SM_152a-154a: Mechanics II-IV Flashcards
Describe the pressure relationship in inspiration
PA < PB and PA < 0
Describe the pressure relationship in expiration
PA > PB
PA > 0
What creates the intrapleural pressure?
Lungs held against chest wall by thin layer of pleural fluid
Intrapleural pressure Pip is ______ than atmospheric pressure
Intrapleural pressure Pip is less than atmospheric pressure
What happens when there is a hole in the chest wall?
Pneumothorax
- Pip = PB = 0
- Lungs collapse
- Cheast wall expands
What are the three transmural pressures?
- Transpulmonary: Ptp = PA - Pip
- Transthoracic: Ptt = Pip - PB
- Transrespiratory: Ptr = PA - PB
How does one measure Pip?
With an esophageal ballon
Esophageal ballon pressure is a reasonable surrogate for Pip
What factors produce elastic recoil of the lung?
- Tissue elastance
- Surface tension
What causes tissue elastance?
What fibers are involved?
Stretching of the lung parenchyma
Fibers involved are
- Elastin
- Collagen (type IV) gives strength to alveolar basement membrame (geometry contributes to recoil)
What lung structure is involved in creation of surface tension?
Alveoli
(curved surfaces, lined w/ thin layer of fluid)
Surfactant _______ but does not _______ surface tension
Surfactant reduces but does not abolish surface tension
What is LaPlace’s Law for a soap bubble?
P = 4T / R
You have a small soap bubble and a large soap bubble connected by a stopcock. If the stopcock is turned so that the small and large bubbles are connected, what will happen?
The small bubble will empty into the large bubble
According to LaPlace’s Law, P increases as R ________.
Why?
According to LaPlace’s Law, P increases as R decreases
This occurs b/c surface tension is constant and b/c of geometry (alveoli have curved surfaces)
Why does surface tension contribute to recoil force of the lung? Use a beaker of water as a model.
The surface layer is compressed b/c unequal forces cause the surface layer to move closer to the one below it
How does surface tension produce pressure development in a bubble?
Attractive forces can be separates into horizontal and vertical components
The vertical components sum and point towards the center, which generates pressure
Describe the effect of bubble size on pressure
A larger radius bubble produces less inward pressure
(the vertical components are smaller because the radius is larrger)
Describe the influence of surfactant on surface tension in small versus large alveoli
There is a higher concentratiion of surfactant in small alveoli
- Reduces surface tension more in small alveoli
- Reduces tendency for small alveoli to collapse
Surfactant stabilizes small alveoli more
What occurs when there is a lack of surfactant?
- Produces strong lung rectractile force (reduces compliance of lung)
- Tends to collapse alveoli (millions of tiny curved surfaces, causes atelectasis in premature infants)
What is the formula for compliance?
Compliance = ∆V / ∆P
Greater slope means greater compliance (greater volume for a given change in pressure)
What factors contribute to lung compliance?
- Elastic components of lung tissue (elastin)
- Geometry (mesh) of collagen fibers
- Surface tension
What is the effect of filling a lung with saline?
- Abolished surface tension (removes air-fluid interface)
- Increases compliance (steeper slope)
- Markedly reduces hysteresis (surfactant produces hysteresis)
What is hysteresis?
Different pressure for a given lung volume during inflation compared to exhalation
What is surfactant composed of?
What cells produce surfactant?
30% protein + 70% phospholipid (dipalmitoyl phosphatidyl choline)
Produced by type II alveolar epithelial cells (activated at 30 weeks gestation)
What causes hysteresis?
Addition of surfactant to fluid surface (from micelles) during lung expansion increases the surfactant concentration, which is higher at any specified lung volume during deflation versus inflation
Describe the action of surfactant during lung expansion
- Increased area allows surfactant to spread over surface
- Creates room for micelles to move to surface
- Allows addition of more surfactant
What is the effect of infants w/ RDS lacking surfactant?
Lungs tend to collapse after each breath, so every breath requires a large effort
(normally, surfactant is distributed over the first few breaths, so the effort required for each breath reaches a steady state value which is much lower than for the first breath)
What effects does reduction of surface tension by surfactant have?
- Reduces work of breathing (increases lung compliance)
- Helps stabilize alveoli (reduces tendency for small alveoli to collapse)
- Causes surface tension to vary w/ surface area (i.e. hysteresis)
Describe the total compliance near FRC compared to individual compliances in lung and chest wall
Total compliance < individual compliances
(at FRC compliances of lung and chest wall are the same, like adding resistances in parallel)
Consider an isolated chest wall (no lungs) w/o any external forces acting on it. Making the chest wall stiffer will ________
Have no effect on its volume
Consider an isolated lung (outside the thorax) w/o any external forces acting on it. Making this lung stiffer (e.g. idiopathic pulmonary fibrosis) will ________
Have no effect on its volume
Lung and chest wall compliance determine _____
Lung and chest wall compliance determine FRC
What occurs in a pneumothorax?
- Lungs collapse due to elastic recoil of lungs
- Chest wall expands
Say you stretch the lungs and attach them to the chest wall via a fluid layer. What occurs?
Lungs recoil, which pulls lungs and chest wall to a smaller volume (FRC) and produces a negative Pip
When the lungs recoil to a smaller volume, what occurs?
