This document discusses acid-base disturbances, including simple vs mixed disturbances, and how to approach and interpret acid-base abnormalities. It covers compensations for various metabolic and respiratory disturbances and how to determine the primary cause. Case examples are provided to demonstrate the approach and interpretation of acid-base status. The key steps are reviewing the history, ABG results including pH, pCO2, HCO3, and serum electrolytes. The appropriateness of compensation and causes such as respiratory, renal, gastrointestinal, or endocrine etiologies are considered.

1. Acid and Base DisturbancesAcid and Base Disturbances
D. W. Daugherty, DOD. W. Daugherty, DO
SURGICAL CRITICAL CARESURGICAL CRITICAL CARE

2. Simple vs. MixedSimple vs. Mixed
• SimpleSimple

When compensation is appropriateWhen compensation is appropriate
• MixedMixed

When compensation is inappropriateWhen compensation is inappropriate

3. Simple Acid-Base DisturbancesSimple Acid-Base Disturbances
• When compensation is appropriateWhen compensation is appropriate
 Metabolic acidosis (Metabolic acidosis (↓ HCO↓ HCO33, ↓ pCO, ↓ pCO22))
 Metabolic alkalosis (Metabolic alkalosis (↑ HCO↑ HCO33, ↑ pCO, ↑ pCO22))
 Respiratory acidosis (Respiratory acidosis (↑ pCO↑ pCO22, ↑ HCO, ↑ HCO33))
 Respiratory alkalosis (Respiratory alkalosis (↓ pCO↓ pCO22, ↓ HCO, ↓ HCO33))

4. Stepwise ApproachesStepwise Approaches

History & physical examinationHistory & physical examination
 Arterial blood gas for pH, pCOArterial blood gas for pH, pCO22, (HCO, (HCO33))

Use the HCOUse the HCO33 from ABG to determine compensationfrom ABG to determine compensation
 Serum Na, K, Cl, COSerum Na, K, Cl, CO22 contentcontent

Use COUse CO22 content to calculate anion gapcontent to calculate anion gap

Calculate anion gapCalculate anion gap

Anion gap = {Na - (Cl + COAnion gap = {Na - (Cl + CO22 content)}content)}

Determine appropriate compensationDetermine appropriate compensation

Determine the primary causeDetermine the primary cause

5. Organ dysfunctionOrgan dysfunction
• CNSCNS –– respiratory acidosis (suppression) and alkalosisrespiratory acidosis (suppression) and alkalosis
(stimulation)(stimulation)
• PulmonaryPulmonary –– respiratory acidosis (COPD) and alkalosisrespiratory acidosis (COPD) and alkalosis
(hypoxia, pulmonary embolism)(hypoxia, pulmonary embolism)
• CardiacCardiac –– respiratory alkalosis, respiratory acidosis,respiratory alkalosis, respiratory acidosis,
metabolic acidosis (pulmonary edema)metabolic acidosis (pulmonary edema)
• GIGI –– metabolic alkalosis (vomiting) and acidosismetabolic alkalosis (vomiting) and acidosis
(diarrhea)(diarrhea)
• LiverLiver – respiratory alkalosis, metabolic acidosis (liver– respiratory alkalosis, metabolic acidosis (liver
failure)failure)
• KidneyKidney – metabolic acidosis (RTA) and alkalosis (1– metabolic acidosis (RTA) and alkalosis (1stst
aldosteone)aldosteone)

6. Organ DysfunctionOrgan Dysfunction
• EndocrineEndocrine

Diabetes mellitus –Diabetes mellitus – metabolic acidosismetabolic acidosis

Adrenal insufficiency – metabolic acidosisAdrenal insufficiency – metabolic acidosis

CushingCushing’’s – metabolic alkalosiss – metabolic alkalosis

Primary aldosteronism – metabolic alkalosisPrimary aldosteronism – metabolic alkalosis
• Drugs/toxinsDrugs/toxins

Toxic alcohols – metabolic acidosisToxic alcohols – metabolic acidosis

ASA – metabolic acidosis and respiratory alkalosisASA – metabolic acidosis and respiratory alkalosis

Theophylline overdose – respiratory alkalosisTheophylline overdose – respiratory alkalosis

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

8. 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.

