Physio - Lung Mechanics Flashcards
Identify lung volumes and capacities obtained by spirometry
- Vc = Top peak = max inhilation
- Vc = Bottom peak = max expiratory volume
- Vt = Tital volume = normal breaths
- 500mL (females = slightly less)
*Identify the anatomical components of the thoracic respiratory system
Basic structures:
- Thoracic cage
- Lungs
- Airways
- Pulmonary vasculature
- Site of gas exchange
Airways:
- Gas in & out
Alveoli & Pulmonary Capillaries:
- Gas exchange
* What pressures influence lung inflation and deflation?
-
Inspiration
- movement of air into lungs, Fi
- Air pressure at the mouth must be > than air pressure in the alveoli
- Patm > PA
-
Exhalation
- movement of air out of lungs, Fo
- Air pressure in the alveoli must be > than air pressure at the mouth
- PA > Patm
Definitions and Conventions of air pressures?
Basics:
- Patm = 0 mm Hg
- If F = 0, then PA = Patm
- No net movement into/out of lungs
Inhilation:
- In the case of Fi, PA < 0;
- i.e. negative value (-)
Exhilation:
- In the case of Fo, PA > 0;
- i.e. positive value (+)
Why is the pressure gradient from mouth to alveoli necessary?
Basics:
- Pressure gradient = required to push air thru tubes (airways)
- airways offer resistance to air movement
- Airways = distributed network of tubes
Formula:
- PA ‐ Patm = F x R
- F = flow of air at level of mouth
- R = total resistance in pulm sys
- ↑ R by making beginning of tube smaller
- ↑R –> ↑ change in pressures
* How does the distributed collection of airways of the lungs influence pulmonary airflow?
Pulmonary “Tree”
- Exponential increase in # if airways w/ each successive generation
- 2n
- Each generation = smaller diameter
- Each generation = resistance to airflow of each airway INCREASES
- ie: Rn α 1/Dn
Diameter:
Resistance to airflow
Cross-sectional area:
Velocity of airflow:
What is the relationship btw airway diamter, airway resistance, and velocity of airflow?
Basics:
- Conducting airways = 1-16
- Transitional airways = 17-22
In successive airway branches…
- diameter & length = ↓
- total airway cross-sectional area = ↑
- net result = ↓ aggregate airway resistance
- velocity of airflow = ↓ (dramatically)
Bulk Flow vs. Diffusion
Bulk flow of air
- occurs in conducting airways
Diffusion
- gas exchange takes place in respiratory zone
Note:
- Almost NO movement of air in airways of the transitional & respiratory zones
Lung Aerodynamics
Basics:
- Palv– Po = FAir x RAirway
Healthy Lung:
- RAirway = small
- b/c vast number of parallel airways
- Greatest RAirway = bronchiolar airways
- changes in RAirway in this region of the pulmonary tree = IMPORTANT
- changes in RAirway at very small airways = LESS IMPORTANT
- difficult to detect
Example:
-
Asthma - medium sized airways get smaller
- ↑ resistance dramatically!
- difficulty breathing in & out
* What is the time course for lung volume and alveolar pressure during one complete ventilator cycle?
Basics:
- Palv < 0
- air flows in
- Palv > 0
- air flows out
- Palv = 0
- no airflow
Duration:
- Inspiratory duration (Ti) = ~ less than Te
- Ti/Te <0
Notes:
- Airflow measured via pneumotachograph at the mouth
* How does Boyle’s Law help explain negative and positive alveolar pressure?
Boyle’s Law
- P x V = k
- P1V1 = P2V2
- Two important conditions:
- Gas temp = constant
- Amt of gas = constant
- Two important conditions:
- ↑ volume of gas –> ↓ gas pressure
Factors Leading to Negative Alveolar Pressure:
- Lung ~ balloon in rigid container (chest)
- ↑ volume of thorax (inhalation) –> amt of gas in lung = constant –> pressure in lungs ↓ –> air moves in (Phigh atm –> Plow lung)
- aka - subatmospheric
What is Pip?
Pressure in Intraplural Space (PIP)
- (-) during inspiration = ~-8
- ~-5 during expiration
- ALWAYS SUBATMOSPHERIC during Tv!
