Nephrology and Fluid/Electrolyte Disorders
Core concepts
Master the AKI workup: pre-renal, intrinsic, post-renal; urine output, urinalysis, fractional excretion of sodium (interpret with diuretics in mind), and ultrasound to exclude obstruction. Know KDIGO staging and when to dialyse (refractory hyperkalaemia, acidosis, pulmonary oedema, uraemic complications). Understand sodium disorders by volume status and tonicity, potassium shifts (insulin, beta-agonists, bicarbonate), and the ECG changes of hyperkalaemia. Know CKD staging by eGFR and albuminuria, and the pillars of renoprotection (BP control, ACEi/ARB, SGLT2 inhibitor).
Traps
Questions test pseudo-hyponatraemia (hyperglycaemia, hyperlipidaemia) and the risk of over-rapid correction (osmotic demyelination). Do not give IV calcium for hyperkalaemia with ECG changes and expect it to lower potassium - it protects the myocardium only. Distinguish SIADH (euvolaemic, low serum osmolality, high urine osmolality, urine Na >40) from hypovolaemic hyponatraemia. A rising creatinine after ACEi initiation may be acceptable if <30% and stable.
Rule of thumb
In hyperkalaemia with ECG changes, give IV calcium first (membrane stabilisation), then insulin-dextrose and salbutamol to shift potassium intracellularly, and arrange definitive removal (dialysis, potassium binders).
Worked example
A patient with euvolaemic hyponatraemia and urine osmolality >100: suspect SIADH, treat the cause, restrict fluid, and correct sodium slowly (≤8–10 mmol/L in 24 hours).