Relevance of linearizing nasal prongs for assessing hypopneas and flow limitation during sleep

Respiratory disturbances in patients with the sleep apnea-hypopnea syndrome (SAHS) may be detected by means of nasal prongs (NP) pressure (PNP). Nevertheless, PNP is nonlinearly related to flow (V) over dot. Our aim was to demonstrate the relevance of linearizing PNP for assessing hypopneas and flow limitation in SAHS. (V) over dot was measured with a pneumotachograph during the hypopneas and flow limitation events in a continuous positive airway pressure (CPAP) titration in six patients with severe SAHS. These flow patterns were reproduced by a flow generator through an analog of the nares and recorded by NP. PNP was linearized [(V) over dot NP = (PNP)(1/2)] by a Specially designed analog circuit. For each event we used (V) over dot, PNP, and (V) over dot NP to compute the hypopnea flow amplitude (HFA) and a flow limitation index (FLI). Owing to NP nonlinearity, PNP considerably misestimated HFA and FLI. By contrast, (V) over dot NP provided HFA and FLI values that were very close to those obtained from (V) over dot: HFA (V) over dot NP) = 1.098 (.) HFA((V) over dot - 0.063 (r(2) = 0.98) and FLI ((V) over dot NP) = 1.044 (.) FLl((V)over dot) + 0.004 (r(2) = 0.99). Square-root linearization of NP greatly increases the accuracy of quantifying hypopneas and flow limitation. This procedure, which could be readily carried out in routine practice by means of the analog circuit we developed, is of interest in optimizing the assessment of respiratory steep disturbances in SAHS.