Electrolytes: The Salts of the Earth

Wendy Arneson, MS, MASCP, MLS(ASCP)CM

Disclosures

Lab Med. 2014;45(1):e11-e15. 

In This Article

Clinical Significance

Potassium maintains cardiac rhythm and contributes to neuromuscular conduction. Imbalances in K level, as indicated by hyperkalemia or hypokalemia, will cause cardiac arrhythmias and neuromuscular weakness. Chloride (Cl) helps to maintain electrical neutrality with Na. Cl also contributes to the maintenance of acid-base balance by participating in the isohydric shift to buffer H+ and the Cl shift to maintain electrical neutrality with movement of bicarbonate (HCO3 ) ions. Renal and endocrine disorders often are characterized by electrolyte imbalances that can be reflected in plasma electrolyte levels. Changes in electrolyte levels, compared with total H2O content in the body, are associated with pathological consequences and increased mortality.[2–4]

Hypernatremia is caused by renal and nonrenal disorders; it is closely tied to total H2O levels in the body. A common nonrenal cause for hypernatremia is hypotonic dehydration resulting from severe diarrhea, extensive burns, or excessive sweating without proper fluid replacement. Infants, elderly individuals, and other patients will also experience hypernatremia due to dehydration if they are deprived of sufficient amounts of H2O or are not properly hydrated. Hypernatremia can also be caused by renal loss of H2O, such as from nephrogenic diabetes insipidus. Serum osmolality and urinary Na levels can help to differentiate the cause of hypernatremia, be it renal loss of H2O or nonrenal in nature.[5]

Hyponatremia, also of renal and nonrenal origins, is classified based on extracellular fluid (ECF) levels. However, clinical assessment of hyponatremia and ECF volume status is difficult without measuring the concentration of Na in a spot urine sample.[6] In situations in which there is increased ECF volume, there may be an actual increase in total Na levels in the body but a dilutional effect of plasma Na levels due to ascites, nephrotic syndrome, or congestive heart failure.[7] Urine Na levels are usually normal or decreased in hyponatremia due to edema. Decreased total Na levels in the body, even in the context of decreased ECF levels, are found in salt-losing renal nephritis, renal tubular acidosis, and treatment with certain types of diuretics or in the syndrome of inappropriate antidiuretic hormone secretion (SIADH) with resulting renal loss of Na. These causes of hyponatremia may be evaluated by testing for the presence of excess urinary Na levels.[5] This illustrates the value of measuring urine Na levels along with serum Na levels.[6]

Because of the high concentration of Na in extracellular fluid, such as plasma H2O, Na plays a major role in maintaining osmotic pressure. Extremely high or low Na concentrations in plasma will cause severe osmotic pressure changes that can induce serious consequences to several organs. The most immediate effect is swelling on the brain and potential coma.[5]

Hyperkalemia may be caused by decreased renal excretion in acute or chronic renal failure, treatment with certain diuretics, or hormonal imbalances such as hypoaldosteronism or hypocortisolism. Hyperkalemia may also be caused by ionic shift, as may be observed in cases of diabetic ketoacidosis or other metabolic acidosis; hyperkalemia is also associated with leukemia, excessive muscle activity, and hemolysis. Finally, hyperkalemia is associated with iatrogenic causes of excessive K administered intravenously or orally. Maintaining serum K levels in the normal reference range in patients with cardiac conditions may decrease life-threatening complications because abnormal serum K levels are associated with increased mortality.[4]

Hypokalemia is caused by renal loss such as renal tubular acidosis, hyperaldosteronism, hypercortisolism, and treatment of patients using certain diuretics. Potassium levels can also be low due to gastrointestinal dietary deficit or loss from severe vomiting, diarrhea, nasogastric suctioning, laxatives, and/or malabsorption. Transcellular ionic shifts that take place during insulin overdose and in metabolic alkalosis can also cause hypokalemia.[8]

Chloride is the major extracellular anion that balances with Na to maintain electrical neutrality. Chloride levels outside of reference ranges, even when considered by themselves, usually indicate a more serious underlying metabolic disorder, such as metabolic acidosis or alkalosis. Testing for Cl levels is diagnostically important and should be evaluated in a wide variety of clinical situations, especially those dependent on total H2O levels in the body.[9] Thus, hyperchloremia, like hypernatremia, occurs most commonly due to dehydration.

Hypochloremia may occur due to loss of Cl from the ECF or excess extracellular fluid levels, such as in edema or similar causes including hyponatremia. Cl loss can occur in the gastrointestinal tract, in compensation for respiratory acidosis, or in primary metabolic alkalosis.[10] The typical close inverse relationship between serum Cl and HCO3 concentrations is often less correlated when patients also exhibit significant changes in H2O balances and concurrent acidosis with anion gap disturbances.[11]

HCO3 , the metabolic component of the plasma buffer system, is the major component of the extracellular buffer system; renal tubular cells and erythrocytes serve as its reservoirs. Changes in plasma tCO2 and HCO3 levels indicate the presence of an acid-base disturbance but must be evaluated in the context of clinical information and several sequential electrolyte and anion gap results. Decreased plasma levels of total CO2 or HCO3 usually result from primary metabolic acidosis or as part of compensation for respiratory alkalosis. Extremely low levels of HCO3 in plasma almost always point to metabolic acidosis, such as in ketosis, renal failure, or other metabolic causes that limit compensation for respiratory alkalosis.

Increased plasma levels of total CO2 result from primary metabolic alkalosis or compensation for acute respiratory acidosis, such as in emphysema or respiratory failure.[12] Mixed metabolic and respiratory acidosis due to respiratory failure, in combination with hyponatremia or hypochloremia due to fluid imbalances such as edema, can yield serious complications and require prolonged treatment.[13]

processing....