Reply

Abstract

To the Editor:We are pleased to respond to the comments raised by Drs Townley, Hopp, and Rosenthal.1Townley R.G. Hopp R.J. Rosenthal R.R. The correct methacholine challenge delivery method?.J Allergy Clin Immunol. 2007; 119: 1026-1027Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar We initially would like to clarify some apparent misconceptions. First, we have not written 5 articles on this topic; we have 3 original research publications.2Todd D.C. Davis B.E. Hurst T.S. Cockcroft D.W. Dosimeter methacholine challenge: comparison of maximal versus submaximal inhalations.J Allergy Clin Immunol. 2004; 114: 517-519Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar, 3Cockcroft D.W. Davis B.E. Todd D.C. Smycniuk A.J. Methacholine challenge: comparison of two methods.Chest. 2005; 127: 839-844Crossref PubMed Scopus (66) Google Scholar, 4Allen N.D. Davis B.E. Hurst T.S. Cockcroft D.W. Difference between dosimeter and tidal breathing methacholine challenge: contributions of dose and deep inspiration bronchoprotection.Chest. 2005; 128: 4018-4023Crossref PubMed Scopus (54) Google Scholar The fourth article, the Rostrum article that inspired this correspondence, contained no new data but was basically an opinion paper that included review of data from the first 3 papers.5Cockcroft D.W. Davis B.E. The bronchoprotective effect of inhaling methacholine by using total lung capacity inspirations has a marked influence on the interpretation of the test result.J Allergy Clin Immunol. 2006; 17: 1244-1248Abstract Full Text Full Text PDF Scopus (65) Google Scholar The fifth article, by Wubbel et al,6Wubbel C. Asmus M.J. Stevens G. Chesrown S.E. Hendeles L. Methacholine challenge testing: comparison of the two American Thoracic Society-recommended methods.Chest. 2004; 125: 453-458Crossref PubMed Scopus (27) Google Scholar comes from another group. Second, we were very careful not to argue for or against any method of aerosol generation. Our concerns relate exclusively to the method of inhalation (to total lung capacity) and not the method of nebulization. Although this (total lung capacity breaths) is the inhalation method recommended for the dosimeter technique,7Chai H. Farr R.S. Froehlich L.A. Mathison D.A. McLean J.A. Rosenthal R.R. et al.Standardization of bronchial inhalation challenge procedures.J Allergy Clin Immunol. 1975; 56: 323-327Abstract Full Text PDF PubMed Scopus (1129) Google Scholar, 8Crapo R.O. Casaburi R. Coates A.L. Enright P.L. Hankinson J.L. Irvin C.G. et al.Guidelines for methacholine and exercise challenge testing-1999.Am J Respir Crit Care Med. 2000; 161: 309-329Crossref PubMed Scopus (2180) Google Scholar the dosimeter can be successfully used with submaximal, approximately half inspiratory capacity breaths.2Todd D.C. Davis B.E. Hurst T.S. Cockcroft D.W. Dosimeter methacholine challenge: comparison of maximal versus submaximal inhalations.J Allergy Clin Immunol. 2004; 114: 517-519Abstract Full Text Full Text PDF PubMed Scopus (32) Google ScholarTownley et al1Townley R.G. Hopp R.J. Rosenthal R.R. The correct methacholine challenge delivery method?.J Allergy Clin Immunol. 2007; 119: 1026-1027Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar raise several important points regarding differences between the 2 methods. However, none of these (with 1 possible exception) are relevant to our conclusions. We will nevertheless address these on a point-by-point basis.There is no doubt that the dosimeter method of aerosol generation, as noted by points raised in the correspondence, does (or at least should) produce an identical volume and therefore dose of agent with each measured breath. Subjects inhaling aerosol by the 2-minute tidal breathing technique may well have variations in both number and depth of breaths; however, the duty cycle (ie, the fraction of the total time spent during inspiration) is consistent in healthy subjects at about 0.35.4Allen N.D. Davis B.E. Hurst T.S. Cockcroft D.W. Difference between dosimeter and tidal breathing methacholine challenge: contributions of dose and deep inspiration bronchoprotection.Chest. 