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D Navajas

Publications and source records attributed to D Navajas.

At least 73 records · Page 4Linked to original sources
PubMed1995

Servocontrolled generator to measure respiratory impedance from 0.25 to 26 Hz in ventilated patients at different PEEP levels.

Assessing respiratory impedance (Zrs) in ventilated patients over a wide frequency band, ranging from breathing rates to typical forced oscillation frequencies, during end-expiratory pauses at different positive end-expiratory pressures (PEEP) is of potential interest to assess a patient's respiratory mechanics. Zrs measurements under these conditions are not possible with the present variants of the forced oscillation technique. The aim of this work was to design a forced oscillation generator operating from spontaneous breathing frequencies whilst withstanding PEEP. To this end, we constructed a generator based on a servocontrolled loudspeaker. This allowed the loudspeaker cone to remain at its resting position regardless of the external PEEP applied. The system was optimized by using a mechanical analogue. The clinical applicability of the servocontrolled generator was assessed by measuring Zrs in mechanically-ventilated chronic obstructive pulmonary disease (COPD) patients during end-expiratory pauses at different transrespiratory pressures. The forced oscillation generator designed may be easily applicable in practice since it is small and light. The system is able to withstand transrespiratory pressures of up to 17 hPa and allows the application of forced oscillation of sufficient amplitude ( > 2 hPa peak-to-peak, 0.25-26 Hz) to obtain reliable respiratory resistance and reactance data. The servocontrolled generator permits the assessment of respiratory mechanics over a wide frequency band ranging from breathing frequencies to the most typical forced oscillation frequencies during end-expiratory pauses at PEEPs within the conventional range.

Equipment Design↗
PubMed1994

Effects of halothane and isoflurane on ventilation and occlusion pressure.

BACKGROUND: Isoflurane has been said to be more ventilatory depressant than halothane. However, data for comparing the respiratory effects of halothane and isoflurane in humans are insufficient at this time. The aim of this study was to extend our understanding of the nature of the central, as opposed to peripheral, ventilatory effect of halothane and isoflurane by comparing them at two concentrations. METHODS: Twenty patients were randomly assigned to receive halothane (n = 10) or isoflurane (n = 10). The patients were studied the day before surgery and during anesthesia immediately before surgery. Ventilatory effects were analyzed in terms of breathing pattern, end-tidal carbon dioxide pressure (PETCO2) and inspiratory occlusion pressure. After anesthetic induction and orotracheal intubation with thiopental and succinylcholine patients were allowed to breathe halothane or isoflurane in oxygen spontaneously at 1.2 (low) and 2.0 (high) minimum alveolar concentration (MAC) applied in random order. Inspiratory active impedance during anesthesia was also measured. RESULTS: Significant reduction of minute ventilation between awake and low MAC states was observed for isoflurane (-34.4%; P < 0.001) but not for halothane. Inspiratory occlusion pressure at 100 ms increased significantly between awake and low MAC states, from 1.43 +/- 0.89 to 2.67 +/- 1.05 cmH2O (P < 0.05) for halothane, representing an 87% increase, whereas a nonsignificant increase (16%) was observed for isoflurane. Both anesthetics showed a dose-related ventilatory depressant effect, not attributable to changes in mechanical properties, reflected by significant reductions in minute ventilation (P < 0.001), tidal volume (P < 0.001), and inspiratory occlusion pressure at 100 ms (P < 0.05) and increases in respiratory rate (P < 0.001) and end-tidal carbon dioxide pressure (P < 0.01) when concentration was increased. However, at the higher concentration a significantly greater reduction of minute ventilation (P < 0.01) was observed for isoflurane (-25.6%) than for halothane (-9.4%). We did not observe differences in respiratory rate between the two anesthetics. Significant differences in inspiratory occlusion pressure wave were observed, characterized by a concave-upward tendency for isoflurane and for high concentration. CONCLUSIONS: Our study confirms the stronger ventilatory depression induced by isoflurane compared with that induced by halothane and indicates that halothane at 1.2 MAC induces significantly less ventilatory depression than expected.

