Cough is one of the most common symptoms in asthma patients, although little attention has been paid to its role in asthma diagnosis and follow-up. Some recent studies from Europe have suggested that cough provoked by inhalation challenges may be useful in diagnosing asthma 12, and also in evaluating the response to asthma treatment 3. These studies support the concept that cough could be utilized as a surrogate for bronchoconstriction when studying patients likely to be unable to perform spirometry. However, the relationship between intensity of coughing and level of bronchoconstriction is still a matter of debate.

Sheppard et al studied the relationship between cough and bronchoconstriction caused by inhaled distilled water aerosol in subjects with asthma. Atropine caused inhibition of the water-induced bronchoconstriction, but did not inhibit cough. Their data suggest that water-induced bronchoconstriction involves cholinergic nerves and that water-induced cough is not dependent on bronchoconstriction4. On the other hand, Koskela et al showed that direct, indirect, and combined airway challenges are able to provoke cough, but the significance of the cough response differs considerably among the challenge stimuli2.

Therefore, the utility of cough during bronchial challenges in diagnosing asthma may depend on the stimulus. In the present study, we hypothesized that bradykinin causes more cough and dyspnea. Bradykinin is thought to be an indirect stimulus, i.e., causes airflow limitation by an action on cells other than the effector cells (smooth muscle). Possible mechanisms by which bradykinin may cause bronchoconstriction involve the stimulation of sensory nerves to induce smooth muscle contraction via neural reflex pathways and this may contribute to cough stimulus in asthma 5. The aim of this study was to compare cough and dyspnea intensity induced by bradykinin, methacholine, and exercise challenge.

We studied 20 asthmatic outpatients (10 women; age range: 21 – 46 years). All subjects were on inhaled short-acting β₂-agonists as rescue medication and 14 subjects on inhaled corticosteroid therapy. Characteristics of studied subjects can be seen in Table 1. The geometric mean PC₂₀ MCh was 0.36 (range 0.08 to 2.35 mg/ml), and the geometric mean PC₂₀ BK was 0.68 (range 0.05 to 3.87 mg/ml). The mean (± SD) FEV₁ fall after exercise was 20.45% ± 3.43%.

Table 2 shows intensity of symptoms among stimuli. Cough induced by bradykinin was more intense. Bradykinin also led to more intense breathlessness detected by VAS scale, but not by Borg scale. The pairwise comparisons of challenge tests are shown in Table 3. For the cough evaluation, bradykinin caused more cough in comparison with methacholine and exercise (p < 0.01). There were no differences between exercise and methacholine (p = 0.31). In terms of dyspnea measured by Borg scale, there were no differences among stimuli (p ≥ 0.15). When dyspnea was measured by VAS, bradykinin induced more dyspnea than exercise (p = 0.04) and than methacholine (p = 0.02).

The baseline FEV₁ had effect on cough (estimate 3.10; 95%CI 0.77–5.43; p < 0.01) and on dyspnea evaluated by Borg scale (estimate 6.96; 95%CI 2.95–10.96; p < 0.01) or by VAS (estimate 5.08; 95%CI 0.89–9.26; p = 0.02). These positive-estimate values indicate that higher baseline FEV₁ leads to more symptoms during FEV₁ fall. For the following covariables, no effect has been detected on cough or dyspnea: body mass index, atopy status, age, gender, tests sequence and inhaled corticosteroid dose.

In this cross over study, 20 asthmatic subjects were evaluated. Self-reported ratings of the intensity of dyspnea (assessed by Borg and VAS) and coughing counts were made during bradykinin, methacholine and exercise induced bronchoconstriction. The study showed that bradykinin induced more cough and dyspnea than methacholine and exercise. As expected, dyspnea increased during challenge tests, so did cough. Some studies failed to show this dose-related increase in cough counts during bronchoconstriction 1516.

Our data support the proposal of using cough to evaluate bronchial responsiveness in special groups of patients. Technical difficulties in performing spirometry are common: one out of five elderly subjects cannot perform spirometry according to the international guidelines 17. Similarly, approximately 30% of pre-school children are unable to perform acceptable efforts 18. In the evaluation of challenge induced cough, bradykinin would probably be the best stimulus because it could increase the sensitivity of the test by increasing cough intensity.

In addition, during challenge tests, cough and dyspnea were significantly more intense in subjects with higher baseline pulmonary function (baseline FEV₁). Probably, patients with prolonged airflow obstruction would be less breathless for any given reduction in FEV₁ than those with higher baseline FEV₁, a process known as temporal adaptation 19. This is in favor of the use of cough to define positive bronchial challenge test, given that most patients who need challenge tests are not severe enough to have very low FEV₁.

