Arterial Blood Gases

Dr David Howdijam *

An arterial blood gas (ABG) test measures the acidity and the levels of oxygen and carbon dioxide in the blood from the artery. This test is used to evaluate how well the lungs are able to move oxygen into the blood and to remove carbon dioxide from the blood.

The oxygen and carbon dioxide levels of the blood and the pH balance of the blood can indicate the presence of certain medical conditions, such as cardiac, lung or kidney disorders and the presence of other critical conditions such as:
o uncontrolled diabetes
o haemorrhage
o drug overdose
o shock

A person’s body carefully regulates blood pH, not allowing blood to become too acidic nor too alkaline/basic. 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 the blood) through the lungs. This respiratory component is also the way that the body supplies oxygen to the 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.

Blood gas analysis gives a snapshot of a person’s blood pH and O2 and CO2 content.

Blood gases examined in an ABG test

The following components can be determined by blood gas analysis:

o 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).

o Partial pressure of O2 (PaO2): Measures the amount of oxygen gas in the blood.

o 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 alkaline/basic.

o O2 saturation (O2 Sat or SaO2): The percentage of haemoglobin that is carrying oxygen. Haemoglobin is the protein in red blood cells that carries oxygen through blood vessels to tissues throughout the body.

o O2 content (O2CT or CaO2): The amount of oxygen per 100 ml of blood.

o Bicarbonate (HCO3-): The main form of CO2 in the body. 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).

o Bases excess/base deficit: A calculated number that represents a sum total of the metabolic buffering agents in the blood. This helps evaluate the total buffering capacity of the lungs and kidneys.

When is it done?

A blood gas analysis is done when someone has symptoms of an oxygen/carbon dioxide or pH imbalance, such as difficulty in breathing, shortness of breath, nausea, or vomiting. It may also be done when someone is known to have a respiratory, metabolic, or kidney disease and is experiencing respiratory distress.

When someone is on ventilator, 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 done when someone has head or neck trauma, which may affect breathing, and when someone is undergoing prolonged anaesthesia – particularly for cardiac bypass surgery or brain surgery – to monitor blood gases during, and for a period after, the procedure.

Sample required

A blood sample is collected from an artery, usually the radial artery in the wrist, arm or groin.

Preparation needed for the test

Typically, no preparation is needed. However, if one is on oxygen therapy, the O2 may either be turned off for 20 to 30 minutes before the collection for a “Room Air” test.

Interpretation of results

The AGB test is done within the shortest possible time (typically within 30 minutes) after collection of sample.

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 they live higher than sea level.

Results from an arterial blood gas test alone are not diagnostic and they should be used in combination with the results of other tests and examinations to evaluate someone for a respiratory, metabolic, or kidney problem.