Acid-Base
Disturbances
Clinical Approach
2006
Pravit Cadnapaphornchai
Simple vs Mixed
Simple
 When compensation is appropriate
 Mixed
 When compensation is inappropriate

Simple Acid-Base Disturbances

When compensation is appropriate
Metabolic acidosis (↓ HCO3, ↓ pCO2)
Metabolic alkalosis (↑ HCO3, ↑ pCO2)
Respiratory acidosis (↑ pCO2, ↑ HCO3)
Respiratory alkalosis (↓ pCO2, ↓ HCO3)
Stepwise Approaches


History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)


Serum Na, K, Cl, CO2 content



Use CO2 content to calculate anion gap
Calculate anion gap


Use the HCO3 from ABG to determine compensation
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause
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)
Organ Dysfunction

Endocrine

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
Stepwise Approaches


History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)


Serum Na, K, Cl, CO2 content



Use CO2 content to calculate anion gap
Calculate anion gap


Use the HCO3 from ABG to determine compensation
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause
pH
< 7.35
7.4
>7.45
Acidosis
Mixed
Alkalosis
Metabolic
Respiratory
Metabolic
Respiratory
Stepwise Approaches


History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)


Serum Na, K, Cl, CO2 content



Use CO2 content to calculate anion gap
Calculate anion gap


Use the HCO3 from ABG to determine compensation
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause
CO2 content
Low
Metabolic acidosis
Resp alkalosis
Normal
Normal
Mixed
High
Metabolic alkalosis
Resp acidosis
A normal CO2 content + high anion gap = metabolic acidosis +
Metabolic alkalosis or metabolic ac + compensatory respiratory ac.
Stepwise Approaches


History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)


Serum Na, K, Cl, CO2 content



Use CO2 content to calculate anion gap
Calculate anion gap


Use the HCO3 from ABG to determine compensation
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause
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
Stepwise Approaches


History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)


Serum Na, K, Cl, CO2 content



Use CO2 content to calculate anion gap
Calculate anion gap


Use the HCO3 from ABG to determine compensation
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause
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
How does the kidney
compensate for metabolic
acidosis?
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
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?
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?

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

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

Stepwise Approaches


History & physical examination
Arterial blood gas for pH, pCO2, (HCO3)


Serum Na, K, Cl, CO2 content



Use CO2 content to calculate anion gap
Calculate anion gap


Use the HCO3 from ABG to determine compensation
Anion gap = {Na - (Cl + CO2 content)}
Determine appropriate compensation
Determine the primary cause
Generation of Metabolic Acidosis
Administration of
HCl, NH4+Cl, CaCl2, lysine HCl
Exogenous acids
ASA
Toxic alcohol
H+
Compensations
Buffers
Endogenous acids
ketoacids
DKA
starvation
alcoholic
Lactic acid
L-lactic
D-lactate
High gap
Loss of HCO3
diarrhea
Lungs
HCO 3
Normal gap
Kidneys
If kidney function is normal, urine anion gap Neg
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
H
Compensations
Buffer
Respiratory
HCO3
Forget the kidney
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
Interpretation of Case 1
Patient has normal gap metabolic
acidosis
Interpretation of Case 1

Next determine the appropriateness of
respiratory compensation
pCO2 = 1.5 x HCO3 + 8 ( ± 2)
 pCO2 = 1.5 x 5 + 8 + 2 = 17.5

 The

patient’s pCO2 is 30
The respiratory compensation is
inappropriate
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

How to differentiate normal
gap acidosis resulting from GI
HCO3 loss (diarrhea) vs dRTA?
Diarrhea vs RTA

Diarrhea
 History
 Urine pH < 5.5
 Negative urine
anion gap

dRTA
 History
 Urine pH > 5.5
 Positive urine
anion gap
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
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
Interpretation of Case 2

This patient has metabolic alkalosis with
appropriate respiratory compensation
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

Vomiting vs Diuretic

Active vomiting





ECF depletion
Metabolic alkalosis
High UNa, UK, low UCl
Urine pH > 6.5
Remote vomiting





ECF depletion
Metabolic alkalosis
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




ECF depletion
Metabolic alkalosis
Low UNa, high UK, low
Cl
Urine pH 5-6
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
Interpretation of Case 3

A normal pH suggests mixed
disturbances
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 15 + 8 = 30-31

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
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.
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
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
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
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
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