Respiratory transfer impedances with pressure input at the mouth and chest

Two methods of measuring respiratory transfer impedance (Ztr) were compared in 14 normal subjects, from 4 to 30 Hz, 1) studying the relationship between transrespiratory pressure (Prs) and flow at the chest when varying pressure at the mouth (Ztrm) and 2) studying the relationship between Prs and flow at the mouth when varying pressure around the chest wall (Ztrw). The similarity of the two relationships was expected on the basis of a T-network model. Almost identical phase responses were obtained from the two methods. Pressure-flow ratios were slightly larger for Ztrw than for Ztrm, but differences did not exceed 2% on average in 11 of 14 subjects. When the data were analyzed with the six-coefficient model proposed by DuBois et al. (J. Appl. Physiol. 8: 587-594, 1956), similar values were found for tissue compliance and tissue inertance but slightly different values for gaseous inertance in the airways (1.97 +/- 0.35 X 10(-2) cmH2O X l-1 X s2 for Ztrw vs. 1.73 +/- 0.26 for Ztrm; P less than 0.01). Similar results were also found for total respiratory resistance but with a slightly larger contribution of airway resistance for Ztrw (64 +/- 14 vs. 57 +/- 10%; P less than 0.05). As a practical conclusion it is recommended to measure Ztrw, which is technically much easier.     

Respiratory impedance to ambient pressure changes at low frequencies

Respiratory impedance may be studied by measuring airway flow (Vaw) when pressure is varied at the mouth (input impedance) or around the chest (transfer impedance). A third possibility, which had not been investigated so far, is to apply pressure variations simultaneously at the two places, that is to vary ambient pressure (Pam). This provides respiratory impedance to ambient pressure changes (Zapc = Vaw/Pam). In that situation airway impedance (Zaw) and tissue impedance (Zt) are mechanically in parallel, and both are in series with alveolar gas impedance (Zg): Zapc = Zaw + Zg + Zaw.Zg/Zt. We assessed the frequency dependence of Zapc from 0.05 to 2 Hz in nine normal subjects submitted to sinusoidal Pam changes of 2-4 kPa peak to peak. The real part of Zapc (Rapc) was of 6.2 kPa.1(-1).s at 0.05 Hz and decreased to 1.9 kPa.1(-1).s at 2 Hz. Similarly the effective compliance (Capc), computed from the imaginary part of Zapc, decreased from 0.045 1.kPa-1 at 0.05 Hz to 0.027 1.kPa-1 at 2 Hz. Breathing against an added resistance of 0.46 kPa.1(-1).s exaggerated the negative frequency dependence of both Rapc and Capc. When values of airway resistance and inertance derived from transfer impedance data were introduced, Zapc was used to compute effective tissue resistance (Rt) and compliance (Ct). Rt was found to decrease from 0.32 to 0.15 kPa.1(-1).s and Ct from 1.11 to 0.64 1.kPa-1 between 0.25 and 2 Hz. Ct was slightly lower with the added resistance. These results are in good agreement with the data obtained by other approaches.     

Stress adaptation and low-frequency impedance of rat lungs

At transpulmonary pressures (Ptp) of 7-12 cmH2O, pressure-volume hysteresis of isolated cat lungs has been found to be 20-50% larger than predicted from their amount of stress adaptation (J. Hildebrandt, J. Appl. Physiol. 28: 365-372, 1970). This behavior is inconsistent with linear viscoelasticity and has been interpreted in terms of plastoelasticity. We have reinvestigated this phenomenon in isolated lungs from 12 Wistar rats by measuring 1) the changes in Ptp after 0.5-ml step volume changes (initial Ptp of 5 cmH2O) and 2) their response to sinusoidal pressure forcing from 0.01 to 0.67 Hz (2 cmH2O peak to peak, mean Ptp of 6 cmH2O). Stress adaptation curves were found to fit approximately Hildebrandt's logarithmic model [delta Ptp/delta V = A - B.log(t)] from 0.2 to 100 s, where delta V is the step volume change, A and B are coefficients, and t is time. A and B averaged 1.06 +/- 0.11 and 0.173 +/- 0.019 cmH2O/ml, respectively, with minor differences between stress relaxation and stress recovery curves. The response to sinusoidal forcing was characterized by the effective resistance (Re) and elastance (EL). Re decreased from 2.48 +/- 0.41 cmH2O.ml-1.s at 0.01 Hz to 0.18 +/- 0.03 cmH2O.ml-1.s at 0.5 Hz, and EL increased from 0.99 +/- 0.10 to 1.26 +/- 0.20 cmH2O/ml on the same frequency range. These data were analyzed with the frequency-domain version of the same model, complemented by a Newtonian resistance (R) to account for airway resistance: Re = R + B/ (9.2f) and EL = A + 0.25B + B . log 2 pi f, where f is the frequency.(ABSTRACT TRUNCATED AT 250 WORDS)     

