2. INTRODUCTION:
Interpreting an arterial blood gas (ABG) is
a crucial skill for physicians, nurses,
respiratory therapists, and other health
care personnel. ABG interpretation is
especially important in critically ill
patients.

3. Arterial blood gasses provide information
about :
• Oxygenation
• ventilation
• acid-base balance.

4. 6 STEPS TO INTERPRET ABG RESULT:
Step 1#: analyze the PaO2 and the SaO2
Step 2#: analyze the pH
Step 3#: analyze the CO2
Step 4#: analyze the HCO3
Step 5#: match the CO2 or the HCO3 with the
pH.
Step 6#: does the CO2 or the HCO3 go
opposite direction of the pH?

5. The PaO2 only tells
you if you need to
give a patient more
oxygen or if you need
to cut back on
supplemental oxygen.
These are normal values for arterial blood gases.
opH= 7.35 – 7.45
oPaCO2= 35 – 45 mmHg
oPaO2= 80 – 100 mmHg
oHCO3= 23 – 26 MEq/L
When
interpreting
ABGs, the most
important values
to look at are pH,
PaCO2, and
HCO3.

7. STEP 1#: ANALYZE THE PAO2 AND THE
SAO2
PaO2 is depend upon :
Age
FiO2
Patm
paO2 : Partial pressure of oxygen molecules
dissolved in plasma.

8. PAO2:
If the patient is hypoxemic, the low oxygen
content in his blood will be reflected in low
PaO2 and SaO2 values.
Mild hypoxemia ,PaO2 of (60-79) mmHg.
moderate hypoxemia, (40-59) mm Hg.
severe hypoxemia, less than 40 mm Hg.

9. SAO2:
Normal range (96% - 100%)

10. P/F RATIO:
P:F ratio is the ratio of arterial oxygen concentration
to the fraction of inspired oxygen. It reflects how
well the lungs absorb oxygen from expired air.
Based on Berlin definition:
• mild (from 200 to 300),
• moderate (from 100 to 200)
• and severe (≤100 mmHg).

11. O2 CONTENT :
Arterial oxygen content CaO2 is the amount of
oxygen bound to hemoglobin plus the amount
of oxygen dissolved in arterial blood.
CaO2 = (Hgb x 1.36 x SaO2) + (0.0031 x PaO2)
Normal CaO2 ranges from (16 – 22) ml O2/dl.

12. SCENARIO:
A patient who presented to the ER with headache and
dyspnea & h/o exposure to smoke in a closed room.
His first blood gases showed Pao2 80 mmHg, PaCO2 38
mmHg, pH 7.43. and oxygenation was judged normal.
He was sent out from the ER and returned a few hours
later with mental confusion.
This time both SaO2 and COHb were measured; paO2 79
mmHg,PaCO2 31 mmHg, pH7.36,SaO2 53%,
carboxyhemoglobin 46%.
 CO poisoning

13. CO POISONING:
• SaO2 = NORMAL “false value on pulse
oximetry”
• PaO2 = NORMAL
Symptoms: Cherry red color instead of cyanosis
(even though there is Hypoxia), its because of the red
pigment of CO, headache and dizzness .
CARBOXYHEMOGLOB
IN TEST
CO-OXIMETRY

14. CARBOXYHEMOGLOBIN TEST:
 Headache and nausea can begin when levels are 10 to
20%.
 Levels > 20% commonly cause vague dizziness,
generalized weakness, difficulty concentrating, and
impaired judgment. Levels > 30% commonly cause dyspnea during exertion,
chest pain (in patients with coronary artery disease), and
confusion.
 Higher levels can cause syncope, seizures, and
obtundation.
 Hypotension, coma, respiratory failure, and death may
occur, usually when levels are > 60%.

16. 6 STEPS TO INTERPRET ABG RESULT:
Step 1#: analyze the PaO2 and the SaO2
Step 2#: analyze the pH
Step 3#: analyze the CO2
Step 4#: analyze the HCO3
Step 5#: match the CO2 or the HCO3 with the
pH.
Step 6#: does the CO2 or the HCO3 go
opposite direction of the pH?

17. STEP 2#: ANALYZE THE pH:
pH
< 7.35 7.35-7.45 > 7.45
Acidosis
Normal or
Compensated
Alkalosis
•This is usually the primary disorder
•Remember: an acidosis or alkalosis may be present even if the pH is in
the normal range (7.35 – 7.45) HOW?? When it is fully compensated and
that will be described later.

