Oxygen delivery and nebuliser setup

PLAB 2 & PRES 3 · practical skills / respiratory · hard · 8 minutes

Candidate brief

You are an FY2 doctor on a medical ward. Mrs Eileen Farrell, 72, has been admitted with an exacerbation of COPD. She is breathless and her oxygen saturations are 86% on air. She is a known carbon dioxide retainer — on her last admission her blood gas showed chronic hypercapnia. Observations: respiratory rate 26, heart rate 108, blood pressure 146/88 mmHg, temperature 37.8 °C, alert. The registrar has prescribed oxygen to a target saturation range and a salbutamol nebuliser. Please set up her oxygen and nebuliser, explain what you are doing, and tell the examiner how you will reassess her. Verbalise each step.

What examiners look for

Oxygen is a drug: it has a prescription, a dose (device and flow), a target range, and toxic effects. In patients at risk of hypercapnic respiratory failure — known COPD with previous CO2 retention is the archetype — the British Thoracic Society target is 88–92%, and every decision in this station flows from that number. The paradox must be explainable to the patient in one breath: "Your lungs struggle to clear the waste gas. Too much oxygen can make your breathing slow down and let that gas build up, which makes you drowsy. So we aim for enough, carefully." A candidate who can teach it understands it. Venturi masks give controlled, fixed-performance oxygen — but only at the flow rate printed on the valve. Setting a 28% Venturi at the wrong flow silently changes the delivered concentration; the number on the valve is part of the prescription. The nebuliser's driving gas is the hidden oxygen prescription. An oxygen-driven nebuliser at 8 L/min is uncontrolled high-dose oxygen therapy for as long as it runs. In hypercapnia-risk patients: drive with air, add nasal oxygen to hold target saturations, and if oxygen must drive it, keep it brief and return to controlled oxygen straight after. Drowsiness is the alarm bell. In a COPD patient on oxygen, new drowsiness means rising CO2 until proven otherwise — it triggers an urgent gas and senior review, never reassurance that she is "settling". And complete the symmetry: the opposite error also kills. Hypoxaemia harms faster than hypercapnia; the correct response to CO2 fear is controlled oxygen with monitoring and a timed ABG — never leaving a patient at 86% untreated while you deliberate. Reassessment is scheduled, not vibes-based: observations continuously, ABG 30–60 minutes after starting or changing FiO2 (sooner if drowsy), and a stated escalation route to NIV assessment if acidosis develops despite maximal therapy.

Guideline references

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