Acid-base Disorders

1. Acid-base Disorders Dr Michael Murphy FRCP Edin FRCPath Senior Lecturer in Biochemical Medicine

2. Outline of lecture • Basic concepts • Definitions • Respiratory problems • Metabolic problems • How to interpret blood gases

4. Questions • What is being regulated? • Why the need for regulation? • Buffering: why is bicarbonate so important? • How is acid-base status assessed?

5. 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!

6. Buffering of H+ Is only a temporary measure (“sponge”) • H+ + HCO3- H2CO3  CO2 + H2O • H+ + Hb- HHb • H+ + HPO42-  H2PO4- • H+ + NH3 NH4+

7. 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-

8. Problem: how do we recover bicarbonate?

9. Problem: how do we regenerate bicarbonate?

10. A wee trip down memory lane! H+ + HCO3- H2CO3  CO2 + H2O [H+] = K[H2CO3] [HCO3-] [H+]  pCO2 [HCO3-]

11. What are the ‘arterial blood gases’? • H+ • pCO2 • HCO3- • pO2

12. Why do they have to be arterial?

13. A word about units…

14. A word about units… Reference interval

15. …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-

16. 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-]

17. 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+

18. Respiratory compensation for metabolic acidosis H+ + HCO3- H2CO3  CO2 + H2O

19. Metabolic compensation for respiratory acidosis H+ + HCO3- H2CO3  CO2 + H2O

20. Metabolic compensation for respiratory acidosis

21. Patterns of compensation [H+]  pCO2 [HCO3-]

22. Respiratory disorders

23. Respiratory acidosis

24. Compensation for respiratory acidosis

25. Causes of respiratory acid-base disorders

26. Metabolic disorders

27. Metabolic disorders and their compensation

28. Causes of metabolic acid-base disorders

29. Putting it all together…

30. First, identify the primary problem…

31. …then, look to see if there’s compensation

32. Let’s apply this to a few examples…

33. 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

34. 31yo woman during acute asthmatic attack. [H+] = 24 nmol/L pCO2 = 2.5 kPa [HCO3-] = 22 mmol/L Case 1

35. 31yo woman during acute asthmatic attack. [H+] = 24 nmol/L pCO2 = 2.5 kPa [HCO3-] = 22 mmol/L Uncompensated respiratory alkalosis Case 1

36. 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

37. 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

38. 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

39. 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

40. 71yo man with stable COPD. [H+] = 44 nmol/L pCO2 = 9.5 kPa [HCO3-] = 39 mmol/L Case 4

41. 71yo man with stable COPD. [H+] = 44 nmol/L pCO2 = 9.5 kPa [HCO3-] = 39 mmol/L Compensated respiratory acidosis Case 4

42. 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!)