Arterial Blood Gas Sampling Technique

Updated: May 19, 2016
  • Author: Mauricio Danckers, MD, FCCP; Chief Editor: Vincent Lopez Rowe, MD  more...
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Technique

Approach Considerations

Healthcare personnel should wear gloves and eye protection for the duration of the arterial blood gas (ABG) sampling procedure and should follow hospital policies regarding management of body fluid samples. The operator should have all the required equipment at the bedside before beginning the procedure.

The arterial pulse is felt with the middle and index fingers of the nondominant hand. Both fingers should be proximal to the desired puncture site; placing the nondominant middle finger distally and the nondominant index finger proximally, with the needle insertion site in between, is strongly discouraged, because of the increased risk of needle stick injury.

If the ABG syringe is to be used, the protective needle sleeve and needle should be placed onto the syringe, the prefilled heparin expelled, and the vented plunger repositioned to the 2-mL mark.

Arterial blood samples should be obtained in strict anaerobic conditions and should be placed on ice and held at 0° C until analysis. [9] Any air bubbles introduced during the sampling procedure will lead to overestimation of arterial oxygen tension (PaO2) and underestimation of arterial carbon dioxide tension (PaCO2). [9]

Keeping the sample at lower temperatures slows cellular metabolism and reduces ongoing consumption of oxygen. [9] This is especially important in patients with leukocytosis. [10]

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Radial Artery Sampling

The radial artery is most easily accessible medial to the radial styloid process and lateral to the flexor carpi radialis tendon, 2-3 cm proximal to the ventral surface of the wrist crease (see the image below).

Anatomic location of radial artery. Anatomic location of radial artery.

The procedure is performed as follows. First, perform a modified Allen test in the limb selected for the procedure (see Periprocedural Care, Preprocedural Planning).

Palpate the patient's radial pulse with the index and middle finger pads of the nondominant hand (see the first image below). Visualize the direction of the artery, and clean the desired puncture site in an outward circular motion with an antiseptic solution (see the second image below).

Identification of radial pulse. Identification of radial pulse.
Cleaning of desired radial artery puncture site. Cleaning of desired radial artery puncture site.

Uncap the ABG syringe, and hold it with two fingers of the dominant hand. The needle bevel should be facing upward. Insert the needle just under the skin at a 45º angle, aiming in the direction of the artery, while palpating the radial pulse proximal to the puncture site with the nondominant hand (see the image below). Angling the needle in this fashion minimizes trauma to the vessel and allows smooth-muscle fibers to occlude the puncture site after the procedure.

Insertion of needle at radial artery puncture site Insertion of needle at radial artery puncture site.

Advance the needle slowly. Once the needle enters the lumen of the radial artery, the arterial blood flow starts to fill the syringe (see the image below). At this point, remove the nondominant hand from the field. It is not necessary to pull back the plunger, unless an unvented plunger with a small (25-gauge) needle is being used or the patient has a weak pulse.

Radial artery puncture. Radial artery puncture.

After 2-3 mL of arterial blood has been obtained, remove the needle. At the same time, use a small piece of gauze, held in the nondominant hand, to apply firm occlusive local pressure at the puncture site for 5 minutes (see the image below). Avoid checking the puncture site until local pressure has been maintained for at least 5 minutes. In patients who have a coagulopathy or are on anticoagulation therapy, it may be necessary to apply local pressure for a longer time. Check for hemostasis, and apply an adhesive bandage over the puncture site.

Removal of needle from radial artery puncture site Removal of needle from radial artery puncture site and application of local pressure for hemostasis.

Apply the needle protective sleeve (see the first image below), then untwist the sleeve and place it in the sharp object container (see the second image below).

Application of needle protective sleeve. Application of needle protective sleeve.
Disposal of needle. Disposal of needle.

Remove the excess air in the syringe by holding it upright and gently tapping it, allowing any air bubbles present to reach the top of the syringe, from where they can then be expelled (see the first image below). Cap the syringe, place it in the ice bag, and send it for analysis (see the second image below).

