Methacholine Airway Responsiveness Decreases during Exercise in Asthmatic Subjects1- 3
M. D. INMAN, R. M. WATSON, K. J. KILLIAN, and P. M. O'BYRNE4
Introduction
Exercise-induced bronchoconstriction may be caused by alterations of the temperature of the airways and/or the osmolarity of the lining fluid caused by the inhalation of large volumes of inadequately conditioned air during exercise (1, 2). These changes in the local environment appear to result in the release of constrictor mediators from cellswithin the airways, which causes bronchoconstriction in asthmatic subjects (3). Most asthmatic subjects have symptoms within 2 to 5 min after the completion of exercise, but not during the period of exercise when the stimulus is greatest. Indeed, transient bronchodilation is seen during (4) and immediately after exercise (5). This suggests that the release of the constrictor mediators may not occur until the exercise is complete, or that other mechanisms exist in the airways that protect against bronchoconstriction during exercise. Although this effect has been recognized (4, 6), the potency of this protection remains unknown. If the protection is both potent and utilizes mechanisms previously unexploited, their isolation may be clinically useful. Toquantify the protection against bronchoconstriction during exercise, wemeasured the bronchoconstrictor response to increasing concentrations of methacholine at rest and during two levels of exercise in subjects with mild asthma. Methacholine waschosen because high inhaled concentrations can be administered without systemic side effects. Methods Subjects Seven atopic subjects with current or seasonal asthma were studied at a time when the asthma was mild and controlled by bronchodilators alone (table I). There was no current exposure to allergens to which they were sensitized (with the exception of house-dust mite), and there had been no exacerbations of Mtbma for at least 4 wk. The baseline FEV I was. > 80.,. predicted normal in all subjects 1414
SUMMARY In many asthmatic subjects, bronchoconstriction develops 2 to 5 min after exercise, reaches a maximum at approximately 10 min, and declines over the next 60 min. However, bronchoclilation is typically observed during and immediately after exercise. We measured the bronchoconstrictor responses to increasing concentrations of inhaled methacholine at rest and during two levels of exercise in seven asthmatic subjects to determine the protection against bronchoconstriction afforded by exercise. On the first day, an incremental Stage 1 exercise test was performed to determine the work capacity (Wcap) of each subject. On the second, third, and fourth days, methacholine was inhaled at rest or during steady-state exercise at one-third or two-thirds of Wcap. The bronchoconstrictor response to methacholine was significantly reduced during exercise (p < 0.0001). The concentration of methacholine required to produce a 20% reduction in FEV1 (PC20) Increased from 2.80 mg/ml (%SEM, 1.62)at rest to 7.29mg/ml (%SEM, 1.43)during exercise at onethird Wcap, and to 31.03mg/ml (%SEM, 1.74)during exercise at two-thirds Wcap (p < 0.001). This study has demonstrated that there is greater than tenfold protection against bronchoconstriction by methacholine during exercise, and the magnitude of the protection depends on the intensity of exercise performed. The mechanism of this protection is not known, but may haveclinical utility. AM REV RESPIR DIS 1990; 141:1414-1417
on all study days. This project was approved by the Ethics Committee of McMaster University Medical Center, and each subject gave written informed consent before taking part in the study.
Study Design Subjects attended the laboratory on 4 study days, separated by at least 1 day. On the first day, an incremental Stage 1 exercise test was performed to determine the work capacity (Wcap) of each subject. On the second, third, and fourth days, a methacholine inhalation test was performed while the subject was sitting on the bicycle at rest or during steadystate exercise at one-third or two-thirds of the subject's Wcap. The methacholine inhalations were started after the subject had been exercising for 5 min. Because of the risks of profound bronchoconstriction caused by a combination of exercise and methacholine, subjects were treated with inhaled 132-agonists immediately after the challenges were completed. These 3 study days were performed in random order. Inhaled bronchodilators were withheld for at least 6 h before each study.
Exercise Tests All testing was performed while the subject was seated on a stationary bicycle ergometer (Ergomed 740; Siemens, Erlangen, FRO). The initial Stage 1 exercise test was performed as described by Jones (7), where the work loads are increased by 100 kpm/min until exhaustion. The subjects breathed room air (tem-
perature, 22 to 23 0 C). The work loads representing one-third and two-thirds Wcap for each subject were determined from the initial Stage 1 exercise test. The duration of the steady-state exercise tests varied between subjects, depending on the number of methacholine inhalations needed to cause a 20070 fall in the FEV I ' For steady-state exercise at onethird Wcap, the test varied between 20 and 40 min (mean, 29 min), whereas at two-thirds Wcap the test varied between 24 and 48 min (mean, 35 min).
