Influence of upper airway shunt on total respiratory impedance in infants.

When input impedance is determined by means of the forced oscillation technique, part of the oscillatory flow measured at the mouth is lost in the motion of the upper airway wall acting as a shunt. This is avoided by applying the oscillations around the subject's head (head generator) rather than at the mouth (conventional technique). In seven wheezing infants, we compared both techniques to estimate the importance of the upper airway wall shunt impedance (Zuaw) for the interpretation of the conventional technique results. Computation of Zuaw required, in addition, estimation of nasal impedance values, which were drawn from previous measurements (K. N. Desager, M. Willemen, H. P. Van Bever, W. De Backer, and P. A. Vermeire. Pediatr. Pulmonol. 11: 1-7, 1991). Upper airway resistance and reactance at 12 Hz ranged from 40 to 120 and from 0 to -150 hPa. l(-1). s, respectively. Varying nasal impedance within the range observed in infants did not result in major changes in the estimates of Zuaw or lung impedance (ZL), the impedance of the respiratory system in parallel with Zuaw. The conventional technique underestimated ZL, depending on the value of Zuaw. The head generator technique slightly overestimated ZL, probably because the pressure gradient across the upper airway was not completely suppressed. Because of the need to enclose the head in a box (which is not required with the conventional technique), the head generator technique is difficult to perform in infants.     

Inverse modeling of dog airway and respiratory system impedances

Mechanical parameters of the respiratory system are often estimated from respiratory impedances using lumped-element inverse models. One such six-element model is composed of an airway branch [with a resistance (Raw) and inertance (Iaw)] separated from a tissue branch [with a resistance (Rt), inertance (It), and compliance (Ct)] by a shunt compliance representing alveolar gas compression (Cg). Even though the airways are known to have frequency-dependent resistance and inertance, these inverse models have been composed of linear frequency-independent elements. In this study we investigated the use of inverse models where the airway branch was represented by a frequency-independent Raw and Iaw, a Raw that is linearly related to frequency and an Iaw that is independent of frequency, and a system of identical parallel tubes the impedance of which was computed from the tube radius and length. These inverse models were used to analyze airway and respiratory impedances between 2 and 1,024 Hz that were predicted from an anatomically detailed forward model. The forward model represented the airways by an asymmetrically branched network with a terminal impedance representative of known Cg, Rt, It, and Ct. For respiratory impedances between 2 and 128 Hz, all models fit the data reasonably well, and reasonably accurate estimates of Cg, Rt, It, and Ct were extracted from these data. For data above 200 Hz, however, only the multiple-tube model accurately fitted respiratory impedances (Zrs). This model fitted the Zrs data best when composed of 27 tubes, each having a radius of 0.148 cm and a length of 16.5 cm.     

Input impedance and peripheral inhomogeneity of dog lungs.

Tracheal pressure, central airflow, and alveolar capsule pressures in cardiac lobes were measured in open-chest dogs during 0.1- to 20-Hz pseudorandom forced oscillations applied at the airway opening. In the interval 0.1-4.15 Hz, the input impedance data were fitted by four-parameter models including frequency-independent airway resistance and inertance and tissue parts featuring a marked negative frequency dependence of resistance and a slight elevation of elastance with frequency. The models gave good fits both in the control state and during histamine infusion. At the same time, the regional transfer impedances (alveolar pressure-to-central airflow ratios) showed intralobar and interlobar variabilities of similar degrees, which increased with frequency and were exaggerated during histamine infusion. Results of simulation studies based on a lung model consisting of a central airway and a number of peripheral units with airway and tissue parameters that were given independent wide distributions were in agreement with the experimental findings and showed that even an extremely inhomogeneous lung structure can produce virtually homogeneous mechanical behavior at the input.     

