Arterial Blood Gas

Aka: Arterial Blood Gas, Blood Gas, ABG, PaCO2, PCO2, PaO2, pO2, Arterial Oxygen Partial Pressure

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II. Labs

  1. See Venous Blood Gas
  2. Arterial Blood Gas
    1. See Arterial Puncture
    2. See Arterial Line

III. Indications

  1. Respiratory Failure
    1. Evaluate alveolar ventilation (PaCO2)
    2. Evaluate blood oyxgenation (PaO2)
    3. Evaluate gas exchange (PaO2, PaCO2, A-a Gradient)
    4. Monitor clinical improvement on NIPPV or Mechanical Ventilation (pH, PaCO2)
  2. Metabolic Conditions
    1. Metabolic Acidosis (e.g. Diabetic Ketoacidosis, Unknown Ingestion)
    2. Metabolic Alkalosis

IV. Images

  1. AcidBaseNomogram
    1. acidBaseNomogramPhVsBicarb.png

V. Causes: Primary and Secondary Acid Base Disorders

  1. Respiratory Acidosis (pCO2 increases)
    1. Uncompensated pH decreased = (Normal HCO3)/(Increased pCO2)
    2. Compensated by Metabolic Alkalosis (HCO3 increases)
      1. Compensated pH normalizes = (Increased HCO3)/(Increased pCO2)
  2. Respiratory Alkalosis (pCO2 decreases)
    1. Uncompensated pH increased = (Normal HCO3)/(Decreased pCO2)
    2. Compensated by Metabolic Acidosis (HCO3 decreases)
      1. Compensated pH normalizes = (Decreased HCO3)/(Decreased pCO2)
  3. Metabolic Acidosis (HCO3 decreases)
    1. Uncompensated pH decreased = (Decreased HCO3)/(Normal pCO2)
    2. Compensated by Respiratory Alkalosis (PCO2 decreases)
      1. Compensated pH normalizes = (Decreased HCO3)/(Decreased pCO2)
  4. Metabolic Alkalosis (HCO3 increases)
    1. Uncompensated pH increased = (Increased HCO3)/(Normal pCO2)
    2. Compensated by Respiratory Acidosis (PCO2 increases)
      1. Compensated pH normalizes = (Increased HCO3)/(Increased pCO2)
  5. Mixed Respiratory Acidosis and Metabolic Acidosis (pCO2 increases and HCO3 decreases)
    1. pH decreased = (Decreased HCO3)/(Increased pCO2)
  6. Mixed Respiratory Alkalosis and Metabolic Alkalosis (pCO2 decreases and HCO3 increases)
    1. pH increased = (Increased HCO3)/(Decreased pCO2)

VI. Interpretation: pH

  1. See ABG Interpretation
  2. See Calculated PaCO2
  3. Normal arterial pH = 7.36 to 7.44
  4. ABG and VBG are equivalent in pH accuracy
    1. VBG pH is consistently 0.03 lower than ABG pH
  5. Metabolic Conditions are suggested if
    1. pH changes in the same direction as pCO2
    2. pH is abnormal but pCO2 remains unchanged
  6. Metabolic Conditions related changes in Bicarbonate
    1. Increase pH by 0.01 (with PaCO2 unchanged)
      1. Bicarbonate increases 0.67 meq/L
    2. Decrease pH by 0.01 (with paCO2 unchanged)
      1. Bicarbonate decreases 0.67 meq/L

