7 Respiratory Mechanics 2

Question 1

Applied forces

Answer 1

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

Question 2

Inspiratory and expiratory muscles

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

Answer 2

• 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

Question 3

How the diaphragm increases the volume of the lungs

Answer 3

• 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

Question 4

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

• Normally
• During inspiration

Answer 4

• 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

Question 5

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

Answer 5

• 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

Question 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

Answer 6

• 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

Question 7

Acitve expiration

Answer 7

• 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

Question 8

Active expiration:
Starting volume

• “Starting volume”
• Expiratory limitation
• At lower volumes, however,…

Answer 8

• “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

Question 9

Active expiration:
Expiratory flow limitation

• Alveolar pressure
• Pleural pressure

Answer 9

• 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

Question 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

Answer 10

• 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

Question 11

Active expiration:
Equal pressure point (EPP)

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

Answer 11

• 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

Question 12

Active expiration:
Expiratory flow limitation

• This critical airway narrowing
• Expiratory flow

Answer 12

• 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

Question 13

Active expiration:
Expiratory flow limitation

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

Answer 13

• (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