Removal of air bubbles from syringe. Removal of air bubbles from syringe.
Capping of syringe. Capping of syringe.
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Femoral Artery Sampling

The femoral artery is best identified in the midline between the symphysis pubis and the anterior superior iliac crest, 2-4 cm distal to the inguinal ligament. The femoral artery is medial to the femoral nerve and lateral to the femoral vein (see the image below).

Anatomy of femoral triangle. Anatomy of femoral triangle.

It is important to assess the distal pulses of the lower limb before attempting femoral puncture. Diminished or absent pedal pulses could be indicative of peripheral arterial disease (PAD). If PAD is a significant possibility, strong consideration should be given to using an alternative arterial puncture site.

The procedure is performed as follows.

Palpate the patient's femoral pulse with the index and middle finger pads of the nondominant hand (see the first image below). Visualize the direction of the artery, and clean the desired puncture site in an outward circular motion with an antiseptic solution (see the second image below).

Identification of femoral artery. Identification of femoral artery.
Cleaning of desired femoral artery puncture site. Cleaning of desired femoral artery puncture site.

Uncap the ABG syringe, and hold it with two fingers of the dominant hand. The needle bevel should be facing upward. Insert the needle just under the skin at a 60-90º angle, aiming in the direction of the artery, while palpating the femoral pulse proximal to the puncture site with the nondominant hand (see the image below).

Insertion of needle at femoral artery puncture sit Insertion of needle at femoral artery puncture site.

Advance the needle slowly. Once the needle enters the lumen of the femoral artery, the arterial blood flow starts to fill the syringe (see the image below). At this point, remove the nondominant hand from the field. It is not necessary to pull back the plunger, unless an unvented plunger with a small (25-gauge) needle is being used or the patient has a weak pulse.

Femoral artery puncture. Femoral artery puncture.

After 2-3 mL of arterial blood has been obtained, remove the needle. At the same time, use a small piece of gauze, held in the nondominant hand, to apply firm occlusive local pressure at the puncture site for 5 minutes (see the image below). In patients who have a coagulopathy or are on anticoagulation therapy, it may be necessary to apply local pressure for a longer time. Check for hemostasis, and apply an adhesive bandage over the puncture site. Recheck the distal pulses.

Removal of needle from femoral artery puncture sit Removal of needle from femoral artery puncture site and application of local pressure for hemostasis.

Apply the needle protective sleeve, then untwist the sleeve and place it in the sharp object container.

Remove the excess air in the syringe by holding it upright and gently tapping it, allowing any air bubbles present to reach the top of the syringe, from where they can then be expelled. Cap the syringe, place it in the ice bag, and send it for analysis.

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Brachial Artery Sampling

The brachial artery is best identified between the medial epicondyle of the humerus and the tendon of the biceps brachii in the antecubital fossa. It can be felt higher in the arm in the groove between the biceps and triceps tendons. The basilic vein and the brachial nerve are located in close proximity (see the image below).

Anatomic location of brachial artery. Anatomic location of brachial artery.

As with femoral artery access, the pulses distal to the brachial artery must be assessed before the procedure. In patients with absent pulses at the wrist (ie, in the radial and ulnar arteries), an alternative site for arterial sampling should be considered.

The procedure is performed as follows.

Palpate the patient's brachial pulse with the index and middle finger pads of the nondominant hand (see the first image below). Visualize the direction of the artery, and clean the desired puncture site in an outward circular motion with an antiseptic solution (see the second image below).

Identification of brachial artery. Identification of brachial artery.
Cleaning of desired brachial artery puncture site. Cleaning of desired brachial artery puncture site.

Uncap the ABG syringe, and hold it with two fingers of the dominant hand. The needle bevel should be facing upward. Insert the needle just under the skin at a 45-60º angle, aiming in the direction of the artery, while palpating the brachial pulse proximal to the puncture site with the nondominant hand (see the image below).

Insertion of needle at brachial artery puncture si Insertion of needle at brachial artery puncture site.

