Dana Children’s Hospital, Tel-Aviv, Israel
Correspondence to: Yakov Sivan, MD, Dana Children’s Hospital, 6 Weizmann St, Tel-Aviv, Israel 64239; e-mail: firstname.lastname@example.org
Ng and colleagues1have raised several issues in their comments on our study.2Based on the study by Trang et al,3 they questioned the validity and sensitivity of the thermistor to detect obstructive hypopneas. The use of a nasal cannula to monitor airflow and to detect apneas and hypopneas has become popular in recent years. This technique may be advantageous in many aspects; however, it has limitations. In their article, Trang and colleagues3showed that the time spent with an uninterpretable cannula signal was significantly longer than the time spent with an uninterpretable thermistor signal (mean uninterpretable time out of total sleep time for the thermistor, 0%, compared to 2 to 4% for the cannula). In addition, mouth breathing was a frequent cause for cannula signal unreliability. More studies are needed to compare the two techniques before the nasal cannula can become the “gold standard” and the only recommended method. The thermistor has been used in many published pediatric studies from the past few years.4–7
We think that the finding that only three subjects had a total of seven obstructive apneas (OAs) [one child had five of the seven OAs] precludes the possibility that the normal distribution of OAs over the 70 cases in the study is possible. Hence, calculating the SD for three cases would be meaningless.
The goal of the study was to establish normal values. Therefore, the study aimed to provide an upper limit value for OAs and obstructive hypopneas, such that all resulting values higher than that number would be considered abnormal. Because only 3 of 70 healthy subjects had a total of seven OAs, calculating the normal upper limit by dividing 7 by the total sleep time of all 70 cases combined will result with an OA index value that would define these three healthy children as abnormal. Using the method described in our study, we presented an upper limit value for the OA index that applies to any child who has OAs.
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