FFinalistSouthampton · 2027
All procedures
GMC #06/Diagnostic procedures taught Y4

Arterial blood gas & acid–base sampling

Insert a needle into a patient's radial artery to take a sample of arterial blood and interpret the results.

Target · Perform under supervision0% ready

Do it on a patient with a supervisor observing you (GMC “direct supervision”).

Overview

Both the *sampling* and the *interpretation* are examinable. Have a systematic ABG approach ready and know how to check collateral flow.

Key numbers

  • Normal ranges: pH 7.35–7.45 · PaCO₂ 4.7–6.0 kPa · PaO₂ 11–13 kPa (on air) · HCO₃⁻ 22–26 mmol/L · BE −2 to +2UK reference ranges
  • Respiratory failure: PaO₂ <8 kPa. Type 1 = normal/low CO₂; Type 2 = PaCO₂ >6.0 kPaBTS
  • Apply firm pressure ≥5 min post-sampling (longer if anticoagulated) — it’s an arteryClinical practice

Indications

  • Suspected respiratory failure / to assess oxygenation & ventilation
  • Acid–base disturbance (DKA, sepsis, renal failure)
  • Any critically unwell patient / rapid electrolyte & lactate

Contraindications & cautions

  • Perform the Allen’s test (collateral flow) before radial sampling
  • Avoid: local infection, AV fistula, absent collateral circulation, severe coagulopathy

Equipment

Pre-heparinised ABG syringe + needleAntiseptic wipe, gauzeLocal anaesthetic (optional)Sharps bin; ice if delayed analysis

Step by step

Prepare
  1. 1Confirm indication, consent; check collateral flow (Allen’s test)
  2. 2Palpate the radial pulse at the wrist with the wrist slightly extended
Sample
  1. 1Clean the skin; insert the needle at 45–90° over the point of maximal pulsation, bevel up
  2. 2Arterial blood pulsates and self-fills the pre-heparinised syringe
  3. 3Withdraw and apply firm pressure for ≥5 minutes (longer if anticoagulated)
  4. 4Expel any air bubbles, cap, and analyse promptly (on ice if >10 min)
Interpret (systematic)
  1. 1Oxygenation: PaO₂ vs FiO₂ (expect PaO₂ ≈ FiO₂% − 10 in kPa). <8 kPa on air = respiratory failure
  2. 2Type 1 resp failure = hypoxaemia + normal/low CO₂ (V/Q mismatch); Type 2 = hypoxaemia + high CO₂ (hypoventilation)
  3. 3pH: acidaemia (<7.35) or alkalaemia (>7.45)
  4. 4Respiratory component: PaCO₂ (raised → respiratory acidosis; low → respiratory alkalosis)
  5. 5Metabolic component: HCO₃⁻ / base excess (low → metabolic acidosis; high → metabolic alkalosis)
  6. 6Compensation, and the anion gap in a metabolic acidosis (raised: DKA/lactate/toxins; normal: GI/renal HCO₃⁻ loss)

Complications

  • Haematoma, arterial spasm/thrombosis, nerve injury, pain (radial > brachial > femoral for safety)

OSCE tips

  • Allen’s test first — a classic marks item.
  • Firm pressure ≥5 min (arterial!).
  • Have a slick, systematic interpretation ready — state pH → resp → metabolic → compensation.

Read

Arterial Blood Gas (ABG) sampling — OSCE GuideGeeky Medics
ABG interpretationGeeky Medics

Sources & credits

Authored to UK finals standard and verified against UK guidance (UK reference ranges · BTS · Clinical practice) and Geeky Medics OSCE guides . Southampton teaching materials © University of Southampton. Always follow current local policy.