ABG interpretation — breathless patient with COPD

PLAB 2 & PRES 3 · respiratory / data interpretation · hard · 10 minutes

Candidate brief

You are an FY2 doctor in the emergency department. Mr Frank Delaney, 68, with known COPD, has been brought in by ambulance with two days of worsening breathlessness and increased sputum after a cold. He is in a cubicle on 2 L/min oxygen by nasal cannulae (put on by the ambulance crew), drowsy but rousable, using accessory muscles. His wife says he is "not himself — sleepy, and his hands keep twitching". Observations: respiratory rate 26/min, SpO2 91% on 2 L, pulse 104/min, BP 146/82 mmHg, temperature 37.8 °C. An arterial blood gas, taken on the 2 L/min oxygen, shows: pH 7.28 (7.35–7.45) PaCO2 8.9 kPa (4.7–6.0) PaO2 7.6 kPa (>10.5 on air) Bicarbonate 30 mmol/L (22–26) Base excess +4 Lactate 1.6 mmol/L Please interpret the gas, explain what it means, and state your immediate actions.

What examiners look for

Read a gas in the same order every time: oxygen first, then pH, then CO2, then bicarbonate, then everything else. The pattern here — low pH, high CO2, high bicarbonate — is acute-on-chronic respiratory acidosis, and the bicarbonate is the historian: kidneys take days to raise it, so a high HCO3 with an acid pH means chronic retention with an acute fall off the cliff. His baseline is his context. A PaCO2 of 8.9 would be alarming in anyone; in a man whose compensated baseline is 7.2 with a normal pH, it tells you exactly what changed and what "back to normal" realistically means — you are treating him back to HIS normal, not the textbook's. Oxygen in COPD is a prescribed drug with a target range, not a kindness to maximise. 88–92% in a known retainer: uncontrolled oxygen worsens hypercapnia (through V/Q changes and the Haldane effect), and the sleepy patient on generous oxygen is an iatrogenic emergency hiding in plain sight. The reverse error — stripping oxygen off a man with a PaO2 of 7.6 — kills faster; controlled is the operative word. Drowsiness is a blood gas you can see. The flap, the bounding pulse, the warm hands and the drifting attention are the clinical shadow of the PaCO2; when the numbers and the patient disagree, believe the patient and repeat the numbers. Treatment is two-track: medicines for the exacerbation (nebulisers, steroids, antibiotics where infective) AND a pre-decided escalation threshold. NIV is not a last resort you improvise at 3 a.m. — it is the stated plan when pH stays below 7.35 despite an hour of maximal therapy, with critical care already aware. And write the target down. Oxygen prescribed with a target range on the drug chart — and a repeat gas time decided before you leave the cubicle — is what stops the next well-meaning cannula change undoing your work at the next handover.

Guideline references

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