84 - Causes and Consequences of Respiratory Centre Depression

Question 1

Where is the central respiratory controller?

Answer 1

Medulla, in brainstem

Question 2

Effectors of respiratory centre

Answer 2

Respiratory muscles

Question 3

Locations of chemoreceptors for breathing
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Answer 3

1) Central (in medulla)

2) Peripheral (EG: in carotid bodies)

Question 4

What do central chemoreceptors detect?

Answer 4

Respond to PaCO2 via CSF [H+]

Question 5

What do peripheral chemoreceptors detect?

Answer 5

Respond to low PaO2, highPaCO2, pH

Question 6

When does the body begin to increase ventilation in response to hypoxia?

Answer 6

Only when arterial O2 concentration is at around 50mmHg

Question 7

Ve

Answer 7

Minute ventilation.
Amount of air breathed in each minute.
Respiratory rate x tidal volume

Question 8

Relationship between Ve and work

Answer 8

Linear increase until ~60% maximum work.

Past 60% maximum work, steeper gradient of Ve increase, to match O2 consumption and CO2 production.

Question 9

Why does Ve increase more steeply after 60% maximum work?

Answer 9

Because anaerobic metabolism begins here, so H+ is produced as well as CO2, requiring more ventilation to compensate decrease in pH.

Question 10

What can lead to hypoventilation? 
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Answer 10

1) Reduced respiratory centre activity
2) Neuromuscular disease
3) Chest wall deformity
4) Obesity
5) Sleep-disordered breathing
6) Respiratory muscle fatigue secondary to severe lung disease

Question 11

Things that can reduce respiratory centre activity

Answer 11

Drugs (EG: opiates)
Trauma
Vascular accidents

Question 12

Examples of neuromuscular diseases that can lead to hypoventilation
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Answer 12

1) Nerve paralysis (polio, Guillian-Barre, trauma)

2) Muscle weakness (motor neurone disease, muscular dystrophy, drugs)

Question 13

Example of a chest wall deformity that can cause hypoventilation

Answer 13

Severe kypho-scoliosis

Question 14

Sleep-disordered breathing
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Answer 14

1) Obstructive sleep apnoea (most common)
2) Central sleep apnoea
3) Obesity hypoventilation syndrome

Question 15

Obstructive sleep apnoea

Answer 15

Transient obstruction of the throat during sleep, preventing breathing and disturbing sleep

Question 16

Why does obstruction of airways occur during sleep?
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Answer 16

Airway muscles relax, especially during REM
Throat already narrowed (from obesity, tonsils, etc.)
Tongue falls backwards (particularly if supine)

Question 17

How can there be a period of ~60 seconds of complete airway obstruction during sleep apnoea?

Answer 17

During deep sleep brain is quite poor at detecting stimuli, so takes a while for it to realise that it is hypoxic.

Question 18

Cycle of events in obstructive sleep apnoea 
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Answer 18

1. Snoring in light sleep
2. Complete obstruction (apnoea) in deep sleep
3. Reduced blood O2, increased CO2, other stimuli
4. Brain “wakes” to lighter sleep (arousal)
5. Muscles contract, airways opens, breathing
   recommences
6. Back into deep sleep, obstructs again
   Often more than 60 events every hour throughout sleep

Question 19

Hypersomnolence

Answer 19

Falling asleep during the day

Question 20

When to suspect sleep apnoea
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Answer 20

1. Snoring
2. Witnessed apnoeas
3. Arousals
4. Choking
5. Symptoms of disturbed sleep (excessive daytime somnolence, mood change, poor memory, poor libido)
6. Difficult to treat hypertension, unexplained respiratory failure etc

Question 21

How can OSA be diagnosed?

Answer 21

Overnight sleep study with a polysomnogram.

Question 22

How does a polysomnogram work?
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Answer 22

Heat-detecting sensor placed over mouth, detect breathing during sleep.

Attach oximeter to ear or finger, see what O2 saturation is like during sleep.

Abdominal movement is detected

Ribcage movement is detected

Question 23

Treatment for OSA
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Answer 23

Nasal CPAP (constant positive air pressure)
Keeps upper airways open (with 4-20cmH2O of pressure)

Can also use mandibular advancement splint (to pull tongue forward, works on mild sleep apnoea)

Surgery is often ineffective

Sleeping on the side

Question 24

Obesity hypoventilation
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Answer 24

1) Usually presents as ventilatory failure +/- right heart failure
2) “Sensitive” to supplemental oxygen
3) Manage with BiPAP and weight reduction (gastric banding)

Question 25

Central sleep apnoea
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Answer 25

1) From reduced drive to breathe
2) Several forms, eg Cheyne Stokes breathing
3) Manage underlying heart failure etc +/- CPAP or BiPAP

Question 26

Patients that you can’t treat with supplemental O2

Answer 26

Those with chronic hypercapnia

Question 27

Why can’t those with chronic hypercapnia be treated with supplemental O2?

Answer 27

They’re dependent on hypoxic drive to breathe (not stimulated by hypercapnia to breathe).
Giving O2 reduces hypoxic drive.