Approach to Acid Base Disorder
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![Page 1: Approach to Acid Base Disorder](https://reader035.fdocuments.in/reader035/viewer/2022062407/56812b13550346895d8f06cb/html5/thumbnails/1.jpg)
Approach to Acid Base Disorder
By
Dr. S. Shivakumar, M.D.
Addl.Professor of Medicine,
Stanley Medical College,
Chennai.
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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 < 7.2 : Severe Acidosis
pH > 7.6 : 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
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Correlation between H+ ion & pH
pH H+ pH 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
Change in 0.3 pH Double or Halve the H+
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Estimating the H+ ion from pH
Within narrow limits
0.01 change in pH ~ 1 mEq / L change of [H+]
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
7.40 = 40 7.39 = 41 7.38 = 42 7.37 = 43
X 0.8 1.25
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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
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Suggestions Step I. Check validity : (Modified Henderson equation)
H = 24 x Pco2
Hco3
H = 24 x 4024
= 40 nEq = pH = 7.4
eg: pH = 7.0 HC03 = 8 PCo2 = 32 H = 24 x 32
8 = 96 nEq = pH 7.0
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Disorder Primary Change
Secondary change
Net effect
M. Acidosis Hco3 Pco2 pH ( H+)
M.Alkalosis Hco3 Pco2 pH ( H+)
R. Acidosis Pco2 Hco3 pH ( H+)
R. Alkalosis Pco2 Hco3 pH ( H+)
Step II: Obtain minimum diagnosis
Look at pH - Acidosis / Alkalosis
Match the Pco2 or Hco3 - Metabolic / Respiratory
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Examples of Simple Disorder
Disorder pH Hco3 Pco2
Metabolic Acidosis 7.15 8 24
Respiratory Acidosis 7.15 30 90
Metabolic Alkalosis 7.7 36 48
Respiratory Alkalosis 7.7 12 10
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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 acidosisCOPD
Respiratory alkalosis Psychogenic hyperventilation, Hepatic Encephalopathy
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Mixed Disorder
Situation DisorderCOPD + Vomiting Respiratory acidosis
+ Metabolic alkalosis
COPD + Diarrhoea Respiratory acidosis
+ Metabolic acidosis
Sepsis Metabolic acidosis
+ Respiratory alkalosis
Cirrhosis + Diuretics Respiratory alkalosis
+ Metabolic alkalosis
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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
AcuteResp.Acidosis
Plasma Hco3 by 1 mEq / L
for each 10 mm Pco2
Chronic.Resp. Acidosis
Plasma Hco3 by 4 mEq / L
for each 10 mm Pco2
Acute.Resp. Alkalosis
Plasma Hco3 by 1 mEq / L
for each 10 mm Pco2
Chronic Resp Alkalosis
Plasma Hco3 by 4 mEq / L
for each 10 mm Pco2
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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
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Mixed acid base disorder
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
for simple disturbance pH = PCo2
Hco3
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Mixed acid base disorder
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
for simple disturbance
Normal or
slightly or (N) pH = PCo2
Hco3
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Examples of mixed acid base disorder
DataDisorder
pH Hco3 Pco2 Po2
6.85 15 (30) 90 50Respiratory acidosis
+ Metabolic acidosis
7.3 45 (30) 90 50Respiratory acidosis
+ Metabolic alkalosis
7.3 6 12 (22) 100Metabolic acidosis
+ Respiratory alkalosis
7.75 40 25 (43) 100Metabolic alkalosis
+ Respiratory alkalosis
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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
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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
