Mechanisms of Bronchial Hyperreactivity in Normal Subjects after Upper Respiratory Tract Infection'-· D. W. EMPEY, 4 L. A. LAITINEN, L. JACOBS, W. M. GOLD, and J. A. NADEL
SUMMARY ________________________________________________________ Inhalation of histamine diphosphate aerosol (1.6 per cent, 10 breaths) produced a 218 ± 54.6 per cent (mean ± SE) increase in airway resistance in 16 normal subjects with colds compared with a 30.5 ± 5.5 per cent increase in II healthy control subjects (P < 0.01). There was no significant difference in mean baseline airway resistance between the two groups. Inhalation of saline produced no significant change in airway resistance in either group. Isoproterenol hydrochloride (0.5 per cent, I breath) or atropine sulfate aerosol (0.2 per cent, 20 breaths) each reversed and prevented the increase in airway resistance by histamine, indicating that the bronchoconstriction was caused by smooth muscle contraction and that post-ganglionic, cholinergic pathways were involved in the mechanism. In 6 subjects with colds, citric acid aerosol (10 per cent, 5 breaths) caused bronchoconstriction that lasted up to 30 sec after inhalation, a significantly greater effect than that observed in control subjects or in the same subjects after recovery (P < 0.05). Prior inhalation of atropine aerosol (0.2 per cent, 20 breaths) prevented the bronchoconstriction after citric acid aerosol in all 6 subjects. The threshold concentration of citric acid that produced cough in 7 subjects with colds was significantly lower than that in control subjects or in the 7 subjects after recovery (P < 0.05), suggesting that the exaggerated cholinergic response was due to a decreased threshold for stimulation of the rapidly adapting sensory receptors in the airways. We have provided evidence that respiratory viral infections that produce airway epithelial damage temporarily cause these subjects to develop more bronchoconstriction after inhaling smaller doses of histamine than do healthy subjects. The fact that atropine prevents this response and that the threshold to cough is temporarily decreased is compatible with our hypothesis that airway epithelial damage by infection exposes and, thus, "sensitizes" the rapidly adapting airway receptors to inhaled irritants, causing increased bronchoconstriction via a vagal reflex. Damage to the airway epithelium may occur as a result of mechanical factors, inhaled chemicals, and pollutants, such as ozone, infections, or perhaps as a result of the action of materials released endogenously (e.g., from mast cells, white blood cells, or platelets). "Sensitization" of rapidly adapting sensory receptors in the airways may be an important factor in asthma and in other diseases of airways.
Introduction Patients with asthma (l-3) and some patients with other chronic obstructive lung diseases (46) develop greater bronchoconstriction after inhaling small doses of various substances than do
(Received in original form October 3, 1975 and in revised form October 31, 1975) J From the Cardiovascular Research Institute and Department of Medicine, University of California Medical Center, San Francisco, Calif. 94143.
healthy persons. The mechanism for this bronchial hyperreactivity is not known, but Simonsson and associates (6) showed that marked bran-
2 Supported in part by NHLI Pulmonary SCOR grant 14201. 3 Requests for reprints should be addressed to J. A. Nadel, M.D., Cardiovascular Research Institute, University of California, San Francisco, Calif. 94143. 4 Recipient of USPHS International Research Fellowship F05 TW 2129. Present address: The London Hospital, Whitechapel, London El lBB.
AMERICAN REVIEW OF RESPIRATORY DISEASE, VOLUME 113, 1976
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choconstriction is produced in asthmatic subjects by maneuvers and inhaled substances that stimulate rapidly adapting sensory receptors in the airways (7) and cause reflex bronchoconstriction in animals (8). The bronchoconstriction in asthmatic patients was reversed or prevented by atropine sulfate (a drug that blocks post-ganglionic, cholinergic pathways), supporting the possibility that a vagal reflex caused the exaggerated response. It has been suggested (8, 9) that abnormalities in airway epithelium increase the sensitivity of the rapidly adapting airway receptors, causing bronchial hyperreactivity in asthmatic wbjects; this could also account for their increased susceptibility to cough when the airways are irritated (10). The study of hyperreactivity in asthmatic patients is complicated, however, by the presence of variable degrees of airway obstruction, treatment with drugs, hypertrophy and hyperplasia of airway smooth muscle, and the occurrence of IgE-mediated antigen-antibody reactions. Therefore, we studied the responses to inhaled irritants in subjects who might be expected to have transient sensitization of the rapidly adapting sensory receptors in the airways but who were otherwise normal. Because infections of the respiratory tract that damage the epithelium and expose nerve endings sensitize these receptors in animals (II) and may change bronchial reactivity in humans (12), we studied the effects in normal subjects of inhaling histamine and citric acid aerosols during and after uncomplicated viral respiratory tract infections. We found that bronchial smooth muscle reactivity increased and cough threshold decreased dramatically during the infection and for several weeks thereafter. Our results are consistent with a mechanism involving sensitization of rapidly adapting sensory receptors in the airways.
Materials and Methods We studied 16 otherwise healthy volunteers who had uncomplicated upper respiratory tract infections (colds); of these, 12 were studied in detail during and after their colds. Diagnosis was made on the basis of symptoms; these included rhinorrhea, unproductive cough, malaise, headache, and fever. Although one subject had influenza A, determined by changes in antibody titer, we were unable to obtain a specific viral diagnosis in the others because of a lack of local virus isolation facilities. Our subjects' ages ranged from 22 to 37 years. Three persons smoked 10 to 20 cigarettes per day, and 2 had a history of hay fever but had no symptoms at the time of the study. None had a personal
or family history of asthma, and none had suffered from any lung diseases. General physical examination was normal. A group of 11 healthy control subjects was drawn from the same population. Ages ranged from 23 to 36 years. Two smoked 10 to 20 cigarettes per day and 2 had a history of hay fever. None had had a cold during the two months preceding the study. All subjects were fully informed volunteers who signed consent forms approved by the Academic Senate Committee on Human Experimentation of the Cniversity of California, San Francisco, before participating in the study. \Ve measured airway resistance and thoracic gas volume using a 900-liter constant-volume whole body plethysmograph (13). Recordings were made using a rapid-writing photographic recorder (DR-12, Electronics for Medicine, White Plains, N. Y.). We expressed the results as airway resistance during panting at 2 cycles per sec (13). Fresh solutions of chemicals were prepared daily. Histamine diphosphate w::.s dissolved in normal saline and buffered to pH 7.0 with sodium bicarbonate. Normal saline, similarly buffered to pH 7.0, was used as a control solution. We diluted citric acid solution (20 per cent in distilled water) with normal saline to concentrations ranging from 0.25 to 20 per cent. We delivered all the solutions as aerosols, using a DeVilbiss no. 40 glass nebulizer and a constant :low of 10 liter per min of compressed air. We did not measure the droplet size of the aerosol, but Mercer and co-workers (14) reported a mass mean diameter of 6.3 I'm using this type of nebulizer under similar conditions. The output of the nebulizer was determined by filling it with solution and weighing it before and after allowing air to flow through it; the output was 0.5 ml of solution per min. The subject, wearing a noseclip, placed the nebulizer between the teeth with the mouth open, took 10 tidal breaths of the aerosol. Subjects were unaware of the sequence of the aerosols and were told that the inhalations might make them feel better or worse or have no effect. All subjects received saline and 1.6 per cent histamine, except for 7 subjects with colds and 7 control subjects in whom we obtained dose-response measurements of airway resistance using 0.4 per cent, 0.8 per cent, and 1.6 per cent histamine. 'Ve made control measurements of airway resistance every 20 sec for 3 min before the inhalation of aerosols and reported the mean of these 9 measurements as the baseline value. We repeated the measurements 2 min after the inhalation of histamine and then every 20 sec for 3 min. The mean of these 9 measurements is reported as the response to the aerosol. Airway resistance was measured every 30 sec thereafter until 6 consecutive measurements were within 10 per cent of the baseline value. If a histamine dose-response relationship was being investigated, inhalation of saline or a stronger concentration
