Blood Gases
How is it used?
Blood gas measurements are used to evaluate your oxygenation and acid/base status.
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 tests are ordered when you have symptoms of an O2/CO2 or pH imbalance, such as difficulty breathing or shortness of breath. Many acute and chronic conditions can cause an imbalance and, while the blood gas tests do not tell your doctor the direct cause of your imbalance, they will point to either a respiratory or metabolic problem.
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 newborn’s condition indicates that he or she may be having difficulty breathing.
Abnormal results of any of the blood gas components may mean that your body is not getting enough oxygen, is not getting rid of enough carbon dioxide, or that there is a problem with kidney function. If left untreated, these conditions create an imbalance that could eventually be life threatening. Your doctor will provide the necessary medical intervention to regain your body’s normal balance, but the original cause of the imbalance must also be addressed.
A pH imbalance, blood that is either too acidic or alkaline 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 (low blood potassium), chronic vomiting (losing acid from the stomach), and sodium bicarbonate overdose.
Is there anything else I should know?
Arterial blood sample collection is usually more painful than regular venipuncture. You will experience moderate discomfort, and a compress is required for some time to prevent any bleeding from the site.
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.