Pathophysiology of respiratory failure

Question 1

Describe what the oxygen cascade is.

Answer 1

The progressive decline in oxygen partial pressure (PO2) as oxygen moves from the atmosphere through various steps to the mitochondria

Question 2

List 4 factors which affect the oxygen cascade.

Answer 2

Humidification

Alveolar gas equation

Diffusion

Physiological shunts

Question 3

What increases the PAO2 (alveolar O2) more than hyperventilation?

Answer 3

An increase in FiO2 (fraction of inspired oxygen) – for example, oxygen therapy

Question 4

What is the alveolar-arterial (A-a) gradient?

Answer 4

The difference between the mean calculated alveolar PO2 (PAO2) from the alveolar gas equation and the systemic arterial PO2 (PaO2) measured using an arterial blood gas

Question 5

In a normal healthy adult, what is the value of the A-a gradient?

Answer 5

<1.5 kPa

Question 6

What will the A-a gradient be like in hypoventilation?

Answer 6

Normal

Question 7

Which term is used to describe a high V/Q mismatch? What does it mean in terms of ventilation and perfusion levels?

Answer 7

Dead space

There is adequate ventilation but not perfusion

Question 8

Which term is used to describe a low V/Q mismatch? What does it mean in terms of ventilation and perfusion levels?

Answer 8

Pulmonary shunt

There is adequate perfusion but not ventilation

Question 9

What are the 4 types of hypoxia?

Answer 9

Hypoxaemic hypoxia

Anaemic hypoxia

Stagnant (circulatory) hypoxia

Cytotoxic hypoxia

Question 10

What is the pathophysiology of hypoxaemic hypoxia?

Answer 10

Low PaO2

Question 11

What is the pathophysiology of anaemic hypoxia?

Answer 11

O2 carrying capacity reduced

Question 12

What is the pathophysiology of stagnant (circulatory) hypoxia?

Answer 12

O2 delivery reduced

Question 13

What is the pathophysiology of cytotoxic hypoxia?

Answer 13

Mitochondria fail to utilize O2 effectively

Question 14

List 5 causes of hypoxaemic hypoxia.

Answer 14

Low partial pressure of inspired O2 e.g. due to altitude

Hypoventilation

V/Q mismatch

Right to left cardiac shunts

Diffusion abnormality e.g. fibrosis

Question 15

List 2 causes of anaemic hypoxia.

Answer 15

Anaemia

Carbon monoxide poisoning

Question 16

List 2 causes of stagnant (circulatory) hypoxia.

Answer 16

Cardiogenic shock – reduced cardiac output

Ischaemia due to local lack of perfusion

Question 17

List 2 causes of cytotoxic hypoxia.

Answer 17

Cyanide poisoning

Sepsis

Question 18

What is the value of severe hypoxaemia found in respiratory failure?

Answer 18

PaO2 <8 kPa

Question 19

What is the difference between Type 1 and Type 2 respiratory failure?

Answer 19

Type 1 – one gas is wrong, oxygen – hypoxaemia (<8 kPa), PaCO2 is normal or low

Type 2 – two gases are wrong, oxygen and CO2 – hypoxaemia and hypercapnia (PaCO2 >6.5 kPa)

Question 20

Type 1 respiratory failure is primarily caused by what?

Answer 20

V/Q mismatch

Question 21

List 6 causes of Type 2 respiratory failure.

Answer 21

Opiates

Chest wall deformity

Obesity

Neurological impairment

COPD

Near-fatal asthma

Question 22

In hypoxaemia, what causes an increase in haemoglobin levels?

Answer 22

Increased erythropoietin production

Question 23

The expected PaO2 of a patient on oxygen should be approximately how many kPa less than the % inspired concentration? For example, if a patient is on 40% oxygen, what would we expect their PaO2 to be?

Answer 23

10 kPa less than inspired oxygen

The patient would be expected to have PaO2 30 kPa