ABG Interpretation

Aka: ABG Interpretation, Arterial Blood Gas Interpretation

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II. Images

  1. AcidBaseNomogram
    1. acidBaseNomogramPhVsBicarb.png

III. 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 (increased acid other than CO2 or bicarbonate loss)
        1. Serum ph decreased
        2. Serum bicarbonate decreased
        3. pCO2 decreased (when compensated with increased Respiratory Rate)
      3. Metabolic Alkalosis (e.g. protracted Vomiting with gastric acid loss)
        1. Serum ph increased
        2. Serum bicarbonate increased
        3. pCO2 increased (when compensated)
    2. Primary Respiratory Disorder
      1. pH changes in opposite direction bicarbonate, pCO2
      2. Respiratory Acidosis (e.g. apnea or Respiratory Failure)
        1. Serum ph decreased
        2. pCO2 increased
        3. Serum bicarbonate increased (when compensated)
      3. Respiratory Alkalosis (e.g. Hyperventilation)
        1. Serum ph increased
        2. pCO2 decreased
        3. Serum bicarbonate decreased (when compensated)
    3. Mixed Disorders
      1. Serum Bicarbonate and pCO2 are altered in opposite directions
      2. Mixed Respiratory Acidosis and Metabolic Acidosis
        1. Serum pH decreased
        2. pCO2 increased
        3. Serum bicarbonate decreased
      3. Mixed Respiratory Alkalosis and Metabolic Alkalosis
        1. Serum pH increased
        2. pCO2 decreased
        3. Serum bicarbonate increased

IV. Technique: Step 2 - Sharpen the diagnosis

  1. Calculate the Anion Gap
    1. Useful in Metabolic Acidosis
    2. Useful in mixed acid-base disorders
    3. Anion Gap = sNa - sCl - sHCO3
      1. Where sNa = Serum Sodium, sCl=Serum Chloride, sHCO3=Serum Bicarbonate
      2. Normal Anion Gap = 8-12 meq/L
    4. Anion Gap is maintained by near balance of key cations (sNa+) and key anions (sCl-, sHCO3-)
      1. In Non-Anion Gap Metabolic Acidosis, only measured cations and anions are affected
        1. In Diarrhea, bicarbonate is lost and compensated by chloride increase
      2. In Anion Gap Metabolic Acidosis, unmeasured anions are increased
        1. Increased Lactic Acid or Ketoacids, for example, result in a significant Anion Gap
  2. Calculate Osmolar Gap
    1. Useful in Metabolic Acidosis with Anion Gap
  3. Calculate Urinary Anion Gap
    1. Useful in Non-Anion Gap Metabolic Acidosis
    2. Distinguishes renal from extra-renal cause

V. 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. Metabolic compensation after a few days of Respiratory Acidosis
    2. pH decreases 0.03 for each 10 mmHg PaCO2 increase from 40 mmHg
    3. 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. Metabolic compensation after a few days of Respiratory Alkalosis
    2. pH increases 0.03 for each 10 mmHg PaCO2 decrease from 40 mmHg
    3. 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

VI. 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

VII. Resources

  1. Acid-Base Interpreter
    1. https://fpnimages.blob.core.windows.net/$web/images/acidBaseApp.html

VIII. 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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