27/8/2015 Arterial Blood Gas Sampling: Overview, Periprocedural Care, Technique

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Arterial Blood Gas SamplingAuthor: Mauricio Danckers, MD; Chief Editor: Vincent Lopez Rowe, MD more...

Updated: Sep 08, 2014

Overview

Background

Arteries are the large vessels that carry oxygenated blood away from the heart. Thedistribution of the systemic arteries is like a ramified tree, the common trunk ofwhich, formed by the aorta, commences at the left ventricle, while the smallestramifications 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 oftenpracticed 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 itan important tool used by physicians to direct and redirect the treatment of theirpatients, especially in patients who are critically ill, to determine gas exchangelevels in the blood related to respiratory, metabolic, and renal function.

ABG sampling is usually performed on the radial artery because the superficialanatomic presentation of this vessel makes it easily accessible. However, thisshould be done only after it has been demonstrated that there is sufficient collateralblood supply to the hand. In cases where distal perfusion is compromised and distalpulses are diminished, femoral or brachial artery puncture can be performedinstead.

The brachial artery commences at the lower margin of the tendon of the teresmajor. 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 atthe bifurcation of the brachial, and passes along the radial side of the forearm tothe wrist.

ABG sampling provides valuable information on the acidbase balance at a specificpoint in the course of a patient's illness. It is the only reliable determination ofventilation success as evidenced by CO2 content. It constitutes a more precisemeasure of successful gas exchange and oxygenation. ABG sampling is the onlyway of accurately determining the alveolararterial oxygen gradient (see the AaGradient calculator).

Because the results of ABG sampling only reflect the physiologic state of thepatient at the time of the sampling, it is important that they be carefully correlatedwith the evolving clinical scenario and with any changes in the patient’s treatment.

Indications

Indications for ABG sampling include the following:

Identification of respiratory, metabolic, and mixed acidbase disorders, withor without physiologic compensation, by means of pH ([H +]) and CO 2 levels(partial pressure of CO 2) [2, 3]

Measurement of the partial pressures of respiratory gases involved inoxygenation and ventilationMonitoring of acidbase status, as in patient with diabetic ketoacidosis (DKA)on insulin infusion; ABG and venous blood gas (VBG) could be obtainedsimultaneously for comparison, with VBG sampling subsequently used forfurther monitoringAssessment of the response to therapeutic interventions such as mechanicalventilation in a patient with respiratory failureDetermination of arterial respiratory gases during diagnostic evaluations [2, 3](eg, assessment of the need for home oxygen therapy in patients withadvanced chronic pulmonary disease)Quantification of oxyhemoglobin, which, combined with measurement ofarterial oxygen tension (PaO 2), provides useful information about theoxygencarrying capacity of the patientQuantification of the levels of dyshemoglobins (eg, carboxyhemoglobin andmethemoglobin)Procurement of a blood sample in an acute emergency setting when venoussampling is not feasible (many blood chemistry tests could be performedfrom an arterial sample [4] )

Contraindications

Absolute contraindications for ABG sampling include the following:

An abnormal modified Allen test (see below), in which case consideration

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should be given to attempting puncture at a different site [2]Local infection or distorted anatomy at the potential puncture site (eg, fromprevious surgical interventions, congenital or acquired malformations, orburns)The presence of arteriovenous fistulas or vascular grafts, in which casearterial vascular puncture should not be attemptedKnown or suspected severe peripheral vascular disease of the limb involved[2]

Relative contraindications include the following:

Severe coagulopathy [2]Anticoagulation therapy with warfarin, heparin and derivatives, directthrombin inhibitors, or factor X inhibitors; aspirin is not a contraindication forarterial vascular sampling in most cases [2]Use of thrombolytic agents, such as streptokinase or tissue plasminogenactivator [2]

Technical Considerations

ABG sampling may be difficult to perform in patients who are uncooperative or inwhom pulses cannot be easily identified. Challenges arise when health carepersonnel are unable to position the patient properly for the procedure. Thissituation is commonly seen in patients with cognitive impairment, advanceddegenerative joint disease, or essential tremor.

