American Journal of Respiratory and Critical Care Medicine

We investigated airway responsiveness to mannitol, a new hyperosmolar challenge, in persons hyperresponsive to airway drying. We studied 36 asthmatic subjects, 18 to 40 yr of age, responsive to exercise (n = 23) and eucapnic hyperventilation (n = 28) defined by a 10% fall in FEV1. Fifteen subjects performed both challenges. All subjects performed a challenge with dry powder mannitol, encapsulated and delivered via a Dinkihaler until a 15% decrease in FEV1 was documented or a cumulative dose of 635 mg was delivered. All subjects responsive to eucapnic hyperventilation and all but one subject responsive to exercise were responsive to mannitol. Sixty-nine percent of subjects had a positive response to mannitol after less than 155 mg (6 capsules) and 94% less than 320 mg (10 capsules). The provoking dose of mannitol required to cause a 15% fall in FEV1 (PD15) was related to the severity of the response to exercise (Pearson's correlation coefficient [rp] = 0.68, p < 0.01) and eucapnic hyperventilation (rp = 0.68, p  < 0.01) in subjects who were not taking inhaled corticosteroids. The mean ( ± SD) maximum percent fall in FEV1 after mannitol was 24.4 ± 6.2% and recovery to bronchodilator occurred within 10 min in most subjects. The mannitol test is simple, inexpensive, faster to perform than hyperpnea with dry air and could become an office-based test. Further studies are now required to determine the sensitivity of mannitol to identify exercise-induced asthma in a random population.

Asthma is a chronic inflammatory disease of the airways characterized by airway hyperresponsiveness. Most individuals with clinically diagnosed asthma experience symptoms when they exercise. Exercise as a bronchial provocation test (BPT) was initially developed for children (1). At the time, the intensity and duration of exercise were recognized as the most important determinants of the severity of exercise-induced asthma (EIA). Later studies demonstrated that the rate of water lost from the airways in conditioning the inspired air was the essential determinant, and exercise per se was not necessary to induce the airway response (2, 3). Thus, the water content of the air inspired and ventilation rate were identified as the factors to be controlled to provoke EIA. This led to development of eucapnic hyperventilation with dry air as a surrogate BPT to identify EIA (4). Eucapnic hyperventilation was standardized by members of the U.S. Army and used to assess recruits for EIA, making testing fast, simple, and less expensive to perform than exercise (5-7).

The stimulus whereby hyperpnea with dry air causes the airways to narrow is the loss of water from the airways in conditioning the inspired air (3, 8, 9). We have previously proposed that the osmolarity of the airway surface liquid (ASL) increases as water is lost by evaporation (3, 9-11). This increase in osmolarity then acts as a signal for water to move from the epithelium toward the airway surface to reestablish the volume and osmolarity of ASL (10, 12). A similar series of events would be expected to occur in response to inhaling a hyperosmolar aerosol.

We have recently developed a new BPT based on inhalation of a hyperosmolar aerosol. We used a dry powder of mannitol suitably prepared for inhalation (13), which is delivered in increasing doses from a dry powder inhaler. Positive responses that are reproducible are obtained in persons with currently active asthma. Inhaled mannitol has the potential to be a fast and simple office-based BPT with the advantage of not requiring expensive equipment and dry air for inhalation.

We wanted to know if mannitol identified persons who responded to exercise and hyperventilation. We hypothesized that the airway effects caused by dehydration during hyperpnea would be similar to the airway effects of inhaling a hyperosmolar aerosol. In that case subjects responding to hyperpnea should also respond to inhalation of a hyperosmolar aerosol. To test this hypothesis, we compared the airway response to inhaled mannitol in clinically recognized young asthmatics who had bronchoconstriction induced by breathing dry air during exercise or hyperventilation.


An advertisement circulated in the local community was used to recruit asthmatic subjects who had exercise-induced asthma. Subjects were interviewed by telephone and were asked to attend the laboratory if they were nonsmokers and had no chest infection 6 wk prior to the study. Subjects were asked to refrain from taking short-acting bronchodilators for 8 h, long-acting bronchodilators for 48 h, and nedocromil sodium or sodium cromoglycate for 24 h prior to the study. No inhaled corticosteroids were taken on the day of the study and no antihistamines for 3 d prior to the study day.

