Asthma is at a crossroads.

Asthma is at a crossroads. Although the clinical syndrome of asthma has been recognized for millennia, it was research performed in the middle of the last hundred that established the primary physiologic abnormality as obstruction of the airways to airflow. In addition to airflow obstruction, it was appreciated that the airways of patients were inflamed, that there was hypertrophy and hyperplasia of the mucus secretory apparatus with abnormal amounts of mucus in the airway lumen and that treatment with adrenergic agonists l to rapid resolution of many of the signs and symptoms of asthma. This latter observation l to the deduction, which still stands, that constriction of the airway sleek muscle was responsible for a substantial proportion of the observ obstruction during an acute asthmatic attack. In the 1970 as research into the physiology of airflow obstruction reached its peak, investigators began to apply the tools of lonely dwelling and molecular biology to understanding the cause causes of airway obstruction. This conclusioned in an explosion of of the present day knowledge about the cell and immunobiology of the airway. (1) There is now an unprecedent array of potential mechanisms that could lead to the airway obstruction that we recognize as asthma, if it were not that there has been little succes in sorting not at home which are the truly active commons Indeed, it is highly likely that the clinical syndrome of asthma derives from a number of distinct molecular mechanisms. The promise of the time to come is that the insights derived from the Human Genome plan will provide the keys exigencyed to achieve this distinction at a causal level

Asthma physiology, especially the consequences that limit maximal expiratory melt has been the primary topic of previous Aspen Lung parleys The physics of forced exhalation are clearly understood. (2) At any point in the airway airflow is limited by the agency of the local wave speed given as:

V = 2/[rho] = [[square cause of([A.sup.3]/([differential]A/[differential][P.sub.TM])]

where V is maximal expiratory run A is the cross-sectional area of the airway, [PsubTM] is the transmural crushing across the airway, ([differential]A/[differential][P.sub.TM]) is airway wall compliance, and [rho] is the density of the airway gas. We know that the physical site of follow limitation, the choke point, during a forced exhalation prevail upons from the central to the peripheral airways. In patients with asthmatic airway obstruction, the site of run limitation is in the small airways, ie, bronchioles, through much of the vital capacity.

Although these relationships have been understood for many years, we continue to learn more about the asthmatic airway. Thickening of the wall limits airflow. (3) In asthma, mechanisms are in play limiting the ability of airways to dilate when a large breath is taken, however their precise nature is not known. (4) Although multiple mechanisms that could eventuate in airway wall thickening have been elucidated, their relative parts remain undefined. Many different inflammatory processe have the potential to thicken the airway wall, by the agency of the accumulation of inflammatory solitary abode; squalids through the deposition of collagen by way of fibroblasts activated to assume a myofibroblastic phenotype, between the sides of hypertrophy and hyperplasia of the mucus secreting apparatus, through changes in the surface properties of the small airways, or according to alterations in the characteristics of the subepithelial connective tissue. (5) The mechanisms active in mild-to-moderate asthma may be distinct from those operative in strict asthma. (6) Multiple mechanisms, including those activated by means of the adaptive immune response (178) or the innate immune reply can elicit these phenotypic changes. Many research disposes have focused on the characters of interleukin-4 or interleukin-13 to mediate these changes, either from one side the cascade of events occurring following antigen exposing or through direct effects of these cytokines forward constitutive airway cells. (1,8)

The number of potential mechanisms that could ultimately impact upon the airway wall is multiplied substantially (9) when single in kind considers the effects of infectious agents like as viruses, especially respiratory syncytial virus, or atypical bacteria, as it was as Mycoplasma or Chlamydia species. (7) These agents can call forth an inflammatory response through as well-as; not only-but also; not only-but; not alone-but adaptive and innate immune mechanisms. A case can be made for many different mechanisms that could modify the airway wall (10); unfortunately, it is difficult to gather evidence that clearly distinguishes among these mechanisms.

A mechanism that merits substantial attention is smooth-muscle activation. (4) Constriction of this tissue, which is repeatedly hypertrophic and hyperplastic in asthma, will diminish airway caliber, if it were not that will also stiffen the airway. If there was no plain muscle, the airway would became true compliant, ie, ([differential]A/[differential][P.sub.TM]) became excessively large, and maximal expiratory liquefys fall. If the airway were to become surpassingly stiff, ie, ([differential]A/[differential][P.sub.TM]) became to a high degree small, flow rates could increase, on the other hand cough would become ineffective (Fig 1) Since the exact tenor will depend on both the lumen size and the airway wall compliance, interventions designed to permanently impact forward smooth muscle, although they could ameliorate airway obstruction, will ne to be viewed with great caution.

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