ABG - ANALYSISDr Jake Turner

Anaesthetic CT2

Objectives1. 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

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⁻

Arterial sampling• Risks

• •

• Considerations• •

• Sampling• •

Arterial sampling• Risks

• Pain• Thrombosis (Allen’s)

• Considerations• •

• Sampling• •

Arterial sampling• Risks

• Pain• Thrombosis (Allen’s)

• Considerations• Gas exchange• Metabolism of RBC

• Sampling• •

Arterial sampling• Risks

• Pain• Thrombosis (Allen’s)

• Considerations• Gas exchange• Metabolism of RBC

• Sampling• Radial, Brachial, Femoral• Arterial lines

ABG Machine• How it works

• Measured values• Derived values

• What it measures• PO₂, PCO₂, pH, Na , K , Cl , Ca²⁺ ⁺ ⁻ ⁺• [HCO₃ ], BE, AG⁻

ABG measured values• Respiratory

•

• Metabolic•

• Biochemical•

ABG measured values• Respiratory

• pH, PO₂, PCO₂, HCO₃⁻

• Metabolic•

• Biochemical•

ABG measured values• Respiratory

• pH, PO₂, PCO₂, HCO₃⁻

• Metabolic• pH, HCO₃ , Lactate, BE, AG⁻

• Biochemical•

ABG measured values• Respiratory

• pH, PO₂, PCO₂, HCO₃⁻

• Metabolic• pH, HCO₃ , Lactate, BE, AG⁻

• Biochemical• Electrolytes, Lactate

Acidosis vs Acidaemia• Acidosis

• Pathological process causing acidaemia

• Acidaemia• Presence of excess H in the blood (⁺ low pH)

• Causes• •

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

Alkalosis vs Alkalaemia• Alkalosis

• Pathological process causing alkalaemia

• Alkalaemia• Presence of H ⁺ deficiency in the blood (high pH)

• Causes• •

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)

Compensation

1. Buffering• •

2. Compensation• •

3. Correction•

Compensation

1. Buffering• Extracellular (HCO₃ , Hb, HPO₄² )⁻ ⁻• Intracellular (HCO₃ , Protein, HPO₄² )⁻ ⁻

2. Compensation• •

3. Correction•

Compensation

1. Buffering• Extracellular (HCO₃ , Hb, HPO₄² )⁻ ⁻• Intracellular (HCO₃ , Protein, HPO₄² )⁻ ⁻

2. Compensation• Respiratory = CO₂ • Renal = H , HCO₃⁺ ⁻

3. Correction•

Compensation

1. Buffering• Extracellular (HCO₃ , Hb, HPO₄² )⁻ ⁻• Intracellular (HCO₃ , Protein, HPO₄² )⁻ ⁻

2. Compensation• Respiratory = CO₂ • Renal = H , HCO₃⁺ ⁻

3. Correction• Original insult

Example 1

• 18 year old female• PC: SOB, low GCS, confusion, abdominal pain

Example 1

opH 6.95oPCO₂ 2.4kPaoPO₂ 16kPaoHCO₃ 8mmol/L⁻oBE -6

• 18 year old female• PC: SOB, low GCS, confusion, abdominal pain

o pH disturbanceo Respiratory?o Metabolic?o Compensation?o Diagnosis

Example 2

• 65 year old male• PC: SOB, low GCS, confusion, agitation

Example 2

opH 7.20oPCO₂ 8.5kPaoPO₂ 6.5kPaoHCO₃ 35mmol/L⁻oBE +4

• 65 year old male• PC: SOB, low GCS, confusion, agitation

o pH disturbanceo Respiratory?o Metabolic?o Compensation?o Diagnosis

Example 3

• 82 year old female• PC: abdominal pain, confusion, PR bleeding

Example 3

• 82 year old female• PC: abdominal pain, confusion, PR bleeding

o pH disturbanceo Respiratory?o Metabolic?o Compensation?o Diagnosis

opH 7.34oPCO₂ 2.4kPaoPO₂ 12kPaoHCO₃ 12mmol/L⁻oBE -4

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-Hasselbalch equation• Causes = Metabolic derangements (primary or compensatory)

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-Hasselbalch equation• Causes = Metabolic derangements (primary or compensatory)

• Anion Gap (8-16mEq/L)• Definition = Σ cations – ⁺ Σ anions⁻• Calculation = ([Na ] + [K ]) – ([Cl ] + [HCO₃ ])⁺ ⁺ ⁻ ⁻• Causes:

• High “unmeasured anion” metabolic acidosis (added acid)• DKA, Lactic acidosis, AKI, CKD, Toxins (methanol, aspirin, Ur, Cy etc)