Sodium

Sodium is the major cation in the extracellular fluid and as such is the major contributor to serum osmolality, which is important in the control of body water distribution. Sodium also functions in acid-base balance, enzyme activity and neuromuscular conductivity. Since serum sodium reflects not only sodium but also water, when interpreting abnormal sodium values the clinician must first determine if the cause of the problem is due to a sodium and/or water imbalance.

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Hyponatremia associated with low total body sodium (dehydration) is caused primarily by depletion of extracellular fluid volume. Common causes include vomiting; diarrhea; gastric suction; fistulas; fluid loss from burns, peritonitis, and pancreatitis; Addison's disease; renal failure; overly aggressive therapy with diuretics and sodium-free IV solutions. This type of hyponatremia can also occur when mannitol or hyperglycemia produce an osmotic diuresis.

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Hyponatremia associated with normal total body sodium is caused by glucocorticoid deficiency, severe hypothyroidism, administration of water to a patient with impaired water excretion capacity, and SIADH. Medications associated with SIADH include carbamazepine, diuretics, narcotics, nicotine, acetaminophen, chlorpropamide and NSAIDs.

•	Hyponatremia associated with high total body sodium (volume excess) is the most common form of hyponatremia. Common causes include edematous states such as CHF, cirrhosis, nephrotic syndrome, and chronic renal failure.

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Patients with hyponatremia associated with low total body sodium exhibit signs and symptoms of dehydration: thirst, dry mucous membranes, cool and clammy skin, weight loss, sunken eyes, diminished urine output and diminished skin turgor; decreased blood pressure; increased pulse; and postural hypotension.

•	Patients with hyponatremia associated with normal total body sodium have no signs of volume abnormalities.

•	Edema, weight gain, hypertension, and neck vein distention are the hallmark signs of hyponatremia associated with high total body sodium (volume excess).

•	Symptoms of severe hyponatremia include agitation, anorexia, apathy, disorientation, lethargy, muscle cramps, depressed deep-tendon reflexes, hypothermia and seizures.

Serum sodium level below 135 mEq/Liter. Serum osmolality is decreased in SIADH and volume excess. Serum osmolality is increased with inadequate intake, insensible losses and diabetes insipidus. Increased BUN:SCr ratio indicates dehydration. Increased specific gravity and osmolality of urine indicate dehydration.

Replace orally if possible. If oral replacement is not possible, administer intravenous normal saline. If hyponatremia is related to volume excess administer diuretics. In severe hyponatremia give hypertonic saline (3% NaCl) in combination with a loop diuretic.

Determine the etiology of the hyponatremia. Hyponatremia due to volume excess is treated by fluid restriction and providing only the daily maintenance sodium requirements (90 to 150 mEq/L) until the serum sodium returns to normal. Hyponatremia due to inadequate sodium intake is treated by increasing the sodium content of the TPN solution. The maximum sodium content per liter of TPN should not exceed 154 mEq. Limit administration of free water.

•	Hypernatremia associated with low total body sodium results when water is lost to a greater extent than is sodium. This often results when hypotonic fluid losses (e.g., profuse sweating or diarrhea) is replaced with inadequate water and salt.

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Hypernatremia associated with normal total body sodium is caused by water loss without sodium loss. Etiologies include increased insensible water losses (fever, burns, ventilator) and central nephrogenic diabetes insipidus. Medications which can cause diabetes insipidus include acetohexamide, amphotericin B, cisplatin, colchicine, gentamicin, sulfonylureas, and diuretics.

•	Hypernatremia associated with high total body sodium is usually caused by excessive intake of sodium due to administration of sodium-containing IV solutions, particularly hypertonic saline.

•	Medications with high sodium content: ampicillin, azlocillin, carbenicillin, penicillin G sodium, piperacillin and ticarcillin.

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Patients with hypernatremia associated with low total body sodium exhibit signs and symptoms of dehydration: thirst, dry mucous membranes, cool and clammy skin, weight loss, sunken eyes, diminished urine output and diminished skin turgor; decreased blood pressure; increased pulse; and postural hypotension.

•	Patients with hypernatremia associated with normal total body sodium have no signs of volume abnormalities.

•	Edema, weight gain, hypertension, and neck vein distention are the hallmark signs of hypernatremia associated with high total body sodium (volume excess).

•	Symptoms of severe hypernatremia include restlessness, irritability, lethargy, hyperreflexia, seizures and coma.

Serum sodium level above 145 mEq/Liter. Serum osmolality is decreased in SIADH and volume excess. Serum osmolality is increased with inadequate intake, insensible losses and diabetes insipidus. Increased BUN:SCr ratio indicates dehydration. Increased specific gravity and osmolality of urine indicate dehydration.

Restrict sodium intake. If fluid losses need to be restored use dextrose 5% in water or hypotonic saline. A diuretic may be used in conjunction with IV solution administration to prevent overcorrection.

Determine the etiology for the hypernatremia. Hypernatremia due to dehydration is treated by administration of additional free water and providing only the daily maintenance sodium requirements (9O to 150 mEq/L). Hypernatremia due to excess sodium intake is treated by reducing or deleting sodium from the TPN solution and all other intravenous fluids until the serum sodium returns to normal.