1. ABG - ANALYSIS Dr Jake Turner Anaesthetic CT2

2. Objectives 1. pH, Acids and Bases 2. Arterial sampling 3. ABG machine and measured values 4. Acidosis vs Acidaemia, Alkalosis vs Alkalaemia 5. Compensation 6. Cases 1, 2, 3 7. 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 PCO ₂ and [HCO ₃⁻ ] 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 PO ₂, PCO ₂, pH, Na ⁺, K ⁺, Cl ⁻, Ca² ⁺ [HCO ₃⁻ ], BE, AG

9. ABG measured values Respiratory Metabolic Biochemical

10. ABG measured values Respiratory pH, PO ₂, PCO ₂, HCO ₃⁻ Metabolic Biochemical

11. ABG measured values Respiratory pH, PO ₂, PCO ₂, HCO ₃⁻ Metabolic pH, HCO ₃⁻, Lactate, BE, AG Biochemical

12. ABG measured values Respiratory pH, PO ₂, PCO ₂, HCO ₃⁻ Metabolic pH, HCO ₃⁻, 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 (CO ₂ retention) = Type 2 respiratory failure Metabolic HCO ₃⁻ 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 (CO ₂ loss) = Tachypnoea Metabolic HCO ₃⁻ accumulation = Vomiting & Renal HCO ₃⁻ retention (Cushing's, contraction alkalosis)

17. Compensation 1. Buffering 2. Compensation 3. Correction

18. Compensation 1. Buffering Extracellular (HCO ₃⁻, Hb, HPO ₄ ² ⁻ ) Intracellular (HCO ₃⁻, Protein, HPO ₄ ² ⁻ ) 2. Compensation 3. Correction

19. Compensation 1. Buffering Extracellular (HCO ₃⁻, Hb, HPO ₄ ² ⁻ ) Intracellular (HCO ₃⁻, Protein, HPO ₄ ² ⁻ ) 2. Compensation Respiratory = CO ₂ Renal = H ⁺, HCO ₃⁻ 3. Correction

20. Compensation 1. Buffering Extracellular (HCO ₃⁻, Hb, HPO ₄ ² ⁻ ) Intracellular (HCO ₃⁻, Protein, HPO ₄ ² ⁻ ) 2. Compensation Respiratory = CO ₂ Renal = H ⁺, HCO ₃⁻ 3. Correction Original insult

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

22. Example 1 o pH 6.95 o PCO ₂ 2.4kPa o PO ₂ 16kPa o HCO ₃⁻ 8mmol/L o BE -6 18 year old female PC: SOB, low GCS, confusion, abdominal pain o pH disturbance o Respiratory? o Metabolic? o Compensation? o Diagnosis

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

24. Example 2 o pH 7.20 o PCO ₂ 8.5kPa o PO ₂ 6.5kPa o HCO ₃⁻ 35mmol/L o BE +4 65 year old male PC: SOB, low GCS, confusion, agitation o pH disturbance o Respiratory? o Metabolic? o Compensation? o Diagnosis

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

26. Example 3 82 year old female PC: abdominal pain, confusion, PR bleeding o pH disturbance o Respiratory? o Metabolic? o Compensation? o Diagnosis o pH 7.34 o PCO ₂ 2.4kPa o PO ₂ 12kPa o HCO ₃⁻ 12mmol/L o BE -4

27. Advanced concepts Base excess/deficit (-2 → +2mEq/L) Definition = Amount of acid needed to neutralise (pH 7.4) 1L blood Base excess = HCO ₃⁻ accumulation Base deficit = HCO ₃⁻ loss or HCO ₃⁻ neutralisation (see anion gap) Calculation = Henderson-Hasselbalch equation 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 = HCO ₃⁻ accumulation Base deficit = HCO ₃⁻ loss or HCO ₃⁻ neutralisation (see anion gap) Calculation = Henderson-Hasselbalch equation Causes = Metabolic derangements (primary or compensatory) Anion Gap (8-16mEq/L) Definition = Σ cations ⁺ – Σ anions ⁻ Calculation = ([Na ⁺ ] + [K ⁺ ]) – ([Cl ⁻ ] + [HCO ₃⁻ ]) Causes: High “unmeasured anion” metabolic acidosis (added acid) DKA, Lactic acidosis, AKI, CKD, Toxins (methanol, aspirin, Ur, Cy etc)