arterial blood gas procedure

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Arterial Blood Gas Obtaining an ABG may be necessary for patients in a critical condition or with a respiratory disease. A sample of arterial blood is taken, usually from the radial or femoral artery, and is analysed by a blood gas analyser. The blood gas analyser draws a number of important readings from the blood sample:

• pH (acidity/alkalinity) • PaO2 (partial pressure of oxygen) • PaCO2 (partial pressure of carbon dioxide) • SaO2 (oxygen saturation) • HCO3 (bicarbonate).

This guide explains how to draw an arterial blood sample from the radial artery.

Check ID and obtain Informed Consent

Introduce yourself to the patient by clearly stating your name and role. Ask the patient their name and date of birth and check this information against their hospital wristband or equivalent. Explain why you need to perform the procedure and what the procedure will involve before obtaining verbal consent from the patient. Ask the patient if they would like any local anaesthetic: "Hello, my name is James Brooks. Can I ask you your name? I am a medical student

and I have been asked to take a sample of your blood so that we can know more about how much oxygen and carbon dioxide is in your blood. To do this, I will need to insert a needle into an artery in your wrist; this is a bit more painful than a normal blood test, but if you prefer we can give you an injection of local anaesthetic to numb the area beforehand. Is that okay with you?"



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Check the Patient's Blood Coagulation Status

Check in the patient's notes for their coagulation screen and platelet count. Ask the patient and check their prescription chart to see if the patient is taking any anticoagulation medication such as warfarin or clopidogrel. If the has patient impaired coagulation, you can still perform arterial blood sampling so long as pressure is applied over the puncture site for a sufficient duration for clotting to occur. You should also ask the patient if they have any allergies: "Are you on any blood thinning medications, such as warfarin or clopidogrel?

Are you allergic to anything?"

Perform the Modified Allen's Test

Before and after performing the modified Allen's test you must wash your hands using soap and water or alcohol gel. The modified Allen's test is always performed prior to taking a sample from the radial artery to ensure there is adequate collateral supply to the hand from the ulnar artery in case of injury to the radial artery during the blood test. Ask the patient to rest their arm on a flat surface. Use two or three fingers from each hand to compress both the radial and ulnar arteries whilst asking the patient to clench and unclench their fist until the palmar skin blanches. Upon palmar blanching, release pressure from the ulnar artery and ask the patient to relax their hand. If colour returns to the hand within 10 seconds: this is a positive modified Allen's test and you may proceed with taking an arterial blood sample from this wrist. If colour does not return to the hand within 10 seconds: this is a negative modified Allen's test and you must not obtain an arterial blood sample from this wrist. Instead, try the Allen's test on the patient's other arm.



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The Modified Allen's Test

Compress the radial and ulnar arteries until palmar blanching (whitening) occurs.

Release pressure on the ulnar artery and ask the patient to relax their hand. Ensure

pressure is maintained on the radial artery.



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Prepare to Obtain an Arterial Blood Sample

Before opening any packaging, wash your hands thoroughly using soap and water or alcohol gel.

Empty all required equipment from its packaging into a clean container. You must check all expiry dates and the integrity of all packaging. You will need:

• Chlorhexidine/alcohol swab • Sterile gloves • ABG needle • Sterile gauze and tape • Thermometer (if recent body temperature not already recorded in charts) • Ice (if you are more than 10 minutes away from a blood gas analyser) • Alcohol hand gel

A clean tray containing (L-R): chlorhexidine/alcohol swab, sterile gloves, ABG needle &

sterile gauze.

Check the patient's temperature using a thermometer or by referring to observation charts. If the patient is currently prescribed oxygen via a mask or nasal cannula, record the concentration of oxygen being delivered.



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Obtain an Arterial Blood Sample

Using a chlorhexidine/alcohol swab, clean the skin overlying the radial artery and allow to dry for 30 seconds.

Wash your hands using alcohol gel and apply sterile gloves.

Palpate the radial artery using three fingers to find the point where the pulse is strongest and remember this location.

Take the arterial blood gas needle, remove any safety caps. If required, push the plunger down fully to expel any residual heparin in the syringe. You may re-palpate the radial artery to ensure you are going to insert the needle at the point where the pulse is felt most strongly. Hold the syringe like a dart and Insert the needle bevel up into the patient's wrist at a 30-45° angle until blood begins to fill the syringe. If the syringe does not fill, encourage filling by retracting the plunger. You should collect a minimum of 1ml blood before withdrawing the needle.

Once the needle has been removed, firm pressure must be applied over the puncture site with sterile gauze for at least 5 minutes. Once bleeding stops, apply a dressing or gauze onto the site. It is often appropriate to ask a colleague to maintain pressure over the injury site whilst you deliver the sample to the arterial blood gas analyser.



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Deliver Sample to the Arterial Blood Gas Analyser

Expel all air bubbles from the syringe before attaching a cap and label. Take the sample to the blood gas analyser within 10 minutes. If you expect a delay, you must place the sample on ice.

Before entering the sample into the blood gas analyser, agitate the sample by inverting and rotating the syringe a few times. The blood gas analyser will require the following information:

• Patient body temperature • Patient name, date of birth and hospital number • If the patient is on oxygen therapy, the % concentration of oxygen given is required

Document the procedure in the patient's notes, stating the rationale for the test, when the test was performed, who did the test and whether there were any complications. Finally, you should document the blood gas results provided by the analyser.

arterial blood gas procedure

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