Interpreting an arterial
blood gas (ABG) is a critical ability for respiratory therapists, doctors, nurses and other healthcare
personnel – including sleep technologists. It is particularly essential to monitor ABGs in
patients who are critically ill. ABGs are a valuable tool for evaluating our patients in the sleep
center presenting with complex health conditions like COPD, obesity hypoventilation, and neuromuscular
conditions.
What Is an Arterial Blood Gas
Test?
The ABG blood test measures
pH or the acidity and the levels of carbon dioxide (CO2) and oxygen (O2) from blood taken from an
artery. It is used for checking a patient’s lung function, and evaluates how well they can move
blood oxygen and remove carbon dioxide.
For the body’s cells to
live, they require oxygen. When your patients inhale and exhale, their lungs are moving oxygen into
their blood and pushing carbon dioxide out. This is referred to as gas exchange and provides the
patients and their cells with the oxygen they require to survive.
Physicians order this test
to obtain clues about the patient’s heart, lung, and kidney function. Usually, they’ll seek
other testing too. And, while the test is usually performed in an ER and
ICU setting, an ABG can also be drawn on patients in other healthcare settings, such as in a sleep
center, depending on their diagnosis.
It may be helpful to have
arterial blood gas testing performed on your sleep patients if they’re having a hard time breathing
to figure out what the issue is. Your patients might have have this test performed if they have a
serious illness or injury. The information can be helpful to you as the sleep
technologist.
ABG Test Done?
The physician might request
an ABG test to:
- Check for severe lung
and breathing problems like cystic fibrosis, asthma or chronic obstructive pulmonary disease
(COPD)
- Determine how lung
problem treatments are working
- Determine if your
patient requires oxygen or other breathing assistance
- Check your
patient’s acid-base balance. They could have extra acid in their body from a serious infection,
kidney failure, diabetes complications, certain toxic ingestions or under-treated sleep
apnea
What Do Arterial Blood Gases
Measure?
An arterial blood gas test
measures how much carbon dioxide and oxygen are in the blood. It might also be used to determine the
blood’s pH to determine how acidic it is. The test is commonly known as an arterial blood gas
(ABG) test or a blood gas analysis.
An ABG measures:
- PaCO2 (partial pressure
of carbon dioxide): Measures carbon dioxide dissolved in blood and carbon dioxide’s ability to
move out of the body.
- PaO2 (partial pressure
of oxygen): Measures oxygen dissolved in the blood and the oxygen’s ability to move into the
blood from the airspace of the lungs.
- pH: Measures H+
(hydrogen ions) in blood. The blood’s pH is typically between 7.35 and 7.45. A less than 7.35 pH
is known as acid and a greater than 7.45 pH is known as basic (alkaline).
- O2CT (oxygen content)
and O2Sat (oxygen saturation) values: The O2 content measures how much oxygen is in the blood,
whereas oxygen saturation is the measurement of the amount of oxygen being carried by the hemoglobin
in red blood cells.
- HCO3 (bicarbonate):
HCO3 is a chemical (buffer) keeping the blood’s pH from becoming too basic or too
acidic.
Your patients’ red blood
cells carry carbon dioxide and oxygen throughout their bodies. These are referred to as blood
gases.
As blood passes through the
lungs, the O2 flows into their blood as the CO2 flows out of their blood and into their lungs. The ABG
test will assist to determine how well their lungs can move the O2 into their blood and remove the CO2
from their blood.
Imbalances in your
patients’ O2, CO2, and pH levels can be an indication of the presence of a specific medical
condition, including…
Sleep Center Related Conditions:
- Chronic Obstructive
Pulmonary Disease (COPD)
- Obesity
Hypoventilation
- Severe Obstructive
Sleep Apnea
- Neuromuscular
conditions
Other Medical
Conditions:
- Heart
failure
- Kidney
failure
- A drug
overdose
- Uncontrolled
diabetes
- Shock
- Chemical
poisoning
- Hemorrhage
Restrictive lung disease and obstructive lung
disease and ABGs also go hand-in-hand. The physician might order an ABG test when the patient
is showing symptoms of any one of these medical conditions.
What Happens During an Arterial Blood Gas
Test?
To perform this test, a
small amount of blood is collected from an artery. It’s a simple and safe procedure that takes
minutes to complete.
Your patient can have an ABG
test in the hospital, in their doctor’s office, and even in your sleep center if you have a
respiratory therapist or nurse on staff. A small needle is used to collect a sample of arterial
blood, typically from thee wrist. In some cases, blood may be taken from an artery on the inside of
their arm above their elbow or in their groin.
Prior to the ABG test, a
modified Allen test may be performed. This is performed by applying pressure to the patient’s
wrist for a few seconds and then releasing pressure to determine if blood flow in the patient’s hand
is normal.
In preparation for the ABG
test, the patient will need to inform you of all medications and supplements they may be taking,
including:
- Supplements
- Medications
- Vitamins
If your patient is on oxygen
therapy, but can breathe without it, it is best to turn their supplemental oxygen off for 20 minutes so
you can perform a “room air” test before drawing blood for the ABG.
You should inform your
patient of any discomfort they may experience during or after the test. Let them know that when blood is
collected from their artery, it could be more uncomfortable than if you were drawing blood from a vein.
This is because their arteries are deeper than their veins and arteries also have sensitive nearby
nerves.
Let the patient know they
may experience a little dizziness or feel faint, lightheaded or nauseated while you’re drawing their
blood. Gently press for several minutes on the area after you remove the needle to lower the chance of
bleeding or bruising.
How to Read Arterial Blood
Gases
The first value you should be looking at is the pH to determine if
your patient’s level is above, below or within the normal range. If your patient’s pH is >
7.45, they are in alkalosis. If the pH is < 7.35, they are in acidosis. Keep in mind the lower the pH
number is, the greater the acid level in the patient’s body. Even if your patient has a normal pH
level, alkalosis or acidosis might still be present, since the body can compensate in order to balance
the pH.
You will next evaluate the
PaCO2 to see if the blood gas changes are metabolically driven or are due to the respiratory
system. Combined with the HCO3 value, you’ll have a complete picture of the blood gas
findings.
Who Draws Arterial Blood
Gases?
Only a health worker for
whom the testing is within the legal scope of practice for that health worker’s position, typically
a Respiratory Therapist, can draw an arterial blood gas.
A sample is obtained through
a catheter placed in the patient’s artery or using a needle and syringe to puncture the
artery. The syringes used for ABGs are pre-heparinized and are made for minimizing air exposure that
will potentially change the blood gas values.
What Are the Normal Arterial Blood Gas
Values?
Arterial blood gas interpretation is performed by a physician.
You should, however, understand the basics of blood gas values.
Blood gas normal values, in general,
include:
- Bicarbonate (22 to 28
milliequivalents per liter)
- Arterial blood pH (7.38
to 7.42)
- Oxygen saturation (94
to 100%)
- Partial pressure of O2
(75 to 100 mm Hg)
- Partial pressure of CO2
(38 to 42 mm Hg)
Your patient’s blood
oxygen levels might be lower if they live above sea level.
Abnormal results could be a
sign of a specific medical condition. For instance, a less than 7.35 blood pH could indicate metabolic
acidosis often caused by shock, diabetic ketoacidosis or kidney failure. A pH greater than 7.45
could indicate metabolic alkalosis often caused by low blood potassium or chronic
vomiting.