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Determinants of maximum expiratory flow

As shown in the previous text and animation flow during a forced expiration is determined by:
1 The elastic properties of the lung:
 
  • the stiffer the lung, i.e. the larger PL,el, the wider the diameter of small, intrapulmonary airways.
  • the stiffer the lung, the greater (for the same flow) the distance between alveoli and that part of the airway where the pressure loss equals = PL,el: the ‘equal pressure point’ is then established in larger and usually stiffer airways.
  • the further the lung is stretched, the larger PL,el.
2 Resistance to flow of smaller intrathoracic airways:
 
  • for the same flow viscous pressure losses increase in proportion to resistance. Hence increased resistance leads to greater pressure losses, and a pressure loss equal to PL,el occurs closer to the alveoli. An ‘equal pressure point’ in smaller and usually more compliant airways will lead to greater dynamic airway compression and hence to greater flow limitation.
  • Airways resistance increases at smaller lung volumes (see 1), and PL,el becomes smaller; therefore maximum expiratory flow decreases during a forced expiration.
3 Airway stiffness at the flow limiting segment.
 
  • the stiffer the airway down from the ‘equal pressure point’, the larger the cross-sectional area of the flow limiting segment and the larger the flow through it.
4 Particularly in elderly subjects airway closure is likely to occur; it occurs in dependent lung regions towards the end of expiration.

The animation illustrates how these factors interact during the recording of a maximum expiratory flow-volume curve.

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