9. Stepwise ApproachesStepwise Approaches

History & physical examinationHistory & physical examination
 Arterial blood gas for pH, pCOArterial blood gas for pH, pCO22, (HCO, (HCO33))

Use the HCOUse the HCO33 from ABG to determine compensationfrom ABG to determine compensation
 Serum Na, K, Cl, COSerum Na, K, Cl, CO22 contentcontent

Use COUse CO22 content to calculate anion gapcontent to calculate anion gap

Calculate anion gapCalculate anion gap

Anion gap = {Na - (Cl + COAnion gap = {Na - (Cl + CO22 content)}content)}

Determine appropriate compensationDetermine appropriate compensation

Determine the primary causeDetermine the primary cause

10. Stepwise ApproachesStepwise Approaches

History & physical examinationHistory & physical examination
 Arterial blood gas for pH, pCOArterial blood gas for pH, pCO22, (HCO, (HCO33))

Use the HCOUse the HCO33 from ABG to determine compensationfrom ABG to determine compensation
 Serum Na, K, Cl, COSerum Na, K, Cl, CO22 contentcontent

Use COUse CO22 content to calculate anion gapcontent to calculate anion gap

Calculate anion gapCalculate anion gap

Anion gap = {Na - (Cl + COAnion gap = {Na - (Cl + CO22 content)} (normal = 8-12)content)} (normal = 8-12)

Determine appropriate compensationDetermine appropriate compensation

Determine the primary causeDetermine the primary cause

11. Compensations for Metabolic DisturbancesCompensations for Metabolic Disturbances
• Metabolic acidosisMetabolic acidosis
 pCOpCO22 = 1.5 x HCO= 1.5 x HCO33 + 8 (+ 8 ( ±± 2)2)
• Metabolic alkalosisMetabolic alkalosis
 pCOpCO22 increases by 7 for every 10 mEq increaseincreases by 7 for every 10 mEq increase
in HCOin HCO33

12. How does the kidneyHow does the kidney
compensate for metaboliccompensate for metabolic
acidosis?acidosis?

13. How does the kidney compensate forHow does the kidney compensate for
metabolic acidosis?metabolic acidosis?
• By reabsorbing all filtered HCOBy reabsorbing all filtered HCO33
• By excreting HBy excreting H++
as NHas NH44
++
(and H(and H22POPO44
--
))
Urine pHUrine pH < 5.5< 5.5
Urine anion gap NegativeUrine anion gap Negative

14. Compensations for Respiratory AcidosisCompensations for Respiratory Acidosis
• Acute respiratory acidosisAcute respiratory acidosis
 HCOHCO33 increases by 1 for every 10 increase inincreases by 1 for every 10 increase in
pCOpCO22
• Chronic respiratory acidosisChronic respiratory acidosis
 HCOHCO33 increases by 3 for every 10 increase inincreases by 3 for every 10 increase in
pCOpCO22

15. Compensations for Respiratory AlkalosisCompensations for Respiratory Alkalosis
• Acute respiratory alkalosisAcute respiratory alkalosis
 HCOHCO33 decreases by 2 for every 10 decrease indecreases by 2 for every 10 decrease in
pCOpCO22
• Chronic respiratory alkalosisChronic respiratory alkalosis
 HCOHCO33 decreases by 4 for every 10 decrease indecreases by 4 for every 10 decrease in
pCOpCO22

16. Mixed Acid-Base DisordersMixed Acid-Base Disorders
• Mixed respiratory alkalosis & metabolicMixed respiratory alkalosis & metabolic
acidosisacidosis

ASA overdoseASA overdose

SepsisSepsis

Liver failureLiver failure
• Mixed respiratory acidosis & metabolicMixed respiratory acidosis & metabolic
alkalosisalkalosis

COPD with excessive use of diureticsCOPD with excessive use of diuretics

17. Mixed Acid-Base DisordersMixed Acid-Base Disorders
• Mixed respiratory acidosis & metabolicMixed respiratory acidosis & metabolic
acidosisacidosis

Cardiopulmonary arrestCardiopulmonary arrest

Severe pulmonary edemaSevere pulmonary edema
• Mixed high gap metabolic acidosis &Mixed high gap metabolic acidosis &
metabolic alkalosismetabolic alkalosis

Renal failure with vomitingRenal failure with vomiting

DKA with severe vomitingDKA with severe vomiting

18. Generation of Metabolic AcidosisGeneration of Metabolic Acidosis
H+
HCO
3
-
Exogenous acids
ASA
Toxic alcohol
Endogenous acids
ketoacids
DKA
starvation
alcoholic
Lactic acid
L-lactic
D-lactate
Administration of
HCl, NH4
+
Cl, CaCl2, lysine HCl
Loss of HCO3
diarrhea
Compensations
Buffers
Lungs
Kidneys
High gap Normal gap If kidney function is normal, urine anion gap Neg

19. 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
Gain of HCO3
Administered HCO3,
Acetate, citrate, lactate
Plasma protein products
Compensations
Buffer
Respiratory
Forget the kidney

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

21. Interpretation of Case 1Interpretation of Case 1
Patient has normal gapPatient has normal gap
metabolic acidosismetabolic acidosis

