Respiratory Mechanics Flashcards
Relationship between pressure and volume at low lung volumes?
At low lung volumes, it takes a fairly big pressure change to make a small increase in volume
What happens once there is some air in the lungs?
Once there is some air in the lungs, a little pressure change produces a large volume change-in other words, it becomes easy to stretch the lungs
When the lung gets close to the TLC?
It again becomes difficult to inflate-so a large pressure change produces a small change in volume
Compliance of the lungs
- What is it?
- How is it measured?
- Measure of the “stretchability” of the lungs
- (change in volume)/(change in pressure)
When is compliance highest?
- The compliance of the lungs is highest in the normal breathing range
- Don’t have to work particularly hard to get a decent Vt
What happens to compliance at either extreme (too small or too large)?
Compliance is low-have to work harder to inflate the lungs at low or high volumes
Thought question
- What is the compliance of the lungs for the first breath?
- How hard does the baby have to work?
- Prior to the first breath, lung volume is very low, compliance is low, and the effort to breath is high
- With successive breaths, the baby’s lungs inflate and become more compliant, so work decreases
Describe the pressure-volume curve of the normal lung and explain the differences observed in inspiration versus expiration
-Respiratory mechanics: pressure and volume relationships
- Something really interesting happens when we allow our lung model to “exhale”-The lung doesn’t deflate the same way it inflated
- Even better, if we inflate the lungs with saline (rather than air), the whole shape changes and the lung mostly deflates the same way it inflated
Why is surfactant important?
Alleviates surface tension-reduces tension in the smallest alveoli more than in larger alveoli
The difference in the inspiratory and expiratory lines on the graph is mostly due to?
the presence of surfactant
Hysteresis
difference between inspiration and expiration
It is believed that the hysteresis is due to?
the molecules of surfactant moving one direction during inspiration but coming back a different way
- surfactant is not evenly distributed on the alveolar surface
- small droplets randomly positioned
- if the alveolar volume is small, the droplets are close together and pull apart during inspiration/fuse during expiration
The slight hysteresis of the saline filled curve is due to?
the resistance of the tissue sliding over one another
Define compliance
-equation?
change in volume/change in pressure
What does low compliance mean?
Low stretchability; hard to inflate
(high compliance is the opposite)
Compare and contrast the compliance of the lung vs. the rib cage vs. the intact system
- Be able to draw the figure!
- Lungs?
Lungs have lots of elastic fibers so they “want” to get smaller (minimal volume)
Rib cage
Due to joint arrangement in body wants to get larger
Intact system
The elastic recoil of the lungs and the rib cage exactly counter each other at FRC
What happens in a pneumothorax?
The lungs get smaller and the rib cage gets larger
Airway resistance equation
R=(8(viscosity)(tube length))/r^4
Contraction of bronchial (airway) smooth muscle changes the radius of the airway and therefore?
changes the resistance
Forced expiratory volume
- Very helpful in identifying changes in the pulmonary mechanics
- Both the FEV itself (total volume out) and how fast the air can be moved out of the lungs is important
FEV1
The amount of air that can be exhaled in the first second is used as one measure of pulmonary function
FEV1:FVC
-normal?
about 80% of the FVC is exhaled in the first second of expiration
FVC, FEV1, and FEV1:FVC in patients with obstructive disease
-2 obstructive diseases?
- 2 obstructive diseases-asthma and emphysema
- It takes far longer to get air out of the lungs
- The FVC is reduced because the air is trapped
- FEV1 is even more reduced
- FEV1:FVC is below 70%
FVC, FEV1, and FEV1:FVC in patients with restrictive disease
-Restrictive disease?
- Interstitial fibrosis
- FVC and FEV1 are both reduced due to inability of the lungs to expand
- FEV1:FVC ratio is higher than normal (>90%) because the elastic recoil of the lungs forces the air out more rapidly
Describe the control of airway resistance
Via the smooth muscle and ANS
Define interdependence
- The structural stability of the small airways and alveoli depends on all the connecting airways alveoli
- The elastic recoil in one alveolus is countered by the elastic recoil of an alveolus with shared walls
Describe how interdependence influences airway resistance
-If you lose the alveolar walls (due to age, smoking, etc) remaining alveoli can collapse during expiration
Describe the work of breathing and how it influences normal respiratory processes
- Work is done to overcome the elastic recoil (emphysema) of the lungs or resistance to airflow
- The elastic recoil is the greater effort (bigger area of the curve)
Describe the factors that alter the work of breathing
?
What happens to air flow as intrapleural pressure becomes more negative?
Air flows into the lungs
