Biomedical subjects

K R Lutchen

Publications and source records attributed to K R Lutchen.

At least 37 records · Page 2Linked to original sources

PubMed1997

Partitioning airway and lung tissue resistances in humans: effects of bronchoconstriction.

The contribution of airway resistance (Raw) and tissue resistance (Rti) to total lung resistance (RL) during breathing in humans is poorly understood. We have recently developed a method for separating Raw and Rti from measurements of RL and lung elastance (EL) alone. In nine healthy, awake subjects, we applied a broad-band optimal ventilator waveform (OVW) with energy between 0.156 and 8.1 Hz that simultaneously provides tidal ventilation. In four of the subjects, data were acquired before and during a methacholine (MCh)-bronchoconstricted challenge. The RL and EL data were first analyzed by using a model with a homogeneous airway compartment leading to a viscoelastic tissue compartment consisting of tissue damping and elastance parameters. Our OVW-based estimates of Raw correlated well with estimates obtained by using standard plethysmography and were responsive to MCh-induced bronchoconstriction. Our data suggest that Rti comprises approximately 40% of total RL at typical breathing frequencies, which corresponds to approximately 60% of intrathoracic RL. During mild MCh-induced bronchoconstriction, Raw accounts for most of the increase in RL. At high doses of MCh, there was a substantial increase in RL at all frequencies and in EL at higher frequencies. Our analysis showed that both Raw and Rti increase, but most of the increase is due to Raw. The data also suggest that widespread peripheral constriction causes airway wall shunting to produce additional frequency dependence in EL.

Adult↗

PubMed1997

Relationship between heterogeneous changes in airway morphometry and lung resistance and elastance.

We present a dog lung model to predict the relation between inhomogeneous changes in airway morphometry and lung resistance (RL) and elastance (EL) for frequencies surrounding typical breathing rates. The RL and EL were sensitive in distinct ways to two forms of peripheral constriction. First, when there is a large and homogeneous constriction, the RL increases uniformly over the frequency range. The EL is rather unaffected below 1 Hz but then increases with frequencies up to 5 Hz. This increase is caused by central airway wall shunting. Second, the RL and EL are extremely sensitive to mild inhomogeneous constriction in which a few highly constricted or nearly closed airways occur randomly throughout the periphery. This results in extreme increases in the levels and frequency dependence of RL and EL but predominantly at typical breathing rates (<1 Hz). Conversely, the RL and EL are insensitive to highly inhomogeneous airway constriction that does not produce any nearly closed airways. Similarly, alterations in the RL and EL due to central airway wall shunting are not likely until the preponderance of the periphery constricts substantially. The RL and EL spectra are far more sensitive to these two forms of peripheral constriction than to constriction conditions known to occur in the central airways. On the basis of these simulations, we derived a set of qualitative criteria to infer airway constriction conditions from RL and EL spectra.

Airway Resistance↗

PubMed1996

Interpretation of respiratory input impedance in healthy infants.

Respiratory input impedance (Zin) is a potentially informative test of pulmonary function in infants who are unable to perform standard tests commonly performed in children and adults Analysis of Zin in dogs using the six-element model of DuBois et al. (J Appl Physiol 8:587, 1956) provides estimates of airways resistance separate from tissue resistance, as well as an estimate of thoracic gas volume. However, reliable estimates of these parameters can only be obtained when Zin displays a distinct antiresonance that is associated with the tissue inertance and alveolar gas compression compliance. To determine whether infants have such an antiresonance. Zin was measured in nine healthy infants (4 < f < 160 Hz). An antiresonance was found at 112.8-10.4 Hz, and the six-element model fit these data well, but the resulting parameters were physiologically unrealistic. We hypothesized that the antiresonance in the measured Zin is the result of shunt compliance proximal to alveolar gas compression compliance. Gas compression in the face mask and nonrigid upper airway walls could provide such a shunt compliance. We investigated another model with four parameters, a single shunt compliance (Cim) representing gas compression in the face mask in parallel with the infant's total respiratory resistance (Rrs) inertance (Irs), and compliance (Crs). This model fits the data well, and the estimated R, (19.3, 4.2 cmH O/L/s) was physiologically reasonable. However, Crs (Crs 1.03-0.58 mL cmH2O) was one order of magnitude smaller than reported Crs. The value for Cim was slightly larger than that based on the estimated volume of gas in the face mask, suggesting an additional influence of upper airway wall shunting. Computer simulations using a model that includes the face mask and upper airway walls confirmed that Cim and the upper airway wall properties significantly influence Zin data over this frequency range. Nevertheless, these simulations suggest that the Rrs estimated from the four-element model is related to airway resistance.

