Control Of Ventilation Flashcards
Describe how ventilatory control is stimulated
Through stimulation of the skeletal muscles on inspiration via the phrenic and intercostal nerves.
Is ventilatory control subconscious?
Yes, but can be subject to voluntary modulation
State the origin of the phrenic nerve
C3-5
Describe the function of the respiratory centres
Set an automatic rhythm of breathing through co-ordinating the firing of smooth and repetitive bursts of action potentials in the Dorsal Respiratory Group.
Can adjust this rhythm in response to stimuli
State the primary output of the Dorsal Respiratory group
To inspiratory muscles
What muscles does the ventral respiratory group innervate?
Muscles for expiration and larynx & pharynx
Describe the pneumotaxic centre
Higher brain centre in the pons that provides inhibitory impulses on inspiration ie helps maintain alternately recurrent inspiration and expiration
Describe how the rhythm of the respiratory centres are modulated
- emotion, via the limbic system in the brain
- voluntary over-ride, via higher centres in the brain
- mechano-sensory input from the thorax, eg stretch reflex
- chemical composition of the blood, detected by chemoreceptors
Where are central chemoreceptors located?
The medulla
What do central chemoreceptors respond directly to?
[H+], which is wholly derived from CO2, therefore directly reflects PCO2
Describe how central chemoreceptors play a role in the control of ventilation
Are the primary ventilatory drive. Detect changes in [H+] in the CSF around the brain, and cause reflex stimulation of ventilation followed by a rise in [H+], driven by raised PCO2
State the name of the condition caused by raised PCO2
Hypercapnia
Why do chemoreceptors not respond to direct changes in plasma [H+]?
Because of the blood-brain barrier. CO2 crosses the barrier not the H+, only once crossed and converted to HCO3- and H+ in the CSF do the chemoreceptors respond.
Where are peripheral chemoreceptors located?
In carotid and aortic bodies
What do peripheral chemoreceptors respond to?
Changes in arterial PO2 (not oxygen content) and [H+]
Describe how peripheral chemoreceptors play a role in the control of ventilation
Cause a reflex stimulation of ventilation following a rise in [H+] or a significant fall in arterial PO2 (eg Hb dissociation or at high altitude)
Describe how changes in plasma pH can alter ventilation via the peripheral chemoreceptors pathways
- If plasma pH falls ([H+] increases), ventilation will be stimulated (drives CO2/HCO3- equation to the left) lowering [H+]
- if plasma pH increases ([H+] falls) ventilation will be inhibited (drives CO2/HCO3- equation to the right) increasing [H+]
Describe how breath holding cannot override involuntary stimuli
As if breath is held long enough consciousness is lost, as is the ability to make decisions so the brain takes over and breaths for you again.
State some common drugs that affect the respiratory centres and how
- barbiturates and opioids depress respiratory centre
- most gaseous anaesthetic agents, increase RR but decrease TV
- nitrous oxide, blunts peripheral chemoreceptors response to falling PaO2.
Why should nitrous oxide not be given to a patient with chronic lung disease
Can send patient into hypoxic drive where the administration of O2 will aggravate the situation by tricking brain into inhibiting ventilation - causes CO2 retention
