1. Approach to Acid Base DisorderBy
Dr. S. Shivakumar, M.D.
Addl.Professor of Medicine,
Stanley Medical College,
Chennai.

2. Normal Values pH : 7.36 – 7.44 (7.40) plasma HCO3 : 24 – 26 mEq/L (25)
pCo2 : 39 – 43 (40)
plasma (H+) : 36 – 44 nEq / L (40 nEq / L)
pH < : Severe Acidosis
pH > : Severe Alkalosis
pH : 7.0 = 1 x 10 – 7 (1 x 1/107)
= 100 x 10 – 9 = 100 nEq / L
Na & K : 10 – 3 = mEq

3. Correlation between H+ ion & pH
Change in 0.3 pH  Double or Halve the H+ pH H+
7.0
100
6.9
125
7.1 80
7.3 50
7.2
62.5
7.4 40
7.6 25
7.5
31.5
7.7 20
7.9
12.5
7.8
15.6
8.0 10

4. Estimating the H+ ion from pH
7.0
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
100 80 62 50 40 32 25 20 16
 1.25
X 0.8
Within narrow limits
0.01 change in pH ~ 1 mEq / L change of [H+]
7.40 = 40
7.39 = 41
7.38 = 42
7.37 = 43

5. Approach Step I : Check validity Step II : Obtain Minimum diagnosis
Step III : Is it a Simple or Mixed acid base disorder
Step IV : Determine Anion gap
Step V : Is it a Triple disorder

6. Suggestions Step I. Check validity : (Modified Henderson equation)
H = 24 x Pco2
Hco3
H = 24 x 40 24
= 40 nEq = pH = 7.4
eg: pH = HC03 = PCo2 = 32
  H = 24 x 32 8
= 96 nEq = pH 7.0

7. Step II: Obtain minimum diagnosis
Look at pH - Acidosis / Alkalosis
Match the Pco2 or Hco3 - Metabolic / Respiratory
Disorder
Primary Change
Secondary change
Net effect
M. Acidosis
 Hco3
 Pco2
 pH ( H+)
M.Alkalosis
 Hco3
 Pco2
 pH ( H+)
R. Acidosis
R. Alkalosis

8. Examples of Simple DisorderpH
Hco3
Pco2
Metabolic Acidosis
7.15 8 24
Respiratory Acidosis 30 90
Metabolic Alkalosis
7.7 36 48
Respiratory Alkalosis 12 10

9. Step III: Is it a simple or mixed Acid-base disorder?
Simple Disorder :
Disorder
Example
Metabolic acidosis
DKA, Renal failure, Lactic acidosis, Methanol poisoning, Diarrhoea
Metabolic alkalosis
Vomiting, Diuretics, Steroids
Respiratory acidosis
COPD
Respiratory alkalosis
Psychogenic hyperventilation, Hepatic Encephalopathy

10. Mixed Disorder Situation Disorder Respiratory acidosis COPD + Vomiting
+ Metabolic alkalosis
COPD + Diarrhoea
+ Metabolic acidosis
Sepsis
Metabolic acidosis
+ Respiratory alkalosis
Cirrhosis + Diuretics
Respiratory alkalosis

11. Apply Compensation Metabolic Acidosis Pco2 should  by 1.2 mm
for each mEq  plasma Hco3
Metabolic Alkalosis
Pco2 should  by 0.6 mm
for each mEq  plasma Hco3
Acute
Resp.Acidosis
Plasma Hco3  by 1 mEq / L
for each 10 mm  Pco2
Chronic.
Resp. Acidosis
Plasma Hco3  by 4 mEq / L
Acute.
Resp. Alkalosis
Plasma Hco3  by 1 mEq / L
for each 10 mm  Pco2
Chronic
Resp Alkalosis
Plasma Hco3  by 4 mEq / L

12. Example pH : 7.00 Pco2 = 32 Hco3  = 8 Fall in Hco3 = 24 – 8 = 16
Compensatory
Fall in Pco2 = 16 x 1.2 = 19
Anticipated Pco2 = 40 – 19 = 21
Estimated : Pco2 = 32 (Pco2  )
Diagnosis
Mixed - Metabolic acidosis + Respiratory acidosis

13. Mixed acid base disorder
Compensation PH
Metabolic Acidosis + Respiratory Acidosis
eg: COPD + DKA
PCo2  & Hco3 
for simple disturbance
pH = PCo2 
Hco3 
Metabolic Alkalosis+ Respiratory Alkalosis
eg: Cirrhosis + Diuretics
PCo2  & Hco3 
pH = PCo2 
Hco3 

14. Mixed acid base disorder
Compensation PH
Metabolic acidosis + Respiratory alkalosis
eg : Sepsis
PCo2  & Hco3 
for simple disturbance
Normal or
slightly  or 
(N) pH = PCo2 
Hco3 
Metabolic alkalosis + Respiratory acidosis
PCo2 & Hco3 
(N) pH = PCo2 
Hco3 

15. Examples of mixed acid base disorder
Data
Disorder pH
Hco3
Pco2
Po2
6.85
15 (30) 90 50
Respiratory acidosis
+ Metabolic acidosis
7.3
45 (30)
+ Metabolic alkalosis 6
12 (22)
100
Metabolic acidosis
+ Respiratory alkalosis
7.75 40
25 (43)
Metabolic alkalosis

16. Step IV: Determine the Anion gap
AG = Na+– (Hco3  + Cl )
Normal = 12 ± 4 ( 8  16 )
Valuable in
Metabolic acidosis – High gap / Normal gap
Metabolic alkalosis – Evaluation of “Starting Hco3”
High gap acidosis : AG > 27 mEq /L
17  26 Suggestive
Eg. – Ketoacidosis, Lactic acidosis,
Methanol intoxication, Renal failure
Normal gap acidosis – Diarrhoea, RTA

