Dehydratation of the organism

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Overview

Dehydration is defined as a decrease in total body water, resulting in disruption of extracellular (ECF) and intracellular fluid (ICF) balance. The clinical presentation of a patient depends on the amount of water lost and the relative loss of electrolytes, which determines the tonicity of the extracellular fluid and the direction in which water shifts between compartments.

Water represents 50 to 60 percent of body weight, and maintaining it at that percentage is essential for circulatory stability, cellular function, and electrolyte homeostasis.

Types of Dehydration

Dehydration is classified according to the osmolarity of the extracellular fluid:

• Isotonic (normotonic)
• Hypotonic
• Hypertonic

These categories describe whether the loss of water and electrolytes is proportional or disproportionate.

Isotonic Dehydration

Definition: Loss of water and electrolytes in equal proportions leading to:

• Reduced extracellular fluid volume
• Normal extracellular fluid osmolarity
• No water shift between compartments

Causes:

• Bleeding
• Burns
• Gastrointestinal losses such as diarrhea and vomiting
• Losses to the third space such as ileus, pancreatitis, and peritonitis

Consequences:

• Reduced venous return
• Reduced cardiac output → hypotension and tachycardia
• Risk of hypovolemic shock

Hypotonic Dehydration

Definition: A greater loss of salts than water leading to:

• Reduced extracellular fluid osmolarity
• Water moving into cells → cellular edema

Causes:

• Replacement of sweat losses with water only
• Addison’s disease (aldosterone deficiency → sodium loss)
• Diuretics
• Renal salt wasting

Consequences:

• Symptoms of hypovolemia
• Brain edema → headache, confusion, seizures
• Nausea, vomiting, muscle cramps

Hypertonic Dehydration

Definition: Greater loss of water than salts leading to:

• Increased extracellular fluid osmolarity
• Water moving out of cells → cellular dehydration

Causes:

• Insufficient water intake
• Fever, heavy sweating, hyperventilation
• Osmotic diuresis (diabetes mellitus with glucosuria)
• Diabetes insipidus (central ADH deficiency or renal ADH resistance)

Consequences:

• Symptoms of hypovolemia
• Intense thirst, dry mucous membranes, dry skin
• Neurological dysfunction: irritability → confusion → coma
• Brain cell shrinkage

Clinical Features

Schematic overview of causes, symptoms, diagnosis and treatment of dehydration

Symptoms:

• Thirst
• Lethargy
• Altered consciousness

Signs:

• Dry skin and mucous membranes
• Tongue furrows
• Sunken eyes
• Sunken fontanelle in infants

Pathophysiological Consequences

Circulatory System:

• Hypovolemia → reduced preload → circulatory failure
• Severe cases → hypovolemic shock

Central Nervous System:

• Hypotonic dehydration → cerebral edema
• Hypertonic dehydration → brain shrinkage with risk of intracranial hemorrhage

Kidneys:

• Reduced renal perfusion → prerenal azotemia
• Prolonged hypoperfusion → acute tubular necrosis

Electrolyte Disturbances:

• Hypernatremia or hyponatremia
• Hyperkalemia or hypokalemia

Hemostasis:

• Increased blood viscosity → thromboembolic risk

Diagnosis

• Physical examination including skin, mucosa, eyes, blood pressure, and heart rate
• Laboratory tests including serum sodium, serum potassium, serum osmolarity, urine osmolarity, urine sodium, creatinine, and urea
• Assessment of body weight and fluid intake and output

Therapy

• Treatment of the underlying cause
• Fluid replacement depending on the type of dehydration:
  • Isotonic dehydration → isotonic saline
  • Hypotonic dehydration → isotonic or hypertonic saline with slow correction
  • Hypertonic dehydration → careful administration of hypotonic fluids to avoid cerebral edema
• Continuous monitoring of consciousness, electrolytes, and urine output

Sources

• AMBOSS
• Pathophysiology lectures
• Guyton and Hall. Textbook of Medical Physiology
• Stefan Silbernagl, Florian Lang. Color Atlas of Pathophysiology
• Osmosis