β2-Agonists at the olympic games

The different approaches that the International Olympic Committee (IOC) had adopted to β₂-agonists and the implications for athletes are reviewed by a former Olympic team physician who later became a member of the Medical Commission of the IOC (IOC-MC). Steadily increasing knowledge of the effects of inhaled β₂-agonists on health, is concerned with the fact that oral β₂-agonists may be anabolic, and rapid increased use of inhaled β₂-agonists by elite athletes has contributed to the changes to the IOC rules. Since 2001, the necessity for athletes to meet IOC criteria (i.e., that they have asthma and/or exercise-induced asthma [EIA]) has resulted in imporved management of athletes. The prevalence of β₂-agonist use by athletes mirrors the known prevalence of asthma symptoms in each country, although athletes in endurance events have the highest prevalence. The age-of-onset of asthma/EIA in elite winter athletes may be atypical. Of the 193 athletes at the 2006 Winter Olympics who met th IOC's criteria, only 32.1% had childhood asthma and 48.7% of athletes reported onset at age 20 yr or older. These findings lead to speculation that years of intense endurance training may be a causative factor in bronchial hyperreactivity. The distinction between oral (prohibited in sports) and inhaled salbutamol is possible, but athletes must be warned that excessive use of inhaled salbutamol can lead to urinary concentrations similar to those observed after oral administration. This article provides justification that athletes should provide evidence of asthma or EIA before being permitted to use inhaled β₂-agonists.     

Clinical concerns with inhaled beta2-agonists: adult asthma

Inhaled beta2-agonists, when used regularly, cause subtle but significant worsening of asthma control. Overuse of inhaled beta2-agonists is associated with increased risk of death from asthma in a dose-response fashion. beta2-Agonists enhance airway responses to allergens, including induced airway hyperresponsiveness and induced airway inflammation. This is a plausible explanation for beta2-agonist-worsened asthma control. These direct effects of inhaled beta2-agonists, including increased airway response to allergen, tolerance, etc., may partially explain the association of overuse with asthma death. However, it is probable that the major reason for the association of beta2-agonists overuse and asthma mortality is an indirect effect. Inhaled beta2-agonists are effective relievers and preventers of bronchoconstriction and asthma symptoms but fail to treat the underlying pathogenesis, namely the airway inflammation. Thus, overuse may mask the true asthma severity and result in both an underappreciation and undertreatment of the disease. This would provide a rational explanation for the relationship of inhaled beta2-agonist use and mortality and also would fit the dose-response pattern. Inhaled beta2-agonists should be used exclusively as needed for relief of symptoms and their requirement should be infrequent: the need for excessive doses of beta2-agonists provides a useful marker of asthma (lack of) control.     

Exercise in elite summer athletes: Challenges for diagnosis

BACKGROUND: There is a high prevalence of asthma and exercise-induced bronchoconstriction (EIB) in elite athletes when the diagnosis is based on symptoms and medication use. Objective measurements are now required by some sporting bodies to support a diagnosis of asthma or EIB to justify use of beta-agonists. Such measurements could include bronchial provocation with methacholine, with eucapnic voluntary hyperpnea (EVH) of dry air (a surrogate for exercise), or both. OBJECTIVE: The aim of the study was to investigate the relationship between asthma symptoms and responses to methacholine and the EVH challenge in a group of unselected elite summer-sport athletes. The outcome would be to inform practitioners of a suitable objective approach to identifying those with asthma and EIB. METHODS: Fifty elite summer-sport athletes with or without asthma were recruited from sporting teams and sports medicine centers throughout Melbourne, Australia. All subjects completed a respiratory questionnaire and, on separate days, underwent a bronchoprovocation challenge test with methacholine and EVH. RESULTS: Forty-two subjects reported one or more respiratory symptoms in the past year, 9 had positive methacholine challenge results (mean PD(20) of 1.69 +/- 2.05 micromol), and 25 had positive EVH challenge results (mean fall in FEV(1) of 25.4% +/- 15%). Although all subjects with positive methacholine challenge results had positive EVH challenge results, methacholine had a negative predictive value of only 61% and a sensitivity of 36% for identifying those responsive to EVH. CONCLUSION: These findings suggest that the pathogenesis of EIB in elite athletes might be different from that of asthma, and as such, neither symptoms nor the methacholine challenge test should be used exclusively for identifying EIB.     

