Key Concepts
Introduction
Potassium (Kโบ) is the dominant intracellular cation, with approximately 98% residing inside cells and only 2% in the extracellular fluid. This steep concentration gradient - maintained by the Naโบ/Kโบ-ATPase pump - is the primary determinant of the resting membrane potential (RMP) of excitable cells, including cardiac myocytes and skeletal muscle fibers. The normal RMP of a cardiac ventricular cell is approximately โ90 mV, generated largely by potassium leak channels that allow Kโบ to flow down its concentration gradient from intracellular to extracellular space. When extracellular Kโบ rises (hyperkalemia), the gradient decreases, the RMP becomes less negative (partially depolarized), and the cell becomes hyperexcitable initially but progressively inexcitable as sodium channels inactivate. When extracellular Kโบ falls (hypokalemia), the gradient steepens, the RMP becomes more negative (hyperpolarized), making the cell harder to depolarize - but paradoxically more prone to ectopic pacemaker activity and re-entrant circuits due to prolonged repolarization. This is why both extremes of potassium produce life-threatening dysrhythmias: they disrupt the precisely calibrated electrical cycling of the heart. Normal serum potassium: 3.5-5.0 mEq/L. Even small deviations (0.5-1.0 mEq/L) can produce clinically...