Abstract
Fog inhalation induces cough and bronchoconstriction in patients with asthma, but only cough in normal subjects; whether it also influences the pattern of breathing is unclear. Nedocromil sodium (NCS) inhibits the cough response to inhalation of several pharmacological agents but its effects on fog-induced cough and changes in the pattern of breathing are unknown. We evaluated the effects of no drug, placebo, and 4- and 8-mg NCS administration on the cough threshold and changes in the pattern of breathing during fog inhalation in 14 healthy subjects. Measurements of tidal volume (Vt), duration of inspiratory and expiratory times (Ti and Te, respectively), total duration of the respiratory cycle (Tt), mean inspiratory flow (Vt/Ti), duty cycle (Ti/Tt), respiratory frequency (f, 60/Tt), and inspiratory minute ventilation (V˙ i) were obtained by inductive plethysmography. Median cough threshold values were unaffected by placebo, but were increased (p < 0.01) by both NCS doses. In no-drug and placebo trials, inhalation of the threshold fog concentration caused increases in both Vt/Ti and V˙ i (p always < 0.05) due to selective increases (p < 0.01) in Vt. These changes were markedly attenuated by both NCS doses administration. Thus, fog induces coughing and increases in Vt, Vt/ Ti, and V˙ i in healthy subjects; NCS possesses antitussive effects and attenuates fog-induced changes in the pattern of breathing, possibly through inhibition of rapidly adapting “irritant” receptors.
Cough and bronchoconstriction are powerful reflex responses that protect the lungs from the noxious effects of inhaled foreign substances. Stimulation of vagally innervated airway receptors by different agents, including nonisosmolar water solutions, may trigger these reflexes as well as increases in respiratory drive and minute ventilation (1-3). Inhalation of ultrasonically nebulized distilled water (fog) induces coughing and bronchoconstriction in patients with asthma (4), whereas it causes only cough in normal subjects (5). The effects of inhalation of nonisosmolar solutions on the pattern of breathing in humans have received little attention. Observations performed a few minutes after exposures to fog have shown that, at variance with normal subjects, patients with asthma consistently displayed an increase in respiratory resistance associated with a rise in both respiratory drive and minute ventilation (6). Noticeably, coughing was commonly observed in both study groups during fog exposure (6). Because coughing is evoked by stimulation of receptors also implicated in the control of the pattern of breathing (1, 2), the lack of ventilatory adjustments during fog inhalation in normal subjects is surprising and deserves further investigation.
Nedocromil sodium, a companion drug to sodium cromoglycate, is widely used in the treatment of reversible obstructive airway diseases. These agents seem to interfere with the transduction of sensory inputs from the lungs, possibly by blocking chloride channels of airway sensory nerve endings (7). Nedocromil sodium has been shown to inhibit bronchoconstriction induced by inhalation of nonisosmolar water solutions (8, 9), whereas its protective role against experimentally induced cough is less clearly established (see Konig [10] for a review).
This study aimed at ascertaining whether inhalation of a fog concentration capable of inducing cough also influences the pattern of breathing in normal subjects. Furthermore, we wished to evaluate whether coughing and the possibly associated changes in the pattern of breathing are affected by prior inhalation of nedocromil sodium.
To avoid the confounding effects on coughing and respiratory activity caused by inhalation of a potentially bronchoconstrictor agent in patients with respiratory diseases (4), experiments were carried out in 14 healthy, nonsmoker volunteers (8 males and 6 females; mean age, 25.4 yr; range, 22 to 30 yr). The study protocol adhered to the recommendations of the Declaration of Helsinki for Human Experimentation and was approved by the local ethics committee; informed consent was obtained from each participant.
Cough was induced by inhalation of ultrasonically nebulized distilled water (fog) produced by a Mist-O2-Gen EN143A ultrasonic nebulizer (Allied Healthcare Products, St. Louis, MO). The nebulizer output could be adjusted by means of a potentiometer and monitored as a direct current (DC) signal. In preliminary trials (11) we determined the actual nebulizer output by increasing it in steps corresponding to 5% of the maximum attainable DC signal. The same experimental set-up was used for both calibration and cough challenges. No respiratory valve was used, and an outlet proximal to the subject's port of the apparatus was provided to avoid rebreathing (11, 12). By increasing the DC signal, the nebulizer could generate progressively increasing fog outputs, hereafter termed “scalar fog concentrations,” ranging from 0.08 (at 30% DC signal) to 4.45 (at 100% DC signal) ml/min (11-13). Subjects were connected to the nebulizer via a mouthpiece and inhaled each scalar fog concentration for 1 min; 2–3 min of rest was scheduled between scalar fog concentrations. On appearance of cough, the test was discontinued and subjects were allowed to recover for 30 min (11-13). The challenge was then restarted with inhalation of the fog concentration immediately below the last administered. If cough could be elicited again at the same fog output that had previously been shown to evoke cough, the challenge was discontinued and that output was taken as the cough threshold of the subject. Conversely, if no cough response could be obtained, the challenge was resumed and continued until cough could be elicited twice at the same fog output. Thus, cough threshold was taken as the lowest fog concentration capable of evoking at least one cough effort during two consecutive challenges separated by a 30-min interval (11-13).
