Arterial blood gas sampling

ABG sampling (particularly radial artery) can be extremely painful and so consider injecting 1 ml of 1% lidocaine, at the needle insertion site prior to sampling.

Indication

• Respiratory failure and directing care
• Post cardiac arrest and determining baseline

Provides

• pH of arterial blood
• partial pressure of both oxygen (PaO₂) and carbon dioxide (PaCO₂)
• oxygen saturation (SaO₂)
• bicarbonate (HCO₃) levels.

Preparation

• If possible wait at least 20 minutes after any change in O₂ therapy before sampling (to achieve a steady-state).
• Explain to the patient why you are doing the test, what it involves and the possible complications (bleeding, bruising, arterial thrombosis, infection and pain); then

Contraindication

• Poor collateral circulation
• Coagulopathy
• Severe distal peripheral vascular disease
• Obtain consent to proceed.

Equipment

• 2 ml Heparinised syringe with cap
• 20-22G needle (Blue needle) - a green needle may be more appropriate for the femoral artery
• Alcohol gel to skin prep
• Sharps disposal container, Gauze
• A plastic bung

Procedure

• Wash hands with soap and water and wear gloves
• identify artery - radial, femoral, brachial and palpate
• The radial is the usual choice using the non-dominant arm

Radial artery

• Perform a modified Allen test to ensure adequate collateral circulation from ulnar artery - this involves compressing both radial and ulnar arteries until the hand goes pale and then releasing the ulnar to see if the hand reperfuses.
• Position the patient's hand with palmar side facing up with the wrist extended 20-30 degrees and identify the radial artery by palpating the pulse
• Choose a site where the pulse is prominent. Clean the sampling site with an alcohol wipe.
• Expel the Heparin from the syringe. Steady your hand on the patient's hand, as shown, then insert the needle at 30 to 45 degrees directed towards the patient against the blood flow, keep the bevel facing up.
• Be gentle and insert needle slowly to minimise pain and the risk of arterial spasm.
• Once inside the artery there is a flashback of pulsating blood will appear in the barrel of the needle. Most ABG syringes will then fill under arterial pressure Obtain at least 3 mL of blood before withdrawing.

Arterial or venous

• venous is low pressure and won't usually self fill the syringe
• venous is dark and has a lower oxygen saturation

Post procedure

• Once a sample has been taken remove the needle and apply firm, direct pressure to the sample site for at least 5 minutes (and until bleeding has ceased).
• Dispose of all sharps and contaminated materials appropriately.
• Get rid of any air bubbles in the sample. Place the sample in ice especially if there is any delay. The sample should be analysed promptly.
• Record and document the fraction of oxygen the patient is taking at that time