A morphometric model of lung mechanics for time-domain analysis of alveolar pressures during mechanical ventilation

In this study we propose, and implement in the time domain, an anatomically consistent model of the respiratory system in critical care conditions that allows us to evaluate the impact of different ventilator strategies as well as of constrictive pathologies on the time course of acinar pressures and flows. We discuss the simplifications of the original Horsfield structure (Horsfield, K., [et_al.] Models of the human bronchial tree. J. Appl. Physiol. 31:207–217, 1971), which were needed to enable the model implementation. The model has a binary tree structure including large airways represented as a combination of wall compliance and laminar resistance, small airways that have the same arrangement but can be heterogeneously constricted, and alveolar compartments that are viscoelastic second-order models to represent the stress adaptation behavior of lung tissue. We have described patient–ventilator interactions modeling the ventilator and the endotracheal tube. In conclusion this model makes it possible to investigate realistically the effect of homogeneous versus heterogeneous constrictive pathologies and the impact of different ventilatory patterns on pressure and flow distribution at the acinar level in the mechanically ventilated patient. © 2002 Biomedical Engineering Society. PAC2002: 8719Uv, 8719Rr, 8710+e     

A microprocessor-controlled tracheal insufflation-assisted total liquid ventilation system

A prototype time cycled, constant volume, closed circuit perfluorocarbon (PFC) total liquid ventilator system is described. The system utilizes microcontroller-driven display and master control boards, gear motor pumps, and three-way solenoid valves to direct flow. A constant tidal volume and functional residual capacity (FRC) are maintained with feedback control using end-expiratory and end-inspiratory stop-flow pressures. The system can also provide a unique continuous perfusion (bias flow, tracheal insufflation) through one lumen of a double-lumen endotracheal catheter to increase washout of dead space liquid. FRC and arterial blood gases were maintained during ventilation with Rimar 101 PFC over 2–3 h in normal piglets and piglets with simulated pulmonary edema induced by instillation of albumin solution. Addition of tracheal insufflation flow significantly improved the blood gases and enhanced clearance of instilled albumin solution during simulated edema.     

Patient-ventilator interaction: A general model for nonpassive mechanical ventilation

A general mathematical model for the dynamic behaviour of asingle-compartment respiratory system in response to an arbitraryapplied inspiratory airway pressure and arbitrary respiratorymuscle activity is investigated. The model is used to computeexplicit expressions for ventilation and pressure variablesof clinical interest for clinician-selected and impedance-determinedinputs. The outcome variables include tidal volume, end-expiratorypressure, minute ventilation, mean alveolar pressure, averagepleural pressure, as well as the work performed by the ventilatorand the respiratory muscles. It is also demonstrated that undersuitable conditions, there is a flow reversal that can occurduring inspiration.     

A model of neonatal tidal liquid ventilation mechanics

Tidal liquid ventilation (TLV) with perfluorocarbons (PFC) has been proposed to treat surfactant-deficient lungs of preterm neonates, since it may prevent pulmonary instability by abating saccular surface tension. With a previous model describing gas exchange, we showed that ventilator settings are crucial for CO(2) scavenging during neonatal TLV. The present work is focused on some mechanical aspects of neonatal TLV that were hardly studied, i.e. the distribution of mechanical loads in the lungs, which is expected to differ substantially from gas ventilation. A new computational model is presented, describing pulmonary PFC hydrodynamics, where viscous losses, kinetic energy changes and lung compliance are accounted for. The model was implemented in a software package (LVMech) aimed at calculating pressures (and approximately estimate shear stresses) within the bronchial tree at different ventilator regimes. Simulations were run taking the previous model's outcomes into account. Results show that the pressure decrease due to high saccular compliance may compensate for the increased pressure drops due to PFC viscosity, and keep airway pressure low. Saccules are exposed to pressures remarkably different from those at the airway opening; during expiration negative pressures, which may cause airway collapse, are moderate and appear in the upper airways only. Delivering the fluid with a slightly smoothed square flow wave is convenient with respect to a sine wave. The use of LVMech allows to familiarize with LV treatment management taking the lungs' mechanical load into account, consistently with a proper respiratory support.     

