Influence of external loading and assisted ventilation on chest wall mechanical properties

Respiratory impedance response to continuous negative airway pressure in awake controls and OSAS

The aim of the study was to determine whether the response of respiratory impedance (Zrs) to decreasing levels of continuous negative airway pressure (CNAP) during wakefulness, differs in controls and subjects with obstructive sleep apnoea syndrome (OSAS). Zrs was measured by the forced oscillation technique (4-32 Hz) in 15 controls and 21 patients with OSAS (apnoea/hypopnoea index >20 per sleep hour) with normal lung function, in the basal state and with application of decreasing CNAP of -5, -10, and -15 hPa. Respiratory resistance was extrapolated to 0 Hz (R0) and estimated at 16 Hz (R16) by linear regression analysis of respiratory resistive impedance versus frequency. Respiratory elastance (Ers) and inertance (Irs) were estimated by multilinear regression analysis of respiratory reactance versus frequency, and resonance frequency (RF) was determined as RF=(1/2pi)(Ers/Irs)0.5. In both groups, R0, R16, Ers and RF significantly increased as the CNAP level decreased (p <0.0001 for all). R0, Ers, and RF increased significantly more in OSAS than in controls (p < 0.01, 0.001, and 0.0001, respectively), independently of the severity of obesity. Receiver operator characteristic curves showed that the parameter which best detected OSAS was RF, with a sensitivity of 81% and 93% specificity for the 13.6 Hz cut-off point. The results of the present study suggest that the response of respiratory impedance to decreasing continuous negative airway pressure levels, might allow detection of obstructive sleep apnoea syndrome in subjects with normal lung function.

Removal of thermal artifact in alveolar pressure measurement during forced oscillation

Thoracic gas volume (TGV) may be measured and total respiratory impedance (Zrs) may be partitioned into its airway and tissue (Zti) components by combining forced oscillations and plethysmographic measurements of alveolar gas compression (Vpl) (Peslin, Duvivier, 1998, J. Appl. Physiol. 84, 553-561 and 862-867). The method requires that the thermal component of Vpl (Vpl,th) be eliminated by conditioning inspired gas to BTPS. We have evaluated a technically simpler method where Vpl,th is corrected using a thermal gain (G) and a thermal time constant (theta). Zrs and the relationship between Vpl and flow (Hpl) were measured in 15 healthy subjects at frequencies from 4 to 29 Hz. G was obtained from the breathing component of Vpl in phase with volume. Zti and TGV (TGVos) were computed from Zrs and from Hpl after the latter was corrected using G and different values of theta. With theta = 27.5 ms both the difference between TGVos and standard plethysmographic TGV, and the difference between Zti spectra obtained with and without gas conditioning were minimal. We conclude that the simpler method is adequate for both purposes.

Partitioning of airway and respiratory tissue mechanical impedances by body plethysmography

We have tested the feasibility of separating the airway (Zaw) and tissue (Zti) components of total respiratory input impedance (Zrs,in) in healthy subjects by measuring alveolar gas compression by body plethysmography (Vpl) during pressure oscillations at the airway opening. The forced oscillation set up was placed inside a body plethysmograph, and the subjects rebreathed BTPS gas. Zrs,in and the relationship between Vpl and airway flow (Hpl) were measured from 4 to 29 Hz. Zaw and Zti were computed from Zrs,in and Hpl by using the monoalveolar T-network model and alveolar gas compliance derived from thoracic gas volume. The data were in good agreement with previous observations: airways and tissue resistance exhibited some positive and negative frequency dependences, respectively; airway reactance was consistent with an inertance of 0.015 +/- 0.003 hPa.s2.l-1 and tissue reactance with an elastance of 36 +/- 8 hPa/I. The changes seen with varying lung volume, during elastic loading of the chest and during bronchoconstriction, were mostly in agreement with the expected effects. The data, as well as computer simulation, suggest that the partitioning is unaffected by mechanical inhomogeneity and only moderately affected by airway wall shunting.