march-2026-abg-station Arterial Blood Gas Sampling — OSCESup
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Procedure April 2026

Arterial Blood Gas Sampling

Patient Mr David Carr
Age / Sex 64M
Setting Medical Admissions
Year Group Y2 / Y3
Duration 10 minutes

Clinical Scenario

Candidate Briefing

Mr David Carr is a 64-year-old man with a known history of COPD (MRC grade 3, on home nebulisers and tiotropium) who has been brought in by ambulance with a 48-hour history of worsening breathlessness and a productive cough with green sputum. He smokes 20 cigarettes per day and has done so for 40 years.

On arrival he appears distressed and is using his accessory muscles. He is currently on room air.

SpO₂ 86% (air)
RR 26 /min
HR 108 bpm
BP 136/82 mmHg
Temp 38.1 °C
GCS 15/15

The registrar has asked you to perform a radial arterial blood gas sample and interpret the result. A simulated arm is provided. The examiner will provide you with a printed ABG result after the procedure.

Allen's Test

Must be performed before radial ABG — assesses ulnar collateral circulation

  1. 1. Ask the patient to clench their fist tightly. Compress both the radial and ulnar arteries simultaneously with your thumbs until the palm blanches.
  2. 2. Ask the patient to open their hand (not fully extended — a partially open hand avoids false positives).
  3. 3. Release the ulnar artery only. The palm should flush pink within 5–7 seconds — this confirms adequate ulnar collateral circulation.
  4. 4. Positive (normal) test: hand flushes within 5–7 seconds → safe to proceed with radial puncture.
  5. 5. Negative (abnormal) test: hand remains pale or takes >10 seconds → poor ulnar collateral supply → do NOT use this radial artery → use the other wrist or an alternative site (femoral, brachial).

Equipment Checklist

Non-sterile glovesCorrect size
Pre-heparinised ABG syringe kitSelf-filling; most modern kits are pre-heparinised and self-venting
23G (blue) needleFiner gauge reduces patient discomfort at radial artery
Alcohol wipe70% isopropyl · Allow 30 sec to dry
Gauze / cotton wool
Adhesive plaster / tape
Sharps binWithin reach before starting
Rolled towel or wrist supportTo dorsiflex the wrist and expose the radial artery
Patient label + specimen bagLabel at bedside immediately after collection
Ice / cold water bagOnly if analysis delayed >15 min; most modern analysers don't require ice

Mark Scheme

Competency Marks Awarded
Introduction & Consent
Introduces self (name and role); confirms patient name and date of birth against wristband 1
Explains the procedure accurately, including that it is more painful than venepuncture; obtains verbal consent
e.g. "I need to take blood from an artery in your wrist — it's a bit more uncomfortable than a standard blood test, but it gives us very important information about your breathing"
 1
Allen's Test
Performs Allen's test — correct technique: compresses both radial and ulnar arteries simultaneously, patient clenches fist, hand blanches 1
Releases ulnar artery only; correctly interprets result (flush within 5–7 sec = positive = safe to proceed) 1
Preparation
Gathers all equipment; places sharps bin within reach; positions wrist in dorsiflexion over rolled towel 1
Performs hand hygiene (6-step technique) and applies non-sterile gloves 1
Technique
Locates the radial pulse with the index and middle fingers of the non-dominant hand; confirms pulsatile flow 1
Cleans the skin with alcohol wipe; allows minimum 30 seconds to dry 1
Warns the patient; inserts needle bevel-up at 30–45° angle, directed toward the pulse
Steeper angle than venepuncture — the radial artery is deeper and pulsatile entry is more reliable at 30–45°
 1
Arterial blood fills the syringe passively (pulsatile flash) without aspiration; collects 1.5–2 ml 1
Withdraws needle in one smooth motion; immediately applies firm pressure with gauze for a minimum of 5 minutes
10 minutes if patient is anticoagulated or has a coagulopathy
 2
Disposes of needle directly into sharps bin without re-sheathing 2
Post-Procedure
Expels any air bubbles from syringe immediately; caps the syringe 1
Labels sample at the bedside (patient name, DOB, hospital number, date, time, FiO₂ at time of sampling) 1
States sample must be analysed within 15 minutes at room temperature (or placed on ice if delayed) 1
Documents procedure in notes; checks pressure site after 5 minutes for haematoma or ongoing bleeding 1
ABG Interpretation (see result below)
States pH and correctly identifies acidosis (pH 7.31 < 7.35) 1
Identifies the primary respiratory cause (PaCO₂ 8.4 kPa — raised → respiratory acidosis) 1
Identifies metabolic compensation (HCO₃⁻ 29 mmol/L — raised → kidneys retaining bicarbonate to compensate) 1
Identifies hypoxaemia (PaO₂ 7.1 kPa on room air — below normal range of 10–13 kPa) 1
Correct conclusion and immediate management: Type 2 respiratory failure; start controlled oxygen 24–28% via Venturi mask; target SpO₂ 88–92% in known COPD 2
Total 23 marks

