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ABG & VBG Interpretation

ABG & VBG Interpretation. Tom Heaps Consultant Acute Physician. ABGs/VBGs: Indications. Unexpected deterioration of sick patient Hypoxaemia on sats monitoring Reduced conscious level Exacerbation of COPD Monitoring of ventilated patient Sepsis Metabolic or electrolyte problem e.g. DKA

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ABG & VBG Interpretation

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  1. ABG & VBG Interpretation Tom Heaps Consultant Acute Physician

  2. ABGs/VBGs: Indications • Unexpected deterioration of sick patient • Hypoxaemia on sats monitoring • Reduced conscious level • Exacerbation of COPD • Monitoring of ventilated patient • Sepsis • Metabolic or electrolyte problem e.g. DKA • Drug Overdose

  3. Step 1: Assess Oxygenation • Normal PaO2 11.5-13.5 kPa ON AIR • Correct hypoxaemia immediately (target SpO2) • Respiratory failure defined as PaO2 <8kPa (SpO2 <93%) • Type I – normal PaCO2 • Type 2 – elevated PaCO2 (ventilatory failure) • Significant respiratory failure may be present despite ‘normal’ or high PaO2 • Predicted PaO2 normally ~10kPa below FiO2 • e.g. 40% venturi, PaO2 should be ~30kPa • Document oxygen use on ABG result!

  4. Step 2: is there an acidosis or alkalosis? • Look at the pH (normal range 7.35-7.45) • pH <7.35 = acidaemia/acidosis • pH >7.45 = alkaemia/alkalosis • Acidosis or alkalosis may still be present with a ‘normal’ pH if the body has already buffered = compensation • CO2 is acidic and HCO3- is alkaline • Normal compensation for acidosis is to decrease CO2 (rapid)and increase HCO3- (takes longer) • Normal compensation for alkalosis is to decrease HCO3- (and increase CO2)

  5. Step 3: What is the respiratory component? • Normal range for PaCO2 is 4.5-6.0kPa • Acidosis (pH <7.35) • PaCO2 >6.0kPa = respiratory acidosis • PaCO2 <6.0kPa = metabolic acidosis • Alkalosis (pH >7.45) • PaCO2 >4.5kPa = metabolic alkalosis • PaCO2 <4.5kPa = respiratory alkalosis

  6. Step 4: What is the metabolic component? • Normal range for HCO3- is 22-26mmol/L • Normal range for base excess (BE) is -2 to +2 • Acidosis (pH <7.35) • HCO3- <22mmol/L (BE < -2) = metabolic acidosis • HCO3- >22mmol/L (BE > -2) = respiratory acidosis • Alkalosis (pH >7.45) • HCO3->26mmol/L (BE > +2) = metabolic alkalosis • HCO3- <26mmol/L (BE < +2) = respiratory alkalosis

  7. Step 5: Combine the information from previous steps • What is the primary disturbance? • Is there any compensation? • Is there a mixed picture? • e.g. in acidosis (pH <7.35) • PaCO2 >6.0kPa with HCO3- >26mmol/L = • Respiratory acidosis with partial metabolic compensation • PaCO2 >6.0kPa with HCO3- <22mmol/L = • Mixed respiratory and metabolic acidosis

  8. Other information from ABG • Lactate (normal range 0-2mmol/L) • Elevated levels often associated with acidosis • tissue hypoperfusion/anaerobic metabolism, liver/renal failure or drugs (e.g. metformin) • Degree of elevation correlates directly with mortality in sepsis • Response to fluids also important • Haemoglobin (Hb) • Potassium (K+) and Sodium (Na+) • Glucose (not on AMU analyzer)

  9. Case Scenario 1: 78-year-old male admitted with IECOPD becomes more drowsy and confused whilst on AMU. Oxygen is being delivered via a non-rebreathe bag and mask.

