1. ABG Interpretation
Steven Podnos MD

2. Why Do ABG’s?
1) Check oxygenation
2) Check the pH (acid base balance)
3) Ventilatory Status
4) Determine the treatment

3. Basic ABG Components
• pH
• PaCO2
• HCO3
• PaO2

4. pH
• Normal pH is maintained by balancing the
H2CO3 (carbonic acid) and HCO3- (bicarb)
• Normal blood pH = 7.35-7.45
• pH < 7.35 = acidosis
• Ph > 7.45 = alkalosis

5. PaCO2
• CO2 has several forms in the blood.
• Like oxygen, some is dissolved directly into the plasma. The
PaCO2 is the measurement of the partial pressure of carbon
dioxide dissolved in the plasma. It is measured in mm Hg
(millimeters of mercury).
• The rest is found in the red blood cells on a hemoglobin
molecule.

6. Buffer System
Carbonic Acid - Bicarbonate Buffer System
CO2 + H2O <--> H2CO3 <--> (HCO3-) + (H+)
carbon dioxide + water <--> carbonic acid <--> bicarbonate + hydrogen ion
NOTE THAT “CO2” on Lyte panel is HCO3
• Note: The two headed arrows indicate that the process is reversible

7. Acid/Base-General Considerations
PH = Increased - Alkalosis
Decreased - Acidosis
PCO2 = Increased – Acidosis
Decreased – Alkalosis
HCO3 = Increased – Alkalosis
Decrease - Acidosis

8. Acid Base Balance
• Understanding the cause of an acid-base imbalance is the
key to treating it.
• The Respiratory component of acid base balance affects the
pH within minutes.
• The Metabolic component of acid base balance can take
days to affect pH.

9. A. Metabolic Acidosis
• Results from renal failure, Lactic
acidosis, DKA
B. Respiratory Acidosis
Hypoventilation
• Results from respiratory failure, instrinsic or
extrinsic.

10. C. Metabolic Alkalosis
• Results from Gastric Suction, vomiting,
D. Respiratory Alkalosis
Hyperventilation
• Results from Head Trauma, Fever,
Emotions, Salicylate Ingestion, Shock
at an early stage

11. PaO2
• About 3% of the body's oxygen is dissolved in the plasma.
• PaO2 is a measurement of the partial pressure of oxygen
dissolved in the plasma only. It is measured in mm Hg.
• The PaO2 does not tell us about the body's total oxygen
content, but it does indicate how much oxygen was
available in the alveoli to dissolve in the blood.

12. Oxygen and Hemoglobin
• CO2's affinity for hemoglobin is much less than O2's
affinity for hemoglobin.
• When CO2 and O2 are both available, hemoglobin will
accept oxygen rather than CO2.
• In the oxygen rich environment of the
alveoli, hemoglobin carries oxygen.
• Oxygenated blood then travels through the body.

13. SaO2
• The remainder of the body's oxygen is carried attached
to hemoglobin molecules.
• SaO2, or oxygen saturation, measures the degree to
which oxygen is bound to hemoglobin.
• Sa02 is expressed as a percentage.

15. OxyHemoglobin Dissociation Curve
• This curve describes the relationship between available
oxygen and amount of oxygen carried by hemoglobin.
• Oxygen-Hemoglobin affinity changes with:
• variation in pH *CO2 *temperature *2,3,-DPG
• Once the PaO2 reaches 60 mm Hg the curve indicates that
there is little change in saturation above this point.
– So, PaO2 of 60 or more is usually considered adequate.
– At PaO2 of less than 60 even small changes will greatly reduce the
SaO2.

16. Steps to ABG Interpretation
1) Determine Acidosis or Alkalosis.
2) Evaluate the Respiratory Mechanism
3) Evaluate the Metabolic Mechanism

17. Normal ABG Values
Norms Quick Reference
• pH = 7.35-7.45 7.4 (+/- 0.5)
• PaCO2 = 35-45 40 (+/- 5)
• HCO3 = 22-26 24 (+/- 2)
• PaO2 = 80-100 90 (+/- 10)
• SaO2 = 94-100 97 (+/- 3)

18. STEP 1
Step 1.
Use pH to determine ph
Acidosis or Alkalosis.
< 7.35 7.35-7.45 > 7.45
Normal or
Acidosis Alkalosis
Compensated

19. STEP # 2
Step 2. PaCO2
Use PaCO2 to
look at the < 35 35 -45 > 45
Respiratory
Mechanism
• Tends toward • Tends toward
alkalosis Normal acidosis
• Causes high or • Causes low
pH Compensated pH
• Neutralizes • Neutralizes
low pH high pH

20. STEP 3
Step 3. HCO3
Use HCO3 to look
at the < 22 22-26 > 26
Metabolic
Mechanism
• Tends toward • Tends toward
acidosis Normal alkalosis
• Causes low or Causes high
pH Compensated pH
• Neutralizes • Neutralizes
high pH low pH

21. Interpretation
High pH Low pH
Alkalosis Acidosis
High HCO3 Low PaCO2 High PaCO2 Low HCO3
Metabolic Respiratory
Respiratory Metabolic

22. Compensation
• “Compensation" is the body's normal response to normalize
pH
– By neutralizing the opposite acid base mechanism.
• Example: If the pH is high because of respiratory alkalosis
(low CO2):
– Alkaline HCO3- will decrease to neutralize the pH.
– In this case, the abnormal bicarb is not a metabolic problem; it is a
metabolic solution to a respiratory problem.
• It is important to determine which is the cause and which is
the effect.
• If you treat the compensatory abnormality, you make the pH
even more abnormal.

23. Combined Disturbances
• A “Combined Disturbance” occurs when the PaCO2 and
HCO3- both alter the pH in the same direction.
• A high PaCO2 and low HCO3- (acidosis).
• Low PaCO2 and high HCO3- (alkalosis).
• RARE

24. Question
• If the pH is 7.30, the PaCO2 is 50, and the
HCO3 is 24 what is the likely diagnosis?

25. Question 1
• If the pH is 7.30, the PaCO2 is 50, and the
HCO3 is 24 what is the likely diagnosis?
Acute uncompensated RESPIRATORY
ACIDOSIS

26. Question
If the pH is 7.49, the PaCO2 is
25, and the HCO3 is 22 what is the
likely diagnosis?

27. Question #2
• If the pH is 7.49, the PaCO2 is 25, and the
HCO3 is 22 what is the likely diagnosis?
ACUTE RESPIRATORY ALKALOSIS,

28. Question
• If the the pH is 7.56, the PaCO2 is 39, and the
HCO3 is 38, what is the likely diagnosis?

29. Question # 3
• If the the pH is 7.56, the PaCO2 is 39, and
the HCO3 is 38, what is the likely diagnosis?
METABOLIC ALKALOSIS

30. Question
• If the pH is 7.35, the PaCO2 is 25, and the
HCO3 is 9, what is the likely diagnosis?

31. Question # 4
• If the pH is 7.35, the PaCO2 is 25, and the
HCO3 is 9, what is the likely diagnosis?
COMPENSTATED METABOLIC ACIDOSIS

32. Question
• If the pH is 7.30, the PaCO2 is 25, and the
HCO3 is 9, what is the likely diagnosis?

33. Question # 5
• If the pH is 7.30, the PaCO2 is 25, and the
HCO3 is 9, what is the likely diagnosis?
PARTIALLY COMPENSTATED
METABOLIC ACIDOSIS