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Identification and monitoring of acid-base disturbances
Measurement of the partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2)
Assessment of the response to therapeutic interventions (eg, insulin in patients with diabetic ketoacidosis)
Detection and quantification of the levels of abnormal hemoglobins (eg, carboxyhemoglobin and methemoglobin)
Procurement of a blood sample in an acute emergency setting when venous sampling is not feasible (most tests can be performed from an arterial sample)
An abnormal modified Allen's test (explained below under 'Site selection')
Local infection or distorted anatomy at the puncture site (eg, previous surgical interventions, congenital or acquired malformations, burns, aneurysm, stent, arteriovenous fistula, vascular graft)
Severe peripheral vascular disease of the artery selected for sampling
Active Raynaud's syndrome (particularly sampling at the radial site)
Considerations before starting
Arterial blood is required for an ABG. It can be obtained by percutaneous needle puncture or from an indwelling arterial catheter. Written consent is not usually required for arterial needle stick puncture but is required for the insertion of an indwelling catheter. Regardless, the risks and benefits of each procedure should be explained to the patient.
Ultrasound is not routinely used but can be used to direct access when sampling by the standard approach has been unsuccessful or is not feasible (eg, weak pulses, patient on multiple vasopressors, obese patients). When used, ultrasound-guided access may increase the operator's ability to enter the vessel and helps minimize injury to the artery and adjacent nerves and veins.
The initial step in percutaneous needle puncture is locating a palpable artery. Common sites include the radial, femoral, brachial, dorsalis pedis, or axillary artery. There is no evidence that any site is superior to the others. However, the radial artery is used most often because it is accessible and more comfortable for the patient than the alternative sites.
Evaluate collateral circulation
One of the risks associated with arterial puncture is ischemia distal to the puncture site. Although rarely performed in practice, identifying collateral flow to the region supplied by the artery can be used by clinicians prior to puncture. While limited studies have found variable accuracy associated with such evaluations, we believe that patients, and in particular high risk patients, undergoing radial or dorsalis pedis artery puncture should have the collateral flow to those vessels evaluated.
Modified Allen's test: The patient's hand is initially held high with the fist clenched. Both the radial and ulnar arteries are compressed firmly by the two thumbs of the investigator. This allows the blood to drain from the hand. The hand is then lowered and the fist is opened (the palm will appear white). Overextension of the hand or wide spreading of the fingers should be avoided because it may cause false-normal results. The pressure is released from the ulnar artery while occlusion is maintained on the radial artery. A pink color should return to the palm, usually within six seconds, indicating that the ulnar artery is patent and the superficial palmar arch is intact. Although the timing of return of circulation to the palm varies considerably, the test is generally considered abnormal if ten seconds or more elapses before color returns to the hand.
The Allen's test: The Allen's test (from which the modified Allen's test evolved) is performed identically, except these steps are executed twice: once with release of pressure from the ulnar artery while occlusion is maintained on the radial artery, and once with release of pressure from the radial artery while occlusion is maintained on the ulnar artery.
An Allen's test to assess the collateral circulation of the posterior tibialis is performed by elevating the leg until the plantar skin blanches followed by compression of dorsalis pedis pulse by the clinician's thumb and lowering of leg to dependency. The foot rapidly resumes its normal color if the posterior tibial artery flow is adequate.
As for all procedures, the equipment necessary should be brought to the bedside prior to the procedure. This includes:
- Non sterile gloves
- Antiseptic skin solution (eg, chlorhexidine and povidone-iodine are solutions)
- ABG kit OR a pre-heparinized 3 mL ABG syringe with a 22 to 25-gauge needle and syringe cap
- 2 × 2 inch sterile gauze
- Adhesive bandage
- Plastic hazard bag with ice (if not provided in the kit)
- Sharp object container
Lidocaine (eg, 1 or 2 percent) without epinephrine may be required should the clinician feel that anesthesia is necessary or the patient requests it.
ABG kits (picture 2) are used by clinicians in most institutions to draw arterial blood. Kits contain a heparinized plastic syringe with the plunger already pulled back to allow for the collection of 2 mL of blood, a protective needle sleeve, a needle, syringe cap, and ice bag. The sleeve, while attached to the syringe, locks the needle within itself to prevent direct contact between operator and needle. It is removed to expose the needle. The prefilled heparin is expelled (incomplete dismissal of heparin falsely lowers the partial pressure of carbon dioxide), and the plunger is then repositioned at the 2 mL mark.
Alternatively a heparinized ABG syringe can be used. Approximately 2 mL of lithium heparin (1000 units/mL) can be aspirated into a syringe through a 22 to 25 gauge needle and then pushed out; the plunger should be left leaving a small empty volume (eg, usually 2 mL) in the syringe.
The planned puncture site should be sterilely prepared.
Local analgesia with injectable 1 to 2 percent lidocaine can be administered but is not usually performed. If local anesthesia is employed (eg, requested by the patient, difficult or prolonged needle stick is pre-empted), 0.5 to 1 mL of the anesthetic is injected to create a small dermal papule at the site of puncture; using larger amounts or injecting the anesthetic into deeper planes may distort the anatomy and hinder identification of the vessel. Traditionally, it was believed that the injection of lidocaine is as painful as the procedure itself so many clinicians avoid using it for this reason. However, in our experience, when performed by personnel experienced in arterial draws, no anesthesia is typically needed.
ABG kits are used by clinicians in most institutions to draw arterial blood. Alternatively, a heparinized syringe can be used. The kit or syringe is prepared as described above. (See 'Equipment' above.)
One or two fingers should be used to gently palpate the artery while holding the needle in the other 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 not recommended, because of the increased risk of needle stick injury. The artery should be punctured with the needle at a 30 to 45 degree angle (radial, brachial, axillary, dorsalis pedis) or at a 90 degree angle (femoral artery) relative to the skin. The syringe fills on its own (ie, pulling the plunger is usually unnecessary). Approximately 2 to 3 mL of blood should be removed.
For patients with poor distal perfusion (eg, hypovolemia, shock, vasopressor therapy) who may exhibit a weak arterial pulse, the operator may need to pull back the syringe plunger, although this increases the risk of venous blood sampling.
If arterial flow is lost during the arterial draw, the needle may have moved outside the vessel lumen. The needle may be pulled back slightly and repositioned to a point just below the skin; subsequent redirection using the maneuver described above should be attempted to re-access the artery. Multiple blind or stabbing movements of the needle while it is inserted deeply in the patient's limb should be avoided since this increases the risk of local injury and pain.
After withdrawing a sufficient volume of blood, the needle should be removed while simultaneously applying pressure to the puncture site with sterile gauze until hemostasis is achieved. This usually takes five minutes in a non-anticoagulated patient; avoid checking the puncture site until local pressure has been maintained for at least this period as this increases the risk of hemorrhage or a hematoma. In patients who have a coagulopathy or are on anticoagulation therapy, it may be necessary to apply local pressure for a longer time. Once hemostasis is achieved, apply an adhesive bandage over the puncture site.
When ABG kits are used, apply the needle protective sleeve then untwist the sleeve and place it in the sharp object container. When an ABG syringe is used, recap, remove, and discard the needle, being careful to avoid a needle stick injury. After discarding the 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. Cap the syringe, roll it between the hands for a few seconds to allow blood to mix with the heparin (prevents clotting), then place on ice in the hazard bag and send it for analysis.