Modeling of the transfer function of the flow transducer used in ventilatory impedance measurements

The frequency response of the ventilatory system can be studied by applying pressure oscillations at the mouth and measuring the total respiratory impedance. Flow is measured by a transducer made of a pneumotachograph connected to a differential pressure transducer. To characterize amplitude and phase distortion introduced by the flow transducer, its transfer function is described by a second-order equation. Determination of the model parameters is performed by a nonlinear minimization technique, in both the time and frequency domains. Time and frequency approaches give nearly identical values of the parameters, provided appropriate filtering is applied to pressure and flow according to their bandwidth and degree of coherence. The use of these dual techniques is likely to improve reliability of the model identification procedure.     

Mechanical Properties of the Respiratory System Derived From Morphologic Insight

This paper aims to provide the mechanical parameters of the respiratory airways (resistance, inertance, and compliance) from morphological insight, in order to facilitate the correlations of fractional-order models with pathologic changes. The approach consists of taking into account wall thickness, inner radius, tube length, and tissue structure for each airway level to combine them into a set of equations for modeling the pressure drop, flow, wall elasticity, and air velocity (axial and radial). Effects of pulmonary disease affecting the inner radius and elastic modulus of bronchial tree are discussed. A brief comparison with the circulatory system, which poses similarities with the respiratory system, is also given. The derived mechanical parameters can serve as elements in a transmission line equivalent, whose structure preserves the geometry of the human respiratory tree. The mechanical parameters derived in this paper offer the possibility to evaluate input impedance by altering the morphological parameters in relation to the pulmonary disease. In this way, we obtain a simple, yet accurate, model to simulate and understand specific effects in respiratory diseases; e.g., airway remodeling. The final scope of the research is to relate the variations in airway structure with disease to the values of fractional-order model parameters.      

A computer-controlled research ventilator for small animals: designand evaluation

The understanding of the mechanical properties of the mammalian respiratory system and how they change under the influence of drugs and in disease are frequently pursued in small animals, since they can be easily obtained in large numbers as pure-bred strains. However, conventional experimental set-ups for studying small animals are generally limited in their ability to measure gas flow into the lungs. Here, the authors present a computer-controlled research ventilator for small animals which can provide conventional mechanical ventilation as well as arbitrary flow perturbations with a bandwidth from 0-55 Hz. Respiratory impedance is estimated from the displacement of the piston and the pressure it generates, thereby obviating the need for a direct flow measurement. The performance of the device was tested on mechanical loads whose impedances were calculated theoretically. The measured and predicted loads agreed within less than 5% up to 30 Hz. Furthermore, the measured impedance of two mechanical loads in series precisely matched the sum of their individual impedances     

A Theoretical Study on Modeling the Respiratory Tract With Ladder Networks by Means of Intrinsic Fractal Geometry

Fractional order modeling of biological systems has received significant interest in the research community. Since the fractal geometry is characterized by a recurrent structure, the self-similar branching arrangement of the airways makes the respiratory system an ideal candidate for the application of fractional calculus theory. To demonstrate the link between the recurrence of the respiratory tree and the appearance of a fractional-order model, we develop an anatomically consistent representation of the respiratory system. This model is capable of simulating the mechanical properties of the lungs and we compare the model output with in vivo measurements of the respiratory input impedance collected in 20 healthy subjects. This paper provides further proof of the underlying fractal geometry of the human lungs, and the consequent appearance of constant-phase behavior in the total respiratory impedance.      

Development of a time-cycled volume-controlled pressure-limited respirator and lung mechanics system for total liquid ventilation

Total liquid ventilation can support gas exchange in animal models of lung injury. Clinical application awaits further technical improvements and performance verification. Our aim was to develop a liquid ventilator, able to deliver accurate tidal volumes, and a computerized system for measuring lung mechanics. The computer-assisted, piston-driven respirator controlled ventilatory parameters that were displayed and modified on a real-time basis. Pressure and temperature transducers along with a lineal displacement controller provided the necessary signals to calculate lung mechanics. Ten newborn lambs (<6 days old) with respiratory failure induced by lung lavage, were monitored using the system. Electromechanical, hydraulic, and data acquisition/analysis components of the ventilator were developed and tested in animals with respiratory failure. All pulmonary signals were collected synchronized in time, displayed in real-time, and archived on digital media. The total mean error (due to transducers, analog-to-digital conversion, amplifiers, etc.) was less than 5% compared with calibrated signals. Components (tubing, pistons, etc.) in contact with exchange fluids were developed so that they could be readily switched, a feature that will be important in clinical settings. Improvements in gas exchange and lung mechanics were observed during liquid ventilation, without impairment of cardiovascular profiles. The total liquid ventilator maintained accurate control of tidal volumes and the sequencing of inspiration/expiration. The computerized system demonstrated its ability to monitor in vivo lung mechanics, providing valuable data for early decision making.     

