Arterial Blood Gases (ABG)
Arterial Blood Gases (ABG)
An arterial blood gas (ABG test) measures the levels of oxygen and carbon dioxide in the blood. Additionally, it reports the level of acidity or alkalinity of the blood, the pH. An ABG is performed in order to diagnose or monitor respiratory, kidney, or metabolic disorders.
An ABG may be ordered to monitor the status of a patient in surgery or after a trauma. The test may also be used to monitor how a patient is responding to oxygen therapy. Additionally, an ABG may also help in the evaluation of a variety of symptoms, including shortness of breath.
If the patient is on supplemental oxygen, no changes should be made to the setting for a full twenty to thirty minutes prior to drawing the ABG sample. If the sample needs to be drawn with the patient off of supplemental oxygen (that is, on “room air”), then the patient should be removed from oxygen and should be off of oxygen for a full twenty to thirty minutes prior to the blood draw.
If the blood will be drawn from the artery at the wrist, the radial artery, then a simple test (the Allen test) should be performed prior to the blood draw to ascertain that the patient has good blood circulation at the wrist. Pressure is applied to the two main wrist arteries (the radial and ulnar arteries) for several seconds. The pressure is then released from one and then the other, and the patient’s hand is observed to verify that if turns a bit red (flushes) as blood returns through those arteries into the hand. If the flushing is not adequate, then the arteries at the other wrist should be tested the same way. If good circulation at either wrist cannot be verified, then the elbow or groin arteries should be considered.
The individual who is drawing the blood should be well-aware if the patient is on any kind of blood thinning medication, since this may make the patient more prone to bleeding or bruising after the blood draw.
Most blood tests involve blood that is drawn from a vein; however, because this test needs to look at the oxygen-carrying capacity of the blood, the sample needs to be drawn from an artery either at the wrist, the elbow crease, or the groin. If the patient has a central line (an intravenous line that goes directly into the heart), the blood sample can be drawn from that. When the radial artery (the artery at the wrist where one checks the pulse rate) is being used for the test, the sample can usually be drawn by a nurse or phlebotomist (an individual who has been trained to draw blood). When an artery at the elbow (the brachial artery), the groin (femoral artery), or a central line is involved, a doctor may be required to
draw the sample. Because arteries run deeper than veins, the needle stick of an ABG is more painful than other blood tests. In some cases, a local anesthetic may be used to numb the area around the artery to be used. The site of the needle stick is cleaned with antiseptic, and the needle is inserted. The blood is collected in vacuum tubes. After collection, the needle is withdrawn, and a cotton ball is usually pressed onto the blood draw site for about 10 minutes, to stop any bleeding and to decrease bruising. A pressure bandage is then applied over the puncture site, and should be left in place for about an hour to decrease bleeding and bruising.
In newborn babies, blood may be obtained from the umbilical artery and umbilical vein for testing, or whole blood from a heel stick may be utilized.
There are no restrictions on diet or physical activity, either before or after the blood test.
As with any blood tests, discomfort, bruising, and/or a very small amount of bleeding is common at the puncture site. Arteries run deeper than veins and the blood pressure within an artery is higher, therefore there is a greater chance for pain, bleeding, and bruising from an ABG than from other blood tests that draw blood from a vein. Immediately after the needle is withdrawn, it is very important to put significant pressure on the puncture site for about 10 minutes, until the bleeding has stopped. This decreases the chance of significant bruising. Warm packs may relieve minor discomfort. Some individuals may feel briefly woozy after a blood test, and they should be encouraged to lie down and rest until they feel better. For about 24 hours after an ABG is drawn, the individual should avoid vigorous exercise or heavy lifting.
Basic blood tests do not carry significant risks, other than slight bruising and the chance of brief dizziness. An arterial blood draw is more painful and more inclined to bleed and bruise, so the risks of these complications are slightly higher after an ABG is drawn.
Brachial— Referring to the arm; the brachial artery is an artery that runs from the shoulder to the elbow.
Central line— An intravenous line that goes directly into the heart.
Femoral— Pertaining to the thigh of the leg. The femoral artery is an artery that runs through the groin area.
