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The Maximum Expiratory Flow-Volume Curve with Air and a Low ...

The Maximum Expiratory Flow-Volume Curve with Air and a Low ...

morning was different

morning was different from the afternoon or that a consistent learning or other progressive change in measurement existed. The within-subject CVs for the V50, V50, and VisoV were 8.7 percent, 31.5 percent, and 65.2 percent, respectively. The contributions to varialion within a subject were estimated from the reading-to-reading, observer-to-observer, time-to-time, and day-to-day sources of variation ( Table 4) and compared with the amount of variation between subjects. It can be seen that for the V50 the intraobserver, inter-observer and time effects were low compared with the subject effect. For the iV5o, the observer effects were a little larger, although still small compared with the main sources of time and subject effects. For the VisoV, the observer effects were relatively much larger. However, they contributed less than 10 percent of all sources of variation within subjects, and there was little additional variation due to inter-observer differences compared with intra-observer differences. The major effect on the VisoV was the time effect, and there was little additional variation due to subject effects compared with observer and time effects within subjects, ie, within-subject variability accounted for virtually all the variability observed. For all three measurements, the day-to-day variation was not muchiarger than the time-to-time vanation. (For the zV5o, it actually happened to be a little smaller. ) This indicates that they measure the same thing, ie, variability due to repetition of the test regardless of time of day or of day. When looking for a true change in an individual’s measurement or the difference between two individuals that is not due simply to deviations from one reading to another or day-to-day fluctuation, the upper and lower limits of the 95 percent confidence intervals for the 5O, zV5O, and VisoV are the observed measurements ± 1.25 L/sec, ± 42.02 percent and ±23.45 percent, respectively. When these are expressed as a percentage of an average reading, the results are ±26.5 percent, 89.4 percent, and 192.2 percent respectively. These confidence intervals apply to one test read by one observer only once on each of the days compared, or each of the individuals compared and a different observer for each. These intervals become shorter when two tests are performed on each of two consecutive days for each of the times compared or individuals compared. For the V50, zV5O, and Viso’T, the intervals become ±0.73 L/sec, ±21.02 percent and ± 12.21 percent, respectively ( or expressed as a percent of an average reading, ± 15.3 percent, ±44.7 percent and ± 100.1 percent, respectively.) All of the previous intervals would be divided by 1.414 when calculating confidence intervals about one estimate instead of a difference between two estimates. Table 5-Sum of Squared Deviations by Component Parts for the V50 Source Sum Squares DF MS F Between subjects 1 147.538 9 127.504 863.46t Between days within subject Between days Days by subjects 56.809 1 1 .547 45.262 30 27 3 1.894 3.849 1.676 2.30 20.02t Between time within days within subjects Between time Time by subject Time by day Time by subject by day 45.435 0.007 11.145 4.553 29.730 40 1 9 3 27 1.136 0.009 1.238 1.518 1.101 0.01 23.45t 1.38 29.76t Between observers within time within days within subjects Between observers Observers by subjects Observers by day Observers by time Observers by subjects by time Observers by subjects by day Observers by day by time Observers by subject by day by time 11.261 0.267 2.658 0.926 0.009 1.047 4.095 0.257 2.002 160 2 18 6 18 54 54 2 6 0.070 0.133 0.148 0.154 0.005 0.058 0.076 0.043 0.037 0.90 8.68t 2.04 0.08 3.42t 4.46t 1.16 2.18t Residual 8.167 480 0.017 Total 1269.210 *DF = degree of freedom, MS = mean square. tSignificant (P

DIscussIoN The variability of any physiologic measurement includes subject variability, observer variability, and factors introduced by equipment and technique. In this study, no attempt was made to quantify the latter. The first third of the MEFV curve is effort-dependent, whereas the last two-thirds are thought to be relatively effort-independent.”12 Several studies have shown a constant within-subject CV for maximum flows throughout the VC range.13’14 However, McCarthy et al7 have shown that effort-dependent tests tend to be more reproducible than effort-independent tests. The present study examined three measurements derived from the last two-thirds, that is, the relatively effort-independent part of the MEFV curve. The V50 was found to be a stable test within subjects with little observer and time to time variability. The major variability was between subjects and the factors governing intersubject variability have been examined previously.8’14’15 The confidence interval for the ‘5O suggests that in a clinical situation, the performance of the test on one occasion initially and on restudy will be sufficient to decide whether a change is significant. The within-subject CV for V50 was much higher than for V50. This is expected, since a number of factors contribute to the variability of 15O. First, it is derived from two curves, each with its own variability; second, when vital capacities obtained with air and He-02 are not absolutely identical, superimposition of the curves must introduce errors. By consistently matching curves at residual volume, such errors are randomized.5 Third, with the high rates of flow of normal healthy young adults, the slope of the curves at the 50 percent VC point is steep. Therefore, even small errors in localizing the 50 percent VC points on the curve may lead to considerable errors in the flow measurement. It is therefore surprising that Zeck, et al10 found an identical CV (6 percent) for both V50 and V5o. However, their methodology was not stated, so it is difficult to know if techniques different from those used in this study were a factor in their low CV for V5o. It is likely that the use of direct writing x-y recorders can diminish the thickness of the traces and reduce some variability. The variability of the V5o from time to time was very large. The confidence limits suggest that a battery of tests ( probably in excess of four on each occasion ) is necessary if meaningful results are to be obtained. However, since there was little difference in the AM-to-PM variability compared with the day-to-day variability, it is probable that repetition of the test can be performed at any time, even consecutively. It is of interest that the very large range of values obtained in normal people,5 ie approximately 20 to 100 percent, may be largely a reflection of the time-to-time variability of the test. When the test was performed on eight occasions as in this study, the range for the ten subjects reduced to approximately 31 to 61 percent ( Table Table 6-Sum of Squared Deviations by Component Parts for V50 Source Sum Squares DF MS F Between subjects 43365.586 8 5420.698 95.76* Between days within subject Between days Days by subjects 44068.000 3391 .957 40676.043 27 3 24 1632.148 1130.652 1694.835 0.61 24.30k Between time within days within subjects Between time 70313.000 2098.080 36 1 1953.139 2098.080 1.02 Time by subject 1 6480.086 8 2060.01 1 28.50* Time by day 6494.204 3 2164.735 1.15 Time by subject by day 45240.630 24 1885.026 21.90* Between observers within time within days within subjects 10734.001 144 74.542 Between observers 143.614 2 71.807 1.31 Observers by subjects 875.302 16 54.706 10.85* Observers by day Observers by time 705.719 85.392 6 2 1 17.620 42.696 1.84 0.56 Observers by subjects by time Observers by subjects by day 1215.525 3060.364 16 48 75.970 63.758 15.07* 12.65* Observers by day by time 61 1 .991 6 101 .998 1.21 Observers by subject by day by time 4036.094 48 84.085 16.68* Residual 15380.666 432 5.042 Total 183861.253 *Significant (P

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