Arterial Blood Gases

Summary

An arterial blood gas (ABG) study is a blood test that measures the levels of different gases in the blood, including oxygen and carbon dioxide. Because the test requires oxygen-rich blood, the blood sample must be taken from an artery rather than a vein. Some people find this method of drawing blood to be more painful than the standard method.Many diseases cause similar symptoms, such as chest pain, dizziness, shortness of breath, hypoventilation (slow, shallow breathing) or hyperventilation (rapid, deep breathing). The ABG can help a physician determine whether those symptoms are caused by metabolic, respiratory or other conditions. The test is also used to assess how well a patient’s lungs are functioning after they have been on a heart-lung machine during open-heart surgery. Furthermore, the test may be ordered for patients who are undergoing respiratory therapy or who are on a ventilator (a machine that breathes for the patient), and to assess how well the kidneys are functioning.

About arterial blood gas (ABG) tests

An arterial blood gas (ABG) study is a blood test that measures the levels of many different gases in oxygen-rich blood. Some of these levels are measured directly while others are calculated from the measurements of other gases. By measuring the gases in arterial blood, a physician can determine its contents before the blood nourishes the body. The analysis of arterial gases provides the physician with information about: How well the body’s lungs are working during or after a procedure (e.g., open-heart surgery, which requires the use of a heart-lung machine). Whether a ventilator (a device that breathes for a patient) is set to deliver the proper amount of oxygen needed by a patient. How well the body’s metabolic system is working. Whether the body’s acid/base levels are in balance.

The effectiveness of respiratory therapy and other treatments for patients with chronic heart, lung or metabolic disorders. Whether a premature infant is getting too much oxygen, which can cause blindness or lung disorders. How well the kidneys are functioning. For the test, blood will be drawn from an artery, usually the radial artery on the side of the wrist. Other possible sites are the brachial artery (in the arm) and femoral artery (in the groin). Before the test, a healthcare professional will use a piece of cotton to clean the area and will answer any questions that the patient may have. The healthcare professional will then insert a small needle into the artery to draw blood into an attached syringe. Because the blood is being drawn from a blood vessel that has more nerves and is thicker than a vein, the patient may experience more pain than from a standard blood test. It is not uncommon to feel a throbbing sensation after the test is over.

Once the needle is removed, the healthcare provider will apply a small patch of cotton over the area and will typically ask the patient to apply gentle pressure to the area. After about five to 10 minutes, any minor bleeding should have stopped. In the meantime, the blood sample is swiftly taken to a laboratory to be tested. One type of machine measures partial pressure of oxygen (PaO2), partial pressure of carbon dioxide (PaCO2) and pH levels. Another machine measures hemoglobin levels. From these measurements, other important values can be calculated. The typical ABG test will list the following measurements: PaO2 (partial pressures of oxygen).

This measure is an indication of how well the lungs are infusing blood with fresh oxygen. At sea level, normal levels of this gas in arterial blood are 75 to 100 millimeters of mercury (mmHg). PaCO2 (partial pressures of carbon dioxide). This measure is an indication of how well the lungs are removing carbon dioxide from the blood. At sea level, normal levels of this gas in arterial blood are 35 to 45 mmHg. The pH level. This is a measure of acid (acidity) and base (alkalinity) on a scale of 0 to 14, with “7” being a neutral balance between acid and base. Normal pH levels of arterial blood are 7.35 to 7.42. O2CT (oxygen content). Normal oxygen content in arterial blood is 15 to 23 percent. SaO2 (oxygen saturation). Normal oxygen saturation in arterial blood is 94 to 100 percent. HCO3 (bicarbonate). Normal levels of this gas in arterial blood are 22 to 26 milliequivalents per liter (mEq/liter).

Factors that may affect results of ABG tests

Factors that may interfere with the accuracy of the ABG test include: Hyperventilation (rapid and deep breathing). This can cause lower than usual PaCO2 levels. While this is a symptom of several diseases, hyperventilation can also occur due to pain or anxiety. Smoking. Tobacco smoke contains about 2,000 gases and chemicals. These toxins can interfere with test results. Carbon monoxide inhalation. Carbon monoxide (CO) is a colorless, odorless, tasteless gas that is produced from the incomplete burning of fuels (e.g., from home furnaces, car engines and indoor barbecues). High CO levels in the body can cause CO poisoning and quickly lead to death. CO levels in the body will also affect an ABG test.