133
BROJ\CHIAL HYPERREACTIVITY AFTER UPPER RESPIRATORY TRACT IJ\FECTION
of histamine was then given, but the inhalations were never less than 10 min apart. To determine whether the increase in airway resistance produced by histamine could be reduced or prevented, we administered isoproterenol hydrochloride aerosol (0.5 per cent, l breath) to 6 subjects and atropine sulfate aerosol (0.2 per cent, 20 breaths) to 6 other subjects. We then obtained new baseline measurements of airway resistance, and 10 min after inhalation of isoproterenol or atropine, we administered histamine aerosol and measured airway resistance as before. \Vhenever we attempted to prevent the responses to histamine, further studies were performed later the same day or the next day to demonstt·ate that hyperreactivity to histamine was still present. To determine whether the bronchoconstriction induced by histamine could be reversed, we gave isoproterenol aerosol (0.5 per cent, l breath) to 7 subjects and atropine aerosol (0.2 per cent, 20 breaths) to 9 subjects 5 min after inhalation of histamine. We then measured airway resistance as before and used the mean of the 9 values obtained after either sequence to determine the effect Of each drug on the histamine-induced bronchoconstriction. After inhalation of atropine, the subjects rinsed their mouths with water to minimize systemic absorption of the drug. We modified the pmcedure for inhalation of citric acid aerosol because this stimulus causes only a transient bronchoconstriction (6). Six subjects with colds and 6 control subjects inhaled 5 breaths of 10 per cent citric acid. Measurements of airway resistance were made immediately after inhalation and again every 10 sec for 3 min. The changes at each 10-sec period after inhalation of citric acid were compared to the baseline values. Subjects were studied again 6 weeks after recovery. To investigate the effect of cholinergic blockade, we used the same procedure 10 min after inhalation of atropine (0.2 per cent, 20 breaths); new baseline values for airway resistance were obtained before inhalation of citric acid (10 per cent, 5 breaths). We studied the effects of citric acid on cough in 7 subjects with colds and in 12 control subjects using a modification of the method described by Bickerman and Barach (10). In this part of the study, the subjects were unaware that we were interested in cough, and we administered aerosols in a modified random, double blind manner. The subjects were seated in a quiet room and were told before each test to breathe out slowly to residual volume, and then to inhale the aerosol (saline or 0.25 to 20 per cent citric acid) rapidly until they reached total lung capacity. The number of coughs after this maneuver was recorded, and the lowest concentration of citric acid that reproducibly (3 times) elicited more than 2 involuntary coughs was considered to be a threshold dose. Weaker concentrations of citric acid were
always used first. There was a !-min interval after each inhalation. To investigate the effect of abolition of bronchial smooth muscle tone on the threshold for cough, we instructed 7 subjects to inhale 2 breaths of 0.5 per cent isoproterenol or saline 5 min before inhalation of citric acid. Studies were performed during the cold and again I month after recovery when the subjects were no longer hyperreactive. Significance of the results was established by means of "Student's" t test applied to paired or unpaired measurements of airway resistance, thoracic gas volume, and specific airway conductance. The cough thresholds were compared by the Wilcoxon signed rank test or the Mann-Whitney test as appropriate for paired and unpaired observations (15). Results
Inhalation of 10 breaths of 1.6 per cent histamine aerosol produced a 218 ± 54.6 per cent (mean ± SE) increase in airway resistance in 16 subjects with colds compared with a 30.5 ± 5.5 per cent increase in II control subjects (P
0.5).
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EMPEY, LAITINEN, JACOBS, GOLD, AND NADEL
0.0 l ). Inhalation of a saline aerosol produced no significant change in airway resistance in either group (P > 0.5). Of the 16 persons with colds, 12 had individual increases in airway resistance that were greater than 100 per cent (figure l); these persons were studied in detail during their colds and for as long as 7 weeks afterward. The remaining 4 had significant but smaller increases in airway resistance and were not studied further. There was no significant difference in the mean baseline airway resistance of the 12 subjects with colds compared with that of the ll control subjects (P > 0.5; figure 1). In the subjects with colds whose response to histamine was significant, thoracic gas volume increased by a mean of 0.5 liter (P < 0.01) as compared with that of the control subjects, who had a mean increase of only 0.05 liter (P > 0.5). The peak increase in airway resistance was achieved within 2 to 3 min after histamine inhalation; it persisted at the maximal level for as long as 5 min, then gradually declined and returned to normal within 20 to 30 min. All of the subjects with colds had similar airway resistances in the control state, but the response to inhaled histamine varied markedly (figure l ), indicating that the degree of responsiveness in these subjects did not depend on differences in baseline resistance. The presence or absence of smoking or of hay fever alw did not appear to be a primary determinant, because the 3 most hyperreactive subjects did not have hay fever and did not smoke. Further-
more, the 2 smokers had some of the lowest responses in the group. Hyperreactivity to histamine appeared within 2 or 3 days after the onset of a cold. Although symptoms of the infection disappeared within 4 to 8 days, hyperreactivity remained in all 12 subjects for a week, in ll subjects for 3 weeks, in 9 subjects for 4 weeks, in 4 subjects for 5 weeks, and in l subject for 6 weeks. By the seventh week all subjects reacted normally (figure l ). The baseline airway resistance of these subjects did not differ significantly after recovery from that obtained during infection (P > 0.5). Those subjects who had the largest histamineinduced increase in airway resistance coughed, complained of tightness in the chest and difficulty in breathing after inhaling histamine, but experienced no flushing or headache. Some subjects in both the control and the experimental groups had minor throat irritation after histamine. Dose-response curves to histamine obtained from subjects with colds were markedly different from those of control subjects. In 7 control subjects 10 breaths of 0.4 per cent histamine aerosol produced only a 7 ± 5.7 per cent increase in airway resistance; in 7 subjects with colds, the same dose produced a 71 ± 27.4 per cent increase (P < 0.05). Similarly, 10 breaths of 0.8 per cent histamine aerosol produced a 33 ± 8.4 per cent increase in control subjects, compared with a mean increase of 155 ± 29.5 per cent in subjects with colds (P < 0.01). After recovery,
12
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Fig. 2. Effects of isoproterenol (0.5 per cent, I breath; left panels) and atropine (0.2 per cent, 20 breaths; right panels) on airway resistance (Raw) after inhalation of histamine aerosol (1.6 per cent, 10 breaths; shaded bars) by subjects with colds. Each panel shows the effect of his· tamine alone ( 0) and after administration of isoproterenol or atropine (at the arrows, • ) in the same subject. Upper panels show the effects of isoproterenol and atropine in reversing the effects of histamine; lower panels show the same drugs preventing the responses to histamine.