A new expiratory support system for resolving air trapping in lungs during mechanical ventilation: a lung model study.

The aim of the study was to assess a new expiratory support (ES) system in resolving air trapping in the lungs during mechanical ventilation. The ES system consisted of a cylinder and two valves that were connected to the ventilatory circuit. The movements of them were synchronized with the ventilator. The cylinder aspirated gas during expiration. We compared the effects of the ES on air trapping between a narrower and an ordinary size endotracheal tube (ETT) (internal diameter (ID): 5 and 8 mm). In addition, we compared the effects of the ES on air trapping between normal and high airway resistance of the lungs (5 and 20 cm H(2)O/L/s). The ventilatory settings were as follows: volume controlled ventilation with constant inspiratory flow rate; tidal volume, 0.5 L; inspiratory time, 1.0 s; expiratory time, 1.0, 1.5, or 2.0 s; and PEEP, 0 cm H(2)O. The ES normalized the end-expiratory alveolar pressure of the 5 mm ID ETT at a level similar to that of the 8 mm ID ETT. The ES also resolved the air trapping induced by the high airway resistance of the lungs. In conclusion, the ES system resolved the air trapping associated with a narrow endotracheal tube and high airway resistance of the lungs.     

A mathematical model for carbon dioxide elimination: an insight for tuning mechanical ventilation

Purpose

The aim is to provide better understanding of carbon dioxide ( $\mathrm{CO}_2$ ) elimination during ventilation for both the healthy and atelectatic condition, derived in a pressure-controlled mode. Therefore, we present a theoretical analysis of $\mathrm{CO}_2$ elimination of healthy and diseased lungs.

Methods

Based on a single-compartment model, $\mathrm{CO}_2$ elimination is mathematically modeled and its contours were plotted as a function of temporal settings and driving pressure. The model was validated within some level of tolerance on an average of 4.9 % using porcine dynamics.

Results

$\mathrm{CO}_2$ elimination is affected by various factors, including driving pressure, temporal variables from mechanical ventilator settings, lung mechanics and metabolic rate.

Conclusion

During respiratory care, $\mathrm{CO}_2$ elimination is a key parameter for bedside monitoring, especially for patients with pulmonary disease. This parameter provides valuable insight into the status of an atelectatic lung and of cardiopulmonary pathophysiology. Therefore, control of $\mathrm{CO}_2$ elimination should be based on the fine tuning of the driving pressure and temporal ventilator settings. However, for critical condition of hypercapnia, airway resistance during inspiration and expiration should be additionally measured to determine the optimal percent inspiratory time (%TI) to maximize $\mathrm{CO}_2$ elimination for treating patients with hypercapnia.     

Total liquid ventilation provides superior respiratory support to conventional mechanical ventilation in a large animal model of severe respiratory failure

Total liquid ventilation (TLV) has the potential to provide respiratory support superior to conventional mechanical ventilation (CMV) in the acute respiratory distress syndrome (ARDS). However, laboratory studies are limited to trials in small animals for no longer than 4 hours. The objective of this study was to compare TLV and CMV in a large animal model of ARDS for 24 hours. Ten sheep weighing 53 ± 4 (SD) kg were anesthetized and ventilated with 100% oxygen. Oleic acid was injected into the pulmonary circulation until PaO2:FiO2 ≤ 60 mm Hg, followed by transition to a protective CMV protocol (n = 5) or TLV (n = 5) for 24 hours. Pathophysiology was recorded, and the lungs were harvested for histological analysis. Animals treated with CMV became progressively hypoxic and hypercarbic despite maximum ventilatory support. Sheep treated with TLV maintained normal blood gases with statistically greater PO2 (p < 10(-9)) and lower PCO2 (p < 10(-3)) than the CMV group. Survival at 24 hours in the TLV and CMV groups were 100% and 40%, respectively (p < 0.05). Thus, TLV provided gas exchange superior to CMV in this laboratory model of severe ARDS.     