Methacholine Inhalation Tests Methacholine inhalation tests were carried out by a method described by Cockcroft and coworkers (8). Aerosols ofthe test solution were generated from a Wright nebulizer with an output of 0.13 ml/min. After the initial control solution of saline, increasing doubling concentrations of methacholine from 0.03 to (Received in original form August l7. 1989 and in revised form December 12, 1989) I From the Department of Medicine, McMaster University, Hamilton, Ontario, Canada. 1 Supported by grants from the Medical Research Council of Canada. J Correspondence and requestsfor reprints should be addressed to Dr. P. M. O'Byrne, Department of Medicine, Rm 3U2, Health Sciences Center, McMaster University, 1200Main St. West, Hamilton, Ontario, Canada L8N 3Z5. 4 Recipient of a Scientist Award from the Medical Research Council of Canada.
1415
AIRWAY RESPONSIVENESS DURING EXERCISE
Subject No. 1 2 3 4 5 6 7
TABLE 1
256
SUBJECT CHARACTERISTICS
128
Age (yr)
Height (cm)
Weight (kg)
FEV 1 (% pred)
Maximal Work Load (watts)
39 16 23 34 18 37 47
178 186 178 178 163 179 165
69 83 70 70 62 73 65
103 91 86 92 96 82 90
333 317 250 267 150 385 150
64 32
Methacholine 16 PC 20 8 lmg/mll
4 2
.5 .25 REST
256.0 mg/ml were inhaled by tidal mouth breathing for 2 min at intervals of 5 min. The bronchoconstrictor response was measured using the FEV 1, which was measured at 30 and 90 s; if the FEV 1 declined during the two measurements, forced expired maneuvers were repeated at 2-min intervals to insure measurement of the lowest value after each inhalation. The test wasstopped when the FEV 1 had fallen by at least 20010, or when the maximal concentration had been inhaled. The results were expressed as the provocation concentration causing a fall in the FEV 1 by 20% (PC 20 ) . Spirometric measurements were made with a Collins 9-L water spirometer (Warren E. Collins Inc., Braintree, MA). The voltage signal was channeled through an A/D converter (Metrabyte, Dash 16) and sampled at 32 Hz by a microcomputer equipped with a data collection and analysis package. Spirometry volume signals were calibrated with a 2-L syringe using two-point calibration. This was linear over the range measured.
Analysis of Bronchomotor Responses To analyze the effects of exerciseon the bronchoconstrictor response to methacholine, two methods were used. (1) Multiple linearregression. Because the baseline PC 20 ranged from 0.25 to 11.73 mg/ml, dummy variables were added to allow for the variability betweensubjects in their responses to methacholine. The FEV 1, expressed as a percentage of control, constituted the dependent variable. The logarithmic transformation of the methacholine concen-
Fig. 1. The effect of exerciseat one-third Wcap and two-thirds Wcap on methacholine airway responsiveness in a single asthmatic subject. Increasing intensity of exercise provides increasing protection against methacholine-induced bronchoconstriction.
.... oII:
... Z
o
(J
'iJI.
.; W II.
tration and the exercise intensity constituted the independent contributors. (2) Analysis of variance. Methacholine PC 20 values werelog-transformed before analysis; therefore, summary statistics are expressed as the geometric mean and percent standard error (%SEM). Comparisons ofthe logarithmic transformation of methacholine PC 20 values between rest and the two levels of exercise were made using a two-way analysis of variance. Having established significance, the Tukey method was used to determine where the differences lay. Statistical significance was accepted as p < 0.05.
Results
Increasing exercise intensity was associated with a highly significant (p < 0.0001) and potent protective effect against the bronchoconstricting effect of methacholine. The bronchoconstrictor response to increasing concentrations of methacholine for a typical subject at rest and during exercise are shown in figure 1. Multiple linear regression yielded the following relationship: OJoFEV l = 86.4 6.2710g[methacholine concentration] + 0.23 OJoWcap (r = 0.716). The coefficient quantifying the bronchoconstrictor effect of methacholine was - 6.26, with a standard error of 0.67, indicating the variable methacholine airway responsiveness between the subjects
EXERCISE
EXERCISE
1/3 Wmax
'/3 Wmax
Fig. 2. Methacholine airway responsiveness,expressed as the methacholine PC,o, at rest and at one-third Wcap and two-thirds Wcap. Increasing intensity of exercise progressively increased the methacholine PC,o in all subjects.
