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. Is only a temporary measure (“sponge”)
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

11. What are the ‘arterial blood gases’?
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 nmol/L
pCO kPa
HCO mmol/L
pO kPa

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

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

36. 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

37. 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

38. 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

39. 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

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

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

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!)