Arterial Blood Gas Sampling

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. For more information about the relevant anatomy, see Arterial Supply Anatomy.

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,[1] 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. ABG sampling is the only way of accurately determining the alveolar-arterial oxygen gradient (see the A-a Gradient calculator).

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.

Indications for ABG sampling include the following[2, 3] :

• 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) ^[4] 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)

VBG sampling may be an acceptable alternative to ABG sampling in critically ill patients who are hemodynamically stable and do not have severe acid-base disturbances.[5]

The American Association for Respiratory Care (AARC) has published a clinical practice guideline on blood gas analysis and hemoximetry.[6]

Absolute contraindications for ABG sampling include the following:

• 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

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.

Healthcare personnel should explain the arterial blood gas (ABG) sampling procedure to the patient, with particular attention to the associated risks and benefits. However, this may not be possible in certain clinical scenarios, such as a critically ill patient with rapid decompensation or a patient with an altered level of consciousness (eg, from encephalopathy or advanced dementia). Written consent is not necessary.

Patients should be asked not to manipulate the area involved in the procedure and should be instructed to alert healthcare personnel if new symptoms develop, such as skin color changes, persistent or worsening pain, active bleeding, impaired movement, or sensation of the limb.

Planning for the procedure focuses on the choice of a puncture site and accurate delineation of the vascular anatomy. If radial artery sampling is to be performed, the adequacy of the ulnar collateral circulation must be confirmed.

Selection of puncture site

Puncture of the radial artery is usually preferred because of the accessibility of the vessel, the presence of collateral circulation, and the artery's superficial course proximal to the wrist, which makes it easier for the clinician to identify the vascular structure and hold local pressure after the procedure is finished.

If radial artery sampling is not feasible, femoral artery puncture is a possible alternative. When femoral artery puncture is being considered, the potential risk of infection at the entry site and the artery's proximity to the femoral vein and nerve must be taken into account. The deeper the vascular structure, the higher the risk of damage to adjacent structures.

Femoral artery puncture necessitates prolonged monitoring and therefore is recommended only in the inpatient setting. Some physicians recommend that femoral artery puncture be avoided whenever possible. This consideration may play a bigger role for patients who will be undergoing an intervention that involves femoral access (eg, cardiac catheterization or intra-aortic balloon pump placement) in the near future.

The brachial artery runs deeper in the arm than the radial artery does. Consequently, its structures are typically harder to identify, and achieving hemostasis when necessary is more difficult. Furthermore, the brachial artery is a relatively small-caliber vessel and does not have extensive collateral circulation. For these reasons, the brachial artery is the least preferred site for puncture.

Repeated arterial blood sampling at the same site increases the risk of hematoma, laceration of the artery, and scarring. It also increases the chances of inadvertent venous blood sampling. If recurrent sampling is required, healthcare personnel should alternate puncture sites. If more frequent sampling is necessary, the healthcare provider should consider placing an indwelling arterial catheter through arterial cannulation.

Confirmation of vascular anatomy

ABG sampling can be difficult in patients with feeble pulses or distorted anatomic landmarks or in situations where sampling of a deep vascular structure (eg, the femoral or brachial artery) is required. In these scenarios, the use of ultrasonography (US) to guide ABG sampling should be considered,[7] especially when sampling by the standard approach has been unsuccessful or is not feasible.

US is a noninvasive technique with an excellent safety profile. It is an important option in cases where vascular sampling proves difficult. US enables more accurate recognition, delineation, and targeting of the chosen vascular structure. It minimizes the risk of vascular laceration and damage to surrounding structures.

Although US guidance is a safe and effective tool for patients with weak pulses, a better option for patients in profound shock or in the midst of cardiopulmonary resuscitation is to obtain arterial blood from the femoral artery using bony landmarks alone. When the tip of the fifth finger is placed on the symphysis pubis and the tip of the thumb on the anterior superior iliac spine, the femoral artery always lies beneath the middle finger.

Assessment of collateral circulation (modified Allen test)

If puncture of the radial artery is planned, a modified Allen test should be performed beforehand when feasible to assess the collateral circulation. Although the anatomy of the radial artery in the forearm and the hand is variable, most patients have adequate collateral flow should radial artery thrombosis occur.[1] The modified Allen test is performed as follows.[1]

Firm occlusive pressure is held on both the radial artery and the ulnar artery (see the first image below). The patient is asked to clench the fist several times until the palmar skin is blanched (see the second image below), then to unclench the fist. Overextension of the hand or wide spreading of the fingers should be avoided, because it may cause false-normal results.[8] The pressure on the ulnar artery is released while occlusion of the radial artery is maintained (see the third image below). The time required for palmar capillary refill is noted.

Modified Allen test: digital occlusion of radial and ulnar artery.

Modified Allen test: clenching of hand.

Modified Allen test: ulnar artery occlusion released.

The test is then repeated, but this time the radial artery is released while the ulnar artery remains compressed (inverse modified Allen test; see the image below).

Modified Allen test: radial artery occlusion released.