Human respiratory input impedance between 32and 800Hz, measured by interrupter technique and forced oscillations

Frey, Urs, Bela Suki, Richard Kraemer, and Andrew C. Jackson. Human respiratory input impedance between 32 and 800 Hz,measured by interrupter technique and forced oscillations. J. Appl. Physiol. 82(3):1018-1023, 1997.Respiratory input impedance (Zin) over a widerange of frequencies (f) has beenshown to be useful in determining airway resistance (Raw) and tissueresistance in dogs or airway wall properties in human adults. Zinmeasurements are noninvasive and, therefore, potentially useful ininvestigation of airway mechanics in infants. However, accuratemeasurements of Zin at these f valueswith the use of forced oscillatory techniques (FOT) in infants aredifficult because of their relatively high Raw and large compliance ofthe face mask. If pseudorandom noise pressure oscillations generated bya loudspeaker are applied at the airway opening (FOT), the power of theresulting flow decreases inversely withf because of capacitive shunting intothe volume of the gas in the speaker chamber and in the face mask. Westudied whether high-frequency respiratory Zin can be measured by using rapid flow interruption [high-speed interrupter technique(HIT)], in which we expect the flow amplitude in the respiratorysystem to be higher than in the FOT. We compared Zin measured by HIT with Zin measured by FOT in a dried dog lung and in five healthy adultsubjects. The impedance was calculated from two pressure signalsmeasured between the mouth and the HIT valve. The impedance could beassessed from 32 to 800 Hz. Its real part at lowf as well as thef and amplitude of the first andsecond acoustic resonance, measured by FOT and by HIT, were notsignificantly different. The power spectrum of oscillatory flow whenthe HIT was used showed amplitudes that were at least 100 times greaterthan those when FOT was used, increasing atf > 400 Hz. In conclusion,the HIT enables the measurement of high-frequency Zin data ranging from 32 to 800 Hz with particularly high flow amplitudes and, therefore, possibly better signal-to-noise ratio. This is particularly important in systems with high Raw, e.g., in infants, when measurements have tobe performed through a face mask.

     

Flutter in flow-limited collapsible tubes: a mechanism for generation of wheezes

We studied flutter in collapsible tubes as a possible mechanism for the generation of respiratory wheezes. The pressure-flow relationships and the wall oscillations of thick-walled [wall thickness (h)-to-lumen radius (r) ratio 1:1.7 to 1.3] self-supporting latex and Silastic tubes mounted between rigid pipes were measured. A high-impedance vacuum pump was connected to the downstream end. Upstream and downstream valves were used to control corresponding resistances. We found loud honking sounds and tube wall oscillations that occurred only when the tubes were buckled and flow limiting, i.e., when the flow became constant and independent of downstream driving pressure. The overall range of oscillatory frequencies was 260-750 Hz for airflow, presenting as sharp peaks of power on the frequency spectrum. The oscillatory frequencies (f) were higher at higher fluid velocities (u) and with narrower distance between opposing flattened walls (2b), resulting from increasing downstream suction pressure and the transmural pressure becoming more negative. The effect of u and b on f for a latex tube (h-to-r ratio 1:1.7) were found to be f = 228 + 0.021 (u/b). These relationships were valid throughout the range of oscillations in this tube (283-720 Hz) and with flow rates of 12-64 l/min. The experimental data were compared with predictions of the fluid dynamic flutter theory and the vortex-induced wall vibrations mechanism. We conclude that viscid flutter in soft tubes is the more probable mechanism for the generation of oscillations in the soft tube model and is a possible mechanism for the generation of respiratory wheezes.     

Liquid plug flow in straight and bifurcating tubes.

A finite-length liquid plug may be present in an airway due to disease, airway closure, or by direct instillation for medical therapy. Air forced by ventilation propagates the plug through the airways, where it deposits fluid onto the airway walls. The plug may encounter single or bifurcating airways, an airway surface liquid, and other liquid plugs in nearby airways. In order to understand how these flow situations influence plug transport, benchtop experiments are performed for liquid plug flow in: Case (i) straight dry tubes, Case (ii) straight pre-wetted tubes, Case (iii) bifurcating dry tubes, and Case (iv) bifurcating tubes with a liquid blockage in one daughter. Data are obtainedfor the trailing film thickness and plug splitting ratio as a function of capillary number and plug volumes. For Case (i), the finite length plug in a dry tube has similar behavior to a semi-infinite plug. For Case (ii), the trailing film thickness is dependent upon the plug capillary number (Ca) and not the precursor film thickness, although the shortening or lengthening of the liquid plug is influenced by the precursor film. For Case (iii), the plug splits evenly between the two daughters and the deposited film thickness depends on the local plug Ca, except for a small discrepancy that may be due to an entrance effect or from curvature of the tubes. For Case (iv), a plug passing from the parent to daughters will deliver more liquid to the unblocked daughter (nearly double, consistently) and then the plug will then travel at greater Ca in the unblocked daughter as the blocked. The flow asymmetry is enhanced for a larger blockage volume and diminished for a larger parent plug volume and parent-Ca.     