BRONCHIAL HYPERREACTIVITY AFTER UPPER RESPIRATORY TRACT INFECTION
the dose-response data of these subjects were not significantly different from those of the control subjects. In each of 6 subjects with colds, inhalation of isoproterenol (0.5 per cent, l breath) before inhalation of histamine decreased the baseline airway resistance only slightly but completely prevented bronchoconstriction (P > 0.3; figures 2 and 3). When we administered isoproterenol (0.5 per cent, l breath) to 7 subjects with colds 5 min after inhalation of histamine, the increase in airway resistance was reversed completely (figures 2 and 3). The mean airway resistance after inhalation of histamine increased significantly (P < 0.001) but decreased to baseline values after subsequent inhalation of isoproterenol (P > 0.4). In each of the 6 subjects with colds, prior ad-
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munstration of atropine aerosol (0.2 per cent, 20 breaths) markedly reduced the increase iu airway resistance after inhalation of histamine (figures 2 and 3). After atropine the mean baseline airway resistance decreased only slightly from the original mean value, and there was no significant bronchoconstriction after inhalation of histamine (P > 0.3; figures 2 and 3). Nine subjects with colds inhaled atropine (0.2 per cent, 20 breaths) 5 min after inhalation of histamine. The increased airway resistance produced by histamine was reduced in each case (figures 2 and 3). The mean airway resistance after atropine did not differ significantly from the mean airway resistance before histamine (P > 0.1). Six subjects with colds inhaled citric acid aerosol (I 0 per cent, 5 breaths). This elicited an increase in airway resistance and a concomitant increase in thoracic gas volume that lasted for 30 sec; both were significantly greater than the changes observed in the control subjects, or in these subjects after recovery from their colds (P
0
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Fig. 3. Effects of isoproterenol (0.5 per cent, 1 breath; left panels) and atropine (0.2 per cent, 20 breaths; right panels) on modifying responses to inhalation of histamine aerosol (1.6 per cent, 10 breaths) in subjects with colds. The line of identity is drawn; points above the line indicate bronchoconstriction, below the line bronchodilation and on the line no change in airway resistance (Raw). Each point is the mean of 9 measurements of Raw in one subject. Each panel shows the response to inhalation of histamine aerosol before ( o) and after (e) drug intervention. Both isoproterenol and atropine reversed (upper panels) and prevented (lower panels) the increase in Raw after inhalation of histamine aerosol.
-0.12 0
20 Time offer citric acid inhololion (sec)
40
Fig. 4. Changes in airway conductance/thoracic gas volume (Gaw /Vtg) from baseline values after inhalation of citric aerosol (10 per cent, 5 breaths) plotted against time after the end of inhalation of citric acid. Each point represents the mean± SE (.).The mean of changes seen in 6 healthy control subjects (•), 6 subjects with colds ( o ), and the same 6 subjects after recovery (e) are shown. The means for control subjects and subjects after recovery from colds are not significantly different at any point (P > 0.05). The means for the subjects with colds are significantly different from the means of the control subjects at 10, 20, and 30 sec after the inhalation, and from the means of the subjects after recovery from their colds at 10 and 20 sec after the inhalation (P < 0.05).
136
EMPEY, LAITINEN, JACOBS, GOLD, AND NADEL
< 0.05; figure 4). Because the increase in thoracic gas volume minimized any increase in airway resistance, we expressed the results as a change in specific conductance. Atropine inhalation (0.2 per cent, 20 breaths) prevented the bronchoconstriction seen after citric acid aerosol in all6 subjects (P < 0.05). The threshold concentration of citric acid that produced cough in the 7 subjects with colds (median, 2 per cent) was lower than that of the control subjects (median, 3.75 per cent) or that of the same subjects after recovery (median, 3.75 per cent; P < 0.05; figure 5). Prior inhalation of isoproterenol (0.5 per cent, 2 breaths) increased the cough threshold in each group, but the subjects with colds still had a lower threshold (median, 2.5 per cent) than did the control subjects (median, 5 per cent) or the same subjects after recovery from their colds (median, 5 per cent; P < 0.05; figure 5). Discussion
Our study demonstrates that spontaneous, presumably viral, upper respiratory tract infections cause striking increases in bronchial reactivity to inhaled histamine and citric acid aerosols in otherwise normal subjects. This bronchial hyperreactivity was present during infection and for several weeks after recovery, but all the subjects returned to normal within 7 weeks. This confirms earlier studies that suggested that such infections increased bronchial reactivity but that did not study possible mechanisms (12, 16). Parker and associates (12) showed an increased bronchial reactivity by a reduction in forced expiratory volume in 1 sec after 5 breaths of 2.5 per cent methacholine in 4 of 8 normal subjects and in 2 of 2 subjects who had hay fever together with acute upper respiratory infections. Laitinen (16) measured peak expiratory flow and found that 2 of 6 atopic subjects without asthma were hyperreactive to 16 breaths of 1.6 per cent histamine when they had just recovered from viral upper respiratory tract infections. In this study we can exclude several mechanisms that have been suggested to explain bronchial hyperreactivity in patients with asthma and chronic bronchitis, because they are not applicable to our subjects. Because resistance to airflow increases inversely with the fourth power of the radius, the response to a bronchoconstrictor stimulus could have a dramatically greater effect on airflow resistance when a significant degree of airway narrowing is already present. This was not a factor in the present study because the
baseline airflow resistance was normal in our wbjects both during infection and after full recovery. Furthermore, other studies of viral respiratory tract infections have failed to show evidence of significant diffuse airway obstruction (17-19); only minor changes, suggesting localized or peripheral airway obstruction, have been shown in some smokers (20). Although airway resistance was reduced slightly by isoproterenol and atropine, such a small change in airway caliber alone could not account for the greatly reduced response to histamine and citric acid. The fact that isoproterenol and atropine reversed the histamine-induced bronchoconstriction indicates that the effects of these drugs are not due simply to small changes in baseline airway caliber or resting smooth muscle tone. Patients with asthma (21, 22) and chronic bronchitis (23) have hypertrophy and hyperplasia of bronchial smooth muscle, which could magnify their response to bronchoconstrictor stimuli and contribute to their bronchial hyperreactivity. Our results cannot be explained on this basis because our subjects were normal persons without lung disease and their bronchial hyperreactivity was transiertt. Only 2 of our 16 subjects had a history of allergy, and because all subjects returned to normal, chronic allergic disease and IgE-mediated reactions cannot be implicated in their transient hyperreactivi ty by causing effects on
--?
20
After isoproterenol
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Without isoproterenol
Fig. 5. Threshold concentrations of citric acid (%) that produced cough in 7 subjects with colds ( o ), in the same subjects after recovery (e), and in 12 control subjects (.&) (abscissa) plotted on a log-log scale against the threshold concentrations of citric acid (%) required to produce cough after inhalation of isoproterenol (0.5 per cent, 2 breaths) 5 min earlier (ordinate). The line of identity is drawn; points on the line show no change, and points above the line show an increase in cough threshold after inhalation of isoproterenol.
