Renal Disorders, Fluids, Electrolytes, and Acid-Base
Renal Disorders
Staging and drug dosing
Acute kidney injury is staged by KDIGO using creatinine rise and urine output (under 0.5 mL/kg/h for 6 hours is stage 1). Serum creatinine lags 24–48 hours behind injury, so a rising creatinine means the insult already happened and a normal creatinine does not exclude injury in a patient with low muscle mass.
Concepts to own
- Distinguish prerenal, intrinsic, and postrenal causes; check bladder volume, urinalysis, and fractional excretion of urea in diuretic-treated patients.
- Indications for renal replacement (AEIOU): refractory acidosis, electrolyte derangement (especially hyperkalemia), intoxication with a dialyzable toxin, volume overload, uremic complications.
- Continuous therapy for hemodynamically unstable patients; intermittent hemodialysis for stable patients needing rapid clearance.
- Dose-adjust renally cleared drugs (most beta-lactams, vancomycin, enoxaparin, DOACs, gabapentin) using Cockcroft-Gault with an appropriate weight; remember augmented clearance in young septic patients can require higher doses.
- Avoid nephrotoxins (NSAIDs, aminoglycosides when alternatives exist, high-dose contrast without indication) and hold metformin in significant AKI.
Traps
Believing diuretics improve renal recovery or mortality - they treat volume, not injury; using a fixed enoxaparin dose in severe renal impairment; forgetting that hepatically cleared drugs need no renal adjustment; assuming a normal creatinine excludes AKI; giving potassium before checking magnesium in refractory hypokalemia.
Worked example
A 60-year-old woman, 60 kg, creatinine 2.0 mg/dL: estimated CrCl ≈ (140 − 60) × 60 / (72 × 2.0) ≈ 33 mL/min (multiply by 0.85 for female sex). Many antimicrobials need dose reduction at this level.
Rule of thumb
If CrCl is under 30 mL/min, check every renally cleared drug on the list before the next dose is due.
Fluids, Electrolytes, and Acid-Base
Approach to the number
Acid-base questions are answered in a fixed order: pH, primary disorder, compensation, then anion gap and delta-delta. Compute the anion gap as Na − (Cl + HCO3), normal roughly 8–12. For metabolic acidosis, check whether the gap is elevated and whether the delta ratio suggests a mixed disorder.
Electrolytes
- Hyponatremia: assess volume status and osmolality; symptomatic seizures or coma warrant hypertonic saline bolus (100–150 mL of 3%) with correction limits around 8–10 mEq/L in 24 hours (4–6 in high-risk patients) to avoid osmotic demyelination.
- Hyperkalemia: stabilize the membrane with calcium, shift with insulin/dextrose and albuterol, remove with diuretics, binders, or dialysis; bicarbonate only helps if acidemic.
- Refractory hypokalemia usually means hypomagnesemia - replace magnesium.
- Correct measured sodium for hyperglycemia; pseudohyperkalemia and pseudohyponatremia are lab artifacts.
Fluids
Balanced crystalloids are preferred over 0.9% saline for most resuscitation. Avoid hypotonic fluid for resuscitation and avoid starch solutions.
Traps
Using Winter's formula incorrectly; assigning a high anion gap to lactate without checking ketones, uremia, or toxins; correcting sodium too quickly in chronic hyponatremia; giving flumazenil-like reflexive antidotes without thinking; forgetting an osmolar gap suggests toxic alcohol.
Worked example
pH 7.20, HCO3 10, pCO2 24: expected pCO2 = 1.5 × 10 + 8 ± 2 = 21–25, so respiratory compensation is appropriate - a pure high-anion-gap metabolic acidosis. If pCO2 were 35, there would also be a respiratory acidosis.
Rule of thumb
Winter's formula: expected pCO2 = 1.5 × HCO3 + 8 ± 2.