West's 9th ed - Chapter 5 - Ventilation-Perfusion (1)

Question 1

What are the four causes of hypoxaemia?

Answer 1

1) Hypoventilation
2) Diffusion limitation (e.g. altitude, pulmonary disease)
3) Shunt (e.g. VSD)
4) Ventilation-perfusion mismatch (e.g. PE)

Question 2

What happens to alveolar PO2 during hypoventilation?

Answer 2

It decreases. This is because the alveolar PO2 is taken up by the pulmonary capillaries at basically an unchanging rate at resting conditions. Therefore, if you reduce ventilation, you reduce O2 replenishment of the alveoli, and you therefore reduce alveolar PO2. (and you increase the alveolar PCO2).

Question 3

What are some causes of hypoventilation?

Answer 3

1. Morphine and barbiturates (depressed central drive) 2. Damage to the chest wall / resp muscles 3. Resistance to lung expansion (e.g. underwater)

Question 4

What is the alveolar gas equation? Just write the equation.

Answer 4

P_AO₂ = P_IO₂ - (P_ACO₂ / R)

R is the respiratory quotient and is usually 0.8

Question 5

How does the ventilation / perfusion ratio change throughout the areas of the lung?

Answer 5

V:Q ratio reduces from apex to base due to regional differences in ventilation and blood flow (in the standing patient)

Question 6

What is the normal partial pressure of oxygen in inspired air? What is the normal P_AO₂ ?

Answer 6

Inspired air: 150mmHg

Alveolar: 100mmHg

This is due to alveolar replenishment and removal into capillaries

Question 7

In a hypoxaemic patient with a shunt, what happens when you give 100% oxygen?

Answer 7

The hypoxaemia responds poorly. The arterial PO₂ does not rise to the expected level - a useful diagnostic test.

Question 8

If you know the Ventilation (in g/min), and you know the Perfusion (in L/min), how can you figure out the concentration of a gas in the alveolus?

Answer 8

Concentration = V/Q

(in g/L)

Question 9

What is the normal ventilation-perfusion ratio?

Answer 9

About 1.

Alveolar ventilation is about 5L/min, and perfusion is also about 5L/min, at rest.

Question 10

What is the PO₂ and the PCO₂ of mixed venous blood entering the alveolar capillary?

Answer 10

PO₂ = 40mmHg

PCO₂ = 45mmHg

Question 11

What is meant by “mixed venous blood”?

Answer 11

This refers to the blood that arrives at the alveolus, the blood of the pulmonary arterioles. It is called “mixed venous” because it is a mixture of all the systemic venous blood returning to the right heart. It excludes shunted blood.

Question 12

What is the alveolar PO₂ at the apex of the lung compared to in the base of the lung (in a standing patient)? How about the alveolar PCO₂?

Answer 12

As you move from the apex to the base, the V/Q ratio decreases (because blood flow increases more than ventilation does). At the apex, the alveolar PO₂ is as high as 130mmHg, and at the base, as low as 90mmHg. At the apex, the alveolar PCO₂ is as low as 30mmHg, and at the base, as high as 42mmHg.

Question 13

How do you calculate the A-a gradient?

Answer 13

P_AO₂ - P_aO₂

The P_aO₂ is found from an ABG.

The P_AO₂ is calculated using the alveolar gas equation:

P_AO₂ = PiO₂ - (P_ACO₂ / R)

BUT you just use the PaCO₂ in place of the P_ACO₂ for the purposes of calculation.

Question 14

What is a normal value for the A-a gradient?

Answer 14

About 5-10 mmHg

Question 15

What happens to the A-a gradient in ventilation-perfusion mismatch?

Answer 15

It widens (its value increases), regardless of whether you have blocked perfusion or blocked ventilation. This is because V/Q mismatch causes hypoxia, i.e. it causes a low P_aO₂

Question 16

Why is the normal A-a gradient not zero?

Answer 16

Because in the normal upright lung, it is normal to have some degree of ventilation-perfusion mismatch (V/Q decreases as you go down the lung). The best-perfused region of the lung is the most poorly oxygenated. This mismatch is fairly trivial though, making the A-a gradient only about 5mmHg.

Question 17

In V/Q mismatch, which is affected more: O₂ uptake, or CO₂ elimination? Why?

Answer 17

The respiratory rate is increased (due to central chemoreceptors sensing the rising CO₂), and this often allows CO₂ elimination to be maintained as normal. However, O₂ uptake is still affected (making P_aO₂ low) - this is because of differences in the dissociation curves of the two gases. (Note that in order to maintain a raised respiratory rate, it is very tiring, and so you can become hypercapnoeic when you tire)

Question 18

What are the two causes of hypercapnoea (CO₂ retention)?

Answer 18

1) Hypoventilation
2) Ventilation-perfusion mismatch

Question 19

If a patient is hypoxic, and you give them 100% oxygen, but their PaO2 doesn’t improve, what can you assume is the cause of their hypoxaemia?

Answer 19

Shunt. This is the only cause of hypoxaemia that cannot be improved by 100% oxygen.