  10. Case Scenario 1: ABG Results FiO2 0.4 (40%) 0.21 (air) pH 7.21 7.35-7.45 PaO2 15.7kPa 11.5-13.5kPa PaCO2 8.9kPa 4.5-6.0kPa HCO3- 31mmol/L 22-26mmol/L BE +8 -2 to +2 Lactate 1.8mmol/L <2.0mmol/L

  11. Case Scenario 1: Interpretation • Relative hypoxaemia (PaO2 should be ~30kPa) • Respiratory failure = type 2 (elevated PaCO2) • Acidosis (pH <7.35) • High PaCO2 = respiratory acidosis • High HCO3- = partial metabolic compensation (likely chronic) • Acute-on-chronic type 2 respiratory failure with respiratory acidosis and partial metabolic compensation

  12. Case Scenario 2: 29-year-old female with type 1 diabetes. Admitted with 48h history of diarrhoea and vomiting. Rapid respiratory rate. CBG 27mmol/L and blood ketones 4.9mmol/L.

  13. Case Scenario 2: ABG Results FiO2 0.21 (21%) 0.21 (air) pH 7.36 7.35-7.45 PaO2 13.7kPa 11.5-13.5kPa PaCO2 3.2kPa 4.5-6.0kPa HCO3- 14mmol/L 22-26mmol/L BE -12 -2 to +2 Lactate 2.8mmol/L <2.0mmol/L

  14. Case Scenario 2: Interpretation • Normal PaO2 on air i.e. no respiratory failure • Normal pH however; • Low HCO3- and BE with high lactate and ketones = metabolic acidosis • Low PaCO2 = respiratory compensation (Kussmaul respiration) • Fully compensated metabolic acidosis due to DKA

  15. Case Scenario 3: 36-year-old male with alcohol dependence and ALD. Admitted to AMU following a staggered co-codamol overdose. GCS 8/15 with small pupils and respiratory rate of 10/min.

  16. Case Scenario 3: ABG Results FiO2 0.35 (35%) 0.21 (air) pH 7.19 7.35-7.45 PaO2 11.7kPa 11.5-13.5kPa PaCO2 7.5kPa 4.5-6.0kPa HCO3- 17mmol/L 22-26mmol/L BE -8 -2 to +2 Lactate 4.2mmol/L <2.0mmol/L

  17. Case Scenario 3: Interpretation • Relative hypoxaemia (PaO2 should be ~25kPa) • Respiratory failure = type 2 (elevated PaCO2) • Acidosis (pH <7.35) • High PaCO2 = respiratory acidosis • Low HCO3- and BE = metabolic acidosis • Mixed respiratory and metabolic (lactic) acidosis • Hypoventilation due to reduced GCS (hepatic encephalopathy) and opiate overdose • Lactic acidosis due to liver failure/drug overdose

  18. Case Scenario 4: 56-year-old female admitted with acute breathlessness. Recent marital stress. Heart rate 120/min, respiratory rate 28/min, SpO2 96% on air. Chest clear. Looks anxious ++

  19. Case Scenario 4: ABG Results FiO2 0.21 (40%) 0.21 (air) pH 7.49 7.35-7.45 PaO2 10.1kPa 11.5-13.5kPa PaCO2 3.8kPa 4.5-6.0kPa HCO3- 25mmol/L 22-26mmol/L BE +2 -2 to +2 Lactate 0.7mmol/L <2.0mmol/L

  20. Case Scenario 4: Interpretation • Hyperventilation with respiratory alkalosis • No metabolic compensation • May be due to anxiety/panic attack but relative hypoxaemia in this case suggests alternative cause e.g. PE, pneumonia, acute asthma

  21. VBG • Cannot be used to assess pO2 or pCO2 • Normal pCO2 on VBG excludes hypercapnia • Good correlation with ABG for other parameters • pH, lactate, BE, HCO3-, electrolytes, Hb • except if patient shocked/peri-arrest • VBG usually adequate in all other situations • Obtain VBG in all acutely unwell patients • Especially sepsis, DKA, UGIB, AKI, overdose

  22. VBG: caveats • Sample must not be shaken and should be analysed immediately (<10min) to prevent haemolysis • lowers pH and increases K+ • K+ up to 0.5mmol lower than lab value • Na+ up to 6mmol lower than lab value • Hb 5g/L higher than lab value on average

  23. Summary - Any Questions?

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