2005; 128: 4018-4023Crossref PubMed Scopus (54) Google Scholar, 9Stromberg N.O. Gustafsson P.M. Breathing pattern variability during bronchial histamine and methacholine challenges in asthmatics.Respir Med. 1996; 90: 287-296Abstract Full Text PDF PubMed Scopus (8) Google Scholar Thus, over a period of 2 minutes, the exposure to the aerosol is consistent and reproducible, as is the response to methacholine.10Cockcroft D.W. Measurement of airway responsiveness to inhaled histamine or methacholine: method of continuous aerosol generation and tidal breathing inhalation.in: Hargreave F.E. Woolcock A.J. Airway responsiveness: measurement and interpretation. Astra Pharmaceuticals Canada Ltd, Mississauga1985: 22-28Google ScholarWe agree that in subjects with greater airway hyperresponsiveness, there is less (indeed no) bronchoprotective effect of maximal inhalations. That is, in fact, what has been demonstrated by our 3 studies.2Todd D.C. Davis B.E. Hurst T.S. Cockcroft D.W. Dosimeter methacholine challenge: comparison of maximal versus submaximal inhalations.J Allergy Clin Immunol. 2004; 114: 517-519Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar, 3Cockcroft D.W. Davis B.E. Todd D.C. Smycniuk A.J. Methacholine challenge: comparison of two methods.Chest. 2005; 127: 839-844Crossref PubMed Scopus (66) Google Scholar, 4Allen N.D. Davis B.E. Hurst T.S. Cockcroft D.W. Difference between dosimeter and tidal breathing methacholine challenge: contributions of dose and deep inspiration bronchoprotection.Chest. 2005; 128: 4018-4023Crossref PubMed Scopus (54) Google ScholarOur data cannot be explained by a dose effect. We have redrawn our figure (Fig 1) with the results from the 2 methods expressed as a PD20, and the marked difference with deep inhalation bronchoprotection persists. There are 7 subjects whose PD20 exceeds 1.44 mg, which equates to the tidal breathing cutoff of a PC20 = 16 mg/mL.8Crapo R.O. Casaburi R. Coates A.L. Enright P.L. Hankinson J.L. Irvin C.G. et al.Guidelines for methacholine and exercise challenge testing-1999.Am J Respir Crit Care Med. 2000; 161: 309-329Crossref PubMed Scopus (2180) Google Scholar In these 7 subjects, despite having normalized the graph for dose, the deep inhalation dosimeter technique produced a PD20 on average almost 10-fold larger than the tidal breathing PD20. If one uses the dosimeter upper limit of normal, which we presume to be a 20% FEV1 fall at 25 mg/mL, this equates to a noncumulative PD20 of 1.13 mg. In this case, there are then 10 subjects with a dosimeter PD20 in the normal range averaging 8-fold larger than their tidal breathing PD20 despite the dose correction. With this cutpoint, we also observed 1 subject with a normal tidal breathing PD20. Further analysis of the data in Fig 1 confirms the point made in the previous paragraph. Although there is no longer any significant difference between the geometric mean PD20 values by the 2 methods, there is a difference when the data are dichotomized at the midpoint of the data, a tidal breathing PD20 of 0.2 mg. In the 28 more hyperresponsive subjects (PD20 < 0.2 mg), the dosimeter PD20 is significantly lower than the tidal breathing PD20 (P = .019). By contrast, in the 27 less hyperresponsive subjects (PD20 > 0.2 mg), the dosimeter PD20 is significantly higher than the tidal breathing PD20 (P = .039). These results not only are consistent with lack of a bronchoprotective effect in the hyperresponsive half of our population but also support the probability of a more efficient aerosol deposition and/or retention with the dosimeter technique. The difference, in the less hyperresponsive half of our population, relates primarily to the bronchoprotective effect (despite the same more efficient deposition and retention of aerosol) seen in the 10 subjects noted.The point about timing of the spirometric maneuvers is valid and, in fact, is the only observation that may impinge on our findings. It is recognized that a delay in timing of the FEV1 will enhance the apparent bronchoconstriction produced by methacholine.11Simard B. Turcotte H. Cockcroft D.W. Davis B.E. Boulay M.E. Boulet L.P. Deep inspiration avoidance and methacholine response in normal subjects and patients with asthma.Chest. 