Tests for the Acid-base Balance Status

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Clinical assesment[✎ edit | edit source]

  • 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.


  • 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[✎ edit | edit source]

  • 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-])

Example.jpg

  • 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[✎ edit | edit source]

Acid-Base Disorders by Walmsley Koay and Watkinson

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