Airway Resistance↗

PubMed1996

How inhomogeneities and airway walls affect frequency dependence and separation of airway and tissue properties.

It has been proposed that during mild-to-moderate bronchoconstriction one can partition airway and tissue properties on the basis of input impedance (Zin) acquired from 0.1 to 5 Hz (K.R. Lutchen, B. Suki, Q. Zhang, F. Peták, B. Daróczy, and Z. Hantos. J. Appl. Physiol. 77: 373-385, 1994). The approach is to apply a homogeneous lung model that contains airway resistance and viscoelastic tissue damping and elastance parameters. The tissue parameters account for the frequency dependence in lung resistance (RL) and elastance (EL). We present an anatomically consistent asymmetrically branching airway model to address two key questions: 1) How will lung inhomogeneities, airway wall shunting, and tissue viscoelasticity contribute to increased frequency dependence and levels of RL and EL during lung constriction? and 2) How much can lung inhomogeneities and airway wall shunting contribute to our assessment of airway, tissue, and overall lung properties derived from Zin? The model incorporates nonrigid airway walls and allows for explicit control over the type and degree of inhomogeneous airway constriction or tissue changes. Our results indicate that, from 0.1 to 5 Hz, airway wall shunting does not become important unless the entire lung periphery experiences significant constriction. Mild-to-moderate inhomogeneous peripheral airway constriction produces a relatively minor additional frequency dependence in RL and EL beyond that due to the tissues alone. With more extreme constriction, however, there is a marked frequency-dependent increase in EL. This phenomenon may render it impossible to distinguish from a single frequency measurement whether an increase in EL during bronchoconstriction is a consequence of a true increase in tissue stiffening or simply a consequence of airway phenomena. Finally, Zin from 0.1 to 5 Hz can be used to provide a reasonable separation of airway and tissue properties for mild-to-moderate homogeneous or inhomogeneous lung constriction. However, during more severe disease, inhomogeneities and/or wall shunting will produce substantial overestimation of tissue damping and hysteretic properties. In fact, the only reliable indicator of a real change in the tissues may be a change in the estimate of tissue elastance that is based on data extending to a sufficiently low frequency.

Airway Resistance↗

PubMed1996

Airway inhomogeneities contribute to apparent lung tissue mechanics during constriction.

Recent studies have suggested that part of the measured increase in lung tissue resistance after bronchoconstriction is an artifact due to increased airway inhomogeneities. To resolve this issue, we measured lung impedance (ZL) in seven open-chest rats with the lungs equilibrated on room air and then on a mixture of neon and oxygen (NeOx). The rats were placed in a body box with the tracheal tube leading through the box wall. A broadband flow signal was delivered to the box. The signal contained seven oscillation frequencies in the 0.234- to 12.07-Hz range, which were combined to produce tidal ventilation. The ZL was measured before and after bronchoconstriction caused by infusion of methacholine (MCh). Partitioning of airway and tissue properties was achieved by fitting ZL with a model including airway resistance (Raw), airway inertance, tissue damping (G), and tissue elastance (H). We hypothesized that if the inhomogeneities were not significant, the apparent tissue properties would be independent of the resident gas, whereas Raw would scale as the ratio of viscosities. Indeed, during control conditions, the NeOx-to-air ratios for G and H were both 1.03 +/- 0.04. Also, there was a small increase in lung elastance (EL) between 0.234 and 4 Hz that was similar on air and NeOx. During MCh infusion, Raw and G increased markedly (45-65%), but the increase in H was relatively small ( < 13%). The NeOx-to-air Raw and H ratios remained the same. However, the NeOx-to-air G ratio increased to 1.19 +/- 0.07 (P < 0.01) and the increase in EL with frequency was now marked and dependent on the resident gas. These results provide direct evidence that for a healthy rat lung airway inhomogeneities do not significantly influence the lung resistance or EL vs. frequency data. However, during MCh-induced constriction, a large portion of the increase in tissue resistance and the altered frequency dependence of EL are virtual and a consequence of the augmented airway inhomogeneities.