Cough was detected by means of the electromyographic (EMG) activity of the abdominal muscles recorded via surface Ag–AgCl electrodes positioned on the anterolateral region of the abdomen along the line of right obliquus externus fibers (11-13). The EMG signals were amplified, full-wave rectified, and passed through a “leaky” integrator (low-pass RC filter, time constant 50 ms) to obtain a “moving average” of the activity, or “integrated” EMG (IEMG) activity. The IEMG signals were only qualitatively analyzed.
The pattern of breathing was recorded by means of a respiratory inductive plethysmograph (Respitrace; Non-invasive Monitoring Systems, Miami Beach, FL). The respiratory inductive plethysmograph (RIP) was calibrated according to a procedure devised by Sackner and coworkers (14); validity of calibration was evaluated both by analyzing RIP waveforms during an isovolume maneuver and by comparing changes in tidal volume amplitude and end-expiratory lung volume measured by spirometry with RIP values (14). The sum of ribcage and abdominal RIP signals, which closely reflects the tidal volume measured at the airway opening, was fed to an eight-channel chart recorder (HP 7758A; Hewlett-Packard, Palo Alto, CA). On paper recordings we measured, on a breath-by-breath basis, the tidal volume (Vt), the duration of inspiratory and expiratory times (Ti and Te, respectively), and the total duration of the respiratory cycle (Tt). The mean inspiratory flow (Vt/Ti), the duty cycle (Ti/Tt), and inspiratory minute ventilation (V˙i) were subsequently calculated. The partial pressure of end-tidal CO2 (Pet CO2 ) was also continuously monitored (Normocap CD 102; Datex, Helsinki, Finland).
Both placebo and nedocromil sodium were administered by metered dose inhalers (MDIs) in conjunction with a Volumatic spacer device (Glaxo Wellcome, Ware, UK). MDIs were actuated by trained nonmedical personnel shortly after the beginning of a slow, deep inspiration (15). On four different sessions at least 3 d apart, participants were given either no drug (control condition) or two puffs of placebo, or two puffs of nedocromil sodium (corresponding to 4 mg of the active agent), or four puffs of the same agent.
This study was performed in a double-blind, cross-over fashion. Participants were comfortably seated on a dentist's chair and were allowed to relax for 20–30 min. Under control conditions, that is, in no-drug trials, each of the two consecutive fog challenges required for assessing the cough threshold was preceded by recordings of the baseline breathing pattern (60–90 s, 20–25 breaths). In placebo and nedocromil sodium runs, each of the two fog challenges was preceded by similar breathing pattern recordings performed before and 30 min after administration of the scheduled agent. In all trials, breathing pattern recordings were continuously performed during fog challenges. During collection of breathing pattern data, participants were requested to breathe normally with as constant a pattern as possible. To facilitate relaxation and minimize the impact of auditory stimuli from the laboratory environment on the pattern of breathing of the subjects, they continuously listened to white noise via lightweight headphones (16).
For breathing pattern studies, all recorded breaths were considered, except for those presenting sighs or short apneas and those immediately preceding and following a cough effort. To evaluate the effects of placebo and nedocromil sodium administration on respiratory activity, values of breathing pattern variables recorded in each subject before and 30 min after administration of placebo and the scheduled nedocromil sodium dose were compared by means of the nonparametric analysis of variance followed by the Dunn test for multiple comparisons. Mean values of breathing pattern variables recorded in each subject immediately before each of the two fog challenges did not significantly differ (paired t tests) and were therefore pooled and considered as a whole. Given the consistency of mean values of breathing pattern variables recorded during each of the two challenges required for cough threshold assessment, these data were also pooled. To evaluate the effects of fog inhalation on respiratory activity, differences between pooled values of breathing pattern variables recorded on each study day before and during inhalation of each fog concentration were compared by using the nonparametric analysis of variance followed by the Dunn multiple comparisons test. The latter was also used to compare cough threshold values observed on each study day. All reported values are means ± SD, unless otherwise stated; in all instances, p < 0.05 was taken as significant.
