© 2003 American Thoracic Society
Time for standardizationTo the Editor:Beydon and colleagues have published their interrupter resistance values (Rint) and measurements of bronchodilator responsiveness (BDR) in healthy children (1). Their baseline data are not dissimilar to others (2). We have shown in preschool children with mild respiratory symptoms but asymptomatic at the time of testing, that the difference in Rint between expiration and inspiration (geometric mean ratio) was 4% (p < 0.01) (3), a similar difference. This was weakly correlated with the average measurement. We concluded that it made little difference whether the measurement was made in expiration or inspiration but that one or other should be chosen as a standard. Secondly, a mean BDR of 15% predicted in healthy children was demonstrated. If this is compared with the much larger response demonstrable in children with even mild respiratory symptoms, then it need not necessarily complicate the interpretation of BDR, as suggested. We have shown that a 22% change pre:post bronchodilator Rint has a sensitivity and specificity profile of 80/76% for previous wheeze in preschool children compared with healthy children (4). Finally, change was expressed as percent predicted rather than a difference or ratio. Normative data are available and so this seems sensible, another reason for standardization. Now that this technique is being used as a clinical and research tool, it is time for formal recommendations for making measurements and expressing results.
Royal London Hospital London, United Kingdom REFERENCES
From the Authors:Bridge and McKenzie make pertinent comments about interrupter resistance (Rint) measurements in young children. First, they comment on our Rint normative data (1). These data (1) are not significantly different from those of Lombardi and coworkers in white preschool children (2). We are pleased to learn that our data are not inconsistent with those of Bridge and McKenzie. Nevertheless, a statistical analysis taking into account regression coefficient variability would be needed to confirm the similarity of the models. Second, Bridge and McKenzie raise the issue of the difference between inspiratory (Rintinsp) and expiratory (Rintexp) resistance values in preschool children. They suggest that one or the other can be used as a standard (3). Our data do not support this suggestion. We found that mean Rintinsp and mean Rintexp were not significantly different in healthy young children (1), in keeping with the study by Lombardi and coworkers (2). However, our data showed that the difference between Rintinsp and Rintexp decreased significantly with age (1), so that the Rintexp/Rintinsp ratio increased significantly (p < 0.006). Rintexp was 8% lower, on average, than Rintinsp at 3 years of age and 12% higher at 7 years. Given the coefficient of variation of Rintexp (12%) (1), using Rintinsp instead of Rintexp would double the intrinsic variability of Rint measurement. Therefore, separate reporting of Rintinsp and Rintexp data is desirable for research and clinical purposes. The third comment by Bridge and McKenzie is about bronchodilator responsiveness measured using methods that do not require active cooperation. Similar to others (4, 5), we found some degree of bronchomotor tone in healthy preschool children, as shown by a mean 12% fall in Rintexp expressed as percent of predicted (1). When McKenzie and coworkers assessed bronchodilator effects based on the ratio of postbronchodilator over baseline values, they found that a 22% change discriminated between young children with and without a history of wheezing (6). However, studies of Rint (4) or resistance measured by the forced oscillations technique (4, 5) as tools for discriminating between young children with and without asthma have found different cut-offs. Therefore, we feel that further studies are needed before recommendations can be made about the optimal bronchodilator responsiveness cut-off for discriminating between young children with and without respiratory disorders. An international workshop on pulmonary function testing in preschool children would be timely. Several issues could be debated, including testing methods, diagnostic criteria, and ways to express bronchodilator responsiveness.
Hôpital Robert Debré, Université Paris VII Paris, France REFERENCES
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