Sodium is the major cation (90%) in extra cellular fluid; potassium, the major cation in intracellular fluid. During repolarization, the sodium-potassium pump continually shifts sodium into the cells and potassium out of the cells; during depolarization, it does the reverse.
Sodium cation functions include maintaining tonicity and concentration of extracellular fluid, acid-base balance (reabsorption of sodium ions and excretion of hydrogen ions), nerve conduction and neuromuscular function, glandular secretion, and water balance.
The body requires only 2 to 4 g of sodium daily. However, most Americans consume 6 to 10 g daily (mostly sodium chloride, as table salt), excreting excess sodium through the kidneys and skin.
A low-sodium diet or excessive use of diuretics may induce hyponatremia (decreased serum sodium concentration); dehydration may induce hypernatremia (increased serum sodium concentration).
Sodium imbalance can result from several causes.
One of the main causes of hyponatremia is excessive GI loss of water and electrolytes. This can result from vomiting, suctioning, or diarrhea; excessive perspiration or fever; potent diuretics; or use of tap-water enemas.
When such losses decrease circulating fluid volume, increased secretion of antidiuretic hormone (ADH) promotes maximum water reabsorption, which further dilutes serum sodium. These factors are especially likely to cause hyponatremia when combined with too much electrolyte-free water intake.
Excessive drinking of water, infusion of I.V. dextrose in water without other solutes, malnutrition or starvation, and a low-sodium diet can also cause hyponatremia, usually in combination with one of the other causes.
Trauma, surgery (wound drainage), and burns, which cause sodium to shift into damaged cells, can lead to decreased serum sodium levels, as can adrenal gland insufficiency (Addison's disease), hypoaldosteronism, and cirrhosis of the liver with ascites.
Syndrome of inappropriate antidiuretic hormone secretion (SIADH), resulting from brain tumor, cerebrovascular accident, pulmonary disease, or neoplasm with ectopic ADH production, can lead to hyponatremia also. Certain drugs, such as chlorpropamide and clofibrate, may produce an SIADH-like syndrome.
Decreased water intake can cause hypernatremia. When severe vomiting and diarrhea cause water loss that exceeds sodium loss, serum sodium levels rise, but overall extracellular fluid volume decreases.
Other causes include excess adrenocortical hormones, as in Cushing's syndrome, and ADH deficiency (diabetes insipidus). Salt intoxication-an uncommon cause-may result from excessive ingestion of table salt.
Signs and symptoms
You could experience the following:
Hyponatremia is defined as a serum sodium level less than 135 mEq/L; hypernatremia, as a serum sodium level greater than 145 mEq/L. However, additional laboratory studies are necessary to determine etiology and differentiate between a true deficit and an apparent deficit resulting from sodium shift or from hypervolemia or hypovolemia.
In true hyponatremia, supportive values include urine sodium greater than 100 mEq/24 hours, with low serum osmolality; in true hypernatremia, urine sodium is less than 40 mEq/24 hours, with high serum osmolality.
The type of treatment varies with the severity of the imbalance.
Treatment for mild hyponatremia usually consists of restricted electrolyte free water intake when it results from hemodilution, SIADH, or conditions such as congestive heart failure, cirrhosis of the liver, and renal failure. If fluid restriction alone fails to normalize serum sodium levels, demeclocycline or lithium, which blocks ADH action in the renal tubules, can be used to promote water excretion.
In extremely rare instances of severe symptomatic hyponatremia, when the serum sodium level falls below 110 mEq/L, treatment may include an infusion of 3% or 5% saline solution.
Treatment with saline infusion requires careful monitoring of venous pressure to prevent potentially fatal circulatory overload. The aim of treatrnent of secondary hyponatremia is to correct the underlying disorder.
Primary treatment of hypernatremia is administration of salt-free solutions (Much as dextrose in water) to return serum sodium levels to normal, followed by infusion of 0.45% sodium chloride lo prevent hyponatremia.
Other measures include a sodiumrestricted diet and discontinuation of drugs that promote sodium retention.
A person can take these actions to help prevent sodium imbalance:
A person who has the flu and uncontrollable vomiting needs to be monitored carefully. He or she may need to be admitted to a hospital to get fluids to prevent dehydration and sodium imbalance .
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