Hyponatraemia workup begins with serum osmolality to exclude pseudo- and hypertonic causes. True hyponatraemia (low osmolality) is then classified by volume status and urine sodium. SIADH is the commonest euvolaemic cause. The correction rate ceiling of 8 mEq/L/day prevents osmotic demyelination syndrome (ODS).
ECG changes in hyperkalaemia progress with rising K⁺. Peaked T-waves are the earliest finding. Calcium gluconate is membrane-stabilising (onset 1–3 min) and must be given first when ECG changes are present — it does not lower potassium. Definitive K⁺ lowering requires insulin-glucose, salbutamol, or dialysis.