This document provides an overview of ABG (arterial blood gas) interpretation. It begins by explaining the importance of maintaining pH homeostasis and the roles of carbon dioxide (CO2) and bicarbonate (HCO3) in buffering pH. It then discusses acid-base imbalances including respiratory and metabolic acidosis and alkalosis. The document uses a "tic-tac-toe" chart to demonstrate how to determine if an ABG result is normal, acidotic, or alkalotic; respiratory or metabolic; and uncompensated, partially compensated, or fully compensated based on the pH, PaCO2, PaO2, and HCO3 levels. Examples are provided to illustrate each

1. ABG Interpretation Lacie M. Crone, RN, MSEd, CCRN, CPN April 10, 2009

2. Why look at ABGs? The body works very hard to maintain homeostasis. Part of that homeostasis is maintaining a serum pH of 7.35 to 7.45. When all is well, the body adjusts the amounts of CO2 (carbon dioxide) and HCO3 (bicarbonate) to keep pH within normal limits.

4. The pH scale looks at how many hydrogen ions are present.

5. The lower the pH, the more hydrogen ions are present. The more acidic the solution.

7. What is up with carbon dioxide? CO2 is a byproduct of cellular respiration. Remember the Kreb’s cycle? When the cell generates ATP it also generates waste products of CO2 and H2O. CO2 acts as an acid in the body. CO2 , in the presence of H2O, can become H2CO3, carbonic acid. You can read more about this in your book. Incidentally, CO2 is an acid in the environment also. CO2 emissions combine with H2O to create H2Co3, hence acid rain although sulfur and nitrogen are much more responsible than CO2 for acid rain.

8. I thought Hydrogen was acidic… It is true that the greater the number of hydrogen ions present, the lower the pH and therefore the greater the level of acidity. The body employs a delicate interplay with CO2 and H and HCO3 to maintain a normal pH. Basically, CO2 acts as an acid, H acts as an acid, and HCO3 acts as a base. There will be more to come on this in a bit.

9. What is up with bicarbonate? HCO3 acts as a base in the human body. The kidneys help regulate the level of HCO3 in the body. Basically, when the body has too many H ions present, the buffer system releases more HCO3 into circulation. There is an enzyme (carbonic anydrase) found concentrated in red blood cells that helps convert CO2 into HCO3 and thus assists in regulating the body’s pH. You can read more about this enzyme by looking it up in your book.

10. Acidosis vs. Alkalosis Basically, when we are talking serum, an acidosis is any pH that is less than 7.35 and an alkalosis is any pH that is greater than 7.45. Remember that there are other fluids in the human body that have a higher or lower pH. We are only talking about serum here. When the body is outside of the 7.35-7.45 range, there are a few things that we as medical personnel can do to help the body come around. The body will tolerate small fluctuations, but large ones can result in death.

11. My bologna has a first name… ABGs are identified by three names. First we identify the last name. Is it an acidosis, an alkalosis, or is it normal? If it is normal we are finished~yippee! Right…normal…that seldom happens.  Next we identify the middle name. Is it respiratory or metabolic? Finally we identify the first name. Is it uncompensated, partially or fully compensated?

13. PaCO2 for PCO2

14. SaO2 for sats obtained from a sat probe22-26

15. Lets play some tic-tac-toe! The object of this is to take your lab values and insert them into the correct box. Once you have placed all the values you will look to see which ones correlate to start naming your gas results. Example #1 pH 7.43 PaO2 85 PaCO2 42 HCO3 25 7.43 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 42 25 This is a normal blood gas!

16. Tic tac toe #2 No Example #2 pH 7.47 PaO2 84 PaCO2 34 HCO3 21 7.47 34 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 21 This is respiratory alkalosis Many gasses will be abnormal. In this case, the pH and the PaCO2 agree because they are both in the alkalosis column. When the PaCO2 and the pH agree then the problem is with the respiratory system in the body. If the pH and the HCO3 had been in alignment the problem would be metabolic.

