Renal, Electrolyte and Acid-Base Disorders
This domain is arithmetic-heavy and unforgiving: staging, formulas and correction rates are all directly examinable. Expect KDIGO staging, dialysis indications, sodium correction limits and a full arterial blood gas interpretation.
Staging and the first question
KDIGO stages AKI by creatinine (1.5-1.9 times baseline, 2.0-2.9, or 3 times, or 4 mg/dL or more, or initiation of dialysis) and by urine output (below 0.5 mL/kg/h for 6-12 hours, below 0.3 for 24 hours, anuria for 12 hours). Then ask: prerenal, intrinsic or postrenal? Ultrasound excludes obstruction; urine sodium and fractional excretion help when diuretics are not confounding; the urea-to-creatinine ratio supports prerenal states; muddy brown casts support acute tubular necrosis. Remove nephrotoxins and dose-adjust drugs.
Dialysis indications
Refractory acidosis, electrolyte derangement (potassium above 6.5 with ECG changes or refractory to medical therapy), dialysable intoxication, fluid overload unresponsive to diuretics, and uraemic complications such as encephalopathy, pericarditis and bleeding. Continuous renal replacement suits haemodynamically unstable patients; intermittent dialysis is faster and adequate for stable ones, with no mortality difference in trials.
Sodium and potassium
Hyponatraemia: treat by symptoms. Seizure or coma means 3% saline 100-150 mL bolus, then correction of 4-6 mmol/L in the first hours and at most 8-10 mmol/L in 24 hours to avoid osmotic demyelination. Chronic asymptomatic hyponatraemia corrects slowly. Distinguish hypovolaemic, euvolaemic (SIADH: urine osmolality above 100, urine sodium above 30) and hypervolaemic causes. Hyperkalaemia: calcium first for ECG changes, then insulin-glucose, salbutamol, bicarbonate if acidotic, and dialysis if refractory.
Acid-base
For metabolic acidosis, Winter's formula gives expected PaCO2 = 1.5 x HCO3 + 8 +/- 2; a mismatch reveals a second disorder. Anion gap = Na - Cl - HCO3 (normal 8-12), corrected upward by 2.5 for every 1 g/dL fall in albumin. Delta ratio = (measured gap - 12) / (24 - measured HCO3): below 1 suggests coexisting non-gap acidosis, above 2 coexisting metabolic alkalosis.
Worked example
pH 7.28, PaCO2 30, HCO3 14, Na 138, Cl 100. Expected PaCO2 = 1.5 x 14 + 8 = 29 +/- 2, so respiratory compensation is appropriate. Anion gap = 138 - 100 - 14 = 24. Delta ratio = 12/10 = 1.2, a pure high-gap acidosis. Had chloride been 112, the gap would be 12 and the diagnosis a hyperchloraemic non-gap acidosis.
Traps
Correcting sodium faster than 8-10 mmol/L per 24 hours. Believing calcium treats hyperkalaemia - it stabilises the myocardium but does not lower potassium. Using the anion gap without albumin correction.