Flow-volume curve

A. Draw an expiratory flow volume curve for a forced expiration from total lung capacity. Describe its characteristics in people with normal lungs, as well as those with obstructive and restrictive lung disease (00B3)

B. Draw a respiratory flow/volume loop and outline how it is obtained. Briefly explain the physiological mechanisms involved in the concept of flow limitation. (96B6)

C. Draw a flow/volume curve for a maximum forced expiration in a person with healthy lungs from: (a) Total lung capacity; (b) Function Residual capacity. Explain your curves. (98A2)

 

Flow-volume curve

Flow-volume curve is drawn with flow being the y-axis and volume being the x-axis.

• when lung volume is high, effort can increase expiratory flow
• when lung volume is low, there is a limit to which effort can increase expiratory flow

Obstructive pathology

In lungs with obstructive pathology,

• total lung capacity is increased
  - increased airway trapping
• FEV1 is decreased
• FVC is decreased
• FEV1/FVC ratio is decreased because FEV1 decrease more significant
• The latter part of curve has a scooped-out appearance

Restrictive pathology

In lungs with restrictive pathology

• total lung capacity is decreased
  - decreased compliance
• FEV1 is decreased
• FVC is decreased
• FEV1/FVC ratio is normal or often increased because FVC decrease more significant
• The latter part of the curve has abnormally high flow relative to the volume

Dynamic compression of airway

The reason for the effort-independent portion of the flow-volume curve of a forced expiration is because of dynamic compression of airway.

Airways past Z11 has no structural rigidity and rely on the radial traction from the surrounding tissue

=> these airways can be compressed by a reversed transmural pressure gradient

During a forced expiration

=> increase in intrapleural pressure

=> compression on both alveoli AND airway

=> increase in alveolar pressure and airway pressure

Initially, airway are held open because airway pressure is greater than intrapleural pressure.

Because of resistance to airflow, airway pressure decreases progressively when moving away from alveoli.

Eventually equal pressure point is reached where the pressure inside the airway is the same as that outside - airway is only held open by elastic recoil of lung parenchyma or structural rigidity.

Downstream of the equal pressure point, transmural pressure gradient is reversed.

During expiration, as lung volume decreases, equal pressure point moves from larger airways towards alveoli

=> when it moves into the smaller airways (Z11 and beyond), the reversed transmural pressure gradient causes collapse

 

Starling resistor mechanism

"Starling resistor mechanism" - driving pressure for flow is the difference between alveolar pressure and intrapleural pressure, rather than that between alveolar and mouth.

=> because effort will increase both alveolar pressure and intrapleural pressure

=> this driving pressure remains the same even with increased expiratory effort

=> maximum flow is constant for each particular lung volume

=> maximum flow decreases as lung volume decreases

 

Factors affecting dynamic compression

• Airway resistance
  => increased resistance
  => more rapid airway pressure drop
  => earlier airway collapse
• Compliance
  => increased compliance
  => reduced recoil
  => reduced driving force
• Lung volume
  => reduced volume
  => reduced lung recoil
  => reduced driving force

Lung volume also affect airway resistance

=> the smaller the lung volume, the greater the airway resistance

Additional

????recoil decrease => intrapleural pressure increase, airway pressure decrease => earlier collapse

 

Examiner's comment

• pneumotachography
• concept of flow limitation and closing capacity
• correct flow-volume curve, with correct labeling of TLC, FRC, RV
• factors affecting dynamic compression: airway resistance, lung compliance, lung volume
• Need to discuss why peak flow is less during expiration from lower lung volume
• (extra) volume-dependent airway closure, its variation with age and compliance
• (extra) lung volume's effect on: airway resistance, lung elastic recoil, driving pressure
• (extra) equal pressure point, movement during expiration and with changing airway resistance
• Need to mention: intrathoracic pressure affecting alveolar pressure
• Need to mention: the pressure drop along the airway due to resistance to flow
• Need to mention: extramural force exceeding intramural force leads to compression and limit flow

Need to add

pneumotachography, i.e. measurement of flow-volume curve