The modified Allen test has been the method most frequently used for clinical assessment of the adequacy of ulnar artery collateral circulation and the patency of the palmar arches of the hand. However, there is some controversial evidence suggesting that it can predict ischemic complications in the setting of radial artery occlusion.[1]

Given the low positive predictive value of the modified Allen test, the examiner should consider further testing to assess patency of circulation, such as finger pulse plethysmography, Doppler flow measurements, and measurement of the arterial systolic pressure of the thumb.[8]

Whether the modified Allen test is clinically reliable as a screening test for adequate collateral circulation of the hand is controversial. A wide range of values for hand reperfusion have been noted, and normal values are not consistent (ranging from 3 to 15 seconds)[1, 8, 9] ; furthermore, there has been conflicting evidence regarding the validity of the modified Allen test as a standard of care.[9, 10]

The materials needed for ABG sampling (see the first image below) include the following:

• Gloves - Nonsterile gloves may be used, but care must be taken not to touch the puncture site after cleaning the area
• Syringe for sampling - A standard syringe with a 25-gauge needle and a 3-mL capacity (a higher-capacity syringe may be difficult to maneuver, and smaller needle sizes may increase the risk of traumatic hemolysis, decreasing the accuracy of hemoglobin and potassium measurements); a 23-gauge needle may work as well ^[11]
• Lithium heparin - 1-2 mL lithium heparin (1000 U/mL) should be aspirated into the syringe through the needle and then pushed out; the plunger should be left depressed to allow the arterial blood flow to fill up the syringe
• ABG syringe (alternative) - Some ABG kits contain a prefilled heparinized syringe along with a protective needle sleeve and a syringe cap (see the second image below); the sleeve, while still attached to the syringe, locks the needle within itself to prevent direct contact between operator and needle; some syringe models have a vented plunger that allows the operator to preset a specific amount of blood to be withdrawn, and with these models, the plunger is placed midway through the syringe and is not pulled back while the puncture is performed; before the procedure, the prefilled heparin is expelled, and the vented plunger is then repositioned at the 2-mL mark
• Antiseptic skin solution - Chlorhexidine and povidone-iodine are solutions commonly used
• Syringe cap - Usually included in the ABG syringe kit
• Sterile gauze, 2 × 2 in. (5 × 5 cm)
• Adhesive bandage
• Bag with ice
• Sharp object container
• Lidocaine HCl 1% without epinephrine (optional)
• 25-gauge needle with syringe for local anesthetic (optional)

Arterial blood gas sampling equipment.

Arterial blood gas syringe kit.

Anesthesia

Local anesthesia with a subcutaneous injection of lidocaine HCl 1% without epinephrine may be used. Local anesthesia is not frequently employed, however, because the administration of the anesthetic is as painful as the procedure itself.[12]

If local anesthesia is employed, 0.5-1 mL of the anesthetic is injected so as 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. After the skin is punctured but just before the anesthetic is injected, the clinician should pull back the plunger to confirm that the needle is not inside a blood vessel; intravascular placement will be signaled by blood filling up the anesthetic syringe.

A randomized controlled trial was conducted to compare the effectiveness of three anesthetic methods (anesthetic cream, cryoanalgesia, and subcutaneous mepivacaine) with standard practice (no anesthesia) in reducing pain caused by radial artery puncture in patients requiring an ABG test in the emergency department (ED).[13] When the control group (no anesthesia) was compared with the three intervention groups, mepivacaine and cryoanalgesia were associated with significantly lower pain scores; anesthetic cream did not produce a statistically significant median difference in pain.

A randomized controlled study by Gur et al found that topical application of 10% lidocaine provided effective analgesia for radial artery blood gas sampling.[14]

Positioning

For radial artery blood sampling, the patient should be in the supine position, with the arm lying at his or her on a hard surface. The forearm should be supinated and the wrist dorsiflexed at 40º. A gauze roll may be placed under the wrist to make the patient more comfortable and to bring the radial artery to a more superficial plane. Overextension of the wrist is discouraged, because interposition of flexor tendons may make the pulse difficult to detect.

For femoral artery blood sampling, the patient is supine on a stretcher, and the patient's leg is placed in neutral anatomic position.

For brachial artery blood sampling, the arm is placed on a firm surface with the shoulder slightly abducted. The elbow is extended, with the forearm in full supination.

After the blood sampling procedure, healthcare personnel should monitor the patient for early and late signs and symptoms of potential complications. Active profuse bleeding at the puncture site might raise suspicion of vessel laceration. Femoral artery bleeding carries an increased risk of circulatory compromise because of the large caliber and deep location of the vessel, which allow larger amounts of blood to accumulate without initially giving rise to clinical findings.

A rapidly expanding hematoma may compromise regional circulation and increase the risk of compartment syndrome, especially in the forearm. This manifests as pain, paresthesias, pallor, and absence of pulses. Paresis and persistent pain may indicate a nerve lesion. Limb skin color changes, absent pulses, and distal coldness may be seen in ischemic injury from artery occlusion caused by thrombus formation or vasospasm. Infection at the puncture site should be considered in the presence of regional erythema and fever.

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.[15] 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).[15]

Keeping the sample at lower temperatures slows cellular metabolism and reduces ongoing consumption of oxygen.[15] This is especially important in patients with leukocytosis.[16]

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.

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

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.

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.

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 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.

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.

Capping of syringe.

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.

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.

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 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.

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 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.

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.

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.

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 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.

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 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.

Complications of ABG sampling include the following[2] :

• Local hematoma
• Arterial vasospasm
• Arterial occlusion
• Air or thrombus embolism
• Local anesthetic anaphylactic reaction
• Infection at the puncture site
• Needle-stick injury to healthcare personnel
• Vessel laceration
• Vasovagal response
• Hemorrhage
• Local pain

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.[17, 18, 19] 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.