Bronchial challenges in athletes applying to inhale a beta2-agonist at the 2004 Summer Olympics

BACKGROUND: The International Olympic Committee Medical Commission required a medical justification for athletes to inhale a beta2-agonist before an event at the Summer Games in Athens in 2004. OBJECTIVE: We sought to establish the percentage of athletes applying to use an inhaled beta2-agonist on the basis of the results of objective tests to establish a diagnosis of asthma or exercise-induced bronchoconstriction. We also sought to compare this percentage with the percentage of athletes simply notifying the intention to use a beta2-agonist at the previous Summer Games in Sydney in 2000. METHODS: An analysis was made of tests that measured the change in FEV1 in response to a bronchodilator or in response to a provoking stimulus, such as exercise, eucapnic voluntary hyperpnea, hypertonic saline, or methacholine. RESULTS: Ten thousand six hundred fifty-three athletes competed in Athens; 4.2% were approved to use a beta2-agonist, and 0.4% were rejected. This approval rate was 26% less than the notifications in 2000 in Sydney (5.7%). Compared with Sydney 2000, there was a significant reduction of submissions and approvals for athletes from the United States, New Zealand, Australia, and Canada and in triathlon and swimming sports. CONCLUSION: The need to provide objective testing has resulted in a reduction in the number of athletes seeking approval to use an inhaled beta2-agonist. Objective evidence has provided information for the doctor that is likely to improve the health of the athlete because many athletes appeared to be undertreated at the time of testing. CLINICAL IMPLICATIONS: We show that documentation of airway narrowing in athletes, particularly in response to exercise or surrogate stimuli for exercise, aids in the diagnosis and management of asthma by providing evidence of bronchial hyperresponsiveness that will respond to treatment with inhaled corticosteroids and is usually associated with a reduction in respiratory symptoms on exercise.     

Responses to bronchial challenge submitted for approval to use inhaled beta2-agonists before an event at the 2002 Winter Olympics

BACKGROUND: There has been an increase in the number and percentage of athletes competing in Olympic Games notifying use of beta2-agonists, from 1.7% at Los Angeles (1984) to 5.5% at Sydney (2000). For Salt Lake City (2002), the International Olympic Committee requested objective evidence to use beta2-agonists for asthma or exercise-induced asthma (EIA). OBJECTIVE: The objective of this study was to evaluate the evidence submitted for approval to use a beta2-agonist. METHODS: Objective evidence for asthma or EIA included (1) an increase of 12% or more of the predicted FEV1 in response to bronchodilator, (2) a reduction in FEV1 of 10% or greater from baseline in response to exercise or eucapnic voluntary hyperpnea, (3) a PD20 FEV1 to methacholine or histamine at a dose of less than 200 microg (2 mg/mL) or less than 1320 microg (13.2 mg/mL) for those taking inhaled corticosteroids for 3 months. RESULTS: There were 165 applications. Of these, 147 (89%) included evidence of a challenge, bronchodilator response, or both, and 163 test results were submitted. One hundred thirty (5.2%) applications were approved. For those with positive responses, the median value (1) was 16.2% of predicted FEV1 for response to a bronchodilator (n = 13), (2) was a 15.9% decrease in FEV1 for response to a physical challenge (n = 36), and, (3) for PD20 FEV1, was 173 microg for response to a pharmacologic challenge (n = 45). CONCLUSION: The analysis demonstrated that it is feasible to request objective evidence to justify use of beta2-agonists on the medical grounds of asthma or EIA.     

Exercise and other indirect challenges to demonstrate asthma or exercise-induced bronchoconstriction in athletes

The prevalence of exercise-induced bronchoconstriction is reported to be high among recreational and elite athletes, yet diagnosis is often symptom-based. Indirect challenges such as the laboratory exercise challenge provide objective criteria for proper diagnosis and treatment. However, a standardized protocol using appropriate exercise intensity, duration, and dry air inhalation is often not implemented, and thus a false-negative test may result. This article reviews and describes the symptom-based diagnosis, the exercise challenge, and other indirect challenges such as eucapnic voluntary hyperpnea, hypertonic saline inhalation, and inhaled powdered mannitol as methods to diagnose and evaluate exercise-induced bronchoconstriction. Advantages and disadvantages of each diagnostic procedure are presented.     

Positioning of glucocorticosteroids in asthma and allergic rhinitis guidelines (versus other therapies)

Asthma and allergic rhinitis are both characterized by airway inflammation, and glucocorticosteroids form the cornerstone of their pharmacologic treatment. All patients with asthma should be prescribed rapid-acting inhaled beta2-agonists as needed to use as rescue therapy in case of symptoms. As soon as patients experience symptoms at least once a week, controller medications should be started on a daily basis to achieve and maintain control of their asthma. Intranasal corticosteroids are given as first-line therapy for moderate to severe persistent rhinitis. Depending on the dominant symptom, H1-antihistamines, decongestants, or ipratropium can be added after re-evaluation.     

