Why would you draw blood from an artery

Medically Reviewed by Sabrina Felson, MD on September 09, 2021

An arterial blood gas (ABG) test measures oxygen and carbon dioxide levels in your blood. It also measures your body’s acid-base (pH) level, which is usually in balance when you’re healthy.

You may get this test if you’re in the hospital or if you have a serious injury or illness.

The test gives your doctor clues about how well your lungs, heart, and kidneys are working. You’ll probably get other tests along with it.

Every cell in your body needs oxygen to live. When you breathe in (inhale) and breathe out (exhale), your lungs move oxygen into your blood and push carbon dioxide out. That process, called gas exchange, provides the oxygen we (and all of our cells) need to survive.

If you are having a hard time breathing, your doctor may use an arterial blood gas (ABG) test to help figure out what the problem is.

Your doctor may ask for an arterial blood gas test to:

You are likely to have an arterial blood gas test in a hospital, but your doctor may be able to do it in their office.

Your doctor or another health care worker will use a small needle to take some of your blood, usually from your wrist. Sometimes they take it from an artery in your groin or on the inside of your arm above your elbow.

Before the arterial blood gas test, they may apply pressure to the arteries in your wrist for several seconds. The procedure, called the modified Allen test, checks that blood flow to your hand is normal.

In order to prepare for an arterial blood gas test, tell your doctor about all medications, supplements, and vitamins you’re taking.

If you’re on oxygen therapy but are able to breathe without it, they might turn off your oxygen for 20 minutes for a “room air” test before the blood gas test.

You may have a few minutes of discomfort during or after the test. Collecting blood from an artery typically hurts more than drawing it from a vein. Arteries are deeper than veins, and there are sensitive nerves nearby.

You also may feel lightheaded, faint, dizzy, or nauseated while your blood is drawn. To lower the chance of bruising, you can gently press on the area for a few minutes after the needle comes out.

Any test that involves using a needle carries some risks, including bleeding, infection, and bruising. You might have some soreness where the needle went in.

Results of your arterial blood gas test usually are available in less than 15 minutes. But your doctor can’t diagnose you based on an arterial blood gas test alone. So you’ll probably get other tests, too.

Your arterial blood gas test results may show whether:

You are are getting enough oxygen
Your lungs are exhaling enough carbon dioxide
Your kidneys are working properly
You have a serious infection effecting perfusion (blood delivery) to your organs

The numbers for normal results vary. Your doctor will interpret your ABG results as they relate to your medical history and your current condition. Your ABG results will play a part in your diagnosis and treatment.

Arterial blood draw, or hemogasanalysis, is an operation that is usually used to evaluate the effectiveness of gaseous exchanges or oxygen therapies, or to verify a suspected acid-base imbalance.

In detail, arterial blood sampling allows to measure:

the pH of the blood;
partial pressure of arterial oxygen;
the partial pressure of carbon dioxide;
oxygen saturation;
blood levels of bicarbonate (HCO3).

This activity is considered to be of medical competence, but when can it also be performed by nurses? Let's see what the reference standards say.

Arterial withdrawal by nurses: regulations

On January 23, 2005, the Superior Health Council of the Ministry of Health expressed its favorable opinion on the exercise of the arterial blood draw by nurses as well, however, setting two conditions:

the nurse must have acquired complete competence in the procedure, according to the procedures defined by current regulations (L.42 / 1999 art. 1 point 2 paragraph 2);
for the related medical and nursing implications, the existence - in the operating unit or in the reference healthcare facility - of a correctly drafted, shared and approved operating protocol must always be provided.

This protocol must be able to:

ensure good practice of arterial sampling technique from the radial artery for haemogasanalysis;
ensure the adoption of all useful complication prevention measures and necessary treatment, in addition to the timely management of related risks.

Hemogasanalysis: the importance of using effective arterial catheters

Hemogasanalysis is an operation that can be associated with several risks, for example bleeding, infections, vessel rupture, distal ischemia.

In addition, arterial puncture can also be painful for the patient, therefore an anesthetic ointment is usually applied to reduce pain.

A fundamental aspect, before the hemogasanalysis procedure, is to evaluate the patency of the ulnar and radial artery, using the Allen test. It involves the following steps:

the patient maintains a clenched fist for about 30 seconds;
the nurse simultaneously compresses the ulnar and radial artery;
the patient reopens his hand, which in the meantime appears pale;
the nurse releases the compression on the ulnar artery.

The hand recoloring time should be 5-7 seconds, otherwise it is not recommended to prick or cannulate the radial artery.

The use of arterial cannula for the measurement of blood pressure and hemogasanalysis allows to reduce the number of punctures and also decrease the risks of infection for the patient and the healthcare professional.

The arterial catheters of the Alfacath line guarantee:

a safe and uncomplicated positioning;
atraumatic artery puncture;
excellent detection of the pressure wave throughout the patient monitoring phase.

To find out more, click here or contact us for more information on configurations, diameters and sizes.

The information given here supplements that given in Chapters 2 and 3. Users of these guidelines should read Chapters 2 and 3 before reading the information given below. This chapter covers background information (Section 5.1), practical guidance (Section 5.2) and illustrations (Section 5.3) relevant to arterial blood sampling.

