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

Abstract

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 (V̇). Our aim was to demonstrate the relevance of linearizing PNP for assessing hypopneas and flow limitation in SAHS. V̇ 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 [V̇NP = (PNP)1/2] by a specially designed analog circuit. For each event we used V̇, PNP, and V̇NP 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, V̇NP provided HFA and FLI values that were very close to those obtained from V̇: HFA (V̇NP) = 1.098 · HFA(V̇) - 0.063 (r2 = 0.98) and FLI(V̇NP) = 1.044 · FLI(V̇) + 0.004 (r2 = 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 sleep disturbances in SAHS.