ABG analysis - in 6 steps. Just the basics.

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