Arterial Blood Gas

Question 1

What are the main indications for arterial blood gas sampling?

Answer 1

• assess oxygen levels and respiratory function
• suspected hypoxia and/or low saturation levels
• check acid / base balance
• check baseline CO2 for patients who are at risk of retaining CO2

Question 3

What are other indications for arterial blood gas sampling?

Answer 3

• airway issues - once the airway is clear
• metabolic derangement - i.e. in DKA
• point of care electrolytes / Hb / lactate results whilst waiting for serum lab results

Question 4

What are contraindications and precautions before arterial blood gas sampling?

Answer 4

• local infection
• distorted anatomy or previous surgical treatment to the artery
• presence of arteriovenous (AV) fistulas
• peripheral vascular disease of the limb to be sampled
• severe coagulopathy or recent thrombolysis
• conditions that affect patency and reactivity of small arteries (e.g. Reynauds)

Question 5

When should you check local guidelines before performing an ABG?

Answer 5

check local guidelines if the patient is on warfarin or IV heparin

Question 6

What are 3 important complications that are important to consider in arterial blood gas sampling?

Answer 6

• discomfort and pain
• infection
• possible ischaemia in patients, particularly those with poor collateral flow

Question 7

Why is ABG more painful than venepuncture?

What steps should be taken to try and minimise pain and discomfort?

Answer 7

it is more painful due to the proximity of nerve fibres near the puncture site

some advocate the use of local anaesthetic

care should be taken when choosing the puncture site and avoid redirecting the needle once it has punctured the wrist

most trusts advocate no more than 3 attempts at each site

Question 8

How can the risk of infection be minimised when performing an ABG?

Answer 8

the procedure must be performed under strict asepsis

local guidelines vary but will either be performed aseptically (using sterile gloves, a sterile field and opening sterile equipment into the field)

or aseptic non-touch technique (ANTT) where the key parts of the equipment are not touched

Question 9

What test must be performed prior to ABG to ensure the patient has adequate collateral flow?

Why?

Answer 9

an Allen’s test must be performed prior to ABG to ensure the patient has adequate collateral flow

this is due to the risk of possible ischaemia

Question 10

What is involved in Allen’s Test?

What is its purpose?

Answer 10

it is used to find out if the blood flow to the hand is adequate whenever intravascular access to the radial artery is planned

if the radial artery was to spasm during the procedure then we need to make sure that the hand is adequately perfused by the ulnar artery

Question 11

What 7 things are needed to perform an ABG?

Answer 11

• an indication for the test
• an assistant
• PPE
• dressing trolley or clear surface to arrange sterile field / equipment
• kit
• blood gas analysing machine
• a patient who has consented to, and is suitable for the procedure

Question 12

What is meant by it being essential that you have an “indication” for performing an ABG?

Answer 12

it is important to understand why you are performing this test

this must be documented in the patients notes

Question 13

Why is an assistant required when performing an ABG?

Answer 13

to help position the patient’s wrist and make the patient comfortable afterwards

it is important that adequate pressure is applied after the procedure for an appropriate amount of time to facilitate adequate clotting

this should be at least 5 minutes

patients themselves do not usually apply enough pressure or for long enough

Question 14

What equipment / kit is needed to perform an ABG?

Answer 14

• sharps bin
• PPE
• pre-heparinised syringe
• skin cleansing agent

Depending on local protocol, these may also be needed:

• sterile field - e.g. within sterile dressing pack
• sterile gloves - these may be in the sterile pack

Always check the integrity of packaging & expiry dates before opening

Question 15

How should the patient be positioned for an ABG and why?

What should already have been done prior to starting the procedure?

Answer 15

the patient should be relaxed with their wrist fully extended

they ideally should be supine in the case of a drop in blood pressure / vasovagal event

you should have already performed an Allen’s test on the patient and have palpated / located their radial artery

Question 16

How should an ABG be performed from first meeting the patient to preparing the ABG equipment?

Answer 16

• wash hands
• introduce yourself, including your name and role
• perform positive patient identity check
• explain the procedure and allow patient to give their consent
• perform Allen’s test
• clean hands with alcohol gel
• position patient’s arm with wrist extended and palpate radial artery

Question 17

What stages are involved in performing an ABG after palpating the radial artery up to inserting the needle?

Answer 17

• clean the puncture site with ChloraPrep using crosshatch technique for 30 seconds and allow to dry
• prepare sterile dressing trolley and open all equipment
• gel hands and apply sterile gloves
• prepare ABG syringe
• fix artery between index and middle fingers of non-dominant hand being careful not to de-sterilise gloves
• warn patient to expect a scratch

Question 18

What are the stages involved in ABG from inserting the needle to disposing of the needle and equipment?

Answer 18

• insert needle at an angle, in the opposite direction to blood flow, until you obtain pulsatile flashback
• allow syringe to fill with 2ml of blood
• withdraw needle, placing gauze over site
• apply firm pressure for at least 5 minutes (longer if coagulopathy or on anticoagulants)
• dispose of needle, replace with filter cap and expel any air from syringe holding the syringe vertically
• thank the patient, tidy away and dispose of equipment into the relevant waste

Question 19

What should be done with the ABG sample after equipment has been disposed of?

Answer 19

• the sample should immediately be sent for analysis
• ensure it is labelled with patient details, date, time, inspired O2 and temperature
• do not dispose of the sample until analysis is complete

Question 20

What maximises the chances of getting an arterial sample?

Answer 20

careful location of the radial artery before performing the procedure maximises chance of getting an arterial sample

Question 21

How much blood do you need to aim to get within the ABG sample?

Answer 21

aim to get around 2ml of blood

anything less than 1ml will be difficult for the machine to analyse and the test may need to be repeated

Question 22

How should the syringe fill once it is in the artery?

Answer 22

when in an artery the syringe should self-fill in a pulsatile motion

if you need to aspirate to get a sample it could indicate that you are in a vein

Question 23

What should be documented about the procedure after it has been performed?

Answer 23

• which limb was sampled
• Allen’s test result
• consent
• number of attempts
• the results, your general interpretation of the results and your plan for the patient

Question 24

What are normal ABG results for pH and PaO2?

Answer 24

pH of 7.35 - 7.45

PaO2 of 11 - 13 kPa

this is normal range for a patient breathing room air (21% O₂)

Question 25

What are normal ABG results for PaCO2, HCO3 and base excess?

Answer 25

PaCO2 of 4.7 - 6 kPa

HCO3 of 22 - 26 mmol/L

base excess of -2 mmol/L to +2 mmol/L