Wash your hands, introduce yourself with your name and role, and confirm the patient's full name and date of birth. Explain the procedure, check the patient's understanding and obtain consent.
Identify a strong radial pulse. This will be the wrist you will be sampling from. Ensure that it is adequately exposed.
Ask the patient if they have any allergies, specifically to latex. Also ask whether they are taking any medications, specifically anticoagulants, and if they have had any surgery recently that may affect the arm you take blood from.
Ask the patient if they are in any pain and are comfortable. Check for any use of accessory breathing muscles, breathing symmetry and lung hyperexpanstion, as this may give clues to the patient’s oxygen and CO2 status. Carefully note the colour of the patient. They may show signs of bronchitis (cyanosis, blue lips) or carbon monoxide poisoning (cherry red lips). Assess the age of the patient and note any paraphernalia around the bedside, looking for clues such as cigarettes, oxygen, inhalers, nebulisers, sputum pots or ventilators.
If the patient is on any oxygen therapy, check how much this is and for how long they have been on it, as this will impact interpretation of your results.
Collect a plastic tray, and clean both the inside and outside of the box thoroughly with an alcohol surface disinfectant wipe. Determine whether or not you think you will need local anaesthesia for the procedure. You will then need to collect a number of items. Ensure that they are all in date.
For arterial blood sampling
Collect the following equipment:
- A pair of non-sterile gloves.
- Alcohol wipe (2% chlorhexidine in 70% alcohol).
- A pre-heparinized syringe.
- Needle (23G).
- A plastic bung.
- Sharps bin.
- Gauze or cotton wool.
- Sterile tape or plaster.
If you choose to give local anaesthesia, then in addition to the above, collect:
- At least 1mL 1% lidocaine solution drawn up in a syringe.
- Subcutaneous needle (25–27 gauge).
Modified Allen's test
Before attempting to sample the radial artery a modified Allen’s test should be carried out to determine the competency of the ulnar artery. This is performed as follows:
- 1. Ask the patient to elevate their hand above their heart and form a tight fist.
- 2. Locate the radial and ulnar arteries and compress them using the index and middle finger of each of your hands.
- 3. Ask the patient to release their fist, keeping their hand elevated. The palm should appear white and pale due to the now obstructed blood supply.
- 4. Release the hand blocking the ulnar artery, allowing blood to flow through it once more.
If colour appears back into the hand within 5-15s, the ulnar artery has sufficient enough blood flow to compensate should the radial supply be compromised. This indicates that it is okay to take an arterial blood sample from the radial artery of this hand.
If colour does not appear back into the hand after 15s, the ulnar circulation to the hand is inadequate and the radial artery should not be compromised. If this is the case, then you must either consider taking a sample from the other hand (after conducting a separate Allen’s test) or another site (such as the femoral artery).
Put on your apron and remove all equipment from its packaging so it is ready to use. Position yourself and the patient so that you are both sitting comfortably, and the patient’s wrist is exposed. Ask the patient to slightly extend their wrist. Expose the patient’s wrist adequately.
Locate the radial artery by palpating the radial pulse. Track the pulsation towards its strongest point whilst trying to visualise the course of the radial artery. Remember this location for later.
Before injecting the anaesthetic, confirm with the patient that they have had no previous reactions to local anaesthetic. Using proper technique, inject up to 0.5mL of lidocaine solution subcutaneously at the site, remembering to draw back initially for blood.
If not already, attach your needle to the pre-heparinised syringe. Flush the heparin through the needle.
Palpate the radial artery with your non-dominant hand. This time start from further away and stop just before the area where you previously felt the strongest pulsation. This is the insertion site, and you want to avoid touching it as much as possible.
Stretch the skin slightly with your non-dominant hand and inform the patient that they may feel a sharp scratch (or a pushing sensation if they have received anaesthetic).
Holding the syringe needle like a dart using your thumb, index and middle fingers. At a 30-45o angle, pierce the skin at the insertion site. Once you have entered the radial artery you will feel a slight give, observe some bubbling or see flash back.
The arterial pressure should cause your syringe to fill up quickly, if not start to draw the syringe back slightly.
Once you are satisfied with your sample, carefully withdraw the needle and apply firm pressure to the area with some gauze or cotton wool to stop the bleeding. Immediately cover the needle with the plastic bung. Apply tape to keep the gauze in place.
Discard the needle straight away into a sharps bin. Cap the syringe, push out any air and remove your gloves.
Send your sample for analysis as soon as possible. If you foresee a delay, you may wish to keep it in ice.
Thank the patient and ensure that they are comfortable. Record the time that you took the arterial blood sample in the notes, whether or not the patient was on oxygen, and whether or not they had a fever.