L21. Causes and Consequences of Respiratory Centre Depression and Hypoventilation

Question 1

What is the oxygen delivery and carbon dioxide clearance of the respiratory system at rest?

Answer 1

200 mL/min O2
250 mL/min CO2
and the body uses only aerobic metabolism at this rate

Question 2

What is the oxygen delivery and carbon dioxide clearance of the respiratory system during exercise?

Answer 2

> 4L/min O2
4L/min CO2
and the body uses both aerobic and anaerobic metabolism

Question 3

What are two major consequences of failure of the respiratory system?

Answer 3

Hypoxia
Hypercapnoea (and respiratory acidosis)

Question 4

What are the three major components of the respiratory system?

Answer 4

1. Controller (brainstem)
2. Pump (Respiratory muscle and airways)
3. The Gas exchangers (AC membrane)

Question 5

What are the sensors of the respiratory system and what do they measure?

Answer 5

Chemoreceptors in the medulla and they respond to changes to PaCO2 (via CSF H measurements)

Peripheral receptors in the aorta and carotid and they respond to changes in PaO2, PaCO2 and pH

Question 6

What is the ventilatory response to hypercapnoea (draw the graph)

Answer 6

It is a linear relationship that shows the higher the CO2 the higher the minute ventilation

Question 7

What is the ventilatory response to hypoxia (draw the graph)

Answer 7

As PaO2 decreases, there is little to no change in minute ventilation until the PaO2 reaches about 50mmHg (which corresponds to about 90% saturation). At this point the minute ventilation rises with increasing hypoxia

Question 8

What is minute ventilation?

Answer 8

The amount of air going in and out of the lungs in one minute

Question 9

What is the minute ventilation dependent on? What is it analogous to in the cardiovascular system?

Answer 9

It is analogous to cardiac output

VE = RR x TV

Question 10

Ventilation increases with work. Why is this?

Answer 10

Increasing work, increases the use (demand) of O2 and for CO2 clearance. Ventilation increases to match these demands and thus keep the arterial levels at baseline

Question 11

What is the anerobic threshold? What happens to minute ventilation when this point is reached?

Answer 11

Anaerobic threshold is when the body begins to use anaerobic mechanisms (pyruvate–> lactic acid) as a source of energy. Thus generating more acid.

The ventilation increases more steeply (graph) with increasing work past this point. And the ventilation begins to fail in keeping the CO2 and pH at baseline (as CO2 decreases slightly to compensate for higher lactic acid - decrease in pH also seen)

Question 12

What are the causes of hypoventilation? [6]

Answer 12

1. Reduced activity in the respiratory centre (reduced drive, drugs, trauma, stroke)
2. Neuromuscular diseases (nerve paralysis, muscle weakness)
3. Chest wall deformities
4. Obesity (gross)
5. Sleep disordered breathing
6. Secondary to respiratory failure

Question 13

What are the three types of sleep disordered breathing?

Answer 13

1. Obstructive sleep apnoea
2. Central sleep apnoea
3. Obesity hypoventilation syndrome

Question 14

Describe obstructive sleep apnoea

Answer 14

A transient obstruction of the throat during sleep which prevents breathing (oxygenation and clearance of CO2) and disturbs sleep

Question 15

Why does airway obstruction occur in sleep apnoea?

Answer 15

Airway muscle relaxation (especially during REM sleep)
Narrowing of the throat (obesity, tonsils, etc)
Tongue falls backwards (especially in supine position)

Question 16

What are the main stages of OSA? [5]

Answer 16

1. Snoring in light sleep
2. REM sleep: complete obstruction (apnoea) occurs
3. Reduced O2 and increased CO2 and other stimuli cause the brain to ‘wake’ into a lighter sleep: arousal
4. Muscles contract (tense) and airways open and breathing recommences
5. Person falls back into a deep sleep and apnoea occurs again

Question 17

Is there any other dysfunction of the respiratory system in OSA (apart from the apnoea causing obstruction)?

Answer 17

No. The respiratory mechanisms and control centre are still fully functional (the person is trying to breath). So during the apnoea, the respiratory centre becomes progressively more active until arousal.

Question 18

Why is there a delay between the onset of apnoea and arousal?

Answer 18

Because during deep sleep, the brain is relatively non-receptive to messages coming from the periphery. Thus it takes about 30-60 seconds in order to wake the brain.

Question 19

When should you suspect OSA?

Answer 19

Snoring
Witnessed apnoeas
Symptoms of disturbed sleep
Arousals
Choking
Difficult to treat hypertension, unexplained respiratory failure

Question 20

What are the main measurements taken in a polysomnogram?

Answer 20

1. Breathing (expiration detected as hot air)
2. Saturation (oximetry)
3. Chest wall and abdominal wall expansion (tests whether it is obstructive vs. central sleep apnoea)

Question 21

What is the best way to describe the oximetry of people with sleep apnoea during REM sleep?

Answer 21

Cyclical desaturation

Question 22

What is the management of OSA?

Answer 22

Continuous Positive Airways Pressure: CPAP delivered through a mask (4-20cm H20 pressure) which is solely to provide a pressure stent to the airways and keep them open.
Others include mandibular advancement splint (brings jaw and tongue forward), sometimes surgery and mainly lying on the side.

Question 23

What is obesity hypertension apnoea?

Answer 23

Extra weight on the diaphragm from the abdomen increasing work on the muscle for respiration. And extra weight on the chest meaning extra work of respiration.

Question 24

What is a major consequence of chronic sleep apnoea?

Answer 24

The respiratory centre will sometimes accept some degree of hypoxia and hypercapnoea in order to achieve some level of sleep by setting up a new set point to allow for sleep and can eventually ignore signals for hypoxia and hypercapnoea.

Question 25

Why is it detrimental to give people with chronic hypercapnoea supplemental oxygen?

Answer 25

These people rely on the HYPOXIC DRIVE to keep them alive.
They’re body has adapted to maximise its respiration in response to decreased levels of oxygen. By providing them with supplemental oxygen removes the stimulus on the respiratory sensors and their compensatory mechanisms fail and they can enter respiratory failure very quickly.