Dehydration

Definition

• Free water deficit

Mechanism

• Loss of hypotonic fluid with insufficient replacement of losses

Causes

• Sweating, GIT loss (vomiting an diarrhoea), dliute urine (diabetes insipidus)*
  
  NB: as long as a conscious person has access to water and is able to drink they usually can compensate for this through the thirst mechanism (c.f the bed-bound, semiconscious person post-operative patient who is being fasted and receiving inadequate IV fluid therapy)

Complications

• hyperosmolality>cell shrinkage>dysfunction, hypovolaemia * (when severe)>shock

Clinical features

• dry mucous membrane (mouth, eyes) > doughy skin > tachycardia > postural BP fall > confusion, hypotension

Investigations

• Plasma levels: ^ sodium (a good indicator), ^ osmolality, ^ urea, ^ creatinine (if severe).
• Urine levels: ^^ sodium, ^^osmolality

Treatment

• Replenish free water deficit (oral or intravenous) ***
  
  * any fluid or electrolyte losses are usually from three possible sources (skin, gut, kidnyes) +/- artificially created communications to these (fistulas and stomas)
  
  ** hypovolaemia occurs faster if there is isotonic loss because there is no driving osmotic gradient that result in water movement into the vascular space
  
  *** often actual losses include some salt, if this is not replaced as well then you may cause dangerous hyponatraemia and hypo-osmolality following treatment leading to cellular oedema e.g. endurance athletes who drink water only rather than salt containing drinks

Important note

• Serum sodium concentration is a function of both total body sodium divided by total body water. Usually it rises due to a deficit of water, not an increase in salt intake.