Effect of Allergen Inhalation on the Maximal Response Plateau of the Dose-Response Curve to Methacholine 1 •2
WATCHARA BOONSAWAT, CHERYL M. SALOME, and ANN J. WOOLCOCK Introduction
Bronchial hyperresponsiveness (BHR) can be assessed in asthmatic subjects by plotting a dose-response curve (DRC) to histamine or methacholine. In normal subjects, the DRC has a sigmoidal shape on which a maximal response plateau can be demonstrated after a mild degree of airway narrowing (1, 2).In asthmatic subjects the DRC is characterized by a leftward shift in position, and in moderate or severe asthma, there is no evidence of a maximal response plateau even when the FEV 1 drops by more than 60% (1). The maximal response and the position of the DRC seem to be controlled by different mechanisms (3-6). ~r Agonist aerosols shift the position of the DRC to the right without any effect on the maximal response (4); corticosteroids have been shown to reduce the maximal response with little effect on the position of the DRC (5). The maximal response is probably more important than the position of the DRC in causing symptoms in asthmatic subjects (7). Exposure to allergen is now considered to playa major role in the pathogenesis of asthma. Challenge with allergen in the laboratory shifts the position of the DRC to the left by two or more doubling doses, especially in subjects who have a late asthmatic response (LAR) (8, 9). This observation led to the hypothesis that repeated exposure to allergen may induce the permanent increase in BHR that is seen in subjects with persistent asthma (10). Because of the clinical relevance of the maximal response, this study was designed to investigate the effect of allergen challenge on the maximal response in atopic subjects. Methacholine challenges were performed to establish the maximal response plateau before and 24 h after an allergen challenge in nine mild asthmatic and seven rhinitic subjects. Methods Subjects A total of 16 atopic subjects, nine asthmatic
SUMMARY Methacholine dose response curves (ORC)In asthmatic subjects are characterized by a leftward shift and increased maximal response. Allergen Inhalation in atopic subjects shifts the ORCto the left, but the effect on the shape Is unknown. This study was designed to Investigate the effect of allergen Inhalation on the maximal response plateau of the methacholine ORC In 16 atopic subjects; nine had mild asthma and seven had rhinitis. They were challenged with allergen and with control solutions In a single-blind design. Methacholine challenges (up to 199 umol) were performed at baseline and 24 h after the control and allergen challenges. A plateau of the ORC was defined as a difference of less than 5% in FEV1 between the last two or more doses. The maximal response was obtained by averaging the values on the plateau and was reached by all except one subject. Allergen Inhalation Induced an early asthmatic response (EAR) In all subjects and an additional late asthmatic response (lAR) In 6 subjects. In subjects with an EAR alone the maximal response to methacholine 24 h after allergen challenge was not different from control (mean difference, 2.9% fall In FEV,; p > 0.05). In subjects with lAR, the mean value for the maximal response Increased from 28.5% after control to 36.5% after allergen (mean difference, 8.0%; p < 0.05). Of six subjects who developed lAR two lost the plateau on the ORCafter allergen challenge. We con: elude that allergen Inhalation Increases the maximal response to methacholine in those subjects who have alAR. AM REV RESPIR DIS 1992; 146:565-569
and sevenrhinitic, from the staff and students of the University of Sydney volunteered for this study. Their baseline characteristics are shown in table 1. Asthmatic subjects had a history of wheezing or chest tightness and werepreviously diagnosed by a doctor as having asthma. All had well-controlled symptoms. 1\vo subjects had stopped using bronchodilators for more than 6 months; the others rarely used them as needed. Three subjects used inhaled corticosteroid regularly as prophylactic treatmen 1. Rhinitic subjects had nasal symptoms without a history of wheezing or chest tightness and had never used asthma medications. Four subjects used intranasal steroids, two used antihistamines, and one took no treatment for rhinitis. We chose very mild asthmatic and rhinitic subjects because wecould safely measure the maximal response plateau in these subjects. All subjects had a positive skin prick test (> 3 mm wheal) to one or more airborne allergens. All had FEV l values more than 800/0 of predicted (11) and PD zo valuesto methacholine between 2.6 and> 200 umol. Only two subjects had a PD zo in asthmatic range (less than 8 umol); the others had PD zo within normal range (greater than 8 umol). There was no history of upper respiratory tract infection for at least 4 wk before the study. Subjects did not use aerosol bronchodilators for 6 h and antihistamine or aerosol steroid for 48 h before testing. All subjects gavetheir written consent to the study. The study protocol
was approved by the Ethics Committee of the Royal Prince Alfred Hospital.
Study Design During the preliminary visit, a histamine inhalation test and skin prick tests were performed. The study consisted of five visits. On the first visit the high-dose methacholine challenge was performed to establish the level of the plateau. At Visit 2, 1 to 2 wk after Visit 1, a control challenge with glycerin was performed in the same way as the allergen challenge. The control challenge was used to differentiate the early asthmatic response (EAR) from the nonspecific irritating effect of glycerine and differentiate the late asthmatic response from the normal fluctuation in lung function. At Visit 3, 24 h after Visit 2, the high-dose methacholine challenge was performed. At Visit 4, 1 to 2 wk after Visit 3, the subjects were challenged with the allergen that caused the biggest wheal on skin prick
(Received in originalform September 18. 1991 and in revised form March 1, 1992) 1 From the Department of Medicine, University of Sydney, New South Wales, Australia. 1 Correspondence and requests for reprints should be addressed to Professor Ann J. Woolcock, Institute of Respiratory Medicine, Royal Prince Alfred Hospital, Camperdown, NSW 2050, Australia.
565
566
BOONSAWAT, SALOME, AND WOOLCOCK
TABLE 1 CHARACTERISTICS OF THE SUBJECTS Subject No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Sex
Age
Diagnosis
PD20 (Methacholine)
FEV 1 (% of predicted)
Treatment
M M M F M F M F M F M M F M M M
29 25 42 29 19 29 33 29 57 23 32 44 37 22 32 24
Asthma Asthma Asthma Rhinitis Asthma Rhinitis Asthma Rhinitis Asthma Asthma Asthma Rhinitis Rhinitis Asthma Rhinitis Rhinitis
63 10 > 200 25 27 200 26 4 15 20 10 > 200 > 200 2.6 > 200 > 200
120 108 101 101 107 112 136 104 110 108 94 91 122 91 125 113
A BOP, A A None A BOP nasal None BOP nasal None A BOP, A BOP, IB (nasal) BOP nasal BOP Anti-H Anti-H
Definition of abbreviations: P0 20 = provocative concentration of methacholine producing a 20% fall in FEV.; A = albuterol inhaler, as needed; BOP = beclomethasone dipropionate inhaler; IB = ipratropium bromide spray; Anti·H = antihistamine tablet.