studied; the coefficient describing the protective effect of exercisewas 0.23, with a standard error of only 0.03, indicating a consistent protective effect during exercise between the subjects. The methacholine PCzo increased from 2.80 mg/ml (OJoSEM, 1.62) when measured at rest to 7.29 mg/ml (OJoSEM, 1.43) at one-third Wcap, and to 31.03 mg/ml (OJoSEM, 1.74) at two-thirds Wcap (p < 0.(01) (Figure 2). The PCzo measured during exercise at one-third Wcap was significantly higher than PC zo at rest (p < 0.05), and the PCzo during exerciseat twothirds Wcap was significantly higher than the PCzo at one-third Wcap (p < 0.05). The baseline FEV 1 was within normal limits for all subjects, ranging from 82 to 103010 normal predicted value (table 1). Baseline FEV 1 values did not change significantly over the study period, being 3.77 ± 0.13 L at rest, 3.85 ± 0.13 L prior to exercise at one-third Wcap, and 3.78 ± 0.13 L prior to exercise at twothirds Wcap. Discussion
This study demonstrated that the bronchoconstrictor response to inhaled methacholine in asthmatic subjects is reduced during exercise and that the magnitude of the reduction increases with the intensity of exercise. Although this effect has .". '. ....... been previously recognized (4, 6), the ...•...".'", . magnitude of the effect has not been pre'. , ...• viously established. Furthermore, the \ \••~ '" w... ". w... protective effect was consistent across all the subjects, but the degree of protection ~ was not significantly related to the baseR••t line airway responsiveness, which ranged , , , , ...J.......t~'......._~-~!--_-'-_....L.._.......L from mildly to severely increased. 4 Control 8 18 32 The potency of this protection against METHACHOLINE mg/ml inhaled methacholine is comparable to
I""";~~:~:~:..,.,.
1416
that observed after pretreatment with an inhaled 132-adrenoceptor agonist. Bandouvakis and coworkers (9) demonstrated a 15.9-fold and Ruffin (10)a ninefold mean increase in methacholine PC 20 after pretreatment with inhaled fenoterol 800 ug, This is similar to the 11.1-fold mean increase in methacholine PC 20 demonstrated at two-thirds Wcap. Minute ventilation was not measured because the measurement of ventilation was not essential for the purposes of this study and because the subjects were required to inhale methacholine, making its measurement technically difficult. However, the reduced airway responsiveness is likely related to the increased ventilation and not a function of exercise itself. Stirling and coworkers (4) showed that the bronchoconstriction caused by inhaled histamine in asthmatic subjects is inhibited by both exercise and isocapnic hyperventilation. Also, Freedman and colleagues (6) demonstrated that bronchoconstriction caused by inhaled methacholine can be reversed by exercise and isocapnic hyperventilation. The total dose of methacholine deposited in the airways and the site of deposition when methacholine is inhaled at rest and during exercise were not evaluated during the study. A smaller dose deposited or more peripheral deposition in the airways may explain a reduced responsivenessto methacholine. This is not likely to account for the differencesin methacholine airway responsiveness demonstrated during exercise. This is because the increased inspired volumes during exercise could not reduce the total inhaled dose of aerosol. The reduced inspired time and increased flow rates will cause more central deposition of the aerosol, which should result in a larger fall in FEV 1 (11). Therefore, an increase in methacholine airway responsivenessrather than a reduction might have been expected during exercise. Donna and coworkers (12)have, however, demonstrated that in subjects with mild asthma, similar to those in our current study, enhanced aerosol deposition does not relate to the magnitude of the response to methacholine. Lastly,exercisecan reverse the bronchoconstriction caused by methacholine already deposited in the airways prior to exercise beginning (6). The results of this study may explain why exercise-induced bronchoconstriction does not typically develop until after exercise is complete. The stimuli for bronchoconstriction are at their greatest during exercise (conditioning large re-
INMAN, WATSON, KILLIAN, AND O'BYRNE
spired volumes [3]),but exercise-induced bronchoconstriction usually begins 2 to 5 min after exercise (13). Bronchoconstrictor mediators released as a consequence of these stimuli do not appear to result in bronchoconstriction, presumably because of the protection afforded by exercise. The mechanism of protection is not known. Neural, humoral, mechanical, or a combination of all three, are possible mechanisms. A reduction in vagal tone and increased beta-adrenergic stimulation occur systematically during exercise (14, 15).Beta-adrenergic stimulation and reduction of vagal tone could reduce the bronchoconstrictor response to methacholine and playa role in this protection. However, ifthe protection achieved during exercise is a consequence of hyperventilation rather than exercise per se, as suggested by Stirling and coworkers (4), these mechanisms are lesslikely to be important. The