Surgical Fluids, Electrolytes & Nutrition
A board-focused walkthrough of surgical fluid management — resuscitation vs maintenance, IV fluid selection, and the 4-2-1 rule — plus the classic post-op electrolyte disturbances (hyponatremia, NG-suction alkalosis, hypocalcemia, hyperkalemia, refeeding) and the enteral-vs-parenteral nutrition decision, each framed as a next-best-step.
Overview: Two Jobs — Resuscitate and Maintain
Surgical patients bleed, third-space fluid into inflamed or obstructed bowel, and lose volume through NG suction, fistulas, fever, and open cavities. The boards want you to separate two jobs. Resuscitation replaces existing deficits and ongoing losses with isotonic crystalloid, titrated to urine output and hemodynamics. Maintenance covers baseline water and electrolyte needs with a hypotonic, potassium-containing fluid. Layered on top are the predictable post-op electrolyte traps — ADH-driven hyponatremia, hypokalemic hypochloremic alkalosis from NG losses, and hypocalcemia after neck surgery — plus the nutrition question of when and by what route to feed. Almost every vignette collapses to a single next-best-step: give a fluid bolus, replace the specific electrolyte, or pick the right feeding route (enteral beats parenteral whenever the gut works).
- Resuscitation fluid = isotonic crystalloid — LR or NS; balanced LR is preferred for large-volume resuscitation.
- Large-volume NS → hyperchloremic non-anion-gap metabolic acidosis.
- Maintenance rate (4-2-1 rule): 4 mL/kg/hr for the first 10 kg + 2 mL/kg/hr for the next 10 kg + 1 mL/kg/hr for each kg >20.
- Typical maintenance fluid: D5 ½NS + 20 mEq KCl/L (once the patient is making urine).
- Urine output is the best bedside gauge of resuscitation — goal ≥0.5 mL/kg/hr in adults.
- Post-op oliguria is hypovolemia until proven otherwise — give a 500 mL–1 L isotonic bolus and reassess before diagnosing ATN.
- Third-spacing (bowel obstruction, pancreatitis, peritonitis, burns) sequesters intravascular volume and demands generous isotonic replacement.
- Do not resuscitate with D5W or hypotonic fluid — it distributes across total body water and barely expands the intravascular space.
IV Fluid Composition Cheat-Sheet
| Fluid | Na / Cl (mEq/L) | Tonicity | Key use / pearl |
|---|---|---|---|
| 0.9% NS | 154 / 154 | Isotonic | Resuscitation; only crystalloid co-infused with blood; large volumes → hyperchloremic metabolic acidosis |
| Lactated Ringer | 130 / 109 (K 4, Ca 3, lactate 28) | Isotonic | Preferred large-volume resuscitation; lactate → HCO₃⁻; caution in hyperkalemia/renal failure |
| 0.45% NS (½NS) | 77 / 77 | Hypotonic | Maintenance (usually as D5½NS + KCl); not for resuscitation |
| D5W | 0 / 0 (50 g/L dextrose) | Hypotonic free water | Corrects free-water deficit / hypernatremia; useless volume expander |
| 3% saline | 513 / 513 | Hypertonic | Symptomatic (seizing) hyponatremia only |
Vignette: POD 1 after laparotomy for small-bowel obstruction, a 70-kg man's urine output falls to 15 mL/hr over 3 hours. HR 110, BP 98/60, dry mucous membranes; he received little IV fluid overnight and has ongoing third-space losses.
Diagnosis: prerenal hypovolemia — the dominant cause of early post-op oliguria — not ATN. (0.5 mL/kg/hr for 70 kg = 35 mL/hr, so 15 mL/hr is frankly oliguric.)
Next best step: give a 500 mL–1 L bolus of isotonic crystalloid (LR) and reassess urine output and vitals. A brisk rise in urine output confirms hypovolemia. Furosemide, dopamine, or dialysis are wrong here, and a diuretic would worsen the prerenal state. Only if urine output stays low despite adequate volume do you work up intrinsic renal failure (urine electrolytes, FeNa, muddy-brown casts).
- NG suction / vomiting → hypokalemic, hypochloremic metabolic alkalosis with paradoxical aciduria; correct with NS + KCl (chloride-responsive alkalosis, urine Cl <20).
- Post-op hyponatremia: surgical stress, pain, nausea, and opioids drive ADH, and excess hypotonic IV fluid makes it worse. Correct chronic hyponatremia slowly — ≤8 mEq/L per 24h — to avoid osmotic demyelination (central pontine myelinolysis).
- Seizing/comatose hyponatremia → 3% saline regardless of chronicity, then slow the rate.
- Hypocalcemia after thyroid/parathyroid surgery: perioral tingling, Chvostek and Trousseau signs, prolonged QT → IV calcium gluconate.
- Hyperkalemia ECG progression: peaked T waves → widened QRS → sine wave → arrest.
- Refractory hypokalemia will not correct until magnesium is repleted.
Classic hyperkalemia sequence — "C BIG K" — in three phases: stabilize → shift → eliminate.
- C — Calcium gluconate: stabilizes the myocardial membrane within minutes. Give it first when there are ECG changes. It does not lower serum K.
- B — Bicarbonate (mainly if acidemic) and Beta-agonist (albuterol): shift K intracellularly.
- I + G — Insulin + Glucose: the fastest reliable shift; glucose prevents hypoglycemia.
- K — K-binders (patiromer / SZC / Kayexalate), plus loop diuretics and dialysis: the only steps that actually remove potassium from the body.
Key trap: calcium and the shifters buy time but do not reduce total-body potassium — you still need an elimination step, and dialysis is definitive in renal failure.
Vignette: A cachectic man with alcohol use disorder is admitted after a fall and started on nutrition support. On day 2–3 he develops weakness, confusion, and a cardiac arrhythmia; labs show phosphate 1.0 mg/dL with low potassium and magnesium.
Diagnosis: refeeding syndrome — reintroduced carbohydrate triggers an insulin surge that drives phosphate, potassium, and magnesium intracellularly; the hallmark lab is hypophosphatemia.
Next best step: slow or hold calorie advancement, aggressively replete phosphate, K, and Mg, and give thiamine before glucose (Wernicke prophylaxis). Prevent it in high-risk patients (alcoholism, anorexia, prolonged starvation) by starting calories low and advancing gradually with close electrolyte monitoring.
- "If the gut works, use it" — enteral nutrition beats parenteral whenever the GI tract is functional: it preserves mucosal integrity, causes fewer infections, and costs less.
- TPN indications: a nonfunctional or inaccessible gut — prolonged ileus, short-bowel syndrome, high-output enterocutaneous fistula, or inability to eat for a prolonged period (well-nourished patients tolerate about 5–7 days; malnourished patients need support sooner).
- TPN complications: catheter-related bloodstream infection (most feared), hyperglycemia, refeeding syndrome, hepatic steatosis/cholestasis, and acalculous cholecystitis from gut disuse (biliary stasis).
- Feed early enterally after most operations once the patient tolerates it; a brief post-op ileus is not a reason to default to TPN.
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