tests. Visit 5 was 24 h after Visit 4, when the high-dose methacholine challenge was performed. All tests were within 1 month and were performed at the same time of the day in each individual. Inhalation tests. A Vitalograph dry spirometer (Vitalograph, Buckingham, UK) was used to measure the FEV 1 and FVC. At each test the measurement was repeated until two curves with FEV 1 reproducible to 100ml were obtained, and the highest ofthese values was recorded. High-dose methacholine tests. Methacholine tests were carried out using a modification of the method described by Chai and coworkers (12).Methacholine in concentrations of 3, 6, 25, 50, 100, and 200 mg/rnl was administered in cumulative doses ranging from 0.8 to 199 umol with a DeVilbiss No. 646 nebulizer (DeVilbiss,Somerset, PA), with the vent open, attached to oxygen at 138kPa (20 psi). The length of each nebulization was controlled by a nebulization dosimeter (Rosenthal French, Baltimore, MD) at a setting of 0.6 s per inhalation. This gave an output of 0.01 ± 0.0011 ml per inhalation. Inhalation were taken slowly from slightly below FRC toward TLC followed by a breath hold of 3 s. The FEV 1 was measured 60 s later and followed immediately by the next dose. The challenge was stopped when the FEV 1 dropped by more than 600,10 or when the maximal dose had been administered. Allergen inhalation tests. Allergen inhalation tests were performed using a hand-held glass nebulizer (DeVilbiss No. 40) with an output of 0.0017 ± 0.0004 ml per puff. Allergen extracts used were commercially glycerinated allergen (Hollister-Stier, Elkhart, IN). Allergens werediluted to a fourfold dilution with 500,10 glycerin. The initial dose for the allergen inhalation test was two puffs of the concentration causing a 3 mm wheal with a skin prick test, and FEV 1 was measured 3 min af-
ter allergen dose. Doubling doses of allergen were given at 3-min intervals until the FEV 1 dropped by 200,10 or more. The FEV 1 was recorded at 5, 10, 15, and 30 min and then at 30-min intervals until it returned to normal. The subjects then left the laboratory and continued to record peak expiratory flow rate (PEFR; Airmed, London, UK) by miniWright peak flowmeter every hour until bedtime. An emergency plan was givento the subjects to institute if a severe LAR occurred. The subjects were considered to have aLAR if the PEFR values dropped by more than 200,10 from the baseline of that visit more than 3 h after allergen challenge. Skin pick tests. Atopic status was measured by skin prick tests using 14 common allergens applied to the forearm according to the method of Pepys (13). The allergens(HollisterSteir, Miles Inc., Elkhart, IN) tested were house dust, cockroach, house dust mite (Dermatophagoides farinae and Dermatophagoides pteronyssinusi, animals (cat, dog, horse, and feather mix), pollens (timothy, ryegrass, ragweed mix, and plantain), and molds (Alternaria tenuis and Aspergillus jumigatus). Histamine and glycerinated saline were used as positive and negative con-
troIs. After 15 min, wheal size was recorded as the long axis and its perpendicular. Mean wheal sizes greater than 3 mm were considered positive. The allergen causing the largest wheal was used for allergen challenge. Data Analysis Dose-response curves to methacholine were constructed using percentage fall in FEV 1 from baseline plotted against the logarithm of the doses of methacholine in micromoles. The position of the DRC was obtained by calculating the provocative dose that caused 200,10 fall in FEV 1(PD 1o FEV 1) by linear interpolation. A maximal response plateau wasdefined as a difference of less than 5070 in FEV 1 between the last two or more doses (14). The maximal response (MFEV 1) was calculated by averaging the points on the plateau. In the absence of the plateau the final fall in FEV 1 was used to calculate the maximal response. The logarithmic unit (log) of PD 10 was used in the analysis. The differences in PD 10 between visits were expressed as mean change and 950,10 confidence interval (CI) in doubling doses (15). Two-way analysis of variance and subsequent Student's paired t test (two-tailed) were used to compare the difference in variables within the groups. The difference in variables between the group with a definite LAR and no LAR werecompared using unpaired t tests.
Results
A total of 15subjects reached a maximal response plateau to methacholine at baseline (Visit 1) (figure 1). Subject 16 had a maximal response at only 12070 fall in FEV 1 but did not have a plateau according to the criteria. Of nine asthmatic subjects eight had a maximal response plateau greater than 20070 fall in FEV 1, and five of seven rhinitic subjects had a maximal response plateau at less than 20070. The results of control and allergen challenge are shown in table 2. After control challenge with glycerin no early asthmatic responses or late asthmatic responses developed. Allergen challenge induced EAR greater than 20070 fall in FEV 1 in all except Subject 13, who appeared to
50
-::J --.J
«u,
40 30
~
Fig. 1. DRC to methacholine on Visit 1 (baseline). All but one subject had a maximal response plateau. Dotted lines represent rhinitic subjects and solid lines represent asthmatic subjects.
20
GJ
u,
10
0
1
10
100
DOSE METHACHOLINE (urnol)
567
EFFECT OF ALLERGEN INHALATION ON METHACHOLINE ORe
TABLE 2
TABLE 3
RESULTS OF ALLERGEN INHALATION TESTS
P0 2oFEV, AND MAXIMAL RESPONSES TO METHACHOLINE AT BASELINE, 24 H AFTER CONTROL INHALATION, AND 24 H AFTER ALLERGEN INHALATION IN SUBJECTS WITH AN EAR ONLY AND IN SUBJECTS WITH AN LAR
Control
Allergen Baseline
Subjects
EAR
EAR alone
1 3 5 7 8 9 10 13 15 16
3 5 5 4 0 0 6 0 6 4
10 10 2 6 8 0 2 2 2 8
21 27 23 24 43 42 28 15 25 31
3 5 7 17 15 6 5 14 12 12
Ryegrass OP OP OP Ryegrass Ryegrass Ryegrass Ryegrass Ryegrass OP
0 0 0 0 0 5
3 0 6 3 12 8
24 34 31 30 32 49
27 26 20 28 22 28
OP OP Ryegrass OP OP OP
LAR
2 4 6 11 12 14
24 h After Control
LAR EAR LAR Allergen Used
Definition of abbreviations: EAR = ;arly asthmatic response, the greatest fall in FEV, in the first hour after challenge; LAR = late asthmatic response, the greatest fall in PEFR between 3 and 24 h after challenge; DP = D. pteronyssinus.
MFEV, (% fall)
P0 20 (pmo/)
MFEV, (% fall)
P0 20 (pmo/)
MFEV, (% fall)
63 200 27 26 4 15 20 200 200 200 49.3 (3.9)
24.1 13.1 31.8 28.0 37.9 31.1 31.5 3.7 9.4 12.8* 22.3 (11.63)
49 200 84 99 16 30 7 200 200 200 68.2 3.3
25.3 6.8 22.4 19.8 27.0 26.7 41.2* 3.0 11.7 5.0 18.89 (12.11)
40 200 150 80 3.1 17 7 200 200 200 56.0 (4.7)
20.5 7.2 21.6* 22.3 37.1 33.7 47.1 6.3 15.5 6.7 21.80 (13.9)
10 25 200 10 200 2.6 25.25 (5.99)
32.0 24.5 9.6 28.4 15.9 49.6 26.6 (13.62)
11 25 200 10 70 0.5 16.37 (7.85)
34.9 25.5 8.4 27.3 22.6 52.3 28.5 (14.53)
2.6 10 145 11 30 0.06 6.48 (14.15)
39.6 30.0 22.8* 39.1 * 30.8 56.9 36.5 (11.8)
EAR only
1 3 5 7 8 9 10 13 15 16 Mean (Sot) LAR
2 4 6 11 12 14 Mean (SO)
have a maximal response at 15.6070 fall in FEV r- Six subjects (three asthmatic and three rhinitic) also developed LAR. The results of methacholine challenge before and after control and allergen challenge are shown in table 3 and figure 2. After control challenge with glycerin the DRC to methacholine did not differ from baseline. The mean difference in maximal response was 1.47% (p > 0.3) and the mean difference in PD zo was 0.05 doubling dose (95% CI = -0.7 to 0.6 doubling dose, p > 0.8). After allergen challenge the results were different between subjects who developed only the EAR and those who developed the LAR. In the group with EAR alone, the DRC to methacholine 24 h after allergen challenge did not differ from baseline or control (figure 2, top). The mean difference in the maximal fall in FEY. was 2.9% from control (p > 0.05) (table 3). The PD zo increased 0.3 doubling doses from control (95% CI = - 0.6 to 1.2doubling doses, p > 0.4). One subject did not have a plateau on the DRC after allergen challenge, but the maximal fall in FEY. at 199 umol methacholine was less than at the baseline methacholine challenge. In the group with the LAR, the DRC to methacholine 24 h after allergen challenge changed significantly from baseline and control (figure 2B). The mean maximal fall in FEV. increased from 28.5070 after control to 36.5070 after allergen challenge with a mean increase of
24 h After Allergen
P020 (pmo/)
• No plateau according to the criteria.
