Arterial Blood Gas (ABG) Test

• Arterial blood gas (ABG) tests measure the gas-exchange ability of the lungs and the buffering capacity of the blood.
• Partial pressure of oxygen (Po2) expresses the absolute amount of oxygen.
• Partial pressure of carbon dioxide (Pco2) expresses the carbon dioxide level.
• Impaired gas exchange in the lungs is indicated by a lower-than-normal Po2 and a higher-than-normal Pco2.
• Inadequate oxygen to the body's tissues is indicated by abnormal ABG values.

Equipment for Arterial Puncture

• Arterial blood is collected in a heparin-pretreated syringe to prevent coagulation.
• Glass or gas-impermeable plastic syringes must be used.
• Syringe sizes range from 1 to 5 mL, depending on the blood volume needed.
• A 21- or 22-gauge needle, 1 to 1½ inches long, is typical for collection.
• The risk of serious infection is greater with arterial puncture than with venipuncture.
• Both alcohol and povidone-iodine or chlorhexidine are used to clean the site because of the risk of infection.
• A subcutaneous injection of 0.5 mL of lidocaine, a local anesthetic, using a 25- to 26-gauge needle on a 1-mL syringe can lessen pain.
• Due to the pressure of arterial blood, it may spray out of the puncture.
• A fluid-resistant gown, face protection, and gloves are needed.
• A puncture-resistant container for sharps is needed.
• A Luer tip covers the syringe top after needle removal.
• Other equipment includes a slurry of crushed ice/water, gauze pads, pressure bandages, a thermometer, and a transport container.

Site Selection

• Phlebotomists collect from the radial or brachial arteries only.
• Physicians or specially trained professionals collect from other arteries
• Other arteries that may be used include the femoral and dorsalis pedis.
• A 23-gauge needle is the minimum size acceptable, and the artery for collection must be near the skin surface and large enough.
• The region distal to the collection site should have collateral circulation.

Radial Artery

• The radial artery, supplying the hand, is the preferred choice.
• It has good collateral circulation and is easily accessible along the thumb side of the wrist, despite being smaller than the brachial or femoral arteries.
• The small size of the radial artery can be a disadvantage in patients with low cardiac output because it is hard to locate.

Brachial Artery

• The brachial artery is large, easy to palpate and puncture, and is located in the antecubital fossa, below the basilic vein and near the insertion of the biceps muscle.
• It has adequate collateral circulation, although less than the radial artery.

Femoral Artery

• The femoral artery is the largest artery used, located in the groin area above the thigh, lateral to the pubic bone.
• It is used when other sites are not available due to its large size and high volume make it useful when cardiac output is low.
• The femoral artery has poor collateral circulation.

Dorsalis Pedis Artery

• The dorsalis pedis artery in the foot is an alternative site in adults.
• The posterior tibial pulse must be checked for adequate pulse when puncturing the dorsalis pedis.
• Blood may be collected from both the umbilical artery and vein in newborns who have difficulty breathing.

Testing Collateral Circulation

• Modified Allen Test: The wrist is extended over a towel, and the patient makes a fist.
• The pulses of both the ulnar and radial arteries are located and compressed.
• The patient repeatedly opens and closes their fist.
• The pressure from the ulnar artery is released.

Respiratory Steady State

• A respiratory steady state is required before radial artery puncture.
• The patient must have received a specified amount of oxygen and refrained from exercise for at least 30 minutes.
• Maintaining this state during collection requires keeping the patient calm and ensuring they are not hyperventilating.

Radial Artery Puncture

• The patient should be prepared and the requisition form completed, including recording the patient's temperature and respiration rate if required.
• A modified Allen test is performed to assess collateral circulation in the hand.
• A positive result on the Allen test indicates that the procedure can proceed.
• The arm should be fully extended, with the anterior surface facing upward.
• Palpate for the artery with either the middle or the index finger to locate the greatest maximum pulsation.
• To perform the puncture, cleanse the fingers of your nondominant hand and place them over the area where the needle should enter the artery to stabilize it.
• The syringe is held like a dart with the dominant hand, with the needle tip pointed bevel up toward the upper arm.
• Blood should appear in the hub as the needle is inserted into the artery.
• The operator should withdraw the needle after the puncture with their dominant hand, and apply direct pressure to the site with a folded gauze square with the nondominant hand for at least 5 minutes.
• The plunger should be depressed slightly to expel any air that may have entered the needle.
• After 5 minutes (or 15 minutes for patients on anticoagulant therapy), the site should be checked to ensure that the bleeding has stopped.
• Medications that increase bleeding times include anticoagulants (e.g., heparin and warfarin) and thrombolytics (e.g., tissue plasminogen activator, streptokinase, and urokinase).
• Dispose of the needle in the sharps container, label the specimen with a waterproof pen, and deliver the specimen to the laboratory immediately.

Arterial Puncture Complications

• Arteriospasm: spontaneous constriction of an artery in response to pain.
• Embolism: blood vessel obstruction caused by an air bubble or dislodged clot in the artery.
• Hematoma: due to inadequate pressure on the site.
• Hemorrhage
• Infection: from skin contaminants.
• Lightheadedness, nausea, or fainting.
• Nerve damage: caused by inadvertent contact with a nerve.
• Severe pain
• Thrombosis: clot formation within the artery.

Sampling Errors

• The most significant source of error is failure to deliver the sample to the laboratory immediately or to properly store the sample on ice if delivery will be delayed.
• Blood cells continue to respire after collection, leading to changes in analyte values (Po2, Pco2, and pH).
• Samples collected in a plastic syringe that are not iced must be analyzed within 30 minutes of collection.

Specimen Rejection

• Air bubbles in the specimen
• Clotting
• Failure to ice the specimen
• Improper or absent labeling
• Inadequate volume of specimen for the test
• Too long a delay in delivering the specimen to the laboratory
• Use of the wrong syringe

Capillary Blood Gas Testing

• Capillary blood gas (CBG) testing is an alternative to ABG testing when arterial collection is not possible or is not recommended.
• Capillary blood is not as desirable a specimen for blood gas testing because it is a mixture of blood from the capillaries, venules, and arterioles and is mixed with tissue fluid.
• The sample site should be cleaned with the appropriate antiseptic swab, using the antiseptic required by the institution.
• The sample should be collected in a heparinized glass pipet
• The tube should be filled completely with blood so that no air bubbles remain.
• The sample should be mixed well using the magnet and flea, and transported the specimen to the laboratory on ice.