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

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Presentation on theme: "Acid-base Disorders Dr Michael Murphy FRCP Edin FRCPath Senior Lecturer in Biochemical Medicine."— Presentation transcript:

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

3

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 + + HCO 3 - H 2 CO 3 CO 2 + H 2 O H + + Hb - HHb H + + HPO 4 2- H 2 PO 4 - H + + NH 3 NH 4 +

7 Why is bicarbonate so important? H + + HCO 3 - H 2 CO 3 CO 2 + H 2 O Other buffer systems reach equilibrium Carbonic acid (H 2 CO 3 ) removed as CO 2 Only limit is initial concentration of HCO 3 -

8 Problem: how do we recover bicarbonate?

9 Problem: how do we regenerate bicarbonate?

10 A wee trip down memory lane! H + + HCO 3 - H 2 CO 3 CO 2 + H 2 O [H + ] = K[H 2 CO 3 ] [HCO 3 - ] [H + ] pCO 2 [HCO 3 - ]

11 What are the arterial blood gases? H + pCO 2 HCO 3 - pO 2

12 Why do they have to be arterial?

13 A word about units… A word about units…

14 Reference interval

15 …and a bit of terminology Acidosis: increased [H + ] Alkalosis: decreased [H + ] Respiratory:the primary change is in pCO 2 Metabolic:the primary change is in HCO 3 -

16 So you can have… Respiratory acidosis: [H + ] due to pCO 2 Respiratory alkalosis: [H + ] due to pCO 2 Metabolic acidosis: [H + ] due to HCO 3 - Metabolic alkalosis: [H + ] due to HCO 3 - [H + ] pCO 2 [HCO 3 - ]

17 Another word…about compensation! H + + HCO 3 - H 2 CO 3 CO 2 + H 2 O When youve got too much H +, lungs blow off CO 2 When you cant blow off CO 2, kidneys try to get rid of H +

18 Respiratory compensation for metabolic acidosis H + + HCO 3 - H 2 CO 3 CO 2 + H 2 O

19 Metabolic compensation for respiratory acidosis H + + HCO 3 - H 2 CO 3 CO 2 + H 2 O

20 Metabolic compensation for respiratory acidosis

21 Patterns of compensation [H + ] pCO 2 [HCO 3 - ]

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 theres compensation

32 Lets apply this to a few examples…

33 Reference intervals for arterial blood gases H + 36-44 nmol/L pCO 2 4.7-6.1 kPa HCO 3 - 22-30mmol/L pO 2 11.5-14.8kPa

34 Case 1 31yo woman during acute asthmatic attack. [H +] = 24 nmol/L pCO 2 = 2.5 kPa [HCO 3 - ] = 22 mmol/L

35 Case 1 31yo woman during acute asthmatic attack. [H +] = 24 nmol/L pCO 2 = 2.5 kPa [HCO 3 - ] = 22 mmol/L Uncompensated respiratory alkalosis

36 Case 2 23yo man with dyspepsia & excess alcohol whos been vomiting for 24h. [H +] = 28 nmol/L pCO 2 = 7.2 kPa [HCO 3 - ] = 48 mmol/L

37 Case 2 23yo man with dyspepsia & excess alcohol whos been vomiting for 24h. [H +] = 28 nmol/L pCO 2 = 7.2 kPa [HCO 3 - ] = 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 pCO 2 = 2.8 kPa [HCO 3 - ] = 8 mmol/L

39 Case 3 50yo man with 2 week history of vomiting and diarrhoea. Dry. Deep noisy breathing. [H +] = 64 nmol/L pCO 2 = 2.8 kPa [HCO 3 - ] = 8 mmol/L Partially compensated metabolic acidosis

40 Case 4 71yo man with stable COPD. [H +] = 44 nmol/L pCO 2 = 9.5 kPa [HCO 3 - ] = 39 mmol/L

41 Case 4 71yo man with stable COPD. [H +] = 44 nmol/L pCO 2 = 9.5 kPa [HCO 3 - ] = 39 mmol/L Compensated respiratory acidosis

42 Final thoughts ALWAYS match blood gases to the history You cant over-compensate physiologically Can over-compensate by IV bicarbonate or artificial ventilation (but thats not really compensation!)


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