lecture 8: breathlessness and control of breathing Flashcards
what are the functions of the respiratory muscles?
- maintaining PO2, PCo2,PH
- defence of the airways and the lungs
- exercise
- speech
- control of intra thoracic and infra abdominal pressures
what controls breathing?
breathing is controlled by the medulla, the cortex and the reflexes
show a graph of tidal breathing :
what does it show?
- the graph shows a single resp cycle
- VT (tidal volume)
- TTHOT (total time for resp cycle)
-VE (minute ventilation)
(tidal volume x frequency
***how to work out the frequency?
1/ TTOT
what does V.E = VT/T1 X T1/TOTT show?
- minute ventilation = tidal volume x frequency
can also be seen as
VE = VT x 60/TTOT
TTOT can be split into inspiratory TI and expiratory TE
VT/T1 = mean inspiratory flow
how powerfully the muscles contract
T1/TOTT = inspiratory duty cycle
the proportion of time actually inspiring
how to calculate inspiratory duty cycle?
T1/TTOT
what is minute ventilation in normal subjects?
normal VE - 6L/min
normal tidal volume - ).5 L
normal inspiratory duty cycle - 40%
how does use of nose clip affect breathing rate
- reduces breathing rate while VE remains roughly the same but VT increases as breathing is deeper.
how does use of a tube affect breathing rate?
increases dead space which increases VE, VT and frequency to clear dead space.
what happens to tidal breathing in disease?
- Intrathoracic airways are narrowed so difficulty ventilating lungs MORE on expiration.
- Lower TV as less air can fill the lungs but compensated by a faster breathing rate (equal TI/TTOT).
- People with COPD breathe much shallower and faster but NOT harder.
- REMEMBER: people with obstructive lung disease have difficulty expiring.
where does involuntary control of breathing happen?
- the medulla
- this always overrides the behavioural centre
- responds to the metabolic demands for and producing CO2
determines a set point for Co2
The Limbic System (survival responses), frontal cortex (emotions), sensory inputs (pain, startle) and sleep (reticular formation) may influence the metabolic centre.
where does voluntary control of breathing happen?
The motor area of cerebral cortex.
Can control acts such as breath holding and singing.
***show a diagram of the organisation of breathing control: ›
what is present in the metabolic controller?
H ion receptors
what nerves are the respiratory spinal motor neurones?
the phrenic nerves
what are the 2 main feedbacks in the metabolic controller?
- Chemoreceptors in the carotid bodies sense hydrogen ion levels in the blood from levels of O2 and CO2. This feeds back to the metabolic controller and has a DOMINANT influence.
a. Removal of carotid bodies reduces response to CO2 levels by up to 40%. - The stretch and irritant receptors on the lungs can influence the metabolic control of breathing.
what is the peripheral chemoreceptor ?
- This carotid body chemoreceptor is a well vascularised bundle of cells at the junction of the internal and external carotid arteries in the neck
- This serves as the rapid response system for detecting changes in arterial PO2 and PCO2
where does the pacemaker of breathing come from?
- 10 groups of neurones in the medulla
-
what is the pre botzinger complex?
in ventro-cranial medulla, seems essential for generating the respiratory rhythm and is known as the ‘gasping centre’
what re the 6 groups of neurones which control the phases of the resp cycle?
o Early inspiratory – initiates inspiratory flow via respiratory muscles.
o Inspiratory augmenting – may dilate pharynx, larynx and airways.
o Late inspiratory – signals the end of inspiration.
o Expiratory decrementing – may brake passive expiration by abducting larynx and pharynx.
o Expiratory augmenting – activate expiratory muscles when ventilation increases UPON EXERCISE.
o Late expiratory – signal the end of expiration, may dilate the pharynx in preparation for inspiration.
what nerve afferents from nose and face?
5th nerve
what nerve afferents from from pharynx and larynx ?
9th nerve
what nerve afferents from bronchi and bronchioles?
10th nerve
what is the hering breuer reflex?
o Hering-Breuer reflex from pulmonary stretch receptors in the lungs, terminates inspiration/expiration but this is weak in humans.
** explain this graph
♣ The slope (S) is an index of chemo-sensitivity.