fact that exercise-induced bronchoconstriction occurs within 2 to 5 min after exerciseis completed suggests that any protective mechanism that operates during exercise may be very transient. The protection afforded by 132-stimulation lasts for several hours (15), making it unlikely that the protection occurs through stimulation of airway 132-adrenoceptors by an increase in circulating catecholamines. Deep inspiration has been demonstrated to reduce airway resistance in normal humans (16). The increase in tidal volume that occurs during exercise and isocapnic hyperventilation may also protect against bronchoconstriction. However, deep inspiration has also been shown to cause transient bronchoconstricti on in some asthmatic subjects (17). Therefore, the importance of mechanical factors caused by increases in tidal volume in causing the protection during exercise is not yet known. Inhibitory mediators released during exercise may be responsible for part of this protection. Bronchodilation, which immediately occurs after exercisein asthmatic subjects, is at least in part caused by prostaglandin release (5). In addition, there is a refractory period after exercise in asthmatic subjects that is a consequence of prostaglandin release (18, 19). Also, inhibitory prostaglandins such as prostaglandin E 1 have been shown to reduce methacholine airway responsiveness (20). The role of inhibitory mediators in protecting against bronchoconstriction during exerciseremains to be established. In summary, this study has demon-
strated that a potent mechanism exists that protects asthmatic subjects against bronchoconstriction during exercise. However, the fact that bronchoconstriction occurs within 5 min after exercise is completed in many asthmatic subjects suggests that this effect is transient. References 1. Gilbert lA, Fouke JM, McFadden ER Jr. Heat and water flux in the intrathoracic airways and exercise-induced asthma. J Appl Physiol 1987; 63:1681-91. 2. Anderson SD, Schoeffel RE, Black JL, Daviskas E. Airway cooling as the stimulus to exerciseinduced asthma: are-evaluation. Eur J Respir Dis 1985; 67:20-30. 3. Anderson SD. Exercise-induced asthma. Chest 1985; 87S:191-5. 4. Stirling DR, Cotton DJ, Graham BL, Hodgson WC, Cockcroft DW, Dosman JA. Characteristics of airway tone during exercisein patients with asthma. J Appl Physiol 1983; 54:934-42. 5. Gelb AF, Tashkin DP, Epstein JD, Gong H Jr, Zamel N. Exercise-induced bronchodilation in asthma. Chest 1985; 87:196-201. 6. Freedman S, Lane R, Gillett MK, Guz A. Abolition of methacholine induced bronchoconstriction by the hyperventilation of exercise or volition. Thorax 1988; 43:631-6. 7. Jones NL. Conduct ofthe stage 1test. In: Jones NL, ed. Clinical exercisetesting. Philadelphia: WB Saunders, 1988;135-44. 8. Cockcroft DW, Killian DN, Mellon JJA, Hargreave FE. Bronchial reactivity of inhaled histamine: a method and clinical survey. Clin Allergy 1977; 7:235-43. 9. Bandouvakis J, Cartier A, Roberts R, Ryan G, Hargreave FE. The effect of ipratropium and fenoterol on methacholine- and histamine-induced bronchoconstriction. Br J Dis Chest 1981; 75: 295-305. 10. Ruffin RE. Therapeutic implications of hyperreactivity. In: Hargreave FE, ed. Airway reactivity: mechanisms and clinical relevance. Mississauga: Astra Pharmaceuticals Canada Ltd, 1980; 223-8. 11. Ruffin RE, Dolovich MB, Wolff RK, Newhouse MT. The effects of preferential deposition of histamine in the human airway.Am RevRespir Dis 1978; 117:485-92. 12. Donna E, Danta I, Kim CS, Wanner A. Relationship between deposition of and responsiveness to inhaled methacholine in normal and asymptomatic subjects. J Allergy Clin Immunol 1989; 83: 456-61. 13. Smith EE, Guyton AC, Manning RD, White RJ. Integrated mechanisms of cardiovascular response and control during exercise in the normal human. In: Sonnenblick EH, Lesch M, eds. Exercise and heart disease. New York: Grune and Stratton, 1977;1-23. 14. Chryssanthopoulos C, Barboriak JJ, Fink IN, Stekiel WJ, Maksud MG. Adrenergic responses of asthmatic and normal subjects to submaximal and maximal work levels. J AllergyClin Immunol1978; 61:17-22. 15. Anderson S, Seale JP, Ferris L, Schoeffel R, Lindsay DA. An evaluation of pharmacotherapy for exercise-induced asthma. J Allergy Clin Immunol 1979; 64:612-24. 16. Nadel JA, Tierney DF. Effect of a previous deep inspiration on airway resistancein man. J Appl Physiol 1961; 16:717-9.
1417
AIRWAY RESPONSIVENESS DURING EXERCISE
17. Orehek J, Nicoli MM, Delpierre S, Beaupre A. Influence of the previous deep inspiration on the spirometric measurement of provoked bronchoconstriction in asthma. Am Rev Respir Dis 1981; 123:269-72. 18. O'Byrne PM, Jones GL. The effect of indo-
methacin on exercise-induced bronchoconstriction and refractoriness after exercise. Am Rev Respir Dis 1986; 134:69-72. 19. Margolskee OJ, Bigby BG, Boushey HA. Indomethacin blocks airway tolerance to repetitive exercisebut not to eucapnic hyperpnea in asthmatic
subjects. Am Rev Respir Dis 1988; 137:842-6. 20. Manning PJ, Lane eG, O'Byrne PM. The effect of oral prostaglandin E 1 on airway responsivenessin asthmatic subjects. Pulmon Pharmacol1989; 2:121-4.
