Dr Leigh Martens

General Practitioner

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Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Arterial Blood Gases


At a Glance

Why Get Tested?

To evaluate lung function by measuring blood pH, oxygen (O2) and carbon dioxide (CO2); to monitor treatment for lung diseases; to detect an acid-base imbalance in your blood, which may indicate a respiratory, metabolic, or kidney disorder; to evaluate the effectiveness of oxygen therapy

When to Get Tested?

When you have symptoms of a respiratory problem such as difficulty breathing, shortness of breath, or rapid breathing; when you are being treated for a lung disease; when an acid-base imbalance is suspected; periodically when you have a condition that causes an acute or chronic oxygen shortage and you are on oxygen therapy; during certain surgeries to monitor your blood's O2 and CO2 levels

Sample Required?

Most often a blood sample collected from an artery, usually the radial artery in your wrist; sometimes a blood sample drawn from a vein in your arm; capillary blood from a heelstick may be used for babies.

Test Preparation Needed?

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. If this cannot be tolerated, or if your health practitioner wants to check your oxygen levels with the O2 on, the amount of oxygen being taken will be recorded.

The Test Sample

What is being tested?

Blood gases are a group of tests that are performed together to measure the pH and the amount of oxygen (O2) and carbon dioxide (CO2) present in a sample of blood, usually from an artery, in order to evaluate lung function and help detect an acid-base imbalance that could indicate a respiratory, metabolic or kidney disorder.

A person's body carefully regulates blood pH, maintaining it within a narrow range of 7.35-7.45, not allowing blood to become too acidic (acidosis) nor too alkaline/basic (alkalosis). The body's regulation of acids and bases has two main components. The first component involves both metabolism and the kidneys: the cellular process of converting one substance to another for energy produces large amounts of acid that the kidneys help eliminate.The second component of regulating pH balance involves eliminating carbon dioxide (an acid when dissolved in blood) through the lungs.This respiratory component is also the way that the body supplies oxygen to tissues.The lungs inhale oxygen, which is then dissolved in the blood and carried throughout the body to tissues.

These processes of gas exchange and acid/base balance are also closely associated with the body's electrolyte balance.In a normal state of health, these processes are in a dynamic balance and the blood pH is stable. (For more on this, see Acidosis and Alkalosis).

There is a wide range of acute and chronic conditions that can affect kidney function, acid production, and lung function, and they have the potential to cause a pH, carbon dioxide/oxygen, or electrolyte imbalance.Examples include uncontrolled diabetes, which can lead to ketoacidosis and metabolic acidosis, and severe lung diseases that can affect CO2/O2 gas exchange. Even temporary conditions such as shock, anxiety, pain, prolonged vomiting, and severe diarrhea can sometimes lead to acidosis or alkalosis.

Blood gas analysis give a snapshot of a person's blood pH and O2 and CO2 content. The following components can be determined by blood gas analysis:

  • pH. A measure of the balance of acids and bases in the blood. Increased amounts of carbon dioxide and other acids can cause blood pH to decrease (become acidic). Decreased carbon dioxide or increased amounts of bases, like bicarbonate (HCO3-), can cause blood pH to increase (become alkaline).
  • Partial pressure of O2 (PaO2). Measures the amount of oxygen gas in the blood.
  • Partial pressure of CO2 (PaCO2). Measures the amount of carbon dioxide gas in the blood. As PaCO2 levels rise, blood pH decreases, making the blood more acidic; as PaCO2 decreases, pH rises, making the blood more basic (alkaline).
  • O2 saturation (O2Sat or SaO2). The percentage of hemoglobin that is carrying oxygen.Hemoglobin is the protein in red bloods cells that carries oxygen through blood vessels to tissues throughout the body.
  • O2 content (O2CT or CaO2). The amount of oxygen per 100 mL of blood.
  • Bicarbonate (HCO3-). The main form of CO2 in the body. It can be calculated from the pH and PaCO2. It is a measurement of the metabolic component of the acid-base balance. HCO3- 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 in the blood, so does the pH (becomes alkaline).
  • Base excess/base 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). Anions are regulated to compensate for imbalances in blood pH.The health practitioner will look at the HCO3- and base excess/deficit results to evaluate the total buffering capacity of the lungs and kidneys when deciding on a treatment to correct an imbalance.

How is the sample collected for testing?

Arterial blood is almost always used for blood gas analysis but, in some cases, as with babies, whole blood from a heelstick is collected instead. Blood may also be taken from the umbilical cord of a newborn. Since arterial blood carries oxygen to the body and venous blood carries waste products to the lungs and kidneys, the gas and pH levels will not be the same in both types of blood samples.

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 the pulse can be felt. A circulation test called an Allen test will be done before the collection to make sure that there is adequate circulation in the person's wrist. The test involves compressing both the radial and the ulnar wrist arteries, then releasing each in turn to watch for "flushing," the pinking of the skin as blood returns to the hand. If one hand does not flush, then the other wrist will be tested. Blood can also be collected from the brachial artery in the elbow or the femoral artery in the groin, although these sample locations require special training to properly access.