ABG Result

ABG — Mr David Carr · 64M · Room air (FiO₂ 21%) Print this card and hand to candidate after procedure
pH
7.31
Ref: 7.35–7.45
↓ Low
PaCO₂
8.4 kPa
Ref: 4.7–6.0 kPa
↑ High
PaO₂
7.1 kPa
Ref: 10.0–13.3 kPa
↓ Low
HCO₃⁻
29 mmol/L
Ref: 22–26 mmol/L
↑ High
Base Excess
+4.5
Ref: −2 to +2
↑ High
SaO₂
88%
Ref: 95–100%
↓ Low
Lactate
1.2 mmol/L
Ref: <2.0 mmol/L
Normal
FiO₂
0.21
Room air
—

Interpretation

1
Is the pH normal?
Normal range 7.35–7.45. pH 7.31 is below the lower limit.
→ Acidosis
2
What is the primary cause?
PaCO₂ is raised (8.4 kPa; normal 4.7–6.0). Raised CO₂ = respiratory acidosis. HCO₃⁻ is also raised — but is this the cause or compensation?
→ Primary respiratory acidosis (raised PaCO₂ drives the pH down)
3
Is there compensation?
HCO₃⁻ is raised at 29 mmol/L and BE is +4.5 — the kidneys are retaining bicarbonate to buffer the acidosis. This is chronic metabolic compensation, suggesting this patient has long-standing CO₂ retention (consistent with COPD).
→ Chronic metabolic compensation (partial — pH still acidotic, not fully corrected)
4
What is the oxygenation status?
PaO₂ 7.1 kPa on room air (normal 10–13.3 kPa). Type 1 respiratory failure = hypoxaemia alone (PaO₂ <8 kPa, normal CO₂). Type 2 = hypoxaemia + hypercapnia (PaCO₂ >6 kPa).
→ Type 2 respiratory failure (hypoxaemia + hypercapnia)
5
Summary & clinical context
Respiratory acidosis with partial metabolic compensation, hypoxaemia, and hypercapnia — consistent with an acute-on-chronic COPD exacerbation.
→ Type 2 respiratory failure on a background of chronic CO₂ retention (COPD)
Immediate Management
  • Controlled oxygen: start 24% or 28% Venturi mask — aim SpO₂ 88–92% (not 94–98%) to avoid suppressing the hypoxic drive in chronic CO₂ retainers.
  • Reassess: repeat ABG in 30–60 minutes to assess response to oxygen therapy.
  • Bronchodilators: back-to-back nebulised salbutamol 2.5 mg and ipratropium 500 mcg.
  • Steroids: prednisolone 30–40 mg PO (or IV hydrocortisone if unable to swallow).
  • Antibiotics: if infective exacerbation (fever, purulent sputum) — co-amoxiclav or doxycycline per local guideline.
  • Consider NIV (BiPAP): if pH <7.35 with PaCO₂ >6 kPa and not improving after 1 hour of medical therapy — refer to ITU/respiratory team early.
  • Escalation plan: discuss ceiling of care and NIV suitability with senior early.

