Quality of Dyspnea in Bronchoconstriction Differs from External Resistive Loads

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Abstract
To test the hypothesis that patients perceive the same quality of dyspnea during mild bronchoconstriction and external resistive loads, we studied subjects with asthma under two conditions: ( 1 ) during methacholine bronchoprovocation to mimic the broncho- spasm of mild asthma and ( 2 ) while breathing on a circuit to which was added a range of external resistors to mimic the me- chanical load of mild asthma. During each of these stimuli, respira- tory variables, overall dyspnea intensity on a modified Borg scale, and the qualitative descriptors of breathlessness from a 19-item questionnaire were assessed. The "chest tightness" and "constric- tion" responses were significantly more frequent in the methacho- line trials as compared with the external load trials (p , 0.0001). The "chest tightness" or "constriction" response was chosen dur- ing 92% of the 26 trials of methacholine bronchoconstriction com- pared with 3% of the 72 trials of breathing against the external re- sistors. Changes in functional residual capacity were not significantly different between the two conditions. We conclude that in mild asthma, the sensation of chest tightness is distinct from the sensa- tion of work and effort and is not attributable to the mechanical load imposed on the respiratory system. In individuals with asthma there is only a modest correlation between changes in lung function, as measured by FEV 1 , and symptoms of breathing discomfort. A controversy exists among researchers in this field regarding the source of the respiratory sensations in patients with asthma. Given the range of physio- logical mechanisms proposed for dyspnea (1, 2), investigators have postulated that airway resistance alone is not the primary derangement responsible for breathing discomfort associated with bronchospasm. A number of investigations have exam- ined the dyspnea of asthma under varying conditions and uti- lizing different models of airway obstruction. Studies in patients challenged with doses of inhaled metha- choline sufficient to produce moderate to severe airway obstruc- tion have suggested that the dyspnea of asthma is characterized by the sensation of "an unsatisfied breath," "unrewarded in- spiratory effort," or an "inability to get a deep breath" (3). These findings are similar to those in studies performed in patients with emphysema (4) and have been attributed to the mechanical load on the system, specifically hyperinflation, and to the experience of breathing at one extreme of the respiratory system's pres- sure-volume curve (3, 4). These results, obtained in patients with moderate to severe airway obstruction, were associated with changes in functional residual capacity (FRC) that approxi- mated 2 L. In contrast to these findings, studies of patients with asthma who are questioned about the breathing discomfort they experi- ence with their disease have revealed that the sensations of "chest tightness" or "constriction" are prominent (5-7). The dis- crepancy between the findings in studies of moderate to severe bronchoconstriction as compared with those in which patients completed dyspnea questionnaires may be due to the level of bronchoconstriction and accompanying hyperinflation under these different conditions. Furthermore, inhaled lidocaine has been shown to reduce the intensity of dyspnea resulting from chemically induced bronchoconstriction (8). These results sug- gest that information from pulmonary receptors rather than af- ferent feedback from stimulation of chest wall receptors by hy- perinflation and mechanical loads may be important in giving rise to the sensations of breathlessness when only mild airway obstruction is provoked. This controversy is further fueled by the preliminary results of a study by Homma and coworkers (9) in which vibrators were applied to the chest wall of individuals with a history of asthma. In five subjects, chest wall vibration resulted in a sensation that was described as similar to the respiratory discomfort associated with an acute asthma attack. These results suggest that chest wall affer- ents may be the source of breathlessness in patients with asthma. Mild asthma is characterized by increased airway resistance in the absence of significant hyperinflation. If mechanical fac- tors, specifically those associated with increased airway resis- tance, are responsible for the dyspnea of mild asthma, we pos- tulated that the quality of breathing discomfort associated with mild bronchospasm would be the same as that associated with external resistive loads. To investigate this question, we studied the quality of dyspnea in patients with asthma in whom mild bronchospasm, at levels of airway obstruction not expected to produce significant hyperinflation, was induced by inhalation of methacholine. The quality of dyspnea was also assessed in these subjects while breathing on a circuit to which was added a range of external resistors to mimic the mechani- cal load of mild asthma. Finally, to systematically examine the qualitative aspects of the dyspnea associated with chest wall vibration, physiotherapy vibrators were applied to these sub- jects and the associated respiratory sensations were assessed.

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