Dehydration
Maintenance of fluid and electrolyte balance is essential to body function and health. Water content decreases from birth to old age; in neonates, approximately 75% of the body weight is water content; by adulthood, the percentage has decreased to approximately 60%; in the elderly, only about 55% of the body weight is water content. The sharpest decline in fluid volume occurs within the first 10 years of life.
Many conditions and factors can affect a person’s fluid and electrolyte status. Dehydration—excessive loss of fluid from the body—occurs when the loss of fluid exceeds the fluid intake.
Causes
Excessive fluid loss, reduced fluid intake, third-space fluid shift, or a combination of these factors can cause fluid volume losses.
Other causes of fluid loss include:
hemorrhage
excessive perspiration
acute renal failure with polyuria
abdominal surgery
vomiting or diarrhea
nasogastric drainage
excessive GI suctioning
diabetes mellitus with polyuria or diabetes insipidus
fistulas
excessive use of laxatives
aggressive diuretic therapy
fever
excessive fluid removal with hemodialysis or peritoneal dialysis therapy.
Possible causes of reduced fluid intake include:
dysphagia
coma
environmental conditions preventing fluid intake
psychiatric illness.
Fluid shifts may be related to:
initial phase of a burn
acute intestinal obstructions
acute peritonitis
pancreatitis
crushing injuries
pleural effusion
ascites
accumulation of blood around a hip fracture.
The pathophysiology of dehydration involves a number of mechanisms. The loss of body fluids causes an increase in blood solute concentration that increases osmolality. Serum sodium levels rise. In an attempt to regain fluid balance between intracellular and extracellular spaces, water molecules shift out of the cells into the concentrated blood. This process, combined with increased water intake and increased water retention in the kidneys, usually restores fluid balance. But without sufficient water in the extracellular space, fluid continues to shift out of the cells into the extracellular space, causing the cells to shrink and impairing cellular function.
Complications
Complications include hypotension, risk of falls related to hypotension, and decreased cardiac output and perfusion to tissues and organs. Severe dehydration can progress to hypovolemic shock. (See What happens in hypovolemic shock.)
Other complications include renal failure from decreased renal perfusion and, without intervention, death.
Other complications include renal failure from decreased renal perfusion and, without intervention, death.
What happens in hypovolemic shock
In hypovolemic shock, vascular fluid volume loss causes extreme tissue hypoperfusion. Internal fluid losses can result from hemorrhage or third-space fluid shifting. External fluid loss can result from severe bleeding or from severe diarrhea, diuresis, or vomiting.
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