Adult↗
PubMed1994

Human lung impedance from spontaneous breathing frequencies to 32 Hz.

Lung impedance (ZL) was measured from 0.1875 to 32 Hz in spontaneously breathing healthy subjects by spectral analysis of the pressure and flow signals generated simultaneously by the muscular generator of breathing and by a forced oscillation system. This method did not require cooperation from the subject to perform panting or special ventilatory maneuvers and therefore allowed us to analyze the frequency dependence of lung resistance, reactance, and elastance (-2 pi.frequency.reactance) at the physiological conditions of normal breathing. Resistance and elastance parameters were also computed by multiple linear regression of the time-domain pressure and flow data on a simple resistance-elastance model. Resistances and elastances computed at the breathing frequency by spectral analysis and by multiple linear regression were similar (nonsignificant differences < 4 and 10%, respectively). The results obtained when comparing ZL from the breathing component (0.1875-0.75 Hz) of the recorded signals and from the forced oscillation component (2-32 Hz) were fairly consistent. ZL (0.1875-10 Hz) was interpreted in terms of a model consisting of an airway compartment, including a resistance and an inertance, in series with a viscoelastic tissue compartment (J. Hildebrandt. J. Appl. Physiol. 28: 365-372, 1970) characterized by two parameters. The model analysis provided parameter values (resistance 2.49 +/- 0.58 hPa.l-1.s, inertance 1.70 +/- 0.29 Pa.l-1.s2, Hildebrandt parameters 4.87 +/- 2.28 and 0.73 +/- 0.99 hPa/l) consistent with the hypothesis that lung tissue in healthy humans during spontaneous breathing behaves as a viscoelastic structure with a hysteresivity of approximately 0.10.

Adult↗
PubMed1994

Influence of site of tracheal pressure measurement on in situ estimation of endotracheal tube resistance.

In situ measurement of distal tracheal pressure (Ptr) via an intraluminal side-hole catheter (IC) has been used to determine endotracheal tube (Rett) and intrinsic patient (Rpt) resistances in intubated subjects. Because of differences in cross-sectional area between the endotracheal tube (ETT) and trachea, fluid dynamic principles predict that IC position should critically influence these results. Accordingly, the aim of this study was to determine the effect of IC position on Rett. Ptr was recorded in vitro through an IC from 2 cm inside, at the tip of, or 2 cm outside an ETT (7, 8, and 9 mm ID) situated within an artificial trachea (13, 18, and 22 mm ID). A reference value of Rett was also obtained. Results were unaffected by IC position during inspiration, overestimating Rett by 7.9 +/- 0.7% (SE). In contrast, during expiration, Rett fell as IC position changed from outside to inside the ETT and was underestimated by 41.3 +/- 3.6% with Ptr recorded inside the ETT. Varying ETT or tracheal size had little effect on the relative error in Rett. The IC itself did increase Rett due to a reduction in effective cross-sectional area, the change varying directly with IC size and inversely with ETT caliber. In vivo values in 11 intubated patients were comparable to in vitro results. In summary, IC position and size can have important consequences on in situ measurements of Ptr and should be considered when clinically monitoring Rett or Rpt.

Aged↗
PubMed1993

Validity of the esophageal balloon technique at high frequencies.

The reliability of the esophageal balloon technique in measuring high-frequency changes in pleural pressure (Ppl) was investigated in six normal subjects by studying the amplitude ratio (A) and phase angle (phi) of esophageal (Pes) and mouth (Pm) pressures during airway occlusion and while pseudorandom pressure variations (2-32 Hz) were applied to the chest. The measurements were made with a common esophageal balloon-catheter system connected to a high-impedance piezoresistive transducer. When the cheeks were firmly supported, A averaged 1.08 +/- 0.063 at 2 Hz and 1.06 +/- 0.11 at 32 Hz. Pes increasingly led Pm with increasing frequency, and phi averaged 20.8 +/- 4.0 degrees at 32 Hz. Washing the airways with 80% He-20% O2 reduced phi by 50%. When the cheeks were not supported, A exhibited a strong positive frequency dependence, averaging 1.71 +/- 0.34 at 32 Hz, whereas phi increased much faster below 20 Hz and tended to decrease afterward. Because the esophageal transfer function Pes/Ppl = (Pes/Pm)/(Ppl/Pm), we could estimate Pes/Ppl by computing for individual subjects the pressure difference between the pleura and the mouth based on the lung and upper airway wall properties that were measured separately. The results suggest that the ratio of Pes and Ppl remains close to unity from 2 to 32 Hz, but Pes lags slightly behind Ppl (phi equals about -7 degrees at 32 Hz).

