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The volume-time tracing and flow-volume loop ascertain the technical adequacy of a maneuver and therefore the quality of the data (see Box 3 ) as well as identifying the anatomic location of airflow obstruction. The volume-time tracing is most useful in assessing whether the end-of-test criteria have been met, whereas the flow-volume loop is most valuable in evaluating the start-of-test criteria. The technique of back-extrapolation of the start of the test to establish a zero time point on the volume-time tracing has been carefully defined and provides a uniform start point for timed measurements. It corrects for delayed or hesitant starts that might otherwise be mistaken for a falsely reduced FEV 1 . Standards for acceptability define limits for the degree of hesitation that can still yield an acceptable FEV 1 (see Box 3 ). The loss of elastic recoil characteristic of emphysema results in airflow limitation during the maximal forced exhalation that may be grossly underestimated if the patient applies less than maximal expiratory force. Such efforts may still be deemed acceptable using the criteria of extrapolated volume. The time to peak flow appears to have excellent usefulness in identifying such efforts in this population (time to peak flow will be greater than 120 msec when effort is submaximal), but it is not yet a recommended acceptability criterion ( Fig. 3 ).

Define steroids function

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