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Interpreting ABGs: Simple and Easy


Interpreting ABGs

Arterial blood gasses, or ABGs, may be among the most complex and confusing parts of nursing practice. Everyone can read an electrolyte level, but when faced with a list of ABGs, most nurses wilt. Fortunately, there are some easy ways to remember how to decipher these important lab results. Without knowing them, your patient could easily go downhill when the answer was in front of you all along.

These easy questions are all you need to know in order to interpret ABGs effectively and help your patient, should a problem arise.

What is Normal?

Firstly, it is important to know what values you are looking at and what is considered normal. The first value is the pH, which measures how many hydrogen ions are in the sample. This determines if the blood is acidotic or alkalotic. Normal values for pH range from 7.35 to 7.45. The next value is the carbon dioxide level, and this will tell you if the problem is a respiratory one. Normal range for CO2 is 35 to 45mmHg. Finally, bicarbonate ions, or HCO3-, will tell you if the problem is related to metabolic changes in your patient. Normal is considered to be from 22 to 26mmol/L. When the numbers fall out of these ranges, you use them to determine what type of problem the patient is experiencing.

Acidic or Alkalotic?

If the numbers are not within normal range, you have to ask yourself if the patient’s pH is acidotic or alkalotic. It may seem counter-intuitive, but the lower the number, the more acidotic the patient is. For instance, a pH of 3 is severely acidotic and requires emergency intervention. Alkalosis is the opposite. The higher the number, the more base is in the blood sample. Although this doesn’t sound as bad as a high acid count, it can still disrupt the normal functioning of the body. Once you’ve determined whether there is too much acid or too much base, you can move on to discover the cause of it.

Respiratory or Metabolic?

After you’ve determined whether the sample is acidic or alkaline, you need to work out if it’s due to respiratory or metabolic causes. This is where it can get tricky. For respiratory problems, the CO2 will be out of the normal range, whereas for metabolic problems the HCO3- will be abnormal. Low CO2 points to respiratory alkalosis, and high HCO3- can indicate metabolic alkalosis.

Compensated or Uncompensated?

Compensation can be thought of as the body’s attempt at correcting an imbalance – is one system in the body trying to compensate for an abnormality in another system? We can examine this by looking at the opposing system (the opposite component of the problem). For example, in an acidosis, we’d look at what the HCO3- is doing. Whereas in an alkalosis, to determine if the body is compensating, we’d look at what the CO2 is doing. If the other system is within normal ranges, then the problem is non-compensated or uncompensated. Ultimately, the body is yet to fix the problem or has been unable to fix the problem.

Interpreting ABGs - Arterial Blood Gases

However, if the other component has gone outside its normal reference ranges, we can think of it as compensation occurring (the body is trying to fix the problem). To assess how well it has been able to do this we need to refer back to the pH. If the pH is not within or close to the normal ranges, then a partial-compensation exists. If the pH is back within normal ranges then a full-compensation has occurred.

A non-compensated or uncompensated abnormality usually represents an acute change occurring in the body. The term partial or fully compensated is used to describe the level of compensation and does not necessarily mean the patient’s ABG is normal or that they are healthy!


To Simplify:

  • Respiratory acidosis: low pH, high CO2
  • Respiratory alkalosis: high pH, low CO2
  • Metabolic acidosis: low pH, low HCO3-
  • Metabolic alkalosis: high pH, high HCO3-
  • Compensated respiratory acidosis: normal pH, high CO2
  • Compensated metabolic acidosis: normal pH, low HCO3-
  • Compensated respiratory alkalosis: normal pH, low CO2
  • Compensated metabolic alkalosis: normal pH, high HCO3-

Case Study 1

Consider the following:
pH = 7.50
CO2 = 47
HCO3- = 32

1) Is it an acidosis or an alkalosis?

Show Answer

The pH is 7.50. This is higher than normal, so we have an alkalosis.

2) Is the problem respiratory or metabolic in nature?

Show Answer

Where else is there an alkalosis? The HCO3- is 32, which is high. So we have a metabolic alkalosis.

3) Is there any compensation occurring? Has the body tried to fix the problem?

Show Answer

We need to look at the other component, in this case, what is the CO2? The CO2 is outside its normal ranges. It’s 47, which is high. So the body is trying to fix the problem. However, the pH is not yet back within normal ranges so a partial compensation exists.

Reveal Conclusion

This ABG is an example of a partially compensated metabolic alkalosis.


So Let’s Recap…

Q1) Is it an acidosis or an alkalosis?
What is the pH? Is there an acidosis (<7.35) or alkalosis (>7.45)?

Q2) Is the problem respiratory or metabolic in nature?
Look for the match. If the CO2 matches the altered pH the problem is associated with the respiratory system. If the HCO3- corresponds with the altered pH, then the problem is metabolic in nature.

Q3) Is there any compensation occurring? Has the body tried to fix the problem?
Has the other component (opposite to where the problem lies) gone outside of its reference range? And to what effect? Has the body done a good job of fixing the problem?

  • Compensated = the pH is close to or within normal ranges (i.e. the body has used it’s buffer system to normalise the pH)
  • Uncompensated = the pH is outside of normal ranges (i.e. the body’s buffer system is yet to, or has failed to bring the pH back within normal ranges)

Case Study 2

Consider the following:
pH = 7.30
CO2 = 50
HCO3- = 30

1) Is it an acidosis or an alkalosis?

Show Answer

The pH is 7.30. This is lower than normal, so we have an acidosis.

2) Is the problem respiratory or metabolic in nature?

Show Answer

What else is acidotic? The CO2 is 50, which is high. So we have a respiratory acidosis.

3) Is there any compensation occurring? Has the body tried to fix the problem?

Show Answer

We need to look at the other component, being HCO3- in this case. Is the HCO3- outside its normal ranges? Yes, normal HCO3- is between 22-26. So the body is trying to fix this problem. Has the body done a good job at fixing the problem? Is the pH back within normal ranges? No, the pH is not within normal ranges, so there is partial compensation occurring.

Reveal Conclusion

This ABG is an example of a partially compensated respiratory acidosis.


Note: ABGs should be thought of as a snapshot of how the body is interacting with its environment at a particular time. They should be interpreted as part of a wider assessment of a patient’s respiratory function.

Show References

References

  • Anderson, AH, Bednarek, S, Brady, CL, Burns, SN, Haynes, N, Kennedy, LD, Lopez, C, Madara, M, Ralph, J & York, DR 2012, Pathophysiology Made Incredibly Easy 2012, 5th edn, Lippincott Williams & Wilkins: Ambler, PA, USA.
  • Chang, E, Daly, J & Elliot, D 2006, Pathophysiology Applied to Nursing Practice, 2nd edn, Mosby Australia, Port Melbourne, VIC, Australia.

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