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Karan - MBBS Year 1: Respiratory Week 1 > Control of Ventilation > Flashcards

Control of Ventilation Flashcards

1
Q

What is the hierarchy of control of ventilation?

A
  • Brainstem automatic
  • Brainstem response to stimuli
  • Higher brainstem
  • Learned responses
  • Voluntary
  • Input from multiple sources
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2
Q

Describe neural respiratory control with respect to the medulla.

A
  • Medulla generates rhythm - inspiration then expiration
  • Ventilation ceases - section below medulla
  • Retained - section above medulla
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3
Q

What is believed to generate the breathing rhythm?

A
  • Network of neurons - Pre-Botzinger complex
  • Located near upper end of medullary respiratory complex
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4
Q

What gives rise to inspiration neurally?

A
  • Rhythm generated by Pre-Botzinger complex
  • Excites dorsal respiratory group neurones (inspiratory)
  • Contraction of inspiratory muscles
  • Firing stops - passive expiration
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5
Q

Describe the Pre-Botzinger complex.

A
  • Ventral group of neurones expressed bilaterally
  • Brain slice studies - rhythmic discharge produces respiratory pattern
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6
Q

Describe muscle contraction during inspiration.

A
  • Thoracic volume increased - diaphragm contraction and flattens
  • External intercostal muscle contraction lifts ribs and moves out sternum - ‘bucket handle’ mechanism
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7
Q

How is active expiration during hyperventilation controlled neurally?

A
  • Increased firing of dorsal neurones excites ventral respiratory group neurones
  • Excites internal intercostals, abdominals causing forced expiration
  • Expiratory muscles not activated in normal breathing
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8
Q

How do neurones in the pons generate rhythms in the medulla?

A
  • Pneumotaxic centre stimulated when dorsal respiratory neurones fire
  • Inspiration inhibited
  • APNEUSIS occurs when PC not present - prolonged breathing
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9
Q

Describe the apneustic centre.

A
  • Impulses from neurones excite inspiratory area of medulla
  • Prolonged inspiration
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10
Q

Describe some regions that can send stimuli that influence the respiratory centres.

A
  • Higher brain centres e.g cerebral cortex, hypothalamus
  • Stretch receptors in walls of bronchi and bronchioles
  • Juxtapulmonary receptors - stimulated by pulmonary oedema
  • Baroreceptors - increased ventilatory rate in response to reduced BP
  • Central and peripheral chemoreceptors
  • Nose and pharynx
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11
Q

Describe feedback loop control in respiratory rhythm.

A
  • Inflation of lung stops inspiration
  • Deflation induces inspiration
  • Dependent on vagal inputs
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12
Q

Describe pulmonary stretch receptors in reflex modification of breathing.

A
  • Activated during inspiration
  • Afferent discharge inhibits inspiration - Hering-Breuer reflex
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13
Q

Describe joint receptors.

A
  • Impulses from moving limbs reflexly increases breathing
  • Probably contribute to increased ventilation during exercise
  • Work alongside tendon/muscle/intracellular pH receptors
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14
Q

How might ventilation increase during exercise?

A
  • Reflexes originating from body movement
  • Adrenaline release
  • Impulses from cerebral cortex
  • Increase in body temperature
  • Accumulation of CO2 and H+ generated by active transport
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15
Q

Describe the cough reflex and how it is controlled.

A
  • Activated by irritation of airways
  • Afferent discharge causes short intake of breath, followed by closure of larynx, contraction of abdominals (raised intra-alveolar pressure).
  • Larynx opens and expulsion of air at high speed.
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16
Q

What do peripheral chemoreceptors detect?

A
  • Oxygen and CO2 tension
  • [H+] in blood
17
Q

Describe peripheral chemo-sensitive areas. PART 1

A
  • Occurs in carotid and aortic bodies
  • Receives high amount of blood flow
  • Oxygen sensitive K+ channels, haem based mitochondrial cytochrome enzymes - responsive to local PO2 concentration
  • Nerve output from carotid body goes to brainstem through vagus
18
Q

Describe peripheral chemo-sensitive areas. PART 2

A
  • Carotid body responds to arterial PO2 of <60mmHg. Above this, little response
  • Ventilatory response occurs within seconds
  • Ventilation increases for 5-10 minutes. Some PCO2 and pH responses also occur
19
Q

Describe central chemoreceptors and the nature of the substance that they detect.