Airway Resistance↗

PubMed1995

Application of linear and nonlinear time series modeling to heart rate dynamics analysis.

The linear autoregressive (AR) model is often used to investigate the pathophysiologic mechanisms controlling heart rate (HR) dynamics. This study implemented parametric models new to this field to determine if a more appropriate HR dynamics modeling structure exists. The linear AR and autoregressive-moving average (ARMA) models, and the nonlinear polynomial autoregressive (PAR) and bilinear (BL) models were fit to instantaneous HR time series obtained from nine subjects in the supine position. Model orders were determined by the Akaike Information Criteria (AIC). Model residual variance was used as the primary intermodel comparison criterion, with significance evaluated by a chi 2 distributed statistic. The BL model best represented the HR dynamics, as its residual variance was significantly (p < 0.05) smaller than that of the corresponding AR model for nine out of nine data sets. In all cases, the BL model had a smaller residual variance than either the ARMA or PAR models. The bilinear model was ineffective at data forecasting, however, we show that this cannot reflect BL model validity because poor prediction is inherent to the BL model structure. The apparent superiority of the nonlinear bilinear model suggests that future heart rate dynamics studies should put greater emphasis on nonlinear analyses.

Adult↗

PubMed1995

Relationship between frequency and amplitude dependence in the lung: a nonlinear block-structured modeling approach.

During lung constriction, there is an increase in both the frequency and tidal volume (VT) dependences of lung tissue resistance (Rti) and elastance (Eti). This suggests that 1) significant alterations take place in the mechanisms contributing to both the linear and nonlinear characteristics of lung tissues; and 2) the frequency and VT dependences of Rti and Eti are coupled. We examined these issues for the case of sine wave and special pseudorandom inputs by utilizing the theory of nonlinear block-structured systems. Two basic model structures were considered: the Hammerstein and the Wiener structures. The Hammerstein structure is a cascade connection of a nonlinear zero-memory (N) system and a linear dynamic process (L). This structure predicts that frequency and VT dependences of Rti and Eti are decoupled. The Wiener structure is an inverse cascade of these two blocks (i.e., L-N) in which the frequency and VT dependences of Rti and Eti are coupled. These two structures were combined with a nonlinear airway compartment and fitted to measured airway opening and alveolar capsule pressure-flow time domain data in dogs before and after histamine-induced constriction. The best lung model was a linear airway compartment combined with a Wiener structure consisting of a constant-phase linear tissue impedance in cascade with a polynomial nonlinearity, suggesting that frequency and VT dependences of Rti and Eti are indeed coupled during control and constricted conditions. Moreover, histamine caused much larger changes in the linear tissue parameters than in the nonlinear coefficients.(ABSTRACT TRUNCATED AT 250 WORDS)

Air Pressure↗

PubMed1995

Partitioning of airway and lung tissue properties: comparison of in situ and open-chest conditions.

The purpose of this study was to investigate under physiological breathing conditions whether airway and lung tissue properties are different in situ and in open-chest conditions. We measured lung input impedance in dogs from 0.2 to 8 Hz with an optimal ventilator waveform at four tidal volumes (VT; from 75 to 450 ml) in intact animals using an esophageal balloon as well as after opening the chest. The lung impedance from both conditions was partitioned into airway and tissue compartments as characterized by airway resistance and inertance (Iaw) and tissue damping (G) and elastance (H) parameters respectively. All parameters except Iaw depended to some extent on VT. The in situ tissue G and H slightly decreased with VT while in the open-chest condition; G decreased and H increased slightly with VT. We found small but significant differences between the mechanical properties of the airway and lung tissues in situ and in open-chest conditions. Over the total population, the G, airway resistance, and Iaw parameters were 13% (not significant), 35% (P < 0.001), and 31% (P < 0.001) smaller, respectively, in situ than in the open-chest condition. However, the H was 15% larger in situ (P < 0.001). Although we cannot completely rule out certain artifacts, these differences most likely reflect real alterations in the lung due to the different configurations and possible differences in the distribution of pleural pressures in the two conditions. The G being smaller and E being larger in situ resulted in hysteresivity (G/H) 36% smaller in situ (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Airway Resistance↗

