How to Draw a Venous Blood Gas Sample

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.

5.1. Background information on 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.

5.1.1. Choice of site

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.

5.1.2. Complications related to arterial blood sampling

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.

5.1.3. Sampling errors

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.

5.2. Practical guidance on arterial blood sampling

5.2.1. Equipment and supplies

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.

5.2.2. Procedure for arterial blood sampling using radial artery

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

1. Approach the patient, introduce yourself and ask the patient to state their full name.

2. 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.

3. 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.

4. 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.

5. Disinfect the sampling site on the patient with 70% alcohol and allow it to dry.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. Label the sample syringe.

14. Remove gloves and wash hands thoroughly with soap and water, then dry using single-use towels; alternatively, use alcohol rub solution.

15. Check the patient site for bleeding (if necessary, apply additional pressure) and thank the patient.

16. Transport the sample immediately to the laboratory, following laboratory handling procedures.

5.3. Illustrations for arterial blood sampling

Figure 5.1 Arterial blood sampling

How to Draw a Venous Blood Gas Sample

Source: https://www.ncbi.nlm.nih.gov/books/NBK138661/

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