Pathophysiology of respiratory failure

Question 1

Define acute respiratory failure

Answer 1

A state in which the pulmonary system can no longer meet the metabolic demands of the body

Can be Type 1 (hypoxaemic with PaO2 of less than 8kPa) or Type 2 (hypercapnic PaCO2 of more than 6.7kPa)

Question 2

What causes type 1 respiratory failure?

Answer 2

Reduced diffusion or diffusion capacity.
Can be caused by low pressure of inspired oxygen (e.g. at altitude), surface area (affecting V/Q ratio) or diffusion coefficient (alveolar membrane)

Question 3

What causes type 2 respiratory failure?

Answer 3

Reduced alveolar ventilation (hypoventilation) resulting in hypercapnia

[Nb. can also be caused by hyperventilation with a reduced AVR]

Question 4

What is the oxygen cascade?

Answer 4

The process of declining oxygen tension from atmosphere to mitochondria

Question 5

What is the alveolar-gas equation?

Answer 5

Calculates the Arterial Partial Pressure Oxygen (PaO2) - influenced by changes in:
Diffusion capacity
Lung perfusion
Ventilation-perfusion matching

Question 6

What is the alveolar-arterial gradient?

Answer 6

A measure of the difference between the alveolar concentration (A) of oxygen and the arterial concentration (a) of oxygen.
Can be used to calculate the extent of hypoxemia

Question 7

What is the result of hypoventilation?

Answer 7

Oxygen in the alveolar is not replenished and CO2 is not removed.
Causes fall in PaO2 in alveolar and arteries causing fall in oxygen saturation.
Also causes rise in PaCO2 in alveolar and arteries.

Question 8

What is impaired diffusion?

Answer 8

Caused by an abnormality of the alveolar membrane or a reduction in the number of capillaries resulting in a reduction of alveolar surface area.

Question 9

What can cause impaired diffusion?

Answer 9

Acute respiratory distress syndrome
Alveolitis

[Nb. it is uncommon]

Question 10

What is the hallmark of impaired diffusion?

Answer 10

Desaturation on exercise

Question 11

What are the mechanisms underlying hypoxemia?

Answer 11

Low inspired oxygen concentration
Hypoventilation
V/Q mismatch
Diffusion abnormality

Question 12

What are the physiological consequences of respiratory failure?

Answer 12

Respiratory compensation
Sympathetic stimulation
Tissue hypoxia
Haemoglobin desaturation 
Hypercapnoea (high PaCO2)

Question 13

What sympathetic stimulation is a result of respiratory failure?

Answer 13

Increased HR
Increased BP
Sweating

Question 14

What are the signs of tissue hypoxia?

Answer 14

Altered mental state
Lactic acidosis
Decreased HR and BP

Question 15

How is haemoglobin desaturation identified?

Answer 15

Cyanosis 
Pulse oximetry (detects oxygen saturation)

Question 16

What are the signs of hypercapnoea?

Answer 16

Flapping tremor
Confusion/ coma
Sympathetic stimulation
Respiratory acidosis

Question 17

What is the respiratory quotient?

Answer 17

Ratio of O2 consumed and CO2 eliminated

Normally 0.8 so for every 10L of O2 consumed, 8L of CO2 is eliminated

Question 18

What are the normal values of an alveolar-arterial gradient?

Answer 18

16 years = 1.1 KPa

80 years = 3.1 KPa