Seven steps help to obtain blood gas samples with safe and consistent results
Arterial blood gases (ABGs) provide crucial information for assessing acid-base status and evaluating a patients ventilation and oxygenation requirements. Due to the important information gained by ABG measurements, it has become the most frequently ordered test in the intensive care areas.1 It is virtually impossible to manage critically ill and ventilated patients without accurate and current ABG data. Mixed venous blood is unacceptable for evaluating these parameters because it can be misleading due to the effects of metabolic activity at the limb in which the sample was taken. While ABGs represent the gold standard for evaluating ventilatory status, each ABG needle stick represents a problematic situation for both the patient and the clinician because it is potentially painful for the patient and more technically difficult to perform than a venipuncture.
the need to assess ventilatory status, acid-base balance, and oxygenation and the oxygen-carrying capacity of the blood;
to assess a patients response to therapeutic intervention (ventilator management) or the progression of a disease process; and
for diagnostic evaluation (as in pulmonary rehabilitation or pulmonary stress tests).
a site in an area with a shunt, fistula, or lesion should never be selected;
a negative result on a modified Allen test;
the need for close monitoring of a femoral puncture site for an extended period after the procedure negates the use of that site outside the hospital; and
administration of medium to high levels of anticoagulants can represent a relative contraindication.
Evaluate the Patient and the orders
Clinicians should understand the reason why they are obtaining any ABG. In some cases, noninvasive monitoring such as pulse oximetry, transcutaneous gas monitoring, or capnography may be suggested. In other cases, the procedure may be reconsidered if it is not clear that the results of the test will alter the course of treatment. An arterial line should be suggested when multiple blood gas studies are anticipated.
The patients activity level, breathing pattern, and supplemental oxygen all affect the outcome of an ABG; therefore, the procedure should be performed with the patient in a steady state. The interface of any supplemental oxygen should always be evaluated for correct settings and proper application.
It is essential to ease patient apprehensions by discussing the reasons for the test and explaining the procedure in terms they understand. Most patients will have experience with venipunctures, but may not realize that an arterial stick is not the same thing.
Choose the Site
Arterial blood samples are normally obtained from adults at the radial, brachial, femoral, or dorsalis pedis arteries. Because radial artery puncture is relatively safe and the site easily accessible as well as convenient for checking collateral circulation, this site is preferable.2 If collateral circulation is absent, the radial artery should never be used. The brachial artery is the second choice, as it is relatively large and easy to palpate, and has good collateral circulation; however, it lies deeper and its proximity to the basilic vein and median nerve makes it easy to hit them by mistake. In addition, the lack of underlying ligaments or bone support increases the risk of hematoma following the procedure. The femoral artery is the third choice because it is relatively easy to palpate and it is sometimes the only site where sampling will be possible; however, it lies close to the femoral vein, poses increased risk of infection, and requires prolonged monitoring after puncture. It should be selected as a last resort and only within a hospital setting.
Acquiring a blood sample at the dorsalis pedis artery is easily performed, reliable and relatively safe.3 Clinicians may wish to explore the procedures unique to this often overlooked location and gain proficiency in utilizing the dorsalis pedis artery as a useful site for ABG punctures when a radial artery sample is not obtainable.
Even an easy stick can prove challenging and clinicians should never attempt the maneuver from an awkward position. A comfortable setting is essential to proper technique. The patient should be lying down or sitting with the arm well supported and the clinician should also be seated if possible.
A rolled towel positioned beneath the wrist helps hyperextend the site while the pulse is carefully palpated. Note that sometimes palpating too firmly can occlude the artery enough to prevent blood from flowing into the syringe, even though the artery has been penetrated.2
If the syringe needs to be repositioned, the tip should be withdrawn to the subcutaneous tissue to prevent severing the artery or tendons with the needle. Repeated puncture of a single site increases the likelihood of hematoma, scarring, or laceration of the artery. Care should be exercised to use alternate sites for patients requiring multiple punctures.4
Perform a Modified Allen Test
A modified Allen test is always performed to assess collateral circulation before performing a radial artery puncture. The clinician uses his fingertips to obstruct blood flow from both the radial and ulnar arteries while the patient is directed to form a clenched fist. Next, the patient is directed to open the hand, revealing a blanched palm and fingers. Pressure is removed from the ulnar artery and the hand pinks up, confirming collateral blood flow.