In newborns that experience difficulty in breathing right after birth, blood may be collected from both the umbilical artery and vein and tested separately.

After an arterial blood draw, pressure must be firmly applied to the site for at least 5 minutes. Since blood pumps through the artery, the puncture will take awhile to stop bleeding. If someone is taking blood thinners or aspirin, it may take as long as 10-15 minutes to stop bleeding. The person collecting the sample will verify that the bleeding has stopped and will put a wrap around the wrist, which should be left in place for about an hour.

Is any test preparation needed to ensure the quality of the sample?

Typically, no test preparation is needed.However, if someone is 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 health practitioner wants to check oxygen levels with the O2 on, the amount of oxygen being taking will be recorded. This is usually expressed as fraction of inspired (inhaled) oxygen in percent (FiO2) or as liters of O2 flowing per minute.

The Test

How is it used?

Blood gas measurements are used to evaluate a person's lung function and acid/base balance.

They are typically ordered if someone is having worsening symptoms of a respiratory problem, such as difficulty breathing or shortness of breath, and a condition such as asthma or chronic obstructive pulmonary disease (COPD) is suspected.Blood gases may also be used to monitor treatment for lung diseases and to evaluate the effectiveness of supplemental oxygen therapy.

Blood gases are used to detect an acid-base imbalance, such as can occur with kidney failure, heart failure, uncontrolled diabetes, severe infections, and drug overdose. They may be ordered along with other tests, such as electrolytes to determine if an electrolyte imbalance is present, glucose to evaluate blood sugar concentrations, and BUN and creatinine tests to evaluate kidney function.

When is it ordered?

A blood gas analysis is ordered when someone has symptoms of an oxygen/carbon dioxide or pH imbalance, such as difficulty breathing, shortness of breath, nausea, or vomiting. It may also be ordered when someone is known to have a respiratory, metabolic, or kidney disease and is experiencing respiratory distress.

When someone is "on oxygen" (ventilation), blood gases may be measured at intervals to monitor the effectiveness of treatment. Other treatments for lung diseases may also be monitored with blood gases.

Blood gases may also be ordered when someone has head or neck trauma, which may affect breathing, and when someone is undergoing prolonged anesthesia – particularly for cardiac bypass surgery or brain surgery – to monitor blood gases during, and for a period after, the procedure.

Checking blood gases from the umbilical cord of a newborn may uncover respiratory problems as well as determine acid/base status. Testing is usually only done if a newborn is having difficulty breathing.

What does the test result mean?

Normal values will vary from lab to lab. They are also dependent on elevation above sea level as a person's blood oxygen level will be lower if they live higher than sea level.

Results from an arterial blood gas analysis are not diagnostic; they should be used in combination with the results of other tests and exams to evaluate someone for a respiratory, metabolic, or kidney problem.

Abnormal results of any of the blood gas components may indicate one or more of the following issues:

  • A person is not getting enough oxygen
  • A person is not getting rid of enough carbon dioxide
  • There is a problem with a person's kidney function

A low partial pressure of oxygen (PaO2) suggests that a person is not getting enough oxygen, while results that are within normal range usually mean that oxygen intake is sufficient.

All other components of the blood gas analysis (pH, PaCO2, HCO3-) are interrelated and the results must be considered together. Certain combinations of results, if abnormal, may indicate a condition that is causing acidosis or alkalosis. These may include the following:

  • Respiratory acidosis is characterized by a lower pH and an increased PaCO2 and is due to respiratory depression (not enough oxygen taken in and carbon dioxide removed). 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 PaCO2, 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-, causing the blood to be too acidic for proper metabolic/kidney function. 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.

Examples of test results associated with the above conditions are summarized below:

pH result Bicarbonate result PaCO2 result Condition Common causes
Less than 7.35 Low Low Metabolic acidosis Kidney failure, shock, diabetic ketoacidosis, intoxication with methanol, salicyate, ethanol
Greater than 7.45 High High Metabolic alkalosis Chronic vomiting, low blood potassium, heart failure, cirrhosis
Less than 7.35 High High Respiratory acidosis Narcotics, lung diseases such as asthma, COPD, airway obstruction
Greater than 7.45 Low Low Respiratory alkalosis Hyperventilation, pain, anxiety, brain trauma, pneumonia, certain drugs (salicylate, catecholamines)

If left untreated, these conditions can create an imbalance that can eventually become life-threatening. A health practitioner can provide the necessary medical intervention to regain normal acid/base balance, but the underlying cause of the imbalance must also be addressed.

Is there anything else I should know?

Arterial blood sample collection is usually a bit more painful than regular venipuncture. You may 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 catheterization labs and by transplant services. Careful interpretation of the results is required. Peripheral venous blood, such as that taken from a vein in the arm, is of no use for oxygen status because it has decreased oxygen content due to the fact that it is composed of blood returning to the heart.