1. Interpretation: Compensated and Uncompensated Blood Gas Analysis
James Barnett, RN, MSN
Clinical Educator – Neuroscience PCC
Vanderbilt University Medical Center
May 2007

2. Compensatory Mechanisms
Compensation is the body’s way of restoring a normal blood pH
Remember: Acid + Base  Neutrality
Compensation DOES NOT treat the root of the problem – the reason for the acid-base imbalance is STILL THERE!!!

3. Compensatory Mechanisms
The body has three means to try to compensate for an acid-base imbalance
Chemical
Respiratory
Renal

4. Chemical Compensation
Chemicals within the blood act within seconds to correct respiratory or metabolic imbalances
Used up quickly – not effective long-term
Chemical buffers in the blood include
Bicarbonate
Phosphate
Proteins

5. Respiratory Compensation
Used to compensate for metabolic imbalances only
Chemoreceptors respond to changes in H+ concentrations  alters respiratory rate and depth
Remember CO2 is an acid

6. Respiratory Compensation
Respiratory Rate will…
Increase when blood H+ is increased (acidic pH)
CO2 is “blown off”
Amount of acid in blood is decreased
Decrease when H+ is decreased (alkaline pH)
CO2 is retained
Amount of acid in blood is increased

7. Respiratory Compensation
This means
Metabolic acidosis causes an increase in rate and depth of ventilation as the body attempts to get rid of acid (CO2)
Metabolic alkalosis causes a decrease in rate and depth of ventilation as the body attempts to retain acid (CO2)

8. Renal Compensation Used to compensate for respiratory imbalances
Remember: HCO3- is a base
Kidneys respond to changes in blood pH
Excrete H+ and retain HCO3- when acidemia is present (1:1 ratio)
Retain H+ and excrete HCO3- when alkalemia is present (1:1 ratio)

9. Renal Compensation This means
A respiratory acidosis will make the kidneys excrete acid (H+) and retain base (HCO3-)
A respiratory alkalosis will make the kidneys excrete base (HCO3-) and retain acid (H+)

10. Renal Compensation This is the slowest compensation
May take hours to days
Most powerful method of compensation
Ineffective in patients with renal failure

11. The body is very smart and will not overcompensate for an imbalance
Note on Compensation
The body is very smart and will not overcompensate for an imbalance

12. Degrees of Compensation
An acid-base imbalance will be compensated for in one of three ways
Uncompensated
Partially compensated
Fully compensated

13. Degrees of Compensation
Uncompensated
Body has made no attempt to correct the acid-base imbalance
Partially compensated
Body is attempting to correct the imbalance
Blood pH remains abnormal in spite of the attempt

14. Degrees of Compensation
Fully compensated
The body is correcting the imbalance
Blood pH is normal
Other blood gas values remain abnormal until the root cause is treated and corrected

15. Uncompensated Acid-Base Imbalance

16. Uncompensated Imbalance
pH abnormal
Either PaCO2 OR HCO3- abnormal
All other values normal
If PaCO2 is abnormal
Problem is respiratory
If HCO3- is abnormal
Problem is metabolic

17. Uncompensated Imbalance
Uncompensated respiratory acidosis
pH < 7.35
PaCO2 > 45
HCO3- WNL
Uncompensated respiratory alkalosis
pH > 7.45
PaCO2 < 35
HCO3- WNL
Remember that CO2 is an acid and that the more of it there is the worse is
the acidemia. Notice that with uncompensated respiratory, the HCO3 is
normal – this is because the body has not began to compensate for the
alterations in CO2

18. Uncompensated Imbalance
Uncompensated metabolic acidosis
pH < 7.35
PaCO2 WNL
HCO3- < 22
Uncompensated metabolic alkalosis
pH > 7.45
PaCO2 WNL
HCO3- > 26
Remember that HCO3 is a base and that the more of it there is the more
alkalotic you will be. Notice that in the case of uncompensated metabolic
the PaCO2 is normal indicating that the body has not began to compensate.