2005; 127: 135-142Crossref PubMed Scopus (17) Google Scholar Whether delaying the spirogram until 3 minutes after completion of the inhalation would have allowed reversal of the deep breathing effect remains to be tested.The question of the deep inhalation required before performing an FEV1 maneuver has been raised many times in the past. If one chooses to evaluate the effect of a bronchoconstrictor by looking at the FEV1, one is always faced with this conundrum. It is possible that the deep inhalation required to measure the FEV1 may, in fact, produce a bronchodilator effect. This is at least constant between methods and cannot explain any of our differences. It is, however, one of the reasons why we have always preferred to use the lowest (technically acceptable) FEV1 after methacholine compared with the lowest postsaline FEV1 in the calculation of the percent change in FEV1.The final point relates to the issue of being able to calculate a PD20 as opposed to a PC20. Although the dosimeter technique does allow one theoretically to calculate a PD20, that is, the amount of methacholine that has been nebulized, we suspect that even this will be inaccurate. Nebulizer output is traditionally determined by weighing the nebulizer before and after a period of use. In fact, this is the only easy way to calibrate nebulizers. Unfortunately, a significant proportion of the weight loss that occurs during nebulization is actually evaporation.12Cockcroft D.W. Hurst T.S. Gore B.P. Importance of evaporative water losses during standardized nebulized inhalation provocation tests.Chest. 1989; 96: 505-508Crossref PubMed Scopus (46) Google Scholar This is a fraction that is usually somewhere around a quarter to a third of the measured mass loss, an amount that contains no medication. Thus, with any nebulizer, the output will be significantly overestimated by extrapolation from the mass loss after a period of nebulization. The important messages from this are at least 2-fold. The first is that with a calibrated nebulizer, the output will be consistent but always overestimated. The second point is that with nebulization, the concentration of methacholine remaining in the nebulizer will increase based on the amount of evaporation. This is important in that it is not appropriate to reuse the solution. The issue of dose is further complicated by the fact that with either method, an unknown dose will actually enter the lungs. This is almost certainly less than 50% even for the more efficient dosimeter technique. It is therefore impossible in simple clinical settings to assess the lung dose or even, in fact, the nebulized dose.For construction of the figure accompanying this reply, we have determined the noncumulative PD20 by multiplying the 2 PC20 values by different constants. The dosimeter PC20 values were multiplied by 0.045 mL per dose step-up, and the tidal breathing PC20 values were multiplied by 0.090 mL per dose step-up. We are not certain what units are expected for the dosimeter PD20. We have chosen to express PD20 values in milligrams (rather than micromoles) and noncumulatively, because doubling concentrations/doses of methacholine administered at 5-minute intervals are closer to noncumulative than to cumulative in terms of their effect on FEV1.13Juniper E.F. Frith P.A. Dunnett C. Cockcroft D.W. Hargreave F.E. Reproducibility and comparison of responses to inhaled histamine and methacholine.Thorax. 1978; 33: 705-710Crossref PubMed Scopus (378) Google ScholarIn conclusion, we agree that there are some important differences between methods and that these have not been completely examined. However, aside from the possibility of delaying the measurement of FEV1 for several minutes after completion of inhalations, none of the issues outlined by Townley et al1Townley R.G. Hopp R.J. Rosenthal R.R. The correct methacholine challenge delivery method?.J Allergy Clin Immunol. 2007; 119: 1026-1027Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar are relevant to the issue of deep inspiration bronchoprotection occurring in a significant proportion of subjects with asthma. This causes the appearance of false-negative methacholine tests. We emphasize that it is possible to perform methacholine inhalation challenges without deep inhalations either by the tidal breathing technique or by a modified dosimeter technique. To the Editor: We are pleased to respond to the comments raised by Drs Townley, Hopp, and Rosenthal.1Townley R.G. Hopp R.J. Rosenthal R.R. The correct methacholine challenge delivery method?.J Allergy Clin Immunol. 