Immunopathology of exercise-induced bronchoconstriction in athletes — A new modified inflammatory hypothesis

Elite athletes have a higher prevalence of exercise-induced bronchoconstriction than the general population. The pathogenesis of exercise-induced bronchoconstriction is not fully elucidated. Increasing evidence suggests that airway inflammation plays a major role in the immunopathogenesis of exercise-induced bronchoconstriction. The aim of our review is to discuss existing evidence and to present a new, modified inflammatory hypothesis of exercise-induced bronchoconstriction. Exercise alters the number and function of circulating immune cells. Episodes of upper respiratory symptoms in elite athletes do not follow the usual seasonal patterns. Moreover, they have an unusual short-term duration, which suggests a non-infectious etiology. If the pro-inflammatory response to exercise has the potential to induce symptoms that mimic respiratory tract infection, it definitely up-regulates pro-inflammatory cytokine expression in the airways. We can conclude that exercise up-regulates airway cytokine expression in a way that favors inflammation and allergic reactions in bronchi and lowers the threshold for bronchoconstriction to different stimuli like cool, dry air, allergens, and pollutants.     

Allergy and asthma in elite summer sport athletes

Exercise may increase ventilation up to 200 L/min for short periods of time in speed and power athletes, and for longer periods in endurance athletes, such as long-distance runners and swimmers. Therefore highly trained athletes are repeatedly and strongly exposed to cold air during winter training and to many pollen allergens in spring and summer. Competitive swimmers inhale and microaspirate large amounts of air that floats above the water surface, which means exposure to chlorine derivatives from swimming pool disinfectants. In the summer Olympic Games, 4% to 15% of the athletes showed evidence of asthma or used antiasthmatic medication. Asthma is most commonly found in endurance events, such as cycling, swimming, or long-distance running. The risk of asthma is especially increased among competitive swimmers, of which 36% to 79% show bronchial hyperresponsiveness to methacholine or histamine. The risk of asthma is closely associated with atopy and its severity among athletes. A few studies have investigated occurrence of exercise-induced bronchospasm among highly trained athletes. The occurrences of exercise-induced bronchospasm vary from 3% to 35% and depend on testing environment, type of exercise used, and athlete population tested. Mild eosinophilic airway inflammation has been shown to affect elite swimmers and cross-country skiers. This eosinophilic inflammation correlates with clinical parameters (ie, exercise-induced bronchial symptoms and bronchial hyperresponsiveness). Athletes commonly use antiasthmatic medication to treat their exercise-induced bronchial symptoms. However, controlled studies on their long-term effects on bronchial hyperresponsiveness and airway inflammation in the athletes are lacking. Follow-up studies on asthma in athletes are also lacking. What will happen to bronchial hyperresponsiveness and airway inflammation after discontinuation of competitional career is unclear. In the future, follow-up studies on bronchial responsiveness and airway inflammation, as well as controlled studies on both short- and long-term effects of antiasthmatic drugs in the athletes are needed.     

Long-Acting β- Agonist Treatment in Patients with Persistent Asthma Already Receiving Inhaled Corticosteroids

International guidelines recommend that long-acting β-agonists should be considered in patients who are symptomatic despite moderate doses of inhaled corticosteroids. When combined with inhaled corticosteroids they improve asthma symptoms and lung function and reduce exacerbations. The evidence suggests that they are well tolerated. However, they are less effective than inhaled corticosteroids as monotherapy and should not be used alone, although the addition of a long-acting β-agonist may permit a small reduction in the corticosteroid dose. Both salmeterol and formoterol appear equally effective in improving asthma control. Formoterol, however, has a rapid onset of action and is now being promoted for the relief of acute asthma symptoms. Both drugs provide prolonged protection against exercise-induced bronchospasm. However, this effect rapidly diminishes with continuous therapy and if this is the main aim of treatment, intermittent use may be preferable. When compared with alternative treatments, inhaled long-acting β-agonists are more effective in controlling asthma symptoms than either theophylline or antileukotriene agents. Bambuterol, an oral prodrug of terbutaline, appears to be as effective as the inhaled long-acting β-agonists and has the advantage of once daily oral administration. However, the inhaled long-acting β-agonists are less likely to have systemic adverse effects. There are theoretical concerns that regular β-agonist treatment may lead to tolerance and a failure to respond to emergency asthma treatment. While there is no doubt that tolerance occurs, there is currently little evidence that this is a clinical problem. Insights into pharmacological as well as therapeutic interactions between inhaled corticosteroids and β-agonists are providing justification for their use in combination. Guidelines for the management of patients with chronic persistent asthma are likely to require modification to reflect these developments.     

