Consequences of Airway Obstruction

Question 1

How does inspiration work? What is the typical oxygen use for WOB?

Answer 1

Stimulation of the diaphragm by the phrenic nerves & external intercostals by the intercostal nerves which increases the volume of the thorax, creating a negative intrapulmonary pressure that sucks air into the lungs

At rest WOB is usually low (3% of total oxygen use)

Question 2

What is resistive WOB?

Answer 2

Airway obstruction - higher pressures need to be generated to overcome the obstruction

Question 3

What are the consequences of airway obstruction?

Answer 3

1. Recruitment of accessory muscles of inspiration
2. Increased oxygen consumption by respiratory muscles
3. Risk of respiratory muscle fatigue if airway obstruction is severe

Question 4

What are the pressures in the lungs during breathing?

Answer 4

• Intra-alveolar pressure less than Patm during inspiration
• Intra-alveolar pressure greater than Patm during expiration
• Intra-alveolar pressure = Patm at end of inspiration and expiration
• Intra-pleural pressure always less than intra-alveolar pressure because of elastic recoil of lungs and chest wall

Question 5

What is spirometry?

Answer 5

Measure of forced expiratory volume vs time

- if FEV1.0 is less than 70% of expected = airflow obstruction

Question 6

What is ventilation/perfusion matching?

Answer 6

Gas exchange is most efficient when ventilation and perfusion are matched, i.e. V/Q = 1 for all individual A-C units
- want an equal amount of oxygen and haemoglobin that needs to be oxygenated

Question 7

What is V/Q mismatch and low V/Q?

Answer 7

Low V/Q units receive relatively less ventilation than perfusion –> as a result O2 binding site on haemoglobin may not all be filled

• reduced PaO2
• some of the blood returning to the LA will not be fully oxygenated

Question 8

What is the most commonest cause of hypoxaemia?

Answer 8

Low V/Q

Decreased PaO2 due to low V/Q responds to supplemental O2

Question 9

What is shunt?

Answer 9

An extreme form of low V/Q unit (V/Q = 0)

It does not respond to oxygen - complete obstruction

Question 10

What are the compensatory mechanisms for low V/Q?

What can be a consequence of this?

Answer 10

Vasoconstriction occurs in areas of low ventilation to reduce the hypoxaemic effects of low V/Q units
- can elevate pulmonary artery pressure

Question 11

What is high V/Q?

Answer 11

When there is more ventilation of units than is required to oxygenate any blood passing through

• wasted ventilation, increases physiological dead-space
• little effect on PaO2

Question 12

What is the A-a gradient?

Answer 12

The gradient between alveolar and arterial O2 tension.

Elevated gradient can demonstrate a gas exchange defect

Question 13

What are the two types of respiratory failure?

Answer 13

Type 1 - low PaO2, normal/low PaCO2

Type 2 - low PaO2, high PaCO2