On the first day of the study, the clinical diagnosis of asthma was confirmed by a staff respiratory physician by examination and history. Subjects performed spirometry and were required to have a baseline forced expiratory volume in one second (FEV1) of greater than 60% of predicted FEV1 (14). Subjects performed a challenge with exercise or eucapnic hyperventilation and were entered into the study if they had a positive response defined as a 10% or greater fall in FEV1 from the prechallenge value (6). Some subjects who exercised also had a challenge with eucapnic hyperventilation.

Thirty-nine asthmatic subjects were recruited and 36 (20 females and 16 males), ranging in age from 18 to 40 yr, were entered into the study (Table 1). All subjects had been prescribed medication for their asthma. Sixteen were taking inhaled corticosteroids on a daily basis and 35 were taking β2-adrenoceptor agonists (Table 1). All subjects had at least one positive skin prick test (2-mm wheal or more) in response to a common aeroallergen (dust, mite, grass, fungi, animal dander).


Subject No.Age (yr)GenderHeight (cm)Weight (kg)Baseline FEV1% Pred Mannitol ChallengeMedicationSteroids (μg/d )Mannitol PD15(mg)Exercise % Fall in FEV1 ISH PVE10 (L/min)
 137F165  66  101.9S, BEC 1,000   772546
 218M181  70   66.0S, BEC 1,000  1492353
 321F168  61   94.0S, TS  1015947
 422F166  60   66.0S, BUD800   494850
 523M172  75   71.2S  2203627
 620M169  58   77.4S, BEC, SCG 1,000   504643
 728F175  62   73.9TS   436436
 824M179  74   84.7Nil  5091071
 920F167  65   75.8S  1931737
1037F167  60   87.9S  1001357
1119M180  75   83.0S, SCG   756544
1220M176  70   76.7TS   146517
1333F168  66   76.9S   385030
1440F173  62  109.8S, BUD 1,600  2914169
1520F165  60   99.5TS   834346
1622M181  82  120.3S, BUD 1,60010
1733M180  84   76.2TS, BUD800   9213
1826M196  81   92.0S   6665
1922M170  72   68.6S   6948
2033M166  59   71.8S, BEC400   2129
2132F172  59   69.8S, FLU 1,500   2742
2219F171  67   86.2S, BEC400  13053
2319F163  66   86.2S  19754
2419M178 101   98.6S  43881
2520M193 105  103.7S  31494
2619F172  76   82.0S, BECprn   2852
2718M175  55   72.0S   8245
2819F150  57   93.0S, NS  10616
2926F173  70  102.4S, BUD400   5279
3019F160  72   78.6TS, BUD800   8526
3119F165  64   79.0S, BEC400   8062
3218F165  59   73.8S, BUD400   2419
3329M183  84   73.8S, BEC200   7048
3427M168 112   69.6S   8852
3530F173  54   75.2S   4944
3631F168  70   81.9S   3538

Definition of abbreviations: S = salbutamol; BUD = budesonide; BEC = beclomethasone; FLU = fluticasone; ISH = isocapnic hyperventilation; SCG = sodium cromoglycate; NS = nedocromil sodium; nil = no medication; prn = on an “as needed” basis; TS = terbutaline sulfate.

The study was approved by the Central Sydney Area Health Service Ethics Committee (X93-0061), and all subjects signed a consent form prior to commencement of the study. The study was performed under the Clinical Trials Notification scheme No. 97-185 of the Therapeutic Goods Administration of Australia.

Study Design

Subjects were asked to attend the laboratory on up to three occasions with exercise or eucapnic hyperventilation being performed first, followed by challenge with inhaled mannitol. Three subjects known to have exercise-induced asthma had the mannitol challenge performed first. In 15 subjects, all three challenges were performed and the time taken to do this documented. For each subject, there was a minimum of 2 d between each challenge, and all challenges were completed in each subject within a 35-d period (6 to 34 d).

