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Educational draft · awaiting clinical reviewUse Rapid for revision, not patient-care decisions. Check current national and local guidance and the BNF or BNFC before acting.

Arterial blood gases and respiratory acid-base disorders

Respiratory · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

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Escalate

Treat the patient while interpreting the sample. Severe hypoxaemia, exhaustion, reduced consciousness, shock, a rapidly falling pH, or acute hypercapnic respiratory failure requires immediate senior and critical-care assessment; controlled oxygen and ventilatory support must not wait for a perfect acid-base label.

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.
  • A high PaCO2 drives respiratory acidosis; a low PaCO2 drives respiratory alkalosis. A low bicarbonate drives metabolic acidosis; a high bicarbonate drives metabolic alkalosis.
  • Compensation does not normally overshoot the pH. A near-normal pH with clearly abnormal PaCO2 and bicarbonate may represent compensation or two opposing primary disorders.
  • Type 1 respiratory failure is hypoxaemia without hypercapnia; type 2 includes hypercapnia and usually reflects inadequate alveolar ventilation.
  • For most acutely ill adults target SpO2 94-98%; if at risk of hypercapnic respiratory failure use 88-92% pending blood gases, unless a patient-specific target applies.
  • In COPD, persistent pH below 7.35 with PaCO2 above 6.5 kPa despite optimal initial treatment is a standard trigger to consider NIV; do not delay escalation when the trajectory is adverse.
  • An anion gap helps organise metabolic acidosis: calculate Na - (Cl + HCO3), interpret against the laboratory range and consider albumin because hypoalbuminaemia lowers the expected gap.
  • Repeat the gas after a material change in oxygen, ventilation or treatment; the trend often matters more than one value.

    • Key red flags

    Acute hypercapnic respiratory failure

    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.

    Chronic or acute-on-chronic hypercapnia

    Raised PaCO2 with raised bicarbonate and a less depressed pH suggests renal compensation. A new pH fall on a chronically high bicarbonate signals acute decompensation; previous stable gases are valuable.

    High-anion-gap metabolic acidosis

    Low pH and bicarbonate with a raised anion gap suggests added unmeasured acids: lactate, ketones, renal failure or selected toxins. Identify the cause immediately rather than giving bicarbonate reflexively.

    Investigation priorities

    01
    ABCDE assessment plus pulse oximetry and documented oxygen deliveryFirst step

    Put gas values into the patient's physiological context.

    Management branches

    First passA reproducible seven-step interpretation

    Every arterial gas.

    1. 1. Verify patient, arterial source, time, temperature if relevant, oxygen device/flow or FiO2, and whether treatment has just changed.
    2. 2. Decide acidaemia, alkalaemia or near-normal pH; then identify whether PaCO2 and/or bicarbonate explains it.
    3. 3. Judge compensation and deliberately look for a mixed disorder when the pH or compensation does not fit one process.
    4. 4. Assess PaO2 in relation to FiO2 and clinical work of breathing; then review lactate, anion gap, glucose, ketones and electrolytes.
    5. 5. State a synthesis and action: for example, 'acute-on-chronic hypercapnic respiratory acidosis with persistent hypoxaemia on 28% oxygen—controlled oxygen, optimal COPD treatment and urgent NIV assessment'.

    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.Oxygen is prescribed to a target, not a fixed saturation maximisation. In critical illness start high-concentration oxygen if needed, then titrate rapidly once reliable oximetry/gases are available; repeat gases after 30-60 minutes in hypercapnic-risk patients.
    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.Aim for adequate breathing, not full arousal. Acute withdrawal, agitation and recurrent toxicity can occur because naloxone may wear off first; infusion and prolonged observation may be needed. Verify the locally approved concentration and protocol.
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    Sources and review status4 sources · checked 27 Aug 2026 · clinical review pending+
    Sources

    Sources and review status

    National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

    • BTS guideline for oxygen use in healthcare and emergency settingsNational target-saturation and repeat-gas recommendations.
    • BTS/ICS guideline for ventilatory management of acute hypercapnic respiratory failureNIV thresholds, monitoring and escalation principles.
    • NICE NG115: COPD diagnosis and managementBlood-gas, oxygen and exacerbation recommendations.
    • BTS enhanced emergency assessment pathways for asthma and COPDPublished July 2026; operational acute hypercapnic respiratory-failure pathway.

    Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

    Authoring stateRapid draftClinical stateAwaiting reviewJurisdictionUnited Kingdom