M. D. INMAN, R. M. WATSON, K. J. KILLIAN, and P. M. O'BYRNE4
Introduction
Exercise-induced bronchoconstriction may be caused by alterations of the temperature of the airways and/or the osmolarity of the lining fluid caused by the inhalation of large volumes of inadequately conditioned air during exercise (1, 2). These changes in the local environment appear to result in the release of constrictor mediators from cellswithin the airways, which causes bronchoconstriction in asthmatic subjects (3). Most asthmatic subjects have symptoms within 2 to 5 min after the completion of exercise, but not during the period of exercise when the stimulus is greatest. Indeed, transient bronchodilation is seen during (4) and immediately after exercise (5). This suggests that the release of the constrictor mediators may not occur until the exercise is complete, or that other mechanisms exist in the airways that protect against bronchoconstriction during exercise. Although this effect has been recognized (4, 6), the potency of this protection remains unknown. If the protection is both potent and utilizes mechanisms previously unexploited, their isolation may be clinically useful. Toquantify the protection against bronchoconstriction during exercise, wemeasured the bronchoconstrictor response to increasing concentrations of methacholine at rest and during two levels of exercise in subjects with mild asthma. Methacholine waschosen because high inhaled concentrations can be administered without systemic side effects. Methods Subjects Seven atopic subjects with current or seasonal asthma were studied at a time when the asthma was mild and controlled by bronchodilators alone (table I). There was no current exposure to allergens to which they were sensitized (with the exception of house-dust mite), and there had been no exacerbations of Mtbma for at least 4 wk. The baseline FEV I was. > 80.,. predicted normal in all subjects 1414
SUMMARY In many asthmatic subjects, bronchoconstriction develops 2 to 5 min after exercise, reaches a maximum at approximately 10 min, and declines over the next 60 min. However, bronchoclilation is typically observed during and immediately after exercise. We measured the bronchoconstrictor responses to increasing concentrations of inhaled methacholine at rest and during two levels of exercise in seven asthmatic subjects to determine the protection against bronchoconstriction afforded by exercise. On the first day, an incremental Stage 1 exercise test was performed to determine the work capacity (Wcap) of each subject. On the second, third, and fourth days, methacholine was inhaled at rest or during steady-state exercise at one-third or two-thirds of Wcap. The bronchoconstrictor response to methacholine was significantly reduced during exercise (p < 0.0001). The concentration of methacholine required to produce a 20% reduction in FEV1 (PC20) Increased from 2.80 mg/ml (%SEM, 1.62)at rest to 7.29mg/ml (%SEM, 1.43)during exercise at onethird Wcap, and to 31.03mg/ml (%SEM, 1.74)during exercise at two-thirds Wcap (p < 0.001). This study has demonstrated that there is greater than tenfold protection against bronchoconstriction by methacholine during exercise, and the magnitude of the protection depends on the intensity of exercise performed. The mechanism of this protection is not known, but may haveclinical utility. AM REV RESPIR DIS 1990; 141:1414-1417
on all study days. This project was approved by the Ethics Committee of McMaster University Medical Center, and each subject gave written informed consent before taking part in the study.
Study Design Subjects attended the laboratory on 4 study days, separated by at least 1 day. On the first day, an incremental Stage 1 exercise test was performed to determine the work capacity (Wcap) of each subject. On the second, third, and fourth days, a methacholine inhalation test was performed while the subject was sitting on the bicycle at rest or during steadystate exercise at one-third or two-thirds of the subject's Wcap. The methacholine inhalations were started after the subject had been exercising for 5 min. Because of the risks of profound bronchoconstriction caused by a combination of exercise and methacholine, subjects were treated with inhaled 132-agonists immediately after the challenges were completed. These 3 study days were performed in random order. Inhaled bronchodilators were withheld for at least 6 h before each study.
Exercise Tests All testing was performed while the subject was seated on a stationary bicycle ergometer (Ergomed 740; Siemens, Erlangen, FRO). The initial Stage 1 exercise test was performed as described by Jones (7), where the work loads are increased by 100 kpm/min until exhaustion. The subjects breathed room air (tem-
perature, 22 to 23 0 C). The work loads representing one-third and two-thirds Wcap for each subject were determined from the initial Stage 1 exercise test. The duration of the steady-state exercise tests varied between subjects, depending on the number of methacholine inhalations needed to cause a 20070 fall in the FEV I ' For steady-state exercise at onethird Wcap, the test varied between 20 and 40 min (mean, 29 min), whereas at two-thirds Wcap the test varied between 24 and 48 min (mean, 35 min).
