Dehydration Physiology

Dehydration: Definition & Types - Defining Dryness

• Definition: Loss of total body water (TBW), with or without electrolytes, leading to ↓TBW & potential hypovolemia.
• Types (based on serum $Na^+$ & osmolality):
  • Isotonic (Isonatremic):
    • Loss of $H_2O$ & $Na^+$ in proportion.
    • Serum $Na^+$: 130-150 mEq/L.
    • Most common.
  • Hypotonic (Hyponatremic):
    • Greater $Na^+$ loss than $H_2O$.
    • Serum $Na^+$: < 130 mEq/L.
  • Hypertonic (Hypernatremic):
    • Greater $H_2O$ loss than $Na^+$.
    • Serum $Na^+$: > 150 mEq/L.

⭐ Hypertonic dehydration often presents with more pronounced neurological symptoms (e.g., irritability, seizures) due to brain cell shrinkage.

Dehydration: Etiology - Why We Wilt

• Reduced Fluid Intake:

  • Poor access/water availability
  • Impaired thirst (elderly, infants, hypothalamic)
  • Nausea, vomiting, dysphagia
  • Altered consciousness

• Increased Fluid Losses:

  • Gastrointestinal (GI): Diarrhea (e.g., cholera, rotavirus), vomiting, NG suction, fistulas.
  • Renal: Diuretics, osmotic diuresis (DKA, HHS), Diabetes Insipidus (DI), adrenal insufficiency.
  • Skin/Insensible: Sweating (fever, exercise), burns.
  • Third Space: Sepsis, pancreatitis, peritonitis, ascites.

⭐ Globally, infectious diarrhea (especially rotavirus) is the most common cause of dehydration in pediatric populations.

Dehydration: Pathophysiology - Body's Battle

• Core Imbalance: Water loss > intake OR Solute gain > water.
• Initial Impact: ↓ Total Body Water (TBW) → ↑ ECF osmolality & ↓ ECF volume.
  • Water shifts: ICF → ECF (if hyperosmolar ECF), leading to cellular dehydration.
• Compensatory Cascade:

• Key Mediators:
  • ADH (Vasopressin): ↑ water permeability in collecting ducts.
  • RAAS: Angiotensin II (vasoconstriction, thirst, aldosterone) & Aldosterone (Na⁺/water retention).
  • Thirst: Stimulated by hyperosmolality & Angiotensin II.
• Cellular Stress: Shrinkage, impaired function.

⭐ The primary defense against hyperosmolality is thirst, while ADH is crucial for regulating water excretion.

Dehydration: Clinical Features - Spotting the Signs

• General: Thirst, dry mucous membranes, ↓ urine output.
• Mild (3-5% loss):
  • Alert, restless.
  • Vitals stable.
  • Slightly dry mucosa.
• Moderate (6-9% loss):
  • Irritable/lethargic.
  • Postural hypotension, tachycardia (↑HR).
  • Dry mucous membranes, sunken eyes.
  • ↓ Skin turgor (slow recoil).
  • Oliguria.
• Severe (≥10%** loss):**
  • Lethargic/comatose.
  • Hypotension (↓BP), shock.
  • Rapid, weak pulse.
  • Parched mouth, deeply sunken eyes.
  • Markedly ↓ skin turgor (tenting >2s).
  • Anuria/severe oliguria.
  • Cool, clammy skin; delayed capillary refill (>3s).

⭐ Skin turgor assessment (e.g., abdominal skin pinch) is a crucial bedside indicator of dehydration severity.

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Dehydration: Diagnosis & Complications - Tests & Troubles

• Diagnosis:
  • Clinical: ↓Skin turgor, dry mucous membranes, tachycardia, hypotension, ↓urine output (<0.5 mL/kg/hr).
  • Labs: ↑Serum Osmolality (>295 mOsm/kg), ↑Urine Specific Gravity (>1.025), ↑BUN/Creatinine ratio (>20:1), electrolyte disturbances (Na⁺, K⁺).
• Complications:
  • Hypovolemic shock, Acute Kidney Injury (AKI).
  • Severe electrolyte imbalance → arrhythmias, seizures.
  • Thromboembolic events.
  • ⚠️ Cerebral edema (esp. with rapid hypotonic fluid rehydration).

⭐ Loss of >10% body weight indicates severe dehydration.

High‑Yield Points - ⚡ Biggest Takeaways

• Dehydration: Water deficit > intake, leading to hypovolemia & cellular dysfunction.
• Isotonic (diarrhea): Most common; proportional Na+ & water loss.
• Hypertonic (fever, DI): Water loss > Na+ loss; ↑ serum osmolality, intracellular dehydration.
• Hypotonic (diuretics): Na+ loss > water loss; ↓ serum osmolality, cellular swelling.
• Key responses: ↑ ADH & aldosterone (RAAS) to conserve Na+ & water.
• Signs: Tachycardia, hypotension, oliguria, ↓ skin turgor; severe cases cause shock/AKI.