Analysis and Simulation of an Adaptive System for Forced Ventilation of the Lungs

A design technique, based on the model reference adaptive control approach for the long term ventilation of the lungs is presented. The design objective is to minimize the harmful effects (e.g., interference in the circulatory system, mechanical damage, etc.) due to possible change in the patient's respiratory parameters (i.e., the airway resistance and the lung and chest wall compliance) during the long term ventilation of the lungs. A model, consisting of a fixed resistance capacitance, RC, analog network is used to generate a ``desire' alveolar pressure profile. The instantaneous difference in the alveolar pressures, obtained from the comparison of the actual patient and his ``desired' behavior, is fed to an ``adaptive controller.' The controller, in turn, will adjust the respirator's output pressure (to the patient) in such a way, that the instantaneous difference in alveolar pressure is reduced to zero. The stability of this newly designed adaptive system is ensured by using Lyapunov's direct method in obtaining the updating laws for the adaptive controller. Using a similar design approach, a respiratory parameters identification scheme is introduced. This identification process is able to generate, indirectly, a continuous estimation of the patient's alveolar pressure (which normally is not monitorable in the actual patient) for the purpose of comparison, in this newly designed adaptive system. Digital simulations of the respirator's pressure control and the identification process, as well as the simulation of the combined system, were performed. The result has indeed demonstrated the ability of a speedy performance of this adaptive system.     

A unified approach for EIT imaging of regional overdistension and atelectasis in acute lung injury

Patients with acute lung injury or acute respiratory distress syndrome (ALI/ARDS) are vulnerable to ventilator-induced lung injury. Although this syndrome affects the lung heterogeneously, mechanical ventilation is not guided by regional indicators of potential lung injury. We used electrical impedance tomography (EIT) to estimate the extent of regional lung overdistension and atelectasis during mechanical ventilation. Techniques for tidal breath detection, lung identification, and regional compliance estimation were combined with the Graz consensus on EIT lung imaging (GREIT) algorithm. Nine ALI/ARDS patients were monitored during stepwise increases and decreases in airway pressure. Our method detected individual breaths with 96.0% sensitivity and 97.6% specificity. The duration and volume of tidal breaths erred on average by 0.2 s and 5%, respectively. Respiratory system compliance from EIT and ventilator measurements had a correlation coefficient of 0.80. Stepwise increases in pressure could reverse atelectasis in 17% of the lung. At the highest pressures, 73% of the lung became overdistended. During stepwise decreases in pressure, previously-atelectatic regions remained open at sub-baseline pressures. We recommend that the proposed approach be used in collaborative research of EIT-guided ventilation strategies for ALI/ARDS.     

A Control System for Long-Term Ventilation of the Lungs

A respirator control system based on a variant process model and optimization of system performance is described. The system attempts to minimize the harmful effects of positive pressure ventilation while meeting the ventilatory requirement of the patient. As alveolar pressure is indicative of respiratory dynamics, it has been used as control parameter. Desired alveolar pressure is derived from a fixed parameter RC lung model while actual alveolar pressure is estimated from the variant lung model which is continuously updated through on-line computation of respiratory mechanical parameters. The controller gain is optimally adjusted so as to minimize error index. The system has been simulated on a digital computer and several representative cases of sudden and gradual parameter variation have been studied. It has been shown that in case of changes in the process, the error quickly damps out to zero.     

An adaptive lung ventilation controller

Closed loop control of ventilation is traditionally based on end-tidal or mean expired CO₂. The controlled variables are the respiratory rate RR and the tidal volume V_T. Neither patient size or lung mechanics were considered in previous approaches. Also the modes were not suitable for spontaneously breathing subjects. This report presents a new approach to closed loop controlled ventilation, called adaptive lung ventilation (ALV). ALV is based on a pressure controlled ventilation mode suitable for paralyzed, as well as spontaneously breathing, subjects. The clinician enters a desired gross alveolar ventilation (V_gA' in l/min), and the ALV controller tries to achieve this goal by automatic adjustment of mechanical rate and inspiratory pressure level. The adjustments are based on measurements of the patient's lung mechanics and series dead space. The ALV controller was tested on a physical lung model with adjustable mechanical properties. Three different lung pathologies were simulated on the lung model to test the controller for rise time (T₉₀), overshoot (Y_m), and steady state performance (Δ_max ). The pathologies corresponded to restrictive lung disease (similar to ARDS), a “normal” lung, and obstructive lung disease (such as asthma). Furthermore, feasibility tests were done in 6 patients undergoing surgical procedures in total intravenous anesthesia. In the model studies, the controller responded to step changes between 48 seconds and 81 seconds. It did exhibit an overshoot between 5.5% and 7.9% of the setpoint after the step change. The maximal variation of V _gA' in steady-state was between ±4.4% and ±5.6% of the setpoint value after the step change. In the patient study, the controller maintained the set V_gA' and adapted the breathing pattern to the respiratory mechanics of each individual patient     

