Download - Acid-base Disorders
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Acid-base DisordersAcid-base Disorders
Dr Michael Murphy FRCP Edin FRCPathDr Michael Murphy FRCP Edin FRCPathSenior Lecturer in Biochemical MedicineSenior Lecturer in Biochemical Medicine
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Outline of lectureOutline of lecture
• Basic concepts
• Definitions
• Respiratory problems
• Metabolic problems
• How to interpret blood gases
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QuestionsQuestions
• What is being regulated?
• Why the need for regulation?
• Buffering: why is bicarbonate so important?
• How is acid-base status assessed?
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What is being regulated?What is being regulated?
Hydrogen ion concentration ([H+], pH)
• 60 mmol H+ produced by metabolism daily
• Need to excrete most or all of this
• So normal urine profoundly acidic
• [H+] 35 to 45 nmol/L…regulation thus very tight!
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Buffering of HBuffering of H++
Is only a temporary measure (“sponge”)
• H+ + HCO3- H2CO3 CO2 + H2O
• H+ + Hb- HHb
• H+ + HPO42- H2PO4
-
• H+ + NH3 NH4
+
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Why is bicarbonate so important?Why is bicarbonate so important?
H+ + HCO3- H2CO3 CO2 + H2O
• Other buffer systems reach equilibrium
• Carbonic acid (H2CO3) removed as CO2
• Only limit is initial concentration of HCO3-
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Problem: how do we recover bicarbonate?Problem: how do we recover bicarbonate?
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Problem: how do we regenerate bicarbonate?Problem: how do we regenerate bicarbonate?
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A wee trip down memory lane!A wee trip down memory lane!
H+ + HCO3- H2CO3 CO2 + H2O
[H+] = K[H2CO3]
[HCO3-]
[H+] pCO2
[HCO3-]
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What are the ‘arterial blood gases’?What are the ‘arterial blood gases’?
• H+
• pCO2
• HCO3-
• pO2
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Why do they have to be arterial?Why do they have to be arterial?
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A word about units…A word about units…
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A word about units…A word about units…
Reference interval
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……and a bit of terminologyand a bit of terminology
• Acidosis: increased [H+]
• Alkalosis: decreased [H+]
• Respiratory: the primary change is in pCO2
• Metabolic: the primary change is in HCO3-
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So you can have…So you can have…
• Respiratory acidosis: [H+] due to pCO2
• Respiratory alkalosis: [H+] due to pCO2
• Metabolic acidosis: [H+] due to HCO3-
• Metabolic alkalosis: [H+] due to HCO3-
[H+] pCO2
[HCO3-]
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Another word…about compensation!Another word…about compensation!
H+ + HCO3- H2CO3 CO2 + H2O
• When you’ve got too much H+, lungs blow off CO2
• When you can’t blow off CO2, kidneys try to get rid of H+
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Respiratory compensation for metabolic acidosisRespiratory compensation for metabolic acidosis
H+ + HCO3
- H2CO3 CO2 + H2O
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Metabolic compensation for respiratory acidosisMetabolic compensation for respiratory acidosis
H+ + HCO3- H2CO3 CO2 + H2O
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Metabolic compensation for respiratory acidosisMetabolic compensation for respiratory acidosis
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Patterns of compensationPatterns of compensation
[H+] pCO2
[HCO3-]
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Respiratory disordersRespiratory disorders
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Respiratory acidosisRespiratory acidosis
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Compensation for respiratory acidosisCompensation for respiratory acidosis
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Causes of respiratory acid-base disordersCauses of respiratory acid-base disorders
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Metabolic disordersMetabolic disorders
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Metabolic disorders and their compensationMetabolic disorders and their compensation
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Causes of metabolic acid-base disordersCauses of metabolic acid-base disorders
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Putting it all together…Putting it all together…
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First, identify the primary problem…First, identify the primary problem…
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……then, look to see if there’s compensationthen, look to see if there’s compensation
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Let’s apply this to a few examples…Let’s apply this to a few examples…
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Reference intervals for arterial blood gasesReference intervals for arterial blood gases
• H+ 36-44 nmol/L
• pCO2 4.7-6.1 kPa
• HCO3- 22-30 mmol/L
• pO2 11.5-14.8 kPa
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Case 1Case 1
• 31yo woman during acute asthmatic attack.
• [H+] = 24 nmol/L• pCO2 = 2.5 kPa
• [HCO3-] = 22 mmol/L
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Case 1Case 1
• 31yo woman during acute asthmatic attack.
• [H+] = 24 nmol/L• pCO2 = 2.5 kPa
• [HCO3-] = 22 mmol/L
• Uncompensated respiratory alkalosis
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Case 2Case 2
• 23yo man with dyspepsia & excess alcohol who’s been vomiting for 24h.
• [H+] = 28 nmol/L• pCO2 = 7.2 kPa
• [HCO3-] = 48 mmol/L
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Case 2Case 2
• 23yo man with dyspepsia & excess alcohol who’s been vomiting for 24h.
• [H+] = 28 nmol/L• pCO2 = 7.2 kPa
• [HCO3-] = 48 mmol/L
• Partially compensated metabolic alkalosis
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Case 3Case 3
• 50yo man with 2 week history of vomiting and diarrhoea. Dry. Deep noisy breathing.
• [H+] = 64 nmol/L• pCO2 = 2.8 kPa
• [HCO3-] = 8 mmol/L
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Case 3Case 3
• 50yo man with 2 week history of vomiting and diarrhoea. Dry. Deep noisy breathing.
• [H+] = 64 nmol/L• pCO2 = 2.8 kPa
• [HCO3-] = 8 mmol/L
• Partially compensated metabolic acidosis
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Case 4Case 4
• 71yo man with stable COPD.
• [H+] = 44 nmol/L• pCO2 = 9.5 kPa
• [HCO3-] = 39 mmol/L
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Case 4Case 4
• 71yo man with stable COPD.
• [H+] = 44 nmol/L• pCO2 = 9.5 kPa
• [HCO3-] = 39 mmol/L
• Compensated respiratory acidosis
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Final thoughtsFinal thoughts
• ALWAYS match blood gases to the history
• You can’t over-compensate physiologically
• Can ‘over-compensate’ by IV bicarbonate or artificial ventilation
(but that’s not really compensation!)