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.
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.
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).
Ask the patient if they are in any pain and are comfortable. Check for any use of accessory breathing muscles, breathing symmetry and lung hyperexpansion, 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.
Palpate the radial pulse on either wrist, just lateral to the tendon of the flexor carpi radialis. Identify the site of strongest pulsation. Remember this location for later. Track the pulsation proximally visualise the course of the radial artery.
Ensure that the wrist is adequately exposed.
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).
Once you are satisfied with the artery, put on your apron and remove all equipment from its packaging so it is ready to use. Attach your needle to the pre-heparinised syringe and flush the heparin through the needle. Wash your hands and put on your non-sterile gloves. Position yourself and the patient so that you are both sitting comfortably. Ask the patient to slightly extend their wrist such that the skin over the artery is as taught as possible.
Repalpate the radial artery until you are comfortable with its course.
Clean the area with an alcohol wipe.
If injecting an anaesthetic agent, confirm with the patient that they have had no previous reactions to local anaesthesia. Using proper technique, inject up to 0.5mL of lidocaine solution subcutaneously at the site of strongest pulsation, remembering to draw back initially for blood.
Palpate the radial artery with your non-dominant hand just distal to the point of strongest pulsation.
Stretch the skin slightly with your dominant forearm by dorsiflexing the wrist 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 the thumb, index and middle fingers of your dominant hand. At a 30-45° angle, pierce the skin at the insertion site. Continue to advance the needle. Once you have entered the radial artery you will feel a slight give, and blood should rush back into the syringe.
The arterial pressure should cause your syringe to fill up quickly, if not, it is possible you have gone through the artery (most likely) or are not deep enough.
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.
Dispose of 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 results and 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.