BRONCHIAL HYPERREACTIVITY AFTER UPPER RESPIRATORY TRACT INFECTION
airway smooth muscle, mast cells (24), or partial {l-adrenergic receptor blockade (25), as they have in asthma. Unlike most patients with asthma, none of our subjects was taking any drugs, so this did not complicate interpretation of the results. The increase in airway resistance in subjects who inhaled the aerosols can be attributed to contraction of airway smooth muscle, rather than to production of mucus or mucosal swelling, because isoproterenol totally abolished and prevented the response. It has been suggested that partial {l-adrenergic receptor blockade could lead to bronchial hyperreactivity (25, 26). This is an unlikely explanation for our results because a single breath of 0.5 per cent isoproterenol prevented the response to histamine. Also, Zaid and Beall (27) failed to show any increase in the bronchial reactivity of normal subjects to histamine and methacholine after treating them with the {l-adrenergic blocking drug, propranolol. Inhalation of atropine sulfate both reversed and largely prevented the response, indicating that post-ganglionic cholinergic pathways were involved. These pathways have been shown by others to be involved in the hyperreactivity of asthmatics to such stimuli as histamine, citric acid (6), and prostaglandin F 2 a (28). This is not a new concept; indeed, Eppinger and Hess (29) suggested "vagotonia" as an explanation of asthma 60 years ago, and atropine-like drugs were used for symptomatic relief of asthma for many years before that (30); they are regaining popularity (31 ). We have suggested that the primary abnor· mality is on the afferent side of a vagally mediated reflex, i.e., that the rapidly adapting airway epithelial receptors are "sensitized" in asthma and have a lower threshold for firing (8, 9). This hypothesis is based on the facts that all stimuli that cause exaggerated bronchomotor responses in asthmatics stimulate rapidly adapting airway epithelial sensory receptors in animals (7) and cause bronchoconstriction in animals (8), and that asthmatics have an increased susceptibility to cough (l 0). An alternative explanation for the effect of atropine, besides that of a reflex, is that an abnormality exists in the airway smooth muscle, and that acetylcholine and the stimulus (e.g., histamine) "interact" on the smooth muscle to produce an exaggerated response. Atropine could prevent this reponse by blocking the acetylcholine effect and preventing the interaction (32). This is unlikely to be
137
the explanation for our results because of the increased susceptibility to cough after low doses of citric acid in our subjects with viral infections. This was not abolished by a bronchodilator, so it is not an effect secondary to bronchoconstriction; it points to "sensitization" of the rapidly adapting airway epithelial receptors, rather than to an abnormality in the smooth muscle caused by the infection. In addition, the pathology of viral upper respiratory tract infections favors the epithelium, rather than the deeper structures, as the main site of damage (33-35). Each of these studies has shown involvement of the airway epithelial cells, sometimes with extensive loss of cells down to the basement membrane, for as long as 3 weeks after viral infections of the respiratory tract, but there is no evidence for viral involvement of the airway smooth muscle. It is possible that this damaged epithelium could lead to increased absorption of histamine, thus increasing the amount available to the airway smooth muscle and leading to an exaggerated bronchoconstrictor response. This is unlikely, however, because it was not possible to produce such large changes in airway resistance as we found, in entirely normal and healthy subjects, even when they inhaled or received intravenously doses of histamine that cause hypotension, flushing, headache, and nausea (36, 37). Histamine may hilVe both a direct effect on airway smooth muscle (38) and a reflex effect mediated by the vagus nerves (39). We suggest that both effects occur when histamine is inhaled, but the local effect may predominate in healthy persons and the reflex effect in persons whose airway epithelial receptors are "sensitized," as in our subjects with viral infections and possibly in patients with asthma. That the local effect of histamine is also present in persons with "sensitized" receptors is suggested by the fact that a small increase in airway resistance occurs in some persons after the reflex effect has been blocked by atropine (figures 2 and 3). We suggest that the increased bronchoconstrictor response to histamine noted in guinea pigs previously exposed to ozone (40) is, similarly, due to epithelial damage and subsequent sensitization of airway receptors. We also suggest that some industrial and ambient irritants may sensitize airway epithelial receptors and decrease the threshold for bronchoconstriction. Many patients with asthma have IgE-mediated antigen-antibody reactions and bronchial hyperreactivity. It has usually been assumed that these
138
EMPEY, LAITINEN, JACOBS, GOLD, AND NADEL
2 phenomena are part of the same process. It is possible, however, that the two are separate. Thus, IgE-mediated antigen-antibody reactions may occur with the subsequent release of the mediators into the airway. However, if bronchial hyperreactivity does not exist, it is conceivable that the subjects will have little or no clinical manifestation of this release phenomenon. Similarly, subjects such as those in our study (with v.iral infections) may have bronchial hyperreactivity, but unless a bronchoconstrictor stimulus is introduced, no bronchoconstriction appears. However, if mediators (e.g., histamine) are released or inhaled and airway hyperreactivity exists, then manifest bronchoconstriction can be expected. A further increase in bronchial hyperreactivity may be an important factor in the wellrecognized deterioration in the clinical state of asthmatic and bronchitic patients when they suffer viral respiratory tract infections (41).
Acknowledgment The writers thank Dr. J. Schieble of the California State Department of Public Health Laboratories, Berkeley, for help with viral diagnosis. References I. Curry, J. J.: The action of histamine on the respiratory tract in normal and asthmatic subjects, J Clin Invest, 1946, 25, 785. 2. Herxheimer, H.: Bronchial obstruction induCitd by allergens, histamine and acetyl-beta-methylcholinechloride, Int Arch Allergy Appl Immunol, 1951,2, 27. 3. Tiffeneau, R.: Hypersensibilite cholinergo-histaminique pulmonaire de l'asthmatique. Relation avec l'hypersensibilite allergenique pulmonaire, Acta Allergol (Kbh), 1958, 5 (Supplement, p.l87). 4. DeVries, K., Booij-Noord, H., Goei, J. T., Grobler, N. J., Sluiter, H. J., Tammeling, G. J., and Orie, N. G. M.: Hyperreactivity of the bronchial tree to drugs, chemical and physical agents, in Bronchitis II, N. G. M. Orie and H. J. Sluiter, ed., Royal Van Gorcum, Assen, 1964, p.l67. 5. Simonsson, B. G.: Clinical and physiological studies on chronic bronchitis. Ill. Bronchial reactivity to inhaled acetylcholine, Acta Allergol (Kbh), 1965,20, 325. 6. Simonsson, B. G., Jacobs, F. M., and Nadel, J. A.: Role of autonomic nervous system and the cough reflex in the increased responsiveness of airways in patients with obstructive airway disease, J Clin Invest, 1967,46, 1812. 7. Mills, J. E., Sellick, H., and Widdicombe, J. G.: Epithelial irritant receptors in the lungs, in Breathing: Hering-Breuer Centenary Symposi-
urn, R. Porter, ed., Churchill, London, 1970, p.77. 8. Nadel, J. A.: Structure-function relationships in the airways: Bronchoconstriction mediated via vagus nerves or bronchial arteries; peripheral lung constriction mediated via pulmonary arteries, Med Thorac, 1965, 22,231. 9. Nadel, J. A.: Neurophysiologic aspects of asthma, in Asthma: Physiology, lmmunopharmacology, and Treatment, K. F. Austen and L. M. Lichenstein, ed., Academic Press, New York, 1973, p.29. 10. Bickerman, H. A., and Barach, A. L.: The experimental production of cough in human subjects induced by citric acid aerosols. Preliminary studies on the evaluation of antitussive agents, Am J Med Sci, 1954, 228, 156. ll. Widdicombe, J. G.: Action potentials in vagal efferent nerve fibers to the lungs of the cat, Naunyn Schmiedebergs Arch Exp Pathol, 1961,
241,415.
12. Parker, C. D., Bilbo, R. E., and Reed, C. E.: Methacholine aerosol as test for bronchial asthma, Arch Intern Med, 1965,115,452. 13. DuBois, A. B., Botelho, S. Y., and Comroe, J. H., Jr.: A new method for measuring airway resistance in man using a body plethysmograph. Values in normal subjects and patients with respiratory disease, J Clin Invest, 1956,35, 327. 14. Mercer, T. T., Goodard, R. F., and Flores, R. L.: Output characteristics of several commercial nebulizers, Ann Allergy, 1965,23,314. 15. Snedecor, G. W., and Cochran, W. G.: Statistical Methods, ed. 6 Iowa State University Press, Ames, 1967, p. 91. 16. Laitinen, L. A. 1.: Histamine and methacholine challenge in the testing of bronchial reactivity, Scand J Respir Dis, 1974, Supplement 86. 17. Johanson, W. G., Jr., Pierce, A. K., and Sanford JeoP.: Pulmonary function in uncomplicated influenza, Am Rev Respir Dis, 1969,100, 141. 18. Picken, J. J., Niewoehner, D. E., and Chester, E. H.: Prolonged effects of viral infections of the upper respiratory tract upon small airways, Am J Med, 1972,52,738. 19. Cate, T. R., Roberts, J. S., Russ, M. A., and Pierce, J. A.: Effects of common colds on pulmonary function, Am Rev Respir Dis, 1973, 108, 858. 20. Fridy, W. W., Jr., Ingram, R. H., Jr., Hierholzer, J. C., and Coleman, M. T.: Airways function during mild viral respiratory illnesses; The effect of rhinovirus infection in cigarette smokers, Ann Intern Med, 1974, 80, 150. 21. Dunnill, M. S., Massarella, G. R., and Anderson, J. A.: A comparison of the quantitative anatomy of the bronchi in normal subjects, in status asthmaticus, in chronic bronchitis, and in emphysema, Thorax, 1969,24, 176. 22. Hossain, S.: Quantitative measurement of bron-
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23.
