Fluid and electrolyte management

Maintenance Fluids - The Daily Drip

• Calculating Rate: Use the Holliday-Segar ($4/2/1$) rule for hourly rate.
  • $4$ mL/kg/hr for first 10 kg
  • $2$ mL/kg/hr for next 10 kg
  • $1$ mL/kg/hr for remaining weight
• Daily Needs:
  • 📌 Think 100 Na+, 50 K+, 100 Cl- (mEq) and 100g sugar.
  • Typical maintenance fluid: D5 1/2 NS + 20 mEq KCl/L.
  • Monitor urine output: goal 0.5-1.0 mL/kg/hr.

⭐ Dextrose (as D5) in maintenance fluids is not for calories but to prevent muscle catabolism and starvation ketosis.

IV Fluid Types - Saline & Friends

• Crystalloids: Solutions with small molecules (e.g., electrolytes, dextrose) that pass easily from the bloodstream to tissues. Used for resuscitation and maintenance.
  • Isotonic: 0.9% Normal Saline (NS), Lactated Ringer's (LR).
  • Hypotonic: D5W (isotonic in the bag, but physiologically hypotonic as dextrose is metabolized).
  • Hypertonic: 3% NaCl.
• Colloids: Solutions with larger molecules (e.g., Albumin) that stay in the vascular space longer.

Fluid Status Assessment - Reading the Signs

⭐ Tachycardia is often the earliest sign of hypovolemia in a post-operative patient.

Electrolyte Mayhem - The Usual Suspects

• Hyponatremia (↓Na+)

  • Causes: SIADH, excess hypotonic fluids, adrenal insufficiency.
  • ⚠️ Max correction 8-10 mEq/L/24h to prevent Osmotic Demyelination Syndrome (ODS).

• Hypernatremia (↑Na+)

  • Causes: Diabetes Insipidus (DI), dehydration, insensible losses.

• Hypokalemia (↓K+)

  • Causes: Diuretics (furosemide), GI losses (diarrhea, vomiting).
  • 📌 EKG: Flat T waves, prominent U waves.
  • Max infusion: 10 mEq/hr (peripheral), 20 mEq/hr (central).

• Hyperkalemia (↑K+)

  • Causes: Renal failure, tissue damage (rhabdomyolysis), ACE inhibitors.

⭐ Never add potassium to an IV bag for a patient who is anuric or has severe renal failure without confirming their potassium level first.

High-Yield Points - ⚡ Biggest Takeaways

• The post-op stress response (↑ADH, ↑aldosterone) causes sodium and water retention.
• Calculate maintenance fluids with the 4/2/1 rule; a common choice is D5 ½NS + 20mEq KCl.
• Post-op hyponatremia is often iatrogenic from excess free water (hypotonic fluids).
• Third spacing causes intravascular depletion; replete with isotonic crystalloids (LR or NS).
• Lactated Ringer's is preferred over Normal Saline to avoid hyperchloremic metabolic acidosis.
• Target urine output of >0.5 mL/kg/hr to ensure adequate end-organ perfusion.