Decrease in extracellular fluid volume

Extracellular fluid volume depletion is a decrease in extracellular fluid volume caused by loss of water and total body sodium. Possible causes include vomiting, increased sweating, diarrhea, burns, diuretic use, and renal failure. Clinical manifestations include decreased skin turgor, dry mucous membranes, tachycardia, and orthostatic hypotension. Diagnosis is based on clinical findings. Treatment includes water and sodium replacement.

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Causes decrease in extracellular fluid volume

A decrease in extracellular fluid volume (hypovolemia) does not correspond to a decrease in plasma volume. A decrease in plasma volume may be observed with a decreased extracellular fluid volume, but it also occurs with an increase in extracellular fluid volume (e.g., in heart failure, hypoalbuminemia, capillary leak syndrome). With a decrease in extracellular fluid volume, sodium loss is usually observed; sodium loss always causes water loss. Depending on many factors, plasma sodium concentration may be high, low, or normal despite a decrease in total body sodium.

Extrarenal causes

• Bleeding.
• Dialysis: hemodialysis, peritoneal dialysis.
• Gastrointestinal: vomiting, diarrhea, nasogastric aspiration.
• Skin: increased sweating, burns, exfoliation.
• Fluid retention in spaces: intestinal lumen, intraperitoneal, retroperitoneal.

Renal/Adrenal Causes

• Acute renal failure: diuresis phase during recovery.
• Adrenal diseases: Addison's disease (glucocorticoid deficiency), hypoaldosteronism.
• Bartter syndrome.
• Diabetes mellitus with ketoacidosis or high glucosuria.
• Taking diuretics.
• Salt-wasting renal diseases (juvenile nephronophthisis, interstitial nephritis, some cases of pyelonephritis and myeloma)

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Symptoms decrease in extracellular fluid volume

Extracellular fluid volume depletion should be suspected in patients with a history of inadequate fluid intake (especially in comatose or disoriented patients); increased fluid losses; diuretic therapy; or renal or adrenal disease.

With a mild decrease in extracellular fluid volume (5%), the only sign may be decreased skin turgor. The patient may complain of thirst. Dry mucous membranes are not always associated with a decrease in extracellular fluid volume, especially in the elderly or in patients who breathe primarily through the mouth. Oliguria is typical. With a decrease in extracellular fluid volume by 5-10%, orthostatic tachycardia, hypotension, or a combination of both are usually observed, although orthostatic changes can also be observed in patients without a decrease in extracellular fluid volume, especially in debilitated and bedridden patients. Skin turgor (best assessed on the upper body) may be decreased. If dehydration exceeds 10%, signs of shock (tachypnea, tachycardia, hypotension, impaired consciousness, poor capillary refill) may be observed.

Diagnostics decrease in extracellular fluid volume

Diagnosis is usually based on clinical presentation. If the cause is obvious and easily corrected (eg, acute gastroenteritis in an otherwise healthy patient), laboratory testing is not necessary; otherwise, serum electrolytes, blood urea nitrogen, and creatinine should be measured. Plasma osmolality, urine sodium, creatinine, and urine osmolality are measured when clinically significant electrolyte imbalances are suspected that are not detected by serum analysis, and in patients with cardiac or renal disease. Invasive monitoring is necessary in patients with preexisting unstable heart failure or arrhythmias.

With a decrease in extracellular fluid volume, central venous pressure and pulmonary artery occlusion pressure usually decrease, but they are rarely measured.

When extracellular fluid volume is reduced, normally functioning kidneys conserve sodium, so that the urine sodium concentration is usually less than 15 mEq/L; the fractional excretion of sodium (urine Na/serum Na divided by urine creatinine/serum creatinine) is usually less than 1%; and the urine osmolality is often greater than 450 mOsm/kg. If extracellular fluid volume reduction is associated with metabolic alkalosis, the urine sodium concentration may be high; in such cases, a urine chloride concentration of less than 10 mEq/L more reliably indicates extracellular fluid volume reduction. However, high urine sodium (usually >20 mEq/L) or low urine osmolality may result from renal sodium losses due to renal disease, diuretic therapy, or adrenal insufficiency. As extracellular fluid volume decreases, blood urea nitrogen and plasma creatinine levels often increase, with a blood urea nitrogen to creatinine ratio greater than 20:1. Hematocrit is often elevated as extracellular fluid volume decreases, but is difficult to analyze without basal values.

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Treatment decrease in extracellular fluid volume

Correction of the cause of extracellular fluid volume reduction is necessary, as well as fluid administration to compensate for existing fluid deficits, ongoing fluid losses, and to ensure daily body requirements. In cases of mild to moderate fluid deficits, oral water and sodium may be administered if the patient is conscious and not suffering from severe vomiting. If there is a significant fluid deficit or oral hydration is ineffective, intravenous administration of 0.9% saline is performed.