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 Change in 0.3 pH  Double or Halve the 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

Estimating the H+ ion from pH 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  1.25 X 0.8 Within narrow limits 0.01 change in pH ~ 1 mEq / L change of [H+] 7.40 = 40 7.39 = 41 7.38 = 42 7.37 = 43

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 40 24 = 40 nEq = pH = 7.4   eg: pH = 7.0 HC03 = 8 PCo2 = 32   H = 24 x 32 8 = 96 nEq = pH 7.0

Step II: Obtain minimum diagnosis Look at pH - Acidosis / Alkalosis Match the Pco2 or Hco3 - Metabolic / Respiratory Disorder Primary Change Secondary change Net effect M. Acidosis  Hco3  Pco2  pH ( H+) M.Alkalosis  Hco3  Pco2  pH ( H+) R. Acidosis R. Alkalosis

Examples of Simple Disorder pH Hco3 Pco2 Metabolic Acidosis 7.15 8 24 Respiratory Acidosis 30 90 Metabolic Alkalosis 7.7 36 48 Respiratory Alkalosis 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 acidosis COPD Respiratory alkalosis Psychogenic hyperventilation, Hepatic Encephalopathy

Mixed Disorder Situation Disorder Respiratory acidosis COPD + Vomiting + Metabolic alkalosis COPD + Diarrhoea + Metabolic acidosis Sepsis Metabolic acidosis + Respiratory alkalosis Cirrhosis + Diuretics Respiratory 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 Acute Resp.Acidosis Plasma Hco3  by 1 mEq / L for each 10 mm  Pco2 Chronic. Resp. Acidosis Plasma Hco3  by 4 mEq / L Acute. Resp. Alkalosis Plasma Hco3  by 1 mEq / L for each 10 mm  Pco2 Chronic Resp Alkalosis Plasma Hco3  by 4 mEq / L

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 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  pH = PCo2  Hco3 

Mixed acid base 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  (N) pH = PCo2  Hco3 

Examples of mixed acid base disorder Data Disorder pH Hco3 Pco2 Po2 6.85 15 (30) 90 50 Respiratory acidosis + Metabolic acidosis 7.3 45 (30) + Metabolic alkalosis 6 12 (22) 100 Metabolic acidosis + Respiratory alkalosis 7.75 40 25 (43) Metabolic 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 + 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 Triple Disorder = Metabolic acidosis + Resp.acidosis + Met.alkalosis Metabolic acidosis – Renal failure Respiratory acidosis – COPD Metabolic alkalosis – Vomiting

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 Due to Renal failure + Hyperkalemia 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

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