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
Venous blood gas sampling
Consent is not required if the patient lacks capacity or is unable to consent
Brief verbal discussion is recommended if the situation allows
Less complex non-emergency procedure with low risk of complications
Failure or sampling error (air in sample, venous blood, improper mixing, transportation delay)
Arterial injury (haematoma, haemorrhage, pseudoaneurysm, arterial dissection)
Thrombosis and distal ischaemia
Aseptic non-touch technique
PPE: sterile gloves, surgical mask, eye protection, sterile ultrasound probe cover, sterile drape
Resuscitation bay or monitored acute bed
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%
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)
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.
This guideline has been reviewed and approved by the following expert groups:
Emergency Care Institute
Please direct feedback for this procedure to ACI-ECIs@health.nsw.gov.au.
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