7 Respiratory Mechanics 2 Flashcards
1
Q
Applied forces
A
- The pressure required to overcome elastic recoil and viscous forces is normally supplied by the respiratory muscles
2
Q
Inspiratory and expiratory muscles
- Inspiratory muscles
- The most important inspiratory muscle
- Other inspiratory muscles
- Expiratory muscles
A
- Inspiratory muscles
- The pressure required to expand the respiratory system above its resting volume is normally provided by the inspiratory muscles
- The most important inspiratory muscle
- Diaphragm
- Other inspiratory muscles
- External intercostals
- Sternocleidomastoids
- Expiratory muscles
- Abdominal wall muscles
- Internal intercostals
3
Q
How the diaphragm increases the volume of the lungs
A
- First, as it contracts, it flattens from its normal dome-shaped configuration, and, like a piston, increases the volume of the thorax (and lungs) and decreases the volume of the abdominal compartment
- Second, as abdominal volume falls, intra-abdominal pressure increases, and this pushes the lower ribs outward, further increasing lung volume
4
Q
Inspiration:
The change in pleural pressure, alveolar pressure, flow, and volume during a breath
- Normally
- During inspiration
A
- Normally
- Ppl is negative at end-expiration due to the opposing elastic recoil of the lungs and chest wall
- That is, the visceral and parietal pleurae are being pulled away from each other
- During inspiration
- Lung elastic recoil increases
- The pleural surfaces are pulled apart even more
- Ppl becomes progressively more negative
5
Q
Inspiration:
Since the diaphragm increases the volume of the lungs faster than they can fill with air,…
A
- Alveolar pressure falls (becomes negative)
- This creates a pressure gradient between the mouth (mouth pressure = 0) and the alveoli
- As air enters the lungs,…
- Alveolar pressure becomes less negative
- The pressure gradient driving flow decreases
- Both return to zero by the end of inspiration
6
Q
Passive expiration
- Passive expiration
- As gas leaves the lungs and the respiratory system returns toward its equilibrium volume,…
- When the respiratory muscles are relaxed,…
- The change in pressure, flow, and volume during such a passive expiration
A
- Passive expiration
- The pressure required to overcome viscous forces is provided by the elastic recoil of the respiratory system
- As gas leaves the lungs and the respiratory system returns toward its equilibrium volume,…
- Pressure must be supplied only to overcome the viscous forces produced by expiratory airflow
- When the respiratory muscles are relaxed,…
- This pressure is provided solely by the elastic recoil of the lungs and chest wall
- The change in pressure, flow, and volume during such a passive expiration
- Under these conditions, both flow and volume reach zero only when the respiratory system has returned to its equilibrium position
- I.e. total elastic recoil pressure and alveolar pressure are zero
7
Q
Acitve expiration
A
- In many circumstances, of course, expiration is not passive
- In the presence of high ventilation requirements (e.g. exercise) or disorders affecting the lungs or chest wall, the expiratory muscles supply additional pressure to augment flow and shorten expiratory time
8
Q
Active expiration:
Starting volume
- “Starting volume”
- Expiratory limitation
- At lower volumes, however,…
A
-
“Starting volume”
- Maximum flow reached during forced expiration varies directly with the volume at which expiration begins
- This “starting volume” also determines the relationship between maximum flow and muscular effort
-
Expiratory flow limitation
- At volumes greater than about 85 percent of the vital capacity, increasing effort leads to a progressive rise in expiratory flow
- At lower volumes, however,…
- A maximum flow is reached and cannot be increased regardless of how much pressure is generated by the respiratory muscles
- This is referred to as expiratory flow limitation
9
Q
Active expiration:
Expiratory flow limitation
- Alveolar pressure
- Pleural pressure
A
- Alveolar pressure
- The sum of the pressures produced by lung elastic recoil and the pressure within the pleural space
- The total pressure available to overcome the viscous forces produced during expiratory airflow
- Pleural pressure
- Surrounds both the alveoli and the intrathoracic airways
- Determined by…
- The elastic recoil of the lungs and chest wall
- The activity of the expiratory muscles
10
Q
Active expiration:
Expiratory flow limitation
- The physiologic basis of expiratory flow limitation is most easily understood by examining…
- During a passive expiration
- During a forced expiration
A
- The physiologic basis of expiratory flow limitation is most easily understood by examining…
- The pressures surrounding and within the airways during expiration
- During a passive expiration (panel A)
- Pleural pressure remains negative and is always less than the pressure in the airways
- During a forced expiration (panel B)
- Pleural pressure becomes positive
11
Q
Active expiration:
Equal pressure point (EPP)
- Equal pressure point (EPP)
- Airways downstream from the EPP
- Cartilaginous airways
- Non-cartilaginous airways
A
-
Equal pressure point (EPP)
- Since the pressure within the airways must fall as gas moves toward the mouth (where the pressure is zero), the pressure inside and outside the airways must eventually become equal
-
Airways downstream from the EPP
- Airways will become narrowed at some point downstream (mouthward) from the EPP
- Cartilaginous airways
- Large airways
- Supported by cartilage
- Relatively resistant to compression
- Non-cartilaginous airways
- Easily narrowed because they have little structural rigidity
- Normally supported only by the elastic recoil of the surrounding lung parenchyma
12
Q
Active expiration:
Expiratory flow limitation
- This critical airway narrowing
- Expiratory flow
A
- This critical airway narrowing
- Responsible for expiratory flow limitation
- Expiratory flow
- Resistance is very high in the collapsed airway segment and much lower beyond it
- So expiratory flow is driven by the pressure gradient between the alveoli and this “choke point” (in this example, 10 cmH2O)
- Not by the gradient between the alveoli and the mouth
- Increasing effor tdoes not increase the pressure gradient or flow
13
Q
Active expiration:
Expiratory flow limitation
- Increasing expiratory effort
- Direct relationship between lung volume and maximum expiratory flow
- Maximum expiratory flow
A
- (Panel C) increasing expiratory effort (e.g. by increasing Ppl to 20 cmH2O)
- Only serves to move the EPP
- The site of airway compression closer to the alveoli
- The expiratory pressure gradient and the flow rate do not change
- Direct relationship between lung volume and maximum expiratory flow
- Airway pressure at the EPP is equal to Ppl
- Alveolar pressure is equal to lung elastic recoil pressure plus Ppl
- –> expiratory pressure gradient is equal to lung elastic recoil pressure
- Maximum expiratory flow
- Determined solely by lung elastic recoil
- The pressure gradient is ~ equal to lung elastic recoil pressure
- Varies directly with lung volume
- Expiratory flow is determined by lung volume
- Determined solely by lung elastic recoil