All participants coughed in response to fog inhalation; in all instances, however, coughing terminated within 15–20 s after stimulus cessation. In both no-drug and placebo trials, cough threshold values ranged from 0.40 to 3.26 ml/min. When subjects were premedicated with both 4 and 8 mg of nedocromil sodium, this variable ranged from 0.73 to 4.45 ml/min. The corresponding median (interquartile range) cough threshold values were 0.73 (0.90), 0.80 (0.89), 2.28 (2.17), and 2.52 (2.17) ml/ min, respectively. Median cough threshold values observed in no-drug and placebo runs did not significantly differ. Conversely, after administration of both nedocromil sodium doses, median cough threshold values rose to comparable extents, and achieved significantly higher values (p < 0.01).
Individual mean values of baseline breathing pattern variables observed on each study day were similar and not significantly influenced by placebo and nedocromil sodium administration. In no-drug and placebo trials, inhalation of fog concentrations lower than cough threshold caused no significant changes in the pattern of breathing, whereas inhalation of the threshold fog concentration for the cough reflex consistently provoked a progressive rise in Vt, Vt/Ti, and V˙i until the appearance of cough (usually one to four cough efforts). The increase in Vt/Ti and V˙i was mainly due to a rise in Vt; Ti/Tt and Pet CO2 did not change (Table 1). After the fog challenge, both Vt/Ti and V˙i recovered within 20–40 s. Both nedocromil sodium doses markedly attenuated fog-induced changes in breathing pattern variables (Table 1). An example of these phenomena is illustrated in Figure 1.
| No Drug | Placebo | NCS | ||||||
|---|---|---|---|---|---|---|---|---|
| 4 mg | 8 mg | |||||||
| Vt (L) | 0.16 (0.08)† | 0.20 (0.09)† | 0.03 (0.07) | 0.02 (0.06) | ||||
| Vt/Ti (L/s) | 0.11 (0.05)‡ | 0.14 (0.09)‡ | 0.03 (0.05) | 0.04 (0.03) | ||||
| Ti/Tt | 0.03 (0.02) | 0.01 (0.04) | 0.05 (0.04) | 0.03 (0.04) | ||||
| V˙ i (L/min) | 2.09 (1.32)† | 2.96 (1.84)† | 0.33 (0.94) | 0.35 (0.91) | ||||
| Pet CO2 (mm Hg) | 39.54 (1.05) | 38.65 (0.88) | 40.28 (1.08) | 39.24 (1.14) | ||||
Fig. 1. Original recordings of tidal volume (Vt) and integrated electromyographic (IEMG) activity of the abdominal muscles in one representative subject. (A), Vt recordings were obtained 30 min after placebo administration before (left) and during inhalation of the threshold fog concentration (0.73 ml/min) for the cough reflex. Note that fog-induced changes in Vt persisted until the completion of the challenge. (B) Vt recordings were obtained 30 min after administration of 4 mg of nedocromil sodium before (left) and during inhalation of the same threshold fog concentration as in placebo trials (0.73 ml/min, right). (C ) Vt recordings obtained 30 min after administration of 4 mg of nedocromil sodium before (left) and during inhalation of the fog concentration (2.28 ml/min) corresponding to the higher cough threshold value attained after administration of 4 mg of nedocromil sodium.
[More] [Minimize]This study shows that, at threshold stimulus intensity for the cough reflex, fog inhalation consistently causes an increase in V˙i and Vt/Ti, which represents an index of respiratory drive. These ventilatory responses are mainly accounted for by increases in Vt. In addition, after administration of nedocromil sodium, median cough threshold values are significantly increased, whereas fog-induced ventilatory changes are markedly attenuated or even completely abolished.
