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Determinants of maximum inspiratory flow

In a healthy subject everything works together to create large flows during a forced inspiration:
1 The greater the inspiratory force applied, the lower the alveolar pressure relative to pressure at the mouth, and the larger the flow into the lung.
2 During a forced inspiration the (subatmospheric) pleural pressure outside intrathoracic airways is lower than bronchial pressure. The pressure difference is largest at the thoracic outlet and smallest in small intrapulmonary airways. Hence large intrathoracic airways in particular (normally the site of greatest airway resistance) are subjected to a considerable distending force, and this helps in generating high inspiratory flows.

A flow limiting compressed airway segment, such as occurs during a forced expiration, does not occur during inspiration. Obviously, in the case of diffuse airway obstruction, such as in COPD, the increased airway resistance may limit inspiratory flow somewhat. Lung elastic recoil does not play part in inspiratory flow (apart from being a component of pleural pressure); on that account, in the case of pulmonary emphysema but no significant peripheral airway obstruction, inspiratory flow will be normal. This is why in such patients the FEV1 is low, but the FIV1 within normal limits.

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The animation depicts schematically airway dimensions during forced inspirations and expirations.

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