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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 = 4 + 29 = 33 mEq / L
Starting Hco3 > 29 suggests associated
Metabolic Alkalosis in the presence of
Metabolic Acidosis
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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
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Approach with an Example(contd)
Step 1 : Check Validity
H = 24 X Pc02 = 24 X 22 = 155 nEq / L = pH 6.8
Hco3 3.4
Step 2: Obtain minimum diagnosis
pH = 6.8 Pco2 = 22 Hco3 = 3.4
Metabolic acidosis
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Approach with an Example(contd)
Step 3:
Is it a Simple or Mixed Acid base disturbance
By applying compensation
(24 3.4 = 20.6 ; 20.6 1.2 = 24.7; 40 24.7 = 15.3)
Predicted Pco2 = 15.3, but Observed Pco2 = 22
Mixed disorder
Metabolic Acidosis + Respiratory Acidosis
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Approach with an Example (contd)
Step 4:
Determine Anion Gap
Na = 135 Cl = 90 Hco3 = 3.4
AG = 135 – (3.4 + 90) = 41.6
High Gap acidosis
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Approach with an Example (contd)
Step 5:
Is it a triple disorder
Look for metabolic acidosis with AG
AG = Calculated AG - AG normal
= 41.6 - 12.0 = 29.6
Starting Hc03 = Hco3+ AG
= 3.4 + 29.6 = 33 mEq /L (Metabolic alkalosis)
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Diagnosis
Triple Disorder = Metabolic acidosis +
Resp.acidosis + Met.alkalosis
Metabolic acidosis – Renal failure
Respiratory acidosis – COPD
Metabolic alkalosis – Vomiting
pH = 6.8 Pco2 = 22 Hco3 = 3.4 Po2 = 60
Na+ = 135 k + = 6.0 Cl = 90 Urea= 110 S.Cr = 3mg/dl Sugar = 100 mgs
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Example –1
A 40 yr old man is admitted for diarrhoea & breathlesness for 2 days.
pH - 7.2 pCo2 - 32 pO2 - 100 Hco3- 12
Na+ - 138 k - 2.9 Cl- - 115 urea-70
S.Cr. - 1.4 sugar-110
Diagnosis: Normal AG Met.Acidosis (AG – 11)
Due to Diarrhoea + Hypokalemia,
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Example –2
A 40 yr old man is admitted for diarrhoea of 1 week & breathlesness of 1 day
pH - 7.1 pCo2 - 20 pO2 - 100 Hco3- 6
Na+ - 140 k + - 6.9 Cl- - 105 urea-120
S.Cr. - 5.4 sugar-110
Diagnosis: High gap Acidosis (AG - 29)Due to Renal failure + Hyperkalemia
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Example –3
A 45 yr old female is admitted for high fever & breathlessness, diagnosed to have acute Cholecystitis
pH - 7.3 pCo2 - 12 pO2 - 100 Hco3- - 6
Na+ - 140 K+ - 5 Cl - - 105 Urea-45
S.Cr. - 1.6 Sugar-120
(Predicted pCo2 – 18, but observed pCo2 – 12) Diagnosis: Metabolic acidosis + Respiratory
Alkalosis Due to sepsis
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Example –4
A 50 yr old Pt., a known case of COPD is admitted for severe diarrhoea
pH - 6.9 pCo2 - 40 pO2 - 50 Hco3- 9
Na+ - 140 K+ - 4 Cl- - 105 Urea-45
S.Cr. - 1.2 Sugar - 128
(Predicted pCo2 – 25, but observed pCo2 – 40)
Diagnosis: Respiratory Acidosis (COPD ) + Metabolic acidosis (Diarrhoea)
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Example –5
A 50 yr old, known COPD Pt. with Cor pulmonale on treatment with Frusemide and is admitted for severe vomitting.
pH - 7.4 pCo2 - 80 pO2 - 40 Hco3- -48
Na+ - 140 K+ - 4 Cl- - 105 Urea-45
S.Cr. - 1.2 Sugar- 128
(Predicted pCo2 – 54, but observed pCo2 – 80) Diagnosis: Respiratory Acidosis (COPD ) +
Metabolic Alkalosis (Vomiting + Diuretics)
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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 - 7.75 pCo2 - 30 pO2 - 80 Hco3- - 40
Na+ - 135 K+ - 3.5 Cl- - 95 Urea - 45
S.Cr. - 1.2 Sugar -110
(Predicted Hco3 – 20, but observed Hco3 – 40) Diagnosis: Respiratory Alkalosis (Cirrhosis ) +
Metabolic Alkalosis (Vomitting + Diuretics)
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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
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