The document provides a 6-step approach to analyzing arterial blood gases (ABGs). The 6 steps are: 1) analyze pH, 2) analyze pCO2, 3) analyze HCO3, 4) match CO2 or HCO3 to pH, 5) check for opposite direction of compensation, and 6) analyze pO2 and oxygen saturation. Examples are given of how to use the 6 steps to diagnose different acid-base disturbances such as respiratory acidosis, metabolic alkalosis, and mixed disorders. Normal values and concepts like the anion gap and types of respiratory failure are also explained.

1. ABG ANALYSIS IN 6 STEPS
Dr. Padmesh

2. • The 6 Easy Steps to basic ABG Analysis:
• 1. Is the pH normal? Acidic or Alkaline?
• 2. Is the pCO2 normal? Acidic or Alkaline?
• 3. Is the HCO3 normal? Acidic or Alkaline?
• 4. Match the CO2 or the HCO3 with the pH
• 5. Does the CO2 or the HCO3 go the opposite
direction of the pH?
• 6. Are the pO2 and the O2 saturation normal?

3. • Step 1: Analyze the pH
• Normal blood pH is 7.4 (range 7.35 to 7.45).
• pH < 7.35  acidic.
• pH > 7.45  alkaline.
• If it falls into the normal range, label what side of
7.4 it falls on. Lower than 7.4 is normal/acidic,
higher than 7.4 is normal/alkalotic.

4. • Step 2: Analyze the pCO2 :
• Normal pCO2 levels = 35-45mmHg.
• Below 35 is alkaline,
• Above 45 is acidic.

5. • Step 3: Analyze the HCO3:
• Normal HCO3 level is 22-26 mEq/L.
• If the HCO3 is below 22, the patient is acidotic.
• If the HCO3 is above 26, the patient is alkalotic

6. • Step 4: Match the CO2 or the HCO3 with the pH
• If the pH is acidotic, and the CO2 is acidotic, then
the acid-base disturbance is being caused by the
respiratory system.
Therefore, we call it a respiratory acidosis.
• However, if the pH is alkalotic and the HCO3 is
alkalotic, the acid-base disturbance is being
caused by the metabolic (or renal) system.
Therefore, it will be a metabolic alkalosis.

7. • Step 5: Does the CO2 or HCO3 go the opposite
direction of the pH?
• If so, there is compensation by that system.
• For example, the pH is acidotic, the CO2 is
acidotic, and the HCO3 is alkalotic: The CO2
matches the pH making the primary acid-base
disorder respiratory acidosis. The HCO3 is
opposite of the pH and would be evidence of
compensation from the metabolic system.

8. • Step 6: Analyze the pO2 and the O2 saturation.
• If they are below normal there is evidence of
hypoxemia.

9. • NORMAL VALUES:

10. • Interpretation of Values:
Hypoxia Hyperoxia

11. Examples:

12. • Example 1:
• Step 1: pH < 7.35  acidotic.
• Step 2: CO2 > 45  acidotic.
• Step 3: HCO3 is normal.
• Step 4: CO2 matches pH, because both
are acidotic. Therefore the
imbalance is respiratory acidosis.
• Step 5: HCO3 is normal, therefore there is no compensation. If the HCO3 is
alkalotic (opposite direction) then compensation would be present.
• Step 6: PaO2 and O2 sat are low indicating hypoxemia.
• Full diagnosis: Uncompensated respiratory acidosis with hypoxemia.
• This patient has an acute respiratory disorder.

13. • Example 2:
• Step 1: pH > 7.45  alkalotic.
• Step 2: CO2 < 35  alkalotic.
• Step 3: HCO3 is normal.
• Step 4: CO2 matches pH, because
both are alkalotic. Therefore
imbalance is respiratory alkalosis.
• Step 5: HCO3 is normal, therefore there is no compensation. If the HCO3 is
acidotic (opposite direction) then compensation would be present.
• Step 6: PaO2 and O2 sat are normal indicating normal oxygenation.
• The full diagnosis for this blood gas is: Uncompensated respiratory alkalosis.