Lung Function Measurements

Spirometry was performed on an Autospiro AS-300 spirometer (Minato Medical Science Co., Osaka, Japan) for mannitol and eucapnic hyperventilation challenges. For exercise an Autospiro AS-800 (Minato Medical Science Co.) was used. Both spirometers were calibrated on the morning of each study day and were regularly checked for concordance between measurements. FEV1 was used as an index of change in airway caliber. Subjects were required to have a less than 20% variation in baseline FEV1 throughout the study.

Exercise Challenge

The protocol used here has been described in detail previously (15). Before challenge with exercise each subject had an electrocardiogram and blood pressure measurements. Workload consisted of increasing the resistance while peddling on an electronically braked bicycle ergometer (Elema-Shonander, Solna, Sweden). A target workload in watts was estimated from the oxygen consumption required to achieve a minute ventilation of between 40 to 60% of their predicted maximal voluntary ventilation (MVV) (16). The MVV was calculated as 35 × predicted FEV1. Spirometry was performed and the prechallenge FEV1 was documented as the best FEV1 of at least two measurements. Subjects were fitted with a heart rate monitor (Polar Sports Tester; Polar Electro, Oulu, Finland) and exercise was performed for 8 min. Subjects breathed from a gas cylinder of compressed dry medical air via a demand valve (O-two Systems International, Mississauga, ON, Canada), capable of delivering air up to 110 L/min. This was connected to a mouthpiece with a two-way valve (Hans Ruldoph 2700; Hans Ruldoph Inc., Kansas City, MO) and the subjects' mouths breathed while wearing a nose clip. The expired air passed into a gasometer (350L Tissot; W.E. Collins, Braintree, MA) and the ventilation was recorded throughout exercise (Watanabe Miniwriter WTR 771A; Watanabe Instrument Corporation, Tokyo, Japan).

The workload was increased from 60%, 75%, and 90% of the target workload over the first 3 min of exercise, respectively. The final 5 min was set at 100% of the target workload. Subjects were encouraged to achieve a cycle rate of 53 to 64 rpm at these workloads. After exercise, at least two measurements of FEV1 were performed at 3, 5, 7, 10, 15, and 30 min after exercise and the lowest FEV1 recorded over this period was used to calculate the maximal percentage fall from baseline by the following equation:

%Fall in FEV1=(Prechallenge FEV1Lowest FEV1 postexercise)(Prechallenge FEV1)×100

On completion subjects who had a less than 50% fall in FEV1 received bronchodilator in the form of 0.5 mg terbutaline sulfate (Bricanyl; Astra Pharmaceutical, Lund, Sweden) or 200 μg salbutamol (Ventolin; Glaxo Wellcome, Greenford, UK) from a pressurized metered-dose inhaler. Those with a greater than 50% fall were given 0.5 mg ipratropium bromide (Atrovent; Boehringer Ingelheim, Ingelheim, Germany) and 5 mg terbutaline sulfate by nebulization with oxygen. The FEV1 was required to recover to within 5% of the prechallenge value before subjects could leave the laboratory.

Isocapnic Hyperventilation Challenge

The protocol used in this study has been described in detail by Rodwell and colleagues (17). In brief, on arrival at the laboratory, each subject had a baseline measurement of FEV1 and this was repeated 10 min later to confirm that it was stable. The best FEV1 at 10 min was recorded as the prechallenge FEV1. This value was used to calculate MVV, taken as 35 × prechallenge FEV1.

