The effects of posture on the mechanics of the respiratory system are not well known, particularly in terms of total respiratory resistance. We have measured respiratory impedance (Zrs) by the forced random noise excitation technique in the sitting and the supine position in 24 healthy subjects. Spirometry and lung volumes (He-dilution technique) were also measured in both postures. The equivalent resistance (Rrs), compliance (Crs), and inertance (Irs) were also calculated by fitting each measured Zrs to a linear series model. When subjects changed from sitting to the supine position, the real part of Zrs increased over the whole frequency band. The associated equivalent resistance, Rrs, increased by 28.2%. The reactance decreased for frequencies lower than 18 Hz and increased for higher frequencies. Consequently, Crs decreased by 38.7% and Irs increased by 15.6%. All of these parameter differences were significant (P less than 0.001). A covariance analysis showed that a significant amount of the postural change in Rrs and Crs can be explained by the reduction of functional residual capacity (FRC). This indicates that the observed differences on Zrs can in part be explained be a shift of the operating point of the respiratory system induced by the decrease in the FRC.

We have previously demonstrated that upper airway obstruction in sleep apnea/hypopnea syndrome (SAHS) can be accurately assessed in real-time by measuring respiratory impedance (|Z|) with the forced oscillation technique (FOT). The aims of the present study were: (1) to determine the feasibility of identifying the optimal continuous positive airway pressure (CPAP) for patients with SAHS based on analysis of the |Z| signal during conventional polysomnographic CPAP titration studies; and (2) to evaluate practical issues involved in the application of FOT during CPAP titration. We performed CPAP titration in 28 patients with SAHS during polysomnography (PSG) (14 nap and 14 full overnight studies) using a FOT system applied continuously to obtain an on-line measurement of |Z|. FOT was easily implemented and was well-tolerated by the patients. Optimal CPAP levels were determined both in the conventional manner from the standard PSG titration record and during a separate blinded analysis using the FOT signal alone. The mean conventional versus FOT-based optimal CPAP values were similar for both nap studies (10.6 +/- 0.6 [mean +/- SEM] versus 11.1 +/- 0.6 cm H(2)O, respectively, p = 0. 054) and overnight studies (9.9 +/- 0.7 versus 9.9 +/- 0.6 cm H(2)O, respectively, p = 1.00). Subsequent analysis of the PSG record with the FOT signal incorporated demonstrated that artefacts in the |Z| tracing occurred during mask leak, mouth breathing, and movement during arousal. Such abnormalities were readily identified from the flow tracing. These results indicate that, for adequate interpretation, the tracing and values of respiratory impedance obtained by FOT should be evaluated in conjunction with the flow signal. Continuous FOT-guided CPAP titration is feasible and may be a useful adjunct during manual titration. FOT could also potentially serve as the basis for automated CPAP in SAHS.

The increase in the prevalence of chronic respiratory diseases has resulted in a rise in health services provided at home. The forced oscillation technique (FOT) proves to be a useful tool when it is desired to assess lung function noninvasively, and particularly for patients in whom spirometry cannot be applied. As no portable FOT device is currently available, the aim of this study was to design and test a portable FOT system for ambulatory and home care applications.The system devised is based on a microprocessor, which allows online computation of respiratory resistance (Rrs) and reactance (Xrs) and reliability indices. The portable device was compared with a conventional FOT system by using signals from 14 patients with chronic respiratory disease.This device has the same computation capabilities and flexibility as conventional FOT systems and meets the requirements for home application. Rrs (14. This portable forced oscillation technique device could be a useful tool for monitoring respiratory mechanics in ambulatory and home care applications.

Respiratory input impedance (Zrs) was measured from 8 to 256 Hz in 10 healthy subjects by a method that eliminated the shunt impedance of extrathoracic airway walls. It consisted of combining the data obtained with a pressure input at the mouth (standard method, Zst) and with a pressure input around the head (Zhg) Zrs = Zst.(Zp + Zhg)/(Zp + Zst) where Zp is the impedance of the mouthpiece and pneumotachograph. Large quantitative differences were observed between Zrs and Zst, demonstrating that the standard method is unreliable at such frequencies. The real part of Zrs increased from 2.6 +/- 0.8 cmH2O.l-1.s at 8 Hz to a maximum of 38 +/- 19 cmH2O.l-1.s at 158 +/- 49 Hz. The imaginary part exhibited a maximum of 19 +/- 8 cmH2O.l-1.s at 126 +/- 38 Hz, a resonance at 157 +/- 43 Hz, and a minimum of lambda 19 +/- 16 cmH2O.l-1.s at 185 +/- 45 Hz. The data were analyzed with five models featuring alveolar gas compressibility; tissue resistance, inertance, and compliance; and different representations of the airways with lumped and distributed parameters. All except the simplest (lumped frequency-dependent resistance) fitted the data equally well, but none provided reliable estimates of gas compliance. Three models gave a consistent description of the airway in terms of equivalent rigid tubes (cross-sectional area 3.5-3.7 cm2, length 47-51 cm). We conclude that high-frequency input impedance could prove useful in exploring the airways but not the peripheral lung.