Post on 22-Dec-2015
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
• 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
• 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• 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
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
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
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)