A Theoretical Study of Controlled Ventilation

This paper is a theoretical study of controlled ventilation with a respirator. An electric circuit analog of the respiratory system is used with respiratory parameters of the normal adult. Deleterious effects of positive pressure ventilation are considered and related to the model. The effects considered include peak alveolar pressure and its time of occurrence, work, and average alveolar pressure. Different pressure waveforms are used as the driving source for the respirator model. These waveforms are rectangle, ramp, negative ramp, sine, and exponential.     

Design and control of a demand flow system assuring spontaneous breathing of a patient connected to an HFO ventilator

Lung protective ventilation is intended to minimize the risk of ventilator induced lung injury and currently aimed at preservation of spontaneous breathing during mechanical ventilation. High-frequency oscillatory ventilation (HFOV) is a lung protective ventilation strategy. Commonly used high-frequency oscillatory (HFO) ventilators, SensorMedics 3100, were not designed to tolerate spontaneous breathing. Respiratory efforts in large pediatric and adult patients impose a high workload to the patient and may cause pressure swings that impede ventilator function. A Demand Flow System (DFS) was designed to facilitate spontaneous breathing during HFOV. Using a linear quadratic Gaussian state feedback controller, the DFS alters the inflow of gas into the ventilator circuit, so that it instantaneously compensates for the changes in mean airway pressure (MAP) in the ventilator circuit caused by spontaneous breathing. The undesired swings in MAP are thus eliminated. The DFS significantly reduces the imposed work of breathing and improves ventilator function. In a bench test the performance of the DFS was evaluated using a simulator ASL 5000. With the gas inflow controlled, MAP was returned to its preset value within 115 ms after the beginning of inspiration. The DFS might help to spread the use of HFOV in clinical practice.     

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.     

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 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.     

不同环境压强下激光空泡特性研究

构建了激光空泡测量实验平台,使用脉冲激光聚焦击穿水介质产生激光空泡,由水听器对激光空泡溃灭辐射声信号进行接收,利用充气泵对高压水箱内的气压进行精确控制。通过仿真计算和实验对不同环境压强下的激光空泡特征和其溃灭时辐射声信号的峰值变化特性进行了研究。结果表明:当环境压强处在0.1~0.7 MPa 范围内变化时,随着环境压强的增大,激光空泡首次脉动周期和空泡最大半径逐渐减小,两者的变化速率逐渐减小。空泡溃灭时辐射声信号的峰值声压在0.1~0.4 MPa内逐渐增大,在0.4~0.7MPa 内逐渐减小,且增大速率大于减小速率。     

Measurement of fractional order model parameters of respiratory mechanical impedance in total liquid ventilation

This study presents a methodology for applying the forced-oscillation technique in total liquid ventilation. It mainly consists of applying sinusoidal volumetric excitation to the respiratory system, and determining the transfer function between the delivered flow rate and resulting airway pressure. The investigated frequency range was f ∈ [0.05, 4] Hz at a constant flow amplitude of 7.5 mL/s. The five parameters of a fractional order lung model, the existing "5-parameter constant-phase model," were identified based on measured impedance spectra. The identification method was validated in silico on computer-generated datasets and the overall process was validated in vitro on a simplified single-compartment mechanical lung model. In vivo data on ten newborn lambs suggested the appropriateness of a fractional-order compliance term to the mechanical impedance to describe the low-frequency behavior of the lung, but did not demonstrate the relevance of a fractional-order inertance term. Typical respiratory system frequency response is presented together with statistical data of the measured in vivo impedance model parameters. This information will be useful for both the design of a robust pressure controller for total liquid ventilators and the monitoring of the patient's respiratory parameters during total liquid ventilation treatment.     

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).     

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     

High-frequency and low-frequency chest compression: effects on lung water secretion, mucus transport, heart rate, and blood pressure using a trapezoidal source pressure waveform

High-frequency chest compression (HFCC), using an appropriate source (pump) waveform for frequencies at or above 3 Hz, can enhance pulmonary clearance for patients with cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD). Using a trapezoidal HFCC source pressure waveform, secretion of water from epithelial tissue and transport of mucus through lung airways can be enhanced for patients with CF and COPD. At frequencies below 3 Hz, low-frequency chest compression (LFCC) appears to have a significant impact on the cardiovascular system. For a trapezoidal source pressure waveform at frequencies close to 1 Hz, LFCC produces amplitude or intensity variations in various components of the electrocardiogram time-domain waveform, produces changes at very low frequencies associated with the electrocardiogram frequency spectra (indicating enhanced parasympathetic nervous system activity), and promotes a form of heart rate synchronization. It appears that LFCC can also provide additional cardiovascular benefits by reducing peak and average systolic and diastolic blood pressure for patients with hypertension.     

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.      