Adult↗
PubMed1993

Respiratory input impedance up to 256 Hz in healthy humans breathing foreign gases.

Currently available data concerning respiratory input impedance (Zrs) at frequencies up to 300 Hz indicate that Zrs is determined mainly by the airways and, in particular, the gas compressibility in the airways and the airway wall compliance. Hence, measurements of Zrs when breathing gases with different physical properties would be useful in investigating airway mechanics and the role of acoustic propagation. Zrs measured with a standard generator (Zst) and corrected for the upper airway shunt (Zrs*) were measured in nine healthy subjects breathing air or a gas mixture consisting of 20% O2 and 80% He or SF6. The frequency band was extended up to 256 Hz for air and He-O2 and up to 128 Hz for SF6-O2. Zrs exhibited a similar pattern for the three gases, with a shift toward low frequencies as the gas density increased. Moreover, the resonance peaks tended to be narrower and higher as the gas density increased. The second frequency of resonance for He-O2, air, and SF6-O2 were 220, 180, and 50 Hz, respectively, for Zrs* and were systematically higher for Zst. Zrs* and Zst data were interpreted in terms of a tricompartmental model that partitioned the airways into two segments: a central one featuring the acoustic propagation in the airways and a peripheral one that included bronchial wall elasticity (Farré et al. J. Appl. Physiol. 67: 1973-1981, 1989). The model was able to interpret the gas dependence of Zrs* but not that of Zst. The influence of the gas physical properties on both Zrs* and Zst confirms that total Zrs at high frequencies is basically that of the airways and that the second resonance is related mainly to the gas compressibility in the airways.

Adult↗
PubMed1992

Optimized estimation of respiratory impedance by signal averaging in the time domain.

The spontaneous breathing of a subject during measurements of respiratory impedance (Zrs) by the forced oscillation technique (FOT) induces errors that result in biased impedance estimates, especially at low frequencies. Although in standard measurements this bias may be avoided by using special impedance estimators, there are two applications of FOT for which such estimators are not useful: when a head generator is used and when measurements are made during intubation. In this paper we describe a data-processing procedure for unbiased impedance estimation for all FOT setups. The proposed estimator (Z) was devised for pseudorandom excitation and is based on time-domain signal averaging before frequency analysis. The performance of estimator Z was first analyzed by computer simulation of a head generator setup and a setup including an endotracheal tube to measure (2-32 Hz) a resistance-inertance-elastance model mimicking Zrs of a healthy subject. Second, Z was assessed during real measurements in 16 healthy subjects. The results obtained in the simulation (e.g., error in elastance was reduced from 15.6% with most conventional estimators to 3.3% with Z in simulation of head generator setup) and in the measurements in subjects (differences of less than 1.6% between Z and a reference) confirmed the theoretical lack of bias of Z and its practical suitability for the different FOT setups. In addition to its applicability in the situations in which no other unbiased estimators are available, estimator Z is also advantageous in most conventional applications of FOT, since it requires much less computing time and thus allows on-line Zrs measurements.

Adolescent↗
PubMed1992

Dynamic response of the isolated passive rat diaphragm strip.

