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Arterial Blood Gas


The arterial blood gas is useful in acutely unwell patients for assessing acid-base balance and oxygen status. This section outlines the approach for collecting an ABG.
Indications for arterial blood gas collection include to assess respiratory and acid base status, as well as for detection of carboxyhaemoglobin and methaemglobin.
Contraindications include patients with coagulopathy, as they are likely to bleed significantly; and inadequate collateral circulation.

Equipment and Preparation

  • The following equipment is required for collection of an arterial blood gas:
  • Positioning the Patient

    Raise the bed to waist level to allow for comfortable collection; the position of the patient depends on the artery to be used. 
  • Radial artery - ask the patient to place their arm outward with palm up, wrist extended slightly
  • Brachial- ask the patient to place their arm outward as far as possible with the antecubital fossa easily accessible
  • Femoral - ensure privacy, then expose the femoral triangle

Identifying an Artery

When looking for an artery, place the index and middle fingers over the desired artery and feel for the area of maximal impulse.
  • Potential sites for arterial blood gas collection are:
  • Radial artery - laterally on the anterior aspect of the forearmThe most commonly used site
  • Brachial artery - medially within the antecubital fossa
  • Femoral artery - centrally within the femoral triangle, between the inguinal ligament, adductor longus and sartorius
  • Tips for Choosing an Artery

  • Do

  • Try to use the radial artery if possible
  • Perform Allen's test prior to inserting the needle
  • Don't

  • Absolutely do not use the same arm as an AV fistula
  • Don't use the radial artery on a limb with a fractured wrist
  • Avoid areas with inadequate collateral circulation (e.g. peripheral vascular disease)
  • Avoid inserting a needle through cellulitis, burns or oedema
  • Allen's Test

  • This is a clinical test used to determine the patency of the radial and ulnar arteries, and is used as a predictor of distal ischaemia risk post radial arterial cannulation or catheterisation.
Ask the patient to clench their fist, and apply pressure to both the radial and ulnar arteries. Ask the patient to relax their hand, looking for blanching - if the hand does not blanch then both vessels are not occluded. Release pressure on the ulnar nerve, and look for flushing as the hand is reperfused.
If the hand does not flush in 10-15 seconds then the ulnar circulation is inadequate - this is a contraindication to radial puncture on this limb.

Specimen Collection

  • Introduction

  • Preparation

  • Collection

  • Finishing Up


Likely CauseWhat to Do
No flashbackNeedle not in the arteryInsert the needle slightly deeper, or withdraw slightly and reattempt at a slightly different angle
Needle is not self-fillingMay be a venous sampleWithdraw and try again
HaematomaNeedle not in the arteryWithdraw and put pressure on the area for at least five minutes


  • The potential complications of arterial blood gas collection include:
  • Haemorrhage / haematoma formation
  • Distal ischaemia
  • Nerve damage
  • Haemorrhage / Haematoma formation

  • Due to the high pressure within arteries, there is a high risk of bleeding or bruising around the site. 
In order to prevent bleeding, place pressure on the insertion site for 3-5 minutes once the needle is removed. Avoid performing an ABG on a patient with a coagulopathy where possible, or consider reversing anticoagulation / withholding it and waiting. 
If bleeding occurs, compress the site for 3-5 minutes the place a compression bandage over the area. Observe the site, and consider reversing anticoagulation if bleeding is major.
  • Distal Ischaemia

  • Ischaemia of a distal limb may occur following arterial blood gas sampling. This may present with pain, pallor, pulselessness, paralysis, paraesthesia or poikilothermia.
To prevent distal ischaemia, avoid performing an ABG in a limb with peripheral vascular disease and perform Allen's test before inserting the needle.
  • Nerve Damage

  • Very uncommonly, patients may develop numbness or paraesthesia as a result of nerve damage from an ABG. To prevent this, ensure that the pulse is easily palpable before inserting the needle, and avoid advancing the needle multiple times.
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