Lab

Arterial Blood Gas

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

  • Labs
  • Images
  1. AcidBaseNomogram
    1. acidbaseNomogramPhVsBicarb.png
  • 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
  • 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)
  • Interpretation
  • pH
  1. See Calculated PaCO2
  2. Normal arterial pH = 7.36 to 7.44
  3. Metabolic Conditions are suggested if
    1. pH changes in the same direction as pCO2
    2. pH is abnormal but pCO2 remains unchanged
  4. 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
  • Interpretation
  • PaO2 (partial pressure of arterial oxygen)
  1. See A-a Gradient
  2. See Arterial Blood Oxygen Content (CaO2)
  3. 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. Hypoxemia
    1. PaO2 < 50 mmHg
  • 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