Some theories are able to explain why, in patients with asthma, the severity of breathlessness is greater during bradykinin than methacholine or exercise challenge at given levels of airway obstruction: a) bradykinin challenge test causes more cough and this could interfere in dyspnea perception, increasing it; b) methacholine has PC₂₀ lower than bradykinin and this could be associated with faster bronchospasm, shorter sensory duration of the experience; c) intensity of asthma symptoms depends on the mechanisms that are involved in the induction of airway obstruction 10. The difference in mode of action among three stimuli should be considered potential determinant of breathlessness severity. For instance, after exercise, various sensations from physical discomfort could have influenced scoring of perceived breathlessness and it is not known what the effect of exercise itself is on cough or sensation of dyspnea. One solution to these potential problems could be a bronchial provocation challenge that would reproduce the hyperventilation of exercise, e.g. eucapnic voluntary hyperventilation of cold and dry air 20.

Another study has showed that bradykinin inhalation caused cough and retroesternal discomfort, but authors did not evaluate cough quantitatively 16. In a more recent study with 12 subjects with mild asthma, authors recorded and counted coughs episodes. They showed that, in general, bradykinin induced more coughing than did methacholine, however, there were some subjects who rarely coughed to either stimuli, whereas others had a marked cough response regardless of the stimuli 15.

Despite cough is a very common symptom and the mechanisms contributing to it are widely studied, there has been much debate, for instance, surrounding the identity of the airway afferent nerve subtype that precipitates reflex coughing. Studies in experimental animals and in humans show clearly that multiple afferent nerve subtypes (mechanosensors and chemosensors) might be involved in the production of reflex coughing. However, not all stimuli evoke cough under all conditions. This might suggest divergence between multiple reflex pathways or the existence of primary and secondary cough afferent pathways 21. Also, there is a suspicious that a complex allergic reaction in the airway may be involved in the development of antigen-induced increase in cough reflex sensitivity 22. There is evidence of the involvement of airway vagal afferents, such as sensory C-fibers, and rapidly adapting receptors in the cough reflex, as well as in other symptoms of respiratory disease, such as bronchospasm 2324. Bradykinin, capsaicin and citric acid, stimuli that are known to active airway chemosensors, are amongst the most potent tussigenic agent in conscious animals and humans21.

The mechanisms of tussive and bronchoconstrictor responses to bradykinin may be the same, via C-fibers 25. The non-myelinated C-fibers contain the tachykinins substance P, neurokinin A and neurokinin B which, upon release, act on NK1, NK2, NK3 receptors respectively to mediate several functions 26. Whilst inhalation of citric acid stimulates both C-fibers and rapidly adapting receptors, capsaicin appears to stimulate only C-fibers and both these agents have been shown to induce cough, in several species including man, and also bronchoconstriction 212627282930.

In several studies, dyspnea score are usually plotted against percentage of fall in FEV₁ and individual symptoms/FEV₁ ratios are used to represent an index of dyspnea, and their corresponding intercepts, representing baseline symptoms. These variables are calculated by linear regression analysis 1931. The mixed procedure fits a variety of mixed linear models to data and enables the use of these fitted models to make statistical inferences about the data. The linear mixed-effects models, therefore, provides flexibility of modeling not only the means of data (as in the standard linear model) but their variances and covariances as well 14.

Some studies make video recordings or employ simultaneous recordings of flow rate, air volume, subglottic pressure and acoustic signal to evaluate cough. However, the use of different devices could interfere with dyspnea sensation, plus, there is a recent study (comparing video recordings and audio recordings) showing that trained observers are able to achieve good agreement counting cough manually from audio recordings 32. Another study showed that the agreement between simultaneous (at the same time when the test is being conducted) and video counting of coughs is generally good. To ensure reliable simultaneous cough counting, challenge tests should be performed in a quiet environment, applying as little unnecessary equipment and measurements as possible 33.

Bradykinin challenge provides the strongest coughing intensity and breathlessness for a given fall in FEV₁, and is thereby recommended for protocols planned to evaluate cough. Bradykinin may act on neural mechanisms that modulate symptoms, increasing cough and dyspnea in asthmatic patients. These data corroborate previous studies that showed, in experimental models, a role for bradykinin in the mechanism of cough.

FEV₁: forced expiratory volume in one second; VAS: Visual Analogue Scale; PC₂₀: provocative concentration of any stimulus resulting in a 20% fall in FEV₁; BK: bradykinin; MCh: methacholine; EIB: exercise-induced bronchospasm; NK: neurokinin.

The authors declare that they have no competing interests.

TRS recruited the subjects, performed the data collecting and draft the manuscript. EZM and CAG performed the statistical analysis and interpretation of data. EOV participated in conception, design of the study, coordination, helped to draft the manuscript and critical revision. All authors have given final approval of the version to be published.