-
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%
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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
-
Ventilation info from ABG
- HYPERventilation
- -- PaCO2 < 35 mmHg
- -- Resp compensation, Hyperactive
- HYPOventilation
- -- PaCO2 > 45 mmHg
- -- Resp depression, Weak respiratory muscle
-
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)
-
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
-
Acid-base status
- pH – acidosis vs alkalosis
- Primary disorder: Respiratory vs metabolic
- Is compensation adequate or as expected ?
- Reveal a previously undetected metabolic disorder
-
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
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Compensation
- Respiratory compensation
- -- Happens immediately
- Renal compensation
- -- Takes 72 hours
- Is the compensation adequate or as expected?
-
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
-
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
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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.)
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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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