REVISIÉN DE LA DIFERENTES TÉCNICAS PARA LA TOMA DE MUESTRA PARA GASOMETRIA ARTERIAL.

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Arterial Blood Gas Sampling
Author: 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. 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 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, and passes along the radial side of the forearm to
the wrist.
ABG sampling provides valuable information on the acid­base 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 alveolar­arterial oxygen gradient (see the A­a
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
Indications for ABG sampling include the following:
Identification of respiratory, metabolic, and mixed acid­base disorders, with
or 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 in
oxygenation and ventilation
Monitoring of acid­base 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 [2, 3]
(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
oxygen­carrying 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 [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, 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
[2]
Relative contraindications include the following:
Severe coagulopathy [2]
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 [2]
Use of thrombolytic agents, such as streptokinase or tissue plasminogen
activator [2]
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
end­stage 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, 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, although 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
dark­colored 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 the
ultrasound guidance during passage of the needle aids in providing an
accurate roadmap to the vessel and helps minimize inadvertent arterial
injuries
Procedural planning
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

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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.[2] 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 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 achieving
hemostasis when necessary is more difficult. Furthermore, the brachial artery is a
relatively small­caliber 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.[2] It also increases the chances of inadvertent
venous blood sampling. If recurrent sampling is required, health care personnel
should alternate puncture sites. If more frequent sampling is necessary, the health
care 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, ultrasound­guided ABG
sampling should be considered, especially when sampling by the standard approach
has been unsuccessful or is not feasible.
Ultrasonography is a noninvasive technique with an excellent safety profile. It is an
important option in cases where vascular sampling proves difficult. The use of
ultrasonography 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 ultrasound 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 false­normal results.[5] 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.

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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 ulnar
artery 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 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.[5]
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, 5, 6] ; furthermore, there is conflicting evidence regarding the
validity of the modified Allen test as a standard of care.[6, 7]
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.
Periprocedural Care
Patient Education/Informed Consent
Health care 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 health care personnel if new symptoms develop, such
as skin color changes, persistent or worsening pain, active bleeding, impaired
movement, or sensation of the limb.
Equipment
The materials needed for ABG sampling (see the image below) include the
following:
Arterial blood gas sampling equipment.
See the list below:
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 25­gauge needle and a 3­
mL capacity (a higher­capacity 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)
Lithium heparin ­ 1­2 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
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

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Arterial blood gas syringe kit.
Antiseptic skin solution ­ Chlorhexidine and povidone­iodine are solutions
commonly used
Syringe cap ­ Usually included in the ABG syringe kit
2 × 2 in. piece of sterile gauze
Adhesive bandage
Bag with ice
Sharp object container
Lidocaine 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 anatomy
increase the chances of a successful arterial vascular sampling and diminish the risk
of complications.
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.
If local anesthesia is employed, 0.5­1 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.
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.
Monitoring and Follow­up
After the blood sampling procedure, health care 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

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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.
Technique
Approach Considerations
Health care 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.[8] 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).[8]
Keeping the sample at lower temperatures slows cellular metabolism and reduces
ongoing consumption of oxygen.[8] This is especially important in patients with
leukocytosis.[9]
Radial Artery Blood Sampling
The radial artery is most easily accessible medial to the radial styloid process and
lateral to the flexor carpi radialis tendon, 2­3 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 Introduction).
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).

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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. 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 (25­gauge) needle is being used
or the patient has a weak pulse.
Radial artery puncture.
After 2­3 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

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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).

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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 and
the anterior superior iliac crest, 2­4 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.

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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 2 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).

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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 the
plunger, unless an unvented plunger with a small (25­gauge) needle is being used
or the patient has a weak pulse.
Femoral artery puncture.
After 2­3 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.

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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.
Brachial Artery Blood Sampling
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).

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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. 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 (25­gauge) needle is being used
or the patient has a weak pulse.
Brachial artery puncture.
After 2­3 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

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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 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 reaction
Infection 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 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 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 Hg
Partial pressure of carbon dioxide (PCO 2) ­ 35­45 mm Hg
Arterial blood pH ­ 7.38­7.42
Oxygen saturation (SaO 2) ­ 94%­100%
Bicarbonate (HCO 3) ­ 22­26 mEq/L
ABG testing is the criterion standard for determining the adequacy of ventilatory
support and the relationship between pH, pO2, pCO2, and HCO3 in the human
body.[10, 11, 12] 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 partial pressure of oxygen (pO2) shows the level of oxygenation in the
body. The partial pressure of carbon dioxide (pCO2) indicates the degree of CO2
production or elimination via the respiratory cycle. An elevated or decreased pCO2
(ie, respiratory acidosis or respiratory alkalosis, respectively) is an indication of the
appropriateness of ventilation.
The bicarbonate ion (HCO3) 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:

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http://emedicine.medscape.com/article/1902703­overview#a1 16/17
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 (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+ HCO3)
Normal range is 8­16 mEq/L.
Contributor Information and Disclosures
Author
Mauricio 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 Board
Mary L Windle, PharmD Adjunct Associate Professor, University of Nebraska Medical Center College of
Pharmacy; Editor­in­Chief, Medscape Drug Reference
Disclosure: Nothing to disclose.
Chief Editor
Vincent Lopez Rowe, MD Professor of Surgery, Program Director, Vascular Surgery Residency, Department of
Surgery, 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, Society
for Clinical Vascular Surgery, Pacific Coast Surgical Association, Western Vascular Society
Disclosure: Nothing to disclose.
Acknowledgements
A special thank­you to Dr Susan Nathan and Mr Kyle Pursell for their contributions to the realization of this
article.
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