A
  • Situated near surface of medulla of brainstem
  • Responds to [H+] of CSF
  • CSF separated from blood by blood-brain barrier
  • Barrier relatively impermeable to H+ and HCO3-, CO2 diffuses readily
  • CSF - less protein than blood so less buffered
20
Q

Describe the effects of CO2

A
  • If in CSF, increased [H+] and acidosis
  • Effect mediated via pH effect on muscarinic receptors to ACh or on other enzymes
  • Increased PCO2 increases rate, volume of breathing within 5 minutes
  • Increases PCO2 5mmHg doubles ventilation
  • Hypoxia - increased sensitivity to PCO2
21
Q

Describe the variation in CO2 drive. PART 1

A
  • Response to PCO2 reduced during sleep
  • Patients with persistently raised PCO2 - insensitive to PCO2 changes, depend on hypoxic drice - common in COPD
  • If no response and excess oxygen delivered so PO2 raised, PCO2 rises and ventilation falls
22
Q

Describe the variation in CO2 drive. PART 2

A
  • CO2 drive reduced with increasing age
  • Some patients without lung disease may have reduced PCO2 response, depend on hypoxia
  • ASTHMATICS WHO DEVELOP INCREASED PCO2 - abnormal CO2 response even when asthma improves
23
Q

Describe the hypoxic drive of ventilation

A
  • Occurs via peripheral chemoreceptors
  • Stimulated when arterial PO2 falls to low levels
  • Not important in normal respiration
  • Important at high altitudes/patients with chronic CO2 retention i.e COPD
24
Q

What is hypoxia at high altitudes caused by and what is the main response to this hypoxia?

A
  • Caused by decreased partial pressure of inspired oxygen
  • Response - hyperventilation and increased cardiac output
25
Q

What is the effect of pH on ventilation?

A
  • Raised PCO2 increases ventilation
  • Increased acidosis due to renal failure, diabetic ketoacidosis, lactic acidosis and Kussmaul respiration
  • Slow diffusion of H+ into CSF - pH slowly stabilises
  • Alkalosis leads to rising PCO2 - small effect
26
Q

What are the chronic adaptations to high altitude hypoxia?

A
  • Polycythaemia - increased oxygen carrying capacity
  • Increased 2,3-BPG produced within RBC - easier offloading of oxygen to tissues
  • Increased number of capillaries and mitochondria
  • Kidneys conserve acid so arterial pH falls
27
Q

What is the influence of the following chemical factors on peripheral (PC) and central chemoreceptors (CC)?
- ARTERIAL PCO2
- ARTERIAL PO2
- ARTERIAL H+

A
  • PCO2 - weak stimulation of PC, opposite for CC
  • PO2 - PC stimulated when PO2 falls below 8 kPa, severe hypoxia depresses respiratory centre for CC
  • H+ - PC stimulated, H+ doesn’t cross blood-brain barrier for CC
28
Q

Describe hypothalamic control of breathing. PART 1

A
  • Pain, temperature changes and emotions affect ventilation. Temperature increases ventilation
  • Sudden cooling causes apnoea or gasping
29
Q

Describe hypothalamic control of breathing. PART 2

A
  • Increased activation of oestrogen dependent progesterone receptors increases ventilation during pregnancy/luteal phase of menstrual cycle
  • More hypoventilation during sleep in some obese patients
30
Q

How is control of ventilation different in transplanted hearts and lungs?

A
  • No vagal input to brain and no response to lung stretch receptors
  • No Hering Bruer reflex but normal breathing
  • Normal response to irritants in trachea but not smaller airways (which can sometimes appear dilated)
  • Normal yawning, sighing
31
Q

Describe the evidences proving cortical control is involved in ventilation.

A
  • Learned patterns allow people to drink, breathe and talk at same time
  • Babies/patients with cerebral diseases - can choke upon swalling due to poor upper airway control
  • Singers alter their own airflow accurately to hit right notes.
32
Q

Describe Cheyne Stokes respiration.

A
  • Alternating hypo- and hyperventilation with repetitive pattern of breathing
  • Present in stage 1 sleep, neonates, altitude hypoxia, cerebrovascular and neurodegenartive diseases
  • Occurs - increased sensitivity to PCO2, prolongation of circulation from lungs to brain
33
Q

Describe hyperventilation

A
  • In metabolic acidosis, ventilation increases. PCO2 falls, pH increases
  • Tetany (muscle spasms) occur with rises in pH
  • Low brain PCO2 causes cerebral vasoconstriction and unconsciousness

Karan - MBBS Year 1: Respiratory Week 1 (10 decks)

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