Radial— Referring to the lower arm. The radial artery is an artery that runs from the elbow, through the wrist, and into the palm of the hand.
Results from the ABG include a measurement of the partial pressure of oxygen or paO2 (how much oxygen is dissolved in the blood), the partial pressure of carbon dioxide or paCO2 (how much carbon dioxide is dissolved in the blood), and pH. The pH is a number that indicates how acidic or alkaline the blood is. It is a measurement involving the concentration of hydrogen ions in the blood. As the paCO2 levels rise, the pH level drops and the blood becomes increasingly acidic; as the paO2 levels rise, the pH level rises, and the blood becomes increasingly alkaline.
The information obtained from an ABG also allows other important aspects of body chemistry to be evaluated, such as the O2 saturation (a measurement of the percentage of oxygen that is bound to the hemoglobin in red blood cells) and the amount of bicarbonate in the body. Bicarbonate, or HCO3- is processed by the kidneys in response to the pH of the body. When the pH goes down (indicating greater acidity), the kidneys excrete HCO3-, in an effort to counterbalance the acidity. When the pH goes up (indicating greater alkalinity), the kidneys reabsorb more HCO3-, in an effort to counterbalance the alkalinity. A final calculation can help to measure the patient’s base/excess or deficit. This is a measurement of the body’s ability to compensate for pH abnormalities through other “buffering” agents in the blood, such as hemoglobin, proteins, phosphates, and bicarbonate.
Normal ABG results
Normal ABG results are as follows:
- paO2: 75–100 mm Hg (millimeters of mercury
- paCO2: 35-45 mm Hg
- pH: 7.35-7.45
- HCO3-: 24-28 mEq/L (millequivalents per liter)
Abnormal ABG results
Abnormal ABG results include the following:
- Respiratory acidosis is indicated by a low pH and a high pCO2, and usually indicates respiratory depression, a situation in which the individual is not breathing in sufficient O2 and is not breathing out sufficient CO2. Respiratory acidosis may be caused by pneumonia, emphysema, chronic bronchitis, chronic obstructive pulmonary disease, pulmonary edema, interstitial fibrosis, foreign body obstructing the airway; or slowed, shallow breathing due to disorders of the muscles of respiration (myasthenia gravis, muscular dystrophy), nervous system control of the muscles of respiration (amyotrophic lateral sclerosis, polio, Guillain-Barre syndrome, botulism, tetanus, organophosphate poisoning, spinal cord injury); conditions that depress the respiratory center in the brain (such as narcotic drugs, sedatives, anesthesia, blood clot blocking the vertebral artery or increased intracranial pressure).
- Respiratory alkalosis is indicated by a high pH and a low pCO2, and may indicate hyperventilation (fast, shallow breathing), brought on by emotional stress, pain, anxiety, problems with the lung that do not allow normal exchange of gases (such as pneumonia, pulmonary embolus, collapsed lung); drugs (salicylates, xanthines, progesterone, epinephrine, thyroxine, nicotine); conditions involving the central nervous system (tumors, strokes, trauma, infections); liver-disease induced encephalopathy; severe infection (gram negative sepsis); low blood sodium.
- Metabolic acidosis is indicated by a low pH and a low HCO3-, and may indicate diabetes; shock; loss of HCO3- through severe diarrhea or pancreatic fistula; kidney failure; use of drugs such as amiloride, triamterene, spironolactone, and beta-blockers; exposure to toxins (paraldehyde, methanol, salicylate, ethylene glycol).
- Metabolic alkalosis is indicated by a high pH and a high HCO3- and may occur with abnormal electrolyte levels, such as low postassium (hypokalemia) or low magnesium (hypomagnesemia); repeated bouts of vomiting or nasogastric suction (which causes a lot of stomach acid to be lost in the vomit); loss through the stool (as in cystic fibrosis, abuse of laxatives); multiple blood transfusions; Cushing’s syndrome; or an overdose of sodium bicarbonate.
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American Association for Clinical Chemistry, 1850 K Street, NW, Suite 625, Washington, DC, 20006, (800) 892-1400, http://www.aacc.org.
Rosalyn Carson-DeWitt, M.D.