t Standard deviation.
8.0% fall in FEV. (p < 0.01,95% CI = 3.5 to 12.50/0 fall in FEV.). The PD zo decreased by 1.34 doubling dose (95070 CI = 0.15to 2.52 doubling doses, p < 0.05) (table 3). The methacholine DRC in each subject is shown in figure 3. Because the difference in baseline FEY. in some subjects may effect the change in DRC, we plotted the DRC, expressingthe response as percentage of predicted FEV r- Of the six subjects, Subjects 6 and 11 lost the plateau on the DRC after allergen chal-
lenge but did not shift the position of the curves. The group with EAR alone and the group with LAR did not differ significantly at baseline in either PD zo or the maximal response to methacholine (p > 0.5 in both cases). Baseline values for FEV. for all subjects before methacholine challenges are shown in table 4. In the group with EAR alone, baseline FEV. was not significantly different for all visits (p > 0.05). In
50
0 - 0 BEFORE 0··· 'OCONTROL
EAR n=10 -:J
40
e-eALLERGEN
-l
~
30
~
i5 u, Fig. 2. Mean ± SEM ORC to rnethacholine before, 24 h after control, and 24 h after allergen challenge: Top = group with only EAR; bottom = group with the LAR.
-rrH
20
..t~··
10
10
50
100
1000
LARn=6 -:J
40
1_l/· 1 l/o'H .;l/ ::r~T-
-l
LE
30
~
i5 l.L.
20 10
vr a:1 1
10
100
DOSE METHACHOUNE (umol)
1000
568
BOONSAWA~
~_
returned to normal, the position of ORC to methacholine had shifted back to control but the maximal response was still increased (figure 3). The change in baseline FEV 1 was not correlated with the increasein the maximal response (p >0.05). Wecould not find the relationship between the severity of the LAR and the degree of increase in the maximal response because four of six subjects who developed the LAR used bronchodilator during the LAR.
SUBJECT. 2
'--~g-g --.,
"o
~
,
I
1.0
10
SALOME, AND WOOLCOCK
Discussion
Fig. 3. ORC to methacholine in subjects with LAR before, 24 h after control, and 24 h after allergen (and 1 wk after allergen challenge in Subject 14). To avoid distortion of the data from the different baseline FEV1 in some subjects, the response was expressed as percentage of predicted FEV1 • Subjects 6 and 11 lost the plateau on the ORC. In Subject 14, when baseline FE~ returned to normal 1 wk after allergen challenge, the position of the ORC had shifted back to normal but the maximal response remained increased.
the group with LAR, the baseline FEV 1 at 24 h after allergen was slightly lower than on the other days (p < 0.05), although only two subjects had more than
a 10% decrease in baseline FEV i - In Subject 14,the baseline FEV 1 recoveredcompletelyafter 1 wk of inhaled beclomethasone. In this subject, when baseline FEV 1
TABLE 4 FEV1 (L) VALUES BEFORE METHACHOLINE CHALLENGE IN BASELINE, 24 H AFTER CONTROL INHALATION, AND 24 H AFTER ALLERGEN INHALATION IN SUBJECTS WITH AN EAR ONLY AND IN SUBJECTS WITH AN LAR SUbjects
Before
24 h after Control
24 h after allergen
EAR only 1 3 5 7 8 9 10 13 15 16 Mean (SEM)*
5.14 4.75 5.09 5.65 3.08 3.79 4.07 3.96 5.42 4.86 4.58 (0.26)
5.31 4.58 4.77 5.54 3.32 3.83 3.84 3.90 5.42 5.09 4.56 (0.25)
5.33 4.80 5.19 5.31 2.99 3.84 3.84 3.77 5.24 5.03 4.54 (0.27)
LAR 2 4 6 11 12 14 Mean (SEM)
4.92 3.28 4.12 3.56 3.36 3.90 3.86 (0.25)
4.86 3.22 4.03 3.62 3.50 3.28 3.75 (0.25)
4.01t 2.97 3.81 3.47 3.53 2.94t 3.45 (0.18)
• Standard error of the mean. FEV, decrease more than 10% from baseline.