To further our understanding of the mechanisms underlying chest wall mechanics, we investigated the dynamic response of the isolated passive rat diaphragm strip. Stress adaptation of the tissue was measured from 0.05 to 60 s after subjecting the strips to strain steps of normalized strain amplitudes from 0.005 to 0.04. The tissue resistance (R), elastance (E), and hysteresivity (eta) were measured in the same range of amplitudes by sinusoidally straining the strip at frequencies from 0.03125 to 10 Hz. The stress (T) depended exponentially on the strain (epsilon) and relaxed and recovered linearly with the logarithm of time. E increased linearly with the logarithm of frequency and decreased with increasing amplitude. R fell hyperbolically with frequency and showed an amplitude dependence similar to that of E. To interpret the strong nonlinear behavior, we extended the viscoelastic model of Hildebrandt (J. Appl. Physiol. 28: 365-372, 1970) to include an exponential stress-strain relationship. Accordingly, the step response was described by T - Tr = Tr(e alpha delta epsilon - 1)(1 - gamma log t), where delta epsilon is the strain amplitude, Tr is the initial operating stress, alpha is a measure of the stress-strain nonlinearity, and gamma is the rate of stress adaptation. The oscillatory response of the model was computed by applying Fung's quasi-linear viscoelastic theory. This quasi-linear viscoelastic model fitted the step and oscillatory data fairly well but only if alpha depended negatively on delta epsilon, as might be expected in a plastic material.

Animals↗
PubMed1991

Time-domain digital filter to improve signal-to-noise ratio in respiratory impedance measurements.

The mechanical impedance of the respiratory system Zrs is usually measured by forced excitation while the patient breathes spontaneously. Pressure and flow signals due to breathing contaminate the excitation signals, leading to a poor signal-to-noise ratio (SNR) and thus to errors in impedance estimation, especially at low frequencies (up to 8 Hz). To enhance SNR in the recorded signals we designed an infinite impulse response digital filter for the frequent case in which the excitation is pseudorandom. The algorithm is based on narrowband second-order bandpass elements centred at the excitation frequencies. The performance of the filter was assessed in a simulation study by superposing forced excitation signals (2-32 Hz) from a reference model and the signals of breathing recorded from 16 subjects. When compared with a conventional high-pass filtering, the devised filtering resulted in an increase in SNR which was almost constant over the whole frequency band: 6.30 +/- 0.98 dB (mean +/- SD). This improvement in SNR was reflected in an increase in the number of subjects for which the corresponding coherence y2 attained a value greater than the conventional threshold of acceptability (y2 = 0.95). At the lowest frequency (2 Hz) only two (12.5 per cent) simulated subjects had y2 greater than or equal to 0.95 with the conventional high-pass filtering. By contrast, when using the devised comb filter the number of subjects with y2 greater than or equal to 0.95 increased up to 13 (81 per cent). The results obtained suggest that this filter may be useful to improve SNR and thus Zrs estimation.

Airway Resistance↗
PubMed1991

Ventilation-perfusion mismatch after methacholine challenge in patients with mild bronchial asthma.

To investigate the effects of methacholine (MTH) challenge on spirometry, lung mechanics, respiratory gases, and ventilation-perfusion (VA/Q) distributions, 16 subjects 16 to 58 yr of age with stable mild asthma (FEV1, 92 +/- 5% [SEM] predicted; FEF25-75, 71 +/- 7% predicted; respiratory system resistance (Rrs) at 4 Hz, 4.6 +/- 0.4 cm H2O/L-1 s; PaO2, 88 +/- 3 mm Hg; AaPO2, 23 +/- 3 mm Hg) were recruited. Baseline VA/Q distributions were unimodal and relatively narrow in 12 patients and modestly bimodal in the other four. The dispersion of pulmonary blood flow (log SD Q) was slightly enlarged (0.71 +/- 0.09) and that of ventilation (log SD V) was normal (0.57 +/- 0.04) (normal range, 0.3 to 0.6); an index of overall VA/Q heterogeneity (DISP R-E*) was also mildly abnormal (5.3 +/- 0.8) (normal values less than 3.0). After MTH challenge, FEV1, FEF25-75, and PaO2 fell (to 62 +/- 3 and 35 +/- 3% predicted, and to 71 +/- 1 mm Hg, respectively), whereas Rrs (p less than 0.001 each), minute ventilation (p less than 0.02), heart rate (p less than 0.01), and AaPO2 increased (p less than 0.001). VA/Q relationships mildly to moderately worsened (log SD Q increased to 0.98 +/- 0.04 [p less than 0.01], log SD V to 0.79 +/- 0.04, and DISP R-E* to 9.8 +/- 0.6 [p less than 0.001 each]). Qualitatively, the pattern of blood flow distribution was broadly unimodal in 13 patients and modestly bimodal in three, of whom only one had a bimodal baseline distribution.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1991

Active inspiratory impedance and neuromuscular respiratory output during halothane anaesthesia in humans.

