Central control of breathing and breathlessness

Question 1

What variables can be calculated from a volume time graph

Answer 1

1. Minute ventilation: Tidal volume (on expiration) x frequency
2. mean inspiratory flow: gradient
   
   • How rapidly diaphragm is contracting
3. Ti/Ttotal = inspiratory duty cycle - proportion of time spend inhaling
4. Minute ventilation: inspiratory duty cycle x mean inspiratory flow

Question 2

What are the determinants of a tidal breath

Answer 2

• Neural drive

- Timing

Question 3

How does chronic bronchitis and emphysema affect minute ventilation?

Answer 3

• groups with airways obstruction breath quicker and more shallow
• Proportion spent on inspiration same

Question 4

What does the metabolic breathing centre respond to?

Answer 4

• Metabolic demands for and production if CO2
  
  • Determines set point
  • Monitored as PaCO2

Question 5

How does the metabolic controller determine minute ventilation?

Answer 5

programmes mean inspiratory flow rate and inspiratory and expiratory times separetly
adjusted to maintain primarily a constant blood and tissue H+ concentration, and secondarily a constant SaO2 (but not PaO2)

Question 6

How does central breathing signalling occur?

Answer 6

1. Metabolic controller sends specific frequency to spinal motoneurons via phrenic nerves
2. Diaphragm contract and chest expands
3. Switches off inspiration and switch on expiration alternatingly
4. Upper airway muscles also under MC control
   
   • dilate on inspiration and narrow on expiration to allow for smooth airflow
5. Carotid bodies feedback chemical and mechanoreceptors in stretch, irritant receptors in lung and respiratory muscles also feedback to MC

Question 7

Where does feedback for breathing control come from?

Answer 7

1. Higher brain centres
2. Mechanoreceptors in lung and upper airway
3. Peripheral chemoreceptors in carotid bodies
4. Central chemoreceptor on ventral medullary surface

see notes

Question 8

How is the rate of breathing controlled neurally?

Answer 8

PreBotzinger Complex; contains network of cells (not a pacemaker) that reciprocally inhibit each other to generate rhythm; present on rostroventrolateral surface of medulla to detect cerebrospinal pH based on CO2 levels

Question 9

At what phases of breathing do the neurons of the Pre botzinger complex depolarise?

Answer 9

Early inspiratory: initiates inspiratory flow via respiratory muscles
- May dilate pharynx, larynx, airwas
Late inspiratory: signals end of inspiration and stop start of expiration
Expiratory decrementing: stop passive expiration by adducting larynx and pharynx
Expiratory augmenting: activate expiratory muscles when ventilation inc e.g exercise
Late expiratory: signal end of expiration and onset of inspiration
- May dilate pharynx

Question 10

What is the Hering Breuer reflex?

Answer 10

mechanoreceptors present in the bronchi and pleura detect stretch, so that upon maximal lung expansion these signal to the medulla/pons via the Vagus nerve to prevent overinflation; pneumotaxic centre of the pons inhibits apneustic centre to stop inspiration

Question 11

What are the parts of the metabolic controller?

Answer 11

• central part in medulla responding to H+ ion of ECF

- peripheral part at carotid bifurcation, the H+ receptors of the carotid body

Question 12

What is the minute ventilation relationship in PaCO2 and Pa02

Answer 12

• Body more sensitive to changes in PaCO2 and H+

- PaO2 not as tightly controlled, SaO2 is important

Question 13

What determines the concentration of hydrogen ions?

Answer 13

[H+] = constant x PaCO2/HCO3-

Strong ion difference: [Na+ + H+] – Cl-

Question 14

What parts of the body regulate pH?

Answer 14

• Lung: fast

- Kidney: slow

Question 15

What are compensatory mechanisms for acidosis?

Answer 15

Ventilatory stimulation lowers PaCO2 and H+
Renal excretion of weak (lactate and keto) acids
Renal retention of chloride to reduce strong ion difference

Question 16

What are compensatory mechanisms for alkalosis?

Answer 16

Hypoventilation raises PaCO2 and H+

• Renal retention of weak (lactate and keto) acids
• Renal excretion of chloride to increase strong ion difference

Question 17

What are the types of respiratory acidosis?

Answer 17

• Acute: hypoventilation causes PaO2 ↓, PaCO2 and H+ ↑ which stimulates metabolic centre (and carotid body) to increase minute ventilation and restore blood gas and H+ levels.
• Chronic: ventilatory compensation may be inadequate for PaCO2 homeostasis but a) renal excretion of weak acids (lactate and keto), b) renal retention of chloride to reduce strong ion difference, returns H+ to normal, even though PaCO2 remains high and PaO2 low.

Question 18

What is the mechanisms behind respiratory alkalosis?

Answer 18

ventilation in excess of metabolic needs

Question 19

What are causes of alkalosis?

Answer 19

• Chronic hypoxaemia
• Excess H+ (metabolic causes)
• Pulmonary vascular disease
• Chronic anxiety (psychogenic)

Question 20

What are the types of breathlessness?

Answer 20

• Tightness: difficulty in inspiring due to airway narrowing; a feeling that the chest is not expanding normally.
• Increased work and effort: breathing at a high minute ventilation, or at a normal minute ventilation but at a high lung volume, or against an inspiratory or expiratory resistance
• Air hunger: sensation of a powerful urge to breath, e.g a breath hold during exercise
  Mismatch between VE demand and output

Question 21

How is respiratory demand coordinated?

Answer 21

Metabolic controller sends signal to spinal motoneurons

Question 22

How is respiratory output coordinated?

Answer 22

Afferents from lung, chest wall and chemoreceptors

Question 23

Where does voluntary breath control occur?

Answer 23

motor cortex, and controls breath holding, singing, talking etc - can be overridden by involuntary

Question 24

Where does involuntary breath control occur?

Answer 24

brainstem, and adjusts ventilation rate in response to pH levels in the blood

Question 25

What are the parts of the metabolic centre?

Answer 25

• Medulla (Muscles): primary respiratory control centre, with a ventral respiratory group to stimulate expiration and dorsal respiratory group to stimulate inspiration (also controls other reflexes)
• Pons (VT): located under medulla to control rate of breathing, with an apneustic centre that stimulates long, deep breathing and increases tidal volume (inhibited by stretch receptors), as well as a pneumotaxic centre that inhibits inspiration by limiting phrenic nerve activity and inhibiting the apneustic centre to lower tidal volume

Question 26

What are the parts of the behavioural centre?

Answer 26

• Motor Cortex: neural centre for voluntary respiratory control, with different regions that control different muscles
• Hypothalamus: can override signals to increase respiratory rate for fight or flight
• Nerves involved: phrenic nerve supplies the diaphragm (autonomic), Vagus nerve innervates diaphragm, larynx and pharynx (autonomic) and posterior thoracic nerves innervate the intercostals (somatic)

Question 27

Where are respiratory chemoreceptors found and what do they detect?

Answer 27

• Central chemoreceptors: located on ventrolateral surface of medulla to detect CSF pH (can be desensitised by chronic hypoxia/hypercapnia)
• Aortic chemoreceptors: detect oxygen and CO2
• Carotid chemoreceptors: detect oxygen, pH and CO2