Blood Gases
The Test
How is it used?
Your body will try to restore any imbalance by itself, but if you have an overwhelming acute or a chronic (long term) problem, you may need medical intervention, such as being given concentrated, pure O2 or, in extreme cases, being put on a respirator to gain additional oxygen. If you are on continuing oxygen therapy, your doctor may order blood gas tests to monitor the effectiveness of that therapy.
When is it ordered?
Blood gas measurements may be ordered if you are known to have a respiratory, metabolic, or kidney disease and are experiencing respiratory distress to evaluate your oxygenation and acid/base balance. Patients who are on oxygen (have supplemental oxygen) may have their blood gases measured at intervals to monitor the effectiveness of treatment. Blood gases may also be ordered for patients with head or neck trauma, injuries that may affect breathing. Patients undergoing prolonged anesthesia particularly for cardiac bypass surgery or brain surgery may have their blood gases monitored during and for a period after the procedure.
Checking the blood gases from the umbilical cord of newborns may uncover respiratory problems as well as determine the baby's acid/base status. Testing is usually only done if a newborns condition indicates that he or she may be having difficulty breathing.
What does the test result mean?
A pH imbalance, blood that is either too acidic (state of acidosis) or alkaline (state of alkalosis) will be primarily due to a condition or disease that affects either respiration (breathing, your lungs) or your metabolic processes. Regardless of what causes the pH imbalance, since the respiratory and metabolic systems are interrelated, one system will compensate for the other to bring the pH back into balance.
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, characterized by a raised pH and a decreased PCO2, is due to over ventilation 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.
Is there anything else I should know?
Sometimes, mixed venous blood taken from a central line is used in particular situations, such as in cardiac catheterisation labs and by transplant services. Careful interpretation of the results is required. Peripheral venous blood is of no use for oxygen status.