Prepare the Site
A 70% isopropyl alcohol wipe should be used to thoroughly clean the skin around the site. Local anesthetic is not generally considered necessary for single punctures,4 as experienced clinicians who have carefully prepared and palpated the site can complete the procedure in a matter of seconds.
DOS AND DONTS
Use the Right Tools
Needle sticks are the most frequent source of transmission of bloodborne disease in health care workers.4 Although relatively safe capless syringes have been available for years, some institutions initially hesitated to provide them because they cost more than inferior syringes. In November 2000, President Clinton signed the Needlestick Safety and Prevention Act (HR 5178, S 3067) mandating revisions in the OSHA Bloodborne Pathogens Standard, and effectively required institutions to use the safer devices to reduce the risk of an exposure incident.
The syringes are only safer when used correctly by capping the syringe with one hand. Clinicians should never use a free hand to press the sheath over the needle, because the risk of puncturing oneself is high. Used needles should be immediately deposited in an approved sharps container.
In addition to providing common sense safeguards from needle sticks, most blood gas kits now use a dry lithium heparin that is neutralized for ionic calcium. This potentially eliminates most of the adverse effects of the anticoagulant while still preventing clotting.3
Use of latex gloves by the clinician is strongly recommended, but it should be remembered that they provide protection from blood splashes only and are virtually worthless in preventing accidental needle sticks. A cotton ball or folded 2×2 gauze can be used to apply pressure to the site for at least 5 minutes, then taped in place as a temporary pressure dressing.
Handling Samples Properly
Blood is living tissue in which oxygen continues to be consumed and carbon dioxide continues to be produced even after the blood is drawn into a syringe. If air bubbles are not removed immediately, oxygen can diffuse into the sample and compromise the results. Air bubbles that mix with a blood sample will result in gas equilibration between the air and the blood. Thus, air bubbles may significantly lower the PCO2 of the blood sample with subsequent increase in pH and cause the PO2 to approach 150 Hg.3
It is essential that the ABG sample be run within 10-15 minutes or immediately be put in a container of ice. An ABG sample can remain stable on ice for at least 1 hour. It may be argued that an iced sample can remain stable for up to several hours, but at that point it is no longer representative of the patients current status and its value as a clinical tool is severely diminished. Failure to cool the sample properly is a common source of preanalytic error.3
Correcting patient temperature, once commonly applied to ABG samples, is no longer the standard. Temperature-corrected PO2 values do not improve our ability to make clinically relevant interpretations.3 A diagnostic and trouble-shooting protocol appropriate to the specific ABG machine used is essential to maintain quality control and accurate results. Commercially prepared controls provide a range of known values typical of those encountered in the clinical setting; pH values range from extreme acidosis to extreme alkalosis. The PO2 and PCO2 ratings are represented in low, medium, and high ranges.2
The newest machines run calibrations, perform self-cleaning maneuvers, and even run quality controls improving accuracy and reliability of measurements with their automated calibration and internal controls.2
Thousands of ABGs are obtained by RCPs every week using tried and true methods that have been refined with years of experience. As an important aspect of quality management, RCPs need to be trained and periodically recertified in universal precautions, proper syringe preparation, site determination, puncture technique, sample acquisition, storage and disposal of blood specimens, and postsampling care of the puncture site.4 Obtaining ABGs can be technically challenging and properly handling the sample is essential. The training and experience of RCPs make them ideally suited to performing the procedure with consistent results, while minimizing patient discomfort and maintaining a safe environment.
John A. Wolfe, RRT, is a contributing writer for RT Magazine.
1. Ventriglia W. Arterial blood gases. Emerg Med Clin North Am. 1986;4:235-235.
2. Burton G, Hodgkin J, Ward J. Respiratory Care: A Clinical Practice Guide. Philadelphia: Lippincott Co; 1991.
3. Shapiro B, Peruzzi W, Kozelowski-Templin R. Clinical Application of Blood Gases. St Louis: Mosby Inc; 1994.
4. AARC Clinical Practice Guidelines. Sampling for arterial blood gas analysis. Respir Care. 1992;37:913-917.