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Approach to ABGs

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  1. Predict the expected acid-base disturbance from patient’s history and exam

    • if patient is vomiting or underwent vigorous NG suctioning – expect metabolic alkalosis
    • if patient admitted with COPD exacerbation and deteriorates – expect respiratory acidosis

  2. Look at pH and decide whether patient is acidemic or alkalemic

    • assume normal pH is 7.40
    • if pH > 7.40, then patient is alkalemic
    • if pH < 7.40, then patient is acidemic
    • pH may be normal in the presence of a mixed acid base disorder, particularly other parameters of the ABG are abnormal

  3. Decide whether overriding disturbance is respiratory or metabolic

    • if pH is acidemic, expect pCO2>40 if primarily respiratory or HCO3<24 if primarily metabolic.
    • if pH is alkalemic, expect pCO2<40 if primarily respiratory or HCO3>24 if primarily metabolic

  4. If the overriding disturbance is respiratory, decide whether acute or chronic using formulas below

    • In acute respiratory acidosis, ↓pH = 0.008 × ∆ PCO2
    • In chronic respiratory acidosis, ↓pH = 0.003 × ∆ PCO2

  5. If a metabolic acidosis is present, calculate the anion gap

    • AG = Na – (Cl- + HCO3-), assume normal AG = 12+/-2
    • If elevated, calculate Delta-Delta Gap = ∆ Anion gap/∆ [HCO3-]
      • If ∆/∆ ratio between 1 – 2, then pure elevated anion gap acidosis
      • If < 1, then there is a simultaneous normal anion gap acidosis present
      • If > 2, then there is a simultaneous metabolic alkalosis present or a compensated chronic respiratory acidosis
    • Another way to look at the ∆/∆ gap would be as follows:
      • If [(pt’s AG) – 12 + (pt’s HCO3-)] is >24, then there is a concomitant alkalosis present
      • If [(pt’s AG) – 12 + (pt’s HCO3-)] is <24 then there is a concomitant acidosis present

  6. If a metabolic disturbance is present, make sure there is adequate respiratory compensation

    • for metabolic acidosis use Winters formula pCO2 = (1.5 x HCO3 ) + 8 +/- 2 to calculate the expected pCO2. If calculated pCO2 is different from actual pCO2, then a second disorder is present
    • for metabolic alkalosis use the following formula: pCO2 = (0.8 x HCO3 ) +16 +/- 4. If calculated pCO2 is different from actual pCO2, then a second disorder is present