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29.
30. 31.
32.
chial muscle in men with asthma, Am Rev Respir Dis, 1973,107,99. Hossain, S., and Heard, B.: Hyperplasia of bronchial smooth muscle in chronic bronchitis, J Pathol, 1970,101, 171. Austen, K. F.: A review of immunological, biochemical and pharmacological factors in the release of chemical mediators, in Asthma: Physiology, 1mrnunopharmacology and Treatment, K. F. Austen and L. M. Lichenstein, ed., Academic Press, New York, 1973, p. 109. Reed, C. E.: Beta-adrenergic blockade, bronchial asthma and atopy, J Allergy, 1968, 42, 238. Szentivanyi, A.: The beta-adrenergic theory of the atopic abnormality in bronchial asthma, J Allergy, 1968, 42, 203. Zaid, G., and Beall, G. M.: Bronchial response to beta-adrenergic blockade, N Engl J Med, 1966, 275,580. Patel, K. R.: Atropine, sodium cromoglycate, and thymoxamine in PgF 2..-induced bronchoconstriction in extrinsic asthma, Br Med J, 1975, 2, 360. Eppinger, H., and Hess, L.: Vagotonia: A Clinical Study in Vegetative Neurology, W. M. Krau~ and S. E. Jelliffe, trans., The Nervous and Mental Disease Publishing Co., New York, 1915. Sigmund, G. G.: Stramonium and its use for smoking, Lancet, 1836, 2, 392. Storms, W. W., doPico, G. A., and Reed, C. E.: Aerosol Sch 1000: An anticholinergic bronchodilator, Am Rev Respir Dis, 1975,111,419. Douglas, J. S., Dennis, M. W., Ridgeway, P., and Bouhuys, A.: Airway constriction in guinea pigs:
139
Interaction of histamine and autonomic drugs,
J Pharmacal Exp Ther, 1973,184, 169.
33. Hers, J. F. Ph., and Mulder, J.: Broad aspects of the pathology and pathogenesis of human influenza, Am Rev Respir Dis, 1961,83, 84. 34. Walsh, J. J., Dietlein, L. F., Low, F. N., Burch, G. E., and Mogabgab, W. J.: Bronchotracheal response in human influenza, Arch Intern Med, 1961,108, 376. 35. Hoorn, B., and Tyrrell, D. A. J.: On the growth of certain "newer" respiratory viruses in organ cultures, Br .J Exp Pathol, 1965,46, 109. 36. Newhall, H. H., and Keiser, H. R.: Relative effects of bradykinin and histamine on the respiratory system of man, J Appl Physiol, 1973, 35, 552. 37. DeVries, K.: The physiopathology of chronic bronchitis, in Bronchitis, N. G. M. Orie and H. J. Sluiter, ed., Royal Van Gorcum, Assen, 1961, p.221. 38. Dale, H. H., and Laidlaw, P.P.: The physiological action of ,8-iminazolylethylamine, J Physiol, 1910,41' 318. 39. DeKock, M.A., Nadel, J. A., Zwi, S., Colebatch, H. J. H., and Olsen, C. R.: A new method for perfusing bronchial arteries: Histamine .bronchoconstriction and apnea, J Appl Physiol, 1966, 21' 185. 40. Easton, R. E., and Murphy, S.D.: Experimental ozone preexposure and histamine, Arch Environ Health, 1967, 15, 160. 41. Lambert, H. P., and Stern, H.: Infective factors in exacerbations of bronchitis and asthma, Br Med J, 1972,3,323.
SUMMARY ________________________________________________________ Inhalation of histamine diphosphate aerosol (1.6 per cent, 10 breaths) produced a 218 ± 54.6 per cent (mean ± SE) increase in airway resistance in 16 normal subjects with colds compared with a 30.5 ± 5.5 per cent increase in II healthy control subjects (P < 0.01). There was no significant difference in mean baseline airway resistance between the two groups. Inhalation of saline produced no significant change in airway resistance in either group. Isoproterenol hydrochloride (0.5 per cent, I breath) or atropine sulfate aerosol (0.2 per cent, 20 breaths) each reversed and prevented the increase in airway resistance by histamine, indicating that the bronchoconstriction was caused by smooth muscle contraction and that post-ganglionic, cholinergic pathways were involved in the mechanism. In 6 subjects with colds, citric acid aerosol (10 per cent, 5 breaths) caused bronchoconstriction that lasted up to 30 sec after inhalation, a significantly greater effect than that observed in control subjects or in the same subjects after recovery (P < 0.05). Prior inhalation of atropine aerosol (0.2 per cent, 20 breaths) prevented the bronchoconstriction after citric acid aerosol in all 6 subjects. The threshold concentration of citric acid that produced cough in 7 subjects with colds was significantly lower than that in control subjects or in the 7 subjects after recovery (P < 0.05), suggesting that the exaggerated cholinergic response was due to a decreased threshold for stimulation of the rapidly adapting sensory receptors in the airways. We have provided evidence that respiratory viral infections that produce airway epithelial damage temporarily cause these subjects to develop more bronchoconstriction after inhaling smaller doses of histamine than do healthy subjects. The fact that atropine prevents this response and that the threshold to cough is temporarily decreased is compatible with our hypothesis that airway epithelial damage by infection exposes and, thus, "sensitizes" the rapidly adapting airway receptors to inhaled irritants, causing increased bronchoconstriction via a vagal reflex. Damage to the airway epithelium may occur as a result of mechanical factors, inhaled chemicals, and pollutants, such as ozone, infections, or perhaps as a result of the action of materials released endogenously (e.g., from mast cells, white blood cells, or platelets). "Sensitization" of rapidly adapting sensory receptors in the airways may be an important factor in asthma and in other diseases of airways.
Introduction Patients with asthma (l-3) and some patients with other chronic obstructive lung diseases (46) develop greater bronchoconstriction after inhaling small doses of various substances than do
(Received in original form October 3, 1975 and in revised form October 31, 1975) J From the Cardiovascular Research Institute and Department of Medicine, University of California Medical Center, San Francisco, Calif. 94143.
healthy persons. The mechanism for this bronchial hyperreactivity is not known, but Simonsson and associates (6) showed that marked bran-
2 Supported in part by NHLI Pulmonary SCOR grant 14201. 3 Requests for reprints should be addressed to J. A. Nadel, M.D., Cardiovascular Research Institute, University of California, San Francisco, Calif. 94143. 4 Recipient of USPHS International Research Fellowship F05 TW 2129. Present address: The London Hospital, Whitechapel, London El lBB.
