C The following guidelines are meant to expand on the material presented in Chapter 3 and to simplify the interpretation of arterial blood gas values. Because memorizing a “cookbook” approach can sometimes be counterproductive if the reason why the approach is being used is not clear, these guidelines are meant to supplement a basic understanding of the underlying physiologic principles. Numerous formulas are used to assess the appropriateness of compensation for a primary acid-base disorder. These formulas are particularly useful for suggesting whether a mixed acid-base disorder is present. Table C-1 lists commonly used formulas that predict the expected degree of respiratory compensation for a primary metabolic problem and metabolic compensation for a primary respiratory problem. These formulas relate arterial PCO2 and measured HCO3−. However, measured values from arterial blood gases include arterial PCO2 and pH, not serum HCO3−. Therefore, to use the formulas in the table, one must either measure serum HCO3− (as part of serum electrolyte values) or use a value calculated from PCO2 and pH according to the Henderson-Hasselbalch equation. Table C-1 EXPECTED COMPENSATION FOR PRIMARY ACID-BASE DISORDERS Adapted from Narins RG, Emmett M: Medicine 59:161–186, 1980. © by Williams & Wilkins, 1980.
Arterial Blood Gases
Guidelines for Interpretation and Sample Problems
Primary Disorder
Compensatory Response
Expected Magnitude of Response
Metabolic acidosis
↓ PCO2
PCO2 = 1.5 × (HCO3−) + 8 ± 2
Metabolic alkalosis
↑ PCO2
PCO2 increases 6 mm Hg for each 10 mEq/L increase in HCO3−
Respiratory acidosis
↑ HCO3−
Acute: HCO3− increases 1 mEq/L for each 10 mm Hg increase in PCO2
Chronic: HCO3− increases 3.5 mEq/L for each 10 mm Hg increase in PCO2
Respiratory alkalosis
↓ HCO3−
Acute: HCO3− falls 2 mEq/L for each 10 mm Hg decrease in PCO2
Chronic: HCO3− falls 5 mEq/L for each 10 mm Hg decrease in PCO2
Thoracic Key
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