Perioperative
Preoperative risk, anticoagulation, SSI prophylaxis, fluids/electrolytes, acid–base, nutrition, and postoperative complications for the Surgery Shelf.
Sources: extracted/theory-perioperative.md (Patient Management); First Aid 2026 (first-aid-usmle-bible).
High-Yield Points
- Preoperative cardiac risk: the golden question is "can you climb a flight of stairs?" = 4 METs. Functional capacity ≥4 METs with no active cardiac symptoms → proceed, no further testing. Routine tests = CBC, basic metabolic panel, ± EKG; coag profile/LFTs/UA/CXR/echo/stress test only for a specific indication.
- Goldman index: the highest-weighted findings are an S3 gallop and elevated JVP (11 pts each) — decompensated CHF is the strongest predictor; recent MI <6 mo = 10 pts. Delay ELECTIVE surgery after an MI (ideally ~60 days).
- Pulmonary risk: smoking and obstructive/restrictive disease; reduce complications with preoperative smoking cessation + incentive spirometry / early mobilization. Atelectasis is the most common early postop pulmonary problem.
- VTE prophylaxis: early ambulation + low-dose heparin and/or sequential compression devices is best prevention.
- Anticoagulation: high thromboembolic risk (e.g., mechanical valve) on warfarin → stop ~5 days pre-op and BRIDGE with heparin/LMWH. Recent drug-eluting stent on dual antiplatelet therapy → delay elective surgery until the DAPT course is complete (interruption → stent thrombosis).
- SSI prophylaxis: give the antibiotic within 60 min before incision (120 min for vancomycin/fluoroquinolone). Colorectal cases need gram-negative + anaerobe coverage (cefoxitin, or cefazolin + metronidazole).
- Hyponatremia: most postop cases are iatrogenic/dilutional → free water restriction. Correcting chronic hyponatremia too fast → central pontine myelinolysis (keep <~20 mEq/L/day).
- Hypernatremia: postop polyuria with dilute urine = diabetes insipidus → replace free water with D5W and treat the cause; correct slowly (too fast → cerebral edema).
- Hypokalemia: EKG = flat T waves, ST depression, U waves; replace K⁺ and correct hypomagnesemia (refractory K⁺ is Mg-dependent).
- Hyperkalemia: EKG = peaked T waves, wide QRS; give IV calcium FIRST (membrane stabilization) → insulin/glucose ± bicarb (shift) → kayexalate/diuretics/dialysis (remove).
- Hypocalcemia: classic post-thyroidectomy/parathyroidectomy; Chvostek & Trousseau signs, prolonged QT → IV calcium.
- Hypercalcemia: IV saline then furosemide (avoid thiazides, which raise calcium); bisphosphonates for malignancy.
- Nutrition: prealbumin (t½ 3–5 d) and transferrin track acute status; albumin (t½ ~21 d) is long-term. Refeeding syndrome → hallmark hypophosphatemia (also ↓K, ↓Mg); advance feeds slowly + replete phosphate/thiamine.
- Postop fever — five Ws: Wind → Water → Walking → Wound → Wonder drugs.
- Wound: salmon-colored serosanguineous drainage = dehiscence; protruding bowel = evisceration → cover with moist sterile saline gauze + EMERGENT OR.
- Transfusion: fever + flank pain + hypotension + hemoglobinuria minutes into a transfusion = acute hemolytic (ABO) reaction → STOP the transfusion, give fluids.
Key Tables / Differentials
Postoperative fever — the five Ws
| Timing | "W" | Cause | Action |
|---|---|---|---|
| POD 1–2 | Wind | Atelectasis (esp. intubated) | Incentive spirometry, mobilize |
| POD 3 | Water | UTI (catheter) | U/A, culture, remove catheter |
| POD 4–6 | Walking | DVT/thrombophlebitis | LE duplex; anticoagulate |
| POD ~5 | Wound | Surgical site infection | Open at bedside, culture, wet-to-dry |
| Any time | Wonder drugs | Drug fever | Diagnosis of exclusion |
Primary acid–base disorders
| Disorder | pH | Primary | Compensation | Classic cause |
|---|---|---|---|---|
| Metabolic acidosis | ↓ | ↓HCO₃⁻ | ↓PCO₂ | High gap = MUDPILES; non-gap = saline/diarrhea |
| Metabolic alkalosis | ↑ | ↑HCO₃⁻ | ↑PCO₂ | Vomiting / NG suction → NS + KCl |
| Respiratory acidosis | ↓ | ↑PCO₂ | ↑HCO₃⁻ | Hypoventilation |
| Respiratory alkalosis | ↑ | ↓PCO₂ | ↓HCO₃⁻ | Hyperventilation/anxiety |
Board Pearls
- Anion gap = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻); normal ~10–12. High gap → MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic, Ethylene glycol, Salicylates).
- IBW < 80% → give preoperative nutritional supplementation to lower morbidity/mortality.
- K⁺ IV rate: ≤10 mEq/hr peripheral, ≤20 mEq/hr central.
- Na⁺ deficit (mEq) = (140 − Na⁺) × 0.6 × wt(kg); Free water deficit (L) = (Na⁺ − 140) × 0.6 × wt / 140.
- SIRS = ≥2 of: T >38/<36 °C, HR >90, RR >20 (or PCO₂ <32), WBC >12k/<4k/>10% bands; sepsis = SIRS + infection.
Classic Vignette Triggers
- "Can climb two flights of stairs, asymptomatic, elective hernia" → proceed, no cardiac testing.
- "18 h post-thyroidectomy, perioral numbness, carpopedal spasm" → hypocalcemia → IV calcium.
- "Renal failure, peaked T waves, wide QRS" → hyperkalemia → IV calcium first.
- "Chronic alcoholic started on TPN, weak/arrhythmia, low phosphate" → refeeding syndrome.
- "POD 7, coughs, gush of pink fluid, bowel through wound" → evisceration → moist gauze + emergent OR.
- "Minutes into PRBC transfusion: fever, flank pain, dark urine, hypotension" → acute hemolytic reaction → stop transfusion.
- "Days of NG suction, pH 7.51, low Cl⁻/K⁺" → contraction (chloride-responsive) metabolic alkalosis → NS + KCl.