Dehydration: excessive loss of body fluid

It is defined as the excessive loss of body fluid. There are three types of dehydration: hypotonic or hyponatremic (primarily a loss of electrolytes, sodium in particular), hypertonic or hypernatremic (primarily a loss of water), and isotonic or isonatremic (equal loss of water and electrolytes). In humans, the most commonly seen type of dehydration by far is isotonic (isonatraemic) dehydration which effectively equates with Hypovolemic, but the distinction of isotonic from hypotonic or hypertonic dehydration may be important when treating people who become dehydrated.

gafacom image Dehydration: excessive loss of body fluid

Physiologically, dehydration, despite the name, does not simply mean loss of water, as water and solutes (mainly sodium) are usually lost in roughly equal quantities to how they exist in blood plasma. In hypotonic dehydration, intravascular water shifts to the extra vascular space, exaggerating intravascular volume depletion for a given amount of total body water loss. Neurological complications can occur in hypotonic and hypertonic states. The former can lead to seizures, while the latter can lead to osmotic cerebral edema upon rapid rehydration.




Hypovolemic
Hypovolemicis specifically a decrease in volume of blood plasma. It defines water deficiency only in terms of volume rather than specifically water.

Signs and symptoms

Symptoms may include headaches similar to what is experienced during a hangover, a sudden episode of visual snow, and dizziness or fainting when standing up due to orthostatic hypotension. Untreated dehydration generally results in delirium, unconsciousness, swelling of the tongue and, in extreme cases, death.

Thirst, dryness of mucous membrane, loss of skin turgor, orthostatic hypotension or tachycardia, reduced jugular venous pressure (JVP) or central venous pressure (CVP) and decreased urine output. In the presence of normal renal function dehydration is associated usually with a urine output of less than 0.5ml kg/hr.

Differential diagnosis

In humans, dehydration can be caused by a wide range of diseases and states that impair water homeostasis in the body. These include:

1. External or stress-related causes  
Prolonged physical activity with sweating without consuming adequate water, especially in a hot and/or dry environment. Prolonged exposure to dry air, e.g., in high-flying airplanes (5%–12% relative humidity) Blood loss or hypotension due to physical trauma
Diarrhea 
Hyperthermia
Vomiting
Burns
Lacrimation
Use of methamphetamine, amphetamine, caffeine and other stimulants 
Excessive consumption of alcoholic beverage 

2. Infectious diseases (Refer to gastrointestinal chapter for details) 
Gastroenteritis
Shigellosis
Yellow fever
Malnutrition
Electrolyte disturbance
Hypernatremia (also caused by dehydration)
Hyponatremia, especially from restricted salt diets
Fasting
Recent rapid weight loss may reflect progressive depletion of fluid volume (the loss of 1 L of fluid results in a weight loss of 1 kg (2.2 lb.)).  
Patient refusal of nutrition and hydration
Inability to swallow (obstruction of the esophagus)
Other causes of obligate water loss

3. Severe hyperglycemia, especially in diabetes mellitus      
Glycosuria
Uremia
Diabetes insipidus
Acute emergency dehydration event
Food borne illness

Tests include:

Blood chemistries (to check electrolytes, especially sodium, potassium, and bicarbonate levels)
Blood urea nitrogen (BUN)
Complete blood count (CBC)
Creatinine
Urine specific gravity
Other tests may be done to determine the cause of the dehydration (for example, blood sugar level to check for diabetes).

Treatment

For some dehydration oral fluid is the most effective to replenish fluid deficit. In more severe cases, correction of fluid deficit is best by intravenous therapy. Solutions used for intravenous rehydration must be isotonic or hypotonic.
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 intravenously with continuing assessment of electrolyte status.




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