2007; 119: 1026-1027Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar We initially would like to clarify some apparent misconceptions. First, we have not written 5 articles on this topic; we have 3 original research publications.2Todd D.C. Davis B.E. Hurst T.S. Cockcroft D.W. Dosimeter methacholine challenge: comparison of maximal versus submaximal inhalations.J Allergy Clin Immunol. 2004; 114: 517-519Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar, 3Cockcroft D.W. Davis B.E. Todd D.C. Smycniuk A.J. Methacholine challenge: comparison of two methods.Chest. 2005; 127: 839-844Crossref PubMed Scopus (66) Google Scholar, 4Allen N.D. Davis B.E. Hurst T.S. Cockcroft D.W. Difference between dosimeter and tidal breathing methacholine challenge: contributions of dose and deep inspiration bronchoprotection.Chest. 2005; 128: 4018-4023Crossref PubMed Scopus (54) Google Scholar The fourth article, the Rostrum article that inspired this correspondence, contained no new data but was basically an opinion paper that included review of data from the first 3 papers.5Cockcroft D.W. Davis B.E. The bronchoprotective effect of inhaling methacholine by using total lung capacity inspirations has a marked influence on the interpretation of the test result.J Allergy Clin Immunol. 2006; 17: 1244-1248Abstract Full Text Full Text PDF Scopus (65) Google Scholar The fifth article, by Wubbel et al,6Wubbel C. Asmus M.J. Stevens G. Chesrown S.E. Hendeles L. Methacholine challenge testing: comparison of the two American Thoracic Society-recommended methods.Chest. 2004; 125: 453-458Crossref PubMed Scopus (27) Google Scholar comes from another group. Second, we were very careful not to argue for or against any method of aerosol generation. Our concerns relate exclusively to the method of inhalation (to total lung capacity) and not the method of nebulization. Although this (total lung capacity breaths) is the inhalation method recommended for the dosimeter technique,7Chai H. Farr R.S. Froehlich L.A. Mathison D.A. McLean J.A. Rosenthal R.R. et al.Standardization of bronchial inhalation challenge procedures.J Allergy Clin Immunol. 1975; 56: 323-327Abstract Full Text PDF PubMed Scopus (1129) Google Scholar, 8Crapo R.O. Casaburi R. Coates A.L. Enright P.L. Hankinson J.L. Irvin C.G. et al.Guidelines for methacholine and exercise challenge testing-1999.Am J Respir Crit Care Med. 2000; 161: 309-329Crossref PubMed Scopus (2180) Google Scholar the dosimeter can be successfully used with submaximal, approximately half inspiratory capacity breaths.2Todd D.C. Davis B.E. Hurst T.S. Cockcroft D.W. Dosimeter methacholine challenge: comparison of maximal versus submaximal inhalations.J Allergy Clin Immunol. 2004; 114: 517-519Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar Townley et al1Townley R.G. Hopp R.J. Rosenthal R.R. The correct methacholine challenge delivery method?.J Allergy Clin Immunol. 2007; 119: 1026-1027Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar raise several important points regarding differences between the 2 methods. However, none of these (with 1 possible exception) are relevant to our conclusions. We will nevertheless address these on a point-by-point basis. There is no doubt that the dosimeter method of aerosol generation, as noted by points raised in the correspondence, does (or at least should) produce an identical volume and therefore dose of agent with each measured breath. Subjects inhaling aerosol by the 2-minute tidal breathing technique may well have variations in both number and depth of breaths; however, the duty cycle (ie, the fraction of the total time spent during inspiration) is consistent in healthy subjects at about 0.35.4Allen N.D. Davis B.E. Hurst T.S. Cockcroft D.W. Difference between dosimeter and tidal breathing methacholine challenge: contributions of dose and deep inspiration bronchoprotection.Chest. 