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Volume depletion, or ECF volume contraction, occurs as a result of loss of total body Na. 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 Na and water.
Because water crosses plasma membranes in the body via passive osmosis, loss of the major extracellular cation (Na) quickly results in water loss from the ECF space as well. In this way, Na loss always causes water loss. However, depending on many factors, serum Na concentration can be high, low, or normal in volume-depleted patients (despite the decreased total body Na 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
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.
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 Na, creatinine, and osmolality are measured when there is suspicion of clinically meaningful electrolyte abnormality that is not clear from serum tests and for patients with cardiac or renal disease. When metabolic alkalosis is present, urine Cl 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 Na. Thus, the urine Na concentration is usually < 15 mEq/L; the fractional excretion of Na (urine Na/serum Na 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 Na concentration may be high because large amounts of HCO3 are spilled in the urine, obligating the excretion of Na to maintain electrical neutrality. In this instance, a urine Cl concentration of < 10 mEq/L more reliably indicates volume depletion.
Misleadingly high urinary Na (generally > 20 mEq/L) or low urine osmolality can also occur due to renal Na losses resulting from renal disease, diuretics, or adrenal insufficiency.
Volume depletion frequently increases the BUN and serum creatinine concentrations with the ratio of BUN to creatinine often > 20:1. Values such as Hct often increase in volume depletion but are difficult to interpret unless baseline values are known.
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 Na 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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