t
The results of this study demonstrate that allergen inhalation can induce a significant increase in the maximal response plateau of the methacholine ORC in both asthmatic and rhinitic subjects. The increase occurred only in subjects who developed the LAR. This supports the hypothesis that inflammatory changes associated with the LAR contribute to the increase in the maximal response plateau in asthma. Allergen-induced shifts of ORC to the left are well established (8, 9), but this is the first study to document that allergen inhalation also increases the maximal fall in FEV 1 at the plateau or causes the loss of the plateau. Rhinitic subjects had a maximal response plateau on the ORC to methacholine at the same level as normal subjects (16), but the asthmatic subjects had a higher maximal response plateau. There was a tendency for subjects with a low P0 20 to have a higher maximal response plateau. Although the maximal response correlates with the position of the curve (14), changes in the maximal response and the position do not necessarily occur together (4-6). Bel and colleagues (3) found that inhaled leukotriene increased the maximal response without an effect on the position and inhaled corticosteroids decreased the maximal response with minimal effects on the position. On the other hand, J3ragonist aerosols dramatically shift the ORC to the right without an effect on the maximal response (4). In our study, the increase in maximal response after allergen inhalation was associated with a shift of the ORC to the left. The increase in maximal response was not merely the result of the left shift in the position since the shape of the DRC was also affected, tending. to lose its plateau. Moreover, the increase in maximal response remained abnormally increased in one subject, evenwhen the position of the ORC shifted back to nearly normal position. These results confirm previous findings that the max-
569
EFFECT OF ALLERGEN INHALATION ON METHACHOLINE DRC
imal response and the position are not totally controlled by the same mechanisms (4-6). The importance of the maximal response plateau has been stressed recently (7, 17). The severity of asthma symptoms seem to be mainly determined by the maximal response (7, 17), and it has been suggested that treatment of asthma should aim at decreasing the maximal response (7). Corticosteroids, which have been shown to decrease the maximal response, were recommended in preference to Pragonists (5). Factors that increase the maximal response should be considered most hazardous to asthmatic subjects. In our study a single exposure to allergen induced an increase in maximal response and even caused a loss of plateau. The maximal response remained abnormally high in one subject even when the position of the DRC shifted back to normal. Thus, measuring only PD 20 is likely to underestimate the hazardous effect of the allergen. The mechanisms controlling the maximal response are still unclear (7, 12, 16). Allergen inhalation has been shown to induce an influx of inflammatory cells into the airways in subjects who developed LAR (18, 19)but cause little or not inflammation in subjects with only EAR (20). Airway inflammation is considered responsible for the allergen-induced LAR and the left shift of the DRC (21, 22). Our results showed that the maximal response increased only in subjects who developed LAR. This suggests that airway inflammation not only shifts the DRC to the left but also increases the maximal response. There are several mechanisms by which airway inflammation could contribute to the increase in maximal response. First, airway inflammation may increase the strength of airway smooth muscle, resulting in a shift in position as wellas an increase in maximal response of the DRC. There is evidence that dog tracheal smooth muscle increases in velocity and magnitude of shortening after allergen challenge (23). Second, increased airway wall thickness from cellular infiltration and edema could cause the increase in baseline resistance and the increase in the maximal response (24). In our study, the subjects had a small decrease in baseline FEV 1 when they had an increase in the maximal response. An-
other possibility is that airway inflammation increases the maximal response by abolishing the inhibitory function of nonadrenergic-noncholinergic nerves (NANC). It is believed that NANC is a braking system in the airway to prevent excessiveairway narrowing (25, 26). Airway inflammation could destroy neurotransmitters in the NANC (25) and abolish this braking system, resulting in an increase in the maximal response and/ or loss of the plateau on the DRC. Finally, Macklem (27) suggested that airway inflammation per se increases the maximal response by decreasingor uncoupling the interdependence between lung parenchyma and airway. An increase in the degree of uncoupling and the increase in the airway wall thickness caused by the LAR are the most likely mechanisms. The findings in this study underscore the important role of allergens in asthma. Exposure to allergen not only shifted the DRC to the left but also increased the maximal response and even caused a loss of the plateau on the DRC. Even though airway inflammation is considered responsible for the increase in the maximal response, the true mechanism needs further investigation. References 1. Woolcock Al, Salome CM, Yan K. The shape of the dose-response curve to histamine in asthmatic and normal subjects. Am RevRespirDis 1984; 130:71-5. 2. Sterk PJ, Daniel EE, Zamel N, Hargreave FE. Limited maximal airway narrowing in nonasthmatic subjects. Role of neural control and prostaglandin release. Am Rev Respir Dis 1985; 132:865-70. 3. BelEH, VeenHVD, Kramps lA, Dijkman JH, Sterk PJ. Maximal airway narrowing to inhaled leukotriene D4 in normal subjects. Comparison and interaction with methacholine. Am Rev Respir Dis 1987; 136:979-84. 4. BelEH, Zwinderman AH, Timmers MC, Dijkman JH, Sterk PJ. The effect of a beta-adrenergic bronchodilator on maximal airway narrowing to bronchoconstrictor stimuli in asthma and chronic obstructive pulmonary disease. Thorax 1991; 46: 9-14. 5. Bel EH, Timmers MC, Dijkman JH, Sterk PJ. The effect of inhaled corticosteroids on the maximal degree of airway narrowing to methacholine in asthmatic subjects. Am Rev Respir Dis 1991; 143:105-8. 6. Macklem PT. Mechanical factors determining maximal bronchoconstriction. Eur Respir J 1989; 2(Suppl 6:516s-9s). 7. Macklem PT. The clinical relevance of respiratory muscle research. Am Rev Respir Dis 1986; 134:812-5. 8. Cartier A, Thomson NC, Frith PA, Roberts R, Tech M, Hargreave FE. Allergen-induced increase
in bronchial responsiveness to histamine relationship to the late asthmatic response and change in airway caliber. J Allergy Clin Immunol 1982; 70: 170-7. 9. Cockcroft DW, Ruffin RE, Dolovich J, Hargreave FE. Allergen-induced increase in non allergic bronchial reactivity.Clin Allergy 1977; 7:503-13. 10. Cockcroft DW. Mechanism of perennial allergic asthma. Lancet 1983; 2:253-5. 11. Morris JF, Koski A, Johnson LC. Spirometric standards for healthy nonsmoking adults. Am Rev Respir Dis 1971; 103:57-67. 12. Chai H, Farr RS, Froehlich LA, et al. Standardization of bronchial inhalation challengeprocedures. J Allergy Clin Immunol 1975; 56:323-7. 13. Pepys J. Types of allergic reaction. Clin Allergy 1973; 3:491-509. 14. Sterk PJ, Timmers MC, Dijkman JH. Maximal airway narrowing in humans in vivo. Histamine compared with methacholine. Am Rev Respir Dis 1986; 134:714-8. 15. Chinn S. Repeatability and method comparison. Thorax 1991; 46:454-6. 16. Sterk PJ, Daniel EE, Zamel N, Hargreave FE. Limited bronchoconstriction to methacholine using partial flow-volume curves in nonasthmatic subjects. Am Rev Respir Dis 1985; 132:272-7. 17. Sterk PT, Bel EH. Bronchial hyperresponsiveness: the need for a distinction between hypersensitivity and excessiveairway narrowing. Eur Respir J 1989; 2:267-74. 18. De Monchy JGR, Kanffman HF, VengeP, et al. Bronchoalveolar eosinophilia during allergeninduced late asthmatic reactions. Am Rev Respir Dis 1985; 131:373-6. 19. Metzger WJ, Richerson HB, Worden K, Monick M, Hunninghake GW. Bronchoalveolar lavage of allergic asthmatic patients following allergen bronchoprovocation. Chest 1986; 89:477-83. 20. Diaz P, Gonzalez MC, Galleguillos FR, et al. Leukocytes and mediators in bronchoalveolar lavage during allergen-induced late-phase asthmatic reactions. Am Rev Respir Dis 1989; 139:1383-9. 21. Marsh WR, Irvin CG, Murphy KR, Behrens BL, Larsen GL. Increase in airway reactivity to histamine and inflammatory cellsin bronchoalveolar lavage after the late asthmatic response in an animal model. Am Rev Respir Dis 1985; 131:875-9. 22. Chung KF, Becker AB, Lazarus SC, Frick OL, Nadel JA, Gold WM. Antigen-induced airway hyperresponsiveness and pulmonary inflammation in allergic dogs. J Appl Physiol 1985;58:1347-53. 23. Antonissen LA, Mitchell RW, Kroeger EA, Kepron W, TseKS, Tephens NL. Mechanical alterations of airway smooth muscle in a canine asthmatic model. J Appl Physiol 1979; 46(4):681-7. 24. James AL, Pare PD, Hogg JC. The mechanics of airway narrowing in asthma. Am Rev Respir Dis 1989; 139:242-6. 25. Barnes PJ. Neural control of human airways in health and disease. Am Rev Respir Dis 1986; 134:1289-314. 26. Thompson DC, Szarek JL, Altiere RJ, Diamond L. Nonadrenergic bronchodilation induced by high concentrations of sulfur dioxide. J Appl Physiol 1990; 69:1786-91. 27. Macklem PT. A hypothesis linking bronchial hyperreactivity and airway inflammation: implications for therapy. Ann Allergy 1990; 64:113-6.
