Ventilation and lung mechanics Flashcards
What is ventilation?
the process of inspiration and expiration
What occurs in the respiratory centre of the brain?
neurones generate automatic rhythmic impulses which are responsible for the normal involuntary rhythmic breathing patter
How do the impulses travel in the body to initiate breathing?
via the spinal cord and peripheral nerve to the inspiratory muscles
How is air drawn in the airways in tidal breathing?
air is draw into the airways by active expansion of the external intercostal muscles which expands the lungs
What is necessary in quiet inspiration (pressure)?
-pressure in the alveoli must be lower than atm pressure
-
Just before you breathe in, what is the pressure inside your lungs?
it is atmospheric pressure (then when you breath and increase volume, pressure drops below atm pressure drawing air in)
Why do the lungs move with the chest wall? (surface tension)
-normally want to recoil but because of pleural fluid found between visceral and parietal pleura in the inter pleural space and forms a seal between the lungs and thoracic wall so lungs expand with the thoracic cavity
What does the diaphragm do in tidal inspiration?
contracts and flattens
Why do the intercostal muscles need to contract in tidal breathing inspiration?
to overcome the forces of the lung recoil that are creating an inward force
What is the resting expiratory level?
a state of equilibrium where you have just expired and before you inspire again
In the resting expiratory level, what forces are acting on your thoracic cavity?
-lung recoil inwards
-chest wall recoil outwards
-diaphragm pulls down due to passive stretching
therefore you get no movement of the chest wall
What is the pressure in the intrapleural space?
negative (relative to atm)
Why is the pressure in the intrapleural space negative?
due to elastic recoil of lung pulling the visceral pleura in and the chest wall pulling the parietal pleura out
What happens if the integrity of the pleural seal is broken?
lungs will tend to collapse because the fluid surface tension is lost and lungs will recoil
How could the integrity of the pleural seal be broken?
-air in intrapleural space due to chest wound - due to negative pressure in intrapleural space, air would be drawn in from the atm
What is tidal volume?
the volume of air entering and leaving the lungs in a single breath during quiet inspiration and expiration
Whta is IRV?
Inspiratory Residial Volume - The extra volume of air that can be breathed in when effort in applied
What is ERV?
Expiratory Residual volume - The extra volume of air that can be breathed out when effort is applied
What is RV?
Residual volume - The volume of air left in the lungs after maximal expiration *RV = FRC-ERV
What is VC?
Vital capacity - The biggest breath than can be taking in fro the maximal inspiration to the maximal expiration (5L)
What is TLC?
Total Lung Capacity - vital capacity plus residual volume
What is IC?
Inspiratory Capacity - volume of air breathed with maximal inspiration after quiet expiration
What is FRC?
Functional residual capacity - The volume left in the lungs at the end of quiet expiration
What is FRC also known as?
the resting end expiratory level
How is the FRC determined?
the balance of elastic forces of the chest wall (outwards), elasticity and surface tension of the lung (inwards)
What is the net effect of these elastic forces in FRC?
the forces balance each other and crate a negative pressure within the intrapleural space relative to atm
How is quiet inhalation done?
- diaphrgam (70%) contracting and flattening
- external intercostal muscles
How is quiet exhalation done?
passive due to elastic recoil and pleural seal
How is forced inhalation done?
require accessory muscles - SCM, scalene, serrates anterior and petoralis major
How is forced exhalation done?
no longer passive - requires internal and innermost intercostals and abdominal wall muscles (external and internal oblique muscles and the rectus abdominus)
What volume of pleural fluid if found in the intrapleural space?
10ml (not much at all)
What does “work of breathing” refer to?
Energy is expended during inspiration to stretch the lungs and overcome airways resistance
What is compliace?
the stretchiness of the lungs (the volume changer per unit pressure change)
What does high compliance mean?
the higher the compliance, the easier the lungs stretch
How does compliance relate to lung volume?
the higher the lung volume, the higher the compliance
How is compliance of the lung determined?
- elastic tissue in lung
- surface tension forces of fluid lining alveoli
What forces need to be overcome in order to stretch the lungs?
elastic recoil forces on the lungs inwards
What could cause lower compliance? (think about how compliance is determined)
-high surface tension will make the lungs harder to stretch - due to making it harder for the alveoli to expand and therefore lungs to expand
How is surface tension created?
alveoli lined by fluid (limits their expansion)
What is surfactant secreted by?
type 2 pneumocytes in the alveoli
When does surfactant production start?
24 weeks
When is surfactant production adequate?
at 35 weeks!
What does surfactant do?
reduces surface tension which increases lung compliance
-stabilises the lung preventing small alveoli collapsing into big ones
Why are smaller alveoli more favourable than larger alveoli?
In larger alveoli, the surfactant molecules are spread further apart making them less effective at disrupting the surface tension so the surface tension will increase as the alveoli size increases and an increased surface tension decreases compliance making it harder to do forced inspiration rather than quiet inspiration
What is surfactant?
complex mixture of phospholipids and proteins with detergent properties
THE HYDROPHILLIC ENDS OF THESE MOLECULES LIN IN THE ALVEOLAR FLUID
THE HYDROPHOBIC ENDS PROJECT INTO THE ALVEOLAR GAS therefore they float of the surface of the lining fluid
Why is the resistance to air flow through the bronchioles great in expiration that inspiration?
