Condition of low water volume in the body that causes electrolyte imbalance and impairs normal organ and tissue function. Common causes are continuous vomiting, severe diarrhea, and profuse sweating, often due to infection, and bleeding, combined with decreased or absent fluid intake. Hospitalized or infirm patients are at greater risk, as the aging process can contribute to lack of thirst, inability to monitor fluid intake, and swallowing difficulty. Other contributing factors may be dementia, confusion, depression, lack of mobility and communication, medications, or lack of monitoring of medication intake. Children also are at greater risk.

Diagnosis of dehydration may be suspected by percentage weight loss greater than 3% weight and changes in osmolality. The most common presentation of dehydration is hypernatremia.

Volume depletion (hypovolemia) can be caused by water loss or by salt and water loss. Isotonic dehydration occurs from balanced sodium and water loss, such as in fasting. Hypertonic dehydration is caused by imbalance created when fluid loss is greater than sodium loss, resulting in hyperosmolality and hypernatremia.

Signs and symptoms of mild to moderate dehydration include tachycardia, oliguria, orthostatic hypotension, dry mucous membranes, increased respirations, polydipsia, and irritability. Severe and life-threatening dehydration may additionally manifest as decreased blood pressure, sunken eyes or cheeks, anuria, cyanosis, shallow pulse, fasciculations, loss of consciousness, and coma.