On the estimation of pulmonary blood flow from CO2 production time series

It may be possible to estimate a nominal pulmonary blood flow (Q) during an exercise stress test via the algorithm used to estimate breath-by-breath alveolar CO2 production. Recently it has been demonstrated that by relating breath-to-breath fluctuations in alveolar CO2 production to breath-to-breath fluctuations in end-tidal CO2, an optimizing parameter related to Q can be determined that can be used to process the CO2 production fluctuations and minimize their variation. However, the reported values of Q using this procedure appear to be biased low. Using a computer simulation of gas exchange, we demonstrate that the estimate of Q is biased low when the nominal lung volume used in the alveolar gas exchange algorithm is too large. Furthermore, alveolar CO2 transport is determined by an integral of alveolar CO2 over the breath time and, thus, is a path-dependent quantity. The use of end-tidal CO2 fluctuations to approximate fluctuations in this integral contributes to an error in the estimation of Q which yields estimates that are biased low. Alternatively, the use of mean alveolar CO2 fluctuations yield more appropriate Q estimates. These results suggest practical implications for estimating effective pulmonary blood flow during an exercise stress test by using breath-to-breath estimates of mean alveolar CO2.     

Classification of nasal inspiratory flow shapes by attributed finite automata.

In a significant proportion of individuals, the physiologic decrease of muscle tone during sleep results in increased collapsibility of the upper respiratory airway. At peak inspiratory flow, the pharyngeal soft tissues may collapse and cause airflow limitation or even complete occlusion of the upper airway (sleep apnea). While there are plenty of methods to detect sleep apnea, only a few can be used to monitor flow limitation in sleeping individuals. Nasal prongs connected to pressure sensor provide information of the nasal airflow over time. This paper documents a method to automatically classify each nasal inspiratory pressure profile into one without flow limitation or six flow-limited ones. The recognition of the sample signals consists of three phases: preprocessing, primitive extraction, and word parsing phases. In the last one, a sequence of signal primitives is treated as a word and we test its membership in the attribute grammars constructed to the signal categories. The method gave in practical tests surprisingly high performance. Classifying 94;pc of the inspiratory profiles in agreement with the visual judgment of an expert physician, the performance of the method was considered good enough to warrant further testing in well-defined patient populations to determine the pressure profile distributions of different subject classes.     

CO2 elimination as predicted by augmented dispersion and convection in an asymmetrical dog airway model

We have developed a computer model of the dog lung based upon an asymmetrically branching network of tubes to describe gas exchange during high-frequency oscillations. Impedances to oscillatory flows were calculated for each airway segment and used to determine flow distributions in all airway generations. Gas exchange was assumed to occur by convective and augmented dispersive mechanisms. Also included in the model were features commonly found in animal studies including the effects of bias flow of fresh gas at the airway opening and equipment dead-space volume. The magnitude of CO2 elimination predicted by the model closely resembled actual experimental data. Specifically, it predicted that CO2 elimination is proportional to frequency to the 0.82 power and tidal volume to the 1.25 power. It also indicated that the bias flow rate and equipment dead-space volume influence gas exchange characteristics and thus these variables should be considered when comparing data from different studies.     

并列双U型通风方式下采空区瓦斯运移规律研究

为了更好地分析和掌握并列双U型通风方式下采空区瓦斯分布和运移规律,针对某矿综采工作面并列双U型通风系统的特点,建立了CFD模型,数值模拟了并列双U型通风方式下采空区瓦斯运移规律;分析结果表明,在综采工作面采用并列双U型通风方式,由于位于采空区上隅角附近联络巷的作用使采空区的大部分瓦斯可以随漏风进入辅助回风巷连续排出,使采空区和流经上隅角的瓦斯减少,有效解决了综采工作面瓦斯超限的问题。     

The extended Kalman filter as a pulmonary blood flow estiṁator

Pulmonary blood flow is represented as a time-varying parameter in a simple model of the bodily uptake of gases that are soluble in blood. The pulmonary blood flow is estimated on-line using Ljung's modification of the extended Kalman filter. The experimental technique for estimation of pulmonary blood flow is based on Zwart's soluble gas method, modified to permit time-varying ventilation and pulmonary perfusion, and also to permit binary dynamic forcing of the inhaled soluble gas. Computer simulations were performed to determine the convergence time and to choose values of algorithmic parameters. Experiments done in humans gave agreement to within the accuracy of that measured invasively by a thermal dilution technique. The method shows promise for use in many biomedical applications, including patient monitoring, physiological experimentation, and clinical heart function tests.     

