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Overview
In physiology, dehydration is a lack of total body water that disrupts metabolic processes. It occurs when free water loss exceeds intake, often resulting from excessive sweating, health conditions, or inadequate consumption of water. Mild dehydration can also be caused by immersion diuresis, which may increase risk of decompression sickness in divers.
Most people can tolerate a 3–4% decrease in total body water without difficulty or adverse health effects. A 5–8% decrease can cause fatigue and dizziness. Loss of over 10% of total body water can cause physical and mental deterioration, accompanied by severe thirst. Death occurs with a 15 to 25% loss of body water. Mild dehydration usually resolves with oral rehydration, but severe cases may need intravenous fluids.
Dehydration can cause hypernatremia (high levels of sodium ions in the blood). This is distinct from hypovolemia (loss of blood volume, particularly blood plasma).
Chronic dehydration can cause kidney stones as well as the development of chronic kidney disease.
Signs and symptoms
The hallmarks of dehydration include thirst and neurological changes such as headaches, general discomfort, loss of appetite, nausea, decreased urine volume (unless polyuria is the cause of dehydration), confusion, unexplained tiredness, purple fingernails, and seizures. The symptoms of dehydration become increasingly severe with greater total body water loss.
A body water loss of 1–2%, considered mild dehydration, is shown to impair cognitive performance. While in people over age 50, the body's thirst sensation diminishes with age, a study found that there was no difference in fluid intake between young and old people. Many older people have symptoms of dehydration, with the most common being fatigue. Dehydration contributes to morbidity in the elderly population, especially during conditions that promote insensible free water losses, such as hot weather.
Cause
Risk factors for dehydration include but are not limited to: exerting oneself in hot and humid weather, habitation at high altitudes, endurance athletics, elderly adults, infants, children and people living with chronic illnesses.
Dehydration can also come as a side effect from many different types of drugs and medications.
Mechanism
Water content of a human body varies from 70–75% in newborns to 40% and less in obese adults, an average value of 60% is suggested. Within the body, water is classified as intracellular fluid or extracellular fluid. Intracellular fluid refers to water that is contained within the cells. This consists of approximately 57% of the total body water weight. Fluid inside the cells has high concentrations of potassium, magnesium, phosphate, and proteins. Extracellular fluid consists of all fluid outside of the cells, and it includes blood and interstitial fluid. This makes up approximately 43% of the total body water weight. The most common ions in extracellular fluid include sodium, chloride, and bicarbonate.
The concentration of dissolved molecules and ions in the fluid is described as osmolarity and is measured in osmoles per liter (Osm/L). When the body experiences a free water deficit, the concentration of solutes is increased. This leads to a higher serum osmolarity. When serum osmolarity is elevated, this is detected by osmoreceptors in the hypothalamus. These receptors trigger the release of antidiuretic hormone (ADH). ADH resists dehydration by increasing water absorption in the kidneys and constricting blood vessels. It acts on the V2 receptors in the cells of the collecting tubule of the nephron to increase expression of aquaporin. In more extreme cases of low blood pressure, the hypothalamus releases higher amounts of ADH which also acts on V1 receptors. These receptors cause contractions in the peripheral vascular smooth muscle.
This increases systemic vascular resistance and raises blood pressure.
Definition
Dehydration occurs when water intake does not replace free water lost due to normal physiologic processes, including breathing, urination, perspiration, or other causes, including diarrhea, and vomiting. Dehydration can be life-threatening when severe and lead to seizures or respiratory arrest, and also carries the risk of osmotic cerebral edema if rehydration is overly rapid.
The term "dehydration" has sometimes been used incorrectly as a proxy for the separate, related condition of hypovolemia, which specifically refers to a decrease in volume of blood plasma. The two are regulated through independent mechanisms in humans; the distinction is important in guiding treatment.
Physical examination
Common exam findings of dehydration include dry mucous membranes, dry axillae, increased capillary refill time, sunken eyes, and poor skin turgor. More extreme cases of dehydration can lead to orthostatic hypotension, dizziness, weakness, and altered mental status. Depending on the underlying cause of dehydration, other symptoms may be present as well. Excessive sweating from exercise may be associated with muscle cramps. Patients with gastrointestinal water loss from vomiting or diarrhea may also have fever or other systemic signs of infection.
The skin turgor test can be used to support the diagnosis of dehydration. The skin turgor test is conducted by pinching skin on the patient's body, in a location such as the forearm or the back of the hand, and watching to see how quickly it returns to its normal position. The skin turgor test can be unreliable in patients who have reduced skin elasticity, such as the elderly.
Laboratory tests
While there is no single gold standard test to diagnose dehydration, evidence can be seen in multiple laboratory tests involving blood and urine. Serum osmolarity above 295 mOsm/kg is typically seen in dehydration due to free water loss. A urinalysis, which is a test that performs chemical and microscopic analysis of urine, may find darker color or foul odor with severe dehydration. Urinary sodium also provides information about the type of dehydration. For hyponatremic dehydration, such as from vomiting or diarrhea, urinary sodium will be less than 10 mmol/L due to increased sodium retention by the kidneys in an effort to conserve water. In dehydrated patients with sodium loss due to diuretics or renal dysfunction, urinary sodium may be elevated above 20 mmol/L.
Patients may also have elevated serum levels of blood urea nitrogen (BUN) and creatinine. Both of these molecules are normally excreted by the kidney, but when the circulating blood volume is low, the kidney can become injured. This causes decreased kidney function and results in elevated BUN and creatinine in the serum.
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