Added value of tidal flow-volume loop
|The flow-volume curve during a normal inspiration and expiration (‘tidal loop’), and subsequently recorded during forced in- en expirations serve to demonstrate the considerable ventilatory reserves available to a healthy subject. Compared to resting ventilation inspiratory and expiratory flows can still be considerably increased. During exercise the level of ventilation can be increased by greater inspiratory and expiratory flows, as well as by increasing tidal volume and respiratory rate. In general in exercising healthy subjects the end-expiratory lung volume (FRC) remains by and large the same.|
In the case of severe airway obstruction it is immediately obvious that the ventilatory reserves are next to nil. Flow during a normal expiration is nearly the same as that during a forced expiration: each normal tidal breath is in fact a forced expiration and involves much work of breathing. During exercise this patient can increase the inspiratory, but not the expiratory flow; this leads to hyperinflation, i.e. an increase in end-expiratory lung volume probably on top of the hyperinflation already present when at rest. The net gain is questionable:
- as expiratory flow increases little, even at the higher average lung volume, there is little if any benefit to alveolar ventilation.
- the pre-existing hyperinflation in the case of severe airway obstruction is exacerbated, leading to more pronounced flattening of the diaphragm. The flatter the diaphragm, the poorer it functions as an inspiratory muscle. In extreme cases paradoxic inspiratory movements of the lower ribs (‘Hoover’s sign’) occur, signifying that the diaphragm has turned into an expiratory muscle by causing inward motion of the thoracic cage in stead of outward motion: a sign of respiratory insufficiency.
See also: situation after bronchodilator drug