Consequences of Respiratory Centre Depression and Hypoventilation

Question 1

Describe the difference in the capacity of the respiratory system at rest and during exercise.

Answer 1

At rest:
Supplies 250 ml/min oxygen
Removes 200 ml/min carbon dioxide

Glucose + Oxygen –> energy + Water + Carbon dioxide
Aerobic Metabolism RQ = 0.8

During Exercise:
Supplies >4000 ml/min oxygen
Removes >4000 ml/min carbon dioxide

Aerobic and anaerobic metabolism
Pyruvate –> lactate + energy RQ = 1.2-1.5

If there is inadequate function, it can cause:

1. Hypoxaemia
2. Hypercapnoea and Respiratory acidosis

Question 2

What three elements are key in the functional organisation of the respiratory system?

Answer 2

1. Controller
2. Pump
3. Gas Exchanger

Question 3

What is the Controller in the functional organisation of the respiratory system?

Answer 3

CONTROLLER: Respiratory Centre & Peripheral Chemoreceptors:

Question 4

What is the pump in the functional organisation of the respiratory system?

Answer 4

PUMP: Respiratory Muscles, Chest Wall and Airways:

Question 5

What is the gas exchanger in the functional organisation of the respiratory system?

Answer 5

GAS EXCHANGER: Alveolar - Capillary Membrane:

Question 6

What are the elements of the respiratory control centre

Answer 6

• Brain stem (pons & medulla)
• Cortex
• Sensors
• Effectors

Question 7

How are signals delivered to and from the control centre for respiratory control?

Answer 7

Sensors such as chemoreceptors, lung receptors and others feed information to the central controller which stimulates effectors of the respiratory muscles to act.

Question 8

Describe the brain stem and cortex control elements for respiratory control.

Answer 8

1. Brain Stem (pons & medulla)
   - neurones in medulla and pons
   - automatic rhythmic inspiratory stimuli and sometimes expiratory stimuli
   - in-put from peripheral sensors
   - can be over-ridden by the cortex
   - major output is to the phrenic nerves
2. Cortex
   - voluntary hyperventilation –> hypocapnoea
   - to a lesser extent, hypoventilation –> hypercapnoea

Question 9

Describe Chemoreceptors.

Answer 9

Chemoreceptors are situated on the ventral surface of the medulla, surrounded by CSF and respond to changes in [H+] in that CSF. This concentration is a reflection of the carbon dioxide in the cerebral capillaries.
Increased PaCO2 –> Increased CSF [H+] –> Increased ventilation
They do not, however respond to PaO2.

Question 10

What are peripheral Chemoreceptors and where are they located?

Answer 10

Peripheral Chemoreceptors are situated in the carotid bodies at birfurcation of the common carotid arteries in the neck and the aortic bodies around the arch of the aorta. They deliver rapid responses to decreased PaO2, decreased pH, Increased PaCO2 –> increased ventilation

Question 11

What other receptors are involved in controlling respiratory function?

Answer 11

Lung and Other Receptors include the pulmonary stretch, irritant and J receptors as well as upper airway receptors, join and muscle receptors and painful stimuli.

Question 12

What is the ventilatory response to CO2?

Answer 12

A Small increase in Co2 –> rapid Increase in ventilation with significant individual variability.

Question 13

What is the ventilatory response to hypoxia?

Answer 13

A large fall in PaO2 before any significant increase in ventilation with significant individual variability

Question 14

What are some examples of stimuli for ventilation?

Answer 14

• Increased Metabolic Activity –> VE matched to O2 consumption and CO2 production
• Metabolic Acidosis –> VE excessive for O2 consumption and CO2 production, and is aimed at correcting the acidosis
• Anxiety –> VE excessive for O2 consumption and CO2 production & Results in a respiratory alkalosis

Question 15

What is the ventilatory response to exercise?

Answer 15

Ventilation increases with work to maintain PaO2 and PaCO2 at baseline. Beyond the anaerobic threshold, relative increase in VE because of the extra H+ production from lactic acid –> VE matched to O2 consumption and CO2 and H+ production

Question 16

What is hypoventilation?

Answer 16

• Situation where the rate of alveolar ventilation is not meeting metabolic requirements for oxygen consumption and carbon dioxide production.
• -> decreased PaO2 and increased PaCO2
• If acute it causes a respiratory acidosis; if chronic there is a conpensatory metabolic alkalosis

Question 17

What are some causes of hypoventilation?

Answer 17

• Reduced respiratory centre activity
  – Reduced drive (eg low CO2 or high pH)
  – Suppression of activity by drugs, trauma, vascular accidents etc
• Neuromuscular disease
  – nerve paralysis (drugs, polio, Guillian- Barre, trauma etc)
  – muscle weakness (drugs, motor neurone disease, muscular dystrophy)
• Chest wall deformity (gross)
• Obesity (gross)
• Sleep disordered breathing

Question 18

What are three different types of Sleep disordered breathing?

Answer 18

1. Obstructive Sleep Apnoea
2. Central Sleep Apnoea
   – several forms, eg Cheyne Stokes breathing
   – manage underlying heart failure etc +/- CPAP or BiPAP
3. Obesity Hypoventilation Syndrome
   – usually presents as ventilatory failure +/- right heart failure
   – “sensitive” to supplemental oxygen
   – manage with BiPAP and weight reduction (gastric banding)

Question 19

Describe Obstructive Sleep Apnoea?

Answer 19

• Transient obstruction of the throat during sleep preventing breathing, and disturbing sleep
• Occurs in people who snore (but not all snorers have OSA)
• Obstruction occurs during sleep because of :
  – Airway muscles relax (floppy throat - esp REM)
  – Throat already narrowed (obesity, tonsils etc)
  – Tongue falls backwards ( esp if supine)
• Cycle of Events:
  
  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
• Very fragmented sleep –> sleep deprivation
• Bed partner often makes diagnosis; should be suspicious if there is snoring, apneas, arousals, choking, symptoms of disturbed sleep such as mood swings, poor memory, decreased libido or if it is difficult to treat hypertension or unexplained respiratory failure etc.
• Managed via Nasal CPAPs or possible surgery.

Question 20

What are some of the consequences of sleep apnoea?

Answer 20

• Severe sleep Apnoea is a ‘sleep or die’ situation.
• Hypoventilation during sleep with consequent re-setting of the respiratory centre day-time hypoventilation
• This can also occur with other conditions, such as severe COPD, severe pulmonary fibrosis and neuromuscular disease in which there is less ventilation during sleep than when awake
• These patients develop chronic hypoxia, chronic hypercapnoea, and a compensated respiratory acidosis