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 patient’s
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.
Indications
• the need to assess
ventilatory status, acid-base balance, and oxygenation and the
oxygen-carrying capacity of the blood;
• to assess a
patient’s 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).
Contraindications
• 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 patient’s 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.
Get comfortable
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
DON’TS
Do •
communicate with the patient about the purpose of the
procedure • always perform a modified Allen test
prior to drawing blood from a radial artery • apply
pressure to the site for at least 5 minutes • ice an
ABG sample unless it is going to be analyzed within
10-15 minutes
Don’t •
palpate too firmly, inhibiting blood flow •
reposition a needle without first withdrawing the tip to
subcutaneous tissue • ever leave bubbles in an ABG
syringe • fail to adequately heparinize a sample to
prevent clotting |
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 2x2 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
patient’s 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
Conclusion
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.
References
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.