Acid Base Disorders

Question 1

What is the relationship between H+ and pH

Answer 1

pH = -log10[H+]

Question 2

What is the buffering equation for H+?

Answer 2

H+ + HCO3- ⇌ H2CO3 ⇌ CO2 + H2O

Question 3

Pathophysiology of respiratory acidosis

Answer 3

• PCO2 increases
• H+ increases
• renal (metabolic) compensation to increase H+ excretion (48-72 hours to achieve maximal effect)

Question 4

Causes of respiratory acidosis

Answer 4

Causes can be acute or chronic

• Choking
• Bronchopneumonia
• COPD

Question 5

Pathophysiology of respiratory alkalosis

Answer 5

• PCO2 decreases
• H+ decreases
• metabolic compensation = H+ increases and bicarbonate decreases

Question 6

Causes of respiratory alkalosis

Answer 6

• Hysterical overbreathing
• Mechanical overventilation
• Raised intra-cranial pressure

Question 7

Metabolic acidosis pathophysiology

Answer 7

• Increase in H+ or decrease in HCO3-
• Respiratory compensation to decrease CO2 (occurs quickly)

Question 8

General causes of metabolic acidosis

Answer 8

• Impaired H+ excretion (impaired kidney function)
• Increased H+ production (eg diabetic ketoacidosis) or ingestion (eg antacids)
• Loss of HCO3- (gastrointestinal loss)

Question 9

What equation may be useful in metabolic acidosis?

Answer 9

Anion gap = [Na2+ + K+] - [Cl- + HCO3-]

Reference range = 14 - 18

Question 10

Causes of an elevated anion gap in metabolic acidosis

Answer 10

• Renal disease
• Diabetic ketoacidosis
• Lactic acidosis
• Poisioning - ethanol, methanol, ethylene glycol, salicylate

Question 11

Causes of lactic acidosis

Answer 11

• severe hypoxia
• severe anaemia
• shock
• haemorrhage
• hypotension
• congestive cardiac failure
• liver failure
• seizures
• thiamine deficiency

Question 12

Causes of metabolic acidosis with a normal anion gap

Answer 12

• Chronic diarrhoea or intestinal fistula
• Renal tubular acidosis

Question 13

Pathophysiology of metabolic alkalosis

Answer 13

• Decrease in H+ or increase in HCO3-
• Respiratory compensation to decrease CO2 excretion

Question 14

Classify the acid base disorder

H+ 63 nmol/L (35-45)

pH 7.20 (7.35-7.45)

PCO2 10 kPa (4.27-6.40)

PO2 7.0 kPa (11.07-14.40)

HCO3 29 mmol/L (21-25)

Answer 14

Respiratory acidosis with minimal compensation

Question 15

Classify the acid base disorder

H+ 45 nmol/L (35-45)

pH 7.35 (7.35-7.45)

PCO2 8.0 kPa (4.27-6.40)

PO2 7.1 kPa (11.07-14.40)

HCO3 33 mmol/L (21-25)

Answer 15

Compensated respiratory acidosis

Question 16

Classify the acid base disorder

H+ 17 nmol/L (35-45)

pH 7.76 (7.35-7.45)

PCO2 1.7 kPa (4.27-6.40)

PO2 13 kPa (11.07-14.40)

HCO3 18 mmol/L (21-25)

Answer 16

Respiratory alkalosis with minimal compensation

Question 17

Classify the acid base disorder

H+ 123.4 nmol/L (35-45)

pH 6.91 (7.35-7.45)

PCO2 1.11 kPa (4.27-6.40)

PO2 21.1 kPa (11.07-14.40)

Bic 1.6 mmol/L (21-25)

Answer 17

Partially compensated respiratory acidosis

Question 18

Calculate the anion gap

Na 140 mmol/L (135-145)

K 3.9 mmol/L (3.5-5.0)

Cl 111 mmol/L (95-108)

Bic 6 mmol/L (21-28)

Urea 3.4 mmol/L (2.5-7.5)

Creat 87 mmol/L (40-130)

Answer 18

Anion gap = (Na + K) – (Cl + HCO3)

= (140 + 3.9) – (111 + 6)

= 26.9

Question 19

Classify the acid base disorder

H+ 29 nmol/L (36-43)

PCO2 7.1 kPa (4.6-6.0)

PO2 8.5 kPa (10.5-13.5)

HCO3- 58 mmol/L (21-28)

Answer 19

Partially compensated metabolic alkalosis