In addition, some types of medications may affect ABG results: Antacids (especially those containing bicarbonate). Diuretics. Medications that promote the formation of urine in the kidneys, causing the body to flush out fluids and minerals. Antibiotics. Medications that harm or kill microorganisms and are commonly used to treat infections. Corticosteroids. Anti-inflammatory medications containing steroids that are similar to the naturally occurring hormone cortisone. Patients should tell their physician before the test about any medications they are taking.

Abnormal ABG test results

An abnormal level of one blood gas alone is often not enough to make a diagnosis. In combination, these levels can indicate the conditions below (for definitions of these measurements see About arterial blood gas (ABG) tests). Impaired respiratory function. Low PaO2, O2CT and SaO2 levels, combined with high PaCO2, are an indication of impaired respiratory function, which may be caused by the following: Weakness or paralysis of the diaphragm (the muscle that inflates and deflates the lungs) Airway obstruction Brain injury Brain tumor Drug abuse Asthma or emphysema Near drowning Insufficient oxygen intake. Low PaO2, O2CT and SaO2 levels combined with normal PaCO2, are an indication of impaired oxygen intake, which may be caused by: A shunt that allows blood to bypass the lungs Collapsed lung Insufficient blood oxygen levels.

Low O2CT levels combined with normal PaO2, SaO2 and PaCO2 are an indication of insufficient blood oxygen levels, which may be caused by: Severe anemia (deficiency of red blood cells) Decreased blood volume Decreased ability of red blood cells to carry oxygen Respiratory acidosis. A low pH combined with high HCO3 and PaCO2 is an indicator of respiratory acidosis. Symptoms include slow and shallow breathing (hypoventilation), headache, rapid heartbeat (tachycardia), confusion, restlessness and apprehension. This may be caused by: Lung disease, such as pneumonia or chronic obstructive pulmonary disease (COPD) Suffocation or an obstructed airway Drug abuse Injury, surgery or other trauma Heart disease, lung disease or neuromuscular disease Respiratory alkalosis. A high pH combined with low HCO3 and PaCO2 is an indicator of respiratory alkalosis. Symptoms include rapid and deep breathing (hyperventilation), prickling or tingling of the skin, lightheadedness, twitching, anxiety and fear.

This may be caused by: Lung disease, such as asthma or pneumonia A bacterial infection Drug or alcohol abuse Injury or trauma Pain Fever Liver failure Severe anxiety Improper settings on a ventilator Metabolic acidosis. A low pH combined with low HCO3 and PaCO2 is an indicator of metabolic acidosis. Symptoms include rapid and deep breathing (hyperventilation), headache, sweet-smelling breath, unusual tiredness or weakness, drowsiness, prolonged unconsciousness (coma), nausea or vomiting. This may be caused by: Kidney disease Liver disease Diabetes mellitus Shock Drug intoxication Poisoning or overdose from aspirin, antifreeze or wood alcohol Metabolic alkalosis. A high pH combined with high HCO3 and PaCO2 is an indicator of metabolic alkalosis. Symptoms include slow and shallow breathing (hypoventilation), restlessness, twitching, extremely tense muscles, confusion, irritability, seizures, lack of emotion (apathy), muscle spasms and prolonged unconsciousness (coma).

This may be caused by: Primary aldosteronism (or Conn’s disease). An endocrine disorder in which overproduction of the hormone aldosterone causes the kidneys to absorb too much sodium and water and eliminate too much potassium. Cushing syndrome. An endocrine disorder in which the adrenal glands produce an excess of cortisone-like hormones. Steroid overdose. Loss of potassium. Improper diet resulting in too much alkali ingestion. Prolonged vomiting. ABG results can be used to diagnose a variety of other conditions as well, which is why these tests are so widely used in emergency departments of hospitals.

Frequency of ABG testing

The ABG test is generally used as a diagnostic tool, and most patients will not require regular testing. However, it may also be used to monitor the treatment of patients on ventilators, undergoing respiratory therapy or who have severe chronic heart, lung or metabolic disorders.

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