Dear Editor

We thank Drs Dundas and McKenzie for their comments.[1] We agree with them that the interrupter resistance (Rint) is able to detect short- term changes in airway calibre after bronchodilator inhalation. However, we must disagree on their comment that Rint has a poor long-term repeatability and their consequent conclusion that Rint is not useful for routine clinical purposes. The long-term (38 days apart) Rint repeatability (2 SD calculated from the analysis of variance results) that Beelen et al.[2] found in 25 healthy preschool children was actually 0.26 kPa/l.s under field conditions and 0.20 kPa/l.s under laboratory conditions. These values are very similar to the long-term (3 weeks apart) repeatability (2 SD of the difference between two sets of measurements) that Chan et al.[3] found in 72 healthy preschool children (0.23 kPa/l.s) and the long-term (2.5 months apart) repeatability (2 SD of the difference between two sets of measurements) that we found in children with a history of wheezing or cough (0.21 kPa/l.s).[4] In our study, the potential effects of the disease or treatment on long-term Rint variability were carefully avoided and only clinically stable children with no change in treatment were recruited. Assessment of the long-term variability of a lung function test must be undertaken under circumstances in which the true lung function can reasonably be expected not to have changed. This is unlikely to be the case in children with asthma, where lung function is expected to vary with time. The fact that Chan et al.[3] found a much higher long-term Rint variability in 95 children with doctor observed wheeze in the previous 4-6 weeks and on no long-term treatment should not induce one to conclude that Rint is not useful in clinical practice, but is, on the contrary, a piece of evidence that Rint is able to detect long- term changes in airway calibre in children with a recent history of respiratory symptoms. If we add that Rint is also feasible in preschool children,[2-4] we can conclude that Rint is a potentially useful tool in routine clinical practice.

References

(1) Dundas I, McKenzie SA. Is the measurement of lung function using the interrupter technique useful for the clinician? [electronic response to Sly and Lombardi; Measurement of lung function in preschool children using the interrupter technique] thoraxjnl.com 2003http://thorax.bmjjournals.com/cgi/eletters/58/9/742#93

(2) Beleen RMJ, Smit HA, van Strien RT, et al. Short and long term variability of the interrupter technique under field and standardised conditions in 3-6 year old children. Thorax 2003;58:1-4.

(3) Chan EY, Bridge PD, Dundas I, et al. Repeatability of airway resistance measurements made using the interrupter technique. Thorax 2003 ;58 :344-7.

(4) Lombardi E, Sly PD, Concutelli G, et al. Reference values of interrupter respiratory resistance in healthy preschool white children. Thorax 2001;56:691-5.

Dear Editor

Sly and Lombardi [1] in their recent editorial suggest that interrupter resistance (Rint) measurements are useful in the management of lung disease in young children. We believe this claim needs further consideration.

Rint measurements can be helpful when change following an intervention, such as the administration of bronchodilator, is greater than its within-occasion repeatability but for a measurement to be useful for following change with time in the individual it must have acceptable between-occasion repeatability. In the same journal, Beelen et al.[2] have reported between-occasion variability of 0.38 kPa.L-1.s (2 SD of the differences between measurements) in 25 healthy children. This figure is similar to that of Chan et al.[3] who reported 72 measurements in healthy children and 95 measurements in stable mildly asthmatic children . For the healthy children, the between-occasion repeatability was 32% expected for age but for the asthmatics this rose to 52%. As a hallmark of asthma is bronchial lability, this is not unexpected. These figures need to be compared to the change expected with treatment. Pao et al.[4] showed that in an identical group of asthmatic children a change in mean Rint of 16% was demonstrated with ICS treatment. Although this change was confidently demonstrated in a group of children it would not be confidently picked up in the individual because the between-occasion repeatability of Rint is much greater than the change expected.

Rint seems to be a good tool for research and for that reason measurements should be standardized. However, we believe its usefulness for the practicing clinician is quite limited as measurements in the individual are not sufficiently reliable on a day to day basis. It is difficult to imagine that further refinement and standardization of the method will improve this.

References

(1) Sly PD and Lombardi E. Measurement of lung function in preschool children using the interrupter technique. Thorax 2003;58(9):742-4.

(2) Beelen RM, Smit HA, Van Striene et al. Short and long term variability of the interrupter technique under field and standardised conditions in 3-6 year old children. Thorax 2003;58(9):761-4.

(3) Chan EY, Bridge PD, Dundas, I et al. Repeatability of airway resistance measurements made using the interrupter technique. Thorax 2003;58(4):344-7.

(4) Pao CS and McKenzie SA. Randomized controlled trial of fluticasone in preschool children with intermittent wheeze. Am J Respir Crit Care Med 2002;166(7):945-9.