Bolus contaminant dispersion in oscillating flow in curved tubes.

The investigation of longitudinal dispersion of tracer substances in unsteady flows has biomechanical application in the study of heat and mass transport within the bronchial airways during normal, abnormal, and artificial pulmonary ventilation. To model the effects of airway curvature on intrapulmonary gas transport, we have measured local gas dispersion in axially uniform helical tubes of slight pitch during volume-cycled oscillatory flow. Following a small argon bolus injection into the flow field, the time-averaged effective diffusion coefficient (Deff/Dmol) for axial transport of the contaminant was evaluated from the time-dependent local argon concentration measured with a mass spectrometer. The value of (Deff/Dmol) is extracted from the curve of concentration versus time by two techniques yielding identical results. Experiments were conducted in two helical coiled tubes (delta = 0.031, lambda = 0.022 or delta = 0.085, lambda = 0.060) over a range of 2 < alpha < 15, 3 < A < 15, where delta is the ratio of tube radius to radius of curvature, lambda is the ratio of pitch height to radius of curvature, alpha is the Womersley parameter or dimensionless frequency, and A is the stroke amplitude or dimensionless tidal volume. Experimental results show that, when compared to transport in straight tubes, the effective diffusivity markedly increases in the presence of axial curvature. Results also compare favorably to mathematical predictions of bolus dispersion in a curved tube over the ranges of frequency and tidal volume studied.     

Mechanical impedances of lungs and chest wall in the cat.

In nine anesthetized and paralyzed cats, the mechanical impedances of the total respiratory system (Zrs) and the lungs (ZL) were measured with small-volume pseudorandom forced oscillations between 0.2 and 20 Hz. ZL was measured after thoracotomy, and chest wall impedance (Zw) was calculated as Zw = Zrs-ZL. All impedances were determined by using input airflow [input impedance (Zi)] and output flow measured with a body box [transfer impedance (Zt)]. The differences between Zi and Zt were small for Zrs and negligible for ZL. At 0.2 Hz, the real and imaginary parts of ZL amounted to 33 +/- 4 and 35 +/- 3% (SD), respectively, of Zrs. Up to 8 Hz, all impedances were consistent with a model containing a frequency-independent resistance and inertance and a constant-phase tissue part (G-jH)/omega alpha, where G and H are coefficients for damping and elastance, respectively, omega is angular frequency, and alpha determines the frequency dependence of the real and imaginary parts. G/H was higher for Zw than for ZL (0.29 +/- 0.05 vs. 0.22 +/- 0.04, P less than 0.01). In four cats, the amplitude dependence of impedances was studied: between oscillation volumes of 0.8 and 3 ml, GL, HL, Gw, and Hw decreased on average by 3, 9, 26, and 29%, respectively, whereas the change in G/H was small for both ZL (7%) and Zw (-4%). The values of H were two to three times higher than the quasistatic elastances estimated with greater volume changes (greater than 20 ml).     

Flutter in collapsible tubes: a theoretical model of wheezes

A mathematical analysis of flow through a flexible channel is examined as a model of flow-induced flutter oscillations that pertain to the production of wheezing breath sounds. The model provides predictions for the critical fluid speed that will initiate flutter waves of the wall, as well as their frequency and wavelength. The mathematical results are separated into linear theory (small oscillations) and nonlinear theory (larger oscillations). Linear theory determines the onset of the flutter, whereas nonlinear theory determines the relationships between the fluid speed and both the wave amplitudes and frequencies. The linear theory predictions correlate well with data taken at the onset of flutter and flow limitation during experiments of airflow in thick-walled collapsible tubes. The nonlinear theory predictions correlate well with data taken as these flows are forced to higher velocities while keeping the flow rate constant. Particular ranges of the parameters are selected to investigate and discuss the applications to airway flows. According to this theory, the mechanism of generation of wheezes is based in the interactions of fluid forces and friction and wall elastic-restoring forces and damping. In particular, a phase delay between the fluid pressure and wall motion is necessary. The wave speed theory of flow limitation is discussed with respect to the specific data and the flutter model.     

