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Potassium (K[+]) is the major cation of the intracellular fluid. Disturbance of potassium homeostasis has serious consequences. Decrease in extracellular potassium is characterized by muscle weakness, irritability, paralysis, fast heart rate, specific cardiac conduction effects that are apparent by electrocardiographic examination, and eventual cardiac arrest.
More than 90% of hypertensive patients with aldosteronism have a hypokalemia (low K[+]). Low K(+) also is common in vomiting, diarrhea, alcoholism, and folic acid deficiency.
Abnormally high extracellular K+ levels produce symptoms of mental confusion; weakness, numbness and tingling of the extremities; weakness of the respiratory muscles; flaccid paralysis of the extremities; slowed heart rate, and eventually peripheral vascular collapse and cardiac arrest. Hyperkalemia may be seen in end stage renal failure, hemolysis, trauma, Addison's disease, metabolic acidosis, acute starvation, dehydration, and with rapid K(+) infusion.
Measurement of serum potassium is used for evaluation of electrolyte balance, cardiac arrhythmia, muscular weakness, hepatic encephalopathy, and renal failure. Potassium should be monitored during treatment of many conditions but especially in ketoacidosis of diabetes mellitus and any intravenous therapy for fluid replacement.
Plasma K=values less than 3.0 mEq/L are associated with marked neuromuscular symptoms and are evidence of a critical degree of intracellular depletion. K(+) values < 2.5 mEq/L are potentially life-threatening.
It is important to remember that values determined by direct potentiometry/ion selective electrodes may be slightly higher in specimens with high protein and lipid concentrations than the corresponding serum specimen that is assayed by an indirect potentiometric method.
Ion selective electrodes are selective for the ion is question buy are not absolutely specific. Other monovalent cations may interfere but not in the physiologic range.