Clinical concerns with inhaled β2-agonists

Inhaled β₂-agonists, when used regulary, cause subtle but significant worsening of asthma control. Overuse of inhaled β₂-agonists is associated with increased risk of death from asthma in a dose-response fashion. β₂-Agonists enhance airway responses to allergens, including induced airway hyperresponsiveness and induced airway inflammation. This is a plausible explanation for β₂-agonist-worsened asthma control. These direct effects of inhaled β₂-agonists, including increased airway response to allergen, tolerance, etc., may partially explain the association of overuse with asthma death. However, it is probable that the major reason for the association of β₂-agonists overuse and asthma mortality is an indirect effect. Inhaled β₂-agonists are effective relievers and preventers of bronchoconstriction and asthma symptoms but fail to treat the underlying pathogenesis, namely the airway inflammation. Thus, overuse may mask the true asthma severity and result in both an underappreciation and undertreatment of the disease. This would provide a rational explanation for the relationship of inhaled β₂-agonist use and mortality and also would fit the dose-response pattern. Inhaled β₂-agonists should be used exclusively as needed for relief of symptoms and their requirement should be infrequent the need for excessive doses of β₂-agonists provides a useful marker of asthma (lack of) control.     

Pharmacogenetics of the beta 2-adrenergic receptor gene

Asthma is a complex genetic disease with multiple genetic and environmental determinants contributing to the observed variability in response to common antiasthma therapies. One focus of asthma pharmacogenetic research has been the beta2-adrenergic receptor gene (ADR beta 2) and its effect on individual responses to beta agonist therapy. Knowledge about the effects of ADR beta 2 variation on therapeutic responses is evolving and should not alter current Asthma Guideline approaches, which consist of the use of short-acting beta agonists (SABAs) for as-needed symptom-based therapy and the use of a regular long-acting beta agonist (LABA) in combination with inhaled corticosteroid therapy for those asthmatics whose symptoms are not controlled by inhaled corticosteroid alone. These approaches are based upon studies showing a consistent pharmacogenetic response to regular use of SABAs and less consistent findings in studies evaluating LABAs. The emerging pharmacogenetic studies are provocative and should lead to functional studies. Meanwhile, the conflicting data concerning LABAs may be caused by such factors as small sample sizes of study populations and differences in experimental design.     

Mechanisms of exercise‐induced bronchoconstriction in athletes: Current perspectives and future challenges

The evidence of exercise‐induced bronchoconstriction (EIB) without asthma (EIBw_A) occurring in athletes led to speculate about different endotypes inducing respiratory symptoms within athletes. Classical postulated mechanisms for bronchial obstruction in this population include the osmotic and the thermal hypotheses. More recently, the presence of epithelial injury and inflammation in the airways of athletes was demonstrated. In addition, neuronal activation has been suggested as a potential modulator of bronchoconstriction. Investigation of these emerging mechanisms is of major importance as EIB is a significant problem for both recreational and competitive athletes and is the most common chronic condition among Olympic athletes, with obvious implications for their competing performance, health and quality of life. Hereby, we summarize the latest achievements in this area and identify the current gaps of knowledge so that future research heads toward better defining the etiologic factors and mechanisms involved in development of EIB in elite athletes as well as essential aspects to ultimately propose preventive and therapeutic measures.     

Recovery from bronchoconstriction and bronchodilator tolerance

Until recently, researchers believed that tolerance or tachyphylaxis to the bronchodilator effects of β-agonists did not occur. However, recent studies examining the recovery from bronchoconstriction have clearly shown that an impaired response to β-agonists occurs in patients who have been using regular β-agonist treatment. This tolerance develops with both long- and short-acting β-agonists and is not affected by treatment with inhaled steroids. It develops rapidly, reaching a maximum within 1 wk of starting β-agonists, and has been demonstrated after methacholine, hypertonic saline, mannitol, and exercise-induced bronchoconstriction. The observed reduction in the bronchodilator response is proportional to the severity of bronchoconstriction. Therefore, although individuals with stable asthma show little evidence of tolerance, those with severe bronchospasm have a markedly reduced bronchodilator response to β-agonists. Almost all asthmatics show evidence of tolerance when tested in the setting of bronchoconstriction, although the extent of this tolerance varies. The reasons for this interindividual variation are not understood. Bronchodilator tolerance is difficult to study in the clinical setting because nearly every patient has used multiple doses of β-agonist before seeking medical attention. However, there is compelling evidence that the response to rescue β-agonist treatment is reduced in those who use regular long- or short-acting β-agonists. The extent to which this phenomenon contributes to asthma morbidity and mortality remains to be determined.     