VII. Interpretation: PaO2 (Partial Pressure of arterial oxygen)

  1. See ABG Interpretation
  2. See A-a Gradient
  3. See Arterial Blood Oxygen Content (Oxygen Saturation, CaO2)
  4. Normal PaO2
    1. Room air at sea level: 80-100 mmHg
    2. Age Adjusted PaO2 = 100 mmHg – 0.3 * AgeY
      1. Where AgeY is age in years
    3. Adjusted for FIO2
      1. Approximate Normal PaO2 = FIO2 * 5
      2. Normal PaO2/FiO2 >400 mmHg
    4. Normal oxygen pressures drop from atmospheric levels to intracellular levels
      1. Atmospheric oxygen: 160 mmHg
      2. Alveolar capillary oxygen (PAO2): 105 mmHg
      3. Arterial oxygen (PaO2): 95 mmHg
      4. Peripheral interstitial oxygen: 40 mmHg
      5. Peripheral intracellular oxygen: 25 mmHg
        1. Peripheral cells need only PO2 of 2 mmHg for adequate functioning
      6. Venous oxygen: 40 mmHg
  5. Hypoxemia
    1. See Hypoxia
    2. PaO2 < 50 mmHg

VIII. Interpretation: PaCO2

  1. See ABG Interpretation
  2. See Calculated PaCO2 (Winter's Formula)
  3. See End Tidal Carbon Dioxide (EtCO2)
  4. Normal PaCO2: 35-45 mmHg
    1. Normal carbon dioxide pressures change little throughout circulation (and are much higher than atmospheric levels)
      1. Atmospheric CO2: 0.3 mmHg
      2. Alveolar and Arterial CO2: 40 mmHg
      3. Interstitial, Intracellular and Venous CO2: 45 mmHg
    2. Respiratory Acidosis with increased PaCO2 consistently affects pH and bicarbonate
      1. Acute: PaCO2 increase of 10 mmHg increases bicarbonate 1 mEq and decreases pH 0.08
      2. Chronic: PaCO2 increase of 10 mmHg decreases pH 0.03
    3. Increased CO2 production is rapidly compensated by increased alveolar ventilation
      1. An elevated PaCO2 suggests inadequate alveolar ventilation (e.g. Bellows Failure)
      2. Doubling alveolar ventilation results in half the PaCO2
        1. Increasing Respiratory Rate from 12 to 24, decreases PaCO2 from 40 to 20 mmHg
      3. Cutting by half the alveolar ventilation results in double the PaCO2
        1. Decreasing Respiratory Rate from 12 to 6, increases PaCO2 from 40 to 80 mmHg
  5. Hypercapnia (Hypercarbia, CO2 Retention)
    1. See Respiratory Failure

IX. Interpretation: Bicarbonate

  1. See ABG Interpretation
  2. Normal Bicarbonate (HCO3-): 22-28 mmHg
  3. Serum bicarbonate is most accurate (compared with ABG or VBG bicarbonate)

X. Interpretation: Conditions Invalidating or Modifying ABG Results

  1. Delayed analysis
    1. Iced Sample maintains values for 1-2 hours
    2. Un-iced sample quickly becomes invalid
      1. PaCO2 rises 3-10 mmHg/hour
      2. PaO2 falls at a rate related to initial value
      3. pH falls modestly
  2. Excessive Heparin
    1. Dilutional effect on results
    2. Decreases bicarbonate and PaCO2
  3. Large Air bubbles not expelled from sample
    1. PaO2 rises 0-30 mmHg
    2. PaCO2 may fall slightly
  4. Fever or Hypothermia
    1. Machine Temperature approaches 37 C
    2. Patient Temperature shifts oxyhemoglobin curve
  5. Hyperventilation or breath holding (due to anxiety)
    1. May lead to erroneous lab results

XI. Resources

  1. Acid-Base Interpreter (fpnotebook)
    1. https://fpnimages.blob.core.windows.net/$web/images/acidBaseApp.html
  2. ABG Interpretation (unbound medicine)
    1. https://anesth.unboundmedicine.com/anesthesia/view/Pocket-ICU-Management/534207/all/Interpretation_of_Arterial_Blood_Gases
  3. ABG Nomogram
    1. acidbaseNomogramPhVsBicarb.png
    2. Code for the ABG nomogram is included in Jupyter Notebook
      1. AcidBaseNomogram.html

XII. 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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