8. Control of Ventilation Flashcards
What does ventilator control require?
Stimulation of the skeletal muscles of inspiration.
How does stimulation of the muscles of inspiration occur?
Via the phrenic nerve and intercostal nerves
Where does ventilator control reside?
Within ill defined centres located in the pons and medulla
What is ventilator control entirely dependent on?
Signalling from the brain
When will breathing cease?
If the spinal cord I severed above the origin of the phrenic nerve C3-5
What is breathing normally?
Subconscious
Although normally subconscious, what can breathing be subject to?
Voluntary modulation
What are the 2 groups of neurons?
- Dorsal respiratory group
- Ventral respiratory group
Respiratory centre have their rhythm modulated by…
- Emotion ( via limbic system in the brain)
- Voluntary over-ride (via higher centres in the brain)
- Mechano-sensory input from the thorax (e.g stretch reflex)
- Chemical composition of the blood (PCO2, PO2 and pH) detected by chemoreceptors
What is the most significant stimulant affecting respiratory centre rhythm?
Chemoreceptor input
What are the 2 types of chemoreceptors??
- Central
- Peripheral
Where are central chemoreceptors located?
Medulla
What do central chemoreceptors respond to?
Respond directly to H+
directly reflects PCO2
What are the central chemoreceptors responsible for?
Primary ventilator drive
Where are peripheral chemoreceptors located?
Carotid and aortic bodies
What do peripheral chemoreceptors respond to?
Respond primarily to plasma [H+] and PO2
less so to PCO2
What are the peripheral chemoreceptors responsible for?
Secondary ventilator drive
What crosses the blood-brain barrier when arterial PCO2 increases?
Carbon dioxide
What muscles does the dorsal respiratory group of neurons innervate?
Muscles of inspiration
What muscles does the ventral respiratory group of neurones innervate?
Muscles of expiration
What do the respiratory centres do?
- Set an automatic rhythm of breathing through co-ordinating the firing of smooth and repetitive bursts of action potentials in DRG- travel to inspiratory muscles.
- Adjust this rhythm in response to stimuli
What do the central chemoreceptors in the medulla do?
- Detect changes in [H+] in CSF around brain
- Cause reflex stimulation of ventilation following rise in [H+] which is driven by hypercapnia (raise PCO2)
What is ventilation reflexly inhibited by?
A decrease in arterial PCO2 (reduced CSF [H+]) (hyperventilation)
What do the central chemoreceptors not respond to?
Do not respond to direct changes in plasma [H+]
What monitors the PCO2 indirectly in the CSF?
Central Chemoreceptors
What are formed when the chemoreceptors respond to an increase in PCO2?
Bicarbonate and H+
What does feedback via the respiratory centres do in response to an increase in arterial PCO2?
Increase ventilation
What slows ventilation rate?
Decrease arterial PCO2
What do the peripheral chemoreceptors cause?
Reflex stimulation of ventilation following significant fall in arterial PO2 or a rise in [H+]
What classifies as being a significant fall in arterial PO2?
A fall below 60mmHg
What do peripheral chemoreceptors not respond to?
Oxygen content
What does increased [H+] usually accompany?
A rise in arterial PCO2
What will alter ventilation?
Changes in plasma pH
What stimulates ventilation?
Acidosis
Plasma pH falls= increase in [H+]
What inhibits ventilation?
Alkalosis
Plasma pH increases= decrease in[H+]
Can be caused by vomiting
What allows a large degree of voluntary control over breathing?
Descending neural pathways from cerebral cortex to respiratory motor neurons
What cannot be overridden?
Involuntary stimuli such as arterial PCO2 or [H+]
What occurs during hyperventilation?
Ventilation id reflex inhibited by an increase in arterial PO2 or a decrease in arterial PCO2/[H+]
What are some common drugs that can affect respiratory centres?
- Barbiturates
- Opioids
- Anaesthetics
- Nitrous oxide
How do barbiturates and opioids affect respiratory centres?
Depress respiratory centres
What can barbiturate and opioid overdose lead to?
Often leads to death as a result od respiratory failure
What do most gaseous anaesthetic agents do?
Increase respiratory rate by decrease tidal volume so decrease AV
What does nitrous oxide do?
Blunts peripheral chemoreceptor response to falling PaO2.
What problems can arise surrounding nitrous oxide?
-Problematic in those with chronic lung disease
-Patients often have a hypoxic drive
Administering O2 to these patients often aggravates the situation
Why is respiration inhibited during swallowing?
To avoid aspiration of food or fluids into the airways.
Why is swallowing followed by an expiration?
In order that any particles are dislodges outwards from the region of the glottis