The aim of this study was to measure, in 11 patients with healthy lungs, active inspiratory impedance during anaesthesia. In addition, we recorded changes in inspiratory occlusion pressure at 100 ms (P0.1) and ventilatory pattern while awake and during anaesthesia with a mean inspiratory fraction (FI) of 0.017 halothane in O2. The total active inspiratory resistance and elastance values were 5.4 +/- 3.3 hPa.l.1.s and 29.9 +/- 6.2 hPa.l.1, respectively. P0.1 and the ratio between P0.1 and mean inspiratory flow (P0.1/(VT/TI)) increased 124% (p less than 0.001) and 68% (p less than 0.001), respectively, during anaesthesia. Respiratory frequency rose significantly from 12.2 +/- 1.5 (mean +/- SD) to 24.6 +/- 4.6 cycles.min-1, while tidal volume and inspiratory duty cycle lowered significantly from 0.599 +/- 0.195 l and 0.44 +/- 0.04 to 0.372 +/- 0.088 l (p less than 0.001) and 0.40 +/- 0.04 (p less than 0.05), respectively. Minute ventilation (VE) and VT/TI did not change significantly. During halothane anaesthesia with an FI:0.017, the increase in neuromuscular respiratory output appears to compensate for the increased mechanical load, thus resulting in maintenance of VE at levels similar to those of an awake state.

Adult↗
PubMed1990

Respiratory input impedance in anesthetized paralyzed patients.

Respiratory impedance (Zrs) was measured between 0.25 and 32 Hz in seven anesthetized and paralyzed patients by applying forced oscillation of low amplitude at the inlet of the endotracheal tube. Effective respiratory resistance (Rrs; in cmH2O.l-1.s) fell sharply from 6.2 +/- 2.1 (SD) at 0.25 Hz to 2.3 +/- 0.6 at 2 Hz. From then on, Rrs decreased slightly with frequency down to 1.5 +/- 0.5 at 32 Hz. Respiratory reactance (Xrs; in cmH2O.l-1.s) was -22.2 +/- 5.9 at 0.25 Hz and reached zero at approximately 14 Hz and 2.3 +/- 0.8 at 32 Hz. Effective respiratory elastance (Ers = -2pi x frequency x Xrs; in cmH2O/1) was 34.8 +/- 9.2 at 0.25 Hz and increased markedly with frequency up to 44.2 +/- 8.6 at 2 Hz. We interpreted Zrs data in terms of a T network mechanical model. We represented the proximal branch by central airway resistance and inertance. The shunt pathway accounted for bronchial distensibility and alveolar gas compressibility. The distal branch included a Newtonian resistance component for tissues and peripheral airways and a viscoelastic component for tissues. When the viscoelastic component was represented by a Kelvin body as in the model of Bates et al. (J. Appl. Physiol. 61: 873-880, 1986), a good fit was obtained over the entire frequency range, and reasonable values of parameters were estimated. The strong frequency dependence of Rrs and Ers observed below 2 Hz in our anesthetized paralyzed patients could be mainly interpreted in terms of tissue viscoelasticity. Nevertheless, the high Ers we found with low volume excursions suggests that tissues also exhibit plasticlike properties.

Adult↗
PubMed1989

Analysis of the dynamic characteristics of pressure transducers for studying respiratory mechanics at high frequencies.