An arterial blood sample is collected from an artery, primarily to determine arterial blood gases. Arterial blood sampling should only be performed by health workers for whom the procedure is in the legal scope of practice for their position in their country and who have demonstrated proficiency after formal training.

The sample can be obtained either through a catheter placed in an artery, or by using a needle and syringe to puncture an artery. These syringes are pre-heparinized and handled to minimize air exposure that will alter the blood gas values. This chapter describes only the procedure for a radial artery blood draw.

Several different arteries can be used for blood collection. The first choice is the radial artery, which is located on the thumb side of the wrist; because of its small size, use of this artery requires extensive skill in arterial blood sampling. Alternative sites for access are brachial or femoral arteries, but these have several disadvantages in that they:

may be harder to locate, because they are less superficial than the radial artery;
have poor collateral circulation;
are surrounded by structures that could be damaged by faulty technique.

There are several potential complications related to arterial blood sampling. The points below list some of the complications related to the procedure, and how they can be prevented (59).

Arteriospasm or involuntary contraction of the artery may be prevented simply by helping the patient relax; this can be achieved, for example, by explaining the procedure and positioning the person comfortably.
Haematoma or excessive bleeding can be prevented by inserting the needle without puncturing the far side of the vessel and by applying pressure immediately after blood is drawn. Due to the higher pressure present in arteries, pressure should be applied for a longer time than when sampling from a vein, and should be supervised more closely, to check for cessation of bleeding.
Nerve damage can be prevented by choosing an appropriate sampling site and avoiding redirection of the needle.
Fainting or a vasovagal response can be prevented by ensuring that the patient is supine (lying down on their back) with feet elevated before beginning the blood draw. Patients requiring arterial blood sampling are usually inpatients or in the emergency ward, so will generally already be lying in a hospital bed. Children may feel a loss of control and fight more if placed in a supine position; in such cases, it may be preferable to have the child sitting on the parent's lap, so that the parent can gently restrain the child.
Other problems can include a drop in blood pressure, complaints of feeling faint, sweating or pallor that may precede a loss of consciousness.

Inappropriate collection and handling of arterial blood specimens can produce incorrect results. Reasons for an inaccurate blood result include:

presence of air in the sample;
collection of venous rather than arterial blood;
an improper quantity of heparin in the syringe, or improper mixing after blood is drawn;
a delay in specimen transportation.

Assemble the relevant items described in Section 2.2.3, plus the following specimen collection equipment and supplies:

pre-heparinized syringe;
needles (20, 23 and 25 gauge, of different lengths) – choose a size that is appropriate for the site (smaller gauges are more likely to lyse the specimen);
a safety syringe with a needle cover that allows the syringe to be capped before transport, without manually recapping (this is best practice for radial blood sampling);
a bandage to cover the puncture site after collection;
a container with crushed ice for transportation of the sample to the laboratory (if the analysis is not done at the point of care);
where applicable, local anesthetic and an additional single-use sterile syringe and needle.

For sampling from the radial artery using a needle and syringe, follow the steps outlined below.

Approach the patient, introduce yourself and ask the patient to state their full name.
Place the patient on their back, lying flat. Ask the nurse for assistance if the patient's position needs to be altered to make them more comfortable. If the patient is clenching their fist, holding their breath or crying, this can change breathing and thus alter the test result.
Locate the radial artery by performing an Allen test (see Annex I) for collateral circulation. If the initial test fails to locate the radial artery, repeat the test on the other hand. Once a site is identified, note anatomic landmarks to be able to find the site again. If it will be necessary to palpate the site again, put on sterile gloves.
Perform hand hygiene, clear off a bedside work area and prepare supplies. Put on an impervious gown or apron, and face protection, if exposure to blood is anticipated.
Disinfect the sampling site on the patient with 70% alcohol and allow it to dry.
If the needle and syringe are not preassembled, assemble the needle and heparinized syringe and pull the syringe plunger to the required fill level recommended by the local laboratory.
Holding the syringe and needle like a dart, use the index finger to locate the pulse again, inform the patient that the skin is about to be pierced then insert the needle at a 45 degree angle, approximately 1 cm distal to (i.e. away from) the index finger, to avoid contaminating the area where the needle enters the skin.
Advance the needle into the radial artery until a blood flashback appears, then allow the syringe to fill to the appropriate level. DO NOT pull back the syringe plunger.
Withdraw the needle and syringe; place a clean, dry piece of gauze or cotton wool over the site and have the patient or an assistant apply firm pressure for sufficient time to stop the bleeding. Check whether bleeding has stopped after 2–3 minutes. Five minutes or more may be needed for patients who have high blood pressure or a bleeding disorder, or are taking anticoagulants.
Activate the mechanisms of a safety needle to cover the needle before placing it in the ice cup. In the absence of a safety-engineered device, use a one-hand scoop technique (as explained in Annex G) to recap the needle after removal.
Expel air bubbles, cap the syringe and roll the specimen between the hands to gently mix it. Cap the syringe to prevent contact between the arterial blood sample and the air, and to prevent leaking during transport to the laboratory.
Label the sample syringe.
Remove gloves and wash hands thoroughly with soap and water, then dry using single-use towels; alternatively, use alcohol rub solution.
Check the patient site for bleeding (if necessary, apply additional pressure) and thank the patient.
Transport the sample immediately to the laboratory, following laboratory handling procedures.