12. Control of Breathing - Asleep Flashcards
What is apnoea and the apnoeic threshold?
- Apnoea - cessation of breathing
* Apnoeic threshold - threshold over which CO2 level has to be to make sure we breath
What is the difference between sleep and other non-responsive states (e.g. coma)?
Sleep is reversible
What do you use to measure sleep and what is the activity when awake?
- Electroencephalogram (EEG)
* Awake - high frequency, low voltage
Which stage is deep sleep?
Stage 4
When do you dream?
• During REM sleep
What happens to muscle activity when you sleep?
Postural and ocular muscle activity falls
Which muscles are spared the functional paralysis during REM sleep?
- Eye muscles
- Diaphragm
(however it is still more difficult to breath during REM sleep)
Describe a hypogram in a healthy adult
- Fall asleep and go up to Stage 4 (deep sleep)
- After 90 minutes, REM sleep
- Amount of deep sleep decreases and REM sleep increases as you go through the night
How does the level of blood gas differ in a sleep cycle with a patient who has difficulty breathing?
Blood gases different at the start compared to the end
What activity is activated if you suddenly breath in deeply?
- Stretch receptor activity directly from the lungs
- Chemosensitivity from the gases in the deep breath
- Both influence the respiratory centre
What are the 2 ways in which breathing is normally controlled?
- Brainstem - reflex/automatic
- Cortex - voluntary/behavioural
(chemosensitivity can be overridden by behavioural control)
Which system does emotional control of breathing come from?
Limbic system
How is breathing controlled during sleep?
• Brainstem
• No cortical control (motor cortex)
• Some input from the cortex unless you’re in deep sleep
- area in control of voluntary breathing, between shoulders and trunk, can be seen on PET scan
Which area of the brainstem is responsible from the reflex control of breathing?
- Rostral Ventrolateral medullary surface
- Near the CSF
- [H+] in the CSF detectable by respiratory nuclei, changing the firing rate (determined by PCO2)
- Cluster of respiratory nuclei = Pre-Botzinger complex (vital for breathing)
- Reciprocal inhibition - when one set fires, the other doesn’t
- Also have early and late firing neurones
How does breathing and blood gas change when sleeping?
- Less input from respiratory centres
- 10% reduction in ventilation
- Shallower breathing (350mL, not 500mL)
- Same breathing rate
- Little change in SaO2
What gas levels drive your breathing?
More PCO2, not PO2
What happens to SaO2 when sleeping?
- Breathing/PaO2 can change a lot without changing SaO2 (due to ODC) when normally sleeping
- SaO2 and PaO2 drop slightly during REM sleep
Why is sleeping a challenge for people with lung disease?
- REM sleep decreases SaO2 and PaO2
- This is worse for lung disease patients as it decreases ventilation and O2 levels, and increases CO2 levels
- Can cause respiratory failure
- Recommended to increase O2 levels at night
Why is necessary for CO2 levels to increase when sleeping?
- Sensitivity to CO2 decreases when asleep
- More CO2 required when sleeping to trigger breathing
- If CO2 didn’t increase when sleeping => death
How do you test someones ventilatory sensitivity to CO2?
• Keep people breathing in their own air
• More CO2 is expired than inspired
• VT = 500mL at 12 breath per minute at the beginning
- minute ventilation of 5 or 6
- maintains CO2 level
• Breathe in own CO2 => PaCO2 increases => try to breathe more (cycle repeats)
• Steep slope - people try to blow the CO2 off if they get sick (higher CO2 sensitivity)
• Flatter slope - people will retain CO2 more and slip into respiratory failure quicker
What slope in ventilatory sensitivity is more beneficial to someone doing an elite performance sport?
- Steeper slope
- Brisk CO2 response
- Favours certain exercises
Why does CO2 sensitivity change when sleeping?
- Less cortical input going to the respiratory centres
- Causes sensitivity to CO2 to decrease and CO2 levels to rise
- Increases blood gas range before we wake ourselves up (allowing brain to sleep which is important)
- If we were very sensitive during sleep, a slight change in CO2 would wake us up
What is hypercapnia and what is the significance of it in sleep?
- Abnormally elevated CO2 levels in sleep
* Triggers a reflex which increases breathing and access to O2 during sleep
What is the condition in which CO2 doesn’t exceed the apnoeic threshold in sleep?
- Central Sleep Apnoea
- aka Congenital Central Hypoventilation Syndrome (CCHS)
- Treated with artificial ventilation
Why are the upper airways badly designed for breathing?
• Designed for eating and drinking
• Tongue at the front and pharyngeal constrictor muscles around the back
• No cartilage rings until the larynx
• Muscular tube is good for swallowing but bad to breath through
• Muscles relax when sleeping
- negative intraluminal pressure (ILP) and positive extraluminal pressure (ELP) pressing on muscular tube
• More ELP if fat around the neck
How does a recessed jaw affect breathing?
- Smaller airway
* More likely to suffer from sleep apnoea, especially with fat around the neck
What happens when the airways block when sleeping?
- Oxygen levels fall
- Carbon dioxide levels rise
- Nothing wrong with chemosensitivity => hypoxia or hypercapnia wakes you up
What is the difference between obstructive and central sleep apnoea?
- Obstructive - no airflow, still trying to breath (mechanical problem)
- Central - chemosensitivity problem, very rare
Patients are tired throughout the day due to disrupted sleep
How can sleep apnoea affect cardiac conditions?
- Generates massive pressures in the chest
* Exacerbates cardiac conditions
How can heart failure affect breathing?
- Pulmonary oedema
* Exacerbates hyperventilation and difficulty breathing
How do you test the integrity of a metabolic controller?
Ventilatory response to a ramp increase of PaCO2
How does hypoxia and hypercapnia change ventilatory sensitivity?
- Hypoxia (low O2 reaching tissues) - increases ventilatory sensitivity to CO2
- Hypercapnia - increases ventilatory sensitivity to hypoxaemia
How do you determine whether a response is central (won’t breath) or peripheral (can’t breath) in origin?
Distinguished by measuring mouth occlusion pressure or diaphragm EMG during CO2 stimulation
