Kwong Wah Hospital, Hong Kong, SAR
Correspondence to: Daniel Kwok-keung Ng, MMedSc, Department of Paediatrics, Kwong Wah Hospital, 25 Waterloo Rd, Kowloon, Hong Kong, SAR; e-mail: firstname.lastname@example.org
We read with interest the article by Uliel et al in a recent issue of CHEST (March 2004).1The thermistor has been shown by Trang et al2 to be an insensitive tool with which to detect hypopneas. Of the 159 obstructive hypopneas identified by nasal cannula, the thermistor detected only 14%. It was unfortunate that the normal value for obstructive hypopnea was somehow missed by Uliel et al.1 Hence, a study is urgently needed to establish the normal value of obstructive hypopneas as hypopneas are now commonly detected in children using the more sensitive nasal cannula. In the absence of established normal value for obstructive hypopneas, a diagnosis of sleep-disordered breathing using the apnea-hypopnea index would remain arbitrary, floating between scores of 5 to 30.3
The distribution of the obstructive apnea (OA) index was skewed by inspecting the discrepancy between the mean and the median. An asymmetric distribution of the OA index has been reported previously.4 However, in the study by Uliel et al1 readers were unable to picture the accrual normality of the distribution since the authors failed to provide the SD of the mean. There are some statistical tests that are available for the assessment of normality, like the Shapiro-Wilks test. Uliel and colleagues should have assessed the distribution of the OA index by statistical testing to confirm that the data were asymmetrically distributed. Only then should they have selected 97.5 percentile as the suggested cut-off value.
We question the validity of the upper limit for the OA index determined by Uliel et al,1 which was obtained after the exclusion of children with an OA index score of 0. The OA index was strongly right-tailed in the general population.4 The exclusion of children without any obstructive apneic episodes in calculating the normal range is simply not supported by evidence or reasoning. We also disagree with Uliel et al1 in their conclusion that their normal range for the OA index in children was comparable with that reported by Marcus et al.5 Uliel et al1 came up with a new upper limit of normal (ie, 2.36) for the study by Marcus et al,5 by excluding those children having a score of 0. This was much higher than the 0.7 reported by Uliel et al.1
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