1. Acid-Base Disturbances
Clinical Approach
2006
Pravit Cadnapaphornchai

2. Simple vs Mixed Simple When compensation is appropriate Mixed
When compensation is inappropriate

3. Simple Acid-Base Disturbances
When compensation is appropriate
Metabolic acidosis (↓ HCO3, ↓ pCO2)
Metabolic alkalosis (↑ HCO3, ↑ pCO2)
Respiratory acidosis (↑ pCO2, ↑ HCO3)
Respiratory alkalosis (↓ pCO2, ↓ HCO3)

4. Stepwise Approaches History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)
Use the HCO3 from ABG to determine compensation
Serum Na, K, Cl, CO2 content
Use CO2 content to calculate anion gap
Calculate anion gap
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause

5. Organ dysfunction
CNS – respiratory acidosis (suppression) and alkalosis (stimulation)
Pulmonary – respiratory acidosis (COPD) and alkalosis (hypoxia, pulmonary embolism)
Cardiac – respiratory alkalosis, respiratory acidosis, metabolic acidosis (pulmonary edema)
GI – metabolic alkalosis (vomiting) and acidosis (diarrhea)
Liver – respiratory alkalosis, metabolic acidosis (liver failure)
Kidney – metabolic acidosis (RTA) and alkalosis (1st aldosteone)

6. Organ Dysfunction Endocrine Drugs/toxins
Diabetes mellitus – metabolic acidosis
Adrenal insufficiency – metabolic acidosis
Cushing’s – metabolic alkalosis
Primary aldosteronism – metabolic alkalosis
Drugs/toxins
Toxic alcohols – metabolic acidosis
ASA – metabolic acidosis and respiratory alkalosis
Theophylline overdose – respiratory alkalosis

7. Stepwise Approaches History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)
Use the HCO3 from ABG to determine compensation
Serum Na, K, Cl, CO2 content
Use CO2 content to calculate anion gap
Calculate anion gap
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause

8. pH < 7.35 7.4 >7.45 Acidosis Mixed Alkalosis Metabolic
< >7.45
Acidosis
Metabolic
Respiratory
Mixed
Alkalosis
Metabolic
Respiratory

9. Stepwise Approaches History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)
Use the HCO3 from ABG to determine compensation
Serum Na, K, Cl, CO2 content
Use CO2 content to calculate anion gap
Calculate anion gap
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause

10. CO2 content Low Normal High
Metabolic acidosis Normal Metabolic alkalosis
Resp alkalosis Mixed Resp acidosis
A normal CO2 content + high anion gap = metabolic acidosis +
Metabolic alkalosis or metabolic ac + compensatory respiratory ac.

11. Stepwise Approaches History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)
Use the HCO3 from ABG to determine compensation
Serum Na, K, Cl, CO2 content
Use CO2 content to calculate anion gap
Calculate anion gap
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause

12. Calculation of Anion Gap in Metabolic Acidosis
Anion gap = Na – (Cl + HCO3)
Normal 8 ± 2
Correction for low serum albumin
Add (4-serum albumin g/dL) X 2.5
to the anion gap

13. Stepwise Approaches History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)
Use the HCO3 from ABG to determine compensation
Serum Na, K, Cl, CO2 content
Use CO2 content to calculate anion gap
Calculate anion gap
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause

14. Compensations for Metabolic Disturbances
Metabolic acidosis
pCO2 = 1.5 x HCO3 + 8 ( ± 2)
Metabolic alkalosis
pCO2 increases by 7 for every 10 mEq increases in HCO3

15. How does the kidney compensate for metabolic acidosis?

16. How does the kidney compensate for metabolic acidosis?
By reabsorbing all filtered HCO3
By excreting H+ as NH4+ (and H2PO4- )
Interpretations
Urine pH < 5.5
Urine anion gap Negative

17. Compensations for Respiratory Acidosis
Acute respiratory acidosis
HCO3 increases by 1 for every 10 mm increases in pCO2
Chronic respiratory acidosis
HCO3 increases by 3 for every 10 mm increases in pCO2
If you don’t have kidneys, can you have chronic respiratory acidosis?

18. Compensations for Respiratory Alkalosis
Acute respiratory alkalosis
HCO3 decreases by 2 for every 10 mm decrease in pCO2
Chronic respiratory alkalosis
HCO3 decreases by 4 for every 10 mm decrease in pCO2
If you don’t have kidneys can you have chronic respiratory alkalosis?

19. Mixed Acid-Base Disorders
Mixed respiratory alkalosis & metabolic acidosis
ASA overdose
Sepsis
Liver failure
Mixed respiratory acidosis & metabolic alkalosis
COPD with excessive use of diuretics

20. Mixed Acid-Base Disorders
Mixed respiratory acidosis & metabolic acidosis
Cardiopulmonary arrest
Severe pulmonary edema
Mixed high gap metabolic acidosis & metabolic alkalosis
Renal failure with vomiting
DKA with severe vomiting

21. Stepwise Approaches History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)
Use the HCO3 from ABG to determine compensation
Serum Na, K, Cl, CO2 content
Use CO2 content to calculate anion gap
Calculate anion gap
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause

22. Generation of Metabolic Acidosis
Loss of HCO3
diarrhea
Administration of
HCl, NH4+Cl, CaCl2, lysine HCl
Exogenous acids
ASA
Toxic alcohol
Endogenous acids
ketoacids
DKA
starvation
alcoholic
Lactic acid
L-lactic
D-lactate H+
HCO3-
Compensations
Buffers
Lungs
Kidneys
High gap
Normal gap
If kidney function is normal, urine anion gap Neg

