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Disorders of Water and Sodium Balance

Hyponatremia

Volitional intake of water is regulated by thirst. Maintenance intake of water is the amount of water sufficient to offset obligatory losses.

Maintenance Water Needs.

Clinical Signs of Hyponatremia. Confusion, agitation, lethargy, seizures, and coma. The rate of change of concentration during onset of hyponatremia is more important in causing symptoms than is the absolute concentration.

Pseudohyponatremia

A marked elevation of the blood glucose creates an osmotic gradient that pulls water from cells into the extracellular fluid, diluting the extracellular Na. The contribution of hyperglycemia can be estimated using the following formula: Expected change in serum = (Serum glucose - 100) x 0.016. Marked elevation of plasma solids (lipids or protein) can also result in erroneous because of laboratory inaccuracy. The percentage of plasma water can be estimated with the following formula:

Diagnostic Evaluation of Hyponatremia

Pseudohyponatremia should be excluded by repeat testing, then the cause of the should be determined based on history, physical exam, urine osmolality, and urine level. An assessment of volume status should determine if the patient is volume contracted, normal volume, or volume expanded.

Classification Hyponatremic Patients Based on Urine Osmolality

Low urine osmolality (50-180 mOsm/L) indicates primary excessive water intake (psychogenic water drinking).

High Urine Osmolality (urine osmolality >serum osmolality). High urine (>40 mEq/L) and volume contraction indicates a renal source of and fluid loss (excessive diuretic use, salt-wasting nephropathy, Addison's disease, osmotic diuresis).

High urine (>40 mEq/L) and normal volume is most likely caused by water retention due to a drug effect, hypothyroidism, or the syndrome of inappropriate antidiuretic hormone secretion (SIADH). In SIADH, the urine level is usually high, but may be low if the patient is on a salt-restricted diet. SIADH is found in the presence of a malignant tumor or a disorder of the pulmonary or central nervous system.

Low urine (<20 mEq/L) and volume contraction, dry mucous membranes, decreased skin turgor, and orthostatic hypotension indicate an extrarenal source of fluid loss (gastrointestinal disease, burns).

Low urine (<20 mEq/L) and volume-expansion, and edema is caused by congestive heart failure, cirrhosis with ascites, or nephrotic syndrome. Effective arterial blood volume is decreased. Decreased renal perfusion causes increased reabsorption of water.

Treatment of Water Excess

Determine the Volume of Water Excess. Water excess = total body water x [(140/measured) -1]

Treatment

Hypernatremia

Clinical Manifestations of Hypernatremia

Signs of either volume overload or volume depletion may be prominent.

Clinical manifestations include tremulousness, irritability, ataxia, spasticity, mental confusion, seizures, and coma. Symptoms are more likely to occur with acute increases in plasma.