PubMed1994

Estimating respiratory mechanical parameters of ventilated patients: a critical study in the routine intensive-care unit.

Airflow and pressure were measured post-operatively in eight mechanically ventilated patients in the routine intensive care unit. Analysis of the input impedance spectra versus frequency suggested that respiratory data cannot be adequately reproduced using the classic two-element R-C model, as the real part of input impedance decreases with frequency. To fit in with this behaviour, we adopted a three-element model with an additional parallel compliance. The three parameters of this model were estimated separately in the frequency and time domains by minimising suitable least-square criterion functions. The results demonstrate a good agreement between the parameter estimates in the frequency and time domains, and show that the three-element model reproduces the input impedance frequency pattern in the range 0.2-8 Hz. Comparison of different linear models in the time domain demonstrated that the precision of parameter estimates and the quality of best fitting sharply increase from the two-element to the three-element model. The addition of a fourth resistive parameter, like in the Mead model, does not lead to appreciable improvement and makes the model almost unidentifiable. The possible contribution of a ventilator-patient circuit of the upper airway shunting and of the peripheral airway obstruction are also discussed.

Airway Resistance↗

PubMed1994

Influence of waveform and analysis technique on lung and chest wall properties.

To test an approach for measuring respiratory system resistance (R) and elastance (E) during non-sinusoidal forcing, we measured airway and esophageal pressures and flow at the trachea of 9 anesthetized-paralyzed dogs during sinusoidal forcing (SF) and 4 types of non-sinusoidal forcings at 0.15 and 0.6 Hz and 300 ml tidal volume. During SF, calculations of E and R of the lungs, chest wall or total system from discrete Fourier transform (DFT) and two other widely used methods (multiple regression and volume-pressure loop analysis) did not differ from each other (P > 0.05). During forcing with sinusoidal or step inspiration with passive expiration (inspiratory to expiratory ratio, I/E, = 1:1), Es from any analysis method were within 10% of values during SF. Although Rs of the lungs, chest wall or total system were not affected by waveform shape with DFT (P > 0.05), the other analysis methods gave values for R during non-SF that differed (P < 0.05) from those during SF by up to 77%. If I/E was changed to 1:2, with or without an added 10% inspiratory pause, values for E and R differed least from values during SF if DFT was used. During severe pulmonary edema induced by infusion of oleic acid in the right atrium, results for lung properties were similar to controls, despite large increases in E and R of the lungs. We conclude that E and R of the lungs and chest wall can be measured by DFT using nonsinusoidal forcing waveforms available on most clinical ventilators, incurring only modest error.

Airway Resistance↗

PubMed1994

Lung tissue viscoelasticity: a mathematical framework and its molecular basis.

Recent studies indicated that lung tissue stress relaxation is well represented by a simple empirical equation involving a power law, t-beta (where t is time). Likewise, tissue impedance is well described by a model having a frequency-independent (constant) phase with impedance proportional to omega-alpha (where omega is angular frequency and alpha is a constant). These models provide superior descriptions over conventional spring-dashpot systems. Here we offer a mathematical framework and explore its mechanistic basis for using the power law relaxation function and constant-phase impedance. We show that replacing ordinary time derivatives with fractional time derivatives in the constitutive equation of conventional spring-dashpot systems naturally leads to power law relaxation function, the Fourier transform of which is the constant-phase impedance with alpha = 1 - beta. We further establish that fractional derivatives have a mechanistic basis with respect to the viscoelasticity of certain polymer systems. This mechanistic basis arises from molecular theories that take into account the complexity and statistical nature of the system at the molecular level. Moreover, because tissues are composed of long flexible biopolymers, we argue that these molecular theories may also apply for soft tissues. In our approach a key parameter is the exponent beta, which is shown to be directly related to dynamic processes at the tissue fiber and matrix level. By exploring statistical properties of various polymer systems, we offer a molecular basis for several salient features of the dynamic passive mechanical properties of soft tissues.

