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Acid-base Disorders. Dr Michael Murphy FRCP Edin FRCPath Senior Lecturer in Biochemical Medicine. Outline of lecture. Basic concepts Definitions Respiratory problems Metabolic problems How to interpret blood gases. Questions. What is being regulated? Why the need for regulation?
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Acid-base Disorders Dr Michael Murphy FRCP Edin FRCPath Senior Lecturer in Biochemical Medicine
Outline of lecture • Basic concepts • Definitions • Respiratory problems • Metabolic problems • How to interpret blood gases
Questions • What is being regulated? • Why the need for regulation? • Buffering: why is bicarbonate so important? • How is acid-base status assessed?
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!
Buffering of H+ Is only a temporary measure (“sponge”) • H+ + HCO3- H2CO3 CO2 + H2O • H+ + Hb- HHb • H+ + HPO42- H2PO4- • H+ + NH3 NH4+
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-
A wee trip down memory lane! H+ + HCO3- H2CO3 CO2 + H2O [H+] = K[H2CO3] [HCO3-] [H+] pCO2 [HCO3-]
What are the ‘arterial blood gases’? • H+ • pCO2 • HCO3- • pO2
A word about units… Reference interval
…and a bit of terminology • Acidosis: increased [H+] • Alkalosis: decreased [H+] • Respiratory: the primary change is in pCO2 • Metabolic: the primary change is in HCO3-
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-]
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+
Respiratory compensation for metabolic acidosis H+ + HCO3- H2CO3 CO2 + H2O
Metabolic compensation for respiratory acidosis H+ + HCO3- H2CO3 CO2 + H2O
Patterns of compensation [H+] pCO2 [HCO3-]
Reference 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
31yo woman during acute asthmatic attack. [H+] = 24 nmol/L pCO2 = 2.5 kPa [HCO3-] = 22 mmol/L Case 1
31yo woman during acute asthmatic attack. [H+] = 24 nmol/L pCO2 = 2.5 kPa [HCO3-] = 22 mmol/L Uncompensated respiratory alkalosis Case 1
23yo man with dyspepsia & excess alcohol who’s been vomiting for 24h. [H+] = 28 nmol/L pCO2 = 7.2 kPa [HCO3-] = 48 mmol/L Case 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 Case 2
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 Case 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 Case 3
71yo man with stable COPD. [H+] = 44 nmol/L pCO2 = 9.5 kPa [HCO3-] = 39 mmol/L Case 4
71yo man with stable COPD. [H+] = 44 nmol/L pCO2 = 9.5 kPa [HCO3-] = 39 mmol/L Compensated respiratory acidosis Case 4
Final 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!)