Arteries are the large vessels that carry oxygenated blood away from the heart. The distribution of the systemic arteries is like a ramified tree, the common trunk of which, formed by the aorta, commences at the left ventricle, while the smallest ramifications extend to the peripheral parts of the body and the contained organs.
Arterial blood gas (ABG) sampling by direct vascular puncture is a procedure often practiced in the hospital setting. The relatively low incidence of major complications, its ability to be performed at the patient’s bedside, and its rapid analysis make it an important tool used by physicians to direct and redirect the treatment of their patients, especially in patients who are critically ill, to determine gas exchange levels in the blood related to respiratory, metabolic, and renal function.
ABG sampling provides valuable information on the acid-base balance at a specific point in the course of a patient's illness. It is the only reliable determination of ventilation success as evidenced by CO2 content. It constitutes a more precise measure of successful gas exchange and oxygenation.
Because the results of ABG sampling only reflect the physiologic state of the patient at the time of the sampling, it is important that they be carefully correlated with the evolving clinical scenario and with any changes in the patient’s treatment.
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| Arterial blood gas (ABG) |
Indications
Identification of respiratory, metabolic, and mixed acid-base disorders, with or without physiologic compensation, by means of pH ([H +]) and CO 2 levels (partial pressure of CO 2)
Measurement of the partial pressures of respiratory gases involved in oxygenation and ventilation
Monitoring of acid-base status, as in patient with diabetic ketoacidosis (DKA) on insulin infusion; ABG and venous blood gas (VBG) could be obtained simultaneously for comparison, with VBG sampling subsequently used for further monitoring
Assessment of the response to therapeutic interventions such as mechanical ventilation in a patient with respiratory failure
Determination of arterial respiratory gases during diagnostic evaluations (eg, assessment of the need for home oxygen therapy in patients with advanced chronic pulmonary disease)
Quantification of oxyhemoglobin, which, combined with measurement of arterial oxygen tension (PaO 2), provides useful information about the oxygen-carrying capacity of the patient
Quantification of the levels of dyshemoglobins (eg, carboxyhemoglobin and methemoglobin)
Procurement of a blood sample in an acute emergency setting when venous sampling is not feasible (many blood chemistry tests could be performed from an arterial sample)
The American Association for Respiratory Care (AARC) has published a clinical practice guideline on blood gas analysis and hemoximetry.
Contraindications
An abnormal modified Allen test (see Preprocedural Planning), in which case consideration should be given to attempting puncture at a different site
Local infection or distorted anatomy at the potential puncture site (eg, from previous surgical interventions, congenital or acquired malformations, or burns)
The presence of arteriovenous fistulas or vascular grafts, in which case arterial vascular puncture should not be attempted
Known or suspected severe peripheral vascular disease of the limb involved
Relative contraindications include the following:
Severe coagulopathy
Anticoagulation therapy with warfarin, heparin and derivatives, direct thrombin inhibitors, or factor X inhibitors; aspirin is not a contraindication for arterial vascular sampling in most cases
Use of thrombolytic agents, such as streptokinase or tissue plasminogen activator
Technical Considerations
ABG sampling may be difficult to perform in patients who are uncooperative or in whom pulses cannot be easily identified. Challenges arise when health care personnel are unable to position the patient properly for the procedure. This situation is commonly seen in patients with cognitive impairment, advanced degenerative joint disease, or essential tremor.
The amount of subcutaneous fat in overweight and obese patients may limit access to the vascular area and obscure anatomic landmarks.
Arteriosclerosis of peripheral arteries, as is seen in elderly patients and patients with end-stage kidney disease, may cause increased rigidity in the vessel wall.
Anatomy
ABG sampling is usually performed on the radial artery because the superficial anatomic presentation of this vessel makes it easily accessible. However, this should be done only after it has been demonstrated that there is sufficient collateral blood supply to the hand. In cases where distal perfusion is compromised and distal pulses are diminished, femoral or brachial artery puncture can be performed instead.
The brachial artery commences at the lower margin of the tendon of the teres major. Passing down the arm, it ends about 1 cm below the bend of the elbow, where it branches into the radial and ulnar arteries. The radial artery commences at the bifurcation of the brachial artery and passes along the radial side of the forearm to the wrist.
Best practices
The following suggestions may enhance the performance of ABG sampling:
Patients with poor distal perfusion (eg, those in hypovolemic states, with advanced heart failure, or on vasopressor therapy) may not exhibit a strong arterial pulsation; the operator may need to pull back the ABG syringe plunger to get a blood sample, though this increases the risk of venous blood sampling.
If arterial blood flow is not obtained, the operator might slowly pull back the needle; it is possible that the needle has gone through the vessel.
Initial arterial flow may subsequently be lost if the needle moves outside the vessel lumen; reidentification of the arterial pulse, using the nondominant middle and index finger, and repositioning the needle in the direction of the vessel could be attempted; avoid blind movement of the needle while it is inserted deeply in the patient’s body—pull it back to a point just below the skin, and redirect it to the arterial pulse felt with the other hand.
Puncture of venous structures can be identified by lack of pulsatile flow or dark-colored blood, though arterial blood in severely hypoxemic patients can also have a dark appearance; if venous blood is obtained, removal of the needle from the patient might be necessary to expel the venous blood from the syringe.
Excessive skin and abundant soft tissue may obstruct the puncture site; the operator can use the nondominant hand to smooth the skin, or an assistant can remove the subcutaneous tissue from the puncture site field.
Incomplete dismissal of heparin solution from the syringe could cause falsely low values for the partial pressure of CO 2; to avoid this, the operator should expel all heparin solution from the syringe before arterial puncture.
Incomplete removal of air bubbles can cause falsely elevated values for the partial pressure of oxygen; to avoid this, the operator should be sure to completely remove air bubbles from the syringe (vented plungers have an advantage over standard syringes in this regard).
Avoid puncture of the brachial artery or femoral artery in patients with diminished or absent distal pulses; the absence of distal pulses may signal severe peripheral vascular disease.
When femoral or brachial artery puncture is being considered, the use of ultrasonographic guidance during passage of the needle aids in providing an accurate roadmap to the vessel and helps minimize inadvertent arterial injuries.
Complication prevention
Although patients with severe coagulopathy are at higher risk for bleeding complications, no clear evidence on the safety of arterial puncture in the setting of coagulopathy exists. In patients with coagulopathy, careful evaluation of the need for ABG sampling is recommended.
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