A model of ventilation of the healthy human lung

This paper presents a model of the lung mechanics which simulates the pulmonary alveolar ventilation. The model includes aspects of: the alveolar geometry; pressure due to the chest wall; pressure due to surface tension determined by surfactant activity; pressure due to lung tissue elasticity; and pressure due to the hydrostatic effects of the lung tissue and blood. The cross-sectional area of the lungs in the supine position derived from computed tomography is used to construct a horizontally layered model, which simulates heterogeneous ventilation distribution from the non-dependent to the dependent layers of the lungs. The model is in agreement with experimentally measured hysteresis of the pressure-volume curve of the lungs, static lung compliance, changes in lung depth during breathing and density distributions at total lung capacity (TLC) and residual volume (RV). In the dependent layers of the lungs, alveolar collapse may occur at RV, depending on the assumptions concerning lung tissue elasticity at very low alveolar volumes. The model simulations showed that ventilation increased with depth in the lungs, although not as pronounced as observed experimentally. The model simulates alveolar ventilation including all of the mentioned components of the respiratory system and to be validated against all the above mentioned experimental data.     

A new modulation technique and compensation algorithm for mainstream CO2 capnography

Carbon dioxide, as a metabolic product of human, is correlated with a patient’s perfusion and ventilation. In medicine, the end-tidal carbon dioxide partial pressure refers to the measurement of exhaled carbon dioxide, and end-tidal carbon dioxide partial pressure monitoring has become an important tool in clinical monitoring. However, there are some aspects needed to be improved. So, the low-frequency modulation is used to reliably acquire the respiration information. The measurement method is improved based on the Lambert–Beer Law. Also, we compensate the initial absorbance because of the light absorbance of the sensor at different wavelengths. The influence of the temperature change is analyzed on the measurement. All of these methods make the end-tidal carbon dioxide partial pressure accurate and convenient.     

Analytical study of resistive MEM gas flow meters

An analytical study for micro-electro mechanical system (MEMS) type gas flow meters is presented. A bulk micromachined structure for the flow meter is proposed. It consists of a micro resistive heater and two temperature sensors situated at the opposite sides of the heater. The silicon substrate is considered to be thermally isolated from the heater by a stacked (SiO₂/Si₃N₄) membrane on a bulk micromachined cavity. The flow meter is considered to work on the bases of displacement of temperature profile around the heating element due to the gas flow. The obtained equation from the analytical model is applied to a specific device dimension. The calculated results show a linear relationship between the gas flow velocity and the device response when the heater and sensing elements separation is about 80 μm. The linearity decreases for increased separation between the heater and the sensing elements as well as the gas flow velocity. The proposed device is also simulated by finite element method using Ansys/Flotran software. The simulation results are in a good agreement with the analytical results. Our analytical results can assist the MEMS gas flow meter designers to define the position of the sensing elements with respect to the heater accurately, depending on the required output signal linearity and gas flow velocity.     

Couette–Poiseuille flow of a gas in long microchannels

The flow of a compressible, isothermal gas under slightly rarefied conditions in a 2D planar geometry is considered. The gas is shear driven and is also subject to an applied pressure gradient, which is also known as Couette–Poiseuille (CP) flow. In this paper, the full Navier-Stokes (NS) equations are solved using a perturbation expansion up to the first order. The pressure profile is solved numerically. On the basis of the solutions, effects of rarefaction and compressibility on the flow characteristics are investigated in detail. The results show the parallel flow assumption to be invalid for cases with slight rarefaction. The axial and vertical velocity components are found to depend on the degree of rarefaction, applied pressure gradient and wall velocity. The effects of rarefaction on the occurrence of back flow are also discussed. In addition, the results for the Poiseuille and CP flow with and without rarefaction can be easily obtained from our results.     