2005; 128: 4018-4023Crossref PubMed Scopus (54) Google Scholar, 9Stromberg N.O. Gustafsson P.M. Breathing pattern variability during bronchial histamine and methacholine challenges in asthmatics.Respir Med. 1996; 90: 287-296Abstract Full Text PDF PubMed Scopus (8) Google Scholar Thus, over a period of 2 minutes, the exposure to the aerosol is consistent and reproducible, as is the response to methacholine.10Cockcroft D.W. Measurement of airway responsiveness to inhaled histamine or methacholine: method of continuous aerosol generation and tidal breathing inhalation.in: Hargreave F.E. Woolcock A.J. Airway responsiveness: measurement and interpretation. Astra Pharmaceuticals Canada Ltd, Mississauga1985: 22-28Google Scholar We agree that in subjects with greater airway hyperresponsiveness, there is less (indeed no) bronchoprotective effect of maximal inhalations. That is, in fact, what has been demonstrated by our 3 studies.2Todd D.C. Davis B.E. Hurst T.S. Cockcroft D.W. Dosimeter methacholine challenge: comparison of maximal versus submaximal inhalations.J Allergy Clin Immunol. 2004; 114: 517-519Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar, 3Cockcroft D.W. Davis B.E. Todd D.C. Smycniuk A.J. Methacholine challenge: comparison of two methods.Chest. 2005; 127: 839-844Crossref PubMed Scopus (66) Google Scholar, 4Allen N.D. Davis B.E. Hurst T.S. Cockcroft D.W. Difference between dosimeter and tidal breathing methacholine challenge: contributions of dose and deep inspiration bronchoprotection.Chest. 2005; 128: 4018-4023Crossref PubMed Scopus (54) Google Scholar Our data cannot be explained by a dose effect. We have redrawn our figure (Fig 1) with the results from the 2 methods expressed as a PD20, and the marked difference with deep inhalation bronchoprotection persists. There are 7 subjects whose PD20 exceeds 1.44 mg, which equates to the tidal breathing cutoff of a PC20 = 16 mg/mL.8Crapo R.O. Casaburi R. Coates A.L. Enright P.L. Hankinson J.L. Irvin C.G. et al.Guidelines for methacholine and exercise challenge testing-1999.Am J Respir Crit Care Med. 2000; 161: 309-329Crossref PubMed Scopus (2180) Google Scholar In these 7 subjects, despite having normalized the graph for dose, the deep inhalation dosimeter technique produced a PD20 on average almost 10-fold larger than the tidal breathing PD20. If one uses the dosimeter upper limit of normal, which we presume to be a 20% FEV1 fall at 25 mg/mL, this equates to a noncumulative PD20 of 1.13 mg. In this case, there are then 10 subjects with a dosimeter PD20 in the normal range averaging 8-fold larger than their tidal breathing PD20 despite the dose correction. With this cutpoint, we also observed 1 subject with a normal tidal breathing PD20. Further analysis of the data in Fig 1 confirms the point made in the previous paragraph. Although there is no longer any significant difference between the geometric mean PD20 values by the 2 methods, there is a difference when the data are dichotomized at the midpoint of the data, a tidal breathing PD20 of 0.2 mg. In the 28 more hyperresponsive subjects (PD20 < 0.2 mg), the dosimeter PD20 is significantly lower than the tidal breathing PD20 (P = .019). By contrast, in the 27 less hyperresponsive subjects (PD20 > 0.2 mg), the dosimeter PD20 is significantly higher than the tidal breathing PD20 (P = .039). These results not only are consistent with lack of a bronchoprotective effect in the hyperresponsive half of our population but also support the probability of a more efficient aerosol deposition and/or retention with the dosimeter technique. The difference, in the less hyperresponsive half of our population, relates primarily to the bronchoprotective effect (despite the same more efficient deposition and retention of aerosol) seen in the 10 subjects noted. The point about timing of the spirometric maneuvers is valid and, in fact, is the only observation that may impinge on our findings. It is recognized that a delay in timing of the FEV1 will enhance the apparent bronchoconstriction produced by methacholine.11Simard B. Turcotte H. Cockcroft D.W. Davis B.E. Boulay M.E. Boulet L.P. Deep inspiration avoidance and methacholine response in normal subjects and patients with asthma.Chest. 