WATCHARA BOONSAWAT, CHERYL M. SALOME, and ANN J. WOOLCOCK Introduction
Bronchial hyperresponsiveness (BHR) can be assessed in asthmatic subjects by plotting a dose-response curve (DRC) to histamine or methacholine. In normal subjects, the DRC has a sigmoidal shape on which a maximal response plateau can be demonstrated after a mild degree of airway narrowing (1, 2).In asthmatic subjects the DRC is characterized by a leftward shift in position, and in moderate or severe asthma, there is no evidence of a maximal response plateau even when the FEV 1 drops by more than 60% (1). The maximal response and the position of the DRC seem to be controlled by different mechanisms (3-6). ~r Agonist aerosols shift the position of the DRC to the right without any effect on the maximal response (4); corticosteroids have been shown to reduce the maximal response with little effect on the position of the DRC (5). The maximal response is probably more important than the position of the DRC in causing symptoms in asthmatic subjects (7). Exposure to allergen is now considered to playa major role in the pathogenesis of asthma. Challenge with allergen in the laboratory shifts the position of the DRC to the left by two or more doubling doses, especially in subjects who have a late asthmatic response (LAR) (8, 9). This observation led to the hypothesis that repeated exposure to allergen may induce the permanent increase in BHR that is seen in subjects with persistent asthma (10). Because of the clinical relevance of the maximal response, this study was designed to investigate the effect of allergen challenge on the maximal response in atopic subjects. Methacholine challenges were performed to establish the maximal response plateau before and 24 h after an allergen challenge in nine mild asthmatic and seven rhinitic subjects. Methods Subjects A total of 16 atopic subjects, nine asthmatic
SUMMARY Methacholine dose response curves (ORC)In asthmatic subjects are characterized by a leftward shift and increased maximal response. Allergen Inhalation in atopic subjects shifts the ORCto the left, but the effect on the shape Is unknown. This study was designed to Investigate the effect of allergen Inhalation on the maximal response plateau of the methacholine ORC In 16 atopic subjects; nine had mild asthma and seven had rhinitis. They were challenged with allergen and with control solutions In a single-blind design. Methacholine challenges (up to 199 umol) were performed at baseline and 24 h after the control and allergen challenges. A plateau of the ORC was defined as a difference of less than 5% in FEV1 between the last two or more doses. The maximal response was obtained by averaging the values on the plateau and was reached by all except one subject. Allergen Inhalation Induced an early asthmatic response (EAR) In all subjects and an additional late asthmatic response (lAR) In 6 subjects. In subjects with an EAR alone the maximal response to methacholine 24 h after allergen challenge was not different from control (mean difference, 2.9% fall In FEV,; p > 0.05). In subjects with lAR, the mean value for the maximal response Increased from 28.5% after control to 36.5% after allergen (mean difference, 8.0%; p < 0.05). Of six subjects who developed lAR two lost the plateau on the ORCafter allergen challenge. We con: elude that allergen Inhalation Increases the maximal response to methacholine in those subjects who have alAR. AM REV RESPIR DIS 1992; 146:565-569
and sevenrhinitic, from the staff and students of the University of Sydney volunteered for this study. Their baseline characteristics are shown in table 1. Asthmatic subjects had a history of wheezing or chest tightness and werepreviously diagnosed by a doctor as having asthma. All had well-controlled symptoms. 1\vo subjects had stopped using bronchodilators for more than 6 months; the others rarely used them as needed. Three subjects used inhaled corticosteroid regularly as prophylactic treatmen 1. Rhinitic subjects had nasal symptoms without a history of wheezing or chest tightness and had never used asthma medications. Four subjects used intranasal steroids, two used antihistamines, and one took no treatment for rhinitis. We chose very mild asthmatic and rhinitic subjects because wecould safely measure the maximal response plateau in these subjects. All subjects had a positive skin prick test (> 3 mm wheal) to one or more airborne allergens. All had FEV l values more than 800/0 of predicted (11) and PD zo valuesto methacholine between 2.6 and> 200 umol. Only two subjects had a PD zo in asthmatic range (less than 8 umol); the others had PD zo within normal range (greater than 8 umol). There was no history of upper respiratory tract infection for at least 4 wk before the study. Subjects did not use aerosol bronchodilators for 6 h and antihistamine or aerosol steroid for 48 h before testing. All subjects gavetheir written consent to the study. The study protocol
was approved by the Ethics Committee of the Royal Prince Alfred Hospital.
Study Design During the preliminary visit, a histamine inhalation test and skin prick tests were performed. The study consisted of five visits. On the first visit the high-dose methacholine challenge was performed to establish the level of the plateau. At Visit 2, 1 to 2 wk after Visit 1, a control challenge with glycerin was performed in the same way as the allergen challenge. The control challenge was used to differentiate the early asthmatic response (EAR) from the nonspecific irritating effect of glycerine and differentiate the late asthmatic response from the normal fluctuation in lung function. At Visit 3, 24 h after Visit 2, the high-dose methacholine challenge was performed. At Visit 4, 1 to 2 wk after Visit 3, the subjects were challenged with the allergen that caused the biggest wheal on skin prick
(Received in originalform September 18. 1991 and in revised form March 1, 1992) 1 From the Department of Medicine, University of Sydney, New South Wales, Australia. 1 Correspondence and requests for reprints should be addressed to Professor Ann J. Woolcock, Institute of Respiratory Medicine, Royal Prince Alfred Hospital, Camperdown, NSW 2050, Australia.
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BOONSAWAT, SALOME, AND WOOLCOCK
TABLE 1 CHARACTERISTICS OF THE SUBJECTS Subject No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Sex
Age
Diagnosis
PD20 (Methacholine)
FEV 1 (% of predicted)
Treatment
M M M F M F M F M F M M F M M M
29 25 42 29 19 29 33 29 57 23 32 44 37 22 32 24
Asthma Asthma Asthma Rhinitis Asthma Rhinitis Asthma Rhinitis Asthma Asthma Asthma Rhinitis Rhinitis Asthma Rhinitis Rhinitis
63 10 > 200 25 27 200 26 4 15 20 10 > 200 > 200 2.6 > 200 > 200
120 108 101 101 107 112 136 104 110 108 94 91 122 91 125 113
A BOP, A A None A BOP nasal None BOP nasal None A BOP, A BOP, IB (nasal) BOP nasal BOP Anti-H Anti-H
Definition of abbreviations: P0 20 = provocative concentration of methacholine producing a 20% fall in FEV.; A = albuterol inhaler, as needed; BOP = beclomethasone dipropionate inhaler; IB = ipratropium bromide spray; Anti·H = antihistamine tablet.
