7 Respiratory Mechanics 2 Flashcards Preview

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Flashcards in 7 Respiratory Mechanics 2 Deck (13)
1

Applied forces

  • The pressure required to overcome elastic recoil and viscous forces is normally supplied by the respiratory muscles

2

Inspiratory and expiratory muscles

  • Inspiratory muscles
  • The most important inspiratory muscle
  • Other inspiratory muscles
  • Expiratory muscles

  • 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

How the diaphragm increases the volume of the lungs

  • 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

Inspiration:
The change in pleural pressure, alveolar pressure, flow, and volume during a breath

  • Normally
  • During inspiration

  • 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

Inspiration:
Since the diaphragm increases the volume of the lungs faster than they can fill with air,...

  • 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

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

  • 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

Acitve expiration

  • 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

Active expiration:
Starting volume

  • "Starting volume"
  • Expiratory limitation
  • At lower volumes, however,...

  • "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

Active expiration:
Expiratory flow limitation

  • Alveolar pressure 
  • Pleural pressure

  • 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

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

  • 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

Active expiration:
Equal pressure point (EPP)

  • Equal pressure point (EPP)
  • Airways downstream from the EPP
  • Cartilaginous airways
  • Non-cartilaginous airways

  • 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

Active expiration:
Expiratory flow limitation

  • This critical airway narrowing
  • Expiratory flow

  • 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

Active expiration:
Expiratory flow limitation

  • Increasing expiratory effort
  • Direct relationship between lung volume and maximum expiratory flow
  • Maximum expiratory flow

  • (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

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