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ABG - Analysis

ABG - Analysis. Dr Jake Turner Anaesthetic CT2. Objectives. pH, Acids and Bases Arterial sampling ABG machine and measured values Acidosis vs Acidaemia, Alkalosis vs Alkalaemia Compensation Cases 1, 2, 3 Advanced concepts. pH and H+. Proton = H⁺ ( pH = –log (H+) )

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ABG - Analysis

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  1. ABG - Analysis Dr Jake Turner Anaesthetic CT2

  2. Objectives • pH, Acids and Bases • Arterial sampling • ABG machine and measured values • Acidosis vs Acidaemia, Alkalosis vs Alkalaemia • Compensation • Cases 1, 2, 3 • Advanced concepts

  3. pH and H+ • Proton = H⁺ (pH = –log (H+)) • pH increases as [H⁺] decreases (ALKALI) • pH decreases as [H⁺] increases (ACIDIC) • Acids (AH) donate protons = AH → A⁻ & H⁺ • Alkalis (B⁻) accept protons = B⁻ + H⁺→ BH • PCO₂ and [HCO₃⁻] dictate the pH of blood

  4. Arterial sampling • Risks • Considerations • Sampling

  5. Arterial sampling • Risks • Pain • Thrombosis (Allen’s) • Considerations • Sampling

  6. Arterial sampling • Risks • Pain • Thrombosis (Allen’s) • Considerations • Gas exchange • Metabolism of RBC • Sampling

  7. Arterial sampling • Risks • Pain • Thrombosis (Allen’s) • Considerations • Gas exchange • Metabolism of RBC • Sampling • Radial, Brachial, Femoral • Arterial lines

  8. ABG Machine • How it works • Measured values • Derived values • What it measures • PO₂, PCO₂, pH, Na⁺, K⁺, Cl⁻, Ca²⁺ • [HCO₃⁻], BE, AG

  9. ABG measured values • Respiratory • Metabolic • Biochemical

  10. ABG measured values • Respiratory • pH, PO₂, PCO₂, HCO₃⁻ • Metabolic • Biochemical

  11. ABG measured values • Respiratory • pH, PO₂, PCO₂, HCO₃⁻ • Metabolic • pH, HCO₃⁻, Lactate, BE, AG • Biochemical

  12. ABG measured values • Respiratory • pH, PO₂, PCO₂, HCO₃⁻ • Metabolic • pH, HCO₃⁻, Lactate, BE, AG • Biochemical • Electrolytes, Lactate

  13. Acidosis vs Acidaemia • Acidosis • Pathological process causing acidaemia • Acidaemia • Presence of excess H⁺ in the blood (low pH) • Causes

  14. Acidosis vs Acidaemia • Acidosis • Pathological process causing acidaemia • Acidaemia • Presence of excess H⁺ in the blood (low pH) • Causes • Respiratory (CO₂ retention) = Type 2 respiratory failure • Metabolic • HCO₃⁻ loss = Diarrhoea • Acid accumulation = DKA, LA , AKI, CKD, toxin ingestion

  15. Alkalosis vs Alkalaemia • Alkalosis • Pathological process causing alkalaemia • Alkalaemia • Presence of H⁺ deficiency in the blood (high pH) • Causes

  16. Alkalosis vs Alkalaemia • Alkalosis • Pathological process causing alkalaemia • Alkalaemia • Presence of H⁺ deficiency in the blood (high pH) • Causes • Respiratory (CO₂ loss) = Tachypnoea • Metabolic • HCO₃⁻ accumulation = Vomiting & Renal HCO₃⁻ retention (Cushing's, contraction alkalosis)

  17. Compensation • Buffering • Compensation • Correction

  18. Compensation • Buffering • Extracellular (HCO₃⁻, Hb, HPO₄²⁻) • Intracellular (HCO₃⁻, Protein, HPO₄²⁻) • Compensation • Correction

  19. Compensation • Buffering • Extracellular (HCO₃⁻, Hb, HPO₄²⁻) • Intracellular (HCO₃⁻, Protein, HPO₄²⁻) • Compensation • Respiratory = CO₂ • Renal = H⁺, HCO₃⁻ • Correction

  20. Compensation • Buffering • Extracellular (HCO₃⁻, Hb, HPO₄²⁻) • Intracellular (HCO₃⁻, Protein, HPO₄²⁻) • Compensation • Respiratory = CO₂ • Renal = H⁺, HCO₃⁻ • Correction • Original insult

  21. Example 1 • 18 year old female • PC: SOB, low GCS, confusion, abdominal pain

  22. Example 1 • pH 6.95 • PCO₂ 2.4kPa • PO₂ 16kPa • HCO₃⁻ 8mmol/L • BE -6 • 18 year old female • PC: SOB, low GCS, confusion, abdominal pain • pH disturbance • Respiratory? • Metabolic? • Compensation? • Diagnosis

  23. Example 2 • 65 year old male • PC: SOB, low GCS, confusion, agitation

  24. Example 2 • pH 7.20 • PCO₂ 8.5kPa • PO₂ 6.5kPa • HCO₃⁻ 35mmol/L • BE +4 • 65 year old male • PC: SOB, low GCS, confusion, agitation • pH disturbance • Respiratory? • Metabolic? • Compensation? • Diagnosis

  25. Example 3 • 82 year old female • PC: abdominal pain, confusion, PR bleeding

  26. Example 3 • pH 7.34 • PCO₂ 2.4kPa • PO₂ 12kPa • HCO₃⁻ 12mmol/L • BE -4 • 82 year old female • PC: abdominal pain, confusion, PR bleeding • pH disturbance • Respiratory? • Metabolic? • Compensation? • Diagnosis

  27. Advanced concepts • Base excess/deficit (-2→+2mEq/L) • Definition = Amount of acid needed to neutralise (pH 7.4) 1L blood • Base excess = HCO₃⁻ accumulation • Base deficit = HCO₃⁻ loss or HCO₃⁻ neutralisation (see anion gap) • Calculation = Henderson-Hasselbalchequation • Causes = Metabolic derangements (primary or compensatory)

  28. Advanced concepts • Base excess/deficit (-2→+2mEq/L) • Definition = Amount of acid needed to neutralise (pH 7.4) 1L blood • Base excess = HCO₃⁻ accumulation • Base deficit = HCO₃⁻ loss or HCO₃⁻ neutralisation (see anion gap) • Calculation = Henderson-Hasselbalchequation • Causes = Metabolic derangements (primary or compensatory) • Anion Gap (8-16mEq/L) • Definition = Σ cations⁺ – Σanions⁻ • Calculation = ([Na⁺] + [K⁺]) – ([Cl⁻] + [HCO₃⁻]) • Causes: • High “unmeasured anion” metabolic acidosis (added acid) • DKA, Lactic acidosis, AKI, CKD, Toxins (methanol, aspirin, Ur, Cy etc)

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