Cough is a vagally mediated respiratory defense reflex, the most important tussigenic zones being located at the level of the proximal tract of the intrathoracic airways (1, 2, 17). The larynx is also regarded as a sensitive site for cough (1, 17), although data in humans have cast doubts as regards its role in experimentally induced coughing (13, 18). Both the rapidly adapting “irritant” receptors and the pulmonary and bronchial sensory endings of vagal C fibers are likely candidates for cough receptors (2, 17). All these receptor types can be activated by nonisosmolar solutions either injected into the blood supply to the endings (19), or directly administered into the airway lumen (3). Previous studies (3, 4, 20) have suggested that airway receptor stimulation by fog may be due to both the absence of chloride ion (causing cough), and the hypo-osmolarity (causing bronchoconstriction in susceptible people). Which among these fog-related stimuli is also responsible for the observed fog-induced changes in the pattern of breathing remains to be established.
The reflex ventilatory adjustments elicited by the stimulation of rapidly adapting receptors mainly consist of an increase in respiratory drive, possibly associated with a lengthening of Ti (2). In contrast, activation of pulmonary and bronchial C fiber endings induces apnea followed by rapid, shallow breathing (21). Thus, fog-induced changes in the pattern of breathing observed in our subjects are more likely due to stimulation of rapidly adapting receptors. Whether this type of receptor is also solely responsible for the cough responses observed during fog challenges in our subjects remains unclear, because the present outcomes do not allow the exclusion of an additional role played by C fibers (see Widdicombe [17] for a review). Additional studies may be required to determine the relevance of unidentified local mechanisms, such as those possibly involved in the bronchial vasodilation evoked by injections of hyperosmolar saline into the bronchial lumen of dogs (22).
Our finding of increases in Vt/Ti and V˙i during fog challenges in normal subjects is in contrast with previous observations (6) showing that patients with asthma displayed both a bronchoconstrictor response and an increase in Vt/Ti a few minutes after the completion of the fog challenge, whereas control subjects failed to do so. Because present results indicate that fog-induced changes in breathing pattern are short lasting and virtually confined to the inhalation period, the discrepancy between the two studies can be explained by differences concerning the timing of breathing pattern recordings with respect to stimulus administration. We propose that in normal subjects the observed short-lasting hyperpnea is mainly due to direct activation by fog of rapidly adapting receptors, whereas in patients with asthma the reported hyperpnea (6) may be a consequence of longer lasting receptor stimulation by fog-induced bronchoconstriction (1, 2).
The protective action of nedocromil sodium against fog- induced cough has been reported only preliminarily (23, 24). Present results not only confirm the protective effect by nedocromil sodium on fog-induced coughing, but also provide the first evidence that this drug attenuates fog-induced concomitant changes in the pattern of breathing without causing per se any alteration in respiratory activity. This latter finding is consistent with previous observations showing reduced ventilatory responses to exercise in selected children with asthma after sodium cromoglycate administration (25).
To date, the mechanisms by which nedocromil sodium may affect fog-induced respiratory responses in normal subjects remain uncertain. At the level of the airway epithelium, nedocromil sodium reduces either the conductance or the open probability of a Ca2+-dependent chloride channel (7). This phenomenon may explain the reduced nerve depolarization elicited by low-chloride solutions superfusing the isolated vagus nerve (26), as well as decreased airway sensory nerve depolarization induced by inhalation of nonisosmolar aerosols. The same mechanism may also participate in preventing mechanical nerve stimulation due to changes in cell volume and subsequent distortion of the epithelial layer caused by nonisosmolar solutions (7).
The finding that doubling the inhaled nedocromil sodium dose did not further enhance the protective effect against fog-induced coughing is not surprising, because fog-induced changes in respiratory activity turned out to be virtually abolished even with the lower nedocromil sodium dose. In addition, present data are in keeping with previous results (27) showing lack of additional protective action by doubling nedocromil sodium doses against SO2-induced bronchoconstriction in patients with asthma.
In conclusion, in normal subjects inhalation of fog causes coughing associated with an increase in Vt, Vt/Ti, and V˙i. These reflex changes in the pattern of breathing are independent of bronchoconstriction and, therefore, cannot merely represent a consequence of an increase in mechanical load. More likely, they reflect fog-induced activation of airway receptors implicated both in the mediation of coughing and in the control of the breathing pattern. Furthermore, because nedocromil sodium attenuates not only cough receptors sensitivity, but also markedly reduces fog-induced increases in Vt, Vt/Ti, and V˙i, present data, along with previous observations (25), suggest that this agent at least partially inhibits receptors implicated in the mediation of reflex hyperpnea, that is, the rapidly adapting receptors (2). Because of its effect on cough threshold, nedocromil sodium may be useful in the treatment of cough, especially when the use of centrally acting antitussive drugs should be avoided.
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All agents employed in this study were kindly provided by Fisons-Italy.