The amount of subcutaneous fat in overweight and obese patients may limit accessto the vascular area and obscure anatomic landmarks.

Arteriosclerosis of peripheral arteries, as is seen in elderly patients and patients withendstage kidney disease, may cause increased rigidity in the vessel wall.

Best practices

The following suggestions may enhance the performance of ABG sampling:

Patients with poor distal perfusion (eg, those in hypovolemic states, withadvanced heart failure, or on vasopressor therapy) may not exhibit a strongarterial pulsation; the operator may need to pull back the ABG syringeplunger to get a blood sample, although this increases the risk of venousblood samplingIf arterial blood flow is not obtained, the operator might slowly pull back theneedle; it is possible that the needle has gone through the vesselInitial arterial flow may subsequently be lost if the needle moves outside thevessel lumen; reidentification of the arterial pulse, using the nondominantmiddle and index finger, and repositioning the needle in the direction of thevessel could be attempted; avoid blind movement of the needle while it isinserted deeply in the patient’s body—pull it back to a point just below theskin, and redirect it to the arterial pulse felt with the other handPuncture of venous structures can be identified by lack of pulsatile flow ordarkcolored blood, though, arterial blood in severely hypoxemic patients canalso have a dark appearance; if venous blood is obtained, removal of theneedle from the patient might be necessary to expel the venous blood fromthe syringeExcessive skin and abundant soft tissue may obstruct the puncture site; theoperator can use the nondominant hand to smooth the skin, or an assistantcan remove the subcutaneous tissue from the puncture site fieldIncomplete dismissal of heparin solution from the syringe could cause falselylow values for the partial pressure of CO 2; to avoid this, the operator shouldexpel all heparin solution from the syringe before arterial punctureIncomplete removal of air bubbles can cause falsely elevated values for thepartial pressure of oxygen; to avoid this, the operator should be sure tocompletely remove air bubbles from the syringe (vented plungers have anadvantage over standard syringes in this regard)Avoid puncture of the brachial artery or femoral artery in patients withdiminished or absent distal pulses; the absence of distal pulses may signalsevere peripheral vascular diseaseWhen femoral or brachial artery puncture is being considered, the use of theultrasound guidance during passage of the needle aids in providing anaccurate roadmap to the vessel and helps minimize inadvertent arterialinjuries

Procedural planning

Planning for the procedure focuses on the choice of a puncture site and accuratedelineation of the vascular anatomy. If radial artery sampling is to be performed, theadequacy 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 thevessel, the presence of collateral circulation, and the artery's superficial courseproximal to the wrist, which makes it easier for the clinician to identify the vascularstructure and hold local pressure after the procedure is finished.

If radial artery sampling is not feasible, femoral artery puncture is a possible

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alternative. When femoral artery puncture is being considered, the potential risk ofinfection at the entry site and the artery's proximity to the femoral vein and nervemust be taken into account. The deeper the vascular structure, the higher the risk ofdamage to adjacent structures.

Femoral artery puncture necessitates prolonged monitoring and therefore isrecommended only in the inpatient setting.[2] Some physicians recommend thatfemoral artery puncture be avoided whenever possible. This consideration may playa bigger role for patients who will be undergoing an intervention that involvesfemoral access (eg, cardiac catheterization or intraaortic 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 achievinghemostasis when necessary is more difficult. Furthermore, the brachial artery is arelatively smallcaliber vessel and does not have extensive collateral circulation. Forthese 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.[2] It also increases the chances of inadvertentvenous blood sampling. If recurrent sampling is required, health care personnelshould alternate puncture sites. If more frequent sampling is necessary, the healthcare provider should consider placing an indwelling arterial catheter through arterialcannulation.

Confirmation of vascular anatomy

ABG sampling can be difficult in patients with feeble pulses or distorted anatomiclandmarks or in situations where sampling of a deep vascular structure (eg, thefemoral or brachial artery) is required. In these scenarios, ultrasoundguided ABGsampling should be considered, especially when sampling by the standard approachhas been unsuccessful or is not feasible.

