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Explain not getting understand confused between driving pressure airway pressure and how it works explain in detail
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https://pmc.ncbi.nlm.nih.gov/articles/PMC6589814/
Your notes are describing dynamic airway compression during forced expiration, especially in emphysema. The confusing part is that several different pressures are being compared at different places.
1. The three pressures
A. Alveolar pressure, PAlv
Pressure inside the alveoli.
This is the pressure that pushes air from alveoli toward the mouth.
During forced expiration:
[
P_{Alv} = P_{pl} + P_{elastic\ recoil}
]
Where:
- (P_{pl}) = pleural pressure, the pressure outside the lung/inside the thorax
- (P_{elastic\ recoil}) = inward spring-back force of the lung
B. Airway pressure, Paw
Pressure inside an airway such as a bronchus or bronchiole.
It is not the same everywhere:
- Near the alveoli, airway pressure is relatively high.
- As air travels toward the mouth, airway pressure gradually falls because energy is lost overcoming airway resistance.
- At the mouth, pressure is approximately atmospheric, defined as 0.
So during forced expiration:
[
P_{Alv} > P_{airway\ near\ alveoli} > P_{airway\ near\ mouth} = 0
]
C. Pleural pressure, Ppl
Pressure outside intrathoracic airways.
- During quiet expiration, it is usually still negative.
- During a forced expiration, expiratory muscles contract and (P_{pl}) can become positive.
A positive pleural pressure squeezes both the lung and the airways from outside.
2. What is “driving pressure” here?
In these notes, driving pressure means the pressure difference that drives gas from the alveoli to the point where airway collapse begins.
[
\text{Driving pressure} = P_{Alv} - P_{EPP}
]
At the equal pressure point (EPP):
[
P_{EPP} = P_{pl}
]
And because:
[
P_{Alv} = P_{pl} + P_{elastic\ recoil}
]
then:
[
\text{Driving pressure}
= P_{Alv} - P_{pl}
= P_{elastic\ recoil}
]
Key message
During forced expiration, maximum expiratory flow depends strongly on elastic recoil of the lung, not simply on how hard the person tries to exhale.
This is the central idea. The physiology text states exactly this relationship: driving pressure from the alveoli to the EPP equals lung elastic recoil pressure. Murray & Nadel's Textbook of Respiratory Medicine, section “Flow Limitation.”
Do not compare “driving pressure < airway pressure.” That statement in the notes is not a meaningful or correct comparison. They are pressures at different sites and must be compared as a pressure difference.
3. Equal pressure point: the important concept
During forceful expiration, imagine this pressure pattern:
Alveoli ----------------------------------------------> Mouth
PAlv high Paw falls gradually 0
Outside airway: pleural pressure (Ppl) is positive
At some point along the airway:
[
P_{airway} = P_{pl}
]
This is the equal pressure point (EPP).
On the alveolar side of the EPP
[
P_{airway} > P_{pl}
]
Pressure inside the airway is greater than outside pressure, so the airway tends to stay open.
At the EPP
[
P_{airway} = P_{pl}
]
No net pressure is distending or compressing the airway.
On the mouth side of the EPP
[
P_{airway} < P_{pl}
]
Now pressure outside the airway exceeds pressure inside it:
[
P_{transmural} = P_{inside} - P_{outside} < 0
]
The airway is compressed. This is dynamic airway compression.
A respiratory physiology explanation describes the same process: airway pressure falls along the airway during flow, and beyond the EPP pleural pressure exceeds the pressure inside the airway, making that segment prone to collapse. Physiology review
4. Why trying harder eventually does not increase expiratory flow
Use an example.
Forced expiration in a normal lung
Suppose:
[
P_{pl} = +20
]
[
P_{elastic\ recoil} = +10
]
Then:
[
P_{Alv} = 20 + 10 = +30
]
At the EPP:
[
P_{EPP} = P_{pl} = +20
]
Therefore:
[
\text{Driving pressure} = 30 - 20 = 10
]
That 10 cm H₂O is the lung's elastic recoil.