-
Change in Pip –> expansion & compression of alveolar air
- changes Palv:
- Palv < 0: air –> in
- Palv > 0: air –> out
- Palv = 0: no airflow
- changes Palv:
Measurement:
- meausured via esophageal balloon catheter & Palv
- airflow = measured via pneumotachograph at mouth
Why does forced exhalation lead to positive intrapleural pressures and possible collapse of small airways?
Passive expiration (Tv)
- Modest airflow
- Airway always open
Forced expiration
- initial HIGH airflow
- large intrapleural pressure = Pip (+)
- contraction of internal intercostals & accessory muscles lead to positive Pip
- collapse of airways
- usually cartilage does not allow for collaspe in upper airways;
- but smaller airways can collapse due to pressure difference
- ex: emphasyma
Note:
- Airflow from HIGH –> LOW pressure
What is the importance of dynamic compression of airways?
Basics:
- Palv during expiration
- Degree of collapse is limited due to opposite forces
Try to close airway…
- Pip & Dynamic compression from respiratory muscles
Keep airway open…
- Alveolar elastic recoil & Traction
When things go wrong…
- If we have deterioration of CT, can inhibit Traction –> collapse
What happens to the trachea during a cough?
Normal breathing:
- Large CSA
- ↓ Volume flow
- ↓ Linear Velocity
During cough:
- Small CSA
- ↑ Volume flow
- ↑ Linear Velocity
- turbulant airflow
- abdomen contracts –> large Pip –> rapid movement of air out of lungs
What is lung compliance?
Basics:
- Compliance = change in V/ change in P
-
NONLINEAR due to physical properties of lung
- difference btw pressure in alveoulus (Palv) & pressure outside the lung (Pip)
Relationships:
- ↑ compliance = ↑ lung volume & ↓ pressure
- ex: emphasema - increase lung compliance but difficulty breathing out
- ↓ compliance = ↓ lung volume & ↑ pressure
- more force need to expand the lung (↑ volume)
Note:
- Sigmoid curve = need ↑ pressure w/ ↑ volume
What is Pulmonary System Compliance?
Basics:
- At FRC = Chest wall + Lung have EQUAL forces
- inward recoil of lung tissue = outward recoil of chest wall
- At any volume (above or below FRC), net force of respiratory sys = SUM of RECOIL FORCE
Example:
- At RV = small lung volume (during forced expiration)
- ↓ lung volume & ↓ volume in thoracic cavity
- ↑ tendency to spring outward
Note:
- Surface tension plays a large role
What are the effects of a pneumothorax on lung volume and diameter of the thorax?
Normally…
- At FRC, opposing recoil forces of lung + chest wall = (-) = subatmospheric pressure
- draws air in
During pneumothorax…
- At FRC, opposing recoil forces of lung + chest wall = 0 = atmospheric pressure
- no air movement
- Lung collapse
- Chest wall expands outwatd
What are the effects of posture on respiratory system compliance?
Sitting (Upright):
- Chest wall + lungs = LESS compliant
- ↑ in FRC
Supine (Laying down):
- Chest wall + lungs = MORE compliant
- ↓ in FRC
Factors affecting Compliance:
-
Lung Increase (↓P, ↑V)
- Emphysema (weaker lungs)
-
Lung Decrease (↑ P, ↓V)
- Fibrosis (stiffer lungs)
- Pleural effusion
- Pneumothorax
- Chemical/thermal damange
- Pneumonia/Pulm congestion
-
Chest Wall Increase:
- Supine posture
-
Chest Wall Decrease:
- Obesity
- Neuromuscular disease (myasthenia gravis)
- Abdominal distention
- Upright posture
How does surface tension in two interconnected alveoli causes the expansion or collapse of each alveolus?
Basics:
- we want to decrease the air liquid interface = reduce surface tension
-
Law of LaPlace
- pressue inside alv = proportional to tension
- inversely proportional to radius!
- thus…
- small alv tend to collapse & empty air into larger alv
- pressue inside alv = proportional to tension
Solution:
-
Type 2 alv cell produce surfactant = reduces the surface tension
- essential feature of the aveoli & lungs
- stop tendency for alv collapse
-
Reduces transpulmonary pressure –> which needs to be dev. to inflate the lungs
- important in newborns
Note:
- dipalmitoylphosphatidylcholine (DPPC) = principle component of surfactant
-
Premature babies who dont have surfactant yet…
- placed on ventilaor & nebulized w/ surfactant
-
mom given glucocorocoid
- so baby can start to produce surfactant before C-section