Transient behavior of arterial systems in response to flow pulses

The arterial system is characterized geometrically as a system of branched elastic fluid lines whose frequency response is then known in the sense of the Fourier transform. For convenience of visualization the transient response of the individual tube to an input pressure-flow pair is represented in the time domain by kernel functions indicating the hybrid effect of viscosity and momentum on the line impedance and damping characteristics. The system as a whole is then divided into a zone of smaller tubes (below 3 mm) and a zone of larger tubes extending up to the aorta. It is shown that as a system each labyrinth of tubes below the 3 mm size may be replaced by a single impedance transformation which is dominantly resistive-capacitive. In the larger tubes, the transformation of the pulse wave at different stations is considered a point of interest. Therefore hand calculated examples are worked to derive the response of a system involving some of the larger vessels to a pressure or flow pulse of the typical shape seen near the heart. The result suggests that the dicrotic wave seen in the pressure pulse of mammals is due to the hybrid viscosity-momentum nature of the longer fluid lines in relation to the gradation of unmatched terminal impedances with which they are terminated. Damping of the higher frequency components is also accounted for.     

Assessment of heterogeneous airway constriction in dogs: a structure-function analysis

Obstructive lung diseases are often characterized by heterogeneous patterns of bronchoconstriction, although specific relationships between structural heterogeneity and lung function have yet to be established. We measured respiratory input impedance (Zrs) in eight anesthetized dogs using broadband forced oscillations at baseline and during intravenous methacholine (MCh) infusion. We also obtained high-resolution computed tomographic (HRCT) scans in 4 dogs and identified 20-30 individual airway segments in each animal. The Zrs spectra and HRCT images were obtained before and 5 min following a deep inspiration (DI) to 35 cmH(2)O. Each Zrs spectrum was fitted with two different models of the respiratory system: 1) a lumped airways model consisting of a single airway compartment, and 2) a distributed airways model incorporating a continuous distribution of airway resistances. For the latter, we found that the mean level and spread of airway resistances increased with MCh dose. Whereas a DI had no effect on average airway resistance during MCh infusion, it did increase the level of airway heterogeneity. At baseline and low-to-moderate doses of MCh, the lumped airways model was statistically more appropriate to describe Zrs in the majority of dogs. At the highest doses of MCh, the distributed airways model provided a superior fit in half of the dogs. There was a significant correlation between heterogeneity assessed with inverse modeling and the standard deviation of airway diameters obtained from HRCT. These data demonstrate that increases in airway heterogeneity as assessed with forced oscillations and inverse modeling can be linked to specific structural alterations in airway diameters.     

Improved frequency response of pneumotachometers by digital compensation

To measure impedance one measures or estimates flow, which is commonly done by measuring the pressure drop across a pneumotachometer. The frequency response characteristics of standard pneumotachometer/pressure transducers (PPT) limit their use to relatively low frequencies. Also, the frequency response of PPTs has been reported to be "load" dependent. Thus, the frequency response characteristics measured under "no-load" conditions, which theoretically could be used to compensate subsequent measurements, may not be appropriate for measurements made under loaded conditions. Another method of measuring impedance exists which depends on a reference impedance element other than a pneumotachometer. In this method, an oscillatory flow signal with known amplitude is generated and used to force the system being tested. Unlike PPTs, this oscillatory flow generator (OFG) is a closed system that allows measurements to be made only during breath holding. Our objective was to determine whether the frequency response of a PPT could be compensated using measurements made under no-load conditions, such that it accurately measured an impedance load. The frequency response of the PPT under no-load conditions was measured by the OFG and used to compensate the output of the PPT in subsequent impedance measurements. The compensated PPT was used to measure the impedance of a mechanical structure and the impedances of four human subjects. The impedances of the mechanical structure and the subjects were also measured using the OFG.(ABSTRACT TRUNCATED AT 250 WORDS)     

