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.
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.
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:
Less than 7.35
Kidney failure, shock, diabetic ketoacidosis, intoxication with methanol, salicyate, ethanol
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.
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.
This article was last reviewed on October 24, 2013. | This article was last modified on December 29, 2014.
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