West's 9th ed - Chapter 6 - Acid Base (1)

Question 1

Write the Henderson-Hasselbach equation with respect to the dissociation of carbonic acid in respiratory acid-base studies.

Answer 1

[A^-] means base concentration. Substitute this for [HCO₃^-]. Unit is mmol/L.

[HA] means acid concentration. Substitute this for [H₂CO₃]. H₂CO₃ is proportional to CO₂, and CO₂ is proportional to PCO₂ (Henry’s law), so substitute (0.03 x PCO₂), and the unit is mmol/L.

The value of pK_a here is 6.1.

If [HCO₃^-] is 24mmol/L, and PCO₂ is 40mmol/L in arterial blood, then you get a pH of 7.4

Question 2

Given the Henderson Hasselbach equation for carbonic acid dissociation, explain which ratio of plasma values needs to remain constant to maintain a plasma pH of 7.4 ?

Answer 2

As long as the ratio of [HCO₃^-] to (PCO₂ x 0.03) remains 20 (all in mmol/L), the pH will remain at 7.4. This means that an increase in PCO₂ will be buffered by an increase in HCO₃^-.

Question 3

What is a Davenport Diagram?

Answer 3

This shows the relationships between pH, plasma HCO₃^- and PCO₂.

Question 4

What is on the x-axis of a Davenport Diagram? What is on the y-axis?

Answer 4

X-axis: plasma pH

Y-axis: plasma bicarbonate concentration

Question 5

What is increased/decreased to cause respiratory acidosis? How is this compensated?

Answer 5

A rise in PCO₂ gives respiratory acidosis. This is compensated to some degree by a rise in HCO₃^-, which occurs passively by the dissociation of H₂CO₃, and then actively by the kidneys retaining bicarbonate.

Question 6

What is increased/decreased to cause respiratory alkalosis? What compensatory mechanisms occur in response?

Answer 6

A decrease in PCO₂ causes respiratory alkalosis, which can be compensated to some degree by increased renal excretion of bicarbonate.

Question 7

What is increased/decreased to cause metabolic acidosis? How can this be compensated?

Answer 7

An increase in acids in the blood causes a reduction of plasma bicarbonate, causing metabolic acidosis. This can be compensated by increased respiratory rate, which lowers to PCO₂, to maintain the HCO₃^-/PCO₂ ratio, and thus maintain the pH.

Question 8

What is increased/decreased to cause metabolic alkalosis? What compensation occurs?

Answer 8

An increase in plasma bicarbonate causes metabolic alkalosis. The only way to compensate is to reduce ventilation, which increases PCO₂ to maintain the HCO₃^-/PCO₂ ratio, but compensation by this method is minimal.

Question 9

In respiratory acidosis, is there a base deficit or a base excess?

Answer 9

Base excess. Even more so with renal compensation. This is because the plasma bicarbonate is raised.

Question 10

In respiratory alkalosis, is there a base deficit or a base excess?

Answer 10

Base deficit. Even more so with renal compensation. This is because the plasma bicarbonate is reduced.

Question 11

In metabolic acidosis, is there a base deficit or a base excess?

Answer 11

Base deficit. Even more so with respiratory compensation. This is because plasma bicarbonate is reduced.

Question 12

In metabolic alkalosis, is there a base deficit or a base excess?

Answer 12

Base excess. Even more so with respiratory compensation. This is because plasma bicarbonate is increased.

Question 13

Give some examples of respiratory acidosis.

Answer 13

Anything that causes CO₂ retention:

• Ventilation-perfusion mismatch (Chronic lung disease, PE)
• Hypoventilation (Barbiturates, Chest wall disease)

Question 14

Give some examples of respiratory alkalosis.

Answer 14

Hyperventilation (e.g. in anxiety, or at altitude)

Question 15

Give some examples of metabolic acidosis.

Answer 15

Anything that causes accumulation of acids in blood.

• Diabetic ketoacidosis
• Lactic acidosis in tissue hypoxia

Question 16

Give some examples of metabolic alkalosis.

Answer 16

Anything metabolic that increases the bicarbonate:

• Ingestion of alkalis
• Loss of acid gastric secretion by vomiting