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

Acid-base DisordersAcid-base Disorders

Dr Michael Murphy FRCP Edin FRCPathDr Michael Murphy FRCP Edin FRCPathSenior Lecturer in Biochemical MedicineSenior Lecturer in Biochemical Medicine

Outline of lectureOutline of lecture

• Basic concepts

• Definitions

• Respiratory problems

• Metabolic problems

• How to interpret blood gases

QuestionsQuestions

• 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?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 HBuffering 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?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 recover bicarbonate?

Problem: how do we regenerate bicarbonate?Problem: how do we regenerate bicarbonate?

A wee trip down memory lane!A wee trip down memory lane!

H+ + HCO3- H2CO3 CO2 + H2O

[H+] = K[H2CO3]

[HCO3-]

[H+] pCO2

[HCO3-]

What are the ‘arterial blood gases’?What are the ‘arterial blood gases’?

• H+

• pCO2

• HCO3-

• pO2

Why do they have to be arterial?Why do they have to be arterial?

A word about units…A word about units…

A word about units…A word about units…

Reference interval

……and a bit of terminologyand 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…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!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 acidosisRespiratory compensation for metabolic acidosis

H+ + HCO3

- H2CO3 CO2 + H2O

Metabolic compensation for respiratory acidosisMetabolic compensation for respiratory acidosis

H+ + HCO3- H2CO3 CO2 + H2O

Metabolic compensation for respiratory acidosisMetabolic compensation for respiratory acidosis

Patterns of compensationPatterns of compensation

[H+] pCO2

[HCO3-]

Respiratory disordersRespiratory disorders

Respiratory acidosisRespiratory acidosis

Compensation for respiratory acidosisCompensation for respiratory acidosis

Causes of respiratory acid-base disordersCauses of respiratory acid-base disorders

Metabolic disordersMetabolic disorders

Metabolic disorders and their compensationMetabolic disorders and their compensation

Causes of metabolic acid-base disordersCauses of metabolic acid-base disorders

Putting it all together…Putting it all together…

First, identify the primary problem…First, identify the primary problem…

……then, look to see if there’s compensationthen, look to see if there’s compensation

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

Reference intervals for arterial blood gasesReference 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 1Case 1

• 31yo woman during acute asthmatic attack.

• [H+] = 24 nmol/L• pCO2 = 2.5 kPa

• [HCO3-] = 22 mmol/L

Case 1Case 1

• 31yo woman during acute asthmatic attack.

• [H+] = 24 nmol/L• pCO2 = 2.5 kPa

• [HCO3-] = 22 mmol/L

• Uncompensated respiratory alkalosis

Case 2Case 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 2Case 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 3Case 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 3Case 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 4Case 4

• 71yo man with stable COPD.

• [H+] = 44 nmol/L• pCO2 = 9.5 kPa

• [HCO3-] = 39 mmol/L

Case 4Case 4

• 71yo man with stable COPD.

• [H+] = 44 nmol/L• pCO2 = 9.5 kPa

• [HCO3-] = 39 mmol/L

• Compensated respiratory acidosis

Final thoughtsFinal 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!)