[Problems in measurement of forced vital capacity--quality assessment with extrapolated volume]

In 1994, the American Thoracic Society proposed criteria for the start of expiration, middle of expiration and end of expiration in order to improve the accuracy of forced vital capacity measurement. Extrapolated volume (EV) and percent extrapolated volume (%EV), which serve as the criteria for the start of expiration, were studied in this paper. It was demonstrated that the values of %V50 and %FEV1.0% increased incorrectly when EV or %EV were outside the criteria because of an erroneous start point for expiration or poor technique. Similar results were obtained in the same subjects, who underwent FVC measurement more than twice with different effort levels. The present study demonstrated that obstructive ventilatory impairment may be underestimated if EV or %EV is outside the criteria. We concluded that the values of EV and %EV are parameters which indicate a rapid rise of the internal pressure of the thoracic cavity at the start of expiration and should be considered in seeking correct measurement of respiratory function including FVC.