Advance the needle slowly. Once the needle enters the lumen of the brachial artery, the arterial blood flow starts to fill the syringe (see the image below). At this point, remove the nondominant hand from the field. It is not necessary to pull back the plunger, unless an unvented plunger with a small (25-gauge) needle is being used or the patient has a weak pulse.

Brachial artery puncture. Brachial artery puncture.

After 2-3 mL of arterial blood has been obtained, remove the needle. At the same time, use a small piece of gauze, held in the nondominant hand, to apply firm occlusive local pressure at the puncture site for 5 minutes (see the image below). In patients who have a coagulopathy or are on anticoagulation therapy, it may be necessary to apply local pressure for a longer time. Check for hemostasis, and apply an adhesive bandage over the puncture site. Recheck the pulses at the wrist.

Removal of needle from brachial artery puncture si Removal of needle from brachial artery puncture site and application of local pressure for hemostasis.

Apply the needle protective sleeve, then untwist the sleeve and place it in the sharp object container.

Remove the excess air in the syringe by holding it upright and gently tapping it, allowing any air bubbles present to reach the top of the syringe, from where they can then be expelled. Cap the syringe, place it in the ice bag, and send it for analysis.

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Complications

Complications of ABG sampling include the following:

  • Local hematoma [3]
  • Artery vasospasm [3]
  • Arterial occlusion [3]
  • Air or thrombus embolism [3]
  • Local anesthetic anaphylactic reaction
  • Infection at the puncture site [3]
  • Needle stick injury to health care personnel [3]
  • Vessel laceration [2]
  • Vasovagal response [2]
  • Hemorrhage [3]
  • Local pain [2]
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Results

Results are usually available within 5-15 minutes. Aberrant results may result from contamination with room air, resulting in abnormally low carbon dioxide and near-normal oxygen levels. Delays in analysis of the blood tube allow for ongoing cellular respiration and may lead to errors with inaccurately low oxygen and high carbon dioxide levels reported in the results.

The ABG test may determine concentrations of lactate, hemoglobin, electrolytes, oxyhemoglobin, carboxyhemoglobin, and methemoglobin.

Values at sea level include the following:

  • Partial pressure of oxygen (PaO 2) - 75-100 mm Hg
  • Partial pressure of carbon dioxide (PaCO 2) - 35-45 mm Hg
  • Arterial blood pH - 7.38-7.42
  • Oxygen saturation (SaO 2) - 94-100%
  • Bicarbonate (HCO 3 ) - 22-26 mEq/L

ABG testing is the criterion standard for determining the adequacy of ventilatory support and the relationship between pH, PaO2, PaCO2, and HCO3 in the human body. [11, 12, 13] These results help to determine if the patient is in metabolic/respiratory alkalosis/acidosis with or without an anion gap. See the Anion Gap calculator.

The pH level indicates whether a patient is acidemic (pH <7.35) or alkalemic (pH >7.45). The PaO2 shows the level of oxygenation in the body. The PaCO2  indicates the degree of CO2 production or elimination via the respiratory cycle. An elevated or decreased PaCO2 (ie, respiratory acidosis or respiratory alkalosis, respectively) is an indication of ventilation that is insufficient or excessive, respectively, either from a primary respiratory cause or in compensation for an alteration in pH.

The bicarbonate ion (HCO3) level demonstrates the degree of a metabolic disturbance in a patient. For example, a low HCO3 level suggests a metabolic acidosis, whereas a high HCO3 level suggests a metabolic alkalosis. A base excess may then be determined to further delineate the underlying respiratory or metabolic disturbance via the following equation:

  • Base excess = 0.93 × ([HCO 3 ] – 24.4 + 14.8 × [pH – 7.4])

A base excess of more than +2 mEq/L indicates metabolic alkalosis (excess bicarbonate). Less than –2 mEq/L indicates a metabolic acidosis (typically either excretion of bicarbonate or neutralization of bicarbonate by excess acid).

The serum anion gap (AG) is then used to determine the underlying cause of a metabolic acidosis. The equation used commonly is as follows:

  • AG = (Na) – (Cl +  HCO 3 )

Normal range is 8-16 mEq/L.

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