Biomechanical Phenomena↗

PubMed1994

Airway and tissue mechanics during physiological breathing and bronchoconstriction in dogs.

In five open-chest dogs and with four to five alveolar capsules we used an optimal ventilator waveform (OVW) to follow frequency and tidal volume (VT) dependence of lung, airway, and tissue resistance (R) and elastance (E) before and during constant infusion of histamine (16 micrograms.kg-1.min-1). OVW contains sufficient flow energy between 0.234 and 4.7 Hz, avoids nonlinear harmonic interactions, and simultaneously ventilates with physiological VT. Each OVW breath permits a smooth estimate of frequency dependence of R and E for the whole lung. A constant-phase model analysis provided estimates of purely viscous resistance (Rvis), which represents the sum of airway resistance (Raw) and any purely newtonian component of tissue resistance (Rti), and parameters G and H, which govern frequency dependence of Rti and tissue elastance (Eti), respectively. Tissue structural damping (eta) is calculated as G/H. This model was applied to the whole lung and tissue impedance as estimated from each capsule. We found a small but inconsequential purely newtonian component of Rti, even during constriction. Four dogs showed a peak response at approximately 4 min in lung Rvis coupled (in time) to initial increases in G, H, eta, and airway inhomogeneities. In two of these dogs the response was severe. Tissue properties estimated from whole lung impedance (G, H, and eta) were nearly identical to values estimated from unobstructed capsules throughout infusion. By using a technique independent of alveolar capsules, our results indicate that a major if not dominant response to a constrictive agonist occurs in lung tissues, resulting in a large increase in Rti and Eti. With severe constriction, significant increases occur in Raw and airway inhomogeneities as well. Finally, separation of airway and tissue properties using input impedance estimated from the frequency-rich OVW avoids use of alveolar capsules and may prove an effective tool for partitioning airway and tissue properties in humans.

Airway Resistance↗

PubMed1993

Impact of frequency range and input impedance on airway-tissue separation implied from transfer impedance.

In humans, application of the DuBois (DuBois et al. J. Appl. Physiol. 8: 587-594, 1956) six-element model to respiratory transfer impedance (Ztr) data has been proposed as a means to noninvasively estimate airway and tissue properties. This approach requires prior knowledge of alveolar gas compressibility (Cg). With input impedance (Zin), prior knowledge of Cg is not required, but the data do not support a reliable separation of airway from tissue properties. In this study, we investigated the separation of airway and tissue properties when Ztr and Zin data are measured and analyzed simultaneously over a larger frequency range than usual. In 10 healthy adults, we measured Ztr and Zin from 2 to 64 Hz. Zin was measured using both the standard approach with oscillations directly into the airway opening (Zst) and the head generator approach (Zhg) with oscillations applied around the head. With Ztr data alone, we found that the airway resistance and inertance estimates were reliable with only 2- to 32-Hz data and were unaffected by including the additional 32- to 64-Hz data. Conversely, the estimates of tissue resistance and inertance were highly unreliable unless the 32- to 64-Hz data are included. Because of enhanced sensitivity of Ztr to Cg from 32 to 64 Hz, inaccuracies in the assigned Cg will distort the estimated tissue but not airway properties. The Ztr-based parameters predicted Zhg data far better than Zst data, which is consistent with Zhg data being less influenced by upper airway shunting over this frequency range. There was no apparent advantage to combining Ztr and Zhg data during parameter estimation. With Cg unfixed, the estimated Cg was 50-100% higher than expected from an independent measurement of functional residual capacity. These results confirm that Ztr alone can provide a reliable distinction of lumped respiratory airway and tissue properties that are little influenced by upper airway wall shunting but only if 2- to 64-Hz data are analyzed. This distinction, however, requires an accurate prior measurement of Cg, and this requirement cannot be removed by combining Ztr and Zin data.

