Nephrology and Fluid/Electrolyte Disorders
Acute kidney injury
KDIGO defines AKI as a creatinine rise of at least 26 µmol/L within 48 hours, or 1.5 times baseline within 7 days, or urine output below 0.5 mL/kg/hour for 6 hours. Distinguish prerenal, intrinsic and post-renal causes; a bladder scan and catheter relieve obstruction, while urine sodium below 20 mmol/L, fractional excretion of sodium below 1% and urine osmolality above 500 suggest a prerenal state - but these indices mislead in sepsis, CKD, diuretic use and contrast nephropathy.
Hyperkalaemia and acid-base
ECG changes (tented T waves, widened QRS, sine wave) demand immediate calcium gluconate, then insulin-dextrose and nebulised salbutamol, with removal via bicarbonate if acidotic or dialysis if refractory. For metabolic acidosis, calculate the anion gap (Na − [Cl + HCO3]) and apply Winter's formula.
Sodium
Assess volume status and urine osmolality. SIADH is euvolaemic with urine osmolality above 100 mOsm/kg and urine sodium above 30 mmol/L, after excluding hypothyroidism and adrenal insufficiency. Correct chronic hyponatraemia by no more than 8–10 mmol/L in 24 hours to avoid osmotic demyelination.
Indications for urgent dialysis: acidosis, electrolytes (refractory hyperkalaemia), intoxication, overload, uraemia (AEIOU).
Worked example: pH 7.20, HCO3 10, Na 140, Cl 104. Anion gap = 26, so a high-anion-gap acidosis. Expected PaCO2 = 1.5 × 10 + 8 ± 2 = 21–25 kPa; a measured PaCO2 of 40 mmHg indicates a coexisting respiratory acidosis - look for exhaustion or opioid toxicity.