2005; 127: 135-142Crossref PubMed Scopus (17) Google Scholar Whether delaying the spirogram until 3 minutes after completion of the inhalation would have allowed reversal of the deep breathing effect remains to be tested. The question of the deep inhalation required before performing an FEV1 maneuver has been raised many times in the past. If one chooses to evaluate the effect of a bronchoconstrictor by looking at the FEV1, one is always faced with this conundrum. It is possible that the deep inhalation required to measure the FEV1 may, in fact, produce a bronchodilator effect. This is at least constant between methods and cannot explain any of our differences. It is, however, one of the reasons why we have always preferred to use the lowest (technically acceptable) FEV1 after methacholine compared with the lowest postsaline FEV1 in the calculation of the percent change in FEV1. The final point relates to the issue of being able to calculate a PD20 as opposed to a PC20. Although the dosimeter technique does allow one theoretically to calculate a PD20, that is, the amount of methacholine that has been nebulized, we suspect that even this will be inaccurate. Nebulizer output is traditionally determined by weighing the nebulizer before and after a period of use. In fact, this is the only easy way to calibrate nebulizers. Unfortunately, a significant proportion of the weight loss that occurs during nebulization is actually evaporation.12Cockcroft D.W. Hurst T.S. Gore B.P. Importance of evaporative water losses during standardized nebulized inhalation provocation tests.Chest. 1989; 96: 505-508Crossref PubMed Scopus (46) Google Scholar This is a fraction that is usually somewhere around a quarter to a third of the measured mass loss, an amount that contains no medication. Thus, with any nebulizer, the output will be significantly overestimated by extrapolation from the mass loss after a period of nebulization. The important messages from this are at least 2-fold. The first is that with a calibrated nebulizer, the output will be consistent but always overestimated. The second point is that with nebulization, the concentration of methacholine remaining in the nebulizer will increase based on the amount of evaporation. This is important in that it is not appropriate to reuse the solution. The issue of dose is further complicated by the fact that with either method, an unknown dose will actually enter the lungs. This is almost certainly less than 50% even for the more efficient dosimeter technique. It is therefore impossible in simple clinical settings to assess the lung dose or even, in fact, the nebulized dose. For construction of the figure accompanying this reply, we have determined the noncumulative PD20 by multiplying the 2 PC20 values by different constants. The dosimeter PC20 values were multiplied by 0.045 mL per dose step-up, and the tidal breathing PC20 values were multiplied by 0.090 mL per dose step-up. We are not certain what units are expected for the dosimeter PD20. We have chosen to express PD20 values in milligrams (rather than micromoles) and noncumulatively, because doubling concentrations/doses of methacholine administered at 5-minute intervals are closer to noncumulative than to cumulative in terms of their effect on FEV1.13Juniper E.F. Frith P.A. Dunnett C. Cockcroft D.W. Hargreave F.E. Reproducibility and comparison of responses to inhaled histamine and methacholine.Thorax. 1978; 33: 705-710Crossref PubMed Scopus (378) Google Scholar In conclusion, we agree that there are some important differences between methods and that these have not been completely examined. However, aside from the possibility of delaying the measurement of FEV1 for several minutes after completion of inhalations, none of the issues outlined by Townley et al1Townley R.G. Hopp R.J. Rosenthal R.R. The correct methacholine challenge delivery method?.J Allergy Clin Immunol. 2007; 119: 1026-1027Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar are relevant to the issue of deep inspiration bronchoprotection occurring in a significant proportion of subjects with asthma. This causes the appearance of false-negative methacholine tests. We emphasize that it is possible to perform methacholine inhalation challenges without deep inhalations either by the tidal breathing technique or by a modified dosimeter technique.