Is there a problem with inhaled long-acting beta-adrenergic agonists?

Short-acting beta(2)-agonists are effective in relieving acute symptoms of asthma and in the short-term prevention of symptoms from stimuli, such as exercise. They are ineffective when used on a regular schedule to improve asthma control. Long-acting beta(2)-agonists, on the other hand, provide sustained bronchodilation and improve asthma control. Regular use of long-acting beta(2)-agonists is not associated with significant tolerance to their bronchodilator action, impairment in the response to albuterol, decreased baseline pulmonary function, increased response to methacholine, or increased risk of adverse cardiac events. Case-control studies do not suggest an increased risk for death or intensive care unit admissions with use of long-acting beta(2)-agonists. In prospective studies in which there has been an increase in asthma deaths or serious asthma exacerbations, this increased risk has not been observed in subjects using inhaled corticosteroids. Where increased deaths have occurred in relation to either short- or long-acting beta(2)-agonists, the events have not occurred equally throughout the exposed population. This suggests that these outcomes were not a direct toxic effect of the drugs and increases the possibility that they resulted from an interaction between relief of symptoms by beta(2)-agonists and delay in seeking medical care.     

Distinctive bronchial inflammation status in athletes: basophils, a new player

The aim of the study was to establish bronchial inflammation status and to measure eicosanoids in sputum obtained from active elite athletes. A total of 68 subjects were enrolled. Twelve were non-athletes and non-asthmatic (NAtNAs), 21 non-athlete asthmatics (NAtAs), 11 athlete non-asthmatics (AtNAs), and 24 athletes with asthma (AtAs) with positive indirect or direct bronchial challenges. Induced sputum was used to measure cells and eicosanoids. Sputum differential cell counts in all the subject groups revealed eosinophilia with the exception of NAtNAs control subjects. Athletes with and without diagnosed asthma showed a significant increase in bronchial epithelial cells and lymphocytes present in their sputum. Also, flow cytometry revealed that a significantly higher number of basophils were present in sputum from athletes (without and with asthma) when compared with non-athletes (without and with asthma). Asthmatic athletes and non-athletes showed a higher increase in LTC₄ levels and PGE₂ metabolites in sputum when compared with healthy controls. The present study identifies basophils as a new player present in athletes bronchial inflammation defining athlete status and not necessarily associated with exercise-induced bronchoconstriction.     

The effect of passive smoking on asthma symptoms,atopy,and airway hyperresponsiveness in schoolchildren

Passive smoking is a major cause of respiratory morbidity, and is associated with increased bronchial responsiveness in children. To evaluate the effect of smoking by a parent on asthma symptoms, atopy, and airway hyperresponsiveness (AHR), we conducted a cross-sectional survey of 503 schoolchildren that involved questionnaires, spirometry, allergy testing, and a bronchial challenge test. If the PC20 methacholine was less than 16 mg/mL, the subject was considered to have AHR. The prevalence of a parent who smoked was 68.7%. The prevalence of AHR was 45.0%. The sensitization rate to common inhalant allergens was 32.6%. Nasal symptoms such as rhinorrhea, sneezing, nasal itching, and nasal obstruction were present in 42.7%. Asthma symptoms such as cough and wheezing were present in 55.4%. The asthma symptoms were significantly more prevalent in children who had a parent who smoked than in those whose parents did not. The nasal symptoms, atopy, and AHR did not differ according to whether a parent smoked. In a multiple logistic regression model, the asthma symptoms and atopy were independently associated with AHR, when adjusted for confounding variables. Passive smoking contributed to asthma symptoms in schoolchildren and was not an independent risk factor of airway hyperresponsiveness in an epidemiological survey.     

What are the Alternatives to Increasing Inhaled Corticosteroids for the Long Term Control of Asthma?