- Force of lung recoil decreases
- Force of chest wall increases
- FRC occurs when recoil force of lung = chest wall
- Pip = -5 cm H2O
What is the effect of a low compliance (stiff) chest wall?
Low compliance (stiff) chest wall makes
- FRC larger than w/ normal chest wall
- Pip more negative
If the chest wall is stiffer (less compliant) than normal but the lungs are normal, FRC will be ________
If the chest wall is stiffer (less compliant) than normal but the lungs are normal, FRC will be larger than normal
What is the effect of a low compliance (stiff) lung?
Low compliance (stiff) lung
- Decreases FRC (pulls chest to smaller volume)
- Makes Pip more negative
If the lung is more compliant (emphysema) than normal but the chest wall is normal, FRC will be ______
If the lung is more compliant (emphysema) than normal but the chest wall is normal, FRC will be larger than normal
If the lung is more compliant (emphysema) than normal, at FRC the Pip will be _______
If the lung is more compliant (emphysema) than normal, at FRC the Pip will be less negative
Expanding lungs w/ low compliance requires ______ force
Expanding lungs w/ low compliance requires greater force
As you go down the lung, relative ventilation ________ and blood flow _______
As you go down the lung, relative ventilation increases and blood flow increases
Why is there a mismatch of ventilation and blood flow?
Ventilation increases from apex to base b/c gravity but blood flow increases from apex to base by a greater amount
At the apex, alveoli are _______ and _______, so there is _______ ventilation
At the apex, alveoli are larger and less compliant, so there is less ventilation
At the apex, there is ______ resistance, so there is ______ blood flow
At the apex, there is higher resistance, so there is less blood flow
At the base, alveoli are _______ and _______, so there is _______ ventilation
At the base, alveoli are smaller and more compliant, so there is more ventilation
At the base, there is ______ resistance, so there is ______ blood flow
At the base, there is lower resistance, so there is greater blood flow
Alveoli at ____ are pulled open
Alveoli at apex are pulled open
(local compliance is higher for alveoli in base, ∆P is same at top and base of lung while ∆V is larger at base)
At residual volume, apex is ______ ventilated than the base
At residual volume, apex is better ventilated than the base
(Pip is positive at the base: small airways are collapsed, compliance is smaller in the base than the apex)
What occurs in congenital lobar emphysema?
- Overinflation of one or pulmonary lobes
- High compliance of diseased lobes, so diseases lobes expand more readily than healthy lobes, which reduces expansion of healthy lung region
During inspiration, an increase in airway resistance above the level of the glottis (e.g. inflamed tonsils and adenoids will make Pip _______
During inspiration, an increase in airway resistance above the level of the glottis (e.g. inflamed tonsils and adenoids will make Pip more negative
During expiration, inflammation of the tonsils and adenoids will make Pip _______
During expiration, inflammation of the tonsils and adenoids will make Pip less negative or more positive
What is the relationship between flow, PA, and airway resistance?
Flow = (PB - PA) / resistanceairway
Flow is equal to pressure gradient divided by resistance of airway
Lung expansion during inspiration _______ PA
Lung expansion during inspiration decreases PA
PA is closely related to airway resistance and airflow
For a given rate of lung expansion, the greater the resistance, the more PA will _______
For a given rate of lung expansion, the greater the resistance, the more PA will decrease
Decreasing PA makes the lung more _______ and makes Pip more _______
Decreasing PA makes the lung more difficult to expand (stiffer) and makes Pip more negative
Intrapleural pressure changes during a breath depend on _______ and _______
Intrapleural pressure changes during a breath depend on elastance and airway resistance
Effort independence occurs during ______ but not ______
Effort independence occurs during expiration but not inspiration
Peak expiratory flow occurs near _____
Peak expiratory flow occurs near TLC
In the Starling resistor, what occurs when Pe > Pi > PO ?
When Pe > Pi > PO, tube collapses and flow = 0
In the Starling resistor, what occurs when Pi > Po > Pe ?
When Pi > Po > Pe, flow is a function of Pi - Po (∆P)
(pressure in tube greater than pressure in airway)
In the Starling resistor, what occurs when Pi > Pe > Po ?
When Pi > Pe > Po, the tube will flutter open and closed and flow will be continuous and a function of Pi - Pe
Describe mechanical interdependence
Tethering reduces airway (and alveolar) collapse
What causes flow limitation?
Dynamic compression of the airways
On forced expiration, lateral pressure is lower where velocity is ______
On forced expiration, lateral pressure is lower where velocity is higher
Airway compression (when Pairway < Pip) causes ______
Airway compression (when Pairway < Pip) causes flow limitation
Describe dynamic airway compression
- Pip > Pairway
- Airways effectively tethered to chest wall
- At larger lung volumes, tethers are stretched and more effectively stiffen airways
Describe effort independence
- Occurs during expiration but not inspiration
- Influenced by tethering, so volume dependent
- Emphsema increases lung compliance, decreasing tethering
Dynamic compression is effort ________
Dynamic compression is effort independent (flow limited)
- As effort increases, Pip increases, increasing compression and limiting flow
What produces wheezing sounds?
Turbulent flow through narrowed airway produces wheezing sounds