In inspiration, as the alveoli expand, the radial traction on the bronchioles is greater. During expiration, the radial traction will be less and the lump of the bronchioles will be smaller increasing the resistance to air flow in expiration
How does surfactant stabilise the lungs?
smaller alveoli have higher pressure than larger alveoli so more likely to collapse in to them but as surfactant removes the ST in small alveoli they have equal pressure to large ones and don’t collapse in
What part of the respiratory tract has the least resistance?
upper tube as larger tubes
So why are the smaller alveoli low resistnace too?
they are connected in parallel
What is radial traction?
(the parenchyma exerted an outwards pull on the small bronchioles to keep them open - radial traction)
What is elasticity?
the ability of the lungs to return to their normal shape after being stretched (elastic recoil)
How is lung elasticity determined?
by the amount of elastin in the elastic fibres int he connective tissue of the lungs and the surface tension of the fluid lining the alveoli
What is the difference between the structure of the bronchiole and a small bronchus?
Small bronchus has small islands of cartilage and glands in the submucosa - bronchioles have none of that
What keeps the bronchiole lumen open?
surrounding alveoli and the radial traction (outward tugging of the surrounding alveolar walls on the bronchioles)
What keeps the bronchiole lumen open?
surrounding alveoli and the radial traction (outward tugging of the surrounding alveolar walls on the bronchioles)
What are obstructive disorders?
group of disorders characterised by obstruction of normal airflow due to airway narrowing
What are the obstructive disorders?
Asthma COPD Bronchiectasis CF Tumours
What are restrictive disorders?
A condition resulting in stiffer lungs which can’t expand to normal volumes
What are the restrictive disorders?
Pulmonary fibrosis
interstitial lung disease
Sarcoidosis
Asbestosis
What conditions would make airway resistance increase?
- chronic bronchitis
- asthmas
- emphysema
What is interstitial lung disease?
- a disorder characterised by stiff lungs
- end result is tissue injury and fibrosis
- due to fibrous tissue in the interstitium
What is the pathophysiology of interstitial lung disease?
- the lungs are stiffer and harder to expand due to the deposition of collagen fibres which are less stretchy than elastin fibres
- this decreases lung compliance
- the elastic recoil is increased (as considers both elastin and collagen fibres)
- lungs become smaller due to increase elastic recoil
- causing restrictive type defect
On examination, what would you find with someone with interstitial lung disease?
- reduced chest expansion
- IC and VC is reduced
- end inspiratory lung crepitations
- pleural effusion
- clubbing
- tachypnoea (increased resp rate)
- bilateral reduced chest movement
What symptoms would a patient with interstitial lung disease present with?
- dry cough
- SOB
- Dyspnea on exertion
- fatigue
- gradual symptoms
What is the diffusion problem with interstitial lung disease?
Thickening of the alveolar walls increases the distance oxygen has to diffuse from alveolar air to the blood (or CO2 from the blood)
What are some causes of interstitial lung disease?
- asbestos
- drugs (methotrexate)
- Connective tissue disorders
- sarcoidosis
- idiopathic
What is respiratory distress syndrome in the new born?
-caused by deficiency of surfactant in premature babies
Why does surfactant deficiency cause RDS?
no surfactant means surface tension is increased
- makes it harder for the lungs to expand and some alveoli will remain collapsed after birth so no gaseous exchange can occur
- lung compliance is decreased
- increased effort to breath
What are the signs of RSD?
- grunting
- nasal flaring
- intercostal and subcostal retractions
- rapid resp rate
- cyanosis
How is RDS treated?
- steroids to promote surfactant production
- surfactant replacement via an endotracheal tube
What is emphysema?
-loss of elastin and breakdown of alveolar walls due to chronic inflammation of cells and neutrophils releasing elastase which breaks down the elastin
What is the effect of loss of elastin?
- lung compliance is increased
- reduced elastic recoil
What is the most noticeable sign of emphysema?
-barrel chest
On CXR will get flattening of diaphragm too
Why does barrel chest occur?
due to the loss of elastic recoil, the lungs (at rest) are more expand than normal (hyper inflated)
Why are airways narrowed in emphysema?
due to loss of elastic fibres exerting an outward pull on the small bronchioles (radial traction)
What is a rarer cause of emphysema?
alpha 1 anti-trypsin deficiency (inherited condition)
What does A1ATD lead to?
an imbalance in proteinases and antiprloteinases leading to destruction of elatin and emphysema
What is asthma?
a chronic inflammatory process which may be triggered in susceptible individuals by allergic/non-allergic stimuli
What does this chronic inflammation cause?
causes airway narrowing due to bronchial smooth muscle contraction, thickening of airway walls by mucosal oedema and excess mucus production partially blocking the lumen
What is a pneumothorax?
condition where air enters the pleural space with the loss of the pleural seal and lung collapse
Why does a lung collapse if air gets into the pleural space?
pleural seal is broken so the elastic recoil of the lung causes it to collapse inwards as the -ve pressure of the of the intrapleural space and the outward record of the chest wall no longer counteract it
What are some other causes of a collapsed lung?
- atelectasis (incomplete expansion of the lungs (neonatal)) - alveoli don’t expand at birth
- pleural effusion (fluid in the cavity)
- abdiminal distention (compresses alveoli)
- airway obstruction
How is a pneumothorax treated?
drain the air from the pleural space using a chest drain with an underwater seal (this prevents fluid or air from entering the pleural cavity
What can cause hypoventilation?
- due to poor expansion of the thoracic cavity or lungs
- injuries to brain stem (ventilation is controlled by impulses in the respiratory centre)
- severe thoracic wall deformities
- resp failure
- phrenic nerve injury