17. AG in metabolic alkalosis
Valuable in diagnosis of Triple disorder
(Metabolic acidosis,Met.alkalosis & Resp. acidosis)
Delta () AG =
Calculated Anion gap – Normal Anion gap
Hco3 +AG = Starting Hco3
Starting Hco3 > 29 suggests associated Metabolic Alkalosis in the presence of Metabolic Acidosis

18. Example Na+ = 135 Hco3 = 4 cl  = 90 pH = 6.8
AG = Na  ( Hco3  + cl ) = 135 – (4 + 90)
= 41  High gap acidosis
AG = Calculated – AG Normal
= 41 – 12 = 29
Starting Hco3 = = 33 mEq / L
Starting Hco3 > 29 suggests associated Metabolic Alkalosis in the presence of Metabolic Acidosis

19. Approach with an Example
A 50 year old male suffering from COPD & vomiting is admitted for breathlessness.
Biochemical parameters
pH – 6.8
pCo2 - 22
pO2 - 60
Hco3- 3.4
Na+ 135
k – 6.0
Cl- - 90
urea-110
S.Cr. – 3.0
sugar-100

20. Approach with an Example(contd)
Step 1 : Check Validity
H = 24 X Pc02 = 24 X 22 = 155 nEq / L = pH 6.8
Hco
Step 2: Obtain minimum diagnosis
pH = 6.8 Pco2 = 22 Hco3 = 3.4
 Metabolic acidosis

21. Approach with an Example(contd)
Step 3:
Is it a Simple or Mixed Acid base disturbance
By applying compensation
(24  3.4 = 20.6 ;  1.2 = 24.7; 40  = 15.3)
Predicted Pco2 = 15.3, but Observed Pco2 = 22
Mixed disorder
Metabolic Acidosis + Respiratory Acidosis

22. Approach with an Example (contd)
Step 4:
Determine Anion Gap
Na = Cl = Hco3 = 3.4
AG = 135 – ( ) = 41.6
High Gap acidosis

23. Approach with an Example (contd)
Step 5:
Is it a triple disorder
Look for metabolic acidosis with AG
AG = Calculated AG - AG normal
= = 29.6
Starting Hc03 = Hco3+ AG
= = 33 mEq /L (Metabolic alkalosis)

24. Diagnosis Triple Disorder = Metabolic acidosis +
pH = Pco2 = 22 Hco3  = Po2 = 60
Na+ = k + = Cl = 90 Urea= 110
S.Cr = 3mg/dl Sugar = 100 mgs
Triple Disorder = Metabolic acidosis +
Resp.acidosis + Met.alkalosis
Metabolic acidosis – Renal failure
Respiratory acidosis – COPD
Metabolic alkalosis – Vomiting

25. Example –1
A 40 yr old man is admitted for diarrhoea & breathlesness for 2 days.
pH - 7.2
pCo2 - 32 pO
Hco3- 12 Na
k - 2.9 Cl
urea-70
S.Cr
sugar-110
Diagnosis: Normal AG Met.Acidosis (AG – 11)
Due to Diarrhoea + Hypokalemia,

26. Example –2 Due to Renal failure + Hyperkalemia
A 40 yr old man is admitted for diarrhoea of 1 week & breathlesness of 1 day
pH - 7.1
pCo2 - 20 pO
Hco3- 6 Na k Cl
urea-120
S.Cr
sugar-110
Diagnosis: High gap Acidosis (AG - 29)
Due to Renal failure + Hyperkalemia

27. Example –3
A 45 yr old female is admitted for high fever & breathlessness, diagnosed to have acute Cholecystitis
pH - 7.3
pCo2 - 12 pO
Hco3- - 6 Na
K+ - 5 Cl
Urea-45
S.Cr
Sugar-120
(Predicted pCo2 – 18, but observed pCo2 – 12)
Diagnosis: Metabolic acidosis + Respiratory Alkalosis Due to sepsis

28. Example –4
A 50 yr old Pt., a known case of COPD is admitted for severe diarrhoea pH
pCo2 - 40
pO2 - 50
Hco3- 9 Na
K+ - 4 Cl
Urea-45
S.Cr
Sugar - 128
(Predicted pCo2 – 25, but observed pCo2 – 40)
Diagnosis: Respiratory Acidosis (COPD ) +
Metabolic acidosis (Diarrhoea)

29. Example –5
A 50 yr old, known COPD Pt. with Cor pulmonale on treatment with Frusemide and is admitted for severe vomitting. pH
pCo2 - 80
pO2 - 40
Hco Na
K+ - 4 Cl
Urea-45
S.Cr
Sugar- 128
(Predicted pCo2 – 54, but observed pCo2 – 80)
Diagnosis: Respiratory Acidosis (COPD ) +
Metabolic Alkalosis (Vomiting + Diuretics)

30. Example –6
A 50 yr old Pt. with Cirrhosis Liver is admitted for coma & Vomitting. He has been treated with Frusemide recently for ascites. pH
pCo2 - 30
pO2 - 80
Hco Na K Cl
Urea - 45
S.Cr
Sugar -110
(Predicted Hco3 – 20, but observed Hco3 – 40)
Diagnosis: Respiratory Alkalosis (Cirrhosis ) +
Metabolic Alkalosis (Vomitting + Diuretics)

31. Summary Suspect the diagnosis from history
Suspect the disturbance from physical symptoms
Evaluate routine laboratory date :
Sugar, RFT, LFT, Na+, K+
Establish the cause of Acid Base disorder
(Utilize thoughtful differential diagnosis),
Direct management of underlying disorder,
unless pH is in a dangerous range

32. Thank You