This lesson will discuss what the arterial blood gas test is and how it is used to interpret acid/base balance in your body. You'll learn about partial pressures, hypercapnia, acidemia, and alkalosis.
Blood From an Artery
The last time you were in a doctor's office and needed to have blood work done, a nurse probably took some blood from a vein for analysis.
But few people may know that while we routinely take blood from a vein to analyze all sorts of problems within our body, we sometimes take blood from an artery for the same reason. It's not nearly as common, but that doesn't mean it's less important.
In just a little bit, you'll discover what things we look for on a test known as an arterial blood gas and why that's important to figuring out what medical problems a patient may be suffering from with respect to the acid/base balance within your body.
What Is an Arterial Blood Gas?
An arterial blood gas is a test that measures the levels of oxygen, carbon dioxide, and the acidity (pH) of arterial blood, the blood that carries oxygen from your lungs and to your tissues.
This test is most often used in critically ill patients suffering from things like lung disease, kidney problems, or uncontrolled metabolic issues all in order to assess the state of disease and its impact on the body.
This blood test gives certain values, including:
- PaO2, the arterial partial pressure of oxygen, normally 75-100 mm Hg.
- PaCO2, the arterial partial pressure of carbon dioxide, normally 35-45 mm Hg.
- The arterial blood pH, normally around 7.35-7.45. The pH essentially represents the acidic hydrogen ion (H+) concentration.
- HCO3- bicarbonate levels are measured as well. They are supposed to be around 22-28 mEq/L.
The term partial pressure is sometimes also called tension. So if you hear someone talking about the arterial oxygen tension, they're referring to the arterial partial pressure of oxygen. Also, I hope you're dying to know what partial pressure actually is since it's a weird term. The partial pressure of a gas is the amount of pressure a specific gas contributes to the total pressure of all the gases in a certain mixture.
For instance, if you blow up a balloon using the air around you, it will contain things like oxygen, nitrogen, and carbon dioxide. The amount of pressure each individual substance within the balloon exerts on the walls of the balloon is their respective partial pressure. For our lesson, think of it this way: the higher the partial pressure of a gas, the more of it will be dissolved in the blood. That's the bottom line.
Another thing I really hope you're dying to have answered by now is why in the world we don't just use venous blood to measure all of this stuff? That's because the composition of venous blood is dependent on the tissue from which venous blood drains from. This, therefore, causes too much uncertainty and variation in test results when compared to measurements of arterial blood. The latter measurement is obviously more directly dependent upon respiratory gas exchange processes, since arterial blood is the blood that just passed through the lungs.
Interpreting the Results
Let's boil down some things real quick. A low pH (lots of H+) is acidic, and a high pH (little H+) is alkalotic. So, an abnormally low pH of the blood, <7.35, is called acidemia, with '-emia' referring to blood, and an abnormally high pH of the blood, >7.45, is termed alkalemia. Acidosis and alkalosis refer to the pathologic processes that cause acidemia and alkalemia, respectively.
Furthermore, in order to truly understand blood gas interpretation from a pathophysiological standpoint, you must know the following equilibrium:
CO2 + H20 <--> H2CO3 <--> H+ + HCO3-
This is a reaction that represents how acid/base status is maintained in equilibrium within our blood. Meaning, the way by which our body keeps our pH at a normal level so we aren't killed by acidity or alkalinity. The fulcrum of the scale, irrelevant to our discussion, is H2CO3 (carbonic acid).
More importantly, the left side of the scale (namely the acidic CO2) is under respiratory system control, and the right side of the scale (namely the basic HCO3-, bicarbonate) is under metabolic system control (for this blood test, represented by the kidneys). If one system malfunctions, causing the substance it controls to increase or decrease, the other system will cause the substance it controls to move in the same direction in order to compensate for the initial change.
For example, if acidic CO2 levels rise due to a malfunction of the respiratory system that controls levels of CO2 in the body, then the kidneys will retain basic bicarbonate in the body to maintain appropriate acid/base balance. It's like a tag team wrestling match. If CO2 enters the wrestling ring (our body), then it will tag the bicarbonate to enter into the ring as well. If CO2 squeezes through the ropes to run away from the wrestling match, bicarbonate will run away as well. The reverse is true if bicarbonate is the one doing the tagging.