14. • Example 3:
• Step 1: pH < 7.35  acidotic.
• Step 2: CO2 is normal.
• Step 3: HCO3 < 22  acidotic.
• Step 4: HCO3 matches pH, because both are
acidotic. Therefore the imbalance is
metabolic acidosis.
• Step 5: CO2 is normal, therefore there is no compensation. If the CO2 is alkalotic
(opposite direction) then compensation would be present.
• Step 6: PaO2 and O2 sat are normal indicating normal oxygenation.
• Full diagnosis: Uncompensated metabolic acidosis.

15. • Example 4:
• Step 1: pH > 7.45  alkalotic.
• Step 2: CO2 is normal.
• Step 3: HCO3 > 26  alkalotic.
• Step 4: HCO3 matches the pH,
because they are both alkalotic.
Therefore the imbalance is
metabolic alkalosis.
• Step 5: CO2 is normal, therefore there is no compensation. If the CO2 is acidotic
(opposite direction) then compensation would be present.
• Step 6: PaO2 and O2 sat are normal.
• Full diagnosis: Uncompensated metabolic alkalosis.

16. • Example 5:
• Step 1: pH < 7.35  acidotic.
• Step 2: CO2 < 35  alkalotic.
• Step 3: HCO3 < 22  acidotic.
• Step 4: HCO3 matches pH, because both
are acidotic. Therefore imbalance is
metabolic acidosis.
• Step 5: CO2 is alkalotic and goes the opposite direction of the pH, so there is
compensation. Because the pH is not in the normal range the compensation is
called partial.
• Step 6: PaO2 and O2 sat are low indicating hypoxemia.
• Full diagnosis: Partially-compensated metabolic acidosis with hypoxemia.

17. • Expected Compensation for simple Acid-base dis:
Disorder Primary Compensatory Magnitude of expected
event event compensation
Met Acidosis Bicarb pCO2 For every 1 meq/L decrease in bicarb,
pCO2 decreases by 1-1.5 mm Hg
Met Alkalosis Bicarb pCO2 For every 1 meq/L increase in bicarb,
pCO2 increases by 0.5-1 mm Hg
Resp Acidodis pCO2 Bicarb For every 1 mm Hg increase in pCO2,
Bicarb increases by 0.1 – 0.4 meq/L
Resp Alkalosis pCO2 Bicarb For every 1 mm Hg decrease in pCO2,
Bicarb decreases by 0.1- 0.4 meq/L

18. • Appropriate Compensation During Simple Acid-
Base Disorders:
• DISORDER EXPECTED COMPENSATION
• Metabolic acidosis PCO2 = 1.5 × [HCO3
-] + 8 ± 2
• Metabolic alkalosis For each 10 mEq/L increase in serum
[HCO3
-] , PCO2 increases by 7 mm Hg .

19. • Appropriate Compensation During Simple Acid-
Base Disorders:
• DISORDER EXPECTED COMPENSATION
• Respiratory acidosis
• Acute : For each 10-mm Hg increase in PCO2 , [HCO3
-] increases
by 1.
• Chronic : For each 10-mm Hg increase in PCO2 , [HCO3
-]
increases by 3.5 .
• Respiratory alkalosis
• Acute: For each 10mm Hg decrease in PCO2 , [HCO3
-] falls by 2.
• Chronic : For each 10mm Hg decrease in PCO2,[HCO3
-] falls by 4

20. • EXAMPLE 6:
• pH = 7.55,
• pCO2 = 14 mmHg,
• Bicarb = 10 meq/L
• Since CO2 matches pH, disorder is RESPIRATORY,
that is, RESPIRATORY ALKALOSIS.
• Since Bicarb goes in opposite direction to pH, it is
compensatory. But compensation is only
partial,since pH is not in normal range.

