Respiratory therapists routinely use the information from an arterial blood gas to make decisions regarding patient care and mechanical ventilation changes. A respiratory therapist should also be able to safely perform an arterial puncture. Several different arteries can be used for blood collection. These include radial, brachial, femoral and dorsalis pedis. 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 the brachial or femoral arteries, but these have the following disadvantages:

• They may be harder to locate because they are less superficial than the radial artery.
• They have poor collateral circulation.
• They are surrounded by structures that could be damaged by faulty technique.
• They may require longer periods of post sampling pressure/occlusion for the site to clot, especially in the presence of blood thinners.

Prior to performing a radial puncture, a modified Allen test should be performed. The modified Allen test is used to measure arterial competency and should be performed before taking an arterial sample. This ensures that there is adequate collateral circulation through the ulnar artery in the event that the radial artery is damaged during the blood draw^[1].

1. Instruct the patient to clench his or her fist; if the patient is unable to do this, close the person’s hand tightly.
2. Using your fingers, apply occlusive pressure to both the ulnar and radial arteries to obstruct blood flow to the hand.
3. While applying occlusive pressure to both arteries, have the patient relax their hand and check whether the palm and fingers have blanched. If this is not the case, you have not completely occluded the arteries with your fingers.
4. Release the occlusive pressure on the ulnar artery only to determine whether the modified Allen test is positive or negative

Once the Allen test is determined to be positive, then it is safe to perform the arterial blood puncture and retrieve a sample. An arterial blood sample is collected from an artery primarily to determine arterial blood gasses. 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.

Complications Related to Arterial Blood Sampling

There are several potential complications related to arterial blood sampling. The items below list some of the complications related to the procedure and how they can be prevented:

• 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.
• Hematoma 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. Ensure that you always check the chart to see if the patient is on blood thinners.
• 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 their feet elevated before beginning the blood draw. Patients requiring arterial blood sampling are usually inpatients or in the emergency ward, so they 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 a 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, or sweating or pallor that may precede a loss of consciousness.

Sampling Errors

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

• Presence of air in the sample
• Collection of venous blood rather than arterial blood
• An improper quantity of heparin in the syringe or improper mixing after blood is drawn
• A delay in specimen transportation