The apparatus used for delivering the dry gas was developed by Smith and Anderson (18), and was a modification of the system described originally by Phillips and colleagues (4). The subjects inhaled dry compressed air containing approximately 4.9% CO2, 21% O2, and the balance N2. The gas mixture passed through a rotameter (2000 Rotameter; G.E.C. Elliot, Croyden, UK) via a demand valve (Demand resuscitator; CIG, Medisheild, Australia) to a target balloon (Meteorological, 100 g; Kaysam Corp., Paterson, NJ). The demand valve could be set to deliver gas at 30 to 150 L/min and the rotameter permitted the monitoring of the rate up to 110 L/min. Subjects breathed the gas from the target balloon through a mouthpiece attached to a two-way valve (Hans Ruldoph 2700; Hans Ruldoph Inc.). The expired gas was passed through a gasometer (Model 602; Email Westinghouse Pty. Ltd., NSW, Sydney, Australia) in order to measure the volume of air expired so that minute ventilation could be calculated. The subjects were instructed to keep the meteorological balloon filled to a constant volume. The challenge commenced breathing at 30% MVV for 3 min. At least two FEV1 values were obtained at 1, 3, 5, 7, and 10 min after completion. If a 15% decrease in FEV1 was obtained, the challenge ceased. If no fall or a plateau less than 15% was seen after the measurement at 5 min, then the subjects continued the challenge ventilating at 60% MVV for 3 min. If a 15% fall was achieved, the challenge ceased; if not, a final period ventilating at MVV for 3 min was performed.

On completion of the challenge, subjects received 0.5 mg terbutaline sulfate actuated into and inhaled from a Nebuhaler (Astra Pharmaceutical, Lund, Sweden). Spirometry was measured at 5, 10, 20, and 30 min after the challenge or until the subject's FEV1 had returned to within 5% of the baseline FEV1 value. The provoking rate of ventilation to cause a 10% fall in FEV1 (PVE10) was calculated by linear interpolation from ventilation response curves using the lowest FEV1 after each provocation step. The provoking ventilation to induce a 10% fall in FEV1 was calculated as a percentage of the predicted MVV, to take into account subject size.

Mannitol Capsule Challenge

The preparation of the dry powder mannitol has been described in detail previously (13). In brief, mannitol (Mannitol BP; Rhône Poulenc Chemicals Pty. Ltd., Brookvale, NSW, Australia) was prepared by spray drying (Buchii 190 Mini Spray Drier, Flawil, Switzerland) a solution of 50 mg/ml. The particle size of the mannitol powder was measured with a multistage liquid impinger (Astra Pharmaceutical) and assayed by vapor pressure osmometry. The mannitol powder used had 46% of the particles under 7 μm in diameter measured at 90 L/min. The capsules were hand-filled with 5, 10, 20 (± 0.2), and 40 (± 0.5) mg of mannitol powder on an analytical balance (Model BA110S; Sartorius, Gottingen, Germany) as required under controlled conditions (temperature ⩽ 20° C). The capsules were stored in a container with silica gel and kept in a cool and dry environment. A Dinkihaler (Rhône Polenc Rorer, Collegeville, PA) was used for the delivery of the mannitol.

On arrival at the laboratory, each subject had a baseline measurement of FEV1 and this was repeated 10 min later to confirm that it was stable. Before the challenge commenced, subjects practiced inhaling through the Dinkihaler several times so that they could achieve a flow rate of between 90 to 120 L/min. Subjects then performed the challenge with doses consisting of 0 (empty capsule acting as a placebo), 5, 10, 20, 40, 80, 160, 160, and 160 mg of mannitol via the Dinkihaler. The 80 mg and 160 mg were given in multiple doses of 40 mg capsules. At least two FEV1 maneuvers were performed 60 s after each dose and the highest FEV1 was used in the calculation. The FEV1 value measured after the 0 mg capsule was taken as the prechallenge FEV1 and used to calculate the percentage decrease in FEV1 in response to the mannitol challenge. If the subject had a greater than 10% fall in FEV1 in response to a single dose, the same dose was repeated for reasons of safety. The challenge ceased when a 15% fall in FEV1 was documented or a cumulative dose of 635 mg had been administered. The provoking dose of mannitol to cause a 15% fall in FEV1 (PD15) was calculated by linear interpolation of the relationship between the percent fall in FEV1 and the cumulative dose to mannitol required to provoke this.

On completion of the challenge, subjects received 0.5 mg terbutaline sulfate actuated and inhaled from a Nebuhaler. Spirometry was measured at 5, 10, 20, and 30 min after the challenge or until the subject's FEV1 had returned to within 5% of the baseline FEV1 value.

