In 1981, we made the observation that tidal flow versus time has a distinctive pattern in patients with airflow obstruction that is different from the sinusoidal appearance seen in normal subjects. To quantify this difference, we described timing indices derived from the analysis of tidal expiratory flow [1]. These indices, volume expired when the peak tidal expiratory flow reached (VPTEF)/tidal volume (VT) and (time to peak expiratory tidal flow (tPTEF)/ expiratory time (tE) closely correlated with each other and correlated significantly with other indices of airflow obstruction [1]. The index tPTEF/tE, has been demonstrated to be useful in predicting wheezy illness in the new-born [2], in discriminating between asthmatic and nonasthmatic children [3][4][5] and between healthy control subjects and children with cystic fibrosis [5] and showing response to histamine [5,6], methacholine [7] and bronchodilator [3][4][5] in asthmatic children, though recently has not been found to be useful in the diagnosis of narrowed airways in infants [8][9][10].In our original description of the typical flow time pattern of tidal expiration in patients with airflow obstruction, we described three parts to the curve: 1) a rapid rise to maximum flow; 2) a slow decline in flow over most of expiration, followed by; 3) an abrupt fall of flow to zero as the next inspiration is triggered while there is still a measurable expiratory flow. In contrast, in normal subjects, expiratory flow rises more slowly to a maximum, and then decreases gradually to zero, resulting in a smoother appearance of the tidal expiratory flow-time curve.The index tPTEF/tE uses only information from the first part of the expiratory flow-time curve, based on a single point. We have described two further indices [11,12] in an attempt to use the information contained in the period of declining expiratory flow after the maximum flow has been reached (figs. 1 and 2). These indices are: 1) the time constant of the respiratory system (Trs); and 2) extrapolated volume (EV) (the volume of dynamic overinflation), i.e. the extra volume that would have been expired if expiration had continued to the elastic equilibrium volume.The theoretical basis of this analysis is that if expiration is relaxed the decay of flow against time reflects the compliance and resistance of the respiratory system. OTIS et al.[13] described a simple model of the respiratory system consisting of a single compartment of constant elastance served by a pathway of constant resistance. When, in this model, expiratory flow is driven by the relaxation pressures of the lung and chest wall, there is a linear volumeflow relationship. This slope is the time constant of relaxed expiration and by analogy with an electrical model Trs is equal to resistance×compliance of the total respiratory system (lungs+ chest wall). COMROE et al. [14] suggested that volume-flow curves of passive expiration could be used to assess the mechanical properties of the lung and chest wall.In patients with airflow obstruction, there ...

Background High calorie intakes,

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Background -There are times in clinical practice when it would be useful to be able to assess the severity of airways obstruction from tidal breathing. Three indices of airways obstruction derived from analysis of resting tidal expiratory flow have previously been described: (1) Tme/ TE = time to reach maximum expiratory flow/expiratory time; (2) Krs = decay constant of exponential fitted to tidal expiratory flow versus time curve; and (3) EV = extrapolated volume -that is, area under the curve when the fitted exponential is extrapolated to zero flow. In this paper a further index -dt,/TE, time from the beginning of expiration till the rapid decay of flow beginslexpiratory time -is evaluated. The aim of this study was to assess the ability of these indices to detect mild airways obstruction.

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