II. Technique: Step 1 - Determine primary abnormality

  1. Determine Acidosis versus alkalosis
    1. pH <7.35: Acidosis
    2. pH >7.45: Alkalosis
  2. Determine Metabolic versus Respiratory
    1. Primary Metabolic Disorder
      1. pH changes in same direction as bicarbonate, pCO2
      2. Metabolic Acidosis
        1. Serum ph decreased
        2. Serum bicarbonate and paCO2 decreased
      3. Metabolic Alkalosis
        1. Serum ph increased
        2. Serum bicarbonate and paCO2 increased
    2. Primary Respiratory Disorder
      1. pH changes in opposite direction bicarbonate, pCO2
      2. Respiratory Acidosis
        1. Serum ph decreased
        2. Serum bicarbonate and paCO2 increased
      3. Respiratory Alkalosis
        1. Serum ph increased
        2. Serum bicarbonate and paCO2 decreased

III. Technique: Step 2 - Sharpen the diagnosis

  1. Calculate the Anion Gap
    1. Helpful in Metabolic Acidosis
    2. Helpful in mixed acid-base disorders
  2. Calculate Osmolar Gap
    1. Helpful in Metabolic Acidosis
  3. Calculate Urinary Anion Gap
    1. Helpful in Non-Anion Gap Metabolic Acidosis
    2. Distinguishes renal from extra-renal cause

IV. Technique: Step 3 - Determine Compensation

  1. Metabolic Acidosis
    1. PaCO2 decreases 1.2 mmHg per 1 meq/L bicarbonate fall
  2. Metabolic Alkalosis
    1. PaCO2 increases 6 mmHg per 10 meq/L bicarbonate rise
  3. Acute Respiratory Acidosis
    1. Bicarbonate increases 1 meq/L per 10 mmHg PaCO2 rise
  4. Chronic Respiratory Acidosis
    1. Bicarbonate increases 4 meq/L per 10 mmHg PaCO2 rise
  5. Acute Respiratory Alkalosis
    1. Bicarbonate decreases 2 meq/L per 10 mmHg PaCO2 fall
  6. Chronic Respiratory Alkalosis
    1. Bicarbonate decreases 4 meq/L per 10 mmHg PaCO2 fall

V. Technique: Step 4 - Define Associated Abnormalities

  1. Calculated PaCO2
    1. Useful in High Anion Gap Metabolic Acidosis
    2. Defines concurrent respiratory abnormalities
  2. Excess Anion Gap
    1. EAG > 30 mEq/L: Metabolic Alkalosis present
    2. EAG < 23 mEq/L: Metabolic Acidosis present

VI. References

  1. Arieff (1993) J Crit Illn 8(2): 224-46 [PubMed]
  2. Narins (1982) Am J Med 72:496 [PubMed]
  3. Narins (1980) Medicine 59:161-95 [PubMed]
  4. Ghosh (2000) Fed Pract p. 23-33
  5. Rutecki (Dec 1997) Consultant, p. 3067-74
  6. Rutecki (Jan 1998) Consultant, p. 131-42

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