Arterial Blood Gas Sampling Flashcards
What do ABGs assess?
pH, PaO2, PaCO2, HCO3, Base Excess (BE)
What do ABGs detect?
Hypoxaemia, hyper and hypocapnia, metabolic compensation, acid-base balance
What are the normal values for pH, PaO2, PaCO2, HCO3 and BE
pH - 7.35-7.45 PaO2 - 10-13kPa PaCO2 - 4.5-6.0kPa HCO3 - 22-28mmol/L BE - +2 to -2
What sites can an ABG be taken from?
Radial artery (most common)
Brachial artery
Femoral artery
Indications for an ABG?
Resp failure (acute and chronic)
Conditions leading to metabolic acidosis (cardiac/renal failure etc)
Patient in ICU needs monitoring (esp those on O2 therapy or mechanical ventilation)
Patient needs monitoring during cardiopulmonary surgery or anaesthesia
To determine prognosis of critically ill patient
Contraindications for ABGs?
Local infection or broken skin at proposed site, previous recent sampling from site, peripheral vascular disease, absent or poor collateral blood flow to hand, AV (arterio-venous fistula) in upper limb, surgery to axilla, anticoagulation medication, coagulopathies.
Equipment
- Gloves
- Apron
- Could wear mask (face and eye protection if patient is agitated)
- Alcohol swab
- Heparinised blood gas syringe
- Cotton wool
- Tape
- Patient request form
- Local anaesthetic (not for IACSS/DOCSS)
What test do you need to do before commencing the arterial blood gas sampling?
(Modified) Allen Test
How do you perform the Modified Allen Test?
- Wash hands
- Don gloves and apron
- Ask patient EXAMINER to make a tight fist
- Using your fingertips (not thumbs!), palpate BOTH arteries at the wrist and apply pressure to occlude them
- Ask patient to clench and unclench their fist several times
- Ask patient to unclench their fist - hand should appear blanched (maintain pressure on both arteries)
- Release the pressure on the ulnar artery (maintain pressure on radial)
- Observe the patient’s hand - should flush with colour within 5-10 seconds
- Repeat process with radial artery (i.e. releasing the radial artery to see if this one is working too)
In what way is the ‘Modified Allen Test’ modified?
The test is done on both arteries (ulnar and radial)
How would you ‘inform’ the patient about the Modified Allen Test?
‘I am going to carry out a quick test on your wrist to check the blood supply to your hand, before I take some blood from you. This will involve me pressing down firmly on your wrist and getting you to scrunch your hand into a fist a few times’
What should you do if the colour does not return to their hands within 5-10 seconds?
Radial artery should not be punctured on that side.
Performance of the task
- Gather equipment
- Wash hands with gel
- Don gloves and apron
- Complete modified Allen test
NOW ON EXAMINER… - Inspect (and ask them) wrist for contraindications (gloves still on) and correctly expose & position it - slight extension of wrist, pillow as support (state what you are doing)
- Palpate (using index and middle fingers of your non dominant hand) the radial artery slightly proximal to the intended puncture site
- Clean the site (make it clear you know not to re-palpate)
NOTE: Could leave wipe on skin with corner pointing to line of artery so don’t have to re-palpate - Unpackage syringe aseptically (leave cap in package but accessible)
- Pull plunger back to 1.5-1.6 ish (makes syringe self fill)
- Unsheathe needle
- Warn patient of pain
- Puncture the radial artery (angle = 45-90 degrees) - if angle is less than 90 degrees, bevel should be up - safer if less that 90 as can advance needle more safely.
- Slowly advance needle
- Stop advancing needle when see flashback
- Allow syringe to self fill - keep hand away from plunger so it can move back by itself
- Carefully withdraw needle and apply pressure immediately to puncture site with cotton wool
- Put pink protective guard over needle
- Ask patient or assistant to maintain pressure (state you would do this) for 5 mins
- Twist off needle from syringe and dispose of into sharps bin.
- Expel air from syringe (use plunger - be very careful - feather the plunger upwards - don’t push up too quickly in case air suddenly releases!)
- Cap the sample
- Roll/invert syringe to mix sample with heparin
- Apply tape over cotton wool (or sterile dressing)(ask about allergies)
- State “I would analyse this sample immediately, or if there is any delay, I would put the sample in ice”
Aftercare
- Take sample straight to lab/analyser (or in ice if delay)
- Make sure to take request form to analyser at same time
- Check patient’s welfare
What documentation must you complete?
In the patient’s notes:
- result of modified Allen test
- Site used
- Result of blood gas analysis
- FiO2/ flow rate at time sample was taken
- Any complications that occurred
