Control of Ventilation

Question 1

Explain how motor movements are affected by the CNS

Answer 1

• Controlled by centres in pons and medulla: subconscious, can be made voluntary, entirely dependent upon impulses from brain (C3-C5)
• Requires stimulation of skeletal muscles of inspiration: phrenic to diaphragm, intercostals to external intercostals

Question 2

Explain the basis of rhythm in respiration

Answer 2

• Respiratory centres coordinate smooth and rapid AP firing in dorsal respiratory group (DRG in medulla)
• Rhythm can be adjusted in response to stimuli

Question 3

Identify the sites of chemoreceptors and identify the stimuli that activate them

Answer 3

Central;

• Medulla
• Respond to [H+] in CSF of brain (i.e. PCO2)
• Primary ventilatory drive

Peripheral;

• Carotid and aortic bodies
• Respond primarily to plasma [H+] and PO2 (less to PCO2)
• Secondary ventilatory drive

Question 4

Explain how, by monitoring the pH of CSF, the central chemoreceptors serve to regulate the arterial PCO2

Answer 4

• Respond to [H+] in CSF of brain
• Hypoventilation –> hypercapnoea –> rise in [H+] –> stimulates ventilation
• Hyperventilation –> Hypocapnoea –> reduces [H+] –> inhibits ventilation

Question 5

Explain how the peripheral chemoreceptors become important during hypoxia and acid-base imbalance

Answer 5

• Stimulate ventilation following significant fall in PaO2 or rise in plasma [H+]
• Fall in pH ([H+] rises - acidosis) stimulates ventilation
• Rise in pH ([H+] falls - alkalosis e.g. vomiting) inhibits ventilation

Question 6

How do barbiturates and opioids affect the respiratory centres?

Answer 6

• Depress them

- Overdose can result in death as a result of respiratory failure

Question 7

How do gaseous anaesthetic agents affect the respiratory centres?

Answer 7

• Increase RR

- Decrease TV so decrease alveolar ventilation

Question 8

How does nitrous oxide affect the respiratory centres?

Answer 8

• Blunts peripheral chemoreceptors’ response to falling PaO2
• Very dangerous in chronic lung diseases where patients rely on hypoxic drive
• Giving O2 only aggravates the situation