Tracing air flow and diagnosing hypopnoeas in normal subjects

Hypopnoea is a type of sleep-related breathing disorder (SRBD), and the apnoea plus hypopnoea index (AHI) is usually computed to diagnose this condition. We introduce a new method to diagnose flow with internal thermistors located on the same sensors as we use to diagnose obstructive segments in patients with sleep-related breathing disorders. The aim of this study is to investigate whether internal thermistors are reliable for diagnosis of hypopnoeas. Fifteen volunteers participated in a prospective comparative study in healthy subjects without SRBD. We simultaneously measured minute ventilation by a pneumotachograph and indirectly air flow by internal thermistors in awake subjects was manually analysed from both the pneumotachograph and the internal thermistors. We found a close agreement between the different methods for percentage reduction in air flow both for the hypopnoea and the apnoea data. The mean difference between the percentage reduction in flow from normal breathing to hypopnoea measured by the pneumotachograph and the internal thermistors lying supine was 3.8% (SD 7.4). In the lateral position the corresponding figures were 1.0% and 4.6. This study in awake, normal subjects indicates that internal thermistors are as reliable as the pneumotachograph in diagnosing hypopnoeas and we believe that the reliability of this monitoring method is adequate for clinical use.