The Global Initiative for Asthma (GINA) guidelines stated the therapeutic goals for the management of asthma and, through a stepwise approach to treatment, defined the various grades of asthma severity and the therapeutic options available to the clinician at each step. This article considers the options at step 3; the management of a patient with poorly controlled asthma who is already taking low-dose inhaled corticosteroids. Before considering a change in therapy, the clinician should rule out alternative diagnoses, confirm compliance with treatment, explore potential exacerbants in the patient’s environment and, where possible, remove them. If a change in medication is necessary, the choice of drug will depend on the therapeutic goal that needs to be achieved. If the most important goal is the control of symptoms and optimisation of lung function, most studies support the addition of a long-acting β2-agonist to low dose inhaled corticosteroids. If recurrent severe exacerbations are a major feature of the poor control, increasing the dosage of inhaled corticosteroids may be most effective. The addition of a leukotriene antagonist may be the best choice if exercise-induced symptoms are prominent or in the setting of aspirin-sensitive asthma. General recommendations supported by the findings of large therapeutic trials do not allow for significant variability in the individual response to a particular drug. Receptor polymorphisms have recently been discovered that may account for variability in the response to β2-agonists and leukotriene receptor antagonists. However, until more is known about the reasons behind this variability, a therapeutic trial may be the most effective way of determining the best drug for an individual patient. One of the key developments in asthma over the past decade has been the acceptance of the concept of asthma as a chronic inflammatory disorder of the airways. However, the long term significance of this inflammation is not clear and the importance of control of inflammation beyond the suppression of symptoms, reduction of exacerbation frequency and the optimisation of lung function has not been established.     

Effects of inhaled beclomethasone dipropionate on serum IgE levels and clinical symptoms in atopic asthma

BACKGROUND: A high serum immunoglobulin (Ig)E level is considered a potent predictor for the development of asthma and IgE is targeted for treatment of asthma. Although inhaled corticosteroids are well established in the treatment of asthma, the effects of inhaled corticosteroids on serum IgE levels in asthma remain uncertain. METHODS: We therefore examined asthma symptoms, concentrations of total serum IgE and specific IgE antibodies to selected allergens, blood eosinophil counts and lung functions before and 3 months after treatment with either inhaled beclomethasone dipropionate (BDP; 800 microg/day) (n = 7) or inhaled beta2-agonists alone (n = 7) in patients with atopic asthma in a randomized, double-blind, parallel-group controlled trial. RESULTS: Inhaled BDP significantly improved asthma symptom scores and forced expiratory volume in 1 s, and decreased blood eosinophil counts, total serum IgE levels and specific IgE antibodies to house dust mite and cedar. Decreases in total serum IgE significantly correlated with an improvement in asthma symptom scores. In contrast, none of parameters altered in patients with atopic asthma treated with inhaled beta2-agonists alone. CONCLUSIONS: Inhaled corticosteroids may improve the subsequent clinical course of atopic asthma in association with a reduction of serum IgE levels.     

Provoked models of asthma: what have we learnt?

Asthma is a chronic inflammatory disease of the airways characterized by physiological abnormalities of variable airflow obstruction and airway hyperresponsiveness (AHR) to a wide variety of physical and inhaled chemical stimuli and the presence of symptoms. AHR is measured by challenging the airways with a variety of agonists and naturally occurring stimuli, which results in constriction of the airway smooth muscle, leading to airway narrowing and airflow limitation. There are two distinct mechanisms by which the airways can narrow to a constrictor stimulus and these are defined by the pathways they take to induce AHR. Direct stimuli are pharmacological agents administered exogenously (such as histamine or methacholine) that act 'directly' on specific receptors on the bronchial smooth muscle to cause constriction. The other mechanism by which the airway can narrow is via the inhalation of indirect stimuli, which include natural stimuli, such as allergen or exercise, and pharmacological agents such as adenosine monophosphate and hyper-osmotic agents (e.g. hypertonic saline or dry powder mannitol). These stimuli induce airway narrowing 'indirectly' by causing the endogenous release of mediators of bronchoconstriction from airway inflammatory cells. Provoked models of asthma have been extremely valuable in understanding the pathobiology of asthma, in aiding diagnosis, in helping to clarify the mechanisms of actions of effective drugs and in the development of new entities to treat asthma. Some provoked models are valuable clinically, particularly those that measure direct AHR, while others, particularly allergen challenge, have been used in animal models and in humans to study the mechanisms of allergen-induced airway inflammation and the associated physiological changes, as well in the development of new drugs for asthma. An emerging role for measurements of AHR is in the evaluation of the optimal treatment for patients with asthma.