and cardiac death unless treated promptly. Electrolyte disturbances are not specifically listed as a major category among the guidelines for cardiac monitoring in hospitalized patients published in 2004 by the American Heart Association, but patients are frequently admitted to monitored settings for treatment of hyperkalemia. In this ECG Challenges column, the effects...

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