Two Aspects of End of Test
There are two aspects of each maneuver are valuated to determine whether a satisfactory end of a test (EOT) has been achieved. First, forced expiratory time (FET) of a maneuver for evaluating whether the subject has "has tried to exhaled" for 6-seconds (3-seconds for children < 10 years of age). Second, the shape of the volume-time curve is evaluated to determining whether a plateau in the volume time curve has been reached. The ATS/ERS-2005 defines an plateau in the volume-time curve as the point in the volume-time curve where there is no change in volume (within 25 ml) or specifically "less than 25 ml exhaled in the previous second".
End of Test Illustration
To illustrate these issues, test results from an 19-year old female are described. The FVC, FEV1, and FEV1/FVC% values are well within the normal range, above the lower limit of normal.
Below is the flow-volume plot of the three acceptable maneuvers performed.
Below are plots of the three acceptable volume-time maneuvers. Notice that the curves are repeatable both in terms of the FVC and FEV1 values and the shapes of the curves. This younger subject reaches a plateau (essentially no change in volume) after one to two second and fluctuates slightly around this volume for the remaining 4 to 7 seconds. With the dry-rolling sealed spirometer used in NHANES III, volume is collected until the subject removes the mouthpiece. So in this example, the subject has clearly attempted to continue to exhale for longer than the minimum expiratory time of 6-seconds recommended in ATS/ERS-2005. However, the highest volume on the curves occurs before 6-seconds. So, if the FET is defined as the time required to reach the highest volume (FVC), the subject would not have met the requirement of a 6-second FET. A maximal effort at the end of the maneuver would probably do little to increase the volume further from what was observed after 3 to 4 seconds. Some spirometers terminate the maneuver once a plateau has been achieved, and the expiratory effort time is not available and would equal the FET -- much less than 6-seconds.
To determining whether a sufficient exhalation time has been achieved, the ATS/ERS-2005 states: "tried to exhale for 6-seconds" as a goal during test performance. The objective is to determine if the effort lasted a sufficient time for a maximal volume to be achieved. Therefore, the FET (time required to achieve the maximal volume) is not the best parameter to be used to determine if the expiratory effort lasted for a sufficient time.
Plateau Volume
As stated above, the ATS/ERS-2005 defined the plateau as the point in the volume-time curve where there is no change in volume (within 25 ml). This definition was specifically stated as "less than 25 ml exhaled in the previous second" in a proposed clarification letter. Below are two examples of measurement of the plateau volume.
In the first example, the final 3.5 seconds of the maneuver has been enlarged. In this example, the largest volume (FVC) is observed at approximately 4.5 seconds and there is no change in volume, i.e. within 25 ml or the dashed-red lines) for 1.56 seconds. So, the plateau has been achieved since there is no change in volume within 25 ml.
The second example is less clear if a plateau has been achieved as the volume drops below the 25 ml limit as the subject pulls on the hose. The highest volume (FVC) occurs at approximately 5.4 seconds and no change in volume (within 25 ml) around the FVC provides a plateau time of 0.71 seconds and a one-second plateau has not been achieved. However, if the "less 25 ml exhaled in the previous second" criterion is used, the curve meets the EOT requirement and the plateau time would be 3.28, exceeding the one-second plateau requirement. Although the volume does drop below the 25 ml limit, the subject has not exhaled more than 25 ml in the last 3.28 seconds.
For the above curve, it could be argued that the plateau was achieved at 3.3 seconds and so the maneuver is over. Some spirometers would terminate the maneuver at approximately 3.3 seconds when a one-second plateau is observed. In addition, the technician could have stopped coaching when the plateau at 3.3 seconds was observed or the slight inhalation at 3.5 seconds. In these circumstances, the slightly higher volume at 5.45 seconds would never have been observed.
The FET in the above curve would be approximately 5.45 seconds, also less than 6-second. Does the above curve meet the ATS/ERS-2005 end of test requirements? Yes, if the "not exhaled more than 25 ml criteria" is used. No, if the "no change in volume within 25 ml" around the FVC is required and the subject was coached to continue exhaling for 6-seconds.
The plateau determination is further complicated by the volume resolution of the spirometer. If the spirometer cannot resolve as little as 25 ml, but rather only 40 ml (as one manufacturer has said is the limit for their spirometer), a plateau will be easier to achieve and the red-bands of 40 ml would be much wider than the 25 ml used in the above illustrations.
It is important to note that early termination or failure to achieve a plateau in the volume-time curve does not mean the FEV1 cannot be derived from this curve or even the FVC if the FVC from this curve is the largest one observed. Also, some subjects with airways obstruction will not reach a plateau in 15-seconds when the maneuver may be legitimately terminated.
This discussion is presented to illustrate the complexity of determining expiratory effort time, forced expiratory time (FET), and whether a plateau has been achieved. All of these are very subject, technician, and instrument dependent.
For comments, please email John Hankinson at comments@occspiro.com