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. 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
Hypovolemic is 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.
In humans, dehydration can be caused by a wide range of diseases and states that impair
water homeostasis in the body. These include:
External or stress-related causes
o Prolonged physical activity with sweating without consuming adequate water,
especially in a hot and/or dry environment
o Prolonged exposure to dry air, e.g., in high-flying airplanes (5%–12% relative
o Blood loss or hypotension due to physical trauma
o Shock (hypovolemic)
o Use of methamphetamine, amphetamine, caffeine and other stimulants
o Excessive consumption of alcoholic beverages
Infectious diseases (Refer to gastrointestinal chapter for details)
o Yellow fever
o Electrolyte disturbance
o Hypernatremia (also caused by dehydration)
o Hyponatremia, especially from restricted salt diets
o 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)).
o Patient refusal of nutrition and hydration
o Inability to swallow (obstruction of the esophagus)
Other causes of obligate water loss
Severe hyperglycemia, especially in diabetes mellitus
Acute emergency dehydration event
Food borne illness
Blood chemistries (to check electrolytes, especially sodium, potassium, and
Blood urea nitrogen (BUN)
Complete blood count (CBC)
Urine specific gravity
Other tests may be done to determine the cause of the dehydration (for example, blood sugar
level to check for diabetes).
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.