Chapter 18. Electrolyte Disorders

Fluid Homeostasis

Pathophysiology

• Maintain intravascular volume to facilitate tissue perfusion & oxygenation; maintain osmotic equilibrium → tonicity 275–290mOsmol/kg H2O; intracellular = extracellular tonicity
• Distribution of total body water (TBW): % body weight varies with age
  • Adult ♂ = 50–60%; adult ♀ = 45–55% (↓ with age); TBW = 0.5 × weight (kg) ♀; 0.6 × weight (kg) ♂
• Intracellular compartment fluid (ICF) = 2/3 of TBW
• Extracellular compartment fluid (ECF) = 1/3 of TBW; interstitial fluid = ¾ ECF ∼25% of TBW; intravascular fluid = ¼ ECF ∼8% of TBW
• Electrolyte composition of fluid compartments
  • ECF: Na+, Cl−, HCO3−, albumin → Na+ primary determinant of extracellular osmolality albumin provides oncotic pressure to retain intravascular volume
  • ICF: K+, Mg++, PO4− → K+ primary determinant of intracellular osmolality; physiologic control of water & sodium homeostasis

Hyponatremia

Pathophysiology

• Excess free water relative to Na+ in ECF; absolute levels of ECF water & Na+ may be ↑, ↓, or normal depending on etiology
• Pathophysiology varies by etiology; assess measured or calculated serum osmolality → POsm = 2[Na+] + [BUN]/2.8 + [Glucose]/18; ECF volume status; measured urine [Na+] &/or measured urine osmolality
  • ↑ urine [Na+] (>20mEq/L) implicates the kidney as etiologic site (innapropriate Na+ excretion in hyponatremia); ↓ urine [Na+] indicates an appropriate renal response to extrarenal cause
• Symptoms result from cerebral edema; severity of edema depends on rate & extent of Na+ ↓
  • Rapid ↓ (<48h) or large ↓, brain tissue remains hypertonic relative to plasma → draws water from ECF; sx of acute cerebral edema → severe HA, stupor, obtundation, coma, seizures; milder cases → confusion, dizziness, nausea, lethargy
  • Gradual Na+ ↓ allows brain tissue time to equalize ICF-ECF tonicity gradient by organic osmole production (ex. taurine); may be asymptomatic

Diagnosis & Evaluation