To evaluate lung function by measuring blood pH, oxygen (O2) and carbon dioxide (CO2); to monitor treatment for lung diseases; to detect an acid-base imbalance in your blood, which may indicate a respiratory, metabolic, or kidney disorder; to evaluate the effectiveness of oxygen therapy
When you have symptoms of a respiratory problem such as difficulty breathing, shortness of breath, or rapid breathing; when you are being treated for a lung disease; when an acid-base imbalance is suspected; periodically when you have a condition that causes an acute or chronic oxygen shortage and you are on oxygen therapy; during certain surgeries to monitor your blood's O2 and CO2 levels
Most often a blood sample collected from an artery, usually the radial artery in your wrist; sometimes a blood sample drawn from a vein in your arm; capillary blood from a heelstick may be used for babies.
Typically, none; however, if you are on oxygen therapy, the O2 may either be turned off for 20 to 30 minutes before the collection for a "Room Air" test. If this cannot be tolerated, or if your healthcare practitioner wants to check your oxygen levels with the O2 on, the amount of oxygen being delivered will be recorded.
Blood gases are a group of tests that are performed together to measure the pH and the amount of oxygen (O2) and carbon dioxide (CO2) present in a sample of blood, usually from an artery, in order to evaluate lung function and help detect an acid-base imbalance that could indicate a respiratory, metabolic or kidney disorder.
A person's body carefully regulates blood pH, maintaining it within a narrow range of 7.35-7.45, not allowing blood to become too acidic (acidosis) nor too alkaline/basic (alkalosis). The body's regulation of acids and bases has two main components. The first component involves both metabolism and the kidneys: the cellular process of converting one substance to another for energy produces large amounts of acid that the kidneys help eliminate. The second component of regulating pH balance involves eliminating carbon dioxide (an acid when dissolved in blood) through exhalation of the lungs. This respiratory component is also the way that the body supplies oxygen to tissues. The lungs inhale oxygen, which is then dissolved in the blood and carried throughout the body to tissues.
These processes of gas exchange and acid/base balance are also closely associated with the body's electrolyte balance. In a normal state of health, these processes are in a dynamic balance and the blood pH is stable. (For more on this, see Acidosis and Alkalosis).
There is a wide range of acute and chronic conditions that can affect kidney function, acid production, and lung function, and they have the potential to cause a pH, carbon dioxide/oxygen, or electrolyte imbalance. Examples include uncontrolled diabetes, which can lead to ketoacidosis and metabolic acidosis, and severe lung diseases that can affect CO2/O2 gas exchange. Even temporary conditions such as shock, anxiety, pain, prolonged vomiting, and severe diarrhea can sometimes lead to acidosis or alkalosis.
Blood gas analysis gives a snapshot of a person's blood pH, O2 and CO2 content. The following components are generally included in blood gas analysis:
Arterial whole blood is almost always used for blood gas analysis but, in some cases, as with babies, whole blood from a heelstick is collected instead. Blood may also be taken from the umbilical cord of a newborn. Since arterial blood carries oxygen to the body and blood from a vein (venous blood) carries waste products to the lungs and kidneys, the gas and pH levels will not be the same in both types of blood samples. Typically, the largest difference in reported values between venous and arterial blood is the PaO2, and consideration of sample type should be taken into account when reviewing results.
An arterial blood sample is usually collected from the radial artery in the wrist, located on the inside of the wrist, below the thumb, where the pulse can be felt. A circulation test called an Allen test will be done before the collection to make sure that there is adequate circulation in the person's wrist. The test involves compressing both the radial and the ulnar wrist arteries, then releasing each in turn to watch for "flushing," the pinking of the skin as blood returns to the hand. If one hand does not flush, then the other wrist will be tested. Blood can also be collected from the brachial artery in the elbow or the femoral artery in the groin, although these sample locations require special training to properly access. Blood may also be collected from an arterial catheter line but should be taken as to ensure minimal contamination.
In newborns that experience difficulty in breathing right after birth, blood may be collected from both the umbilical artery and vein and tested separately.
After an arterial blood draw, pressure must be firmly applied to the site for at least 5 minutes. Since blood pumps through the artery, the puncture may take awhile to stop bleeding. If someone is taking blood thinners or aspirin, it may take as long as 10-15 minutes to stop bleeding. Following collection, the person taking the sample will verify that the bleeding has stopped and will put a wrap around the wrist, which should be left in place for about an hour.
Typically, no test preparation is needed. However, if someone is on oxygen therapy, the O2 may either be turned off for 20 to 30 minutes before the collection for a "Room Air" test or, if this cannot be tolerated or if the healthcare practitioner wants to check oxygen levels with the O2 on, the amount of oxygen being delivered will be recorded. This is usually expressed as fraction of inspired (inhaled) oxygen in percent (FiO2) or as liters of O2 flowing per minute.
Blood gas measurements are used to evaluate a person's lung function and acid/base balance.
They are typically ordered if someone is having worsening symptoms of a respiratory problem, such as difficulty breathing or shortness of breath, and a condition such as asthma or chronic obstructive pulmonary disease (COPD) is suspected. Blood gases may also be used to monitor treatment for lung diseases and to evaluate the effectiveness of supplemental oxygen therapy.
Blood gases can also be used to detect an acid-base imbalance, which can occur in kidney failure, heart failure, uncontrolled diabetes, severe infections, and drug overdose. They may be ordered along with other tests, such as electrolytes to determine if an electrolyte imbalance is present, glucose to evaluate blood sugar concentrations, and BUN and creatinine tests to evaluate kidney function.