Statistical Analysis

The geometric mean (Gmean) ± 95% confidence interval (CI) were calculated using the log values for PD15 (mg) which were normally distributed. The Pearson's correlation (rp) and significance values were used to investigate the relationship between log PD15 with percent fall to exercise and with PVE10 (19). Spearman rank (rs) correlation was also used to investigate the relationship between the severity of the airway responses to the challenges in the same subjects (19). The relationships were calculated on the total group and on the groups taking inhaled corticosteroids and those not taking inhaled corticosteroids.

Values for FEV1 are expressed as mean ± SD of the percentage of predicted FEV1. The baseline values for FEV1 and the FEV1 following the 0 mg capsule (prechallenge FEV1) were expressed as a percentage of predicted and compared using Student's paired t test (19). The baseline FEV1 and the time to complete the challenge for the 15 subjects who performed all three challenges were compared using analysis of variance (ANOVA) with repeated measures. Student's paired t test was also used to compare the values for FEV1 expressed as percentage from baseline FEV1 during bronchodilator recovery to inhaled mannitol and eucapnic hyperventilation. For this analysis FEV1 values that exceeded the baseline FEV1 were considered as if the FEV1 had returned to the baseline value.

All subjects with a positive response to hyperventilation with dry air and all but one subject with a positive response to exercise were identified using inhaled mannitol. A wide range of severity of responses was documented to all three challenge tests and the individual responses are given in Table 1. The majority of subjects with EIA were identified with the inhalation of a cumulative dose of 155 mg (range, 25 to 635 mg) or 6 (range, 3 to 18) capsules of mannitol and all but two with 10 capsules (320 mg) or less (Figures 1 and 2). The subject with the mildest response to exercise (number 16) had a 10% fall in FEV1, but did not respond to either the mannitol or the eucapnic hyperventilation challenge. In the subjects who exercised, the percent fall in FEV1 after exercise was 40 ± 19% (mean ± SD) compared with a Gmean of 85 mg (CI: 57 to 125 mg). For the subjects who performed hyperventilation, the mean PVE10 was 47.4 L/min (CI: 40 to 55 L/min) compared with a Gmean of 86 mg (CI: 61 to 120 mg). The PVE10 was calculated to be 41 ± 13% of the predicted MVV.

The relationships between the challenges are given in Table 2 for the whole group and for those taking and not taking inhaled corticosteroids. The relationships between the responses to the challenges were closer in subjects not receiving treatment with inhaled corticosteroids than those treated with inhaled corticosteroids (Figures 3 and 4).


Total GroupNo SteroidsSteroidsTotal GroupNo SteroidsSteroids
Number of subjects 22 13 9 28  1711
Pearson's correlation, rp, PD15 (mg)  −0.45*   −0.68  0.11   0.57    0.68  0.40
Spearman's correlation, rs, PD15 (mg)  −0.45  −0.65* −0.20   0.46*    0.52*  0.38

*p < 0.05.

p < 0.01.

There was a small but significant difference between the baseline FEV1 on the mannitol day 83.3 ± 14.3% (mean ± SD) compared with the prechallenge FEV1 on the exercise day 80.3 ± 14.9% (p = 0.03). The same was seen for baseline FEV1 for mannitol day 83.2 ± 12.4% compared with hyperventilation 81.2 ± 13.5% (p = 0.04) days. There was no significant difference between the baseline FEV1 in the 15 subjects who performed all three challenges (p = 0.32). There was, however, a small but significant decrease in FEV1 after the placebo (empty capsule), with the FEV1 mean (± SD) values at baseline 83.3 ± 13.3% predicted and prechallenge 81.1 ± 14.0% predicted (p < 0.001).

The mean maximum percent fall in FEV1 was 24.4 ± 5.1% after mannitol and 23.9 ± 9.3% after hyperventilation (p = NS). With exercise, the mean maximum percent fall after exercise was 40.0 ± 19.0% (range, 10.0 to 65.4%) and this was significantly different from the maximum percent fall achieved with mannitol 25.9 ± 7.1% (range 6.5 to 39.2%) (p < 0.001).

