Airway re-narrowing following deep inspiration in asthmatic and nonasthmatic subjects

Abstract

After bronchoconstriction, deep inspiration (DI) causes dilatation followed by airway re-narrowing. Re-narrowing may be faster in asthmatic than nonasthmatic subjects. This study investigated the relationship between re-narrowing and the magnitude of both DI-induced dilatation and the volume-dependence of respiratory system resistance (Rrs) during tidal breathing.In 25 asthmatic and 18 nonasthmatic subjects the forced oscillation technique was used to measure Rrs at baseline and after methacholine challenge, during 1 min of tidal breathing, followed by DI to total lung capacity (TLC) and passive return to functional residual capacity (FRC). Dilatation was measured as the decrease in Rrs between end tidal inspiration and TLC, re-narrowing as Rrs at FRC immediately after DI, as per cent Rrs at end-tidal expiration, and volume dependent tidal fluctuation as the difference between mean Rrs at end-expiration and end-inspiration.Asthmatic subjects had greater re-narrowing, less dilatation, and greater tidal fluctuations both at baseline and after challenge. Re-narrowing correlated with baseline tidal fluctuation and inversely with dilatation. Both baseline tidal fluctuation and dilatation were significant independent predictors of re-narrowing.Following deep inspiration-induced dilatation, faster airway re-narrowing in asthmatic than nonasthmatic subjects is associated not only with reduced deep inspiration-induced dilatation but also with some property of the airways that is detectable prior to challenge as an increased volume dependence of resistance. It is well recognised that the response of the airways to deep inspiration (DI) differs in asthmatic and nonasthmatic subjects. Following airway narrowing induced by inhaled methacholine, DI causes dilatation of the airways in both asthmatic and nonasthmatic subjects. However, the magnitude of the dilatation is usually less in asthmatic than in nonasthmatic subjects [1,2]. It has been suggested that the inability of DI to overcome bronchoconstriction is a fundamental abnormality of asthma, and could contribute to airway hyperresponsiveness [1].The amount of residual dilatation measured after a DI is likely to be determined both by the degree to which the airways distend in response to the inflationary pressure of a DI and by the rate at which they re-narrow following the release of the distending force. After a single DI, it can take more than a minute for the airways of a healthy subject to renarrow to pre-DI calibre [3]. Using the forced oscillation technique to measure airway resistance continuously throughout a DI, JENSEN et al.[4] measured the extent of both dilatation and re-narrowing, and found that asthmatic subjects both dilated less and re-narrowed more than healthy subjects. They concluded that these differences were attributable to differences in the behaviour of airway smooth muscle (ASM), most likely due to an increase in the stiffness of asthmatic ASM making it unresponsive to stretch. These findings imply that there is an inverse correlation b...

Estimation of the bronchodilatory effect of deep inhalation after a free run in children

“…It was also established, in a preliminary study, that ventilation was close to baseline 5 min after exercise, at the time the measurement was made [11]. An alternative is the measurement of end-inspiratory and end-expiratory values to eliminate the flow dependence of Rrs [5,17]. Upper airway calibre changes during breathing and the tendency for the laryngeal folds to close during expiration probably account for larger Rrs variability compared to that found during inspiration in children [18].…”

“…Rrs or Grs measured using the FOT is increasingly used as an index of airway dimensions during breathing [4][5][6], and represents a potentially powerful tool for evaluating the bronchomotor effects of DI. Two significant physiological sources of Grs variability, however, should be minimised in order to specifically address this effect.…”

“…There are only few studies on the bronchomotor effects of DI in EIAO [10,11], and their correspondence with the Grs response to exercise has not been established. When using a single excitation frequency, the FOT offers a fairly simple way of tracking Grs during breathing, thereby providing an estimate of the effect of DI on airway calibre [5,11], provided the breath-bybreath variability of Grs is taken into account.…”

“…However, few data are available regarding the Grs response to exercise in children in the routine laboratory. Grs measured at frequencies above a few Hertz is thought to express airway conductance to flow [4][5][6], and, in describing this response to exercise, it is important to ensure that the observed decrease in Grs after exercise truly reflects a decrease in airway calibre at the bronchial level. Since the 1980s, the effects of deep breaths on airway mechanics have been the object of intensive research [7,8], and have potentially important applications in the lung function laboratory [9].…”

Airway smooth muscle dynamics: a common pathway of airway obstruction in asthma

“…An advantage of this approach is that it explicitly captures the functional impact of the deep inspiration on the net change in airway diameters averaged over the entire airway tree. Also, one can track the dynamics of airway reconstriction [21,63,141], which may be sensitive to altered ASM function in situ. A disadvantage is that one cannot distinguish airway dilation from either airway or alveolar recruitment.…”