the dynamic pressure fluctuations of breathing do not reduce the responsiveness of an excised, intact human airway. Surprisingly, this is the main result presented by Noble et al. (10) in this issue of the Journal of Applied Physiology. How could this result stand in the face of piles of seemingly contradictory evidence at length scales above and below the intact airway level (e.g., Refs. 1, 4, 12, 14)? By bridging the gap in length scales, the intact airway was positioned to provide the crucial piece of evidence to forever solidify the impact of the dynamic mechanical forces of breathing on airway responsiveness. Conversely, the present study by Noble et al. (10) in human intact airways now rigorously extends our recent study on intact bovine airways (8), both showing that the tidal-like pressure fluctuations of breathing do not reduce airway responsiveness compared with static pressures alone. This raises several provocative questions: Why did these intact airways not behave as predicted? Where did we go wrong in so fervently embracing that they would? What does the ineffectiveness of breathing on airway responsiveness mean for understanding airway hyperresponsiveness in disease?
The genesis of this confounding path can be traced to several provocative studies at the isolated airway smooth muscle (ASM) level confirming that oscillatory strain applied to isometrically activated tracheal ASM strips caused sustained reductions in active force and stiffness (4). In addition, large oscillatory strains before isometric ASM activation reduced the subsequent active force generation (14). Early studies at the intact airway level were simply analogous to these isolated ASM studies in that they applied volume oscillations on airways activated at a fixed volume (5). Hence, it was not surprising that they found a reduction in the airway's active pressure. Meanwhile, at the largest length scale, investigators using a variety of approaches confirmed that asthmatic patients have two well-established distinguishing features from healthy patients: their airways are hyperresponsive to ASM agonist and their airways do not remain dilated after a deep inspiration (DI) (1). Connecting the observations from the isolated ASM to the whole organ was too alluring and the research field was off and running with a new simple hypothesis and supporting data; namely, that the inability to stretch the ASM may be the proximal cause of airway hyperresponsiveness. The conjecture was that the lack of proper oscillatory mechanical strain on the ASM layer (due to airway wall remodeling, for example) prevents the breakage of actin-myosin cross bridges, making the ASM excessively stiff and contractile and propagating the transition from a normal airway to a hyperresponsive airway (7).
But the field was actually examining airway responsiveness in its vaguest sense. The isolated ASM studies really asked: if we activate the ASM but require it to stay at its initial resting length, how will it respond when we expose it to lengths above and below its initial length? From these studies, one would need to extrapolate how the ASM would actually cause an airway to narrow, with a hope that the process of narrowing was simply a translation of ASM shortening and consequent geometric diameter reduction. Such an assumption ignores how the act of narrowing would impact the airway wall itself as a system. The field was set on isometric studies because it was excessively difficult to apply time-varying forces to the ASM that would accurately mimic the ASM in situ embedded within the airway wall. One clever approach was eventually designed to mimic the in vivo pressures on an ASM strip (11), and the ASM strip again behaved as predicted: it substantially re-lengthened when exposed to what appeared to be physiological forces. That study seemed capable of setting the hypothesis into reality but several assumptions and mathematical modeling efforts were needed that were not yet fully validated. At the other end of the length scale, during ASM provocation in animals in vivo (12), the degree of constriction inferred from airway resistance diminished with increased amplitude volume oscillations. As larger tidal volumes were applied to increase the amplitude of volume oscillations, the mean transmural pressure (PTM) also increased. Thus these studies simply could not validate the dynamic ASM hypothesis because one would need to separate the effects of the dynamic oscillations from the effects of the concomitant increase in static mean PTM.
The intact airway studies of Noble et al. (10) and LaPrad et al. (8) removed the need to rely on the aforementioned assumptions and were designed to directly address the hypothesis at hand: if one removes an airway from a lung and exposes it to the known pressures of breathing, how will it constrict when stimulated? These studies precisely controlled the most important input parameter for the present question—the pressure across the airway wall (PTM). On top of that, airway responsiveness was directly measured as luminal diameters or volumes. By altering its caliber, the airway could thus freely respond to PTM fluctuations in order to maintain a complex balance between the stresses created by the PTM, the active stresses created by the ASM, and the passive stresses of the wall tissue within a three-dimensional geometry. Thus, by using an intact airway preparation, these studies (8, 10) no longer needed to extrapolate their results as in the tracheal ASM preparation. Importantly, the Noble et al. study (10) also explicitly used human airways from subjects without airflow obstruction, thus removing any nagging questions of species differences. For these reasons, the results of Noble et al. (10) are tough to refute: the lack of tidal breathing oscillations do not result in a hyperresponsive human airway.
Importantly, Noble's intact airways behaved normally in response to DI. The constricted airways transiently dilated and then renarrowed quickly with a time constant of 5–10 s. This is consistent with DI responses measured in healthy humans in vivo (6), animals in vivo (3), and bovine and porcine excised intact airways (8, 9). As Noble et al. (10) discussed, the short-lived changes in healthy airway caliber from DI may artificially enhance the differences in responsiveness between healthy and asthmatic subjects when assessed by forced expiratory volume. Nevertheless, because DIs only occur 10 times an hour, the transient bronchodilatory response to DI would ultimately be insufficient to result in a sustained reduction in airway responsiveness. Sustained (i.e., >60 s) increases in lung function following DI would then presumably be unrelated to individual airway dilation, but instead would result from the recruitment of a previously closed portion of the airway tree (2).
How then can we put all the pieces back together? Just for a minute, let us set aside our notions of ASM, drop the complex terminology, and think of the ASM as any other soft material. As with any soft material in classical mechanics, the ASM will begin to yield as we expose it to increasing tensile strain. This is exactly what happens during isometric ASM experiments—the well described active processes of actin-myosin cross bridges and cytoskeletal recruitment (15) are indeed fragile (13) and will break easily under applied tension. However, in an intact airway, the ASM length shortens during activation and the ASM material quickly moves away from the tensile strains required for the yielding regime. Only a DI applied to a constricted airway would probe the larger lengths required for ASM yielding. As living cells, however, the ASM can rebuild its infrastructure over time (13). Thus both DIs and tensile strains in isometric experiments are reversible assuming they were not too large to enter into the failure regime of the material. Only by applying continual DIs to constricted intact airways would Noble et al. be able to obtain the same results as isometric experiments; the pressures of normal tidal breathing are just not large enough.
What do these intact airway studies teach us above airway hyperresponsiveness in vivo? The lack of oscillatory mechanical strains during breathing does not cause a hyperreponsive airway. Instead, some other mechanism caused the airway to transition to a hyperresponsive state, and its transient response to DI (or lack thereof) is simply a functional manifestation of the system's current state. In classical mechanics, pulling on a piece of material tells us about its physical properties; in the same way, the inability to dilate from a DI shows us the outcome of a living system gone awry. Thus perhaps the time has come to dissolve the relationship between airway responsiveness and breathing fluctuations and begin a new search for the elusive mechanism of airway hyperresponsiveness.
No conflicts of interest, financial or otherwise, are declared by the author(s).
- Copyright © 2011 the American Physiological Society