Low-frequency respiratory mechanical impedance in the rat

A modified forced oscillatory technique was used to determine the respiratory mechanical impedances in anesthetized, paralyzed rats between 0.25 and 10 Hz. From the total respiratory (Zrs) and pulmonary impedance (ZL), measured with pseudorandom oscillations applied at the airway opening before and after thoracotomy, respectively, the chest wall impedance (ZW) was calculated as ZW = Zrs - ZL. The pulmonary (RL) and chest wall resistances were both markedly frequency dependent: between 0.25 and 2 Hz they contributed equally to the total resistance falling from 81.4 +/- 18.3 (SD) at 0.25 Hz to 27.1 +/- 1.7 kPa.l-1 X s at 2 Hz. The pulmonary compliance (CL) decreased mildly, from 2.78 +/- 0.44 at 0.25 Hz to 2.36 +/- 0.39 ml/kPa at 2 Hz, and then increased at higher frequencies, whereas the chest wall compliance declined monotonously from 4.19 +/- 0.88 at 0.25 Hz to 1.93 +/- 0.14 ml/kPa at 10 Hz. Although the frequency dependence of ZW can be interpreted on the basis of parallel inhomogeneities alone, the sharp fall in RL together with the relatively constant CL suggests that at low frequencies significant losses are imposed by the non-Newtonian resistive properties of the lung tissue.     

Effects of curved inlet tubes on air flow and particle deposition in bifurcating lung models

In vivo bifurcating airways are complex and the airway segments leading to the bifurcations are not always straight, but curved to various degrees. How do such curved inlet tubes influence the motion as well as local deposition and hence the biological responses of inhaled particulate matter in lung airways? In this paper steady laminar dilute suspension flows of micron-particles are simulated in realistic double bifurcations with curved inlet tubes, i.e., 0 degrees < or =theta< or =90 degrees, using a commercial finite-volume code with user-enhanced programs. The resulting air-flow patterns as well as particle transport and wall depositions were analyzed for different flow inlet conditions, i.e., uniform and parabolic velocity profiles, and geometric configurations. The curved inlet segments have quite pronounced effects on air-flow, particle motion and wall deposition in the downstream bifurcating airways. In contrast to straight double bifurcations, those with bent parent tubes also exhibit irregular variations in particle deposition efficiencies as a function of Stokes number and Reynolds number. There are fewer particles deposited at mildly curved inlet segments, but the particle deposition efficiencies at the downstream sequential bifurcations vary much when compared to those with straight inlets. Under certain flow conditions in sharply curved lung airways, relatively high, localized particle depositions may take place. The findings provide necessary information for toxicologic or therapeutic impact assessments and for global lung dosimetry models of inhaled particulate matter.     

Effects of posture and bronchoconstriction on low-frequency input and transfer impedances in humans.

We simultaneously evaluated the mechanical response of the total respiratory system, lung, and chest wall to changes in posture and to bronchoconstriction. We synthesized the optimal ventilation waveform (OVW) approach, which simultaneously provides ventilation and multifrequency forcing, with optoelectronic plethysmography (OEP) to measure chest wall flow globally and locally. We applied an OVW containing six frequencies from 0.156 to 4.6 Hz to the mouth of six healthy men in the seated and supine positions, before and after methacholine challenge. We measured mouth, esophageal, and transpulmonary pressures, airway flow by pneumotachometry, and total chest wall, pulmonary rib cage, and abdominal volumes by OEP. We computed total respiratory, lung, and chest wall input impedances and the total and regional transfer impedances (Ztr). These data were appropriately sensitive to changes in posture, showing added resistance in supine vs. seated position. The Ztr were also highly sensitive to lung constriction, more so than input impedance, as the former is minimally distorted by shunting of flow into alveolar gas compression and airway walls. Local impedances show that, during bronchoconstriction and at typical breathing frequencies, the contribution of the abdomen becomes amplified relative to the rib cage. A similar redistribution occurs when passing from seated to supine. These data suggest that the OEP-OVW approach for measuring Ztr could noninvasively track important lung and respiratory conditions, even in subjects who cannot cooperate. Applications might range from routine evaluation of airway hyperreactivity in asthmatic subjects to critical conditions in the supine position during mechanical ventilation.     

