A blood gas is a test we use to determine how much oxygen and CO2 are in the patient's blood. It's a blood draw where you insert a needle into the patient's radial artery in the wrist area, bracheal artery in the antecubital area (the backside of the elbow) or the femoral artery in the groin (thankfully we don't use this area too often).
About 90 percent of the time we draw this blood from the wrist. We draw arterial blood because this blood is freshly oxygenated blood from the lungs on its way to tissues. We want to know how much oxygen is in this blood. If oxygen is low then we may choose to supply the patient with supplemental oxygen. We can do this with a nasal cannula or a variety of masks.
If the CO2 is high we may need to assist the patient with his ventilations in order to help the patient blow off this CO2. The reason CO2 gets high is because the patient is not taking good enough breaths. He may be pooping out because his lungs are diseases. In this case, we use his CO2 level to help us determine what we can do to help him.
Another thing an ABG does is help us determine the acidity (pH) of the blood. If a patient is in severe respiratory distress his blood may become very acidotic. If this happens, we may need to help the patient breath so that we can get his pH back to normal.
This blood test can also help a doctor diagnose some diseases. For example, if the CO2 is chronically elevated this can be a classic sign of chronic bronchitis or emphysema. Too see a video of an ABG being drawn you can click here.