Examiner Questions

Why do we use a Venturi mask at 24–28% rather than high-flow oxygen for this patient?
Mr Carr is a known COPD patient with chronic CO₂ retention (evidenced by the raised HCO₃⁻ and BE). Some of these patients have shifted to rely partly on hypoxia to drive their ventilation (hypoxic drive). High-flow oxygen can blunt this drive, worsen hypercapnia and precipitate respiratory arrest. The target SpO₂ in known CO₂ retainers is 88–92%, not the standard 94–98%. A Venturi mask delivers a precise, controlled FiO₂ regardless of flow rate.
What is the difference between Type 1 and Type 2 respiratory failure?
  • Type 1: PaO₂ <8 kPa with normal or low PaCO₂ — pure hypoxaemic failure. Causes: pneumonia, PE, pulmonary oedema, ARDS, pneumothorax.
  • Type 2: PaO₂ <8 kPa with PaCO₂ >6 kPa — hypoxaemia plus hypercapnia (ventilatory failure). Causes: COPD exacerbation, severe asthma, neuromuscular disease, obesity hypoventilation, chest wall deformity.
The ABG result comes back and you notice the PaO₂ is very high (e.g. 25 kPa). What might have happened technically?
Air contamination of the sample — if air bubbles are not immediately expelled after collection, atmospheric oxygen (PO₂ ~20 kPa) equilibrates with the sample and causes a falsely elevated PaO₂ and falsely low PaCO₂. This is why expelling bubbles and capping the syringe immediately is a mark-scheme item. The sample should also be analysed promptly, as ongoing cellular metabolism will falsely lower PaO₂ over time.
After 5 minutes of pressure you notice a large firm haematoma forming at the wrist. What do you do?
Continue firm manual pressure for a further 5–10 minutes — do not release too early. If the haematoma is expanding rapidly, consider whether the patient is on anticoagulants (this patient is not, but always check). Elevate the limb. If bleeding is not controlled with sustained pressure, seek senior help — a compressive bandage may be applied, but must not be occlusive. Document the complication and reassess neurovascular status of the hand distally (sensation, capillary refill, radial pulse).
Why is the FiO₂ at the time of sampling clinically important?
The PaO₂ result is only interpretable in the context of how much oxygen the patient was receiving. A PaO₂ of 10 kPa is acceptable on room air (FiO₂ 0.21) but would indicate significant respiratory impairment on 60% oxygen. The alveolar-arterial (A-a) gradient can be calculated to quantify this. Always document and label the FiO₂ on the sample — without it, the PaO₂ value is clinically meaningless.

Safety Points

Fail Skipping Allen's test before radial ABG is a patient safety error. If ulnar collateral circulation is inadequate and the radial artery goes into spasm or thromboses, the hand may become ischaemic. Always check first.
Fail Re-sheathing the needle or placing it on a surface is an automatic fail. Directly into the sharps bin after withdrawal.
Key Point Applying inadequate pressure after arterial puncture causes haematoma — the most common complication. Five minutes of firm, uninterrupted pressure is the minimum. Do not peek too early. If the patient is on anticoagulants, apply pressure for 10 minutes or longer.
Key Point Actively aspirating the syringe (pulling the plunger back) is incorrect technique — arterial blood should fill the syringe passively due to arterial pressure. Aspiration risks haemolysis and venous contamination.
Top Tip When interpreting the result aloud to the examiner, use a structured approach every time — pH → CO₂ → HCO₃ → oxygenation → summary. This signals systematic thinking and is much harder to get wrong than jumping straight to a conclusion.