23. H HCO3 Loss of H+ from GI Vomiting, NG suction Congenital Cl diarrhea
Loss of H+ from kidney
1st & 2nd aldosterone
ACTH
Diuretics
Bartter’s, Gitelman’s, Liddle’s
Inhibition of β – OH steroid deh H
HCO3
Compensations
Buffer
Respiratory
Forget the kidney
Gain of HCO3
Administered HCO3,
Acetate, citrate, lactate
Plasma protein products

24. CASE 1 A 24 year old diabetic was admitted for weakness.
Serum Na 140, K 1.8, Cl 125, CO2 6, anion gap 9.
pH 6.84 (H+ 144) pCO2 30, HCO3 5

25. Interpretation of Case 1
Patient has normal gap metabolic acidosis

26. Interpretation of Case 1
Next determine the appropriateness of respiratory compensation
pCO2 = 1.5 x HCO3 + 8 ( ± 2)
pCO2 = 1.5 x = 17.5
The patient’s pCO2 is 30
The respiratory compensation is inappropriate

27. Interpretation of Case 1
This patient has normal anion gap metabolic acidosis with inappropriate respiratory compensation
The finding does not fit DKA but is consistent with HCO3 loss from the GI tract or kidney

28. How to differentiate normal gap acidosis resulting from GI HCO3 loss (diarrhea) vs dRTA?

29. Diarrhea vs RTA Diarrhea History Urine pH < 5.5
Negative urine anion gap
dRTA
History
Urine pH > 5.5
Positive urine anion gap

30. Case 2 A 26 year old woman, complains of weakness.
She denies vomiting or taking medications.
P.E. A thin woman with contracted ECF.
Serum Na 133, K 3.1, Cl 90, CO2 content 32, anion gap11.
pH 7.48 (H+ 32), pCO2 43, HCO3 32.
UNa 52, UK 50, UCl 0, UpH 8

31. Interpretation of Case 2
Determine the appropriateness of respiratory compensation
For every increase of HCO3 by 1, pCO2 should increase by 0.7
pCO2 = 40 + (32-25) x 0.7 = 44.9
The patient’s pCO2 = 43

32. Interpretation of Case 2
This patient has metabolic alkalosis with appropriate respiratory compensation

33. Interpretation of Case 2
Urine Na+ 52, UK+ 50, Cl- 0, pH 8
Urine pH = 8 suggests presence of large amount of HCO3. The increased UNa and UK are to accompany HCO3 excretion. The kidney conserves Cl
The findings are consistent with loss of HCl from the GI tract
Final diagnosis = Self-induced vomiting

34. Vomiting vs Diuretic Active vomiting Remote vomiting Active diuretic
ECF depletion
Metabolic alkalosis
High UNa, UK, low UCl
Urine pH > 6.5
Remote vomiting
Low UNa, high UK, low Cl
Urine pH 6
Active diuretic
ECF depletion
Metabolic alkalosis
High UNa, UK and Cl
Urine pH 5-5.5
Remote diuretic
Low UNa, high UK, low Cl
Urine pH 5-6

35. Case 3
A 40 year old man developed pleuritic chest pain and hemoptysis. His BP 80/50. pH 7.4, pCO2 25, HCO3 15 and pO2 50

36. Interpretation of Case 3
A normal pH suggests mixed disturbances

37. Interpretation of Case 3
His pCO2 is 25, his HCO3 15
If this is acute respiratory alkalosis his HCO3 should have been 25-{(40-25) x 2/10}= 22
If this is chronic respiratory alkalosis, his HCO3 should have been 25 – {(40-25) x 4/10} = 19
If this is metabolic acidosis, his pCO2 should have been 1.5 x = 30-31

38. Interpretation of Case 3
He has combined respiratory alkalosis and metabolic acidosis
The likely diagnosis is pulmonary embolism with hypotension and lactic acidosis or pneumonia with sepsis and lactic acidosis
Other conditions are ASA overdose, sepsis, liver failure

39. Case 4
A patient with COPD developed CHF. Prior to treatment his pH 7.35, pCO2 was 60 and HCO3 32. During treatment with diuretics he vomited a few times. His pH after treatment was 7.42, pCO2 80, HCO3 48.

40. Interpretation of Case 4
Pt’s data pH 7.35, pCO2 60 and HCO3 32
For acute respiratory acidosis
For every 10 mm elevation of pCO2, HCO3 increases by 1, his HCO3 should have been 25 + (60-40) x 1/10 = 27
He did not have acute respiratory acidosis

41. Interpretation of Case 4
Pt’s data pH 7.35, pCO2 60 and HCO3 32.
For chronic respiratory acidosis
For every 10 mm elevation of pCO2, HCO3 increases by 3
His HCO3 should have been 25 + (60-40) x 3/10 = 31
His HCO3 is 32
He had well compensated chronic respiratory acidosis

42. Interpretation of Case 4
His pH is now 7.42, pCO2 80, HCO3 48
If pCO2 of 80 is due to chronic respiratory acidosis, HCO3 should only be 32 +(80-60) x 3/10=38 and not 48
He had combined metabolic alkalosis and respiratory acidosis after treatment of CHF

43. Case 5
A cirrhotic patient was found to be confused. Serum Na 133, K 3.3, Cl 115, CO2 content 14, anion gap 4
pH 7.44 (H+ 36), pCO2 20, HCO3 13

44. Interpretation of Case 5
Determine the respiratory compensation
For chronic respiratory alkalosis, every 10 reduction in pCO2, HCO3 should decrease by 4
HCO3 should be 25 - (40-20) x 4/10=17
For acute respiratory alkalosis, HCO3 = 21
Patient’s HCO3 is 13, suggesting a metabolic acidotic component is present
Anion gap is 4, even corrected for low albumin, is still low suggesting a normal gap metabolic acidosis
Patient had combined metabolic acidosis and respiratory alkalosis