Differential pressure transducers are commonly used to study respiratory mechanics at physiological frequencies as well as during external forcing at high frequencies. In the latter condition, measuring errors could occur if the input impedance of the pressure transducers is not sufficiently large with respect to that of the respiratory system. In this work we analysed the input impedance Z and the transfer function H of two common pressure transducers (Validyne MP-45 and Celesco LCVR) equipped with membranes of different sensitivities and with connecting tubes of different lengths. Z was measured by the tube method and H was measured by comparison with a flat-response pressure transducer. In agreement with the predictions based on a simple lumped-parameters model, we found that Z reached very low values, especially at the frequencies where H had a resonance peak. For instance, for the widespread Validyne MP-45 transducer (200 Pa) with connecting tubes of 16 cm length and 3.8 mm internal diameter a minimum of Z of 8300 Pa s litre-1 at 96 Hz was measured; at that frequency the amplitude of H attained a value of 3.1. Using the above transducer model we simulated the measurement of a rat input impedance up to 128 Hz using Validyne and Celesco transducers. With the Validyne MP-45 (200 Pa), equipped with the same connecting tubes as above, the computed error reached up to 50 per cent for the real part and 140 per cent for the imaginary part.

Animals↗
PubMed1989

A least squares algorithm to determine the mechanical time constant distribution of the lung during forced expiration.

A method to determine the mechanical time-constant distribution of the lung during a forced expiration manoeuvre is proposed. The method is based on a least squares algorithm constrained to give reasonably smooth non-negative solutions. The smoothing constraint was imposed by minimizing the second derivative of the distribution function in accordance with the physiological meaning of the time-constant distribution. Nevertheless, the obtained solution depends greatly on the relative weights of the two terms in the objective function to be minimized i.e., the error on the fit of the volume signal and the smoothness of the distribution function. To select the optimum smoothing weight, a criterion based on the stability of the reconstructed distribution shape was defined. The performance of the algorithm and that of the defined criterion were evaluated by using simulated signals of forced expired volume. The error of reconstructed distributions was quantified by means of the area enclosed between this distribution and the original one used to generate the simulated volume signal. The results obtained showed that for all the analyzed signals: (1) There is a value of the weight of the smoothing constraint which gives rise to a solution that is optimum in a least squares sense. (2) The proposed stabilization criterion enables us to approach this optimum solution from experimental signals.

Algorithms↗
PubMed1989

Respiratory input impedance during high frequency oscillatory ventilation.

Total respiratory input impedance (Zrs) measured by forced excitation may be computed easily from pressure and flow measurements recorded at the airway opening. The purpose of this paper was to analyse how the information provided by Zrs may be used for monitoring ventilatory mechanics during high frequency oscillatory ventilation (HFOV). We measured impedance (0.125-32 Hz) in six dogs, and in four dogs after infusion of histamine. We interpreted Zrs data in terms of a linear resistance-inertance-elastance (R-I-E) model to estimate the pressure decrease in the airways and the pressure amplitude in the alveolar region. We modelled airways non-linearities and analysed their effect at high flow oscillation amplitudes. We concluded that Zrs measurements may be useful to monitor ventilatory mechanics and to determine the optimum settings of the ventilator during HFOV.

Animals↗
PubMed1989

A correction procedure for the asymmetry of differential pressure transducers in respiratory impedance measurements.

The usual setup for measuring respiratory input impedance requires a differential pressure transducer attached to a pneumotachograph. As, up to now, no data correction procedure has been devised to account for transducer asymmetry, a highly symmetrical transducer is required to obtain reliable impedance data. In this communication, a general model for the measuring system is presented. Its main feature is that differential pressure transducers are modeled as two input-one output systems. From the theoretical model, we defined a dynamic calibration and data correction procedure. This was tested using highly asymmetrical transducers (common-mode rejection ratio between 45 and 27 dB) to measure the impedance of two respiratory analogs. The latter were linear resistance (R), inertance (I), compliance (C) series models simulating a normal subject (R = 3.47 hPa.s.l-1, I = 1.45 Pa.s2.l-1, C = 18.6 ml.hPa-1) and an obstructive patient (R = 11.15 hPa.s.l-1, I = 1.28 Pa.s2.l-1, C = 18.5 ml.hPa-1). Results obtained applying the devised procedure (errors in R, I, and C always less than 4 percent) show that respiratory input impedance can be adequately measured if data are corrected for transducer asymmetry.

Airway Resistance↗
PubMed1989

Human respiratory impedance from 8 to 256 Hz corrected for upper airway shunt.

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.

Adult↗