Analysis of expiratory tidal flow patterns as a diagnostic tool in airflow obstruction

Abstract

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 ...

Relationship Between Spontaneous Expiratory Flow-Volume Curve Pattern and Air-Flow Obstruction in Elderly COPD Patients

“…On the other hand, assessment of the spontaneous expiratory flow-volume (SEFV) curve has been reported as a method to evaluate the severity of airway obstruction, which has the advantage that it is easily measured via spirometry during quiet breathing. [3][4][5][6][7][8][9][10] Shortening of the time to reach the peak expiratory flow 3 and the slope of the late SEFV curve, 4 which can be obtained from the SEFV curve, has been reported to be related to airway obstruction. Moreover, a method to assess the pattern of the SEFV curve has been reported, and concavity of the SEFV curve has been found to be related to expiratory flow limitation.…”

Detecting airways obstruction from the tidal flow profile

“…The results of MORRIS et al [1] and WILLIAMS et al [2] are, therefore, interesting from a clinical perspective be-cause they suggest that the severity of airflow obstruction, a parameter of key concern to the respiratory specialist, may be derived easily and noninvasively, namely from airflow measurements at the mouth. The physiological basis of this approach is not difficult to appreciate.…”

“…The two papers in this issue from the Osler Chest Unit at Oxford University [1,2] exploit the fact that the breathing pattern in humans is affected in characteristic ways when the pulmonary airways are obstructed. MORRIS et al [1] examined the latter part of the tidal expiratory flow pattern in patients with varying degrees of airflow obstruction and found that the expiratory time constant correlated positively with standard measures of airflow obstruction.…”

“…MORRIS et al [1] examined the latter part of the tidal expiratory flow pattern in patients with varying degrees of airflow obstruction and found that the expiratory time constant correlated positively with standard measures of airflow obstruction. WILLIAMS et al [2] also showed that a shape index related to the rate of decay of post-maximal expiratory flow correlated strongly with airflow obstruction, as determined by conventional methods.…”

“…Consequently, it is not surprising that airflow obstruction tends to lead to characteristic changes in the pattern of spontaneous breathing. MORRIS et al [1] and WILLIAMS et al [2] have tried to identify particular features of the expiratory airflow pattern that capture these characteristic changes.…”