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!

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+

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-

Problem: how do we recover bicarbonate?

Problem: how do we regenerate bicarbonate?

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

What are the ‘arterial blood gases’? pCO2 HCO3- pO2

Why do they have to be arterial?

A word about units…

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

Metabolic compensation for respiratory acidosis

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

Respiratory disorders

Respiratory acidosis

Compensation for respiratory acidosis

Causes of respiratory acid-base disorders

Metabolic disorders

Metabolic disorders and their compensation

Causes of metabolic acid-base disorders

Putting it all together…

First, identify the primary problem…

…then, look to see if there’s compensation

Let’s apply this to a few examples…

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

Case 1 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 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

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 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

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 4 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

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!)