18. STEP 3#: ANALYZE THE CO2
PaCO2
< 35 35 -45 > 45
• Tends toward alkalosis
•Causes high pH
Normal or
Compensated
• Tends toward acidosis
•Causes low pH

19. STEP 3#: ANALYZE THE HCO3:
HCO3
< 22 22-26 > 26
•acidosis
•Causes low pH
Normal or
Compensated
•Alkalosis
•Causes high pH

20. STEP 5+6#: MATCH THE CO2 OR THE
HCO3 WITH THE pH.
Acidosis Respiratory pH ↓ PaCO2 ↑ HCO3 N
Acidosis Metabolic pH ↓ PaCO2 N HCO3 ↓
Alkalosis Respiratory pH ↑ PaCO2 ↓ HCO3 N
Alkalosis Metabolic pH ↑ PaCO2 N HCO3 ↑
Acidosis mixed pH ↓ PaCO2 ↑ HCO3 ↓
What is the relationship between the direction of change in the
pH and the direction of change in the PaCO2?
In primary respiratory disorders, the pH and PaCO2 change
inapposite directions; in metabolic disorders the pH and
PaCO2 change in the same direction.

21. Metabolic
Respirator
y
Acute chronicAnion
gap
Alkalosis Acidosis
pH
PaCO2

22. IF IT’S RESPIRATORY: ACUTE OR CHRONIC?
Expected changes in pH for a 10-mmhg change
PaCO2 resulting from either primary respiratory
acidosis or respiratory alkalosis:
• Respiratory acidosis
pH↓ by 0.08 pH↓ BY 0.03
• Respiratory alkalosis
pH↑by 0.08 pH↑by 0.03
Acute Chronic

23. COMPENSATED OR NOT:
The body compensates for acid-base imbalance through
either the lungs or the kidneys.
If PaCO2 is abnormal and pH is normal, it indicates
compensation.
 pH > 7.4 would be a compensated alkalosis.
 pH < 7.4 would be a compensated acidosis.
Respiratory compensation
 Complete within 24hrs
Metabolic compensation
 Complete within several days
Both the respiratory or renal compensation almost never
over compensates.

24. PARTIALLY COMPENSATED:
With partially compensated ABGs, the body has
begun to correct the acid-base problem via
either the lungs or the kidneys.
The pH has not returned to the normal range.

25. CASE #1 :
A patient’s ABGs pH= 7.30; PaCO2=31 mmHg;
PaO2=76 mmHG; HCO3=17 mEq/L.
First, look at the pH. What does this pH value
indicate?
alkalosis
acidosi
s

26. CASE #1 :
A patient’s ABGs pH= 7.30; PaCO2=31 mmHg;
PaO2=76 mmHG; HCO3=17 mEq/L.
The above ABG represents what kind of acidosis ?
Respiratory
acidosis
Metabolic
alkalosis
Metabolic
acidosis
Respiratory
alkalosis

27. CASE #1 :
A patient’s ABGs pH= 7.30; PaCO2=31 mmHg;
PaO2=76 mmHG; HCO3=17 mEq/L.
Fully
compensated
patially
Compensated
Uncompensated

28. CASE #2 :
A patient’s blood gases are pH= 7.35; PaCO2= 33;
PaO2= 88; HCO3= 18.
What does the pH indicate?
alkalosis
acidosi
s

29. CASE #2 :
A patient’s blood gases are pH= 7.35; PaCO2= 33;
PaO2= 88; HCO3= 18.
Based on this information, the above ABG
represents what kind of acidosis?
Respiratory
acidosis
Metabolic
alkalosis
Metabolic
acidosis
Respiratory
alkalosis

30. CASE #2 :
A patient’s blood gases are pH= 7.35; PaCO2= 33;
PaO2= 88; HCO3= 18.
patially
Compensated
Uncompensated
Fully
compensated

31. IF IT’S METABOLIC: ANION GAP
Anion gap measures the difference between anions (-) and
cations (+) present in blood.
Normal value = 10 – 12 mmol/L
AG = Na – (HCO3 + Cl)

32. CAUSES OF HIGH-ANION-GAP METABOLIC
ACIDOSIS Elevated anion
gap represents
metabolic
acidosis.

33. REFERENCES:
• Interpreting Arterial Blood Gases :WILLIAM C. Pruitt
CPFT, MBA, AND MICHAEL JACOBS, RN, CCRN,
CEN,MSN.
• Interpretation of Arterial Blood Gases (ABGs)
David A. Kaufman, MD Chief, Section of Pulmonary,
Critical Care & Sleep Medicine.
• Clinical Blood Gases,2nd Edition, Assessment &
Intervention: W.Malley,Imprint:Saunders. ISBN
:9780721684222
• 6 Easy steps to ABG analysis,E-booklet, David W.
Woodruff, MSN, RN-BC, CNS, CMSRN, CEN.

34. Presented by:
Fay Ali Al-Buainain