A 60-year-old man with a history of COPD presents with increasing breathlessness and confusion. ABG shows: pH: 7.25 PaCO₂: 65 mmHg HCO₃⁻: 30 mmol/L What is the most likely acid-base disorder? A) Respiratory acidosis with metabolic compensation B) Metabolic acidosis with respiratory compensation C) Respiratory alkalosis with metabolic compensation D) Metabolic alkalosis with respiratory compensation E) Mixed respiratory and metabolic acidosis Correct Answer: A) Respiratory acidosis with metabolic compensation Explanation: pH < 7.35 → Acidosis PaCO₂ high (65 mmHg) → Respiratory acidosis HCO₃⁻ high (30 mmol/L) → Metabolic compensation B) Metabolic acidosis → Incorrect. Would have low HCO₃⁻. C) Respiratory alkalosis → Incorrect. Would have high pH and low CO₂. D) Metabolic alkalosis → Incorrect. Would have high HCO₃⁻ and high pH. E) Mixed acidosis → Incorrect. Would have low HCO₃⁻ instead of high. Key Tips for PLAB 1: ✅ COPD + CO₂ retention = Respiratory acidosis. ✅ Compensation: ↑ HCO₃⁻ (but pH remains low). ✅ Treatment: Controlled O₂ therapy + Non-invasive ventilation (BiPAP).