With that in mind, let's interpret the meaning of the values of our test results. The exact pathophysiology of each main disturbance is found in other lessons that I encourage you to watch since we don't have time for that right now.
A low pH and high levels of PaCO2 and HCO3- are indicative of respiratory acidosis. This means too much CO2 is retained in the body, resulting in hypercapnia, or too much CO2 in the blood (sometimes called hypercarbia). The most common cause of this is an inability for the respiratory system to properly 'blow-off' CO2. This can occur due to improper gas exchange as a result of pneumonia, COPD, airway obstruction due to a foreign body, pneumothorax (a collapsed lung), and many other reasons.
Another possibility on the arterial blood gas is a low pH and low levels of PaCO2 and HCO3- that are indicative of metabolic acidosis. This can be due to a loss of bicarbonate due to kidney disease or things like diarrhea. Or it could be due to too much acid (H+) in the body from conditions such as diabetic ketoacidosis.
Further still, a high pH with high PaCO2 and HCO3- levels indicate metabolic alkalosis. In this case, the opposite of what I was talking about occurs. Meaning, there's either too little H+ or too much bicarbonate. Acid can be lost due to a condition called hyperaldosteronism, where elevated levels of a hormone known as aldosterone cause acid (H+) secretion out of the body and thereby also cause a relative increase of bicarbonate. Vomiting also decreases levels of H+ in the body, since so much of your gastric acid is lost when you vomit.
Finally, a high pH with low levels of PaCO2 and HCO3- indicate respiratory alkalosis. This state implies that the lungs are blowing off CO2 way too much, leading to a high pH since the acidic CO2 is now gone. This causes the kidneys to compensate by getting rid of the basic bicarbonate out of the body so the body doesn't become too basic. The only real way for respiratory alkalosis to occur is if the person is hyperventilating, or breathing too quickly as a result of numerous causes, such as being located in high altitudes where the air is thin, causing the person to breathe more quickly to try and gain as much oxygen as possible.
A Quick Memory Trick
If you're overwhelmed at trying to remember which test results indicate what, here's a quick trick. If all three values are pointing in the same direction, that is to say all three are increased abnormally or decreased abnormally, then it has to be a metabolic problem. It's obviously acidosis if the pH is low and alkalosis if it's high. However, if the pH is pointing in the opposite direction of both the bicarbonate and carbon dioxide, it's a respiratory problem.
Let's summarize the important things that were covered. An arterial blood gas is a test that measures the levels of oxygen, carbon dioxide, and the acidity (pH) of arterial blood.
This blood test gives certain values, including:
- PaCO2, the arterial partial pressure of carbon dioxide
- The arterial blood pH
- HCO3-, or bicarbonate
The partial pressure of a gas is the amount of pressure a specific gas contributes to the total pressure of all the gases in a certain mixture.
A low pH (lots of H+) is acidic, and a high pH (little H+) is alkalotic. So, an abnormally low pH of the blood, <7.35, is called acidemia, with '-emia' referring to blood, and an abnormally high pH of the blood, >7.45, is termed alkalemia.
A low pH and high levels of PaCO2 and HCO3- are indicative of respiratory acidosis. This means too much CO2 is retained in the body, resulting in hypercapnia, too much CO2 in the blood (sometimes also called hypercarbia).
Another possibility on the arterial blood gas is a low pH and low levels of PaCO2 and HCO3- that are indicative of metabolic acidosis.
Further still, a high pH with high PaCO2 and HCO3- levels indicate metabolic alkalosis.
Finally, a high pH with low levels of PaCO2 and HCO3- indicate respiratory alkalosis.
Following this lesson, you'll be able to:
- Identify what an arterial blood gas measures and explain why arteries are used to take these measurements
- Define partial pressure
- Explain what can cause abnormally high or abnormally low blood pH
- Describe the pH and levels of PaCO2 and HCO3- that indicate metabolic acidosis, metabolic alkalosis and respiratory alkalosis