Ultrasonography is a noninvasive technique with an excellent safety profile. It is animportant option in cases where vascular sampling proves difficult. The use ofultrasonography enables more accurate recognition, delineation, and targeting ofthe chosen vascular structure. It minimizes the risk of vascular laceration anddamage to surrounding structures.

Although ultrasound guidance is a safe and effective tool for patients with weakpulses, a better option for patients in profound shock or in the midst ofcardiopulmonary resuscitation is to obtain arterial blood from the femoral arteryusing bony landmarks alone. When the tip of the fifth finger is placed on thesymphysis pubis and the tip of the thumb on the anterior superior iliac spine, thefemoral 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 performedbeforehand when feasible to assess the collateral circulation. Although the anatomyof the radial artery in the forearm and the hand is variable, most patients haveadequate collateral flow should radial artery thrombosis occur.[1] The modified Allentest is performed as follows.[1]

Firm occlusive pressure is held on both the radial artery and the ulnar artery (seethe first image below). The patient is asked to clench the fist several times until thepalmar 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 falsenormal results.[5] The pressure on the ulnar artery isreleased while occlusion of the radial artery is maintained (see the third imagebelow). The time required for palmar capillary refill is noted.

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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 ulnarartery remains compressed (inverse modified Allen test; see the image below).

Modified Allen test: radial artery occlusion released.

The modified Allen’s test has been the method most frequently used for clinical

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assessment of the adequacy of ulnar artery collateral circulation and the patency ofthe palmar arches of the hand. However, there is some controversial evidencesuggesting that it can predict ischemic complications in the setting of radial arteryocclusion.[1]

Given the low positive predictive value of the modified Allen test, the examinershould consider further testing to assess patency of circulation, such as finger pulseplethysmography, Doppler flow measurements, and measurement of the arterialsystolic pressure of the thumb.[5]

Whether the modified Allen test is clinically reliable as a screening test for adequatecollateral circulation of the hand is controversial. A wide range of values for handreperfusion have been noted, and normal values are not consistent (ranging from 3to 15 seconds)[1, 5, 6] ; furthermore, there is conflicting evidence regarding thevalidity of the modified Allen test as a standard of care.[6, 7]

Complication prevention

Although patients with severe coagulopathy are at higher risk for bleedingcomplications, no clear evidence on the safety of arterial puncture in the setting ofcoagulopathy exists. In patients with coagulopathy, careful evaluation of the needfor ABG sampling is recommended.

Periprocedural Care

Patient Education/Informed Consent

Health care personnel should explain the arterial blood gas (ABG) samplingprocedure to the patient, with particular attention to the associated risks andbenefits. However, this may not be possible in certain clinical scenarios, such as acritically ill patient with rapid decompensation or a patient with an altered level ofconsciousness (eg, from encephalopathy or advanced dementia). Written consent isnot necessary.

Patients should be asked not to manipulate the area involved in the procedure andshould be instructed to alert health care personnel if new symptoms develop, suchas skin color changes, persistent or worsening pain, active bleeding, impairedmovement, or sensation of the limb.

Equipment

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

Arterial blood gas sampling equipment.

See the list below:

Gloves Nonsterile gloves may be used, but care must be taken not to touchthe puncture site after cleaning the areaSyringe for sampling A standard syringe with a 25gauge needle and a 3mL capacity (a highercapacity syringe may be difficult to maneuver, andsmaller needle sizes may increase the risk of traumatic hemolysis,decreasing the accuracy of hemoglobin and potassium measurements)Lithium heparin 12 mL lithium heparin (1000 U/mL) should be aspiratedinto the syringe through the needle and then pushed out; the plunger shouldbe left depressed to allow the arterial blood flow to fill up the syringeABG syringe (alternative) Some ABG kits contain a prefilled heparinizedsyringe along with a protective needle sleeve and a syringe cap (see theimage below); the sleeve, while still attached to the syringe, locks the needlewithin itself to prevent direct contact between operator and needle; somesyringe models have a vented plunger that allows the operator to preset aspecific amount of blood to be withdrawn, and with these models, theplunger is placed midway through the syringe and is not pulled back whilethe puncture is performed; before the procedure, the prefilled heparin isexpelled, and the vented plunger is then repositioned at the 2 mL mark

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Arterial blood gas syringe kit.

