Tests for the Acid-base Balance Status

Clinical assesment

• The most clinical useful information comes from the clinical description of the patient by the history and physical examination.
  • The H&P usually gives an idea of what acid base disorder might be present even before collecting the ABG sample.

Agap:

• The major causes of simple and mixed disturbances shoud be kept in mind so that such disorders may be predicted from the patient’s clinical picture. For example,
  • patients with severe vomiting and those on long-term thiazide therapy can be expected to have metabolic alkalosis,
  • those with renal failure may have metabolic acidosis
  • and patients with pneumonia or congestive cardiac failure may have respiratory alkalosis…

• The possibility of an acid-base balance disturbance suggested by clinical presentation should then be evaluated by appropriate laboratory tests.

Blood gases

• In order to interpret acid-base disturbances, the following five factors are considered:
  • pH
  • HCO₃-
  • PCO2
  • Anion gap
  • and assessment for compensation

• The first step is to determine if the patient is acidemic or alkalemic, based on pH.

• Second, the primary disorder is determined by evaluating HCO₃– and PCO₂.
  • If HCO₃– is elevated and pH is elevated, there is metabolic alkalosis.
  • If both are decreased, there is metabolic acidosis.

• Next, one must look at the PCO₂ in the context of the HCO₃–.
  • If HCO₃– is within the normal reference range and PCO₂ is elevated but the patient is acidotic,
    • the condition is respiratory acidosis.
  • If bicarbonate is within the normal reference range and PCO₂ is decreased but the patient is alkalotic,
    • the condition is respiratory alkalosis.

• Next determine the anion gap, using standard formulas below, to determine the etiology of metabolic acidosis.
  • Anion gap = [Na] – ([Cl-] + [HCO₃-])

• Finally the pH, HCO₃–, and PCO₂ are considered to determine if compensation is as expected based on the typical ratio of 20:1 for bicarbonate to carbonic acid.
  • For example, both decreased HCO₃– and PCO₂ should produce a slightly decreased or nearly normal pH if they are in metabolic acidosis compensation.
  • To determine the actual ratio of bicarbonate to carbonic acid, PCO₂ is converted to H₂CO₃ using the relationship
    • PCO₂ * 0.03 = H₂CO₃

• Compensation for metabolic acidosis or alkalosis is achieved initially by the respiratory system.
  • Respiratory compensation for acidosis means that the lungs increase the level of alveolar ventilation, which raises the pH toward normal.
    • The increased ventilation eliminates or blos off CO₂, which eliminates carbonic acid.
    • Also, the presence of acidosis normally increases respiratory drive.
    • The respiratory system compensates for a metabolic defect.

• • In metabolic alkalosis, some decrease in ventilation occurs but the PCO₂ generally remains normal since respiratory compensation doesn’t occur until alkalosis has been severe and prolonged.
    • Compensation for metabolic alkalosis is less complete since hypoventilation is not a naturally sustainable condition.

References

Acid-Base Disorders by Walmsley Koay and Watkinson

Clinical chemistry (A laboratory perspective) by Wendy Arneson and Jean Brickell