Blood gas tests tell your doctor if you have enough oxygen in your blood and whether or not your blood pH is balanced - not too acidic (acidosis) or too alkaline/basic (alkalosis).
Blood gas tests directly measure:
1. pH. A measure of the level of hydrogen ion (H+), which indicates the acid/base status of your blood.; the pH of your blood decreases (becomes more acidic) with increased amounts of PCO2 and other acids, and the pH increases (blood becomes more alkaline) with decreased PCO2 or increased amounts of bases like bicarbonate (HCO3-)
2. PO2. The partial pressure of O2 (the amount of oxygen gas dissolved in blood)
3. PCO2. The partial pressure of CO2 (the amount of carbon dioxide gas dissolved in the blood); as PCO2 levels rise, blood pH levels will decrease, becoming more acidic; as PCO2 decreases, pH levels will rise, making the blood more alkaline.
Calculations or measurements also can be done to give other parameters, such as:
1. O2 Saturation: The percentage of hemoglobin saturation or how much oxygen is bound to hemoglobin in the red blood cells and available to be carried through the arteries to nourish the body’s cells.
2. HCO3- or Bicarbonate: The main form of CO2 in the body; 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 (conserved) 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.
3. Base Excess/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). These anions try to compensate for imbalances in the pH (too acidic/acidosis or too alkaline/alkalosis) caused by diseases or conditions that affect the lungs (respiratory acidosis/alkalosis) or kidneys (metabolic acidosis/alkalosis).
Arterial blood is almost always used for blood gas analysis, but in some cases such as for babies, whole blood from heel sticks is used. Since arterial blood carries oxygen to the body and venous blood carries waste products to the lungs, the gas and pH levels will not be the same in both.
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 you can feel your pulse). A circulation test called an Allen test will be done before the collection to make sure that you have 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 return of blood to your hand). If your hand does not flush, your other wrist will be tested. Blood can also be collected from the brachial artery in your elbow or the femoral artery in your groin.
In newborns that experience difficulty in breathing right after birth, blood may be collected from both the umbilical artery and vein and tested separately.
Respiratory acidosis is characterized by a lower pH and an increased PCO2 and is due to respiratory depression (not enough oxygen in and CO2 out). This can be caused by many things including pneumonia, chronic obstructive pulmonary disease (COPD), and over-sedation from narcotics.
Respiratory alkalosis is characterized by a raised pH and a decreased PCO2 and is due to overventilation caused by hyperventilating, pain, emotional distress, or certain lung diseases that interfere with oxygen exchange.
Metabolic acidosis is characterized by a lower pH and decreased HCO3-; the blood is too acidic on a metabolic/kidney level. Causes include diabetes, shock, and renal failure.
Metabolic alkalosis is characterized by an elevated pH and increased HCO3- and is seen in hypokalemia, chronic vomiting (losing acid from the stomach), and sodium bicarbonate overdose.
NORMAL VALUES
• Diabetes.
• Drug or alcohol overdose.
• Kidney failure.
• Lung diseases, such as chronic lung disease, a blood clot in the lungs, or pneumonia.
• Severe dehydration from severe burns, vomiting, diarrhea, or taking water pills (diuretics).
• Severe infection (sepsis).
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