Disorders of Electrolyte Balance

Introduction & Principles - Fluid Foundations

• Body Fluid Compartments: Intracellular Fluid (ICF ~2/3), Extracellular Fluid (ECF ~1/3; interstitial, plasma).
• Osmolarity: Total solute concentration. Normal plasma osmolality: 275-295 mOsm/kg.
  • Calculated: $2 \times [Na^+] + [Glucose]/18 + [BUN]/2.8$
• Tonicity: Effective osmolality; influences cell volume.
• Electrolyte Imbalance Causes: Abnormal intake, absorption, distribution, or excretion.

⭐ ECF volume is primarily determined by total body sodium content.

Sodium Imbalances - Salty Situations

Hyponatremia (Na+ < 135 mEq/L; Severe < 120 mEq/L): CNS symptoms (confusion, seizures). Max correction 8-10 mEq/L/24h.

⭐ Rapid correction of chronic hyponatremia can lead to Osmotic Demyelination Syndrome (ODS).

• Causes: Water loss (Diabetes Insipidus, fever), Na+ gain (iatrogenic).
• Symptoms: Thirst, CNS (irritability, coma).
• Management: Correct water deficit. Water Deficit = $0.6 \times \text{Wt (kg)} \times [(\text{Serum Na}^+ / 140) - 1]$. Hypotonic fluids.

Potassium Imbalances - K-Drama Central

Calcium & Phosphate Imbalances - Bone & Beyond Buddies

• Hypocalcemia (Total Ca < 8.5 mg/dL; Ionized Ca < 4.65 mg/dL)
  • Causes: Vitamin D deficiency, hypoparathyroidism, CKD.
  • Sx: Chvostek's, Trousseau's signs, tetany, paresthesias. ECG: QT prolongation.
  • Corrected Ca: $Corrected Ca (mg/dL) = Measured Ca + 0.8 \times (4.0 - albumin (g/dL))$.
  • Mgmt: IV Calcium gluconate (acute); Oral Ca/Vit D (chronic).
• Hypercalcemia (Total Ca > 10.5 mg/dL)
  • Causes: Primary hyperparathyroidism, malignancy (e.g., PTHrP).

    ⭐ Malignancy and primary hyperparathyroidism are the two most common causes of hypercalcemia in adults.

  • Sx: 📌 "Stones (renal calculi), bones (pain, fractures), groans (abdominal pain, constipation), thrones (polyuria, polydipsia), psychiatric overtones (confusion, fatigue)". ECG: Short QT interval.
  • Mgmt: IV fluids, bisphosphonates, calcitonin.
• Phosphate Imbalances
  • Hypophosphatemia (< 2.5 mg/dL): Causes (refeeding syndrome, antacids), muscle weakness, rhabdomyolysis. Mgmt: Oral/IV phosphate.
  • Hyperphosphatemia (> 4.5 mg/dL): Causes (CKD, hypoparathyroidism), soft tissue calcification. Mgmt: Phosphate binders, dialysis.
• Ca-PO4 Relationship Highlights

  Factor	Serum Ca	Serum PO4
  PTH	↑	↓
  Vitamin D (active)	↑	↑
  CKD (severe)	↓	↑

Magnesium Imbalances - Mighty Mag Moves

• Hypomagnesemia ($Mg^{2+}$ < 1.8 mg/dL or < 0.7 mmol/L)
  • Causes: GI/renal loss, PPIs, alcohol.
  • Features: Neuromuscular hyperexcitability (tetany, seizures), arrhythmias (📌 Torsades de Pointes - TdP). Often coexists with hypokalemia, hypocalcemia.
  • Management: $MgSO_4$.

  ⭐ Hypomagnesemia can cause refractory hypokalemia and hypocalcemia because magnesium is a cofactor for potassium uptake and PTH release/action.

• Hypermagnesemia ($Mg^{2+}$ > 2.6 mg/dL or > 1.1 mmol/L)
  • Causes: Renal failure, iatrogenic Mg administration.
  • Features: ↓ DTRs, respiratory depression, hypotension, bradycardia, cardiac arrest.
  • Management: IV calcium gluconate, dialysis.

High‑Yield Points - ⚡ Biggest Takeaways

• Hyponatremia (<135 mEq/L): SIADH common; rapid correction risks osmotic demyelination.
• Hypernatremia (>145 mEq/L): Free water loss (DI); slow correction prevents cerebral edema.
• Hypokalemia (<3.5 mEq/L): ECG (U waves, flat T), muscle weakness; potentiates digoxin toxicity.
• Hyperkalemia (>5.5 mEq/L): ECG (peaked T, wide QRS); Ca gluconate for cardiac protection.
• Hypocalcemia (<8.5 mg/dL): Tetany, Chvostek/Trousseau, prolonged QT.
• Hypercalcemia (>10.5 mg/dL): "Stones, bones, groans"; short QT.
• Hypomagnesemia: Causes refractory hypokalemia & hypocalcemia.