Adult↗

PubMed1993

Optimal ventilation waveforms for estimating low-frequency respiratory impedance.

We present a broad-band optimal ventilator waveform (OVW), the concept of which was to create a computer-driven ventilator waveform containing increased energy at specific frequencies (f). Values of f were chosen such that nonlinear harmonic distortion and intermodulation were minimized. The phases at each f were then optimized such that the resulting flow waveform delivered sufficient volume to maintain gas exchange while minimizing peak-to-peak airway opening pressure. Simulations with a linear anatomically consistent branching airway model and a nonlinear viscoelastic model showed that respiratory resistance (Rrs) and elastance (Ers) estimates at 0.1-2 Hz from the OVW are far superior to those from a standard step ventilator waveform (SVW) during healthy and obstructed conditions and that the OVW reduces the influences of harmonic interactions. Using a servo-controlled oscillator, we applied individual sine waves, an OVW containing energy at 0.15625-2.4 Hz, and an SVW to healthy humans and one symptomatic asthmatic subject before and after bronchodilation. The OVW was markedly superior to the SVW and always provided smooth estimates of Rrs and Ers. Before bronchodilation in the asthmatic subject Rrs was highly elevated and Ers was markedly increased with f; after bronchodilation the level of Rrs and the f dependence of Ers decreased. Although based on results from only one asthmatic subject, these data suggest a dominant influence of airway constriction and lung inhomogeneities during asthmatic bronchoconstriction that is alleviated by bronchodilators. These and other results indicate that the OVW approach has high potential for simultaneously probing f and amplitude dependence in the mechanical properties of clinical subjects during physiological breathing conditions and perhaps during dynamic bronchoconstriction.

Adult↗

PubMed1993

Alternative model of respiratory tissue viscoplasticity.

Respiratory tissue impedance exhibits both tidal volume and frequency dependences in the ranges of normal breathing. Hildebrandt argued that these indicate tissue viscoplasticity and offered a model in support of his argument consisting of viscoelastic and plastoelastic compartments, both mechanically in parallel (J. Appl. Physiol. 28: 365-372, 1970). Although the model appears to be qualitatively consistent with oscillatory behavior of a wide variety of respiratory tissues, it yields only moderately good quantitative correspondences despite a relatively large number of parameters, eight. One reason may be the model topology, which implies that rate-dependent and amplitude-dependent processes are decoupled. This is contrary to observed behavior. In this study we offer a model in which viscoelastic and plastoelastic compartments are mechanically coupled through a serial arrangement. The total number of parameters in the model is four. Using a least squares technique, we fitted this model to impedance data of chest wall, healthy lungs, and edematous lungs, all measured in vivo. We found that the model could account for the major, as well as the more subtle, features of the chest wall data with fewer parameters and fewer ad hoc assumptions than Hildebrandt's model. Although it lacks anatomic specifics, the model suggests that the observed chest wall behavior may stem from the actin-myosin cross-bridge kinetics. It also seems applicable to lung tissue, although the requirements for the plastoelastic compartment are less certain. In the case of edematous lungs, the applicability of the model is difficult to establish.

Airway Resistance↗

PubMed1993

Low-frequency respiratory mechanics using ventilator-driven forced oscillations.

