Arterial Blood Gas - Procedure

last authored: Dec 2009, David LaPierre




Arterial blood gas sampling provides valuable information regarding respiratory function and acid-base status. It is usually performed via a puncture of the radial artery at the wrist or the femoral artery at the groin. The dorsalis pedis and posterior tibial arteries are also occasionally used.

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  • indications
  • contraindications


Arterial blood gases are commonly performed to:

  • assess respiratory status
  • assess the acid:base status
  • obtain blood for chemical, hematologic analysis or blood culture (though venous blood generally preferred)


Contraindications to the radial site include:

  • weak or absent radial pulse (severe atherosclerosis, reynauds disease)
  • positive Allen test
  • infection or burn overlying puncture site

Contraindications to femoral artery sampling include:

  • presence of a synthetic femoral bypass graft
  • overlying infection
  • coagulopathy or anticoagulated patient





Video - arterial blood gas sampling (Dalhousie University Common Common Currency)

  • materials required
  • radial artery puncture
  • femoral artery puncture

Materials Required

You will require:

  • eye protection and latex gloves
  • prepackaged arterial puncture kit (3 cc syringe with Heparin Ball, 20g x 1 in. Needle) or
  • 3 cc syringe and 1 - 2 cc of 1:1000 Heparin Solution
  • povidone - Iodine Swab, Alcohol Swab,
  • Syringe Cap
  • 2 x 2 Gauze and tape
  • ice

Advise the patient on the procedure. Formal written consent is not generally indicated.


Prepare all equipment. If not using a pre-heparinized syringe, draw 1 cc of Heparin into the syringe to coat the barrel, then expel it.


Because it is easily accessible, compressible and generally has collateral flow through the Ulnar Artery, the Radial Artery of the non dominant hand is usually the first choice for arterial puncture.

Perform the Allen test.

In the majority of patients, the arterial blood supply to the hand is through both the Radial and Ulnar Arteries. This can be a real benefit should the Radial Artery become occluded or damaged during an attempt at cannulation. Some patients, because of disease or aberrant anatomy may have little collateral flow through the Ulnar Artery. The Allen Test is an attempt to document the presence of Ulnar Artery flow prior to performing a Radial Artery stab. The Ulnar Arterial pulse is not usually palpable, even in normal young people.

  • Have the patient squeeze their fist tightly for ~ 10 seconds.

  • With the hand still in a tight fist, place direct pressure over the Radial and Ulnar Arteries at the wrist using the index and middle fingers of your hands.

  • Ask the patient to open their fist. The hand should appear blanched.

  • Release the pressure over the Ulnar Artery and observe for flushing of the palm.

  • If no flushing occurs, release the radial side and observe again.

  • Absence of cutaneous flushing following release of the ulnar artery suggests decreased flow through this artery and indicates that you should try another site if possible.

Although the current research does not support use of the Allen’s Test when drawing every ABG it is imperative that an over all examination for the presence of severe peripheral vascular disease be made to the area distal to the site. There are certain circumstances where it is essential to perform this test. Although many clinicians do not use it, there are still many places where it is Hospital Policy and as such we continue to teach it. IN ALL INSTITUTIONS, HOSPITAL POLICY APPLIES TO ALL STAFF.



Place the patient in a comfortable position. Lying is preferred with the arm extended and supinated. Most prefer to extend the patient’s wrist over a rolled towel or an IV fluid bag. The hand may be secured with tape if the patient is unable to cooperate.

Put on Gloves and Protective Eye wear

From a seated position, identify the radial pulse at the level of the radial styloid, using the index finger of your non-dominant hand. Prepare the site with Povidone - Iodine then alcohol.

Infiltrate a subcutaneous weal with 2% Plain Xylocaine (without epinephrine) ~ 1 cm distal to your index finger. Do not use too much anaesthetic as this may obscure the pulse.

Insert the needle, bevel up, at a 30 - 45 degree angle through the weal and into the radial artery. Stop if you get a flash back of blood into the hub of the needle and allow the arterial pressure to fill the syringe with ~ 2 cc of blood. Do not aspirate.

If blood is not obtained, slowly withdraw the needle. Frequently, you will have passed through the front and back walls of the artery and will get good blood flow on withdrawal. If necessary, withdraw to the skin surface and redirect your needle for another try.

Once blood is obtained, have the patient or an assistant apply pressure over the site for 5 minutes (10 minutes in an anticoagulated patient ).

Insert the needle into the enclosed orange rubber block and remove from the syringe. Discard safely in sharps container. Slowly advance the plunger on the needle to remove any excess air and place the black rubber cap on the open end of the syringe.

Ideally, blood gas analysis should be performed within ~ 20 minutes. Air bubbles left in the syringe may falsely elevate the pO2 of the sample.

Femoral Artery Puncture

Femoral Artery Puncture just below the level of the inguinal ligament is usually a second option, especially where radial puncture is contraindicated. Femoral sites may be more prone to infection and are less easily compressible.

A 22g x 1 ½ in. needle is substituted.


The Femoral pulse is palpated, with the patient supine, ~ 2 cm below the inguinal ligament and entered at a 30 - 45 degree angle.

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Excess Heparin and Air Bubbles in the syringe may give false results to your ABG analysis. Air bubbles will falsely elevate the pO2 whereas heparin will lower the pCO2. Collecting at least 2 cc of blood will reduce this potential error. However, the lab can process as little as 0.5 cc if necessary.


Use a small needle i.e. 25g on infants or small children.


The volar aspect of the wrist is very sensitive. A small amount of 2% Plain Xylocaine (i.e. without epinephrine) will make the procedure less painful for your patient.

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Bleeding - great care must be taken to apply sufficient pressure over the site following the procedure. Avoid the femoral route in patients who have received anticoagulant or thrombolytic therapy.

Infection - uncommon if aseptic technique is followed.

Distal Ischemia - may be secondary to arterial spasm, embolization of atherosclerotic plaque or thrombosis of the vessel. Uncommon in patients without pre-existing disease. Avoid punctures of end arteries without collateral circulation i.e. Brachial Artery.

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Additional Resources




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Topic Development

created: DLP, Aug 09

authors: DLP, Aug 09





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