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Volume Depletion

By James L. Lewis, III, MD, Attending Physician;Attending Physician, Princeton Baptist Medical Center;Brookwood Medical Center

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Volume depletion, or extracellular fluid (ECF) volume contraction, occurs as a result of loss of total body sodium. Causes include vomiting, excessive sweating, diarrhea, burns, diuretic use, and kidney failure. Clinical features include diminished skin turgor, dry mucous membranes, tachycardia, and orthostatic hypotension. Diagnosis is clinical. Treatment involves administration of sodium and water.

Because water crosses plasma membranes in the body via passive osmosis, loss of the major extracellular cation (sodium) quickly results in water loss from the ECF space as well. In this way, sodium loss always causes water loss. However, depending on many factors, serum sodium concentration can be high, low, or normal in volume-depleted patients (despite the decreased total body sodium content). ECF volume is related to effective circulating volume. A decrease in ECF (hypovolemia) generally causes a decrease in effective circulating volume, which in turn causes decreased organ perfusion and leads to clinical sequelae. Common causes of volume depletion are listed in Common Causes of Volume Depletion.

Common Causes of Volume Depletion

Type

Examples

Extrarenal

Bleeding

Trauma

Dialysis

Hemodialysis

Peritoneal dialysis

GI

Nasogastric suctioning

Skin

Excessive sweating

Exfoliation

3rd-space losses

Intestinal lumen

Intraperitoneal

Retroperitoneal

Renal, adrenal, and pituitary

Acute renal failure

Diuretic phase of recovery

Adrenal disorders

Adrenal insufficiency (causing adrenal steroid deficiency), including Addison disease

Hypoaldosteronism

Genetic disorders causing renal sodium and potassium wasting

Hypothalamic or pituitary disorders causing vasopressin (ADH) deficiency

Central diabetes insipidus (eg, due to trauma, tumor, infection)

Osmotic diuresis

Diabetes mellitus with extreme glucosuria

Diuretics

Loop diuretics

Thiazide diuretics

Salt-wasting renal disease

Interstitial nephritis

Medullary cystic disease

Myeloma (occasionally)

Pyelonephritis (occasionally)

Symptoms and Signs

When fluid loss is < 5% of ECF (mild volume depletion), the only sign may be diminished skin turgor (best assessed at the upper torso). Skin turgor may be low in elderly patients regardless of volume status. Patients may complain of thirst. Dry mucous membranes do not always correlate with volume depletion, especially in the elderly and in mouth-breathers. Oliguria is typical.

When ECF volume has diminished by 5 to 10% (moderate volume depletion), orthostatic tachycardia, hypotension, or both are usually, but not always, present. Also, orthostatic changes can occur in patients without ECF volume depletion, particularly patients deconditioned or bedridden. Skin turgor may decrease further.

When fluid loss exceeds 10% of ECF volume (severe volume depletion), signs of shock (eg, tachypnea, tachycardia, hypotension, confusion, poor capillary refill) can occur.

Diagnosis

  • Clinical findings

  • Sometimes serum electrolytes, BUN, and creatinine

  • Rarely plasma osmolality and urine chemistries

Volume depletion is suspected in patients at risk, most often in patients with a history of inadequate fluid intake (especially in comatose or disoriented patients), increased fluid losses, diuretic therapy, and renal or adrenal disorders.

Diagnosis is usually clinical. When the cause is obvious and easily correctable (eg, acute gastroenteritis in otherwise healthy patients), laboratory testing is unnecessary; otherwise, serum electrolytes, BUN, and creatinine are measured. Plasma osmolality and urine sodium, creatinine, and osmolality are measured when there is suspicion of clinically meaningful electrolyte abnormality that is not clear from results of serum tests and for patients with cardiac or renal disease. When metabolic alkalosis is present, urine chloride is also measured.

Central venous pressure and pulmonary artery occlusion pressure are decreased in volume depletion, but measurement is rarely required. Measurement, which requires an invasive procedure, is occasionally necessary for patients for whom even small amounts of added volume may be detrimental, such as those with unstable heart failure or advanced chronic kidney disease.

The following concepts are helpful when interpreting urine electrolyte and osmolality values:

  • During volume depletion, normally functioning kidneys conserve sodium. Thus, the urine sodium concentration is usually < 15 mEq/L; the fractional excretion of sodium (urine sodium/serum sodium divided by urine creatinine/serum creatinine) is usually < 1%; also, urine osmolality is often > 450 mOsm/kg.

  • When metabolic alkalosis is combined with volume depletion, urine sodium concentration may be high because large amounts of bicarbonate are spilled in the urine, obligating the excretion of sodium to maintain electrical neutrality. In this instance, a urine chloride concentration of < 10 mEq/L more reliably indicates volume depletion.

  • Misleadingly high urinary sodium (generally > 20 mEq/L) or low urine osmolality can also occur due to renal sodium losses resulting from renal disease, diuretics, or adrenal insufficiency.

Volume depletion frequently increases the BUN and serum creatinine concentrations; the ratio of BUN to creatinine is often > 20:1. Values such as Hct often increase in volume depletion but are difficult to interpret unless baseline values are known.

Treatment

  • Replacement of sodium and water

The cause of volume depletion is corrected and fluids are given to replace existing volume deficits as well as any ongoing fluid losses and to provide daily fluid requirements. Mild-to-moderate volume deficits may be replaced by increased oral intake of sodium and water when patients are conscious and not vomiting. When volume deficits are severe or when oral fluid replacement is impractical, IV 0.9% saline is given. For typical IV regimens, see Intravenous Fluid Resuscitation; for oral regimens, see Oral Rehydration.

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