Decay of inspiratory muscle activity in chronic airway obstruction

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 ...

Section: Analysis Of Expiratory Tidal Flow Patterns As a Diagnostic Tmentioning

confidence: 60%

mentioning

confidence: 60%

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

Morris

Madgwick²,

Collyer³

et al. 1998

“…In obstructed airways, peak expiratory flow is reached sooner than in those with normal airways. This more rapid rise in expiratory flow is thought to be due to a faster decline in post-inspiratory muscle activity [7,8]. As a consequence, a greater part of expiration is passive in subjects with airway obstruction.…”

mentioning

confidence: 99%

Tidal expired airflow patterns in adults with airway obstruction

Williams

Madgwick²,

Morris³

1998

Earlier studies have shown that time and flow indices derived from tidal expiratory flow patterns can be used to distinguish the severity of airway obstruction. This study was designed to address two aspects of tidal expiratory flow patterns: 1) how do expiratory flow patterns differ between subjects with normal and obstructed airways; and 2) can a sensitive index of airway obstruction be derived from these pattern differences? Tidal expiratory flow patterns from 66 adult subjects with varying degrees of airway obstructive disease with a forced expiratory volume in one second (FEV1) of 20-121% predicted were examined. In each subject, the expired flow pattern from each consecutive breath was scaled and then averaged together to create a single expired pattern. A detailed examination of the scaled flow patterns in 12 subjects (six with normal airways and six with airway obstruction) showed that the shape of the post-peak expiratory flow portion was different in the subjects with airway obstruction. A slope index, S, was derived from the scaled patterns and found to be sensitive to the severity of airway obstruction, correlating with FEV1 (% pred) with r2=0.74 (p<0.05, n=57). The S index also correlated (r2=0.36, p<0.05, n=47) with the functional residual capacity (FRC) (% pred) which was >100% in subjects with severe airway obstruction and lung overinflation. In subjects with normal airways, three further airflow patterns could be distinguished, which were different from the patterns seen in subjects with the severest airway obstruction. Scaled flow patterns from tidal expiration collected from uncoached subjects, can be used to derive an index of airway obstruction.

“…In COPD, electrophysiological studies suggest that the inspiratory muscles "switch off" early in expiration [13,14], but in asthma it appears that tonic inspiratory muscle activity may remain throughout expiration and this may contribute to the hyperinflation, at least when asthma is induced acutely, e.g. with histamine or methacholine.…”

Section: Mechanisms Of Hyperinflationmentioning

confidence: 99%

Pulmonary hyperinflation a clinical overview

Gibson

1996

107

P Pu ul lm mo on na ar ry y h hy yp pe er ri in nf fl la at ti io on n a a c cl li in ni ic ca al l o ov ve er rv vi ie ew w G.J. GibsonPulmonary hyperinflation a clinical overview. G.J. Gibson. ERS Journals Ltd 1996. ABSTRACT: Pulmonary hyperinflation is usually defined as an abnormal increase in functional residual capacity, i.e. lung volume at the end of tidal expiration. As such, it is virtually universal in patients with symptomatic diffuse airway obstruction. Hyperinflation inferred from a standard chest radiograph implies an increase in total lung capacity.The relaxation volume of the respiratory system (Vr) increases in patients with chronic airway disease as a result of changes in the elastic properties of the lungs and chest wall. In addition, a variable degree of dynamic hyperinflation may be present. This results from the onset of inspiration before lung volume has fallen to Vr. Dynamic hyperinflation is frequently present at rest in patients with moderate-to-severe airway obstruction, and it increases further on exercise, thereby increasing the mechanical load on the inspiratory muscles and at the same time reducing their mechanical advantage.Important clinical consequences and associations of hyperinflation include: distortions of chest wall motion; impaired inspiratory muscle function; increased oxygen cost of breathing; greater likelihood of hypercapnia; impaired exercise performance; and greater severity of breathlessness. The symptomatic improvement after treatment with a bronchodilator may be due, in part, to lessening of hyperinflation.