Antiseptic skin solution Chlorhexidine and povidoneiodine are solutionscommonly usedSyringe cap Usually included in the ABG syringe kit2 × 2 in. piece of sterile gauzeAdhesive bandageBag with iceSharp object containerLidocaine HCl 1% without epinephrine (optional)25gauge needle with syringe for local anesthetic (optional)

Patient Preparation

Appropriate positioning of the patient and knowledge of the vascular anatomyincrease the chances of a successful arterial vascular sampling and diminish the riskof complications.

Anesthesia

Local anesthesia with a subcutaneous injection of lidocaine HCl 1% withoutepinephrine may be used. Local anesthesia is not frequently employed, however,because the administration of the anesthetic is as painful as the procedure itself.

If local anesthesia is employed, 0.51 mL of the anesthetic is injected so as tocreate a small dermal papule at the site of puncture; using larger amounts orinjecting the anesthetic into deeper planes may distort the anatomy and hinderidentification of the vessel. After the skin is punctured but just before the anestheticis injected, the clinician should pull back the plunger to confirm that the needle isnot inside a blood vessel; intravascular placement will be signaled by blood filling upthe anesthetic syringe.

Positioning

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

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

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

Monitoring and Followup

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

A rapidly expanding hematoma may compromise regional circulation and increasethe risk of compartment syndrome, especially in the forearm. This manifests aspain, paresthesias, pallor, and absence of pulses. Paresis and persistent pain mayindicate a nerve lesion. Limb skin color changes, absent pulses, and distal coldness

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may be seen in ischemic injury from artery occlusion caused by thrombus formationor vasospasm. Infection at the puncture site should be considered in the presenceof regional erythema and fever.

Technique

Approach Considerations

Health care personnel should wear gloves and eye protection for the duration of thearterial blood gas (ABG) sampling procedure and should follow hospital policiesregarding management of body fluid samples. The operator should have all therequired 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 thenondominant middle finger distally and the nondominant index finger proximally,with the needle insertion site in between, is strongly discouraged, because of theincreased risk of needle stick injury.

If the ABG syringe is to be used, the protective needle sleeve and needle should beplaced onto the syringe, the prefilled heparin expelled, and the vented plungerrepositioned to the 2 mL mark.

Arterial blood samples should be obtained in strict anaerobic conditions and shouldbe placed on ice and held at 0° C until analysis.[8] Any air bubbles introduced duringthe sampling procedure will lead to overestimation of arterial oxygen tension (PaO2)and underestimation of arterial carbon dioxide tension (PaCO2).[8]

Keeping the sample at lower temperatures slows cellular metabolism and reducesongoing consumption of oxygen.[8] This is especially important in patients withleukocytosis.[9]

Radial Artery Blood Sampling

The radial artery is most easily accessible medial to the radial styloid process andlateral to the flexor carpi radialis tendon, 23 cm proximal to the ventral surface ofthe wrist crease (see the image below).

Anatomic location of radial artery.

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

Palpate the patient's radial pulse with the index and middle finger pads of thenondominant 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 antisepticsolution (see the second image below).

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Identification of radial pulse.

Cleaning of desired radial artery puncture site.

Uncap the ABG syringe, and hold it with 2 fingers of the dominant hand. Theneedle bevel should be facing upward. Insert the needle just under the skin at a 45ºangle, aiming in the direction of the artery, while palpating the radial pulse proximalto the puncture site with the nondominant hand (see the image below). Angling theneedle in this fashion minimizes trauma to the vessel and allows smooth musclefibers 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 theplunger, unless an unvented plunger with a small (25gauge) needle is being usedor the patient has a weak pulse.

