19. Control of breathing (asleep)

Question 1

The body becomes functionally paralysed during REM sleep. What 2 muscles are spared?
What implications does this have for breathing?

Answer 1

Eye muscles
Diaphragm
The accessory muscles (e.g. intercostals) are no longer active so it becomes more difficult to breathe

Question 2

How does the control of breathing change when asleep compared to when awake?

Answer 2

When asleep, cortical and emotional control of breathing is inactive.
It’s solely regulated by the respiratory centre.

Question 3

Describe the changes that can be seen between wakefulness and sleep

Answer 3

Hypoventilation
Decrease in tidal volume
Decrease in minute ventilation
Decrease in alveolar ventilation
Oxygen saturation remains ~unchanged

Question 4

Name a complex in the medulla that is involved in regulating the respiratory rhythm.

Answer 4

Pre-Botzinger Complex

Question 5

What plays the biggest role in the control of breathing when awake?

Answer 5

PCO2 levels

Question 6

How does oxygen saturation change when asleep?

Answer 6

Oxygen saturation remains the same: because you are at the flat part of the ODC so despite a decrease in PaO2, oxygen saturation stays the same.

Question 7

How does oxygen saturation change during REM sleep?

Answer 7

PO2 and SO2 drop slightly

Question 8

How does oxygen saturation change when asleep in someone with COPD?

Answer 8

Their awake saturation is lower (on the steeper part of the ODC) so a reduction in PaO2 during sleep can cause a marked reduction in oxygen saturation and can push them into respiratory failure

Question 9

How do carbon dioxide levels change when you go to sleep?

Answer 9

PaCO2 increases (as tidal volume decreases)

Question 10

What mechanism keeps you breathing when asleep?

Answer 10

Reduction in tidal volume and increase in PaCO2

Question 11

How does sensitivity to carbon dioxide change when you go to sleep?

Answer 11

Sensitivity to CO2 decreases when asleep: there is a smaller change in minute ventilation per 1 kPa rise in PCO2

Question 12

What increases likelihood of becoming hypercapnic?

Answer 12

Low CO2 sensitivity

As more likely to retain CO2 and not blow it off

Question 13

What is the apnoeic threshold?

Answer 13

The minimum PaCO2 required to trigger breathing during sleep

Question 14

What happens if you prevent the carbon dioxide levels from exceeding the apnoeic threshold?

Answer 14

You stop breathing

Question 15

Describe the structure of the upper airways (pharynx).

Answer 15

It is a muscular, collapsible tube with no cartilage rings holding it open.

Question 16

Describe what causes obstructive sleep apnoea.

Answer 16

Relaxation of the muscular parts of the upper airways
Generates negative intraluminal pressure downstream
Airways close

Question 17

What can contribute to airway collapse and obstructive sleep apnoea?

Answer 17

If in addition there is positive extralumenal pressure e.g. fat around airway

Question 18

Describe characteristics that predispose to obstructive sleep apnoea

Answer 18

Fat tissue around tongue and neck
Shorter airways anatomically
Older (lose muscle tone in airways, airways become more compliant)

Question 19

What causes snoring?

Answer 19

Turbulent air passing over vocal cords (caused by slight closure of the airways)

Question 20

What defines central sleep apnoea?

Answer 20

No airflow, but no effort to breath

Question 21

What defines obstructive sleep apnoea?

Answer 21

No airflow, but effort to breath

Question 22

Describe the cycle that takes place in someone suffering from obstructive sleep apnoea.

Answer 22

Fall asleep 
Decrease in upper airway muscle activity
Apnoea
High CO2, low O2, still trying to breath
Wake due to effort of trying to breath against closed airway
Cycle repeats

Question 23

How do sleep-related changes in breathing exacerbate heart failure?

Answer 23

Patients hyperventilate

Therefore have a low PaCO2 (below apnoeic threshold) which means they can experience central sleep apnoea

Question 24

How does central sleep apnoea differ from obstructive sleep apnoea?

Answer 24

This is a chemosensitivity issue rather than a mechanical obstruction

Question 25

List 2 causes of central sleep apnoea

Answer 25

Stroke / central lesion

Congenital hypervenilation syndome.

Question 26

What is the main cause of obstructive sleep apnoea

Answer 26

Obesity