Moments analysis in clinical spirography--a cul-de-sac?

Mean transit time (MTT) and some higher amounts of the forced expiratory curve were calculated at routine spirography in 190 patients with respiratory disorders. MTT was closely related to traditional indices of airflow obstruction. Within duplicates, both MTT and an index of dispersion of transit times depended strongly on expiratory time and showed a correspondingly poor repeatability. An additive, exponential model of log normally distributed transit times could, for numerical reasons, be adapted in 61 patients only and led in these to a further impairment of precision. A corresponding model of discrete type, universally applicable by optimization, regenerated volume/time curves inaccurately. None of these infinite models proved able to eliminate the influence of expiratory endurance on transit time characteristics. This finding indicates that forced expiratory curves do not in general behave like composite exponentials.