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Circulation - Arterial blood gas (radial)


Blood gas sampling of arterial pCO2 required


Hypercarbia confirmed by venous blood gas (pCO2 >45mmHg)

Contraindications (absolute in bold)

Proximal traumatic injury (absolute)

Deficient collateral circulation

Site infection



Venous blood gas sampling

Informed consent

Medical emergency

Consent is not required if the patient lacks capacity or is unable to consent

Brief verbal discussion is recommended if the situation allows


Verbal consent

Less complex non-emergency procedure with low risk of complications

Potential complications


Failure or sampling error (air in sample, venous blood, improper mixing, transportation delay)

Arterial injury (haematoma, haemorrhage, pseudoaneurysm, arterial dissection)

Nerve injury

Thrombosis and distal ischaemia


Procedural hygiene

Standard precautions

Aseptic non-touch technique

PPE: sterile gloves, surgical mask, eye protection, sterile ultrasound probe cover, sterile drape


Resuscitation bay or monitored acute bed


Procedural clinician


Ultrasound (recommended)

Rolled towel and tape (for arm positioning)

Drawing up needle for lignocaine, 25g needle for infiltration and 5ml syringe

Pre-heparinised 3ml blood gas syringe and 23g needle

Gauze and tape to cover the puncture site after collection


Supine with arm and wrist extension (radial artery)

Hand and wrist immobilised in mild dorsiflexion (rolled towel under wrist)


1-2ml lignocaine 1%

Sequence (insertion)

Palpate radial pulse or locate with ultrasound

Local anaesthetic infiltration with 25g needle

Massage area to disperse bleb of lignocaine and wait 1-2 minutes

Holding the syringe and needle like a dart, use the index finger or ultrasound to locate the pulse again

Insert 23g needle with attached heparinised syringe at 30-45 degrees to skin

Advance the needle into the radial artery until a blood flashback appears

Allow the syringe to fill to the appropriate level without pulling back the syringe plunger

Withdraw the needle and syringe

Place gauze over the site and have the patient or an assistant apply firm pressure for 3-5 minutes

Remove needle, expel air bubbles, cap the syringe and roll the specimen between the hands to gently mix it

Cap the syringe (preventing contact between the arterial blood sample and the air)

Post-procedure care

Test sample: Immediately at pint of care or transport on ice to lab (discard if >15 minutes delay to testing)

Confirm bleeding controlled after gauze removed

Document (completion, attempts and any immediate complications)


The radial artery is the preferred site due to accessibility and collateral circulation

Local anaesthetic relieves pain and reduces vasospasm aiding placement

Ultrasound can significantly improve first attempt success rate

Extended point pressure to the site may be required for patients with coagulopathy


Arterial blood sampling is a painful procedure which should be avoided whenever possible. Local anaesthetic is recommended, and an arterial line suggested if repeat sampling is likely.

The venous blood gas is an easy screening tool for excluding hypercapnia and hyperlactataemia. These values are always higher on a venous gas than an arterial sample. If a venous gas excludes hypercarbia and hyperlactataemia then an arterial gas is not required.

An arterial blood gas may be indicated when a venous blood gas demonstrates a pCO2 >45mmHg to assesses whether the cause is respiratory failure or a shocked state with poor tissue perfusion. The normal difference between arterial and venous pCO2 is 5mmHg, however this can increase markedly in shocked states as slow tissue perfusion allows more time for C02 loading into venous blood.

For example: a septic patient with an acute exacerbation of COPD is acidotic with a venous pCO2 70mmHg. An arterial blood gas (or arterial line) is indicated to assess whether the cause of hypercarbia is respiratory or poor perfusion (both of which can raise pCO2 on the venous gas).

Electrolytes values on venous blood gases are reliable.

Peer review

This guideline has been reviewed and approved by the following expert groups:

Emergency Care Institute

Please direct feedback for this procedure to


World Health Organization. WHO guidelines on drawing blood: best practices in phlebotomy. Geneva, Switzerland: WHO; 2010.125pp. Available from:

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Giner J, Casan P, Belda J, González M, Miralda RM, Sanchis J. Pain during arterial puncture. Chest. 1996;110(6):1443-1445. doi:10.1378/chest.110.6.1443

Melhuish TM, White LD. Optimal wrist positioning for radial arterial cannulation in adults: A systematic review and meta-analysis. Am J Emerg Med. 2016;34(12):2372-2378. doi:10.1016/j.ajem.2016.08.059

White L, Halpin A, Turner M, Wallace L. Ultrasound-guided radial artery cannulation in adult and paediatric populations: a systematic review and meta-analysis. Br J Anaesth. 2016;116(5):610-617. doi:10.1093/bja/aew097

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