Synopsis
Read an arterial blood gas in a fixed clinical sequence, recognise ventilatory failure and mixed acid-base disease, and act on the cause rather than treating a number in isolation.
- Record the inspired oxygen, device, time and clinical state before interpreting PaO2; an oxygen tension without FiO2 is incomplete information.
- Use one sequence: pH, PaCO2, bicarbonate/base excess, compensation, oxygenation, lactate, then electrolytes and clinical cause.
- Acidaemia is pH below 7.35 and alkalaemia above 7.45; decide which component moves in the direction that explains the pH.
Key red flags
Raised PaCO2 with pH below 7.35 and limited renal bicarbonate retention, often with drowsiness, exhaustion or reduced respiratory effort. In COPD, PaCO2 above 6.5 kPa with persistent acidosis after initial treatment supports urgent NIV assessment.
Investigation priorities
Put gas values into the patient's physiological context.
Management branches
Every arterial gas.
- 1. Verify patient, arterial source, time, temperature if relevant, oxygen device/flow or FiO2, and whether treatment has just changed.
- 2. Decide acidaemia, alkalaemia or near-normal pH; then identify whether PaCO2 and/or bicarbonate explains it.
Key medicines
Controlled oxygenTitrate to SpO2 94-98% for most acutely ill adults; if at risk of hypercapnic respiratory failure use 24% Venturi at 2-3 L/min or 28% Venturi at 4 L/min (or nasal cannulae 1-2 L/min) aiming 88-92% pending gases.
Naloxone when opioid toxicity is causing ventilatory failureTitrate IV in small increments in monitored care; a commonly used initial hospital dose is 100-200 micrograms IV, repeated every 2-3 minutes to adequate ventilation, with larger 400 microgram doses used when dependence is unlikely. Follow the current emergency medicines protocol.