Lung Mechanics Flashcards
Outline the procedure for the volume-time curve
wearing a noseclip, patient inhales to TLC, then exhales as hard and fast as possible for six seconds into vitalograph
List the ranges for the FEV1/FVC ratio
Normal 3.7/5.1 = 73%
Restrictive 3.2/3.3= 97%
Obstructive 1.0/1.9= 53%
How is the volume-time curve affected by obstructive and restrictive disorders
Obstructive: much slower exhalation rate, FEV lower and FEV1:FVC ratio 25% (volume reduced because airways narrowed)
Restrictive: similar rate but lower FVC (airways ok but volume affect
Outline how we obtain readings for the pressure-volume loop
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)
What will the pressure-volume loop look like for a patient with COPD
volume increases as lungs get larger and coving occurs as smaller airways offer lower flow rates - when severe the capacity decreases further and coving increases
Slightly lower PEF peak too
What is the effect of restrictive disorders on the pressure-volume loop
operating at lower volumes and less access to air, with normal/slightly lower PEF peak - filling not moving gas problem
What is the respiratory flow envelope
Inspiration and expiration lines- can never go outside this loop- it is an anatomical limitation.
Summarise the effects of obstructions on the pressure-volume loop
Extrathoracic obstruction: blocked inhalation (decreased inspiratory flow rate)
Intrathoracic obstruction: blocked exhalation (decreased expiratory flow rate)
Fixed airway obstruction: blocked inhalation and exhalation reducing flow rate because narrowed airways - blunting both curves
What can restrictive diseases be
Extra-thoracic
What is meant by obstructive disorders
Airways are narrowed and resistance to air flow is increased
Inflamed and thickened bronchial walls (asthma)
Airways filled with mucus (COPD)
Airway collapse (emphysema)
What is meant by restrictive disorders
Lungs are less able to expand so the volume of gas exchange is reduced
Stiffening of lung tissue (pulmonary fibrosis)
Inadequacy of respiratory muscles (DMD)
What is happening in obstructive disorders
The flow of air into and out of the lung is obstructed
Lungs are operating at higher volumes
This leads to hyperinflation of the lungs as air is trapped behind closed airways.
List the chronic and acute causes of obstructive disorders
Chronic causes:
COPD
Emphysema
Bronchitis
Acute causes:
Asthma
Describe the changes in lung volumes seen in obstructive disorders
RV increases- gas that is trapped cannot leave the lungs
RV:TLC ratio increase
ERV, IRV and TV decrease
In severe cases, vital capacity is decreased
TLC increases
What is happening in restrictive disorders
Infaltion/deflation of the lung or chest wall is restricted
Lungs are operating at lower volumes
All subdivisions of volume are decreased and the RV:TLC ratio will be normal or increased (where vital capacity has decreased more quickly than RV)
List the pulmonary and extrapulmonary causes of restrictive disorders
Pulmonary causes:
Lung fibrosis
Interstitial lung disease
Extrapulmonary causes
Obesity
Neuromuscular disease
What happens if Palveolar= Patm
No air flow occurs (FRC)
Transmural pressures create a negative pleural pressure as they pull out.
o Trans-pulmonary pressure is zero though so no net movement of air at the function residual capacity.
What is pleural pressure normally
approximately -5 cmH2O (atm/alveolar usually 0 cmH2O)
What do we see in forced exhalation
inward muscle force is larger than the outward recoil force, leading to an increase in pleural pressure to -2 cmH2O
What do we see in forced inspiration
outward muscle force is larger than the inward recoil force, leading to the pulling apart of the pleura, increasing the negative pressure to -8cmH2O- sucking air in
Describe the changes in intrapleural pressure in breathing
Fluctuates during breathing- but is approximately 0.5kPa at then end of quiet expiration.
On inspiration, intrathoracic volume is increased; this lowers intrapleural pressure, making it more negative, causing the lungs to expand and air to enter.
On expiration, the muscles in the chest wall relax and the lungs return to their original size by elastic recoil, intrapleural pressure becomes less negative, air leaves.
Describe the flow of air into lungs
§ Flow changes due to changes WE make to the alveolar pressure (as we can’t change atmospheric pressure).
§ When the chest wall rises, we create a negative intrapleural pressure à air flows in.
§ When the alveoli fill up, they return to equilibrium so no pressure difference.
§ At expiration, the chest falls and positive pressure is created which forces the air out (elastic recoil)
Describe the shape of the lung volume change graph with time
see diagram!
Describe the shape of the Palveolar graph with time
see diagram!
describe the shape of the flow rate graph
see diagram!
Describe the shape of the Ppl graph
see diagram!
Describe how we get the bottom graph and its significance
volume graph (inverted) over the pleural pressure graph to show difference between the volume and flow rate
greatest difference in middle
-ve in inspiration
+ in expiration
What is the shape of the transrespiratory pressure
sigmoid
What happens in expiration
we are trying to create positive pressure in chest
pleural pressure increases- trying to compress lungs
What happens in inspiration
Tension created to bring air in
Describe the shape of Prs with restrictive disorders
squashed reduced vital capacity; stretched because more effort to move air in so stretched - chest wall:lung interface less compliant
Describe the shape of Prs with obstructive disorders
operates at higher volumes with smaller vital capacity as tissue is more compliant - operates at smaller volumes too- so more narrow.
