Arterial Blood Gas (ABG)

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josephplam
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10016
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Arterial Blood Gas (ABG)
Updated:
2010-03-25 02:07:04
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Arterial Blood Gas (ABG)
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  1. Arterial Blood Gas (ABG):
    pH/PaCO2/PO2/HCO3- calculated from Henderson-Hasselbach /(O2
    saturation)
    • Arterial Blood Gas (ABG)
    • 7.40 (7.35-7.45)/ 35-45 mmHg/ 80-100 mmHg/ 23-28 mEq/L/ 95%

    • Mixed Venous Blood Gas (VBG)
    • 7.38 (7.33-7.43)/ 45-51 mmHg/ 35-40 mmHg/ 24-28 mEq/L/ 70-75%
  2. What can we learn from ABG ?
    • Ventilation – PaCO2
    • Oxygenation – PaO2, SaO2
    • Acid/base status - Metabolic vs respiratory
    • Is the compensation adequate or as expected ?
    • To reveal a previously undetected metabolic disorder
  3. Ventilation info from ABG
    • HYPERventilation
    • -- PaCO2 < 35 mmHg
    • -- Resp compensation, Hyperactive
    • HYPOventilation
    • -- PaCO2 > 45 mmHg
    • -- Resp depression, Weak respiratory muscle
  4. Oxygenation info from ABG
    • Look at PaO2, O2 sat
    • Hypoxemia:
    • -- PaO2 <80 mmHg
    • -- O2 sat< 92%
    • To assess the integrity of Alveolar-arterial oxygen gradient,
    • calculate A-a gradient (Alveolar-arterial Oxygen gradient)
  5. Calculate A-a gradient (Alveolar-arterial oxygen gradient)
    • A-a gradient
    • = PAlveolar O2 – Parterial O2
    • = PAO2 – PaO2
    • **PAO2 = (Patm – Pwater) FiO2 – PaCO2/0.8
    • = (760 mmHg – 47 mmHg) FiO2 – PaCO2/0.8 (FiO2= Fraction of inspired O2= 21% (0.21) if room air)
    • Expected A-a gradient
    • < (Age / 4) + 4
    • If Calculated A-a gradient < Expected A-a gradient,
    • then no problem with transfer of O2 from alveoli to capillaries

    • If Calculated A-a gradient > Expected A-a gradient, then there is problem with transfer of O2 from alveoli to capillaries
    • eg. diffusion defects, V/Q mismatch or R-to-L shunting
  6. Acid-base status
    • pH – acidosis vs alkalosis
    • Primary disorder: Respiratory vs metabolic
    • Is compensation adequate or as expected ?
    • Reveal a previously undetected metabolic disorder
  7. Acidosis vs Alkalosis (differentiating the primary disorder)
    • pH is < 7.40 – acidosis
    • PaCO2 > 40 mmHg – Respiratory acidosis
    • Serum HCO3-- < 23 mEq/L – Metabolic acidosis

    • pH is > 7.40 – alkalosis
    • PaCO2 < 40 mmHg – Respiratory alkalosis
    • Serum HCO3-- > 23 mEq/L – Metabolic alkalosis
  8. Compensation
    • Respiratory compensation
    • -- Happens immediately
    • Renal compensation
    • -- Takes 72 hours
    • Is the compensation adequate or as expected?
  9. Is the compensation adequate or as expected ? (acidosis)
    • Metabolic acidosis-- Respiratory compensation.
    • Winter’s formula: PaCO2= 1.5(HCO3-) + 8 (+/-2)

    • Respiratory acidosis
    • -- Acute: Change in HCO3- = 0.1 x change in PaCO2
    • -- Chronic: Change in HCO3- = 0.35 x change in PaCO2
  10. Is the compensation adequate or as expected ? (alkalosis)
    • Metabolic alkalosis -- Respiratory compensation (PaCO2 rarely rises >50 mmHg)
    • -- Change in PaCO2= 0.6 (change in HCO3-)

    • Respiratory alkalosis
    • Acute: Change in HCO3- = 0.2 x change in PaCO2
    • Chronic: Change in HCO3- = 0.5 x change in PaCO2
  11. Calculate Anion Gap for everybody
    Anion Gap= Na – Cl – HCO3

    Correct Anion Gap if low serum albumin

    • Equation of Figge:
    • Corrected Anion Gap = Anion Gap + 2.5 (4.4 – albumin g/dL) (Do NOT correct Na for
    • hyperglycemia when calculating Anion Gap.)
  12. Reveal a previously undetected metabolic disorder
    • Calculate an anion gap
    • Normal anion gap= 12

    • Calculate “delta gap”
    • = Calculated anion gap – normal anion gap

    Corrected (serum) HCO3 = serum HCO3 + “delta gap”

    • If corrected HCO3 < 23 – additional non-anion gap metabolic acidosis
    • If corrected HCO3 > 23 – additional metabolic alkalosis

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