SAQ 110 – Metabolic – acid-base disturbance and hyponatraemia

Question 7. acid-base disturbance and hyponatraemia.

a. Calculations

• Anion gap = 101 + 4.7 – 73 – 9 = 23.7, or 101 – 73 – 9 = 19 if potassium is omitted; therefore the anion gap is raised.
• Delta ratio = (anion gap – 12) / (24 – HCO3-) = approximately 0.8 using potassium, or approximately 0.5 without potassium; this suggests combined high anion gap and normal anion gap metabolic acidosis.
• A-a gradient may also be calculated: PAO2 approximately 141 mmHg, PAO2 – PaO2 approximately 50 mmHg, therefore raised.

b. Likely explanations

• Acute decompensation of chronic liver disease, including hepatorenal or metabolic causes.
• Post-seizure state.
• Sepsis, especially respiratory source.
• Toxic ingestion such as ethanol, salicylate, or chlorpromazine.
• Head injury.
• SIADH.

c. Fluid replacement approach

• For severe symptomatic hyponatraemia, consider urgent 3% saline 1-2 mL/kg/hour via central line for 2-3 hours.
• Aim to raise serum sodium by 1-2 mmol/L/hour for the first 3-4 hours.
• Avoid increasing sodium by more than 12 mmol/L in 24 hours.
• Correct volume depletion cautiously with normal saline, for example 150 mL/hour.
• Aim for urine output greater than 0.5 mL/kg/hour and systolic BP greater than 90 mmHg.
• Concentrated albumin may be required to maintain blood pressure in chronic liver disease.