Now the person exhales even harder:
[
P_{pl} = +30
]
[
P_{Alv} = 30 + 10 = +40
]
At EPP:
[
P_{EPP} = +30
]
So:
[
\text{Driving pressure} = 40 - 30 = 10
]
Result: more effort raised both alveolar pressure and pleural pressure by the same amount. The driving pressure did not increase.
Instead, the airway downstream from the EPP is squeezed more. This is why, later in a forced expiration, flow becomes effort-independent. Fishman’s Pulmonary Diseases and Disorders, p. 199.
5. What changes in emphysema?
In emphysema, alveolar walls and elastic fibers are destroyed.
This causes two major problems:
- Reduced elastic recoil
- Reduced radial traction, meaning surrounding lung tissue no longer holds small bronchioles open as effectively
Lower elastic recoil means lower alveolar pressure
For the same forced expiratory effort:
[
P_{Alv} = P_{pl} + P_{elastic\ recoil}
]
If elastic recoil is low, (P_{Alv}) is lower.
Example:
Normal: Ppl +20, recoil +10 -> PAlv +30
Emphysema: Ppl +20, recoil +3 -> PAlv +23
In emphysema:
[
\text{Driving pressure} = P_{elastic\ recoil}
]
So the driving pressure may be only 3 rather than 10 cm H₂O. Air has less pressure available to flow outward.
6. Why the EPP moves toward small airways in emphysema
Because airway pressure drops as air travels from alveoli to mouth, the point where:
[
P_{airway} = P_{pl}
]
occurs earlier, closer to the alveoli.
That means the EPP moves from a relatively larger, better-supported airway to a smaller bronchiole.
Small bronchioles have:
- no cartilage
- thin walls
- less surrounding elastic support in emphysema
So they collapse much more easily.
Normal:
Alveoli ---- small airways ---- large/cartilage-supported airway ---- mouth
EPP
Compression mainly in a sturdier airway
Emphysema:
Alveoli ---- small bronchioles ---- larger airways ---- mouth
EPP
Compression in collapsible bronchioles
This premature small-airway collapse traps gas behind the closed airway, producing air trapping, raised residual volume, and hyperinflation. Emphysema predisposes to this because loss of elastic tissue removes the support that normally holds small airways open. Morgan and Mikhail's Clinical Anesthesiology, p. 932.
7. Correcting the flow chart in your image
A clearer version is:
Forced expiration
↓
Pleural pressure becomes positive
↓
Alveolar pressure rises:
PAlv = Ppl + elastic recoil
↓
Air moves from alveoli to mouth
↓
Airway pressure falls along the airway because of resistance
↓
At EPP: Paw = Ppl
↓
Mouthward of EPP: Paw < Ppl
↓
Dynamic compression of airway
In emphysema
Loss of elastic recoil + loss of alveolar attachments
↓
Lower alveolar pressure for a given expiratory effort
↓
EPP shifts toward peripheral small bronchioles
↓
Small bronchioles collapse prematurely
↓
Air cannot leave completely
↓
Air trapping and hyperinflation
8. One correction to your note
The line:
“Driving pressure opens up alveoli”
is not the best way to state it.
During expiration, the key role of alveolar pressure is to push air out, not to open alveoli. In emphysema, the major problem is that the small airways collapse before the alveoli can empty.
Also, forced expiration can worsen airway compression in COPD/emphysema. This explains why patients often exhale slowly and use pursed-lip breathing. The back pressure generated at the lips helps keep downstream airways open longer and allows more complete emptying.
One-line summary
Airway pressure falls from alveoli to mouth; during forced expiration pleural pressure becomes positive; when outside pleural pressure exceeds pressure inside a small airway, it collapses. In emphysema, reduced elastic recoil shifts this collapse into unsupported bronchioles, causing air trapping.
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