A blood gas analysis is ordered when someone has symptoms of an oxygen/carbon dioxide or pH imbalance, such as difficulty breathing, shortness of breath, nausea, or vomiting. It may also be ordered when someone is known to have respiratory, metabolic, or kidney disease and is experiencing respiratory distress.
When someone is "on oxygen" (ventilation), blood gases may be measured at intervals to monitor the effectiveness of treatment. Other treatments for lung diseases may also be monitored with blood gases.
Blood gases may also be ordered when someone has head or neck trauma, which may affect breathing, and when someone is undergoing prolonged anesthesia – particularly for cardiac bypass surgery or brain surgery – to monitor blood gases during, and for a period after, the procedure.
Checking blood gases from the umbilical cord of a newborn may uncover respiratory problems as well as determine acid/base status. Testing is usually only done if a newborn is having difficulty breathing.
Normal values will vary from lab to lab. They are also dependent on elevation above sea level as a person's blood oxygen level will be lower if he or she lives higher than sea level.
Results from an arterial blood gas analysis are not diagnostic; they should be used in combination with the results of other tests and exams to evaluate someone for a respiratory, metabolic, or kidney problem.
Abnormal results of any of the blood gas components may indicate one or more of the following issues:
A low partial pressure of oxygen (PaO2) suggests that a person is not getting enough oxygen, while results that are within normal range usually mean that oxygen intake is sufficient.
All other components of the blood gas analysis (pH, PaCO2, HCO3-) are interrelated and the results must be considered together. Certain combinations of results, if abnormal, may indicate a condition that is causing acidosis or alkalosis. These may include the following:
Examples of test results associated with the above conditions are summarized below:
|pH result||Bicarbonate result||PaCO2 result||Condition||Common causes|
|Less than 7.35||Low||Low||Metabolic acidosis||Kidney failure, shock, diabetic ketoacidosis, intoxication with methanol, salicyate, ethanol|
|Greater than 7.45||High||High||Metabolic alkalosis||Chronic vomiting, low blood potassium, heart failure, cirrhosis|
|Less than 7.35||High||High||Respiratory acidosis||Narcotics, lung diseases such as asthma, COPD, airway obstruction, pneumonia, myasthenia gravis|
|Greater than 7.45||Low||Low||Respiratory alkalosis||Hyperventilation, pain, anxiety, brain trauma, pneumonia, certain drugs (salicylate, catecholamines)|
If left untreated, these conditions can create an imbalance that can eventually become life-threatening. A healthcare practitioner can provide the necessary medical intervention to regain normal acid/base balance, but the underlying cause of the imbalance must also be addressed.
Arterial blood sample collection is usually a bit more painful than regular venipuncture. You may experience moderate discomfort and a compress may be required for some time to prevent any bleeding from the site.
Sometimes mixed venous blood taken from a central line is used in particular situations, such as in cardiac catheterization labs and by transplant services. Careful interpretation of the results is required. Peripheral venous blood, such as that taken from a vein in the arm, is of no use for oxygen status because it has decreased oxygen content due to the fact that it is composed of blood returning to the heart.
Blood gas analysis, performed by trained personnel, is usually done in a hospital, emergency room, surgical center, ambulance, or large laboratory setting because it should be performed immediately after sample collection and specialized equipment is required. Most doctors' offices do not have such capabilities.
Most cases of pneumonia or asthma can be diagnosed by symptoms and monitored by listening to your chest sounds or by examining the results of spirometry tests or chest x-rays. Most of the time, asthma will respond to prescribed medications and pneumonia will respond to rest and possibly antibiotics. Blood gas analysis may be necessary, however, if you have severe or acute breathing problems or prolonged, chronic ones. In these cases, blood gas analysis is usually done in an emergency room or hospital setting.
A pulse oximeter is a noninvasive way (no needlestick or blood sample required) of continuously monitoring O2 saturation. A small clip-like device (sensor) is attached to the end of the finger or earlobe. The sensor reads light that is transmitted through the skin. Pulse oximeters are useful for monitoring trends in O2 saturation, but their accuracy can be affected by the presence of abnormal forms of hemoglobin, like carboxyhemoglobin (see below), low blood pressure due to poor perfusion (pumping of blood into an organ or tissue), and very low levels of hemoglobin due to severe anemia.
If your blood gases were measured using an instrument known as a co-oximeter, then your lab report may also list results for carboxyhemoglobin and other altered forms of hemoglobin. A co-oximeter is a blood gas analyzer that can measure concentrations of hemoglobin derivatives (like carboxyhemoglobin) in addition to the usual blood gas measurements. A co-oximeter is not always used, so these values are not reported for all blood gas analyses.
Carboxyhemoglobin is an altered form or derivative of hemoglobin that forms when carbon monoxide binds to hemoglobin. Levels of carboxyhemoglobin are often elevated with carbon monoxide poisoning, and a co-oximeter is used to measure carboxyhemoglobin levels and to monitor oxygen therapy. Hemoglobin binds to carbon monoxide about 210 times more strongly than to oxygen. Binding to carbon monoxide significantly decreases hemoglobin's ability to carry oxygen through the body, which can lead to a serious, life-threatening condition.
Other hemoglobin derivatives include sulfhemoglobin (or sulfmethemoglobin) and methemoglobin, which may result from the ingestion of certain medicines or exposure to chemicals. These altered forms of hemoglobin, like carboxyhemoglobin, cannot function properly to carry oxygen to tissues in the body and are commonly measured by a co-oximeter.
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