♣ B is the apnoeic threshold, sensitive to acid-base status (only operates in sleep).
♣ This measures the sensitivity of the metabolic respiratory centre to hydrogen ions by use of a carbon dioxide challenge – breathing into a CO2 primed bag.
♣ The respiration into a closed bag maintains rising CO2 levels which raises the minute ventilation in response.
o NOTE: A 30Lmin-1 rise in VE for every 1kPa rise in PaCO2.
♣ Green = normoxia while orange = hypoxia (which increases sensitivity of acute CO2 response – mediated by the carotid body).
what is Chronic Metabolic Acidosis ?
increases the threshold (x-axis intercept -> left) but does NOT alter sensitivity (gradient).
what is Chronic Metabolic Alkalosis ?
decreases the threshold and again does not alter the gradient.
explain the graph some more ***
♣ The black line represents the VE at which PCO2 is below normal resting level.
o During sleep, ventilation would drop to zero but due to continual CO2 production, after 10-60 seconds, PaCO2 has risen enough (threshold) to restart breathing.
♣ A depressed ventilatory response to PCO2 means a flattening of the slope (decreased sensitivity) and a rise in set point (resting PaCO2 threshold).
o This could be the result of a disease affecting metabolic control or suppressive drugs (anaesthetics).
♣ An important peripheral cause of a reduction in sensitivity would be respiratory muscle weakness which usually progresses to a raised PaCO2.
is the system more sensitive to CO2 or O2
CO2
what happens to minute ventilation when there is a decrease in arterial PO2?
it increases
what does a fall in ventilation result in?
- causes fall in PaO2
- rise in PaCO2
- the fall in Po2 increases the sensitivity of the carotid body to PaCo2 and H+
- this means ventilation increases
- PaO2 increases
why is the body not well adapted to altitude?
hypoxic hyperventilation lowers to PCO2 and inhibits the ventilatory response
what happens to the tidal volume graph for a patient with emphysema or bronchitis?
- their breathing is more shallow and frequent
- so it looks like a normal ventilation
- their more or less normal inspiratory flow suggests their neural drive or force is normal
- however, this is not the case
- looking at the nervous action of the diaphragm
- the diaphragm is working more harder to achieve the same VT/TI
- the minute ventilation is normal
- but breathing is shallow
what happens in response to respiratory acidosis?
- a fall in ventilation leads to a rise in CO2 and hydrogen ions
- this stimulates the metabolic controller to increase breathing
- this is a rapid response system
- there is also a renal system
- this is renal excretion and retention of weak acids
- renal compensation however takes a much longer time
what happens in response to metabolic acidosis ?
- this is when there is an excess of H+ ions in response to metabolism
- the compensatory mechanisms are
- renal excretion
- ventilation stimulation to decrease CO2
what happens in response to metabolic alkalosis?
- hypoventilation increases the CO2 and the H+
- renal retention of weak acids
- renal excretion of chloride ions
- ## may happen in acute asthmatic attacks
what are the types of hypoventilation?
- central
- peripheral
within each section is = acute and chronic
what is central acute hypoventilation?
- metabolic center poisoning
what is central chronic hypoventilation?
- metabolic center congenital issue
- obesity
- chronic mountain sickness
what is peripheral acute hypoventilation?
- muscle relaxant drugs
- myasthenia gravis
what is peripheral chronic hypoventilation?
- resp muscle weakness of neuromuscular
what is COPD in terms of hypoventilation?
- mix of central hypoventilation and peripheral
- caused by lung inefficiency
- the metabolic controller is insensitive
what are the conditions for hyperventilation?
- chronic hypoxemia
- excess H +
- chronic anxiety
- pulmonary vascular disease
what is breathlessness
- subjective so hard to define
- breathlessness at rest = difficulty inspiring
- breathlessness at exercise= cardiac or resp disease
what are the types of breathlessness?
- tightness - difficulty in inspiring
- increased work and effort - high lung volume means high resistance against breathing
- air hunger - powerful urge to breathe
what is air hunger like?
- like suffocation
- ## difference between VE demand and VE achieved
what is a scale for measuring breathlessness?
- borg scale
10 points
what is breath holding time?
- test of behaviour vs metabolic demand
- the break point is the expression of air hunger