17. Tic tac toe #3 Example #3 pH 7.28 PaO2 92 PaCO2 52 HCO3 24 7.28 52 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 24 This is a respiratory acidosis. In this case, the pH and the PaCO2 agree because they are both in the acidosis column. Because the pH and the PaCO2 agree, this is a respiratory acidosis.

18. Tic tac toe #4 Example #4 pH 7.34 PaO2 94 PaCO2 28 HCO3 18 7.34 28 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 18 This is a metabolic acidosis. In this case, the pH and the HCO3 agree because they are both in the acidosis column. Because the pH and the HCO3 agree, this is a metabolic acidosis.

19. But you said three names… I did say that ABGs were known by three names. First we identify if it is normal, an acidosis, or an alkalosis. Next we identify the source by identifying either respiratory (CO2 related) or metabolic (HCO3 related) based upon which of those values correlates with the pH. Now we need to determine if it is compensated, uncompensated, or partially compensated.

20. Compensation-partial or none To determine compensation we look at the value that didn’t come into alignment with the pH. If the value that is not in alignment with the pH is in the normal column, there is no compensation. If the value that is not in alignment with the pH is in the far opposite column, there is partial compensation. The tricky one is full compensation. We will look at that on another set of slides.

21. Tic tac toe #5 Example #5 pH 7.34 PaO2 94 PaCO2 28 HCO3 18 7.34 28 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 18 This is a partially compensated metabolic acidosis. In this case, the pH and the HCO3 agree because they are both in the acidosis column. Because the pH and the HCO3 agree, this is a metabolic acidosis. Because the unrelated value (CO2) is in the far opposite column, there is partial compensation provided by the respiratory system. The patient is blowing off lots of CO2 to blow off an acid to try to raise the pH. You will likely notice this person has a high respiratory rate.

22. Tic tac toe #6 Example #6 pH 7.49 PaO2 89 PaCO2 60 HCO3 30 7.49 60 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 30 This is a partially compensated metabolic alkalosis. In this case, the pH and the HCO3 agree because they are both in the alkalosis column. Because the pH and the HCO3 agree, this is a metabolic alkalosis. Because the unrelated value (CO2) is in the far opposite column, there is partial compensation provided by the respiratory system. The patient is holding onto lots of CO2 to hold onto an acid to try to lower the pH. You will likely notice this person has a low respiratory rate.

23. Tic tac toe #7 Example #7 pH 7.34 PaO2 86 PaCO2 60 HCO3 30 7.34 60 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 30 This is a partially compensated respiratory acidosis. In this case, the pH and the CO2 agree because they are both in the acidosis column. Because the pH and the CO2 agree, this is a respiratory acidosis. Because the unrelated value (HCO3) is in the far opposite column, there is partial compensation provided by the buffer system. The patient is holding onto lots of HCO3 to hold onto a base to try to raise the pH.Increasing this patient’s respiratory rate would also raise the pH.

24. Tic tac toe #8 Example #8 pH 7.34 PaO2 86 PaCO2 60 HCO3 24 7.34 60 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 24 This is an uncompensated respiratory acidosis. In this case, the pH and the CO2 agree because they are both in the acidosis column. Because the pH and the CO2 agree, this is a respiratory acidosis. Because the unrelated value (HCO3) is in the normal column, there is nocompensation provided by the buffer system.

25. So what about full compensation? If a gas shows full compensation, the pH will be normal. If the pH is normal how do you determine if it is an acidosis or alkalosis? You look at which it is closer to.

26. Tic tac toe #9 Example #9 pH 7.38 PaO2 86 PaCO2 60 HCO3 26 7.38 60 Click the mouse to put each value on the chart. Click once more to reveal the name of the gas. 26 This is an fully compensated respiratory acidosis. In this case, the pH is normal but on the acid side of 7.4 (half-way between 7.35 and 7.45.) We consider the problem to be respiratory because PaCO2 is acidic. HCO3 is normal. This gas is fully compensated because the pH is normal but since the PaCO2 is off, it is a fully compensated respiratory acidosis.