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. Also calculate the Anion Gap
    3. PaCO2 = (1.5 * Bicarb) + 8
  2. Metabolic Alkalosis
    1. PaCO2 increases 6 mmHg per 10 meq/L bicarbonate rise
  3. Acute Respiratory Acidosis
    1. pH decreases 0.08 for each 10 mmHg PaCO2 increase from 40 mmHg
    2. Bicarbonate increases 1 meq/L per 10 mmHg PaCO2 rise from 40 mmHg
  4. Chronic Respiratory Acidosis
    1. pH decreases 0.03 for each 10 mmHg PaCO2 increase from 40 mmHg
    2. Bicarbonate increases 3.5 to 4 meq/L per 10 mmHg PaCO2 rise from 40 mmHg
  5. Acute Respiratory Alkalosis
    1. pH increases 0.08 for each 10 mmHg PaCO2 decrease from 40 mmHg
    2. Bicarbonate decreases 2 meq/L per 10 mmHg PaCO2 decrease from 40 mmHg
  6. Chronic Respiratory Alkalosis
    1. pH increases 0.03 for each 10 mmHg PaCO2 decrease from 40 mmHg
    2. Bicarbonate decreases 5 meq/L per 10 mmHg PaCO2 decrease from 40 mmHg
      1. Minimum bicarbonate in respiratory compensation is typically 12-15 meq/L

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. Ghosh (2000) Fed Pract p. 23-33
  2. Killu and Sarani (2016) Fundamental Critical Care Support, p. 93-114
  3. Rutecki (Dec 1997) Consultant, p. 3067-74
  4. Rutecki (Jan 1998) Consultant, p. 131-42
  5. Arieff (1993) J Crit Illn 8(2): 224-46 [PubMed]
  6. Narins (1982) Am J Med 72:496 [PubMed]
  7. Narins (1980) Medicine 59:161-95 [PubMed]

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