Methacholine Inhalation Tests Methacholine inhalation tests were carried out by a method described by Cockcroft and coworkers (8). Aerosols ofthe test solution were generated from a Wright nebulizer with an output of 0.13 ml/min. After the initial control solution of saline, increasing doubling concentrations of methacholine from 0.03 to (Received in original form August l7. 1989 and in revised form December 12, 1989) I From the Department of Medicine, McMaster University, Hamilton, Ontario, Canada. 1 Supported by grants from the Medical Research Council of Canada. J Correspondence and requestsfor reprints should be addressed to Dr. P. M. O'Byrne, Department of Medicine, Rm 3U2, Health Sciences Center, McMaster University, 1200Main St. West, Hamilton, Ontario, Canada L8N 3Z5. 4 Recipient of a Scientist Award from the Medical Research Council of Canada.
1415
AIRWAY RESPONSIVENESS DURING EXERCISE
Subject No. 1 2 3 4 5 6 7
TABLE 1
256
SUBJECT CHARACTERISTICS
128
Age (yr)
Height (cm)
Weight (kg)
FEV 1 (% pred)
Maximal Work Load (watts)
39 16 23 34 18 37 47
178 186 178 178 163 179 165
69 83 70 70 62 73 65
103 91 86 92 96 82 90
333 317 250 267 150 385 150
64 32
Methacholine 16 PC 20 8 lmg/mll
4 2
.5 .25 REST
256.0 mg/ml were inhaled by tidal mouth breathing for 2 min at intervals of 5 min. The bronchoconstrictor response was measured using the FEV 1, which was measured at 30 and 90 s; if the FEV 1 declined during the two measurements, forced expired maneuvers were repeated at 2-min intervals to insure measurement of the lowest value after each inhalation. The test wasstopped when the FEV 1 had fallen by at least 20010, or when the maximal concentration had been inhaled. The results were expressed as the provocation concentration causing a fall in the FEV 1 by 20% (PC 20 ) . Spirometric measurements were made with a Collins 9-L water spirometer (Warren E. Collins Inc., Braintree, MA). The voltage signal was channeled through an A/D converter (Metrabyte, Dash 16) and sampled at 32 Hz by a microcomputer equipped with a data collection and analysis package. Spirometry volume signals were calibrated with a 2-L syringe using two-point calibration. This was linear over the range measured.
Analysis of Bronchomotor Responses To analyze the effects of exerciseon the bronchoconstrictor response to methacholine, two methods were used. (1) Multiple linearregression. Because the baseline PC 20 ranged from 0.25 to 11.73 mg/ml, dummy variables were added to allow for the variability betweensubjects in their responses to methacholine. The FEV 1, expressed as a percentage of control, constituted the dependent variable. The logarithmic transformation of the methacholine concen-
Fig. 1. The effect of exerciseat one-third Wcap and two-thirds Wcap on methacholine airway responsiveness in a single asthmatic subject. Increasing intensity of exercise provides increasing protection against methacholine-induced bronchoconstriction.
.... oII:
... Z
o
(J
'iJI.
.; W II.
tration and the exercise intensity constituted the independent contributors. (2) Analysis of variance. Methacholine PC 20 values werelog-transformed before analysis; therefore, summary statistics are expressed as the geometric mean and percent standard error (%SEM). Comparisons ofthe logarithmic transformation of methacholine PC 20 values between rest and the two levels of exercise were made using a two-way analysis of variance. Having established significance, the Tukey method was used to determine where the differences lay. Statistical significance was accepted as p < 0.05.
Results
Increasing exercise intensity was associated with a highly significant (p < 0.0001) and potent protective effect against the bronchoconstricting effect of methacholine. The bronchoconstrictor response to increasing concentrations of methacholine for a typical subject at rest and during exercise are shown in figure 1. Multiple linear regression yielded the following relationship: OJoFEV l = 86.4 6.2710g[methacholine concentration] + 0.23 OJoWcap (r = 0.716). The coefficient quantifying the bronchoconstrictor effect of methacholine was - 6.26, with a standard error of 0.67, indicating the variable methacholine airway responsiveness between the subjects
EXERCISE
EXERCISE
1/3 Wmax
'/3 Wmax
Fig. 2. Methacholine airway responsiveness,expressed as the methacholine PC,o, at rest and at one-third Wcap and two-thirds Wcap. Increasing intensity of exercise progressively increased the methacholine PC,o in all subjects.