19. Partially Compensated Imbalances
Occur when compensation mechanisms are activated, but have not had sufficient time to normalize the blood pH
NOTE: Some people say that there is no such thing as “partially” compensated – it is kind of like being “a little pregnant” – but it is indicative of a part of the process called compensation

20. Partially Compensated Imbalances
pH is abnormal
Both PaCO2 and HCO3- are abnormal in the same direction (increased or decreased from normal)
If PaCO2 is high (↑ acid), HCO3- will also be high (↑ alkaline) to neutralize the environment
If PaCO2 is low (↓ acid), HCO3- will also be low (↓ alkaline) to neutralize the environment

21. Partially Compensated Imbalances
Partially Compensated Respiratory Acidosis
pH < 7.35
PaCO2 > 45
HCO3- > 26
Partially Compensated Respiratory Alkalosis
pH > 7.45
PaCO2 < 35
HCO3- < 22
In the case of Partially Compensated Resp Acidosis, the pH is low, indicating an
acid environment…when you look at the PaCO2, it too is acidic, which is how you
know that you have a respiratory acidosis. With the HCO3 being high, you can
deduce that the body is raising its base to counteract the acid represented by the pH;
therefore, partially compensated respiratory acidosis.

22. Partially Compensated Imbalances
Partially Compensated Metabolic Acidosis
pH < 7.35
PaCO2 < 35
HCO3- < 22
Partially Compensated Metabolic Alkalosis
pH > 7.45
PaCO2 > 45
HCO3- > 26
With partially compensated metabolic acidosis, you notice first that the pH is low (acidosis).
Ask yourself, which number is representative of an acid condition. In this case it is the low
base (HCO3), so you know you have a metabolic acidosis. You know it is partially compensated
because the PaCO2 is low indicating that CO2 (an acid) is being lost from the body to correct
for the low pH.

23. Compensated Imbalances
Occur when compensatory mechanisms have been able to fully normalize blood pH

24. Compensatory Mechanisms
Both PaCO2 and HCO3- are abnormal, but in the same direction
If PaCO2 is high (↑ acid), HCO3- will also be high (↑ alkaline)
If PaCO2 is low (↓ acid), HCO3- will also be low (↓alkaline)

25. Compensated Imbalances
Compensated Respiratory Acidosis
pH WNL but closer to
PaCO2 > 45
HCO3- > 26
Compensated Respiratory Alkalosis
pH WNL but closer to
PaCO2 < 35
HCO3- < 22
In compensated respiratory acidosis, the pH tends to range between 7.35 and 7.39 – still acidic,
But in the normal pH range. When you look at the PaCO2, you notice that it is high (acidic), but
The HCO3 is also high, indicating that the body has compensated and normalized the low pH.

26. Compensated Imbalances
Compensated Metabolic Acidosis
pH WNL but closer to
PaCO2 < 35
HCO3- < 22
Compensated Metabolic Alkalosis
pH WNL but closer to
PaCO2 > 45
HCO3- > 26

27. Mixed Imbalances
Occur when patient has both metabolic and respiratory disorders that cause an acid-base imbalance
Examples:
Diabetic KetoAcidosis (metabolic acidosis) with decreased respiratory drive (respiratory acidosis)
Severe vomiting (metabolic alkalosis) with high fever (respiratory alkalosis)

28. Mixed Imbalances pH will be normal PaCO2 and HCO3- will be abnormal
PaCO2 will be high with low HCO3- (both tend toward acid side)
PaCO2 will be low with high HCO3- (both tend toward base side)

29. Mixed Imbalances Mixed acidosis pH < 7.35 PaCO2 > 45
HCO3- < 22
Mixed alkalosis
pH > 7.45
PaCO2 < 35
HCO3- > 26
Notice with the mixed acidosis that you have an acidic pH (less than 7.35, with other
Parameters indicating an acid environment. High PaCO2 (too much acid). Low HCO3
(too little base – an acidic environment). This is classic mixed acidosis.

30. Finished You have finished this in-service.
Please go to the next in-service titled:
Effects of Acid Base on Oxygenation