Partitioning of airway and respiratory tissue mechanical impedances by body plethysmography

Peslin, R., and C. Duvivier. Partitioning of airway andrespiratory tissue mechanical impedances by body plethysmography. J. Appl. Physiol. 84(2): 553-561, 1998.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 bybody plethysmography (Vpl) during pressure oscillations at the airwayopening. The forced oscillation setup was placed inside a bodyplethysmograph, and the subjects rebreathedBTPS gas. Zrs,in and the relationship between Vpl and airway flow (Hpl) were measured from 4 to 29 Hz. Zawand Zti were computed from Zrs,in and Hpl by using the monoalveolar T-network model and alveolar gas compliance derived from thoracic gasvolume. The data were in good agreement with previous observations: airway 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 · l¹and tissue reactance with an elastance of 36 ± 8 hPa/l. The changes seen with varying lung volume, during elastic loading of the chest andduring bronchoconstriction, were mostly in agreement with the expectedeffects. The data, as well as computer simulation, suggest that thepartitioning is unaffected by mechanical inhomogeneity and onlymoderately affected by airway wall shunting.

     

Density dependence of respiratory input and transfer impedances in humans

Total respiratory input (Zin) and transfer (Ztr) impedances were obtained from 4 to 30 Hz in 10 healthy subjects breathing air and He-O2. Zin was measured by applying pressure oscillations around the head to minimize the upper airway shunt and Ztr by applying pressure oscillations around the chest. Ztr was analyzed with a six-coefficient model featuring airways resistance (Raw) and inertance (Iaw), alveolar gas compressibility, and tissue resistance, inertance, and compliance. Breathing He-O2 significantly decreased Raw (1.35 +/- 0.32 vs. 1.74 +/- 0.49 cmH2O.l-1.s in air, P less than 0.01) and Iaw (0.59 +/- 0.33 vs. 1.90 +/- 0.44 x 10(-2) cmH2O.l-1.s2), but, as expected, it did not change the tissue coefficients significantly. Airways impedance was also separately computed by combining Zin and Ztr data. This approach demonstrated similar variations in Raw and Iaw with the lighter gas mixture. With both analyses, however, the changes in Iaw were more than what was expected from the change in density. This indicates that factors other than gas inertance are included in Iaw and reveals the short-comings of the six-coefficient model to interpret impedance data.     

Evaluation of phase correction and low gas density to improve thoracic gas volume measurement

We investigated two methods of decreasing the error on plethysmographic determinations of thoracic gas volume (TGV) related to cheeks movements during panting maneuvers: lowering gas density in the airways with an 80% He-20% O2 mixture and computing TGV from the in-phase component of the plethysmographic signal (TGVr). The methods were tested by measuring how TGV estimates varied when panting frequency was raised from 0.8 to 2.5 Hz during the same occlusion. The measurements were performed in 6 normal subjects and 12 patients with chronic bronchitis with and without cheeks support and when the airway was connected to an external device simulating an increased cheeks compliance. A small negative frequency dependence of TGV (delta TGV/delta f = -1.2 +/- 0.8%/Hz with cheeks support), most probably unrelated to upper airway walls, was found in normal subjects. Delta TGV/delta f was positive and algebraically larger in patients than in normals, reaching 2.2 +/- 3.4%/Hz without cheeks support and 11.8 +/- 8.0%/Hz with the additional cheeks. The latter value was only 20% smaller when computed on the basis of TGVr, demonstrating the limited usefulness of the phase-based correction. In contrast, breathing He-O2 decreased delta TGV/delta f to approximately 50% of its air value (P less than 0.01) and appears as an effective way to diminish the error in obstructive patients.