studied; the coefficient describing the protective effect of exercisewas 0.23, with a standard error of only 0.03, indicating a consistent protective effect during exercise between the subjects. The methacholine PCzo increased from 2.80 mg/ml (OJoSEM, 1.62) when measured at rest to 7.29 mg/ml (OJoSEM, 1.43) at one-third Wcap, and to 31.03 mg/ml (OJoSEM, 1.74) at two-thirds Wcap (p < 0.(01) (Figure 2). The PCzo measured during exercise at one-third Wcap was significantly higher than PC zo at rest (p < 0.05), and the PCzo during exerciseat twothirds Wcap was significantly higher than the PCzo at one-third Wcap (p < 0.05). The baseline FEV 1 was within normal limits for all subjects, ranging from 82 to 103010 normal predicted value (table 1). Baseline FEV 1 values did not change significantly over the study period, being 3.77 ± 0.13 L at rest, 3.85 ± 0.13 L prior to exercise at one-third Wcap, and 3.78 ± 0.13 L prior to exercise at twothirds Wcap. Discussion
This study demonstrated that the bronchoconstrictor response to inhaled methacholine in asthmatic subjects is reduced during exercise and that the magnitude of the reduction increases with the intensity of exercise. Although this effect has .". '. ....... been previously recognized (4, 6), the ...•...".'", . magnitude of the effect has not been pre'. , ...• viously established. Furthermore, the \ \••~ '" w... ". w... protective effect was consistent across all the subjects, but the degree of protection ~ was not significantly related to the baseR••t line airway responsiveness, which ranged , , , , ...J.......t~'......._~-~!--_-'-_....L.._.......L from mildly to severely increased. 4 Control 8 18 32 The potency of this protection against METHACHOLINE mg/ml inhaled methacholine is comparable to
I""";~~:~:~:..,.,.
1416
that observed after pretreatment with an inhaled 132-adrenoceptor agonist. Bandouvakis and coworkers (9) demonstrated a 15.9-fold and Ruffin (10)a ninefold mean increase in methacholine PC 20 after pretreatment with inhaled fenoterol 800 ug, This is similar to the 11.1-fold mean increase in methacholine PC 20 demonstrated at two-thirds Wcap. Minute ventilation was not measured because the measurement of ventilation was not essential for the purposes of this study and because the subjects were required to inhale methacholine, making its measurement technically difficult. However, the reduced airway responsiveness is likely related to the increased ventilation and not a function of exercise itself. Stirling and coworkers (4) showed that the bronchoconstriction caused by inhaled histamine in asthmatic subjects is inhibited by both exercise and isocapnic hyperventilation. Also, Freedman and colleagues (6) demonstrated that bronchoconstriction caused by inhaled methacholine can be reversed by exercise and isocapnic hyperventilation. The total dose of methacholine deposited in the airways and the site of deposition when methacholine is inhaled at rest and during exercise were not evaluated during the study. A smaller dose deposited or more peripheral deposition in the airways may explain a reduced responsivenessto methacholine. This is not likely to account for the differencesin methacholine airway responsiveness demonstrated during exercise. This is because the increased inspired volumes during exercise could not reduce the total inhaled dose of aerosol. The reduced inspired time and increased flow rates will cause more central deposition of the aerosol, which should result in a larger fall in FEV 1 (11). Therefore, an increase in methacholine airway responsivenessrather than a reduction might have been expected during exercise. Donna and coworkers (12)have, however, demonstrated that in subjects with mild asthma, similar to those in our current study, enhanced aerosol deposition does not relate to the magnitude of the response to methacholine. Lastly,exercisecan reverse the bronchoconstriction caused by methacholine already deposited in the airways prior to exercise beginning (6). The results of this study may explain why exercise-induced bronchoconstriction does not typically develop until after exercise is complete. The stimuli for bronchoconstriction are at their greatest during exercise (conditioning large re-
INMAN, WATSON, KILLIAN, AND O'BYRNE
spired volumes [3]),but exercise-induced bronchoconstriction usually begins 2 to 5 min after exercise (13). Bronchoconstrictor mediators released as a consequence of these stimuli do not appear to result in bronchoconstriction, presumably because of the protection afforded by exercise. The mechanism of protection is not known. Neural, humoral, mechanical, or a combination of all three, are possible mechanisms. A reduction in vagal tone and increased beta-adrenergic stimulation occur systematically during exercise (14, 15).Beta-adrenergic stimulation and reduction of vagal tone could reduce the bronchoconstrictor response to methacholine and playa role in this protection. However, ifthe protection achieved during exercise is a consequence of hyperventilation rather than exercise per se, as suggested by Stirling and coworkers (4), these mechanisms are lesslikely to be important. The