Limitations of Frequency Dependence as a Measure of Airway Obstruction

The purpose of this study is to develop a pulmonary model and determine the frequency response sensitivity of mechanical parameters such as impedance, dynamic compliance, and dynamic resistance as a function of individual airway properties. Computer simulations of a three compartment model of various physiological cases were used to determine lung parameters as a function of frequency, peripheral airway contribution to total airway resistance, and relative percent obstruction of the peripheral airways. Provided our present concepts of the lung are valid and adequately incorporated into the present model, our results indicate the utility of frequency dependence as a measure of airway obstruction.     

Relations Between Fractional-Order Model Parameters and Lung Pathology in Chronic Obstructive Pulmonary Disease

In this study, changes in respiratory mechanics from healthy and chronic obstructive pulmonary disease (COPD) diagnosed patients are observed from identified fractional-order (FO) model parameters. The noninvasive forced oscillation technique is employed for lung function testing. Parameters on tissue damping and elastance are analyzed with respect to lung pathology and additional indexes developed from the identified model. The observations show that the proposed model may be used to detect changes in respiratory mechanics and offers a clear-cut separation between the healthy and COPD subject groups. Our conclusion is that an FO model is able to capture changes in viscoelasticity of the soft tissue in lungs with disease. Apart from this, nonlinear effects present in the measured signals were observed and analyzed via signal processing techniques and led to supporting evidence in relation to the expected phenomena from lung pathology in healthy and COPD patients.      

Design and calibration of a high-frequency oscillatory ventilator

High-frequency ventilation (HFV) is a modality of mechanical ventilation which presents difficult technical demands to the clinical or laboratory investigator. The essential features of an ideal HFV system are described, including wide frequency range, control of tidal volume and mean airway pressure, minimal dead space, and high effective internal impedance. The design and performance of a high-frequency oscillatory ventilation system is described which approaches these requirements. The ventilator utilizes a linear motor regulated by a closed loop controller and driving a novel frictionless double-diaphragm piston pump. Finally, the ventilator performance is tested using the impedance model of Venegas [1].     

A new estimator to minimize the error due to breathing in themeasurement of respiratory impedance

The authors present a novel respiratory impedance estimator to minimize the error due to breathing. Its practical reliability was evaluated in a simulation using realistic signals. These signals were generated by superimposing pressure and flow records obtained in two conditions: (1) when applying forced oscillation to a resistance-inertance-elastance (RIE) mechanical model; (2) when healthy subjects breathed through the unexcited forced oscillation generator. Impedances computed (4-32 Hz) from the simulated signals with the estimator resulted in a mean value which was scarcely biased by the added breathing (errors less than 1% in the mean R, I and E) and had a small variability (coefficients of variation of R, I, and E of 1.34, 3.5, and 9.6%, respectively). The results suggest that the proposed estimator reduces the error in measurement of respiratory impedance without appreciable extra computational cost     

Instrumentation for Measuring Respiratory Impedance by Forced Oscillations

Recent reports have suggested that the frequency dependence of the respiratory impedance may provide a sensitive method for characterizing early changes in pulmonary mechanics. A modification of the forced-oscillation technique provides an experimental method for obtaining the necessary data. A loudspeaker was used to provide the pressure oscillations, and the magnitudes and phase angle of the transduced pressure and flow signals were measured with a special electronics unit. A test comparing predicted values of a standard impedance (a 5-g bottle) to experimental data indicates that the measured amplitude is within 10 percent and the measured phase within over the frequency range of 1-16 Hz. Dog studies showed that measurements at all frequencies up to 16 Hz were reproducible within a few percent of the mean value on a given animal. Data obtained following bronchoconstriction and its reversal in six dogs indicate that the measurements are sensitive to alterations in pulmonary mechanics. Data from two dog models of clinical disease suggest that the technique may provide meaningful diagnostic information.     