AMERICAN REVIEW OF RESPIRATORY DISEASE, VOLUME 113, 1976
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EMPEY, LAITI:-./E:-1, JACOBS, GOLD, AND 1'\ADEL
choconstriction is produced in asthmatic subjects by maneuvers and inhaled substances that stimulate rapidly adapting sensory receptors in the airways (7) and cause reflex bronchoconstriction in animals (8). The bronchoconstriction in asthmatic patients was reversed or prevented by atropine sulfate (a drug that blocks post-ganglionic, cholinergic pathways), supporting the possibility that a vagal reflex caused the exaggerated response. It has been suggested (8, 9) that abnormalities in airway epithelium increase the sensitivity of the rapidly adapting airway receptors, causing bronchial hyperreactivity in asthmatic wbjects; this could also account for their increased susceptibility to cough when the airways are irritated (10). The study of hyperreactivity in asthmatic patients is complicated, however, by the presence of variable degrees of airway obstruction, treatment with drugs, hypertrophy and hyperplasia of airway smooth muscle, and the occurrence of IgE-mediated antigen-antibody reactions. Therefore, we studied the responses to inhaled irritants in subjects who might be expected to have transient sensitization of the rapidly adapting sensory receptors in the airways but who were otherwise normal. Because infections of the respiratory tract that damage the epithelium and expose nerve endings sensitize these receptors in animals (II) and may change bronchial reactivity in humans (12), we studied the effects in normal subjects of inhaling histamine and citric acid aerosols during and after uncomplicated viral respiratory tract infections. We found that bronchial smooth muscle reactivity increased and cough threshold decreased dramatically during the infection and for several weeks thereafter. Our results are consistent with a mechanism involving sensitization of rapidly adapting sensory receptors in the airways.
Materials and Methods We studied 16 otherwise healthy volunteers who had uncomplicated upper respiratory tract infections (colds); of these, 12 were studied in detail during and after their colds. Diagnosis was made on the basis of symptoms; these included rhinorrhea, unproductive cough, malaise, headache, and fever. Although one subject had influenza A, determined by changes in antibody titer, we were unable to obtain a specific viral diagnosis in the others because of a lack of local virus isolation facilities. Our subjects' ages ranged from 22 to 37 years. Three persons smoked 10 to 20 cigarettes per day, and 2 had a history of hay fever but had no symptoms at the time of the study. None had a personal
or family history of asthma, and none had suffered from any lung diseases. General physical examination was normal. A group of 11 healthy control subjects was drawn from the same population. Ages ranged from 23 to 36 years. Two smoked 10 to 20 cigarettes per day and 2 had a history of hay fever. None had had a cold during the two months preceding the study. All subjects were fully informed volunteers who signed consent forms approved by the Academic Senate Committee on Human Experimentation of the Cniversity of California, San Francisco, before participating in the study. \Ve measured airway resistance and thoracic gas volume using a 900-liter constant-volume whole body plethysmograph (13). Recordings were made using a rapid-writing photographic recorder (DR-12, Electronics for Medicine, White Plains, N. Y.). We expressed the results as airway resistance during panting at 2 cycles per sec (13). Fresh solutions of chemicals were prepared daily. Histamine diphosphate w::.s dissolved in normal saline and buffered to pH 7.0 with sodium bicarbonate. Normal saline, similarly buffered to pH 7.0, was used as a control solution. We diluted citric acid solution (20 per cent in distilled water) with normal saline to concentrations ranging from 0.25 to 20 per cent. We delivered all the solutions as aerosols, using a DeVilbiss no. 40 glass nebulizer and a constant :low of 10 liter per min of compressed air. We did not measure the droplet size of the aerosol, but Mercer and co-workers (14) reported a mass mean diameter of 6.3 I'm using this type of nebulizer under similar conditions. The output of the nebulizer was determined by filling it with solution and weighing it before and after allowing air to flow through it; the output was 0.5 ml of solution per min. The subject, wearing a noseclip, placed the nebulizer between the teeth with the mouth open, took 10 tidal breaths of the aerosol. Subjects were unaware of the sequence of the aerosols and were told that the inhalations might make them feel better or worse or have no effect. All subjects received saline and 1.6 per cent histamine, except for 7 subjects with colds and 7 control subjects in whom we obtained dose-response measurements of airway resistance using 0.4 per cent, 0.8 per cent, and 1.6 per cent histamine. 'Ve made control measurements of airway resistance every 20 sec for 3 min before the inhalation of aerosols and reported the mean of these 9 measurements as the baseline value. We repeated the measurements 2 min after the inhalation of histamine and then every 20 sec for 3 min. The mean of these 9 measurements is reported as the response to the aerosol. Airway resistance was measured every 30 sec thereafter until 6 consecutive measurements were within 10 per cent of the baseline value. If a histamine dose-response relationship was being investigated, inhalation of saline or a stronger concentration
133
BROJ\CHIAL HYPERREACTIVITY AFTER UPPER RESPIRATORY TRACT IJ\FECTION
of histamine was then given, but the inhalations were never less than 10 min apart. To determine whether the increase in airway resistance produced by histamine could be reduced or prevented, we administered isoproterenol hydrochloride aerosol (0.5 per cent, l breath) to 6 subjects and atropine sulfate aerosol (0.2 per cent, 20 breaths) to 6 other subjects. We then obtained new baseline measurements of airway resistance, and 10 min after inhalation of isoproterenol or atropine, we administered histamine aerosol and measured airway resistance as before. \Vhenever we attempted to prevent the responses to histamine, further studies were performed later the same day or the next day to demonstt·ate that hyperreactivity to histamine was still present. To determine whether the bronchoconstriction induced by histamine could be reversed, we gave isoproterenol aerosol (0.5 per cent, l breath) to 7 subjects and atropine aerosol (0.2 per cent, 20 breaths) to 9 subjects 5 min after inhalation of histamine. We then measured airway resistance as before and used the mean of the 9 values obtained after either sequence to determine the effect Of each drug on the histamine-induced bronchoconstriction. After inhalation of atropine, the subjects rinsed their mouths with water to minimize systemic absorption of the drug. We modified the pmcedure for inhalation of citric acid aerosol because this stimulus causes only a transient bronchoconstriction (6). Six subjects with colds and 6 control subjects inhaled 5 breaths of 10 per cent citric acid. Measurements of airway resistance were made immediately after inhalation and again every 10 sec for 3 min. The changes at each 10-sec period after inhalation of citric acid were compared to the baseline values. Subjects were studied again 6 weeks after recovery. To investigate the effect of cholinergic blockade, we used the same procedure 10 min after inhalation of atropine (0.2 per cent, 20 breaths); new baseline values for airway resistance were obtained before inhalation of citric acid (10 per cent, 5 breaths). We studied the effects of citric acid on cough in 7 subjects with colds and in 12 control subjects using a modification of the method described by Bickerman and Barach (10). In this part of the study, the subjects were unaware that we were interested in cough, and we administered aerosols in a modified random, double blind manner. The subjects were seated in a quiet room and were told before each test to breathe out slowly to residual volume, and then to inhale the aerosol (saline or 0.25 to 20 per cent citric acid) rapidly until they reached total lung capacity. The number of coughs after this maneuver was recorded, and the lowest concentration of citric acid that reproducibly (3 times) elicited more than 2 involuntary coughs was considered to be a threshold dose. Weaker concentrations of citric acid were
always used first. There was a !-min interval after each inhalation. To investigate the effect of abolition of bronchial smooth muscle tone on the threshold for cough, we instructed 7 subjects to inhale 2 breaths of 0.5 per cent isoproterenol or saline 5 min before inhalation of citric acid. Studies were performed during the cold and again I month after recovery when the subjects were no longer hyperreactive. Significance of the results was established by means of "Student's" t test applied to paired or unpaired measurements of airway resistance, thoracic gas volume, and specific airway conductance. The cough thresholds were compared by the Wilcoxon signed rank test or the Mann-Whitney test as appropriate for paired and unpaired observations (15). Results
Inhalation of 10 breaths of 1.6 per cent histamine aerosol produced a 218 ± 54.6 per cent (mean ± SE) increase in airway resistance in 16 subjects with colds compared with a 30.5 ± 5.5 per cent increase in II control subjects (P
0.5).
