Transit time analysis of spirograms: which blow is best?

Moment analysis applied to the forced expiratory spirogram describes the distribution of fractions of the forced vital capacity with respect to time (t). The first three moments (based on t1, t2 and t3) are used to derive respectively the mean transit time (MTT), its standard deviation, also related indices and an index of skewness. The moments are all obtained from the same spirogram (out of 3 or 5), but the criteria for selection have not been defined. We have looked to see which curve yields the most reproducible indices. 46 naive male volunteers performed five forced expirations on two occasions nine days apart. The spirograms were truncated with respect to volume at 99% of forced vital capacity, the time of which was taken as forced expiratory time (FET), and separately at 6 s when FET exceeded this. The absolute values and reproducibility between days of the first three moments were not significantly different for the first three compared with all five blows. Transit time indices were extracted from the first three blows in nine different ways; they included deriving a mean curve from the averaged moments, taking the curve with the shortest MTT, that with the largest FVC, etc. Reproducibility was best when the indices were calculated from the averaged moments; that for the curve with the shortest MTT was nearly as good. The curve with the shortest FET, largest peak expiratory flow, largest FEV1/FVC ratio or largest area under the flow-volume curve were for most indices equally reproducible, but the absolute results were not always interchangeable.(ABSTRACT TRUNCATED AT 250 WORDS)