fact that exercise-induced bronchoconstriction occurs within 2 to 5 min after exerciseis completed suggests that any protective mechanism that operates during exercise may be very transient. The protection afforded by 132-stimulation lasts for several hours (15), making it unlikely that the protection occurs through stimulation of airway 132-adrenoceptors by an increase in circulating catecholamines. Deep inspiration has been demonstrated to reduce airway resistance in normal humans (16). The increase in tidal volume that occurs during exercise and isocapnic hyperventilation may also protect against bronchoconstriction. However, deep inspiration has also been shown to cause transient bronchoconstricti on in some asthmatic subjects (17). Therefore, the importance of mechanical factors caused by increases in tidal volume in causing the protection during exercise is not yet known. Inhibitory mediators released during exercise may be responsible for part of this protection. Bronchodilation, which immediately occurs after exercisein asthmatic subjects, is at least in part caused by prostaglandin release (5). In addition, there is a refractory period after exercise in asthmatic subjects that is a consequence of prostaglandin release (18, 19). Also, inhibitory prostaglandins such as prostaglandin E 1 have been shown to reduce methacholine airway responsiveness (20). The role of inhibitory mediators in protecting against bronchoconstriction during exerciseremains to be established. In summary, this study has demon-
strated that a potent mechanism exists that protects asthmatic subjects against bronchoconstriction during exercise. However, the fact that bronchoconstriction occurs within 5 min after exercise is completed in many asthmatic subjects suggests that this effect is transient. References 1. Gilbert lA, Fouke JM, McFadden ER Jr. Heat and water flux in the intrathoracic airways and exercise-induced asthma. J Appl Physiol 1987; 63:1681-91. 2. Anderson SD, Schoeffel RE, Black JL, Daviskas E. Airway cooling as the stimulus to exerciseinduced asthma: are-evaluation. Eur J Respir Dis 1985; 67:20-30. 3. Anderson SD. Exercise-induced asthma. Chest 1985; 87S:191-5. 4. Stirling DR, Cotton DJ, Graham BL, Hodgson WC, Cockcroft DW, Dosman JA. Characteristics of airway tone during exercisein patients with asthma. J Appl Physiol 1983; 54:934-42. 5. Gelb AF, Tashkin DP, Epstein JD, Gong H Jr, Zamel N. Exercise-induced bronchodilation in asthma. Chest 1985; 87:196-201. 6. Freedman S, Lane R, Gillett MK, Guz A. Abolition of methacholine induced bronchoconstriction by the hyperventilation of exercise or volition. Thorax 1988; 43:631-6. 7. Jones NL. Conduct ofthe stage 1test. In: Jones NL, ed. Clinical exercisetesting. Philadelphia: WB Saunders, 1988;135-44. 8. Cockcroft DW, Killian DN, Mellon JJA, Hargreave FE. Bronchial reactivity of inhaled histamine: a method and clinical survey. Clin Allergy 1977; 7:235-43. 9. Bandouvakis J, Cartier A, Roberts R, Ryan G, Hargreave FE. The effect of ipratropium and fenoterol on methacholine- and histamine-induced bronchoconstriction. Br J Dis Chest 1981; 75: 295-305. 10. Ruffin RE. Therapeutic implications of hyperreactivity. In: Hargreave FE, ed. Airway reactivity: mechanisms and clinical relevance. Mississauga: Astra Pharmaceuticals Canada Ltd, 1980; 223-8. 11. Ruffin RE, Dolovich MB, Wolff RK, Newhouse MT. The effects of preferential deposition of histamine in the human airway.Am RevRespir Dis 1978; 117:485-92. 12. Donna E, Danta I, Kim CS, Wanner A. Relationship between deposition of and responsiveness to inhaled methacholine in normal and asymptomatic subjects. J Allergy Clin Immunol 1989; 83: 456-61. 13. Smith EE, Guyton AC, Manning RD, White RJ. Integrated mechanisms of cardiovascular response and control during exercise in the normal human. In: Sonnenblick EH, Lesch M, eds. Exercise and heart disease. New York: Grune and Stratton, 1977;1-23. 14. Chryssanthopoulos C, Barboriak JJ, Fink IN, Stekiel WJ, Maksud MG. Adrenergic responses of asthmatic and normal subjects to submaximal and maximal work levels. J AllergyClin Immunol1978; 61:17-22. 15. Anderson S, Seale JP, Ferris L, Schoeffel R, Lindsay DA. An evaluation of pharmacotherapy for exercise-induced asthma. J Allergy Clin Immunol 1979; 64:612-24. 16. Nadel JA, Tierney DF. Effect of a previous deep inspiration on airway resistancein man. J Appl Physiol 1961; 16:717-9.
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17. Orehek J, Nicoli MM, Delpierre S, Beaupre A. Influence of the previous deep inspiration on the spirometric measurement of provoked bronchoconstriction in asthma. Am Rev Respir Dis 1981; 123:269-72. 18. O'Byrne PM, Jones GL. The effect of indo-
methacin on exercise-induced bronchoconstriction and refractoriness after exercise. Am Rev Respir Dis 1986; 134:69-72. 19. Margolskee OJ, Bigby BG, Boushey HA. Indomethacin blocks airway tolerance to repetitive exercisebut not to eucapnic hyperpnea in asthmatic
subjects. Am Rev Respir Dis 1988; 137:842-6. 20. Manning PJ, Lane eG, O'Byrne PM. The effect of oral prostaglandin E 1 on airway responsivenessin asthmatic subjects. Pulmon Pharmacol1989; 2:121-4.