134
EMPEY, LAITINEN, JACOBS, GOLD, AND NADEL
0.0 l ). Inhalation of a saline aerosol produced no significant change in airway resistance in either group (P > 0.5). Of the 16 persons with colds, 12 had individual increases in airway resistance that were greater than 100 per cent (figure l); these persons were studied in detail during their colds and for as long as 7 weeks afterward. The remaining 4 had significant but smaller increases in airway resistance and were not studied further. There was no significant difference in the mean baseline airway resistance of the 12 subjects with colds compared with that of the ll control subjects (P > 0.5; figure 1). In the subjects with colds whose response to histamine was significant, thoracic gas volume increased by a mean of 0.5 liter (P < 0.01) as compared with that of the control subjects, who had a mean increase of only 0.05 liter (P > 0.5). The peak increase in airway resistance was achieved within 2 to 3 min after histamine inhalation; it persisted at the maximal level for as long as 5 min, then gradually declined and returned to normal within 20 to 30 min. All of the subjects with colds had similar airway resistances in the control state, but the response to inhaled histamine varied markedly (figure l ), indicating that the degree of responsiveness in these subjects did not depend on differences in baseline resistance. The presence or absence of smoking or of hay fever alw did not appear to be a primary determinant, because the 3 most hyperreactive subjects did not have hay fever and did not smoke. Further-
more, the 2 smokers had some of the lowest responses in the group. Hyperreactivity to histamine appeared within 2 or 3 days after the onset of a cold. Although symptoms of the infection disappeared within 4 to 8 days, hyperreactivity remained in all 12 subjects for a week, in ll subjects for 3 weeks, in 9 subjects for 4 weeks, in 4 subjects for 5 weeks, and in l subject for 6 weeks. By the seventh week all subjects reacted normally (figure l ). The baseline airway resistance of these subjects did not differ significantly after recovery from that obtained during infection (P > 0.5). Those subjects who had the largest histamineinduced increase in airway resistance coughed, complained of tightness in the chest and difficulty in breathing after inhaling histamine, but experienced no flushing or headache. Some subjects in both the control and the experimental groups had minor throat irritation after histamine. Dose-response curves to histamine obtained from subjects with colds were markedly different from those of control subjects. In 7 control subjects 10 breaths of 0.4 per cent histamine aerosol produced only a 7 ± 5.7 per cent increase in airway resistance; in 7 subjects with colds, the same dose produced a 71 ± 27.4 per cent increase (P < 0.05). Similarly, 10 breaths of 0.8 per cent histamine aerosol produced a 33 ± 8.4 per cent increase in control subjects, compared with a mean increase of 155 ± 29.5 per cent in subjects with colds (P < 0.01). After recovery,
12
8 4
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Fig. 2. Effects of isoproterenol (0.5 per cent, I breath; left panels) and atropine (0.2 per cent, 20 breaths; right panels) on airway resistance (Raw) after inhalation of histamine aerosol (1.6 per cent, 10 breaths; shaded bars) by subjects with colds. Each panel shows the effect of his· tamine alone ( 0) and after administration of isoproterenol or atropine (at the arrows, • ) in the same subject. Upper panels show the effects of isoproterenol and atropine in reversing the effects of histamine; lower panels show the same drugs preventing the responses to histamine.
BRONCHIAL HYPERREACTIVITY AFTER UPPER RESPIRATORY TRACT INFECTION
the dose-response data of these subjects were not significantly different from those of the control subjects. In each of 6 subjects with colds, inhalation of isoproterenol (0.5 per cent, l breath) before inhalation of histamine decreased the baseline airway resistance only slightly but completely prevented bronchoconstriction (P > 0.3; figures 2 and 3). When we administered isoproterenol (0.5 per cent, l breath) to 7 subjects with colds 5 min after inhalation of histamine, the increase in airway resistance was reversed completely (figures 2 and 3). The mean airway resistance after inhalation of histamine increased significantly (P < 0.001) but decreased to baseline values after subsequent inhalation of isoproterenol (P > 0.4). In each of the 6 subjects with colds, prior ad-
8
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Changes in G0 w / Vtg
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munstration of atropine aerosol (0.2 per cent, 20 breaths) markedly reduced the increase iu airway resistance after inhalation of histamine (figures 2 and 3). After atropine the mean baseline airway resistance decreased only slightly from the original mean value, and there was no significant bronchoconstriction after inhalation of histamine (P > 0.3; figures 2 and 3). Nine subjects with colds inhaled atropine (0.2 per cent, 20 breaths) 5 min after inhalation of histamine. The increased airway resistance produced by histamine was reduced in each case (figures 2 and 3). The mean airway resistance after atropine did not differ significantly from the mean airway resistance before histamine (P > 0.1). Six subjects with colds inhaled citric acid aerosol (I 0 per cent, 5 breaths). This elicited an increase in airway resistance and a concomitant increase in thoracic gas volume that lasted for 30 sec; both were significantly greater than the changes observed in the control subjects, or in these subjects after recovery from their colds (P
0
4
( em HzO per LPS l
Oo
0
135
4
0
0~ i 2
4
Raw before histamine (em H20 per LPS l
Fig. 3. Effects of isoproterenol (0.5 per cent, 1 breath; left panels) and atropine (0.2 per cent, 20 breaths; right panels) on modifying responses to inhalation of histamine aerosol (1.6 per cent, 10 breaths) in subjects with colds. The line of identity is drawn; points above the line indicate bronchoconstriction, below the line bronchodilation and on the line no change in airway resistance (Raw). Each point is the mean of 9 measurements of Raw in one subject. Each panel shows the response to inhalation of histamine aerosol before ( o) and after (e) drug intervention. Both isoproterenol and atropine reversed (upper panels) and prevented (lower panels) the increase in Raw after inhalation of histamine aerosol.
-0.12 0
20 Time offer citric acid inhololion (sec)
40
Fig. 4. Changes in airway conductance/thoracic gas volume (Gaw /Vtg) from baseline values after inhalation of citric aerosol (10 per cent, 5 breaths) plotted against time after the end of inhalation of citric acid. Each point represents the mean± SE (.).The mean of changes seen in 6 healthy control subjects (•), 6 subjects with colds ( o ), and the same 6 subjects after recovery (e) are shown. The means for control subjects and subjects after recovery from colds are not significantly different at any point (P > 0.05). The means for the subjects with colds are significantly different from the means of the control subjects at 10, 20, and 30 sec after the inhalation, and from the means of the subjects after recovery from their colds at 10 and 20 sec after the inhalation (P < 0.05).
