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Renal, Electrolytes, and Acid-Base
TEXTPhysiology I · 10 min read
GFR drives drug dosing. K + Na drive arrhythmias and neurology. Anion gap drives DKA recognition. Cover all three in one mental framework.
After this lesson you can
3 min read8 sections- Identify acute kidney injury intraoperatively.
- Manage common electrolyte derangements.
- Apply acid-base interpretation (MUDPILES, Winter's).
- Plan anesthetic in CKD/ESRD.
GFR and drug dosing
Normal ~120 mL/min/1.73m².
Renal drug handling matters when drug or active metabolite is renally cleared (aminoglycosides, vancomycin, morphine-6-glucuronide, pancuronium, certain non-depolarizing NMBs).
Cisatracurium is the safe NMB in renal failure — Hofmann elimination is organ-independent.
Avoid morphine in ESRD because M6G accumulates.
Anion gap acidosis — MUDPILES
Normal 8-12.
Elevated gap means unmeasured anion in the blood.
- Methanol
- Uremia
- DKA
- Propylene glycol
- Iron/INH
- Lactic acidosis
- Ethylene glycol
- Salicylates
Non-gap acidosis from GI/renal HCO₃ loss (diarrhea, RTA).
Mixed disorders identified by delta-delta ratio.
Always corrected for albumin (add 2.5 to gap per g/dL below 4.0).
Compensation expectations
Chronic: HCO₃ rises 3.5 per 10. Respiratory alkalosis: opposite.
Metabolic acidosis (Winter's formula): expected PaCO₂ = 1.5 × HCO₃ + 8 ± 2. Metabolic alkalosis: PaCO₂ rises 0.7 per 1 mEq HCO₃ above 24. Numbers outside expected = mixed disorder.
Use to diagnose, then treat the underlying cause.
Potassium derangements
- calcium chloride
1 g(membrane stabilizer, 1-3 min onset, ~30-60 min duration) - then shift (insulin 10 U + D50, albuterol neb, NaHCO₃ if acidotic)
- then remove (loop, dialysis, Kayexalate slow)
- T-wave flattening
- U-waves
- arrhythmia threshold lowered
Replace cautiously — IV peripheral max 10 mEq/hr; central max 20-40 mEq/hr.
Sodium derangements
Acute (<48 hr) symptomatic: 3% NaCl 100 mL boluses, target rise 4-6 mEq/L in first 6 hr.
Chronic: correct ≤8-10 mEq/L per 24 hr (faster causes central pontine myelinolysis — irreversible quadriparesis).
- TURP irrigation absorption
- oxytocin antidiuresis
- SIADH
Hypernatremia almost always hypovolemic — restore volume slowly.
Calcium and magnesium
Hypocalcemia in massive transfusion (citrate chelation) — replace 1 g CaCl₂ per 4-6 U PRBC, or by ionized Ca.
Hypercalcemia (malignancy, hyperparathyroid): hydration + bisphosphonates.
10 mEq/L, respiratory depression ~15, arrest ~25-30.Treat with calcium gluconate 1 g IV.
Renal failure types + perioperative AKI prevention
- ATN (hypotension, contrast, nephrotoxin, rhabdo)
- AIN (drug-induced — beta-lactams, NSAIDs)
- glomerulonephritis
KDIGO AKI staging by Cr rise + UOP.
- euvolemia
- MAP ≥65 (higher in chronic HTN)
- avoid nephrotoxins (NSAIDs, aminoglycosides, contrast in pre-existing CKD)
- goal-directed fluid management with balanced crystalloid (LR > NS — chloride-restrictive)
N-acetylcysteine evidence weak.
Avoid hydroxyethyl starches (renal injury signal in critical illness).
Dialysis patient + the OR
Operate the day AFTER dialysis when possible (volume + K + uremia optimized; some heparin clearance complete).
- K <5.5 mandatory
- recent K within hours
- ECG for hyperK signs (peaked T, wide QRS)
- no AV-fistula arm BPs/IVs (preserve fistula)
- assess volume status
- avoid morphine + meperidine
- gabapentin (renally cleared, severe sedation)
- NSAIDs
Use cisatracurium (Hofmann), fentanyl, hydromorphone (better than morphine), remifentanil (organ-independent ester hydrolysis).
Sugammadex safe (drug-rocuronium complex eliminated despite renal clearance slowing).
⚠ Common pitfalls
- Aggressive crystalloid in ESRD — fluid overload; smaller boluses + reassess.
- Forgetting that dialysis-dependent patients have altered drug clearance — case-by-case.
- Standard sux dose in known ESRD — K rise more pronounced.
- Missing the post-renal cause of AKI — Foley/ureteral obstruction often correctable.
💎 Clinical pearls
- Winter's formula (expected PaCO₂ in metabolic acidosis) = 1.5 × HCO₃ + 8 ± 2.
- Anion gap > 12 → MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic, Ethylene glycol, Salicylates).
- Hyperkalemia treatment ladder: calcium first, then shift (insulin + D50, albuterol, bicarb), then remove (kayexalate, dialysis).
- Cis-atracurium or sugammadex-reversed roc are NMBs of choice in renal failure.
Recap
- Winter's formula (expected PaCO₂ in metabolic acidosis) = 1.5 × HCO₃ + 8 ± 2.
- Anion gap > 12 → MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic, Ethylene glycol, Salicylates).
- Hyperkalemia treatment ladder: calcium first, then shift (insulin + D50, albuterol, bicarb), then remove (kayexalate, dialysis).
- Cis-atracurium or sugammadex-reversed roc are NMBs of choice in renal failure.
Mark each section done to complete the module.