The Test Sample
What is being tested?
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 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 give a snapshot of a person's blood pH and O2 and CO2 content. The following components can be determined by blood gas analysis:
- pH. A measure of the balance of acids and bases in the blood. Increased amounts of carbon dioxide and other acids can cause blood pH to decrease (become acidic). Decreased carbon dioxide or increased amounts of bases, like bicarbonate (HCO3-), can cause blood pH to increase (become alkaline).
- Partial pressure of O2 (PaO2). Measures the amount of oxygen gas in the blood.
- Partial pressure of CO2 (PaCO2). Measures the amount of carbon dioxide gas in the blood. As PaCO2 levels rise, blood pH decreases, making the blood more acidic; as PaCO2 decreases, pH rises, making the blood more basic (alkaline).
- O2 saturation (O2Sat or SaO2). The percentage of hemoglobin that is carrying oxygen. Hemoglobin is the protein in red bloods cells that carries oxygen through blood vessels to tissues throughout the body.
- O2 content (O2CT or CaO2). The amount of oxygen per 100 mL of blood.
- Bicarbonate (HCO3-). The main form of CO2 in the body. It can be calculated from the pH and PaCO2. It is a measurement of the metabolic component of the acid-base balance. HCO3- is excreted and reabsorbed by the kidneys in response to pH imbalances and is directly related to the pH level. As the amount of HCO3- rises in the blood, so does the pH (becomes alkaline).
- Base excess/base deficit. A calculated number that represents a sum total of the metabolic buffering agents (anions) in the blood. These anions include hemoglobin, proteins, phosphates, and HCO3- (bicarbonate, which is the dominant anion). Anions are regulated to compensate for imbalances in blood pH. The health practitioner will look at the HCO3- and base excess/deficit results to evaluate the total buffering capacity of the lungs and kidneys when deciding on a treatment to correct an imbalance.
How is the sample collected for testing?
Arterial 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 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.
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.
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 will 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. The person collecting 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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
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 health practitioner wants to check oxygen levels with the O2 on, the amount of oxygen being taking will be recorded. This is usually expressed as fraction of inspired (inhaled) oxygen in percent (FiO2) or as liters of O2 flowing per minute.