(See also Water and Sodium Balance Water and Sodium Balance Body fluid volume and electrolyte concentration are normally maintained within very narrow limits despite wide variations in dietary intake, metabolic activity, and environmental stresses. Homeostasis... read more and Overview of Disorders of Fluid Volume Overview of Disorders of Fluid Volume Because sodium is the major osmotically active ion in the extracellular fluid (ECF), total body sodium content determines ECF volume. Deficiency or excess of total body sodium content causes... read more .)
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 the table Common Causes of volume Depletion Common Causes of Volume Depletion 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... read more .
When fluid loss is < 5% of ECF volume (mild volume depletion), the only sign may be diminished skin turgor (best assessed at the upper torso). Skin turgor may be low in older patients regardless of volume status. Patients may complain of thirst. Dry mucous membranes do not always correlate with volume depletion, especially in older patients 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 is > 10% of ECF volume (severe volume depletion), signs of shock Shock Shock is a state of organ hypoperfusion with resultant cellular dysfunction and death. Mechanisms may involve decreased circulating volume, decreased cardiac output, and vasodilation, sometimes... read more (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. If accurate patient weights immediately before and after fluid loss are known, the difference is an accurate estimate of volume loss; for example, pre- and post-workout weights are sometimes used to monitor dehydration in athletes.
When the cause is obvious and easily correctable (eg, acute gastroenteritis Gastroenteritis Gastroenteritis is inflammation of the lining of the stomach and small and large intestines. Most cases are infectious, although gastroenteritis may occur after ingestion of drugs and chemical... read more 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 Metabolic Alkalosis Metabolic alkalosis is primary increase in bicarbonate (HCO3−) with or without compensatory increase in carbon dioxide partial pressure (Pco2); pH may be high or nearly normal. Common causes... read more 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 Heart Failure (HF) Heart failure (HF) is a syndrome of ventricular dysfunction. Left ventricular failure causes shortness of breath and fatigue, and right ventricular failure causes peripheral and abdominal fluid... read more or advanced chronic kidney disease Chronic Kidney Disease Chronic kidney disease (CKD) is long-standing, progressive deterioration of renal function. Symptoms develop slowly and in advanced stages include anorexia, nausea, vomiting, stomatitis, dysgeusia... read more .
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 (< 15 mmol/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 (> 450 mmol/kg).
When metabolic alkalosis Metabolic Alkalosis Metabolic alkalosis is primary increase in bicarbonate (HCO3−) with or without compensatory increase in carbon dioxide partial pressure (Pco2); pH may be high or nearly normal. Common causes... read more 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(< 10 mmol/L) more reliably indicates volume depletion.
Misleadingly high urinary sodium (generally > 20 mEq/L [> 20 mmol/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 hematocrit 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 sodium and water when patients are conscious and not vomiting; oral rehydration regimens in children Oral Rehydration Oral fluid therapy is effective, safe, convenient, and inexpensive compared with IV therapy. Oral fluid therapy is recommended by the American Academy of Pediatrics and the WHO and should be... read more are discussed elsewhere. Tube feedings can also be used if oral intake is limited or not safe for some reason.
When volume deficits are severe or when oral fluid replacement is impractical, IV 0.9% saline or a buffered electrolyte solution (eg, Ringer's lactate) is given. Both are well tolerated and safe, but Ringer’s lactate and other buffered solutions have lower chloride concentrations and may reduce development of hyperchloremic metabolic acidosis. Recent evidence seems to favor use of Ringer's lactate over saline, but the issue is not settled. Because both of these solutions distribute evenly into the extracellular space, three to four times the deficit in intravascular volume needs to be given. Typical IV regimens Intravenous Fluid Resuscitation Almost all circulatory shock states require large-volume IV fluid replacement, as does severe intravascular volume depletion (eg, due to diarrhea or heatstroke). Intravascular volume deficiency... read more are given elsewhere.