tests. Visit 5 was 24 h after Visit 4, when the high-dose methacholine challenge was performed. All tests were within 1 month and were performed at the same time of the day in each individual. Inhalation tests. A Vitalograph dry spirometer (Vitalograph, Buckingham, UK) was used to measure the FEV 1 and FVC. At each test the measurement was repeated until two curves with FEV 1 reproducible to 100ml were obtained, and the highest ofthese values was recorded. High-dose methacholine tests. Methacholine tests were carried out using a modification of the method described by Chai and coworkers (12).Methacholine in concentrations of 3, 6, 25, 50, 100, and 200 mg/rnl was administered in cumulative doses ranging from 0.8 to 199 umol with a DeVilbiss No. 646 nebulizer (DeVilbiss,Somerset, PA), with the vent open, attached to oxygen at 138kPa (20 psi). The length of each nebulization was controlled by a nebulization dosimeter (Rosenthal French, Baltimore, MD) at a setting of 0.6 s per inhalation. This gave an output of 0.01 ± 0.0011 ml per inhalation. Inhalation were taken slowly from slightly below FRC toward TLC followed by a breath hold of 3 s. The FEV 1 was measured 60 s later and followed immediately by the next dose. The challenge was stopped when the FEV 1 dropped by more than 600,10 or when the maximal dose had been administered. Allergen inhalation tests. Allergen inhalation tests were performed using a hand-held glass nebulizer (DeVilbiss No. 40) with an output of 0.0017 ± 0.0004 ml per puff. Allergen extracts used were commercially glycerinated allergen (Hollister-Stier, Elkhart, IN). Allergens werediluted to a fourfold dilution with 500,10 glycerin. The initial dose for the allergen inhalation test was two puffs of the concentration causing a 3 mm wheal with a skin prick test, and FEV 1 was measured 3 min af-
ter allergen dose. Doubling doses of allergen were given at 3-min intervals until the FEV 1 dropped by 200,10 or more. The FEV 1 was recorded at 5, 10, 15, and 30 min and then at 30-min intervals until it returned to normal. The subjects then left the laboratory and continued to record peak expiratory flow rate (PEFR; Airmed, London, UK) by miniWright peak flowmeter every hour until bedtime. An emergency plan was givento the subjects to institute if a severe LAR occurred. The subjects were considered to have aLAR if the PEFR values dropped by more than 200,10 from the baseline of that visit more than 3 h after allergen challenge. Skin pick tests. Atopic status was measured by skin prick tests using 14 common allergens applied to the forearm according to the method of Pepys (13). The allergens(HollisterSteir, Miles Inc., Elkhart, IN) tested were house dust, cockroach, house dust mite (Dermatophagoides farinae and Dermatophagoides pteronyssinusi, animals (cat, dog, horse, and feather mix), pollens (timothy, ryegrass, ragweed mix, and plantain), and molds (Alternaria tenuis and Aspergillus jumigatus). Histamine and glycerinated saline were used as positive and negative con-
troIs. After 15 min, wheal size was recorded as the long axis and its perpendicular. Mean wheal sizes greater than 3 mm were considered positive. The allergen causing the largest wheal was used for allergen challenge. Data Analysis Dose-response curves to methacholine were constructed using percentage fall in FEV 1 from baseline plotted against the logarithm of the doses of methacholine in micromoles. The position of the DRC was obtained by calculating the provocative dose that caused 200,10 fall in FEV 1(PD 1o FEV 1) by linear interpolation. A maximal response plateau wasdefined as a difference of less than 5070 in FEV 1 between the last two or more doses (14). The maximal response (MFEV 1) was calculated by averaging the points on the plateau. In the absence of the plateau the final fall in FEV 1 was used to calculate the maximal response. The logarithmic unit (log) of PD 10 was used in the analysis. The differences in PD 10 between visits were expressed as mean change and 950,10 confidence interval (CI) in doubling doses (15). Two-way analysis of variance and subsequent Student's paired t test (two-tailed) were used to compare the difference in variables within the groups. The difference in variables between the group with a definite LAR and no LAR werecompared using unpaired t tests.
Results
A total of 15subjects reached a maximal response plateau to methacholine at baseline (Visit 1) (figure 1). Subject 16 had a maximal response at only 12070 fall in FEV 1 but did not have a plateau according to the criteria. Of nine asthmatic subjects eight had a maximal response plateau greater than 20070 fall in FEV 1, and five of seven rhinitic subjects had a maximal response plateau at less than 20070. The results of control and allergen challenge are shown in table 2. After control challenge with glycerin no early asthmatic responses or late asthmatic responses developed. Allergen challenge induced EAR greater than 20070 fall in FEV 1 in all except Subject 13, who appeared to
50
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«u,
40 30
~
Fig. 1. DRC to methacholine on Visit 1 (baseline). All but one subject had a maximal response plateau. Dotted lines represent rhinitic subjects and solid lines represent asthmatic subjects.
20
GJ
u,
10
0
1
10
100
DOSE METHACHOLINE (urnol)
567
EFFECT OF ALLERGEN INHALATION ON METHACHOLINE ORe
TABLE 2
TABLE 3
RESULTS OF ALLERGEN INHALATION TESTS
P0 2oFEV, AND MAXIMAL RESPONSES TO METHACHOLINE AT BASELINE, 24 H AFTER CONTROL INHALATION, AND 24 H AFTER ALLERGEN INHALATION IN SUBJECTS WITH AN EAR ONLY AND IN SUBJECTS WITH AN LAR
Control
Allergen Baseline
Subjects
EAR
EAR alone
1 3 5 7 8 9 10 13 15 16
3 5 5 4 0 0 6 0 6 4
10 10 2 6 8 0 2 2 2 8
21 27 23 24 43 42 28 15 25 31
3 5 7 17 15 6 5 14 12 12
Ryegrass OP OP OP Ryegrass Ryegrass Ryegrass Ryegrass Ryegrass OP
0 0 0 0 0 5
3 0 6 3 12 8
24 34 31 30 32 49
27 26 20 28 22 28
OP OP Ryegrass OP OP OP
LAR
2 4 6 11 12 14
24 h After Control
LAR EAR LAR Allergen Used
Definition of abbreviations: EAR = ;arly asthmatic response, the greatest fall in FEV, in the first hour after challenge; LAR = late asthmatic response, the greatest fall in PEFR between 3 and 24 h after challenge; DP = D. pteronyssinus.
MFEV, (% fall)
P0 20 (pmo/)
MFEV, (% fall)
P0 20 (pmo/)
MFEV, (% fall)
63 200 27 26 4 15 20 200 200 200 49.3 (3.9)
24.1 13.1 31.8 28.0 37.9 31.1 31.5 3.7 9.4 12.8* 22.3 (11.63)
49 200 84 99 16 30 7 200 200 200 68.2 3.3
25.3 6.8 22.4 19.8 27.0 26.7 41.2* 3.0 11.7 5.0 18.89 (12.11)
40 200 150 80 3.1 17 7 200 200 200 56.0 (4.7)
20.5 7.2 21.6* 22.3 37.1 33.7 47.1 6.3 15.5 6.7 21.80 (13.9)
10 25 200 10 200 2.6 25.25 (5.99)
32.0 24.5 9.6 28.4 15.9 49.6 26.6 (13.62)
11 25 200 10 70 0.5 16.37 (7.85)
34.9 25.5 8.4 27.3 22.6 52.3 28.5 (14.53)
2.6 10 145 11 30 0.06 6.48 (14.15)
39.6 30.0 22.8* 39.1 * 30.8 56.9 36.5 (11.8)
EAR only
1 3 5 7 8 9 10 13 15 16 Mean (Sot) LAR
2 4 6 11 12 14 Mean (SO)
have a maximal response at 15.6070 fall in FEV r- Six subjects (three asthmatic and three rhinitic) also developed LAR. The results of methacholine challenge before and after control and allergen challenge are shown in table 3 and figure 2. After control challenge with glycerin the DRC to methacholine did not differ from baseline. The mean difference in maximal response was 1.47% (p > 0.3) and the mean difference in PD zo was 0.05 doubling dose (95% CI = -0.7 to 0.6 doubling dose, p > 0.8). After allergen challenge the results were different between subjects who developed only the EAR and those who developed the LAR. In the group with EAR alone, the DRC to methacholine 24 h after allergen challenge did not differ from baseline or control (figure 2, top). The mean difference in the maximal fall in FEY. was 2.9% from control (p > 0.05) (table 3). The PD zo increased 0.3 doubling doses from control (95% CI = - 0.6 to 1.2doubling doses, p > 0.4). One subject did not have a plateau on the DRC after allergen challenge, but the maximal fall in FEY. at 199 umol methacholine was less than at the baseline methacholine challenge. In the group with the LAR, the DRC to methacholine 24 h after allergen challenge changed significantly from baseline and control (figure 2B). The mean maximal fall in FEV. increased from 28.5070 after control to 36.5070 after allergen challenge with a mean increase of
24 h After Allergen
P020 (pmo/)
• No plateau according to the criteria.
