Blood Gases

The body's regulation of acids and bases has a component that involves metabolism and the kidneys. In the body, the process of converting one substance to another for energy (metabolism) produces large amounts of acid and the kidneys help to eliminate it. The body also regulates pH balance by eliminating carbon dioxide (an acid) through the lungs. This respiratory component is also the way 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 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 are a wide range of acute and chronic conditions that can affect kidney function, acid production, or lung function, and that 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 carbon dioxide/oxygen gas exchanges. Even temporary conditions such as shock, anxiety, pain, prolonged vomiting, and severe diarrhea can sometimes lead to acidosis or alkalosis.

Blood gas tests give a snapshot of the blood's pH and oxygen and carbon dioxide content. They directly measure:

• pH - a measure of the balance of acid and bases in the blood. Blood pH decreases, becoming more acidic, with increased amounts of carbon dioxide (PCO₂) and other acids. Blood pH increases, becoming more alkaline, with decreased carbon dioxide or increased amounts of bases like bicarbonate (HCO₃^-).
• Partial pressure of O₂ (PO₂) - the amount of oxygen gas in blood.
• Partial pressure of CO₂ (PCO₂) - the amount of carbon dioxide gas in the blood. As PCO₂ levels rise, blood pH levels decrease, becoming more acidic; as PCO₂ decreases, pH levels rise, making the blood more basic (alkaline).

Calculations or measurements can also be done to give other parameters, such as:

• O₂ saturation - 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.
• Bicarbonate (HCO₃^-) - the main form of CO₂ in the body, it can be calculated from the pH and PCO₂. 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, so does the pH.
• 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). The anions try to compensate for imbalances in the blood pH. The doctor will look at the HCO₃^- and base excess/deficit results to evaluate the total buffering capacity when deciding on a treatment to correct an imbalance.

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 your 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 your 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. These sample locations require special training to properly access, so the collection is often performed by a doctor.

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 you are taking blood thinners or aspirin, it may take as long as ten to fifteen minutes to stop bleeding. The person collecting the sample will verify that the bleeding has stopped and will put a wrap around your wrist, which should be left in place for an hour or so.