Airflow And. Airways DLA Flashcards
Why is their effort independence in flow volume loops?
This is due to dynamic airway compression
- during forced expiration
- intrapleural pressure at or above alveolar pressure
- Causes dynamic collapsing of the lung airways
- Associated with wheezing of COPD
When are flow volume loops made?
Measured during forced breathing
What causes the effort-dependent loops portion of the flow volume loops?
Due to:
-Vel and force of muscle contraction
- elastic recoil lung
- Airway patency
What causes the effort independent portion of the flow volume loop?
Despite varying effort of exhalation, later parts of curves take same path
Due to:
-Elastic recoil of lung
-Airway resistance (EPP dynamic compression & airway resistance with lung vol)
Why is it impossible to breach flow-volume loop?
- Impossible to breach flow volume loop - I.e. starting slow and then exhaling forcefully result in same path decline
- Something limits expiratory flow over most of lung volume
-Dynamic compression of airways increases resistance to airflow and effectively limits flow during expiration I.e. no matter how hard you try to exhale the flow is limited by the collapsed airways
What is the transmural Pressure formula?
PTA= Paw-Pp
What is the trans pulmonary pressure (PL)?
PL= PA- PI
What is the preinspiration phase of the breathing cycle?
Airway pressure everywhere is 0(no flow)
Intrapleural pressure -5 P (IP), there is a +5 cm H2O holding airways open
What is the role of inspiration in the breathing cycle?
Chest wall expands, diaphragm flattens, intrapleura P(IP) and alveolar pressure PA fail by 2, and air flows into the lungs. Pressure drops along the airway, and so at a point further it is -1. Pressure +6 holding airway open
What are the occurrences of end inspiration of breathing cycle?
Flow is 0. Slightly higher intrapleural pressure PIP.
All of the above - airflow driven by PA and PATM gradient
What is the role of forced expiration and intrapleural pressure in breathing cycle?
Forced expiration, intrapleural pressure increases to +30 (diaphragm and thorax). Lung elastic recoil adds 18 cm H2O pressure to the lungs. As air flows down this steep gradient pressure the airway pressure falls way below the intrapleural pressure PIP causing the airways to collapse above the EPP and airway resistance to increase dramatically
Effective= PA-PIP
What is dynamic compression of the airway ?
Process that results in effort independence of the flow-volume loop
Effort independence region of flow volume loop: Dynamic compression results in resistance to air flow as intrapleural pressure increases. Thus any extra effort not result in increased air flow due to this “valve”
What is Equal Pressure Point (EPP)?
Point at which pressure inside the airway equals pressure outside (intrapleural pressure).
Dynamic airway c9mpression starts after the equal pressure point( EPP)
Summarize EPP
EPP= when pleural pressures= airway pressures
Summarize Dynamic airway compression
Above EPP, upstream towards mouth,there is dynamic airway compression
- decreased radius, increased resistance, thus radius restricts airflow
- occurs at lower lung volumes
- results in effort independent portion of the flow-volume loop
What is the significance of EPP?
Pressure gradient from alveoli to mouth drives air movement during exhalation.
Note: pleural pressures no longer subatomic, but positive during forced expiration
At certain point airway pressure= pleural prrsssure. This is the equal pressure point
As expiration continues the EPP will travel down into the lung
Upstream from EPP (towards alveoli) airways remain open
Downstream from EPP unsupported airways collapse. Larger airways suppo4ted by cartilage
Summarize the function of forced expiration
Upon forced expiration airways collapse and act as a valve rate limiting flow through the airways
-As lung volume declines resistance increases, due to decrease in structural support at low lung volumes (I.e. the inflated lung is more rigid and the floppy airways are held open by this rigidity)
- Therefore, more effort does not produce more flow
- And thus the effort-independent portion of the flow-volume loop
Summarize dynamic compression of the airways
Limits air flow in normal subjects during forced expiration
-May occur in diseased lungs at relatively low expiratory flow rates, thus reducing exercise ability
During dynamic compression, flow is determined by alveolar pressure minus pleural pressure (not mouth) and is therefore independently effort
Is exaggerated in some lung diseases by reduced elastic recoil and loss of radial traction on airways
Compare elastic recoil and compliance of the lung
Normal lungs- EPP
Dynamic compression/restriction typically in airways with cartilage.
High lung compliance I.e. low elasticity e.g. emphysema, COPD
EPP lower down respiratory tree- dynamic compression also lower down tree
- Decreased elastic recoil pressure (ERP) of lung (+2 vs +10)
- Airway pressures less (+32 vs +40)
- EPP further down respiratory treee
Explain pathology of emphysema
In pathological states of high lung compliance eg emphysema, the elastic recoil force of the lung is reduced
During a maximal expiration,
- PIP+ 30
- Elastic recoil pressure of the alveoli now +2 vs +10
- thus, pressure gradient from the Alveoli to the outside is less steep and the TAP along the airway is less
Consequently the EPP is going to occur to at higher lung volumes
At a PA of +32 and a PIP of +30 most of the airways are going to be compressed early in expiratory flow. This explains why patients with emphysema find it so diff8cult to breathe out
Summarize Airway generation
Airway resistance decreases dramatically down respiratory tree
Even though radius falls significantly
When is air resistance highest?
Highest in medium sized bronchi; low in very small airways
Contraction of bronchial smooth muscle narrows airways and increases resistance
Decreases as lung volume rises because the airways are then pulled open
Why is their resistance in straws being blown?
Hard work—> large resistance due to small cross sectional area
Very hard work—> larger resistance due to smaller cross sectional area
Puts lots of straws together- even if individually smaller
- as a group have large cross sectional area
- air flowing through in parallel fashion
Therefore- less resistance
How does lung volume and airway resistance affect each other?
At high lung volumes, there is increased traction on small airways pulling them open
Traction on airway by elastic recoil of alveolar septa= radial traction”
- decline lung volume
- decrease in distending pressure (radial traction) on airways provided by lung inflation
- Airway radius decreases—> resistance to airflow increases
Liken it to trying to blow through a rigid straw vs. a floppy straw
High lung vols
- inflated lung more rigid
- floppy airways are held open by this rigidity
What has the greatest effect on resistance to airflow?
Radius changes have greatest effect
How does Poiseuille’s law effect airflow?
R= 8nl/ pi x r^4
Length increase x2= resistance increases also by 2
Radius most critical
Reducing the radius by half increases resistance by 16
Summarize airway resistance.
Flow= pressure gradient/ resistance V=🔼P/R
In COPD/asthma-large increase in airway resistance
For given difference (🔼P) between alveoli and atmosphere increasing resistance by 3, reduces flow (V) by 3
Therefore- to maintain normal flow the patient must increase the 🔼P by 3 to compensate and restore equivalent flow
How?-breathing harder- increasing thoracic volume even more - I.e. huge increase in workload for the patient
What are the airflow patterns?
Terminal bronchioles airflow truly laminar
Trachea-airflow is turbulent
Transitional flow in the rest of the airways i.e. flow is intermediate between laminar and turbulent
In laminar conditions airflow is proportionate to the pressure gradient (🔼P)
If flow is turbulent-the airflow is proportionate to the square root of 🔼P
Turbulence= greater pressure gradient (🔼P) required to sustain same flow
= an effective increase in resistance
