The forced inspiratory vital capacity (FIVC) consists of:
1) full expiration to residual volume, then
2) rapid maximal inspiratory effort
FIF(25-75%) is the rate of airflow over the middle half of the FIVC. This normally exceeds FEF(25-75%). A fall in the FIF(25-75%) is a more sensitive indicator of respiratory muscle dysfunction or suboptimal effort than the FEF(25-75%).
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