Arterial blood gas

Introduction

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.

Sampling arterial blood is fairly invasive and can be a lot more painful than venepuncture. As a result, it is important to clearly communicate the details of the procedure and its necessity. For example “I need to take a sample of blood to see how acidic it is and what your oxygen levels are like. I will need to take the sample from an artery in your wrist, rather than from a vein, and it will involve a small needle with a sharp scratch. Some patients find that this can be quite painful, so if you wish I can give you something to numb the pain. I will of course do my best to be quick and will talk through what I am doing as I go along.”

Equipment

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:

Note that in most UK hospitals, the syringe, needle and bung come pre-packaged in an arterial blood gas collection set.

For anaesthesia

If you choose to give local anaesthesia, then in addition to the above, collect:

Administration of local anaesthesia can make it difficult to repalpate the radial artery. So should you choose to use this, ensure that you are confident with the course of the artery

Inspection

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.

A history and examination are naturally crucial to interpretation of an arterial blood gas. In a stable patient, you will already have performed these. However, in scenarios where there isn’t time (for example in resuscitation or an acutely deteriorating patient), or if you feel patient that the patient’s condition has changed since their last examination, a quick inspection is essential to properly interpret your findings.

Palpation

Radial pulse

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.

If you are right-handed, it is usually easier to sample from the patient's right wrist. The needle will enter the artery pointing proximally, and so if you are positioned on the right of the patient you will have more room for your wrist and elbow to move. The reverse is true for left-handed practitioners.

To identify the pulse, we suggest using three fingers to lightly palpate until your fingers are parallel to the artery, immediately superior to it. You should be able feel the pulse in each finger. Then, lift your palm up such that you can feel the pulse with just your fingertips such that you are able to form a clear, visual picture of how the artery runs under the skin.

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. 1. Ask the patient to elevate their hand above their heart and form a tight fist.
  2. 2. Locate the radial and ulnar arteries and compress them using the index and middle finger of each of your hands.
  3. 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. 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).

Preparation

Positioning

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.

Ideally you should rest the patient’s arm on a pillow with the forearm supinated and the palm facing upwards.

Repalpation

Repalpate the radial artery until you are comfortable with its course.

Procedure

Clean the area with an alcohol wipe.

Local anaesthetic

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.

Needle insertion

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.

Note that it is easy to miss the artery. If you do this, you will need to withdraw your needle back slightly to see if you can readjust. This is why it is important to visualise the course of the artery, as it can give you an idea of where to direct your needle.

Sample collection

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.

If they are able to, ask the patient to take over applying the pressure to the area such that you are able to focus on safely disposing of the needle. The pressure will need to be maintained for up to 5 minutes before the bleeding stops.

Completion

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.

Ideally, prepare any paper work before you start the procedure as you will need to take your sample for analysis and may forget crucial details.

Interactive markscheme


When assessing each other, click on each list item as you go along. Doing so will turn the list item green. Make careful note of any steps missed at the end.
We recommend completing any examination or procedure in under 10 minutes, but you can adjust the timer to suit your needs.
:

  1. Introduction: “Hello. I’m SimpleOSCE and I’m a medical student. Could I confirm your full name and DOB?”
  2. Explain procedure and obtain consent: “I need to take a sample of blood to see how acidic it is and what your oxygen levels are like. I will need to take the sample from an artery in your wrist, rather than from a vein, and it will involve a small needle with a sharp scratch. Some patients find that this can be quite painful, so if you wish I can give you something to numb the pain. I will of course do my best to be quick and will talk through what I am doing as I go along.”
  3. Wash your hands.
  4. Enquire about allergies (latex), medication (anticoagulants) and whether the patient has had any surgery recently.
  5. Perform a brief inspection and check if the patient is on oxygen therapy.
  6. Optional: Determine whether or not the patient needs local anaesthesia.
  7. Assemble the correct equipment in to a cleaned tray.
  8. Check the expiry dates of the equipment.
  9. Palpate the radial pulse, identifying the point of strongest pulsation.
  10. "Track" the pulsation proximally visualise the course of the radial artery.
  11. Perform the modified Allen’s test.
  12. Wash your hands again and put on a pair of non-sterile gloves.
  13. Prepare your equipment and flush the heparin through the needle.
  14. Position the patient’s hand appropriately, ideally on a pillow with the wrist slightly extended.
  15. Clean the area with an alcohol swab and wait for 30s.
  16. Optional: Inject local anaesthetic subcutaneously at the site of strongest pulsation.
  17. Palpate the radial pulse just distal to the point of strongest pulsation.
  18. Warn the patient about a sharp scratch.
  19. Insert the needle at a 30-45° angle and observe flashback of blood.
  20. Allow syringe to fill and then withdraw the needle.
  21. Apple gauze to the site and apply firm pressure.
  22. Dispose of the needle safely in the sharps bin.
  23. Cap the syringe and remove any air.
  24. Send the blood for sample analysis.
  25. Instruct the patient to apply firm pressure to the gauze for at least 5 minutes.
  26. Record in the notes the results, time of sampling, whether the patient was on oxygen and whether they had a fever.
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