Application of the general linear model for smoothing gas exchange data

The precision of an interpretation of gas exchange records in progressive exercise is limited by the typical breath-to-breath variation in the data. Recently, two procedures have been proposed for minimizing the "noise" in the estimates of alveolar gas exchange time series data. One approach utilizes an estimate of pulmonary blood flow (Q) for smoothing purposes. The other approach utilizes an estimate of effective lung volume (V'L) for smoothing purposes. In this paper, we formulate the smoothing problem as a general linear model and demonstrate the concurrent estimates of both V'L and Q. Furthermore, we investigate the interaction between V'L and Q. Specifically, when a high value of lung volume is used (such as the subject's resting functional residual capacity) in the alveolar gas exchange algorithm, the estimate of Q is biased low and the result is a less effective smoothing of the data. In addition, we demonstrate how the Q estimate can be improved by utilizing more appropriate estimates of arterial carbon dioxide tension.     

Closed-loop control of extracorporeal oxygen and carbon dioxide gas transfer

Additional extracorporeal gas transfer facilitates ultra-protective mechanical ventilation during treatment of severe lung disease. The proposed automation contributes to both patient safety and therapeutic success. A decentralized control system set the oxygen and carbon dioxide gas transfer rates. The controlled variables are estimated using standard measurement devices without direct blood contact. To reduce patient stress, an outer-loop integral controller adjusts the extracorporeal blood flow. The control system was first evaluated in silico and then in vivo using an animal model. Finally, the method is shown to be feasible and its response time is sufficient to meet patients' clinical needs.     

An improved experimental method for local clothing ventilation measurement

A clothing local ventilation measuring device based on the Lotens–Havenith steady state tracer gas method was developed and an improved experimental method for understanding local ventilation mechanisms was proposed. The local ventilation system can measure the arm, chest and back ventilation rates at the same time. Local ventilation mechanisms of an impermeable garment at two activities (static, walking) and two wind speeds (no wind, 1.2 m/s) were studied, with a focus on determining the pathways of ventilation through the different garment openings. The results showed that local ventilation rates of chest, back and arm varied considerably over locations and conditions. As expected, ventilation rates were highest for all locations at walking with wind conditions. Ventilation mechanism changed at different walking and wind conditions. The main air exchange pathway for all locations was through the garment bottom. Wind had a greater impact on clothing local ventilation than walking.     

Increasing test accuracy by varying driver slew rate

A variable slew rate, together with the ability to control ascending and descending slew rates independently, significantly improves the overall accuracy of test and verification systems for application-specific ICs. Although a high slew rate is usually desirable, in some cases, such as ECL devices and devices in circuits of older vintage, a variable rate is advantageous. Essential driver characteristics are identified, and the driver model is described. For a small parts cost, and with only a negligible increase in power requirements, it is estimated that independent control of slew rates for ascending and descending edges can improve tester accuracy by several hundred picoseconds     

Synchronization of a class of nonlinear network flow systems

In this paper, we consider the synchronization of a class of nonlinear network flow systems. Motivated by air distribution problem in air conditioning and mechanical ventilation (ACMV) systems, we propose a class of coupled nonlinear multi‐agent systems that can model a wide class of network flow systems, including air flow in ACMV systems, water flow in irrigation systems, traffic flow in transportation systems, and so on. Then we consider the synchronization problem for the class of nonlinear multi‐agent systems and propose cooperative controllers for the system. Based on graph theory, we derive conditions on the initial values of the state and the control input such that synchronization can be achieved. An application to air ventilation is provided to demonstrate the effectiveness of the cooperative controllers. Copyright © 2015 John Wiley & Sons, Ltd.     

Inspiratory flow shape clustering: an automated method to monitor upper airway performance during sleep

We describe an automated method for monitoring airflow dynamics in the upper airway of a sleeping subject. Its main task is to determine a set of inspiratory flow shape representatives and their relative incidence in a given respiratory airflow material. The flow shape clustering aims at reducing redundant information in the data, and thereby decreases the time needed to score overnight sleep recordings. Compared with previous computer-assisted systems, built on a pre-defined classification of prototype shapes, we require no a priori assumptions of the flow shape clusters to be discovered. The intrinsic flow shape clustering is performed with a modification of the Isodata algorithm, and the K-means clustering is used as a reference in comparison studies. The operation of the method is demonstrated on clinical sleep recordings both from patients with nocturnal breathing disorders and from non-symptomatic individuals. The feasible results obtained in the practical research design suggest that application of clustering algorithms to respiratory airflow measurements could give important insights into the subtle flow shape abnormalities underlying obstructive sleep-disordered breathing.     