136
EMPEY, LAITINEN, JACOBS, GOLD, AND NADEL
< 0.05; figure 4). Because the increase in thoracic gas volume minimized any increase in airway resistance, we expressed the results as a change in specific conductance. Atropine inhalation (0.2 per cent, 20 breaths) prevented the bronchoconstriction seen after citric acid aerosol in all6 subjects (P < 0.05). The threshold concentration of citric acid that produced cough in the 7 subjects with colds (median, 2 per cent) was lower than that of the control subjects (median, 3.75 per cent) or that of the same subjects after recovery (median, 3.75 per cent; P < 0.05; figure 5). Prior inhalation of isoproterenol (0.5 per cent, 2 breaths) increased the cough threshold in each group, but the subjects with colds still had a lower threshold (median, 2.5 per cent) than did the control subjects (median, 5 per cent) or the same subjects after recovery from their colds (median, 5 per cent; P < 0.05; figure 5). Discussion
Our study demonstrates that spontaneous, presumably viral, upper respiratory tract infections cause striking increases in bronchial reactivity to inhaled histamine and citric acid aerosols in otherwise normal subjects. This bronchial hyperreactivity was present during infection and for several weeks after recovery, but all the subjects returned to normal within 7 weeks. This confirms earlier studies that suggested that such infections increased bronchial reactivity but that did not study possible mechanisms (12, 16). Parker and associates (12) showed an increased bronchial reactivity by a reduction in forced expiratory volume in 1 sec after 5 breaths of 2.5 per cent methacholine in 4 of 8 normal subjects and in 2 of 2 subjects who had hay fever together with acute upper respiratory infections. Laitinen (16) measured peak expiratory flow and found that 2 of 6 atopic subjects without asthma were hyperreactive to 16 breaths of 1.6 per cent histamine when they had just recovered from viral upper respiratory tract infections. In this study we can exclude several mechanisms that have been suggested to explain bronchial hyperreactivity in patients with asthma and chronic bronchitis, because they are not applicable to our subjects. Because resistance to airflow increases inversely with the fourth power of the radius, the response to a bronchoconstrictor stimulus could have a dramatically greater effect on airflow resistance when a significant degree of airway narrowing is already present. This was not a factor in the present study because the
baseline airflow resistance was normal in our wbjects both during infection and after full recovery. Furthermore, other studies of viral respiratory tract infections have failed to show evidence of significant diffuse airway obstruction (17-19); only minor changes, suggesting localized or peripheral airway obstruction, have been shown in some smokers (20). Although airway resistance was reduced slightly by isoproterenol and atropine, such a small change in airway caliber alone could not account for the greatly reduced response to histamine and citric acid. The fact that isoproterenol and atropine reversed the histamine-induced bronchoconstriction indicates that the effects of these drugs are not due simply to small changes in baseline airway caliber or resting smooth muscle tone. Patients with asthma (21, 22) and chronic bronchitis (23) have hypertrophy and hyperplasia of bronchial smooth muscle, which could magnify their response to bronchoconstrictor stimuli and contribute to their bronchial hyperreactivity. Our results cannot be explained on this basis because our subjects were normal persons without lung disease and their bronchial hyperreactivity was transiertt. Only 2 of our 16 subjects had a history of allergy, and because all subjects returned to normal, chronic allergic disease and IgE-mediated reactions cannot be implicated in their transient hyperreactivi ty by causing effects on
--?
20
After isoproterenol
10
5
/~
0 0
I
Without isoproterenol
Fig. 5. Threshold concentrations of citric acid (%) that produced cough in 7 subjects with colds ( o ), in the same subjects after recovery (e), and in 12 control subjects (.&) (abscissa) plotted on a log-log scale against the threshold concentrations of citric acid (%) required to produce cough after inhalation of isoproterenol (0.5 per cent, 2 breaths) 5 min earlier (ordinate). The line of identity is drawn; points on the line show no change, and points above the line show an increase in cough threshold after inhalation of isoproterenol.
BRONCHIAL HYPERREACTIVITY AFTER UPPER RESPIRATORY TRACT INFECTION
airway smooth muscle, mast cells (24), or partial {l-adrenergic receptor blockade (25), as they have in asthma. Unlike most patients with asthma, none of our subjects was taking any drugs, so this did not complicate interpretation of the results. The increase in airway resistance in subjects who inhaled the aerosols can be attributed to contraction of airway smooth muscle, rather than to production of mucus or mucosal swelling, because isoproterenol totally abolished and prevented the response. It has been suggested that partial {l-adrenergic receptor blockade could lead to bronchial hyperreactivity (25, 26). This is an unlikely explanation for our results because a single breath of 0.5 per cent isoproterenol prevented the response to histamine. Also, Zaid and Beall (27) failed to show any increase in the bronchial reactivity of normal subjects to histamine and methacholine after treating them with the {l-adrenergic blocking drug, propranolol. Inhalation of atropine sulfate both reversed and largely prevented the response, indicating that post-ganglionic cholinergic pathways were involved. These pathways have been shown by others to be involved in the hyperreactivity of asthmatics to such stimuli as histamine, citric acid (6), and prostaglandin F 2 a (28). This is not a new concept; indeed, Eppinger and Hess (29) suggested "vagotonia" as an explanation of asthma 60 years ago, and atropine-like drugs were used for symptomatic relief of asthma for many years before that (30); they are regaining popularity (31 ). We have suggested that the primary abnor· mality is on the afferent side of a vagally mediated reflex, i.e., that the rapidly adapting airway epithelial receptors are "sensitized" in asthma and have a lower threshold for firing (8, 9). This hypothesis is based on the facts that all stimuli that cause exaggerated bronchomotor responses in asthmatics stimulate rapidly adapting airway epithelial sensory receptors in animals (7) and cause bronchoconstriction in animals (8), and that asthmatics have an increased susceptibility to cough (l 0). An alternative explanation for the effect of atropine, besides that of a reflex, is that an abnormality exists in the airway smooth muscle, and that acetylcholine and the stimulus (e.g., histamine) "interact" on the smooth muscle to produce an exaggerated response. Atropine could prevent this reponse by blocking the acetylcholine effect and preventing the interaction (32). This is unlikely to be
137
the explanation for our results because of the increased susceptibility to cough after low doses of citric acid in our subjects with viral infections. This was not abolished by a bronchodilator, so it is not an effect secondary to bronchoconstriction; it points to "sensitization" of the rapidly adapting airway epithelial receptors, rather than to an abnormality in the smooth muscle caused by the infection. In addition, the pathology of viral upper respiratory tract infections favors the epithelium, rather than the deeper structures, as the main site of damage (33-35). Each of these studies has shown involvement of the airway epithelial cells, sometimes with extensive loss of cells down to the basement membrane, for as long as 3 weeks after viral infections of the respiratory tract, but there is no evidence for viral involvement of the airway smooth muscle. It is possible that this damaged epithelium could lead to increased absorption of histamine, thus increasing the amount available to the airway smooth muscle and leading to an exaggerated bronchoconstrictor response. This is unlikely, however, because it was not possible to produce such large changes in airway resistance as we found, in entirely normal and healthy subjects, even when they inhaled or received intravenously doses of histamine that cause hypotension, flushing, headache, and nausea (36, 37). Histamine may hilVe both a direct effect on airway smooth muscle (38) and a reflex effect mediated by the vagus nerves (39). We suggest that both effects occur when histamine is inhaled, but the local effect may predominate in healthy persons and the reflex effect in persons whose airway epithelial receptors are "sensitized," as in our subjects with viral infections and possibly in patients with asthma. That the local effect of histamine is also present in persons with "sensitized" receptors is suggested by the fact that a small increase in airway resistance occurs in some persons after the reflex effect has been blocked by atropine (figures 2 and 3). We suggest that the increased bronchoconstrictor response to histamine noted in guinea pigs previously exposed to ozone (40) is, similarly, due to epithelial damage and subsequent sensitization of airway receptors. We also suggest that some industrial and ambient irritants may sensitize airway epithelial receptors and decrease the threshold for bronchoconstriction. Many patients with asthma have IgE-mediated antigen-antibody reactions and bronchial hyperreactivity. It has usually been assumed that these
138
EMPEY, LAITINEN, JACOBS, GOLD, AND NADEL
2 phenomena are part of the same process. It is possible, however, that the two are separate. Thus, IgE-mediated antigen-antibody reactions may occur with the subsequent release of the mediators into the airway. However, if bronchial hyperreactivity does not exist, it is conceivable that the subjects will have little or no clinical manifestation of this release phenomenon. Similarly, subjects such as those in our study (with v.iral infections) may have bronchial hyperreactivity, but unless a bronchoconstrictor stimulus is introduced, no bronchoconstriction appears. However, if mediators (e.g., histamine) are released or inhaled and airway hyperreactivity exists, then manifest bronchoconstriction can be expected. A further increase in bronchial hyperreactivity may be an important factor in the wellrecognized deterioration in the clinical state of asthmatic and bronchitic patients when they suffer viral respiratory tract infections (41).
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