Blood Gas Tests
If your doctor suspects that you have symptoms of an oxygen/carbon dioxide imbalance or an acid-base imbalance. These include difficulty breathing, shortness of breath, or rapid breathing (hyperventilation). You may also be tested to monitor the effectiveness of oxygen therapy (used when you have a condition that causes an oxygen shortage) and during operations to monitor your blood's oxygen and carbon dioxide levels. If you have chronic obstructive pulmonary disease (COPD) this test may be used to assess if you need long term oxygen therapy.
A blood sample collected from an artery, usually the radial artery in the wrist (located on the inside of the wrist, below the thumb, where you can feel your pulse). A capillary blood from a heel-prick may be used for babies
Typically, none. However, if you are on oxygen therapy, the O2 may either be turned off for 20 to 30 minutes before the collection for a "Room Air" test or, if this cannot be tolerated or if the doctor wants to check your oxygen levels with the O2 on, the amount of oxygen being taken will be recorded.
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:
- 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 CO2 and other acids, and the pH increases (blood becomes more alkaline) with decreased CO2 or increased amounts of bases like bicarbonate (HCO3-).
- PO2 – the partial pressure of O2; (the amount of oxygen gas dissolved in blood). Decreased PO2 may result in an elevated PCO2 and decreased pH
- 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 can also be done to give other values, such as:
- O2 saturation – a calculation of how much oxygen is bound to haemoglobin in the red blood cells and available to be carried through the arteries to nourish the body’s cells
- HCO3-; (bicarbonate) is excreted and reabsorbed 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
How is the sample collected for testing?
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. Arterial blood is almost always used for blood gas analysis, but in some cases, analysis of venous blood may be carried out. For babies, capillary blood from heel pricks may be used.
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 may 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 who experience difficulty in breathing right after birth, blood may be collected from the blood vessels of the umbilical cord.
After the arterial blood has been collected you will be instructed to hold the site firmly for at least 5 minutes. Since blood pumps through the artery, the puncture will take a while to stop bleeding. If you are taking blood thinners or aspirin, it may take up to ten or fifteen minutes to stop. The doctor will verify that the bleeding has stopped.
Is any test preparation needed to ensure the quality of the sample?
Typically, no test preparation is needed. However, if you are on oxygen therapy, the oxygen may either be turned off for 20 to 30 minutes before the collection for a "room air" test or, if this cannot be tolerated or if the doctor wants to check your oxygen levels with the oxygen on, the amount of oxygen being taking will be recorded. This is usually expressed as fraction of inspired oxygen in percent (e.g., 30% FIO2) or as litres of O2 flowing per minute.
How is it used?
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.
When is it requsted?
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.
What does the test result mean?
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.
Is there anything else I should know?
Can this test be done in a doctor's surgery?
I've had pneumonia before and currently have asthma. Why has my doctor never requested this test on me?
Most cases of pneumonia or asthma can be diagnosed by symptoms and monitored by listening to your chest sounds or by examining the results of spirometry tests or chest x-rays. Most of the time, asthma will respond to your usual medications and pneumonia to rest and possibly antibiotics. Blood gas tests may be necessary if you have severe, sudden or long-term breathing problems. In these cases, blood gas tests are usually done in a hospital setting.
Is there any other way to measure my oxygen levels?
A pulse oximeter is a method of continuously monitoring oxygen saturation which is noninvasive (there is no need to obtain a blood sample). A small clip-like device (called a sensor) is attached to the end of the finger or earlobe. The sensor reads light that is transmitted through the skin. Pulse oximeters are useful for monitoring oxygen saturation, but their accuracy can be affected by the presence of abnormal forms of haemoglobin, like carboxyhaemoglobin (present in carbon monoxide poisoning), poor circulation, and very low levels of haemoglobin due to severe anaemia. Another non-invasive method used in hospitals is transcutaneous (through the skin) oxygen monitoring. This uses a small electrode placed on the skin which measures the oxygen dissolved in the tissue underneath it. However, these methods do not detect an acid-base imbalance.
Why does my laboratory report also list carboxyhaemoglobin? What is it?
If your blood gases were measured using an instrument known as a co-oximeter, then your laboratory report may also list results for carboxyhaemoglobin and other altered forms of haemoglobin. A co-oximeter is a blood gas analyser that can measure concentrations of haemoglobin derivates (like carboxyhaemoglobin) in addition to the usual blood gas measurements. A co-oximeter is not always used, so these values are not reported for all blood gas analyses.
Carboxyhaemoglobin is an altered form of haemoglobin that forms when carbon monoxide binds to haemoglobin. Levels of carboxyhaemoglobin are often elevated with carbon monoxide poisoning and cigarette smoking, and a co-oximeter is used to measure carboxyhaemoglobin levels and to monitor oxygen therapy. Haemoglobin binds to carbon monoxide about 210 times more strongly than to oxygen. Binding to carbon monoxide significantly decreases haemoglobin's ability to carry oxygen through the body, which can lead to a serious, life-threatening condition.
Other haemoglobin derivates include sulfhaemoglobin (or sulfmethaemoglobin) and methaemoglobin, which may result from the ingestion of certain medicines or exposure to chemicals. These altered forms of haemoglobin, like carboxyhaemoglobin, cannot function properly to carry oxygen to tissues in the body and are commonly measured by a co-oximeter.