The subjects who exercised achieved a mean percentage of predicted MVV of 68% (range, 38 to 90%) and this represented a ventilation of 53 to 121 L/min. To achieve this, the mean workload was 197 watts (range, 110 to 340 W) with the mean maximum heart rate of 174 (range, 154 to 208) beats/min. Most subjects could achieve their target workload but not maintain it. Even though workload was often reduced, the required minute ventilation could be and was sustained for 4 min in most subjects.

Recovery to baseline FEV1 following administration of bronchodilator was documented in 23 of the 28 subjects who performed mannitol and hyperventilation and was found to be similar between the two challenges (Figure 5).

The mannitol challenge was completed in a shorter time than both exercise and eucapnic hyperventilation challenges. The time of challenge was compared in the 15 subjects who performed all three challenges. The median time to complete a mannitol challenge was 12 min (range, 8 to 20 min) compared with exercise 40 min (range, 13 to 40 min) and hyperventilation 17 min (range, 10 to 28 min) and these were all significantly different from one another (p < 0.001).

The results of this study clearly show that persons responsive to inhalation of dry air during exercise or hyperventilation are also responsive to inhaled mannitol. Although a positive response is considered with a provoking dose of mannitol to cause a 15% fall in FEV1 (PD15) less than 635 mg, the majority of subjects required less than 155 mg. For most of the subjects, this required the delivery of ⩽ 6 capsules of mannitol over 12 min.

The relationships between the airway response to mannitol and those to exercise and hyperventilation were significant, though not strong. However, the relationships were much closer in the steroid-naive subjects. This is important as they form the clinically relevant group of subjects for a diagnostic test such as mannitol. Inhaled corticosteroids have been shown to reduce responsiveness to mannitol (20), to exercise (21), and to hyperventilation (22), however the rate of reduction in responsiveness to different stimuli may be different (23). In subjects being treated with inhaled corticosteroids such challenges are used to assess response to treatment and a positive response to these challenges would require the reassessment of treatment, preferably by increasing the dose. One subject on a high dose of inhaled corticosteroids achieved the mildest response to exercise of the group (10% fall in FEV1) and was not responsive to mannitol or hyperventilation. Responses such as this may also warrant the reassessment of treatment, by decreasing the dose of inhaled corticosteroids.

Many of the subjects who performed exercise had large decreases in lung function, with eight subjects having a greater than 50% fall in FEV1. Such large percentage falls in FEV1 were not documented with the mannitol or hyperventilation. One advantage of a progressive dose–response challenge is that the test can be stopped before pronounced falls in lung function, which is an important safety feature.

With hyperosmolar challenge the maximal reduction in FEV1 is observed in 1 to 2 min (24), whereas the maximal response can often be delayed until up to 5 to 10 min after cessation of hyperventilation (6) and up to 20 min after cessation of exercise (25). This rapid development of the airway response to hyperosmolar agents is one reason why challenges such as mannitol can be performed in a shorter time than hyperventilation and exercise.

In this study we used a 15% fall in FEV1 as the abnormal response to inhaled mannitol. We have previously reported responses to inhaled mannitol in healthy subjects and found the mean percent fall after a cumulative dose of 635 mg was 3.5 ± 2.0% (13), thus the mean plus 3 standard deviations (i.e., 99% of the normal population) would have a maximal fall less than 10%. We chose 15% fall for the abnormal response as only small numbers of healthy persons have been studied so far. If the same responses are obtained in larger numbers of healthy people, then the abnormal response to mannitol could be considered as a 10% decrease in FEV1. Abnormal responses to exercise are usually considered as 10% (16) or 15% (26) and 10% to hyperventilation (6).

Bronchoconstriction to mannitol can be reversed rapidly with a standard dose of bronchodilator (13). Recovery to baseline FEV1 after mannitol occurs usually within 5 to 10 min and it was similar to the recovery after hyperventilation in the same subjects. Recovery after bronchodilator was not monitored after exercise as there was a greater variation in maximal percent falls in FEV1, making it difficult to compare the recovery to mannitol or hyperventilation, where the responses were milder.

