8. Control of Ventilation Flashcards
What does ventilatory control require?
Stimulation of the (skeletal) muscles of inspiration
Occurs via the phrenic (to diaphragm) and intercostal nerves (to external intercostal muscles)
Where does ventilatory reside?
Ill defined centres in the pons and medulla (respiratory centres)
Ventilatory control is normally _______ and subject to _______
Subconscious
Voluntary modulation
What does the Phrenic nerve supply?
The diaphragm
What is ventilatory control entirely dependent on?
Signalling from the brain
Why is C3,4,5 significant?
If severed above this level breathing ceases
Which muscles does the Dorsal Respiratory group go to?
Muscles for inspiration
Which muscles does the Ventral Respiratory group go to?
Muscles for expiration
What do respiratory centres have their rhythm modulated by?
- Emotion (limbic system)
- Voluntary over-ride (higher centres in brain)
- Mechano-sensory input from the thorax (e.g. stretch reflex)
- Chemical composition of the blood (PCO2, PO2 and pH) - detected by chemoreceptors
Where do Central chemoreceptors come from and what do they respond to?
- Medulla
- Respond directly to H+ (directly reflects PCO2)
- Primary ventilatory drive
Where do Peripheral chemoreceptors come from and what do they respond to?
- Carotid and aortic bodies
- Respond primarily to plasma [H+] and PO2 (less so to PCO2)
- Secondary ventilatory drive
Following the rise in [H+], what do central receptors cause?
Reflex stimulation of ventilation
driven by raised PCO2 = Hypercapnea
What buffer equation can be made for the effect of [H+] in the brain?
CO2 + H2O H2CO3 H+ + HCO3-
What is ventilation reflexly inhibited by?
A decrease in PCO2 (reduces CSF [H+]) HYPERVENTILATION
What causes an increase in the rate and depth of breathing?
Increase [H+]
What happens when arterial PCO2 increases?
CO2 crosses the blood-brain barrier and not H+
What do central chemoreceptors indirectly do in the cerebrospinal fluid?
Monitor PCO2
What two products are formed?
Bicarbonate and H+
What does feedback via the respiratory centres increase and why?
Increases ventilation in response to increased arterial PCO2
What does decreased PCO2 cause?
Slowed ventilation rate
What do Peripheral Chemoreceptors detect changes in and then cause?
Arterial PO2 and [H+]
Cause reflex stimulation of ventilation following significant fall in arterial PO2 (consider haemoglobin dissociation) or a rise in [H+]
Below which PO2 level would stimulation ventilation have the greatest effect?
Below 60mmHg
What doe Peripheral chemoreceptors respond to?
Arterial PO2 not O2 content
What does increased [H+] accompany?
A rise in arterial PCO2
CO2 + H2O H2CO3 HCO3- + H+
Will changes in Plasma pH alter ventilation and if so how?
Yes they will alter ventilation via the peripheral chemoreceptors pathways
What is acidosis?
Plasma pH falls ([H+] increases) and ventilation will be stimulated
What is alkalosis?
Plasma pH increases ([H+] falls-) (e.g. vomiting) and ventilation will be inhibited
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+]
e.g. Breath-holding
Which drugs depress the respiratory centre?
Barbiturates and Opiods
What does an overdose of barbiturates and opioids often result in?
Death as a result of respiratory failure
What do most gaseous anaesthetic agent do?
Increase RR but decrease TV and so decrease AV
What is Nitrous Oxide and what does in do?
A common sedative/light anaesthetic agent
Blunts peripheral chemoreceptor response to falling PaO2
Is Nitrous Oxide safe all the time?
Very safe in most individuals, problematic in chronic lung disease cases where individual often on “hypoxic drive”
Administering O2 to these patients aggravates situation