When can pressures more positive than FRC be created
In forced expiration
What is meant by compliance
The tendency to distort under pressure
Change in volume/ change in pressure
What is meant by elastance
The tendency to recoil to its original volume
Resistance to the stretch
Change in volume/ change in pressure
What is the relationship between elastance and compliance
C=1/E
What is the key difference between fluid-filled lungs and air-filled lungs
Fluid-filled lungs are more compliant than air-filled lungs
They stretch more for a given pressure- and so make ventilation easier than air-filled lungs (can reach max volume with less pressure)
Why are fluid-filled lungs more compliant
There is a small amount of water lining the lungs (surfactant) and the air-water interface exhibits surface tension whereas the fluid-water interface does not exhibit surface-tension.
What is meant by surface tension
A physical property of liquids that arises because fluid molecules have a stronger attraction to each other than the water molecules.
Molecules on the surface are therefore in contact with air and are pulled close together and act like a skin- think of a bubble.
Describe the issue with surface tension in the alveoli
§ The water molecules at the surface have no balancing force on one side which causes a surface tension.
§ Water molecules are more distributed around the outside.
§ On the inside, the water attracts to each side which means if the alveoli recoils too much, it will collapse.
Creates an unbalanced upward force (water-air) this would create large pressures to breathe and would increase the risk of alveolar collapse.
What is the importance of lung surfactant
Surfactant breaks up the surface tension- thus preventing alveolar collapse.
this makes the lungs more compliant.
Air-water interface exhibits surface tension
Fluid-water interface does not
Prevents collapse of small alveoli
Increases compliance (by reducing surface tension)
Reduces the ‘work of breathing’
also prevents the transudation of fluid into the alveoli.
Describe the properties of surfactant
Pulmonary surfactant
80% polar phospholipids
10% non-polar lipids
10% protein
Describe how surfactant is made
By type 2 pneumocytes
It is first stored intracellularly as lamellar bodies and then released as tubular myelin (the storage form of active surfactant). Once in the alveolar air space, the tubular myelin unravels to form a thin layer of surfactant over type 1 and type 2 pneumocytes
What is the main phospholipid in surfactant
dipalmitoyl phosphatidylcholine
How else is alveolar collapse prevented
There is interaction between adjacent groups of alveoli. Therefore, collapsing alveoli pull on adjacent alveoli. This is alveolar interdependence.
Describe the law of Laplace in terms of alveoli
P= 2T/r
Smaller the sphere, the greater the internal pressure needed to keep it inflated
Describe resistance in the airways
You would think that due to Poiseuille’s law, that a decrease in calibre of airway = more resistance but it also depends upon the flow.
§ Each generation is a bifurcation of a small airway and there gets a point (4th generation) when resistance decreases instead of increases because there are so many airways so so much cross-sectional area.
BUT INCREASES UP UNTIL 4TH GENERATION
Describe conductance of the lung
Conductance – how well the airways allow air to pass through.
§ Conductance steadily increases with increasing lung volume (as resistance decreases – as you breathe in, your airways expand).
high volumes increase radial traction, increasing the length and diameter of the airways
Are airways rigid
Airways are not rigid pipes – they dilate as lung volume increases
Describe the flow in collapsible tubes preinspiration
In the pre-inspiratory lung, the transmural pressure is positive and the airway is patent. This continues on throughout inspiration. This is FRC
pressure in lungs and atmosphere is 0 so no-airflow; intraplueral space at -5 due to recoil
Describe the flow in collapsible tubes mid-inspiration
intrapleural tension increased to -8 cmH2O, creating pressure gradient for air to move in
Describe the flow in collapsible tubes at the end of inspiration
intrapleural tension still at -8, but lung pressure and atmospheric are at 0 cmH2O
What happens in forced expiration
However, if you do forced expiration, then there is a massive increase in intrapleural pressure from the forces due to the chest muscles and abdominal muscles à pleural pressure may exceed internal pressure which may shut the airway.
What is transmural pressure
in - out; increases during inspiration (+5 to +8) preventing airway collapse
i.e p.atm - p.pl
How is airway collapse prevented
Hard expiration: creates bigger positive pressure in intra-pleural space; transmural pressure now surpassed collapsing pressure and airway should collapse - cannot maintain pressure and cartilage splinting prevents airway collapse
large airways need cartilage
How would compliance and resistance be affected by chronic obstructive pulmonary disease?
, think of the pathophysiology of COPD. Bronchitis causes a narrowing of the airways and hypersecretion of mucus, which INCREASES RESISTANCE. Also, emphysema breaks down the connective and elastic tissue that give the lungs their structure, so they’d become more floppy and have INCREASED COMPLIANCE.
What is the most likely reason for under-reading FVC in a patient with COPD?
The key in the question here is ‘under-reading’ and not ‘getting a low reading’. Of course you know that responses B, C and D all contribute to reduced FVC, however only A: Poor technique could cause the measurement to be lower than the true value.
Assertion: Resistance to airflow increases as the airways get narrower
Reason: Poiseuille’s Law states that as radius of tube is halved, resistance increases 16-fold
Assertion: This is FALSE – resistance peaks at generation 4 then enormously decreases
Reason: This is TRUE – however the bigger factor here is the exponential increase in airways number, which is associated with a large net increase in cumulative cross sectional area.
Move from A to B, how will Palv, Patm and Ppl change?
Correct!
As A to B is a maximal expiration, the expiratory muscles will contract, pushing the pleural layers together (increasing pleural pressure) and compressing the lung tissue (increased alveolar pressure). Atmospheric pressure is always 0 cmH2O in an open circuit.
What does patency mean
Patency = whether the airways are open and active and able to conduct air.