Radial artery puncture.

After 23 mL of arterial blood has been obtained, remove the needle. At the sametime, use a small piece of gauze, held in the nondominant hand, to apply firmocclusive 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 atleast 5 minutes. In patients who have a coagulopathy or are on anticoagulation

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therapy, it may be necessary to apply local pressure for a longer time. Check forhemostasis, and apply an adhesive bandage over the puncture site.

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

Apply the needle protective sleeve (see the first image below), then untwist thesleeve 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 theycan then be expelled (see the first image below). Cap the syringe, place it in the icebag, and send it for analysis (see the second image below).

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Removal of air bubbles from syringe.

Capping of syringe.

Femoral Artery Blood Sampling

The femoral artery is best identified in the midline between the symphysis pubis andthe anterior superior iliac crest, 24 cm distal to the inguinal ligament. The femoralartery is medial to the femoral nerve and lateral to the femoral vein (see the imagebelow).

Anatomy of femoral triangle.

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

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The procedure is performed as follows.

Palpate the patient's femoral pulse with the index and middle finger pads of thenondominant 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 antisepticsolution (see the second image below).

Identification of femoral artery.

Cleaning of desired femoral artery puncture site.

Uncap the ABG syringe, and hold it with 2 fingers of the dominant hand. Theneedle bevel should be facing upward. Insert the needle just under the skin at a 6090º angle, aiming in the direction of the artery, while palpating the femoral pulseproximal to the puncture site with the nondominant hand (see the image below).

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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 theplunger, unless an unvented plunger with a small (25gauge) needle is being usedor the patient has a weak pulse.

Femoral artery puncture.

After 23 mL of arterial blood has been obtained, remove the needle. At the sametime, use a small piece of gauze, held in the nondominant hand, to apply firmocclusive local pressure at the puncture site for 5 minutes (see the image below). Inpatients who have a coagulopathy or are on anticoagulation therapy, it may benecessary to apply local pressure for a longer time. Check for hemostasis, and applyan adhesive bandage over the puncture site. Recheck the distal pulses.

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Removal of needle from femoral artery puncture site and application of local pressure forhemostasis.

Apply the needle protective sleeve, then untwist the sleeve and place it in the sharpobject 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 theycan then be expelled. Cap the syringe, place it in the ice bag, and send it foranalysis.

Brachial Artery Blood Sampling

The brachial artery is best identified between the medial epicondyle of the humerusand the tendon of the biceps brachii in the antecubital fossa. It can be felt higher inthe arm in the groove between the biceps and triceps tendons. The basilic vein andthe 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 beassessed before the procedure. In patients with absent pulses at the wrist (ie, in theradial and ulnar arteries), an alternative site for arterial sampling should beconsidered.

The procedure is performed as follows.

Palpate the patient's brachial pulse with the index and middle finger pads of thenondominant 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 antisepticsolution (see the second image below).

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Identification of brachial artery.

Cleaning of desired brachial artery puncture site.

Uncap the ABG syringe, and hold it with 2 fingers of the dominant hand. Theneedle bevel should be facing upward. Insert the needle just under the skin at a 4560º angle, aiming in the direction of the artery, while palpating the brachial pulseproximal 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 theplunger, unless an unvented plunger with a small (25gauge) needle is being usedor the patient has a weak pulse.

Brachial artery puncture.

After 23 mL of arterial blood has been obtained, remove the needle. At the sametime, use a small piece of gauze, held in the nondominant hand, to apply firmocclusive local pressure at the puncture site for 5 minutes (see the image below). In

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patients who have a coagulopathy or are on anticoagulation therapy, it may benecessary to apply local pressure for a longer time. Check for hemostasis, and applyan 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 forhemostasis.

Apply the needle protective sleeve, then untwist the sleeve and place it in the sharpobject 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 theycan then be expelled. Cap the syringe, place it in the ice bag, and send it foranalysis.

