The Procedure
Arterial blood gas (ABG) sampling is usually attempted at the radial artery, although the femoral artery is a good alternative.
Protocol:
1. Collect your equipment, introduce yourself to the patient, confirm their identity, explain what you are about to do, and obtain consent. Wash your hands before you start.
2. Allen Test: Always perform the Allen test to ensure that a satisfactory ulnar circulation is available for the hand. To do this, occlude both - the ulnar and the radial arteries, by applying adequate but gentle pressure with your index and middle fingers. Wait for a few seconds, and you will see the hand turning pale (as the veins drain the blood, but compression prevents significant refilling). Release the finger that is compressing the ulnar artery, but continue to keep the radial artery compressed. The normal pink hand colour should return in <5 seconds, indicating that there is sufficient collateral blood flow from the ulnar artery. This is a positive Allen test, and you may then proceed with the procedure. If the test is negative, then you shouldn't attempt arterial blood gas sampling from the radial artery.
3. Make sure the patient is comfortable. Place the patient in the following position, depending on the intended ABG sampling site and locate the artery:
Radial artery: Place their arm on a pillow for support, and extend the wrist. The radial artery is located on the lateral border of the volar surface of the distal wrist
Femoral artery: Lay the patient supine, with the groin and leg extended and slightly abducted. Remember the mnemonic 'NAVY' (Nerve Artery Vein Y-Fronts), from lateral to medial. The artery is located at the mid-inguinal point, halfway between the anterior superior iliac spine and pubic symphysis, 2 cm below the inguinal ligament.
4. It can be helpful to locate the artery prior to putting on your gloves. Always remember to retract the skin with your other hand prior to locating the artery (ensure you retract the skin both above and below the site of the ABG). Once you are happy with the site of the vessel, put on your gloves.
Practical tip: To help 'remember' the site of the artery, it can be useful to mark the skin by gently pressing your nail down into the patient's arm at the site of the vessel.
7. Optional: There are mixed views with regards to the use of local anaesthetic for this procedure - whilst ABGs are undoubtedly painful, the majority of the pain comes from the initial penetration of the skin. Once through the surface, the most painful part of the procedure is over. Whilst local anaesthetic infiltration can reduce the initial pain experienced, it is not a pain-free procedure in itself, and some argue that the anaesthetic will not make any difference to any 'deeper' pain experienced. For the purposes of the exam, it is probably best to say that you would infiltrate local anaesthetic. In 'real-life', discuss this option with the patient and offer them the choice, explaining that the anaesthetic may help, but will not make the procedure totally pain-free.
To infiltrate the anaesthetic, raise a small bleb of local anaesthetic at the proposed skin entry site with the small needle. To learn more about local anaesthetic doses, click on the 'anaesthetics' link on the right.
8. Once the anaesthetic has had time to take effect, once again retract the surrounding skin. Holding the heparin-coated ABG needle & syringe like a pen between your thumb and index finger, insert it at around 45o (or less) to the skin (radial sample). At the femoral artery, use a greater angle (around 60-90o). Whilst most books state angles of around 90o for radial artery sampling, in reality it is easier to take a shallower angle at this site, as this minimises accidentally going straight through the artery.
9. With a finger of the other hand palpating the vessel on either side of the 'target', insert the needle, initially with a firm action to ensure that the skin is penetrated in 'one go'. This part is very painful, and the skin can also be surprisingly 'tough', especially in young patients (or very elastic in older patients). In order to, both, minimise pain and to ensure that the needle actually goes through the skin, make sure that this first action is a controlled and swift manoeuvre. Going in hesitantly will make things more painful for the patient and more frustrating for you, as the needle will just fail to go through the skin!
10. If you do not find the artery straight away, it is possible to 'move around' to try and find the vessel. You may need to partially retract the needle then push it further in (although bear in mind that this can be painful). Once you enter the vessel, hold still, and the ABG syringe will fill automatically. Wait until around 1ml has been filled into the syringe.
11. Withdraw the needle and apply firm pressure over the area with the cotton wool ball for 5 minutes. Expel any air bubbles in the sample, and apply the cap. Make sure you use the specially designed part of the lid of the sharps bin to remove the needle. DO NOT remove it with your hands. Keep the syringe cool if there will be a delay until you get the sample analysed.
12. Thank the patient, and clear up. Ensure you dispose of all of your sharps at once. Take the sample to the nearest blood gas analyser, and follow the necessary instructions to get the sample calibrated and analysed. Wash your hands.