22. Interpretation of Case 1Interpretation of Case 1
• Next determine the appropriateness of respiratoryNext determine the appropriateness of respiratory
compensationcompensation
 pCOpCO22 = 1.5 x HCO= 1.5 x HCO33 + 8 (+ 8 ( ±± 2)2)
 pCOpCO22 = 1.5 x 5 + 8 + 2 = 17.5= 1.5 x 5 + 8 + 2 = 17.5
The patientThe patient’s pCO’s pCO22 is 30is 30
• The respiratory compensation is inappropriateThe respiratory compensation is inappropriate

23. Interpretation of Case 1Interpretation of Case 1

This patient has normal anion gap metabolicThis patient has normal anion gap metabolic
acidosis with inappropriate respiratoryacidosis with inappropriate respiratory
compensationcompensation

The finding does not fit DKA but isThe finding does not fit DKA but is
consistent with HCOconsistent with HCO33 loss from the GI tractloss from the GI tract
or kidneyor kidney

24. How do you differentiate aHow do you differentiate a
normal gap acidosis resultingnormal gap acidosis resulting
from GI HCOfrom GI HCO33 loss (diarrhea)loss (diarrhea)
vs RTA?vs RTA?

25. Diarrhea vs RTADiarrhea vs RTA
• DiarrheaDiarrhea

HistoryHistory

Urine pH < 5.5Urine pH < 5.5

Negative urineNegative urine
anion gapanion gap
• RTARTA

HistoryHistory

Urine pH > 5.5Urine pH > 5.5

Positive urinePositive urine
anion gapanion gap

26. Case 2Case 2
A 26 year old woman, complains of weakness.
She denies vomiting or taking medications.
P.E. A thin woman with contracted ECF.
Na 133
K 3.1
Cl 90
CO2 32
Gap 11
pH 7.48 (H+
32) / pCO2 43 / HCO3 32.
UNa 52 / UK 50 / UCl 0 / UpH 8

27. Interpretation of Case 2Interpretation of Case 2
• Determine the appropriateness of respiratoryDetermine the appropriateness of respiratory
compensationcompensation
 For every increase of HCOFor every increase of HCO33 by 1, pCOby 1, pCO22 shouldshould
increase by 0.7increase by 0.7
 pCOpCO22 = 40 + (32-25) x 0.7 = 44.9= 40 + (32-25) x 0.7 = 44.9
The patientThe patient’’s pCOs pCO22 = 43= 43

28. Interpretation of Case 2Interpretation of Case 2
This patient has metabolic alkalosis withThis patient has metabolic alkalosis with
appropriate respiratory compensationappropriate respiratory compensation

29. Interpretation of Case 2Interpretation of Case 2

Urine NaUrine Na++
52, UK52, UK++
50, Cl50, Cl--
0, pH 80, pH 8

Urine pH = 8 suggests presence of large amountUrine pH = 8 suggests presence of large amount
of HCOof HCO33. The increased UNa and UK are to. The increased UNa and UK are to
accompany HCOaccompany HCO33 excretion. The kidneyexcretion. The kidney
conserves Clconserves Cl

The findings are consistent with loss of HClThe findings are consistent with loss of HCl
from the GI tractfrom the GI tract

Final diagnosis = Self-induced vomitingFinal diagnosis = Self-induced vomiting

30. Vomiting vs DiureticVomiting vs Diuretic
• Active vomitingActive vomiting

ECF depletionECF depletion

Metabolic alkalosisMetabolic alkalosis

High UNa, UK, low UClHigh UNa, UK, low UCl

Urine pH > 6.5Urine pH > 6.5
• Remote vomitingRemote vomiting

ECF depletionECF depletion

Metabolic alkalosisMetabolic alkalosis

Low UNa, high UK, lowLow UNa, high UK, low
ClCl

Urine pH 6Urine pH 6
• Active diureticActive diuretic

ECF depletionECF depletion

Metabolic alkalosisMetabolic alkalosis

High UNa, UK and ClHigh UNa, UK and Cl

Urine pH 5-5.5Urine pH 5-5.5
• Remote diureticRemote diuretic

ECF depletionECF depletion

Metabolic alkalosisMetabolic alkalosis

Low UNa, high UK, lowLow UNa, high UK, low
ClCl

Urine pH 5-6Urine pH 5-6

31. Case 3Case 3
A 40 year old man developed pleuritic chestA 40 year old man developed pleuritic chest
pain and hemoptysis. His BP 80/50.pain and hemoptysis. His BP 80/50.
pH 7.4pH 7.4
pCOpCO22 2525
HCOHCO33 1515
pOpO22 5050

32. Interpretation of Case 3Interpretation of Case 3
A normal pH suggests mixed disturbancesA normal pH suggests mixed disturbances

