Blood Gas Tests

Blood gas measurements are used to evaluate the severity of an O₂/CO₂ or acid-base imbalance.

Your body will try to restore any imbalance by itself, but if you have an overwhelming sudden problem or a long-term drain on your body, you may need medical intervention (for example, receiving additional oxygen, medication or IV fluids). If you are on oxygen therapy, your doctor may request blood gas tests to monitor the effectiveness of that therapy.

Blood gas tests are requested when you have symptoms of an O₂/CO₂ or pH imbalance, such as difficulty breathing or shortness of breath. Many 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 requested if you are known to have a respiratory, metabolic or kidney disease, especially if you experience respiratory distress, to evaluate the amount of oxygen in your blood and acid-base balance. They also may be requested for patients with head or neck trauma, as these injuries may affect breathing. Patients undergoing prolonged anaesthesia – particularly for cardiac bypass surgery or brain surgery – may have their blood gases monitored during and for a period after the procedure, as may patients who require additional oxygen or are on a ventilator to help with their breathing.

Checking the blood gases from the umbilical cord of newborns may uncover respiratory problems as well as determine the acidity of the baby’s blood. Testing is usually only done if a newborn’s condition indicates that s/he 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. The severity of the problem depends on whether the cause is acute (sudden onset and short term), or chronic (long-term), and whether or not your body is able to compensate for the imbalance. Your doctor will provide the necessary medical intervention to regain your body’s normal balance, but s/he must also address the original cause of the imbalance.

An acid-base 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 your breathing (known as the 'respiratory component') or kidney function (known as the 'metabolic component'). 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.

Acid-base imbalances can be divided into four main categories:

Respiratory acidosis is characterised by a low pH and an increased PCO₂ and is due to respiratory disease (not enough oxygen in and carbon dioxide out). This can be caused by many things, including pneumonia, chronic obstructive pulmonary disease (COPD), and over-sedation from drugs. Respiratory alkalosis, characterised by a raised pH and a decreased PCO₂, due to over-ventilation (too much oxygen in and carbon dioxide out). This hyperventilation may be caused by, for example, pain or emotional distress due to a severe infection.

Metabolic acidosis is characterised by a low pH and decreased bicarbonate (HCO₃^-), and occurs when the blood is too acidic due to a metabolic/kidney disorder. Causes include diabetes, shock, drugs and kidney failure.

Metabolic alkalosis is characterised by an elevated pH and increased bicarbonate (HCO₃^-). Causes include hypokalaemia (low blood potassium), chronic vomiting, dehydration and sodium bicarbonate overdose.

Arterial blood sample collection is usually more painful than normal collection of blood from a vein. You will experience moderate discomfort, and the site will need to be pressed for some time to prevent further bleeding.

Blood gas measurements are usually done in a hospital since the analysis should be done immediately after the blood has been collected and requires specialised equipment. Most GPs do not have such equipment in their surgeries.