Ventilation Flashcards
Label graph with lung volumes and capacities
Capacities are the sum of two or more volumes
Volumes are discrete and don’t overlap
What factors affect lung volumes and capacities?
Body size (height and shape)
sex male and female)
Disease (pulmonary and neurological)
age (chronological, physical)
fitness (innate and training)
what are the 3 types of dead space?
Anatomical dead space and Alveolar dead space and Physiological dead space
Anatomical dead space
capacity of the airways incapable of undertaking gas exchange, usually the conducting zone- the first 16 generations of airways (150ml) eg nose pharynx, larynx, trachea, bronchi and bronchioles
The conducting zone is dead space
Alveolar dead space
capacity of the airways that should be able to undertake gas exchange by cannot
There could be alveoli that are not perfused or have collapsed within the respiratory zone - this makes up ALVEOLAR dead space (eg hypoperfused alveoli), should be 0ml in adults
Physiological dead space
sum of the alveolar and anatomical dead space (around 150 ml)
what is dead space?
Dead Space = the part of the airways and lung that DOES NOT PARTICIPATE IN GAS EXCHANGE
What is respiratory zone
Respiratory zone: 7 generations, gas exchange, typically 350 ml in adults, ait reaching here is equivilant to alveolar ventilation
Increase dead space
anaesthetic circuit snorkelling
Decrease dead space
tracheostomy circothyrocotomy
Chest wall relationship
The chest wall has a tendency to spring outwards, and the lung has a tendency to recoil inwards
What happens at FRC?
These forces are in equilibrium at end-tidal expiration (functional residual capacity; FRC), which is the ‘neutral’ position of the intact chest.
Chest recoil =
lung recoil
inspiratory muscle effort + chest recoil >
lung recoil
results in inspiration
Chest recoil
lung recoil + expiratory muscle effort
results in expiration
What are the lungs surrounded by?
The lungs are surrounded by a visceral pleural membrane
The inner surface of the chest wall is covered by a parietal pleural membrane
The pleural cavity (the gap between pleural membranes) is a fixed volume and contains protein-rich pleural fluid
The chest wall and lungs have their own physical properties that in combination dictate the position, characteristics and behaviour of the intact chest wall
Generating flow
Pressures drive flow - without a pressure gradient there would be no flow
NOTE: generally, when we’re talking about lung volumes, we use cm H2O instead of mm Hg or kPa - this is the default for measuring in respiratory physiology
What os negative pressure breathing?
Palv is reduced below Patm
You inspire when there is lower pressure inside the lungs - this is NEGATIVE PRESSURE BREATHING (how we normally breathe)
It is also possible to ventilate using positive pressure breathing
This involves increasing the pressure outside by using a ventilator or CPR
Transmural pressure
TRANSMURAL PRESSURES - this is the pressure across a tissue or several tissues
Pinside – Poutside
A negative transrespiratory pressure will lead to inspiration
Mechanics of ventilation
The effect of the diaphragm is like a syringe
A pulling force in one direction
The effect of the other respiratory muscles is like a bucket handle
An upwards and outwards swinging force
Transpulmonary pressure
There is a TRANSPULMONARY pressure = difference between alveolar and intrapleural pressure
NOTE: you always do the pressure inside MINUS the pressure outside to try and figure out the orientation of the gradient
TRansrespiratory pressure
The TRANSRESPIRATORY PRESSURE is the important one - it tells us whether there will be airflow into or out of the lung
Ventilation
At the start of the cycle there is no transpulmonary pressure (between alveolar and intrapleural pressure) because there is no volume change
The chest wall expands and creates negative pressure so more air flows in
This establishes a pressure gradient down which air flows
Eventually the pressure gradient will equalise again
Chest wall relation, curve
We know that naturally the lung recoils inwards and the chest wall recoils outwards
Residual volume is the smallest volume of air that we can have in our lungs - if we put all the expiratory effort in, we are left with residual volume
FRC - chest wall pressure is -5 cm H2O and lung pressure is 5 cm H2O hence the FRC is 0
It takes relatively little pressure to expand the chest wall to 6L which is relatively easy because the chest wall wants to expand
However, to get the elastic lung to expand, the bigger the volume, the more pressure is needed
So the intact lung has a sigmoid shape in terms of its volume-pressure relationship
When we exercise, it is inefficient to use the whole of our vital capacity because a lot of energy and effort is expended to utilise the inspiratory and expiratory muscles to the maximum
You want to ventilate the lungs to achieve a higher ventilation performance but you don’t want to tire out your muscles
Pulmonary function tests- volume time curve
Patient wears noseclip
Patient inhales to TLC
Patient wraps lips round mouthpiece
Patient exhales as hard and fast as possible
Exhalation continues until RV is reached or six seconds have passed
Visually inspect performance and volume time curve and repeat if necessary.
Look out for:
Slow starts
Early stops
Intramanouever variabiltiy
Peak flow
Patient wears noseclip
Patient inhales to TLC
Patient wraps lips round mouthpiece
Patient exhales as hard and fast as possible
Exhalation does not have to reach RV
Repeat at least twice. Take highest measurement
Flow volume loops
Patient wears noseclip
Patient wraps lips round mouthpiece
Patient completes at least one tidal breath (A&B)
Patient inhales steadily to TLC (C)
Patient exhales as hard and fast as possible (D)
Exhalation continues until RV is reached (E)
Patient immediately inhales to TLC (F)
Visually inspect performance and volume time curve and repeat if necessary. Look out for:
Inconsistencies with clinical picture
Interrupted flow data
FVC?
FVC = Forced Vital Capacity
FEV1?
FEV1 = amount of air forced out of the lungs in 1 second
Healthy Person - around 75% of the air is out within the first second
FET (Forced Expiratory Time)?
FET (Forced Expiratory Time) = the time taken to expel all the air from the lungs
obstructive lung disease
increases residual volume, with reduced IRV, ERV and TV, flow into and out of lung is obstructed so lungs operate at higher volumes (eg asthma, COPD)
Obstructive Lung Disease (e.g. COPD):
FEV1 would be much lower (can’t expel air fast)
FET is much higher (takes longer to expel all air)
FVC is much lower
restrictive disease
Restrictive Lung Disease (e.g. sarcoidosis, lung fibrosis
Imagine you are trying to breathe and then someone gives you a big bear hug from behind - it limits the expansion of the thorax (inflation/ deflation of chest/lungs are restricted and lungs operate at lower volumes)
FVC is lower
FEV1 is relatively high - because their conducting airways are quite clear they can expel air relatively easily
Abnormal flow volume loops
see slides
