Nurses encourage a patient to drink an oral rehydration solution to reduce the combination of dehydration and hypovolemia he acquired from cholera. Cholera leads to GI loss of both excess free water (dehydration) and sodium (hence ECF volume depletion - hypovolemia).
|Classification and external resources|
|Specialty||Critical care medicine|
Dehydration, refers to a deficit of total body water, with an accompanying disruption of metabolic processes. It is also the reason for hypernatremia. The term dehydration must be distinguished from hypovolemia (loss of blood volume, particularly plasma).
Dehydration occurs when free water loss exceeds free water intake, usually due to exercise or disease. Most people can tolerate a three to four percent decrease in total body water without difficulty. A five to eight percent decrease can cause fatigue and dizziness. Over ten percent can cause physical and mental deterioration, accompanied by severe thirst. A decrease more than fifteen to twenty-five percent of the body water is invariably fatal. Mild dehydration is characterised by thirst and general discomfort and usually resolves with oral rehydration.
- Definition 1
- Signs and symptoms 2
- Cause 3
- Prevention 4
- Treatment 5
- See also 6
- References 7
- Notes 8
- External links 9
Dehydration occurs when water intake is insufficient to replace free water lost due to normal physiologic processes (e.g. breathing or urination) and other causes (e.g. diarrhea or vomiting). Hypovolemia is a related condition specifically meaning a decrease in volume of blood plasma— not of total body water. Both (total body water and plasma volume) are regulated through independent mechanisms in humans  and should not be conflated. Some authors have reported three types of dehydration based on serum sodium levels: hypotonic or hyponatremic (referring to this as primarily a loss of electrolytes, sodium in particular), hypertonic or hypernatremic (referring to this as primarily a loss of water), and isotonic or isonatremic (referring to this as equal loss of water and electrolytes). Indeed, in humans, it has been commonly thought that the most commonly seen type of dehydration (by far) is isotonic (isonatraemic) dehydration. This usage is incorrect  and the term isotonic or isonatremic or eunatremic dehydration actually all refer to hypovolemia and should therefore be abandoned in favor of the latter. Hyponatremic dehydration cannot exist because by definition depletion of total body water can only lead to hypernatremia so this term actually refers to coexistence of two separate disorders - hyponatremia and hypovolemia and again the term dehydration must be avoided. A classic example of hyponatremia coexisting with hypovolemia is Addison's disease where cortisol deficiency leads to ADH excess and hyponatremia but mineralocorticoid deficiency simultaneously leads to sodium loss and hypovolemia. The latter subjects are not dehydrated, on the contrary they are over-hydrated (from free water retention due to ADH excess).
Dehydration is thus a term that has been very loosely used to either mean true dehydration or as a proxy for hypovolemia and only the former is the proper use of this term. This is important because total body water is not controlled via sodium regulation, only intravascular volume is so controlled and this distinction is important to guide therapy. 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 overtly rapid.
Signs and symptoms
Patients who lose enough extracellular fluid (ECF) volume to develop skin tenting (loss of skin elasticity), flat neck veins, and orthostatic or frank tachycardia and dizziness or fainting when standing up due to orthostatic hypotension, are often said to be dehydrated or dry. This is incorrect since these findings are not signs of dehydration and indicate ECF depletion, or hypovolemia for short.
Symptoms and signs of hypertonicity are the hallmarks of dehydration and include thirst and neurological changes such as headache, general discomfort, loss of appetite, decreased urine volume (unless polyuria is the cause of dehydration)), confusion, unexplained tiredness, and even seizures. The symptoms of dehydration become increasingly severe with greater total body water loss. In people over age 50, the body’s thirst sensation diminishes and continues diminishing with age. Many senior citizens suffer symptoms of dehydration. Dehydration contributes to morbidity in the elderly especially during conditions that promote insensible free water losses such as hot weather. A Cochrane review on this subject defined water-loss dehydration as people with serum osmolality of 295 mOsm/kg or more and found that the main symptoms in the elderly were expressing fatigue, missing drinks between meals and bioelectrical impedance analysis. However, this Cochrane review was also plagued by the same lack of clarity regarding the distinction between dehydration and hypovolemia seen in the literature, but confusion was avoided to a large extent by their use of hypertonicity to define dehydration. It must be pointed out that dehydration and hypovolemia may occur simultaneously in the same person at the same time and thus explains the lack of clarity in the literature regarding the symptoms and signs associated with these two conditions - however, their distinction is essential to guide therapy.
