An arterial blood gas (ABG) test is a diagnostic tool that measures the acidity (pH) and the levels of oxygen and carbon dioxide in a blood sample taken from an artery. This test offers a precise assessment of how effectively the lungs transfer oxygen into the bloodstream and remove carbon dioxide. It also helps evaluate the body’s acid-base balance, providing insights into a patient’s overall respiratory and metabolic state. Unlike most blood tests, an ABG draws blood directly from an artery, which carries oxygen-rich blood, allowing for accurate measurement of blood gas levels before they are used by tissues.
Why an Arterial Blood Gas Test is Performed
Healthcare providers order an ABG test to evaluate lung function and the body’s acid-base balance, which is maintained by the lungs and kidneys. This test helps diagnose severe breathing problems and lung diseases such as asthma, cystic fibrosis, or chronic obstructive pulmonary disease (COPD). It also assesses how well current treatments for lung conditions are working.
The ABG test can determine if a patient requires supplemental oxygen or mechanical ventilation. Beyond respiratory issues, it helps measure the acid-base balance in individuals with conditions like heart failure, kidney failure, uncontrolled diabetes, sleep disorders, or severe infections. An ABG can also be used in emergency situations, such as drug overdoses, carbon monoxide poisoning, or smoke inhalation injuries.
The Brachial Artery and How the Test is Done
The brachial artery is located in the upper arm, typically running along the inside of the arm near the elbow crease. While the radial artery in the wrist is the most common site for an ABG draw due to its accessibility, the brachial artery is an alternative site, especially in emergency situations or with children.
Before the procedure, a healthcare professional will prepare the patient, who may be seated or lying down with their arm extended and supported. The skin over the brachial artery will be cleaned with an antiseptic solution.
The healthcare provider will palpate the brachial pulse with their non-dominant hand, typically placing two fingers proximal to the intended puncture site to stabilize the artery. A small needle attached to a heparinized syringe is then inserted just under the skin at a 45-60 degree angle, aiming towards the artery. Once the needle enters the artery, arterial blood will typically pulsate into the syringe, indicating successful access.
After blood is collected, the needle is carefully removed, and immediate, firm pressure is applied to the puncture site with gauze. This pressure should be maintained for at least 5 minutes, or up to 10-15 minutes for brachial artery punctures, to prevent bruising or hematoma formation. The syringe is then sealed with a cap and inverted several times to mix the sample with the heparin. The sample should be transported to the lab for analysis.
What the Arterial Blood Gas Results Reveal
An ABG test measures several key parameters that provide a comprehensive picture of a patient’s respiratory and metabolic status. The pH level indicates the acidity or alkalinity of the blood, with a normal range between 7.35 and 7.45. Values below 7.35 suggest acidosis (too much acid), while values above 7.45 indicate alkalosis (too much base).
The partial pressure of oxygen (PaO2) measures the amount of oxygen dissolved in the arterial blood, reflecting how well oxygen moves from the lungs into the bloodstream. Normal PaO2 levels fall between 75 and 100 millimeters of mercury (mmHg). The partial pressure of carbon dioxide (PaCO2) indicates how much carbon dioxide is in the blood and how effectively the lungs remove it from the body. A range for PaCO2 is 35-45 mmHg.
Bicarbonate (HCO3-) is an electrolyte that helps buffer the blood, maintaining its pH balance. This value is calculated from the pH and PaCO2, with normal levels between 22 and 26 milliequivalents per liter (mEq/L). Oxygen saturation (SaO2) measures the percentage of hemoglobin in red blood cells that is carrying oxygen, providing an overall sense of oxygen delivery, with normal values between 94% and 100%. These combined measurements allow healthcare providers to assess gas exchange and acid-base balance to guide treatment decisions.
Potential Risks and Post-Procedure Care
Patients may experience discomfort or sharp pain during the needle insertion, as arteries are deeper and surrounded by nerves compared to veins. Drawing blood from an artery can be more uncomfortable than from a vein because arteries are deeper and surrounded by nerves. It is also common to have some soreness, bruising, or a hematoma, which is a collection of blood under the skin, at the puncture site.
Other risks include excessive bleeding or infection. Some individuals might feel lightheaded, dizzy, or nauseated during or immediately after the procedure. To minimize these risks, patients are advised to keep firm pressure on the puncture site for several minutes after the needle is withdrawn.
Healthcare providers will monitor the site for any signs of continued bleeding or hematoma formation. Patients are recommended to avoid heavy lifting or strenuous activity with the affected arm for at least 24 hours to promote healing and prevent re-bleeding. Any unusual swelling, increasing pain, or signs of infection should be reported to a healthcare professional.