Numerical simulation of sinusoidal flow in a straight elastic tube: effects of phase angles

Numerical simulations of flow in straight elastic (moving wall) tubes subjected to a sinusoidal pressure gradient were performed for conditions prevailing in large and medium sized arteries. The effects of varying the phase angle between the pressure gradient and the tube radius, the amplitude of wall motion, and the unsteadiness parameter (alpha) on flow rate and wall shear stress were investigated. Mean and peak flow rates and shear stresses were found to be strongly affected by the phase angle between the pressure gradient and the tube radius with greater sensitivity at higher diameter variation and higher alpha. In large artery simulations (alpha = 12), means flow rate was found to be 60% higher and peak flow rate to be 73% higher than corresponding rigid tube values for certain phase angles, while a threefold increase in mean wall shear stress and sevenfold increase in peak wall shear stress were observed in a sensitive phase angle range. Significant reversal in the wall shear stress direction occurred in the sensitive phase angle range even when there was negligible flow rate reversal. All effects were greatly diminished in simulations of medium sized vessels (alpha = 4). Some experimental evidence to support the predictions of a strong effect of phase angle on wall shear stress in large vessels is presented. Finally, physiological implications of the present work are discussed from a basis of aortic input impedance data, and a physical explanation for the extreme sensitivity of the flow field to small amplitude wall motion at high alpha is given.     

One-dimensional model for propagation of a pressure wave in a model of the human arterial network: comparison of theoretical and experimental results

Pulse wave evaluation is an effective method for arteriosclerosis screening. In a previous study, we verified that pulse waveforms change markedly due to arterial stiffness. However, a pulse wave consists of two components, the incident wave and multireflected waves. Clarification of the complicated propagation of these waves is necessary to gain an understanding of the nature of pulse waves in vivo. In this study, we built a one-dimensional theoretical model of a pressure wave propagating in a flexible tube. To evaluate the applicability of the model, we compared theoretical estimations with measured data obtained from basic tube models and a simple arterial model. We constructed different viscoelastic tube set-ups: two straight tubes; one tube connected to two tubes of different elasticity; a single bifurcation tube; and a simple arterial network with four bifurcations. Soft polyurethane tubes were used and the configuration was based on a realistic human arterial network. The tensile modulus of the material was similar to the elasticity of arteries. A pulsatile flow with ejection time 0.3 s was applied using a controlled pump. Inner pressure waves and flow velocity were then measured using a pressure sensor and an ultrasonic diagnostic system. We formulated a 1D model derived from the Navier-Stokes equations and a continuity equation to characterize pressure propagation in flexible tubes. The theoretical model includes nonlinearity and attenuation terms due to the tube wall, and flow viscosity derived from a steady Hagen-Poiseuille profile. Under the same configuration as for experiments, the governing equations were computed using the MacCormack scheme. The theoretical pressure waves for each case showed a good fit to the experimental waves. The square sum of residuals (difference between theoretical and experimental wave-forms) for each case was <10.0%. A possible explanation for the increase in the square sum of residuals is the approximation error for flow viscosity. However, the comparatively small values prove the validity of the approach and indicate the usefulness of the model for understanding pressure propagation in the human arterial network.     

The impedance of curved artery models

The impedance (pressure drop/flow rate) of four curved artery models has been determined experimentally for steady and periodic flows simulating conditions in the aortic arch. Steady flow results indicate that very short entry lengths are required for flow development in curved artery models, and impedance is elevated above straight tube values by a factor of 3-4 for mean flow conditions in the aortic arch. Results for periodic flow with a nonzero mean show a significant elevation of mean flow impedance relative to values for steady flow at the mean flow rate--a factor of 2-3 for aortic arch flow conditions. The impedance of the first harmonic of periodic flows follows straight tube theory at high values of the unsteadiness parameter in agreement with available theory for curved tubes. The implications of the impedance measurements for wall shear stress in the aortic arch are discussed.     

Convection-diffusion interaction for oscillatory flow in a tapered tube

Transport of soluble material is analyzed for volume-cycle oscillatory flow in a tapered tube. The equations of motion are solved using a regular perturbation method for small taper angle and order unity amplitude over a range of the Womersley parameter. The transport equation is also solved by a regular perturbation method where uniform end concentrations and no wall flux are assumed. The time-averaged axial transport of solute is calculated for several tapered tubes. There is substantial modification of transport compared to the straight tube case and the results are interpreted with respect to pulmonary gas exchange.