t Standard deviation.
8.0% fall in FEV. (p < 0.01,95% CI = 3.5 to 12.50/0 fall in FEV.). The PD zo decreased by 1.34 doubling dose (95070 CI = 0.15to 2.52 doubling doses, p < 0.05) (table 3). The methacholine DRC in each subject is shown in figure 3. Because the difference in baseline FEY. in some subjects may effect the change in DRC, we plotted the DRC, expressingthe response as percentage of predicted FEV r- Of the six subjects, Subjects 6 and 11 lost the plateau on the DRC after allergen chal-
lenge but did not shift the position of the curves. The group with EAR alone and the group with LAR did not differ significantly at baseline in either PD zo or the maximal response to methacholine (p > 0.5 in both cases). Baseline values for FEV. for all subjects before methacholine challenges are shown in table 4. In the group with EAR alone, baseline FEV. was not significantly different for all visits (p > 0.05). In
50
0 - 0 BEFORE 0··· 'OCONTROL
EAR n=10 -:J
40
e-eALLERGEN
-l
~
30
~
i5 u, Fig. 2. Mean ± SEM ORC to rnethacholine before, 24 h after control, and 24 h after allergen challenge: Top = group with only EAR; bottom = group with the LAR.
-rrH
20
..t~··
10
10
50
100
1000
LARn=6 -:J
40
1_l/· 1 l/o'H .;l/ ::r~T-
-l
LE
30
~
i5 l.L.
20 10
vr a:1 1
10
100
DOSE METHACHOUNE (umol)
1000
568
BOONSAWA~
~_
returned to normal, the position of ORC to methacholine had shifted back to control but the maximal response was still increased (figure 3). The change in baseline FEV 1 was not correlated with the increasein the maximal response (p >0.05). Wecould not find the relationship between the severity of the LAR and the degree of increase in the maximal response because four of six subjects who developed the LAR used bronchodilator during the LAR.
SUBJECT. 2
'--~g-g --.,
"o
~
,
I
1.0
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SALOME, AND WOOLCOCK
Discussion
Fig. 3. ORC to methacholine in subjects with LAR before, 24 h after control, and 24 h after allergen (and 1 wk after allergen challenge in Subject 14). To avoid distortion of the data from the different baseline FEV1 in some subjects, the response was expressed as percentage of predicted FEV1 • Subjects 6 and 11 lost the plateau on the ORC. In Subject 14, when baseline FE~ returned to normal 1 wk after allergen challenge, the position of the ORC had shifted back to normal but the maximal response remained increased.
the group with LAR, the baseline FEV 1 at 24 h after allergen was slightly lower than on the other days (p < 0.05), although only two subjects had more than
a 10% decrease in baseline FEV i - In Subject 14,the baseline FEV 1 recoveredcompletelyafter 1 wk of inhaled beclomethasone. In this subject, when baseline FEV 1
TABLE 4 FEV1 (L) VALUES BEFORE METHACHOLINE CHALLENGE IN BASELINE, 24 H AFTER CONTROL INHALATION, AND 24 H AFTER ALLERGEN INHALATION IN SUBJECTS WITH AN EAR ONLY AND IN SUBJECTS WITH AN LAR SUbjects
Before
24 h after Control
24 h after allergen
EAR only 1 3 5 7 8 9 10 13 15 16 Mean (SEM)*
5.14 4.75 5.09 5.65 3.08 3.79 4.07 3.96 5.42 4.86 4.58 (0.26)
5.31 4.58 4.77 5.54 3.32 3.83 3.84 3.90 5.42 5.09 4.56 (0.25)
5.33 4.80 5.19 5.31 2.99 3.84 3.84 3.77 5.24 5.03 4.54 (0.27)
LAR 2 4 6 11 12 14 Mean (SEM)
4.92 3.28 4.12 3.56 3.36 3.90 3.86 (0.25)
4.86 3.22 4.03 3.62 3.50 3.28 3.75 (0.25)
4.01t 2.97 3.81 3.47 3.53 2.94t 3.45 (0.18)
• Standard error of the mean. FEV, decrease more than 10% from baseline.
