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Step 1 : Is to identify whether the pH is acidic or alkali or normal.

• Normal pH : 7.35 – 7.45

• Normal PCo2 : 36 – 44 mmHg

• Normal Po2: 80 – 100 mmHg

• Normal Bicarbonate : 22- 26mEql/L

• When there is increase in PCO2 or decrease HCO3 it is called respiratory or metabolic acidosis respectively.

• When there is decreases in PCO2 or increase in HC03 it is called respiratory or metabolic alkalosis respectively.

• If there is change in the pH to outside the normal range, the suffix 'emia' is used to describe the acid base derangement; Acidemia is a condition where pH falls <7.36 and alkalemia is a condition where pH rises above 7.44

Step 2: Recognize whether acidosis is metabolic or respiratory ?

• In Respiratory Acidosis there will be decrease in pH and increase in PCo₂ levels

• In Respiratory Alkalosis there will be increase in pH and decrease in PCo2 levels.

• In Metabolic Acidosis there will be decrease in pH and decrease in HCO₃^-.

• In Metabolic Alkalosis there will be increase in pH and increase in HCO3^-.

Note: Here in metabolic acidosis & alkalosis the direction of pH & bicarbonate is in the same direction but whereas in respiratory alkalosis/acidosis it travels in the opposite direction.

Step3: Calculate Anion Gap

• If it is a metabolic acidosis find out the anion gap and classify into High anion gap metabolic acidosis or Normal anion gap metabolic acidosis.

Step 4: Look for Compensation in a case of metabolic acidosis or alkalosis

• If it is a metabolic cause then look for respiratory compensation

• Metabolic Acidosis
  

• Expected PaCO2 = (1.5 * HCO3) + (8 ±2)

                                               OR

• Expected PaCO2 = HCO3 + 15

                                               OR

• PaCO2 will decrease 1.25mmHg per mmol decrease in HCO3

• Metabolic Alkalosis
  

• Expected PaCO2 = (0.7*HCO3)+ (21±2)

                             OR

• PaCO2 will increase by 6 per 10mmol/L rise in Bicarbonate.

STEP 4: Look whether Respiratory acidosis or alkalosis is acute or chronic

• If respiratory look for adequate metabolic compensation and classify it as acute or chronic change.

• The compensatory response to primary changes in PaCO2 takes place in the kidney and involves an adjustment in HC03 reabsorption from proximal tubules.

• But compensatory mechanism is a slow process and begins to appear in 6 -12hrs and is fully developed after a few days. Therefore respiratory acidosis is classified into acute or chronic

Acute

• Expected pH in acute respiratory acidosis = 7.4 - (0.008* [(PaCO2 – 40)]

  OR

• [HCO3-] will increase by 1 mmol/L per 10mmHg increase in PaCO2

• Expected pH in acute respiratory alkalosis = 7.4 + (0.008 *[40- PaCO2] )

or

• [HCO3-] will decrease by 2 mmol/L per 10 mmHg decrease in PaCO2

Chronic

• Expected pH in chronic respiratory acidosis = 7.4 - (0.003* [(PaCO2 – 40)]

OR

• [HCO3-] will increase by 3.5 mmol/L per 10mmHg increase in PaCO2

• Expected pH in chronic respiratory alkalosis = 7.4 + (0.003*[(40- PaCO2)]

  OR

• [HCO3-] will decrease by 4 mmol/L per 10mmHg decrease in PaCO2

STEP 5: Look for any Mixed Acid Base disorder / Third acid base disorder

• Respiratory Acidosis

  • A metabolic acidosis is superimposed on a respiratory acidosis if the decrease in pH is greater than expected based on the PaCO₂. For example ABG of a patient shows a pH 7.1 & PaCO₂ of 60 mmHg. The expected pH for an acute decrease in the PaCO₂ to 60 mmHg is 7.24. Thus, a superimposed metabolic acidosis is present.

  • A metabolic alkalosis is superimposed on a respiratory alkalosis if the increase in pH is greater than expected based on the PaCO_2. . For example ABG of a patient shows a pH 7.55 & PaCO₂ of 30 mmHg. The expected pH for an acute decrease in the PaCO₂ to 30 mmHg is 7.48. Thus, a superimposed metabolic alkalosis is present.

• A combined acidosis and alkalosis is present if the PaCO₂ is increased or decreased and the pH is normal.

• Metabolic Acidosis
  

Step 6: Alveolar- arterial oxygenation gradient

Note

• The H+ concentration at pH 7.4 is 40nmol/L. The relationship between the pH and H+ is linear between pH values of 7.2 to 7.5. The pH change of 0.01 will cause a change of H+ by 1 mmol/L.

• In metabolic acidosis the H+ ions move into cell leading to extrusion of K+ . For each 0.10 decrease in the pH, serum K+ will increase by approximately by 0.5 mEq/L.

Updated on : 21/7/2015

Reference

• The ICU : by Paul marino

• Tintinalli

• Davidson

• The Washington Manual of Critical Care. Second Edition