Diagnostics

• Perform an initial clinical evaluation: to help identify the most likely underlying cause

• Order initial laboratory studies: ABG, BMP [4][5]

• Determine the primary acid-base disorder: i.e., using pH, PCO2, and HCO3-

• Calculate the expected compensatory (or secondary) response.

  • Mixed acid-base disorder: The expected compensatory response differs from the laboratory findings.

  • No mixed acid-base disorder: The expected compensatory response aligns with the laboratory findings.

• Perform further diagnostic workup (to determine the mechanism and the cause), e.g.:

  • In metabolic acidosis: anion gap and delta gap

  • In metabolic alkalosis: urinary chloride and potassium levels

Careful clinical evaluation is an important first step in the assessment of acid-base disorders, as it can provide important diagnostic clues that can help determine the underlying cause.

• Suggested approach

  1. Evaluate blood pH (reference range: 7.35–7.45).

  2. Evaluate HCO3- (reference range: 22–28 mEq/L).

  3. Evaluate PCO2 (reference range: 33–45 mm Hg).

• Interpretation

  • pH < 7.35 (acidemia): Primary disorder is an acidosis.

    • ↓ pH and ↓ HCO3-: metabolic acidosis

    • ↓ pH and ↑ PCO2: respiratory acidosis

  • pH > 7.45 (alkalemia): Primary disorder is an alkalosis.

    • ↑ pH and ↑ HCO3-: metabolic alkalosis

    • ↑ pH and ↓ PCO2: respiratory alkalosis

  • See also “Pathophysiology” above.

• Further considerations (not needed for the assessment of acid-base status but required for blood gas analysis)

  • Evaluate PO2.

    • High: hyperoxemia

    • Low: hypoxemia

  • See also “Respiratory failure.”

SMORE: change in PCO2 in the Same direction as pH → Metabolic disorder; change in PCO2 in the Opposite direction to pH → REspiratory disorder

Compensation (acid-base)

• Definition: physiological changes that occur in acid-base disorders in an attempt to maintain normal body pH

• Compensatory changes

  • In metabolic disorders: rapid compensation within minutes through changes in minute ventilation

  • In respiratory disorders: typically slow compensation over several hours to days through changes in urine pH

    
    

• Assessment and interpretation: Calculate the expected compensation; see “Calculation of compensatory response.”

  • Primary respiratory disorders

    • Measured HCO3- > expected HCO3-: metabolic alkalosis in addition to respiratory disturbance

    • Measured HCO3- < expected HCO3-: metabolic acidosis in addition to respiratory disturbance

  • Primary metabolic disorders

    • Measured PCO2 > expected PCO2: respiratory acidosis in addition to metabolic disturbance

    • Measured PCO2 < expected PCO2: respiratory alkalosis addition to metabolic disturbance