21. • EXAMPLE 6:
• pH = 7.55,
• pCO2 = 14 mmHg,
• Bicarb = 10 meq/L
• Fall in pCO2 = 40-14 = 26
• Expected fall in Bicarb = 0.2 x 26 = 5.2 meq/L
• However,actual fall in Bicarb = 24-10 = 14
• Since actual fall in Bicarb is more than expected, there is
Metabolic Acidosis also.

22. • EXAMPLE 7:
• pH = 7
• pCO2 = 30 mmHg
• Bicarb = 6 meq/L
• Since Bicarbonate matches pH, disorder is
METABOLIC, hence, METABOLIC ACIDOSIS.
• Since pCO2 goes in opposite direction to pH, it is
compensatory. Only Partial compensation as pH is
not normal.

23. • EXAMPLE 7:
• pH = 7
• pCO2 = 30 mmHg
• Bicarb = 6 meq/L
• Fall in Bicarb = 24-6 = 18 meq/L
• Expected fall in pCO2 = 18 x 1.2 = 21.6 mmHg
• However, actual fall in pCO2 is only 40-30 = 10 mmHG
• Therefore, actual fall in pCO2 is much lesser than expected.
Hence, pCO2 is actually higher, which means there is
RESPIRATORY ACIDOSIS also.

24. • Relationship between pO2 and SpO2 :
Mnemonic: 40, 50, 60, 70, 80, 90
pO2 SpO2
40 70
50 80
60 90
90 99

25. • ANION GAP:
• Anion gap indicates the presence or absence of increased levels of
unmeasured anions (proteins, phosphates, SO4, organic anions)
(Na + K + Ca + Mg + unmeasured cations) = (Cl + HCO3 + unmeasured anions)
AG = Measured Cations – Measured Anions
• AG =[ Na – (Cl + HCO3) ] = [ 140 - (104+24) ] = 12 meq/L
• AG =[ (Na + K ) – (Cl + HCO3) ] = [ (140 + 4) - (104+24) ] = 16 meq/L

26. • ANION GAP:
• Anion gap indicates the presence or absence of increased levels of
unmeasured anions (proteins, phosphates, SO4, organic anions)
(Na + K + Ca + Mg + unmeasured cations) = (Cl + HCO3 + unmeasured anions)
AG = Measured Cations – Measured Anions
• AG = [ Na – (Cl + HCO3) ] = 8 - 12 meq/L
• AG = [ (Na + K ) – (Cl + HCO3) ] = 12 - 20 meq/L

27. • ANION GAP:
- Increased Anion Gap Metabolic Acidosis:
-Addition of acid load  Increase in unmeasured anions
- Normal Anion Gap Metabolic Acidosis:
-Loss of bicarbonate.
-However, this loss of bicarbonate is compensated by
corresponding increase in chloride,so that the anion gap
remains unchanged.
AG = [ Na – ( Cl + HCO3 ) ]
(Hyperchloremic Met.Acidosis)

28. • CLASSIFICATION OF ANION GAP:
• High Anion gap M.A Normal Anion gap M.A
High MUD PILES Normal USED CARP
• Methanol -Ureterostomy
• Uremia -Small bowel fistula
• DKA -Extra Chloride
• Paraldehyde -Diarrhea
• IEM, Iron, INH -Carb.anhydrase inhibitor
• Lactic acidosis -Adrenal insufficiency
• Ethanol, ethylene glycol -RTA
• Salicylates -Pancreatic fistula

29. • RESPIRATORY FAILURE:
• Type I Respiratory Failure:
( Diffusion defect; CO2 diffusion is better;
eg: Pnemonia, Pulmonary edema)
• pO2 , but normal pCO2
• Type II Respiratory Failure:
( Respiratory gases not reaching alveoli for proper gaseous
exchange; eg: HMD , Respiratory paralysis in Polio )
• pO2 and pCO2