Fluid and Electrolyte Disorders

Body Fluid Basics - The Aqua Balance

• Total Body Water (TBW): ~60% of body weight in males, ~50% in females.
• Distribution: Intracellular Fluid (ICF) 2/3; Extracellular Fluid (ECF) 1/3 (Interstitial fluid 3/4 of ECF, Plasma 1/4 of ECF).
• Plasma Osmolality: $2 \times [Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$. Normal: 275-295 mOsm/kg.
• Starling forces govern fluid movement across capillaries.

⭐ Main determinant of ECF volume is Sodium; main determinant of ICF volume is Potassium.

• Water homeostasis: regulated by ADH (osmoregulation) & thirst; Volume homeostasis: RAAS & natriuretic peptides (sodium regulation).

Sodium Disorders - Salty Situations

Hyponatremia (Na+ < 135 mEq/L). Symptoms: Nausea, confusion, seizures.

• Diagnostic Algorithm:

• Correction: Acute/symptomatic: 3% NaCl. Chronic: Max 8-10 mEq/L/24h (target 0.5 mEq/L/hr).

⭐ Rapid correction of chronic hyponatremia (Na+ < 120 mEq/L for >48h) risks Osmotic Demyelination Syndrome (ODS).

Hypernatremia (Na+ > 145 mEq/L). Symptoms: Thirst, lethargy, seizures.

• Indicates hyperosmolality. Causes: ↓water intake, ↑water loss (Diabetes Insipidus, osmotic diuresis), ↑Na+ gain.
• Free Water Deficit: $FWD = (0.6 \times \text{Body Wt kg}) \times ((\text{Serum Na}^+ / 140) - 1)$.
• Correction: Hypotonic fluids (D5W, 0.45% NaCl). Max 10-12 mEq/L/24h (target 0.5 mEq/L/hr) to prevent cerebral edema.

Potassium Disorders - $K^+$ Conundrums

• Hypokalemia ($K^+$ < 3.5 mEq/L): Causes: Diuretics, GI loss. ECG: 📌 U wave, flat T. Rx: Oral/IV KCl (max 10-20 mEq/hr; 40 central).
• Hyperkalemia ($K^+$ > 5.5 mEq/L): Causes: Renal failure, ACEi/ARBs. ECG: 📌 Tall T, Wide QRS, Sine wave. Rx: See flowchart.

⭐ Calcium gluconate for cardiac membrane stabilization in hyperkalemia does not lower serum $K^+$LEVELS.

Calcium, Phosphate, Magnesium - Mineral Mayhem

• Corrected Calcium: $Ca_{corrected} = Ca_{measured} + 0.8 \times (4 - albumin)$
• Normal Levels: Ca++ 8.5-10.5 mg/dL; PO4 2.5-4.5 mg/dL; Mg++ 1.7-2.2 mg/dL.

⭐ Hypomagnesemia can cause refractory hypokalemia and hypocalcemia.

Acid-Base Balance - pH Puzzles

• Normal ABG: pH 7.35-7.45; PaCO2 35-45 mmHg; HCO3 22-26 mEq/L.
• Anion Gap (AG): $Na^+ - (Cl^- + HCO_3^-)$; Normal: 8-12 mEq/L.
  • 📌 High AG (MUDPILES): Methanol, Uremia, DKA, Paraldehyde, Iron, Lactic acidosis, Ethylene glycol, Salicylates.
  • 📌 Normal AG (HARDUPS): Hyperalimentation, Acetazolamide, RTA, Diarrhea, Uretero-enteric fistula, Pancreatic fistula, Saline.
• Key Formulas & Compensations:
  • Metabolic Acidosis (Winter's): $PCO_2 = 1.5 \times HCO_3^- + 8 \pm 2$
  • Acute Respiratory Acidosis: For every 10 mmHg ↑ PaCO2, HCO3 ↑ by 1 mEq/L.
• Stepwise ABG Interpretation:

⭐ In chronic respiratory acidosis, for every 10 mmHg ↑ in PaCO2, HCO3 ↑ by 3-4 mEq/L.

High‑Yield Points - ⚡ Biggest Takeaways

• Hyponatremia: Correct slowly to prevent Osmotic Demyelination Syndrome (ODS). SIADH is a key euvolemic cause.
• Hypernatremia: Indicates free water deficit. Correct slowly to prevent cerebral edema. Diabetes Insipidus is a classic cause.
• Hypokalemia: Presents with U waves on ECG and muscle weakness. Diuretics are a common cause.
• Hyperkalemia: Shows peaked T waves on ECG. Renal failure and ACE inhibitors are major causes. Calcium gluconate is cardioprotective.
• Metabolic Acidosis: Calculate Anion Gap (AG). High AG causes include DKA, Lactic Acidosis, Salicylates.
• Metabolic Alkalosis: Often due to vomiting or diuretic use (e.g., loop, thiazide diuretics).