Hemodynamic Phenomena in Fusiform Aneurysms?II: Pulsatile Flow Conditions

Pressure and flow distributions are reported at different pulsatile flows for four latex fusiform aneurysm models. The accuracy of results by Pitot-static techniques is examined in terms of experimental evidence and theoretical considerations. Pressure distributions within the aneurysm are associated with the Reynolds number, the degree of wall divergence, and the degree of sac dilation, and these parameters are interactive. The Bernoulli principle was usually inapplicable, and higher peak pressures may be seen where measured velocities may lead one to predict lower pressures. The presence of such localized high pressures depends on whether disturbed flow occurs at that location. There may be little qualitative difference in the patterns of velocity and pressure within an aneurysm between pulsatile and steady flow conditions. However, with pulsatile flow, the presence of distributed flow may be more obvious and commence closer to the proximal end of the aneurysm.     

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.     

Compact high-energy Q-switched cladding-pumped fiber laser with atuning range over 40 nm

We describe a compact Q-switched diode pumped double-clad ytterbium-doped fiber laser. The fiber laser was bidirectionally pumped by two laser diodes (2 W of output power each) via two side-injecting pump-couplers. We used a large multimode core of 15 μm diameter to increase the laser gain volume and thus to achieve higher pulse energy. Experimentally this laser produced pulses with energy up to 170 μJ with a peak power of 2 kW (at a low repetition rate of 500 Hz) and was tunable from 1060 to 1100 nm     

PBP压电双晶片驱动器力学特性研究

为了获得后屈曲预压缩 （PBP）驱动器的力学特性,该文通过解析模型、有限元模型及实验验证对其进行了研究,3种结果符合较好。结果表明,PBP驱动器可达到10°的输出转角峰 峰值,3倍于施加同样电压的普通压电双晶片,设计空间增大。其一截频率达到178 Hz,远高于普通微小型电动伺服舵机。该文可为PBP舵机驱动器原理样机的研制提供理论基础和实验方法。     

A new method to generate a patient-specific finite element model of the human buttocks.

Finite element (FE) models are very efficient tools to study internal stresses in human structures that induce severe pressure sores. Unfortunately, methods currently used to generate FE models are not suitable for clinical application involving wheelchair users. A clinical-oriented method, based on calibrated-biplanar radiographs, was therefore developed to generate a subject-specific FE model of the buttocks in a non-weighted sitting position. The model was then used to analyze the stress distribution within the buttocks and compare two wheelchair seat cushions designs. Additional radiographs and pressure measurements in a weighted sitting position were acquired to validate the FE model experimentally. Results from the FE model were in good agreement with experimental data and related literature. An internal peak pressure of 45.3 kPa was observed while seated on a flat foam cushion, corresponding to an interface pressure of 23.6 kPa. Both pressures occurred underneath the ischial tuberosities. When compared to the flat foam cushion, the contoured foam cushion reduced internal and interface peak pressures by 18% and 33%, respectively. The method developed in this study has a great potential for clinical use. The FE model, by predicting realistic stress distributions, allows for the selection of a convenient wheelchair seat cushion.     

Effects of diameter, length, and circuit pressure on sound conductance through endotracheal tubes

We evaluated the acoustic frequency response of endotracheal tubes (ETs) to assess their effect on respiratory system sound transmission studies. White noise 150-3300 Hz was introduced into 4.0-, 6.0-, and 8.0-mm ETs and recorded at their proximal and distal ends. Four tubes of each size were studied at their original and normalized lengths, in straight and bent configurations, and at circuit pressures from 0 to 20 cmH2O. The characteristics of the sound transmission were compared using an analysis of variance for repeated measures. The average transmission amplitude varied directly with tube diameter. The position of peaks and troughs on the amplitude frequency distribution depended on tube length but not on tube diameter. The angle of the phase-frequency plot correlated well with the length of the tube and was independent of its diameter. A 90 degrees bend in the tube had no effect on its sound transmission. Increasing the circuit pressure above ambient modified the frequency response only if volume changes occurred in the test lung. When used to conduct sound into the respiratory system an ET affects the incident signal predictably depending on its length and diameter but not on its curvature or circuit pressure.     

A Device for the Calculation of dP/dt/P with Internal Calibration

An analog method for the calculation of dP/dt/P is described that in-corporates a means for internal calibration. Engineering tests of this device are reported as well as tests using pressures from the left ventricle of swine (20-25 Kgm). Interventions included inflation of a balloon in the inferior vena cava (peak pressure reduced by 50 percent), occlusion of the aorta (peak pressure increased by 100 percent), and pacing (from 150 to 210 beats/min). dP/dt/P (P denotes left-ventricular pressure) calculated from the analog device agreed within the noise level when compared with the calculation based on pressure alone using an A-D converter and digital computer.