t
The results of this study demonstrate that allergen inhalation can induce a significant increase in the maximal response plateau of the methacholine ORC in both asthmatic and rhinitic subjects. The increase occurred only in subjects who developed the LAR. This supports the hypothesis that inflammatory changes associated with the LAR contribute to the increase in the maximal response plateau in asthma. Allergen-induced shifts of ORC to the left are well established (8, 9), but this is the first study to document that allergen inhalation also increases the maximal fall in FEV 1 at the plateau or causes the loss of the plateau. Rhinitic subjects had a maximal response plateau on the ORC to methacholine at the same level as normal subjects (16), but the asthmatic subjects had a higher maximal response plateau. There was a tendency for subjects with a low P0 20 to have a higher maximal response plateau. Although the maximal response correlates with the position of the curve (14), changes in the maximal response and the position do not necessarily occur together (4-6). Bel and colleagues (3) found that inhaled leukotriene increased the maximal response without an effect on the position and inhaled corticosteroids decreased the maximal response with minimal effects on the position. On the other hand, J3ragonist aerosols dramatically shift the ORC to the right without an effect on the maximal response (4). In our study, the increase in maximal response after allergen inhalation was associated with a shift of the ORC to the left. The increase in maximal response was not merely the result of the left shift in the position since the shape of the DRC was also affected, tending. to lose its plateau. Moreover, the increase in maximal response remained abnormally increased in one subject, evenwhen the position of the ORC shifted back to nearly normal position. These results confirm previous findings that the max-
569
EFFECT OF ALLERGEN INHALATION ON METHACHOLINE DRC
imal response and the position are not totally controlled by the same mechanisms (4-6). The importance of the maximal response plateau has been stressed recently (7, 17). The severity of asthma symptoms seem to be mainly determined by the maximal response (7, 17), and it has been suggested that treatment of asthma should aim at decreasing the maximal response (7). Corticosteroids, which have been shown to decrease the maximal response, were recommended in preference to Pragonists (5). Factors that increase the maximal response should be considered most hazardous to asthmatic subjects. In our study a single exposure to allergen induced an increase in maximal response and even caused a loss of plateau. The maximal response remained abnormally high in one subject even when the position of the DRC shifted back to normal. Thus, measuring only PD 20 is likely to underestimate the hazardous effect of the allergen. The mechanisms controlling the maximal response are still unclear (7, 12, 16). Allergen inhalation has been shown to induce an influx of inflammatory cells into the airways in subjects who developed LAR (18, 19)but cause little or not inflammation in subjects with only EAR (20). Airway inflammation is considered responsible for the allergen-induced LAR and the left shift of the DRC (21, 22). Our results showed that the maximal response increased only in subjects who developed LAR. This suggests that airway inflammation not only shifts the DRC to the left but also increases the maximal response. There are several mechanisms by which airway inflammation could contribute to the increase in maximal response. First, airway inflammation may increase the strength of airway smooth muscle, resulting in a shift in position as wellas an increase in maximal response of the DRC. There is evidence that dog tracheal smooth muscle increases in velocity and magnitude of shortening after allergen challenge (23). Second, increased airway wall thickness from cellular infiltration and edema could cause the increase in baseline resistance and the increase in the maximal response (24). In our study, the subjects had a small decrease in baseline FEV 1 when they had an increase in the maximal response. An-
other possibility is that airway inflammation increases the maximal response by abolishing the inhibitory function of nonadrenergic-noncholinergic nerves (NANC). It is believed that NANC is a braking system in the airway to prevent excessiveairway narrowing (25, 26). Airway inflammation could destroy neurotransmitters in the NANC (25) and abolish this braking system, resulting in an increase in the maximal response and/ or loss of the plateau on the DRC. Finally, Macklem (27) suggested that airway inflammation per se increases the maximal response by decreasingor uncoupling the interdependence between lung parenchyma and airway. An increase in the degree of uncoupling and the increase in the airway wall thickness caused by the LAR are the most likely mechanisms. The findings in this study underscore the important role of allergens in asthma. Exposure to allergen not only shifted the DRC to the left but also increased the maximal response and even caused a loss of the plateau on the DRC. Even though airway inflammation is considered responsible for the increase in the maximal response, the true mechanism needs further investigation. References 1. Woolcock Al, Salome CM, Yan K. The shape of the dose-response curve to histamine in asthmatic and normal subjects. Am RevRespirDis 1984; 130:71-5. 2. Sterk PJ, Daniel EE, Zamel N, Hargreave FE. Limited maximal airway narrowing in nonasthmatic subjects. Role of neural control and prostaglandin release. Am Rev Respir Dis 1985; 132:865-70. 3. BelEH, VeenHVD, Kramps lA, Dijkman JH, Sterk PJ. Maximal airway narrowing to inhaled leukotriene D4 in normal subjects. Comparison and interaction with methacholine. Am Rev Respir Dis 1987; 136:979-84. 4. BelEH, Zwinderman AH, Timmers MC, Dijkman JH, Sterk PJ. The effect of a beta-adrenergic bronchodilator on maximal airway narrowing to bronchoconstrictor stimuli in asthma and chronic obstructive pulmonary disease. Thorax 1991; 46: 9-14. 5. Bel EH, Timmers MC, Dijkman JH, Sterk PJ. The effect of inhaled corticosteroids on the maximal degree of airway narrowing to methacholine in asthmatic subjects. Am Rev Respir Dis 1991; 143:105-8. 6. Macklem PT. Mechanical factors determining maximal bronchoconstriction. Eur Respir J 1989; 2(Suppl 6:516s-9s). 7. Macklem PT. The clinical relevance of respiratory muscle research. Am Rev Respir Dis 1986; 134:812-5. 8. Cartier A, Thomson NC, Frith PA, Roberts R, Tech M, Hargreave FE. Allergen-induced increase
in bronchial responsiveness to histamine relationship to the late asthmatic response and change in airway caliber. J Allergy Clin Immunol 1982; 70: 170-7. 9. Cockcroft DW, Ruffin RE, Dolovich J, Hargreave FE. Allergen-induced increase in non allergic bronchial reactivity.Clin Allergy 1977; 7:503-13. 10. Cockcroft DW. Mechanism of perennial allergic asthma. Lancet 1983; 2:253-5. 11. Morris JF, Koski A, Johnson LC. Spirometric standards for healthy nonsmoking adults. Am Rev Respir Dis 1971; 103:57-67. 12. Chai H, Farr RS, Froehlich LA, et al. Standardization of bronchial inhalation challengeprocedures. J Allergy Clin Immunol 1975; 56:323-7. 13. Pepys J. Types of allergic reaction. Clin Allergy 1973; 3:491-509. 14. Sterk PJ, Timmers MC, Dijkman JH. Maximal airway narrowing in humans in vivo. Histamine compared with methacholine. Am Rev Respir Dis 1986; 134:714-8. 15. Chinn S. Repeatability and method comparison. Thorax 1991; 46:454-6. 16. Sterk PJ, Daniel EE, Zamel N, Hargreave FE. Limited bronchoconstriction to methacholine using partial flow-volume curves in nonasthmatic subjects. Am Rev Respir Dis 1985; 132:272-7. 17. Sterk PT, Bel EH. Bronchial hyperresponsiveness: the need for a distinction between hypersensitivity and excessiveairway narrowing. Eur Respir J 1989; 2:267-74. 18. De Monchy JGR, Kanffman HF, VengeP, et al. Bronchoalveolar eosinophilia during allergeninduced late asthmatic reactions. Am Rev Respir Dis 1985; 131:373-6. 19. Metzger WJ, Richerson HB, Worden K, Monick M, Hunninghake GW. Bronchoalveolar lavage of allergic asthmatic patients following allergen bronchoprovocation. Chest 1986; 89:477-83. 20. Diaz P, Gonzalez MC, Galleguillos FR, et al. Leukocytes and mediators in bronchoalveolar lavage during allergen-induced late-phase asthmatic reactions. Am Rev Respir Dis 1989; 139:1383-9. 21. Marsh WR, Irvin CG, Murphy KR, Behrens BL, Larsen GL. Increase in airway reactivity to histamine and inflammatory cellsin bronchoalveolar lavage after the late asthmatic response in an animal model. Am Rev Respir Dis 1985; 131:875-9. 22. Chung KF, Becker AB, Lazarus SC, Frick OL, Nadel JA, Gold WM. Antigen-induced airway hyperresponsiveness and pulmonary inflammation in allergic dogs. J Appl Physiol 1985;58:1347-53. 23. Antonissen LA, Mitchell RW, Kroeger EA, Kepron W, TseKS, Tephens NL. Mechanical alterations of airway smooth muscle in a canine asthmatic model. J Appl Physiol 1979; 46(4):681-7. 24. James AL, Pare PD, Hogg JC. The mechanics of airway narrowing in asthma. Am Rev Respir Dis 1989; 139:242-6. 25. Barnes PJ. Neural control of human airways in health and disease. Am Rev Respir Dis 1986; 134:1289-314. 26. Thompson DC, Szarek JL, Altiere RJ, Diamond L. Nonadrenergic bronchodilation induced by high concentrations of sulfur dioxide. J Appl Physiol 1990; 69:1786-91. 27. Macklem PT. A hypothesis linking bronchial hyperreactivity and airway inflammation: implications for therapy. Ann Allergy 1990; 64:113-6.
