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Dr. Ajith Kumar J MD
Dept. of Emergency Medicne
Travancore Medicity, Kollam
India
editor
6 Step Approach to ABG
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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
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-.
| pH | PaCo2 | HCO3 |
Metabolic acidosis | ↓↓ | ↓ | ↓↓ |
Respiratory acidosis | ↓↓ | ↑↑ | ↑ |
Metabolic alkalosis | ↑↑ | ↑ | ↑↑ |
Respiratory alkalosis | ↑↑ | ↓↓ | ↓ |
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
Expected PaCO2 = (1.5 * HCO3) + (8 ±2)
OR
Expected PaCO2 = HCO3 + 15
OR
PaCO2 will decrease 1.25mmHg per mmol decrease in HCO3
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
Key Facts to Remember
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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 PaCO2. . 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.
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
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emmedonline
Dr. Ajith Kumar J MD
Dept. of Emergency Medicne
Travancore Medicity, Kollam
India
editor