Approach to Acid Base Disorder

By

Dr. S. Shivakumar, M.D.

Addl.Professor of Medicine,

Stanley Medical College,

Chennai.

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

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+

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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,

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

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

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)

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)

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)

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