Breathlessness and control of breathing Flashcards
How do you calculate minute ventilation
minute ventilation = tidal volume x frequency
What proportion of breathing is inspiration
40%
How does tidal volume differ between chronic bronchitis sufferers , emphysema sufferers and normal people
Similar inspiration rates
Chronic bronchitis much lower tidal volume
emphysema lower tidal volume
Where is the involuntary / metabolic centre found
medulla/ bulbo-pontine
Where is the voluntary / behavioural centre found
Motor area of the cerebral cortex
Give some features of the metabolic/involuntary centre
Will always override the behavioural
Responds to metabolic demands for and production of carbon dioxide + determines set point
What is the metabolic centre influenced by
Limbic system
Frontal cortex
Sensory inputs
What determines impulse frequency to the respiratory spinal motor neurones
Metabolic controller
Changes based on the H+ concentration in the blood
Describe the response when carbon dioxide increases in concentration
- Increase in H+ is detected by the carotid bodies
- Impulses sent to the metabolic controller
- Increase frequency to the respiratory spinal motor neurones (+ upper airway muscles)
- impulse to respiratory muscles
- Minute ventilation increases
- Stretch and irritant receptors and muscle spindles and tendon organs in the lung feedback to the metabolic controller
How is the diaphragm driven to contract
Metabolic controller via the phrenic nerve
Where does the behavioural controller feed into to control breathing
Respiratory spinal motor neurones
2 situations: breath holding, sneeze and cough
Where do emotions feed into to control breathing
- Frontal cortex
- Limbic system
- reticular formation
- Metabolic receptor
Where is the peripheral chemoreceptor found
The junction between internal and external carotid arteries in the neck
Describe the peripheral chemoreceptor
Responsible for 40% of change to the controllers
Well perfused carotid body acts as a rapid response system
Sends signals to the medulla via cranial nerve IX, glossopharyngeal
What is the function of the Pre-Botzinger complex
(near 4th ventricle)
Gasping centre
Coordination of this with other controllers allows gasping to be orderly and responsive
Where is the central part of the metabolic controller found and what does it respond to
In the medulla
Responds to H+ of extracellular fluid
Which nerves respond to irritants
V - trigeminal (nose + face)
IX - glossopharyngeal (larynx + pharynx)
X - vagus (bronchi + bronchioles)
Why do fast and slow responses to H+ concentration exist
CO2 is very diffusible, and H+ changes mirror PCO2 changes, very rapidly for the hyperperfused carotid body, but more slowly in the ECF bathing the medulla.
lung - fast
kidney - slow
What is the magnitude of response for changes in pCO2 and what happens when pO2 is lower
small changes in pCO2 causes a large change in minute ventilation and response
Increase in sensitivity when pO2 is lower
Describe acute respiratory acidosis
Decrease in PaO2 and PaCO2 while H+ increases
Metabolic centre increases minute ventilation to restore blood gas and H+
Due to metabolic centre poisoning e.g. drugs, anaesthetics
Describe chronic respiratory acidosis
Ventilatory compensation may be inadequate for PaCO2 homeostasis
Resolved by renal excretion of weak acids (lactate and keto) and retention of chloride
What are the causes of chronic respiratory acidosis
Vascular/ neoplastic disease of the metabolic centre
Congenital central hypoventilation syndrome
Obesity hypoventilation syndrome
Chronic mountain sickness
What is peripheral respiratory acidosis caused by
Acute: muscle relaxant drugs, myasthenia gravi
Chronic: neuromuscular with respiratory muscle weakness
COPD
What are the causes of respiratory alkalosis
Chronic hypoxaemia Excess H+ (metabolic) Pulmonary vascular disease Chronic anxiety (psychogenic) High altitude
Describe dyspnoea
Breathlessness with the connotation of discomfort or difficulty
At rest - difficulty with inspiration or expiration
On exercise - excessive breathing for the task
Describe tightness
Difficulty in inspiring due to airway narrowing
Feeling that the chest is not expanding properly
Increased work and effort
Breathing at a high/normal minute ventilation but at a high lung volume, or against inspiratory or expiratory resistance
Describe air hunger
Sensation of a powerful urge to breathe
Experimentally produced by driving breathing with added CO2, while restricting tidal volume by breathing from a bag of fixed volume
Mismatch between VE demanded and the VE achieved
How can breathlessness be measured
Borg CR-10 scale
+ visual analogue
Breath holding time - behavioural vs metabolic controller
Describe the hearing-breuer reflex
Stretch receptors in the airways and intercostal muscles send afferent signals to the medulla via the vagus nerve
These signals dampen down the respiratory centre activity
Decreased firing of the phrenic nerve, decreased respiratory rate
What, other than CO2, can cause an increase in H+ conc
Lactate in infection
Ketones in diabetic ketoacidosis