Automated Analysis of Intratidal Dynamics of Alveolar Geometry From Microscopic Endoscopy

Alveolar parenchyma, the gas exchange area of the respiratory system, is prone to mechanical damage during mechanical ventilation. Development of lung protective ventilation strategies therefore requires a better understanding of alveolar dynamics during mechanical ventilation. In this paper, we propose a novel method for automated analysis of the intratidal geometry of subpleural alveoli based on the evaluation of video frames recorded from alveolar microscopy in an experimental setting. Our method includes the recording with a microscopic endoscope, feature extraction from image data, the analysis of a single frame, the tracking and analysis of single alveoli in a video sequence, and the evaluation of the obtained sequence of alveolar geometry data. Our method enables automated analysis of 2-D alveolar geometry with sufficient temporal resolution to follow intratidal dynamics. The developed method and the reproducibility of the results were successfully validated with manually segmented video frames.      

Digital Simulation of an Adaptive System for Long-Term Ventilation of the Lungs

Digital simulation of a model reference adaptive control scheme for long-term ventilation of the lungs is presented. This adaptive scheme is capable of bringing the instantaneous alveolar pressure of the patient to its normal level within one inspiration period, following a possible change in the patient's respiratory parameters (i.e., airway resistances or lung and chest wall compliances).     

Resolving the hemodynamic inverse problem

The "hemodynamic inverse problem" is the determination of arterial system properties from pressures and flows measured at the entrance of an arterial system. Conventionally, investigators fit reduced arterial system models to data, and the resulting model parameters represent putative arterial properties. However, no unique solution to the inverse problem exists-an infinite number of arterial system topologies result in the same input impedance (Zin) and, therefore, the same pressure and flow. Nevertheless, there are exceptions to this theoretical limitation; total peripheral resistance (Rtot), total arterial compliance (Ctot), and characteristic impedance (ZO) can be uniquely determined from input pressure and flow. Zin is determined completely by Ctot and Rtot at low frequencies, Zo at high frequencies, and arterial topology and reflection effects at intermediate frequencies. We present a novel method to determine the relative contribution of Zo, Ctot, Rtot and arterial topology/reflection to Zin without assuming a particular reduced model. This method is tested with a large-scale distributed model of the arterial system, and is applied to illustrative cases of measured pressure and flow. This work, thus, lays the theoretical foundation for determining the arterial properties responsible for increased pulse pressure with age and various arterial system pathologies.     

A correction procedure for the asymmetry of differential pressuretransducers in respiratory impedance measurements

The usual setup for measuring respiratory input impedance requires a differential pressure transducer attached to a pneumotachograph. Because no data correction procedure has been devised to account for transducer asymmetry, a highly symmetrical transducer is required to obtain reliable impedance data. Here, 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, a dynamic calibration and data correction procedure is defined. This was tested using highly asymmetrical transducers (common-mode rejection ratio between 45 and 27 dB) to measure the impedance of two respiratory analogs. Results obtained show that respiratory input impedance can be adequately measured if data are corrected for transducer asymmetry     

Thoracic electrical impedance tomographic measurements during volume controlled ventilation-effects of tidal volume and positive end-expiratory pressure

The aim of the study was to analyze thoracic electrical impedance tomographic (EIT) measurements accomplished under conditions comparable with clinical situations during artificial ventilation. Multiple EIT measurements were performed in pigs in three transverse thoracic planes during the volume controlled mode of mechanical ventilation at various tidal volumes (V(T)) and positive end-expiratory pressures (PEEP). The protocol comprised following ventilatory patterns: 1) V(T)(400, 500, 600, 700 ml) was varied in a random order at various constant PEEP levels and 2) PEEP (2, 5, 8, 11, 14 cm H2O) was randomly modified during ventilation with a constant V(T). The EIT technique was used to generate cross-sectional images of 1) regional lung ventilation and 2) regional shifts in lung volume with PEEP. The quantitative analysis was performed in terms of the tidal amplitude of the impedance change, reflecting the volume of delivered gas at various preset V(T) and the end-expiratory impedance change, revealing the variation of the lung volume at various PEEP levels. The results showed: 1) an increase in the tidal amplitude of the impedance change, proportional to the delivered V(T) at all constant PEEP levels, 2) a rising end-expiratory impedance change, with PEEP reflecting an increase in gas volume, and 3) a PEEP-dependent redistribution of the ventilated gas between the planes. The generated images and the quantitative results indicate the ability of EIT to identify regional changes in V(T) and lung volume during mechanical ventilation.