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Sly and Lombardi1 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 a 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 issue, Beelen et al2 reported between-occasion variability of 0.38 kPa/l.s (2 SD of the differences between measurements) in 25 healthy children. This figure is similar to that of Chan et al3 who reported 72 measurements in healthy children and 95 measurements in children with stable mild asthma. In the healthy children the between-occasion repeatability was 32% expected for age, but in the asthmatic children this rose to 52%. As a hallmark of asthma is bronchial lability, this is not unexpected. These figures need to be compared with the change expected with treatment. Pao et al4 showed that, in an identical group of asthmatic children, a change in mean Rint of 16% occurred with treatment with inhaled corticosteroids. Although this change was demonstrated in a group of children, it would not be picked up easily 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 standardised. However, we believe its usefulness for the practising 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 standardisation of the method will improve this.
We thank Drs Dundas and McKenzie for their comments. 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 with 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 repeatability (38 days apart) of Rint measurements (2 SD of the difference between two sets of measurements) reported by Beelen et al1 in healthy preschool children was actually 0.37 kPa/l.s in 25 children under field conditions and 0.28 kPa/l.s in 15 children under laboratory conditions. This value is very similar to the long term repeatability (3 weeks apart, 2 SD of the difference between two sets of measurements) reported by Chan et al2 in 72 healthy preschool children (0.23 kPa/l.s) and the long term repeatability (2.5 months apart, 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).3 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 al2 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 lead to the conclusion that Rint is not useful in clinical practice. On the contrary, it provides 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,1–3 we can conclude that it is a potentially useful tool in routine clinical practice.
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