We evaluated the potential for using a fast Fourier transform (FFT) analysis applied to a standard ventilator waveform to estimate (< 2 Hz) frequency dependence of respiratory or lung resistance (R) and elastance (E). In four healthy humans we measured pressure and flow at the airway opening while applying sine wave forcing from 0.2 to 0.6 Hz at two tidal volumes (VT; 250 and 500 ml). We then applied a step inspiratory ventilator flow wave with relaxed expiration at the same VT and only 0.2 Hz. Step waveform data were also acquired from nine mechanically ventilated patients under intensive care unit conditions. Finally, we simultaneously measured total respiratory (rs), lung (L), and chest wall (cw) impedance data from two dogs (0.156-2 Hz) before and after severe pulmonary edema. Rrs and Ers were estimated by the FFT approach. Humans displayed a small frequency dependence in Rrs and Ers from 0.2 to 0.6 Hz, and both Rrs and Ers decreased at the higher VT. The spectral estimates of Rrs and Ers with the step ventilator wave were often qualitatively comparable to sine wave results below 0.6 Hz but became extremely erratic above the third harmonic. Conversely, in dogs the step wave produced reliable and stable estimates up to 2 Hz in all conditions. Nevertheless, Ecw and Ers still displayed clear and correlated oscillations with increasing frequency, whereas EL showed none. This suggests that nonlinear processes, most likely at the chest wall, contribute to periodic-like fluctuations in respiratory mechanical properties when estimated by applying FFT to a step ventilator wave. Moreover, in humans, but not dogs, a ventilator flow cycle contains insufficient signal energy beyond the third harmonic. We show that the amount of energy available at higher frequencies is largely governed by the mechanical time constant contributing to passive expiratory flow. In dogs the shorter time constant contributes to increased energy. In essence, the frequency content of the flow is subject dependent, and this is not a desirable situation for controlling the quality of the impedance spectra available from a standard ventilator wave.

Adult↗

PubMed1992

Confidence bounds on respiratory mechanical properties estimated from transfer versus input impedance in humans versus dogs.

Using parameters typical of a dog, we have shown that estimates for the parameters in the six-element model of Dubois et al. would be very unreliable if either input (Z(in)) or transfer (Ztr) data from only 2-32 Hz were fit. It has subsequently been shown that this model is not appropriate for human Z(in) from 2-320 Hz. However, several studies have continued to apply the model to human Ztr data from only 2-32 Hz. In this study a sensitivity analysis is used to determine whether and why the six-element model could be applicable to lower frequency (less than 64 Hz) Ztr data in humans, but not Z(in) data over any frequency range. We first predicted the joint parameter uncertainty bounds assuming a fit to either 2-32 Hz Z(in) or Ztr data created from literature based mean parameter values. Consistent with previous studies, we predicted that the estimates will be very unreliable if obtained from Z(in) data for humans or dogs, or from Ztr data from dogs. Surprisingly, however, the reliability of several parameter estimates from human Ztr data from only 2-32 Hz are reasonable. We next evaluated the variability in 2-64 Hz based Ztr parameter estimates by comparing experimental variability in two healthy human subjects (over 10 and 13 trials) to theoretical and Monte Carlo numerical predictions based on a single trial. Again, the Ztr parameters were reliable. A simulation study was used to describe the reasons for enhanced reliability when using human Ztr data. It is shown that this reliability is largely dependent on alveolar gas compressibility, Cg.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Pseudorandom signals to estimate apparent transfer and coherence functions of nonlinear systems: applications to respiratory mechanics.

There is an increasing need in physiology to estimate nonparametric linear transfer functions from data originating from biological systems which are invariably nonlinear. For pseudorandom (PRN) input stimuli, we derive general expressions for the apparent transfer (Z) and coherence (gamma 2) functions of nonlinear systems that can be represented by a Volterra series. It is shown that in the case of PRN signals in which the frequency components are integer multiples of other components the estimates of Z are seriously biased due to harmonic distortion and crosstalk among frequency components of the input. When the PRN signal includes components that are not integer multiples of other components harmonic distortion is avoided, but not necessarily cross talk. Here the estimates of Z remain poor without a noticeable influence on gamma 2. To avoid the problems associated with harmonic distortions and minimize the influence of crosstalk, a family of pseudorandom signals is proposed which are especially suited for the estimation of Z and gamma 2 in mechanical measurements of physiological systems at low frequencies. The components in the signals cannot be reproduced as linear combinations of two or more frequency components of the input. In a second-order system, this completely eliminates the bias, while in higher-order, but not strongly nonlinear systems, the interactions among the components are reduced to a level that the response can be considered as if it was measured with independent sine waves of an equivalent amplitude. It is also shown that the values of gamma 2 are not appropriate to assess linearity of the system. The theory is supported by simulation results and experimental examples brought from the field of respiratory mechanics by comparing the input impedance of the respiratory system of a dog measured with various PRN signals.

Airway Resistance↗