Mannitol was generally well tolerated. However, some subjects experienced cough immediately after the 20 mg and/or 40 mg doses. When cough occurred, it was immediately after inhalation and could have been due to the nonrespirable size fraction depositing in the throat. The Dinkihaler is a simple, low-resistance dry powder inhaler that makes inhalation easy, permitting the subjects to inhale at high flow rates. The high inspiratory flow rate and low resistance may have increased the rate of impaction on the throat. Cough also occurred in some subjects well after inhalation, at a time when deposition in the airways would have occurred. In the future this may be minimized by changing the preparation of dry powder inhalers and powders, permitting minimized deposition of powder in the oropharynx and upper airways.

We have previously found a weak relationship between responsiveness to inhaled mannitol and methacholine although all those responsive to inhaled mannitol were responsive to inhaled methacholine (r = 0.5, p < 0.01) (13). We did not investigate the responsiveness to inhaled methacholine or histamine in this study. However we have previously reported 18 of 38 subjects positive to exercise, being negative to histamine (26) so that the presence of exercise-induced asthma was not predicted by responses to pharmacological agents. Others have reported similar findings (27, 28) and these studies highlight the need for tests other than methacholine and histamine to identify exercise-induced asthma.

The results of this study are consistent with the concept that exercise and hyperventilation may provoke the airways to narrow by dehydration and increasing airway osmolarity. Inhaled mannitol has the same, if not greater potential, to increase airway osmolarity because it is not easily absorbed across the epithelium (29). A unifying concept to explain the mechanism whereby hyperpnea with dry air and the inhalation of mannitol cause the airways to narrow is the activation of mechanisms involved in cell volume regulation, and regulatory volume increase in particular. Thus, the intracellular events that occur in order for a cell to regain its volume after shrinkage, will be the same whether the environment is made hyperosmolar by loss of water or by adding a hyperosmolar stimulus. Further, these intracellular events, that include an increase in intracellular calcium, could lead to the release of a wide variety of intracellular mediators from different cells in the airways (12, 30). In vitro studies using mannitol as a stimulus to induce release of mediators from mast cells support this concept (31). Further, work in animals suggests that sensory nerves and release of neuropeptides may be important in response to dry air and hyperosmolar stimuli (32).

Studies investigating the inhibitory effect of specific antagonists suggest that histamine, leukotrienes, and prostaglandins are involved in the airway narrowing provoked by exercise- and hyperventilation-induced asthma (33, 34). Similar findings have been reported when hyperosmolar saline has been used to narrow the airways (35). Drugs without bronchodilator properties, such as nedocromil sodium and sodium cromoglycate, can markedly inhibit exercise, hyperventilation, and hyperosmolar stimuli (12). If hyperosmolar stimuli release a variety of mediators this could explain why positive responses to exercise and hyperventilation can occur in subjects with negative responses to inhaled methacholine and histamine (26-28).

The precise relationship between asthma and responsiveness to hyperosmolar stimuli is not completely understood. However, it is known that persons with active asthma responding to inhaled corticosteroids are also hyperresponsive to hyperosmolar stimuli and these responses can decrease to within the normal range with treatment with steroids (23). Thus, responsiveness to hyperosmolar stimuli may provide an indirect index of airway inflammation.

The mannitol challenge overcomes many of the practical disadvantages of challenge with exercise and hyperventilation, particularly the need for a source of dry air. The equipment requires a dry powder inhaler and capsules containing different doses of mannitol. It is fast, simple, and inexpensive to perform. The responses are repeatable and recovery to baseline lung function after bronchodilator is usually less than 10 min. These practical advantages make the inhalation of mannitol potentially attractive for development as an office-based test or for use in the field. Further studies are now required to define the sensitivity and specificity of mannitol to identify exercise-induced asthma in a random population.

The Australian patent no. 682756 for the use of mannitol described here is held by the Central Sydney Area Health Service (NSW Australia) and international patents have been registered (PCT/AU95/ 00086).

Supported by the National Health and Medical Research Council of Australia (J.D.B.) and the Finnish Academy (H.K.).

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Correspondence and requests for reprints should be addressed to Sandra D. Anderson, Ph.D., D.Sc., Principal Hospital Scientist, PCP9, Royal Prince Alfred Hospital, Missenden Road, Camperdown, NSW 2050, Australia. E-mail:


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