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!
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
[HCO3-]
What are the ‘arterial blood gases’?
• H+
• 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 + H O
2
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!)