Inspiratory dynamic obstruction detected by forced oscillation during CPAP. A model study.

Abstract

Assessment of upper airway mechanics in patients with obstructive sleep apnea/hypopnea (OSA) can be carried out qualitatively from indirect signals (flow pattern, snoring, strain gauges, inductance plethysmography) or quantitatively by means of invasive estimation of esophageal pressure. The forced oscillation technique (FOT) is a noninvasive method of potential interest for quantitatively assessing airway obstruction in the sleeping patient. The aim of this work was to ascertain in a model study whether FOT could provide an index of airway obstruction when applied at the conditions of total and partial occlusions similar to the ones found in patients with OSA. An airway analog closely mimicking upper airway collapsibility was constructed and mechanically characterized by the relationship between its flow, upstream and downstream pressures as well as by means of FOT. We simulated total collapse (apnea), different levels of partial collapse with flow limitation (hypopnea), and release of airway obstruction when the collapsible analog was used as an artificial upper airway in a spontaneously breathing subject submitted to continuous positive airway pressure (CPAP) up to 14 cm H2O.s/L. The results showed that the amplitude of airway impedance measured by FOT was a suitable index to detect obstruction in collapsible segments. We concluded from this realistic model study that FOT could be a valuable tool for quantitatively assessing airway obstruction in patients with OSA treated with CPAP. This noninvasive technique is potentially useful both in studying upper airway mechanics in detail and in automatically monitoring airway obstruction in routine studies.

Analog circuit for real-time computation of respiratory mechanical impedance in sleep studies

“…Forced oscillation (5 Hz) was applied during sleep for CPAP = 4 hPa and for the optimal CPAP in each patient (10-14 hPa) determined by absence of arousals, normal sleep architecture, and awake tidal breathing esophageal pressure. P n and V 0 n were recorded at the nasal mask by a piezoresistive pressure transducer (176 PC, Honeywell) and a Fleisch-type pneumotachograph with a differential pressure transducer [62 hPa, LCVR, Celesco] [7]. These P n and V 0 n signals were fed to the analog circuit designed to compute jZj a (Fig.…”

“…These P n and V 0 n signals were fed to the analog circuit designed to compute jZj a (Fig. 2) and were simultaneously sampled at 100 Hz by a computerized system to calculate impedance by digital signal processing (jZj d ) according to the procedures usually employed in the field of FOT and described in detail in [7]. Briefly, a movingaverage filter with a window length of one FOT period (0.2 s) was used to obtain the oscillatory components of pressure and flow from the sampled P n and V 0 n [7].…”

“…2) and were simultaneously sampled at 100 Hz by a computerized system to calculate impedance by digital signal processing (jZj d ) according to the procedures usually employed in the field of FOT and described in detail in [7]. Briefly, a movingaverage filter with a window length of one FOT period (0.2 s) was used to obtain the oscillatory components of pressure and flow from the sampled P n and V 0 n [7]. jZj d was calculated point-by-point from the fast Fourier transform (FFT) of the filtered pressure and flow signals computed on a cycle-per-cycle basis (0.2 s) [7], [11] and smoothed by a moving-average filter with a window length of 0.4 s. As we used phaseless moving-average digital filters, jZj d was undelayed as regards the recorded P n and V 0 n signals.…”

“…Briefly, a movingaverage filter with a window length of one FOT period (0.2 s) was used to obtain the oscillatory components of pressure and flow from the sampled P n and V 0 n [7]. jZj d was calculated point-by-point from the fast Fourier transform (FFT) of the filtered pressure and flow signals computed on a cycle-per-cycle basis (0.2 s) [7], [11] and smoothed by a moving-average filter with a window length of 0.4 s. As we used phaseless moving-average digital filters, jZj d was undelayed as regards the recorded P n and V 0 n signals. Respiratory impedance computed with the designed analog circuit (jZj a ) and impedance computed digitally (jZj d ), according to conventional procedures were compared.…”

“…The FOT has been widely used to measure Z in a variety of clinical fields such as bronchoconstriction [2] and brochodilation [3] tests, anesthesia [4], and intensive care [5]. As has been shown recently [6], [7], [8], FOT is also useful in assessing inspiratory and expiratory airway obstruction in the recurrent upper-airway collapses found in patients with obstructive sleep apnea/hypopnea (OSA).…”