Impact of aortic repair based on flow field computer simulation within the thoracic aorta

Purpose of this computational study is to examine the hemodynamic parameters of velocity fields and shear stress in the thoracic aorta with and without aneurysm, based on an individual patient case and virtual surgical intervention. These two cases, case I (with aneurysm) and II (without aneurysm), are analyzed by computational fluid dynamics. The 3D Navier-Stokes equations and the continuity equation are solved with an unsteady stabilized finite element method. The vascular geometries are reconstructed based on computed tomography angiography images to generate a patient-specific 3D finite element mesh. The input data for the flow waveforms are derived from MR phase contrast flow measurements of a patient before surgical intervention.The computed results show velocity profiles skewed towards the inner aortic wall for both cases in the ascending aorta and in the aortic arch, while in the descending aorta these velocity profiles are skewed towards the outer aortic wall. Computed streamlines indicate that flow separation occurs at the proximal edge of the aneurysm, i.e. computed flow enters the aneurysm in the distal region, and that there is essentially a single, slowly rotating, vortex within the aneurysm during most of the systole.In summary, after virtual surgical intervention in case II higher shear stress distribution along the descending aorta could be found, which may produce more healthy reactions in the endothelium and benefit of vascular reconstruction of an aortic aneurysm at this particular location.     

MEMS气体流量传感器的简易流量测试装置

为了完成所研制的MEMS气体流量传感器样品的流量测试与标定,设计制作了一种由标准流量发生器和传感器信号读出与数据采集电路组成的简易流量测试装置。标准流量发生器由注射器和可更换的砝码组成,利用不同的砝码配重,在注射器出气口产生合适的恒定气体流速。通过理论分析和Ansys有限元数值仿真,验证了简易标准流量发生器的可行性。传感器信号读出与数据采集电路基于内建多路A/D转换器的单片机实现,具有传感器加热电阻器的恒温控制、流量信号的数字检测和显示的功能。采用该简易流量测试装置对自行研制的MEMS气体流量传感器进行了流量测试与标定,获得了待测器件的标定参数、传感器流量测量的绝对误差和相对误差。     

Large-eddy simulation of streamwise-rotating turbulent channel flow

In many engineering and industrial applications the investigation of rotating turbulent flow is of great interest. Whereas some research has been done concerning channel flows with a spanwise rotation axis, only few investigations have been performed on channel flows with a rotation about the streamwise axis. In the present study an LES of a turbulent streamwise-rotating channel flow at Re_τ = 180 is performed using a moving grid method. The three-dimensional structures and the details of the secondary flow distribution are analyzed and compared with experimental data. The numerical-experimental comparison shows a convincing agreement as to the overall flow features. The results confirm the development of a secondary flow in the spanwise direction, which has been found to be correlated to the rotational speed. Furthermore, the findings show the distortion of the main flow velocity profile, the slight decrease of the streamwise Reynolds stresses in the vicinity of the walls, and the pronounced increase of the spanwise Reynolds stresses at higher rotation rates near the walls and particularly in the symmetry region. As to the numerical set-up it is shown that periodic boundary conditions in the spanwise direction suffice if the spanwise extent of the computational domain is larger than 10 times the channel half width.     

Modeling of indoor airflow and dispersion of aerosols using immersed boundary and random flow generation methods

Numerical modeling of environmental flows involves complex geometry, moving bodies, multi-phase flow, and buoyant jet effects. An in-house CFD code has been developed using finite volume and immersed boundary methods. The transport and dispersion of virus-laden aerosols in a ventilated room is investigated by this numerical code. The uniqueness of this numerical code is that it can efficiently compute small-scale turbulent flow in which most commercial CFD software will suffer from large numerical error. Random flow generation (RFG) [33] is an ideal choice for small-scale turbulence for low-Reynolds number flow in a ventilated room. In addition, Lagrangian stochastic (LS) walk model is applied to directly compute probability density function (PDF) of aerosols and estimate the risk factor of aerosol dispersion from a point source. The present study focuses on aerosols with small diameter (<10 μm) in which the effects of evaporation on the dispersion of aerosols could be neglected. Different location of aerosol sources and a typical ventilation layout are discussed in detail. The numerical results with PDF yield more useful quantitative information to assess the risk area of virus transport in a ventilated room than that shown in random trajectories of particles as widely reported in the literature. This study provides valuable information for ventilation control strategies with respiratory protection, such as enhanced air exchange, air filtration rate, and improved airflow patterns to reduce indoor infection risk via airborne virus laden droplets.