7. Breathlessness and Control of Breathing Flashcards
What is most important function of the respiratory muscles?
Maintain arterial pH
What does breathing out rely on?
The elasticity of the lung and chest wall - This is a passive process
Therefore only breathing in counts towards respiration
Give the equation for Minute ventilation
V.E = VT x f
Minute ventilation = tidal volume x frequency
What does VT mean?
Tidal volume
What does V.E mean?
Minute ventilation
What is TTOT?
Duration of single respiratory cycle
What can TTOT be split into?
Inspiratory and expiratory
Define VT/TI
Mean inspiratory flow and shows how powerfully the muscles contract. This is called the neural drive
Define TI/TTOT
Inspiratory duty cycle - shows the proportion of the cycle actively ventilating (inspiring)
What happens in terms of VT, TI and TTOT when metabolic demands increase?
Increase in ventilation so - increase in VT/TI and decrease in TTOT hence there is an increase in frequency
What is normal tidal volume?
0.5L
What happens in terms of VT, V.E, VT/TI and frequency when artificial dead space is added?
The VT, V.E, VT/TI and frequency increases. The neural drive increases due to a need to satisfy more ventilation
How does chronic bronchitis and emphysema affect ventilation?
Narrows intrathoracic airways making expiration more difficult than inspiration. There is higher RV increasing the stiffness of the chest, hence increasing the work of breathing
What is the VT/TI of people with COPD?
They do not breathe any harder, as VT/TI is more or less the same
How do people with COPD breathe?
They take shorter an shallower breaths
How does shorter and shallower breathes affect the TV and TTOT?
The TV and TTOT decreases
How does exercise affect the TTOT?
It halves the TTOT hence doubling the frequency
In a healthy person how does the TI/TTOT change when exercising?
It increases - more time for inspiration
In a person with COPD how does the TI/TTOT change when exercising?
It decreases - more time for expiration as they have difficulty expiring
What controls involuntary breathing/metabolic centre?
Medulla (bulbo-pontine brain)
What controls voluntary breathing/behavioural centre?
Motor area of cerebral cortex
What centre will always override the other in terms of control of breathing?
The metabolic will always override the behavioural
What is the main driver of breathing?
Diaphragm
Where is the metabolic centre situated?
In the brain stem in the bulbopontine region
Where are the behavioural components situated?
Scattered throughout the mid and upper parts of the brain
What does the metabolic centre respond to?
Metabolic demands and production of CO2
What receptor lies in the metabolic controller?
H+ ion receptor
What nerves move the chest wall?
Phrenic nerves in the cervical region of the upper spinal cord
What is an important feedback to the metabolic controller?
Chemoreceptors in the carotid bodies. The detect H+ ion levels, pO2 and pCO2
How does the metabolic controller control the upper airways muscles?
It dilates the pharynx and the larynx on inspiration and narrow them on expiration (to act as a break on expiratory flow)
What is the pacemaker of breathing?
Made up from 10 groups of neurons in the medulla near the nuclei of cranial nerves IX and X
What is the pre-Botzinger complex?
A group of pacemaker activity essential for generating the respiratory rhythm. Also called the ‘gasping centre’
What is responsible for the generation of a tidal breath
6 groups of neurons in the medulla and brain stem. They discharge at different phases of the respiratory cycle
How does early inspiratory neurons initiate inspiratory flow?
Via the respiratory muscles
What do inspiratory augmenting neurons do?
Dilate pharynx, larynx and airways
What are the important roles of pharyngeal an laryngeal muscles?
They open up the airways and act as brakes in breathing
What is the lack of tone in the pharyngeal muscles cause?
Obstructive sleep apnoea syndrome
What does the afferents of the 5th nerve come from?
Nose and face (irritant)
What does the afferents of the 9th nerve come from?
Pharynx and larynx (irritant)
What does the 10th nerve come from?
Bronchi and bronchioles (irritant and stretch)
Herring-Breuer reflex
What nerves are irritant receptors?
5th,9th and 10th nerves
What do irritant receptors lead to?
Coughing and sneezing which are defensive
What is the function of the Hering-Breuer reflex?
Used as a cut off signal for inspiration - comes from pulmonary stretch activated by large lung inflation
Which nerve is responsible for the Hering-Breuer reflex?
10th
What are the two parts of the metabolic controller?
Central part of the medulla SLOW- H+ conc
Peripheral part at the carotid bifurcation (carotid sinus) FAST - H+ conc
What does H+ concentration correlate to?
Mirrors pCO2 changes
What does a rise in arterial pCO2 cause?
A rise in minute ventilation
How many L/min rise in minute ventilation for every 1kPa rise in arterial pCO2?
30 L/min - very sensitive system
What effect does chronic metabolic acidosis have?
It increases the threshold but not the sensitivity. Shifts left
Define chronic metabolic alkalosis?
Opposite of metabolic acidosis - shifts right. Still about threshold not sensitivity
What happens to ventilation in sleep?
Ventilation would drop to zero but CO2 production continues. Within 10-60 seconds the arterial CO2 would have risen sufficiently above the apnoeic point to restart breathing
What can cause depressed ventilatory response?
A disease affecting the metabolic control centre
Define depressed ventilatory response to pCO2
Decrease in sensitivity to pCO2 - caused by affects on the metabolic control
What is an important peripheral cause of a reduction in sensitivity to pCO2?
Respiratory muscle weakness leads to a raised arterial pCO2
Where is the pre-Botzinger complex found?
Found in the ventro-cranial medulla near the 4th ventricle
What may late inspiratory neurons signal?
The end of inspiration, and ‘brake’ the start of expiration
What can expiratory decrementing neurons do?
They may ‘brake’ passive expiration by abducting the larynx and pharynx
What do expiratory augmenting neurons do?
They may activate respiratory muscles when ventilation increases on exercise
What do late expiratory neurons do?
They may sign the end of expiration and dilate the pharynx in preparation for inspiration
What happens if you lower your arterial pCO2 to below the normal resting level (5.3kPa) ?
You don’t stop breathing - there is a minimal drive to breathe attributed to wakefulness
What is not as tightly regulated compared to PaCO2 and H+?
PaO2 - rather oxygen saturation is better defended
Usually, what does a fall in ventilation cause?
A fall in PaO2 and a rise in PaCO2
What does the fall in PaO2 cause?
It increases the sensitivity of the carotid body to PaCO2 and H+. As a result the ventilation increases and so PaO2 increases
What can inhibit the ventilatory response?
Hypoxic hyperventilation on going to altitude lowers the PCO2. This causes inhibition of the response. Several days of acclimatisation is required for the metabolic centre to adjust a lower O2 set point for increasing the sensitivity of PaCO2 and H+
If the lung is unable to fully respond to metabolic acidosis and alkalosis what system also helps?
The renal excretion and retention of weak acids. The renal compensation is a slow responder
What determines the H+ concentration in the blood?
The ratio of PaCO2:HCO3-
Define metabolic acidosis
When the source of the excess hydrogen ions comes from metabolism rather than inadequate ventilation
What are the compensatory mechanisms for acidosis?
Ventilatory stimulation lowers PaCO2 and H+
Renal excretion of weak acids
Renal retention of chloride to reduce strong ion difference
What is metabolic alkolosis caused by?
Loss of H+ ion: vomiting, diuretics and dehydration
What are the compensatory mechanisms for alkalosis?
Opposite of the compensatory mechanisms of acidosis
What does breathlessness at rest imply?
Difficulty with inspiration and expiration
What are the three types of breathlessness?
Tightness, Increases work and effort, Air hunger