Complications of Procedure

Complications of ABG sampling include the following:

Local hematoma [3]

Artery vasospasm [3]

Arterial occlusion [3]

Air or thrombus embolism [3]Local anesthetic anaphylactic reactionInfection at the puncture site [3]

Needle stick injury to health care personnel [3]

Vessel laceration [2]

Vasovagal response [2]

Hemorrhage [3]

Local pain [2]

Results

Results are usually available within 515 minutes. Aberrant results may result fromcontamination with room air, resulting in abnormally low carbon dioxide and nearnormal oxygen levels. Delays in analysis of the blood tube allow for ongoing cellularrespiration and may lead to errors with inaccurately low oxygen and high carbondioxide levels reported in the results.

The arterial blood gas (ABG) test may determine concentrations of lactate,hemoglobin, electrolytes, oxyhemoglobin, carboxyhemoglobin, and methemoglobin.

Values at sea level include the following:

Partial pressure of oxygen (PO 2) 75100 mm HgPartial pressure of carbon dioxide (PCO 2) 3545 mm HgArterial blood pH 7.387.42Oxygen saturation (SaO 2) 94%100%Bicarbonate (HCO 3) 2226 mEq/L

ABG testing is the criterion standard for determining the adequacy of ventilatorysupport and the relationship between pH, pO2, pCO2, and HCO3 in the humanbody.[10, 11, 12] These results help to determine if the patient is inmetabolic/respiratory alkalosis/acidosis with or without an anion gap. See the AnionGap calculator.

The pH level indicates whether a patient is acidemic (pH < 7.35) or alkalemic (pH>7.45). The partial pressure of oxygen (pO2) shows the level of oxygenation in thebody. The partial pressure of carbon dioxide (pCO2) indicates the degree of CO2production or elimination via the respiratory cycle. An elevated or decreased pCO2(ie, respiratory acidosis or respiratory alkalosis, respectively) is an indication of theappropriateness of ventilation.

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

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Base excess = 0.93 X ([HCO3] 24.4 + 14.8 X [pH 7.4])

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

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

AG = (Na) (Cl+ HCO3)

Normal range is 816 mEq/L.

Contributor Information and DisclosuresAuthorMauricio Danckers, MD Pulmonary and Critical Care Physician, Aventura Medical Center

Mauricio Danckers, MD is a member of the following medical societies: American College of Chest Physicians,American Medical Association

Disclosure: Nothing to disclose.

Coauthor(s)Ethan D Fried, MD, MS Associate Professor of Medicine, Hofstra North ShoreLIJ School of Medicine;Associate Designated Institutional Official, Associate Chair for Education, Department of Medicine, Member,Division of Pulmonary/Critical Care Medicine, Lenox Hill Hospital

Ethan D Fried, MD, MS is a member of the following medical societies: American College of Physicians,Association of Program Directors in Internal Medicine

Disclosure: Nothing to disclose.

Specialty Editor BoardMary L Windle, PharmD Adjunct Associate Professor, University of Nebraska Medical Center College ofPharmacy; EditorinChief, Medscape Drug Reference

Disclosure: Nothing to disclose.

Chief EditorVincent Lopez Rowe, MD Professor of Surgery, Program Director, Vascular Surgery Residency, Department ofSurgery, Division of Vascular Surgery, Keck School of Medicine of the University of Southern California

Vincent Lopez Rowe, MD is a member of the following medical societies: American College of Surgeons,American Heart Association, Society for Vascular Surgery, Vascular and Endovascular Surgery Society, Societyfor Clinical Vascular Surgery, Pacific Coast Surgical Association, Western Vascular Society

Disclosure: Nothing to disclose.

AcknowledgementsA special thankyou to Dr Susan Nathan and Mr Kyle Pursell for their contributions to the realization of thisarticle.

References

1. Brzezinski M, Luisetti T, London MJ. Radial artery cannulation: a comprehensive review of recent anatomicand physiologic investigations. Anesth Analg. 2009 Dec. 109(6):176381. [Medline].

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27/8/2015 Arterial Blood Gas Sampling: Overview, Periprocedural Care, Technique

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