In the elderly, blunted response to thirst and/or inadequate ability to access free water in the face of excess free water losses (especially hyperglycemia related) seem to be the main causes of dehydration. Excess free water or hypotonic water can leave the body in two ways - sensible loss such as osmotic diuresis, sweating, vomiting and diarrhea. Insensible water loss occurs mainly through the skin and respiratory tract. In humans, dehydration can be caused by a wide range of diseases and states that impair water homeostasis in the body. These occur through the following main mechanisms:
Water / hypotonic fluid loss (with decreased thirst and/or impaired access to free water):
- Insensible respiratory losses
- Fluid loss with a sodium plus potassium concentration less than that in the plasma, such as urinary losses in hyperglycemia (osmotic diuresis)
- Transient hypernatremia can occur when water shifts intracellularly caused by activities, such as severe exercise, and on cessation of activities the sodium returns to normal within 5 to 15 minutes
- Hypertonic sodium intake without appropriate water intake leads to hypernatremia as sodium load is excreted in water leading to free water loss. This is the mechanism of free water loss when hypertonic saline is given in SIADH
For routine activities, thirst is normally an adequate guide to maintain proper hydration. With exercise, exposure to hot environments, or a decreased thirst response, additional water may be required. An accurate determination of fluid volume lost during a workout can be made by performing weight measurements before and after a typical exercise session.
Normal water loss occurs through the lungs as water vapor (about 350ml), through the skin by perspiration (100ml) and by diffusion through the skin (350ml), or through the kidneys as urine (1000–2000ml, about 900ml of which is obligatory water excretion that gets rid of solutes). Some water (about 150–200ml, in the absence of diarrhea) is also lost through the feces. In warm or humid weather or during heavy exertion, however, the water loss can increase by a factor of 10 or more through perspiration; all of which must be promptly replaced. In extreme cases, the losses may be great enough to exceed the body's ability to absorb water from the gastrointestinal tract; in these cases, it is not possible to drink enough water to stay hydrated, and the only way to avoid dehydration is to either pre-hydrate or find ways to reduce perspiration (through rest, a move to a cooler environment, etc.)
When large amounts of water are being lost through perspiration and concurrently replaced by drinking, maintaining proper electrolyte balance becomes an issue. Drinking fluids that are hypertonic or hypotonic with respect to perspiration may have grave consequences (hyponatremia or hypernatremia, principally) as the total volume of water turnover increases.
The treatment for minor dehydration, often considered the most effective, is drinking water and stopping fluid loss. Plain water restores only the volume of the blood plasma, inhibiting the thirst mechanism before solute levels can be replenished. Solid foods can contribute to fluid loss from vomiting and diarrhea. Urine concentration and frequency will customarily return to normal as dehydration resolves.
In more severe cases, correction of a dehydrated state is accomplished by the replenishment of necessary water and electrolytes (through oral rehydration therapy or fluid replacement by intravenous therapy). As oral rehydration is less painful, less invasive, less expensive, and easier to provide, it is the treatment of choice for mild dehydration. Solutions used for intravenous rehydration must be isotonic or hypotonic. Pure water injected into the veins will cause the breakdown (lysis) of red blood cells (erythrocytes).
When fresh water is unavailable (e.g. at sea or in a desert), seawater and ethanol will worsen the condition. Urine contains a similar solute concentration to seawater, and numerous guides advise against its consumption in survival situations.
For severe cases of dehydration where fainting, unconsciousness, or other severely inhibiting symptom is present (the patient is incapable of standing or thinking clearly), emergency attention is required. Fluids containing a proper balance of replacement electrolytes are given orally or intravenously with continuing assessment of electrolyte status; complete resolution is the norm in all but the most extreme cases.
Some research indicates that artificial hydration to alleviate symptoms of dry mouth and thirst in the dying patient may be futile.
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