33. Interpretation of Case 3Interpretation of Case 3
 His pCOHis pCO22 is 25, his HCOis 25, his HCO33 1515
 If this is acute respiratory alkalosis his HCOIf this is acute respiratory alkalosis his HCO33
should have been 25-{(40-25) x 2/10}= 22should have been 25-{(40-25) x 2/10}= 22
 If this is chronic respiratory alkalosis, his HCOIf this is chronic respiratory alkalosis, his HCO33
should have been 25 – {(40-25) x 4/10} = 19should have been 25 – {(40-25) x 4/10} = 19
 If this is metabolic acidosis, his pCOIf this is metabolic acidosis, his pCO22 should haveshould have
been 1.5 x 15 + 8 = 30-31been 1.5 x 15 + 8 = 30-31

34. Interpretation of Case 3Interpretation of Case 3

He has combined respiratory alkalosis andHe has combined respiratory alkalosis and
metabolic acidosismetabolic acidosis

The likely diagnosis is pulmonary embolism withThe likely diagnosis is pulmonary embolism with
hypotension and lactic acidosis or pneumoniahypotension and lactic acidosis or pneumonia
with sepsis and lactic acidosiswith sepsis and lactic acidosis

Other conditions are ASA overdose, sepsis, liverOther conditions are ASA overdose, sepsis, liver
failurefailure

35. Case 4Case 4
A patient with COPD developed CHF. Prior toA patient with COPD developed CHF. Prior to
treatmenttreatment
pH 7.35pH 7.35
pCOpCO22 6060
HCOHCO33 3232
During treatment with diuretics he vomited a fewDuring treatment with diuretics he vomited a few
times. His pH after treatment wastimes. His pH after treatment was
pH7.42 / pCOpH7.42 / pCO22 80 / HCO80 / HCO33 4848

36. Interpretation of Case 4Interpretation of Case 4
 PtPt’’s data pH 7.35, pCOs data pH 7.35, pCO22 60 and HCO60 and HCO33 3232

For acute respiratory acidosisFor acute respiratory acidosis

For every 10 mm elevation of pCOFor every 10 mm elevation of pCO22, HCO, HCO33 increases byincreases by
1, his HCO3 should have been 25 + (60-40) x 1/10 =1, his HCO3 should have been 25 + (60-40) x 1/10 =
2727

He did not have acute respiratory acidosisHe did not have acute respiratory acidosis

37. Interpretation of Case 4Interpretation of Case 4
 PtPt’’s data pH 7.35, pCOs data pH 7.35, pCO22 60 and HCO60 and HCO33 3232

For chronic respiratory acidosisFor chronic respiratory acidosis
 For every 10mm elevation of pCOFor every 10mm elevation of pCO22, HCO, HCO33 increases by 3increases by 3
 His HCOHis HCO33 should have been 25 + (60-40) x 3/10 = 31should have been 25 + (60-40) x 3/10 = 31
 His HCOHis HCO33 is 32is 32

He had well compensated chronic respiratoryHe had well compensated chronic respiratory
acidosisacidosis

38. Interpretation of Case 4Interpretation of Case 4
 His pH is now 7.42, pCOHis pH is now 7.42, pCO22 80, HCO80, HCO33 4848
 If pCOIf pCO22 of 80 is due to chronic respiratoryof 80 is due to chronic respiratory
acidosis, HCOacidosis, HCO33 should only be 32 +(80-60) xshould only be 32 +(80-60) x
3/10=38 and not 483/10=38 and not 48

He had combined metabolic alkalosis andHe had combined metabolic alkalosis and
respiratory acidosis after treatment of CHFrespiratory acidosis after treatment of CHF

39. Case 5Case 5
A cirrhotic patient was found to be confusedA cirrhotic patient was found to be confused..
Na 133Na 133
K 3.3K 3.3
Cl 115Cl 115
COCO22 1414
Gap 4Gap 4
pH 7.44 (HpH 7.44 (H++
36)36)
pCOpCO22 2020
HCOHCO33 1313

40. Interpretation of Case 5Interpretation of Case 5

Determine the respiratory compensationDetermine the respiratory compensation
 For chronic respiratory alkalosis, every 10 reduction in pCOFor chronic respiratory alkalosis, every 10 reduction in pCO22,,
HCOHCO33 should decrease by 4should decrease by 4
 HCOHCO33 should be 25 - (40-20) x 4/10=17should be 25 - (40-20) x 4/10=17
 For acute respiratory alkalosis, HCOFor acute respiratory alkalosis, HCO33 = 21= 21

PatientPatient’’s HCO3 is 13, suggesting a metabolic acidotics HCO3 is 13, suggesting a metabolic acidotic
component is presentcomponent is present

Anion gap is 4, even corrected for low albumin, is still lowAnion gap is 4, even corrected for low albumin, is still low
suggesting a normal gap metabolic acidosissuggesting a normal gap metabolic acidosis

Patient had combined metabolic acidosis and respiratory alkalosisPatient had combined metabolic acidosis and respiratory alkalosis