Effect of repeated induced airway collapse during total liquid ventilation

Negative pressure generated during the expiratory phase of total liquid ventilation (TLV) may induce airway collapse. Evaluation of the effect of repeated airway collapse is crucial to optimize this technique. A total of 24 New Zealand White rabbits were randomly divided into four groups. Ventilation was performed for 6 hours with different strategies: conventional gas ventilation, TLV without airway collapse, and TLV with collapse induced in either 75 or 150 sequential breaths. In the treated groups, airway collapse was induced by increasing the perfluorocarbon drainage velocity while maintaining the minute ventilation constant. Airway pressure, gas exchange, and blood pressure were monitored at 30-minute intervals. At the end of the experiment, airway and lung parenchyma specimens were processed for light microscopy. No evidence of fluorothorax was noticed in any of the four groups at autopsy examination. Minimal signs of inflammation were noticed in all airway and lung parenchyma specimens, but no evident structural alteration was visible. Adequate gas exchange and systemic blood pressure were maintained during all the studies. Repeated airway collapse is not associated with structural changes in the respiratory system and does not alter the gas exchange ability of the lungs.     

Secretion movement during manual lung inflation and mechanical ventilation

This project aimed to investigate the direction of artificial sputum movement during mechanical ventilation (MV) and bagging (MH) using a tube model. Three solutions of artificial sputum (ultrasonic gel, viscosity 100, 200 and 300 poise (P)) were prepared. About 1 ml of gel was placed in a glass tube connected to a test lung at one end and, via a pneumotachograph, to either a mechanical ventilator or a self-inflating bag, at the other. The position of the gel in the tube was recorded before and after 20 artificial breaths. Simultaneous breath-to-breath respiratory mechanics were measured. The procedure was repeated three times for each gel viscosity, with a fresh experimental set up for each measurement. Results showed that the distance travelled from the lung was significantly greater with MH compared with MV (P < 0.001). The lower the gel viscosity, the further the gel moved from the lung with both ventilatory modes (P < 0.001). MH was superior to MV for secretion mobilisation in a tube model.     

A microprocessor based feedback controller for mechanical ventilation

A microcomputer feedback system has been developed which adjusts the inspired minute volume of a ventilator based on the patient's end-tidal CO₂ concentration. The feedback controlled ventilator was evaluated in 6 dogs (18–20 kg). Arterial PCO₂ was monitored continuously while end-tidal CO₂ concentration was controlled by the microcomputer system and the following perturbations introduced: [1] NaHCO₃ was infused IV, [2] a pulmonary artery was occluded, [3] one lumen of a double lumen endobronchial tube was occluded, and [4] an air embolism was given. The end-tidal PCO₂ controller kept PaCO₂ within 1.2 mm Hg of the desired value when CO₂ production increased by as much as 44%. Changing the ventilation/perfusion ratios caused differences as large as 22 mm Hg between the arterial and end-tidal PCO₂ and the controller was not effective in keeping PaCO₂ at the desired level. Closed loop control of ventilation based on end-tidal PCO₂ measurements successfully compensated for increases in CO₂ production keeping PaCO₂ constant. The controller did not, however, keep PaCO₂ at the desired level when significant changes occurred in the distribution of blood flow to ventilation. This work was supported by Siemens-Elema, Biochem Int. and Shriners Childrens Hospital.     

Anatomical hip model for the mechanical testing of hip protectors

An anatomical hip model has been developed to simulate the impact load on the hip of a falling person wearing a hip protector. The hip consists of an artificial pelvis made of aluminium, linked by a ball-and-socket joint to an anatomically shaped steel femur (thigh bone). The femur is embedded in silicone material with a hip-shaped surface to allow realistic positioning of the protectors with accessory underwear. Additionally, the silicone simulates the damping and load-dispersal effect of soft tissue. A triaxial load sensor is integrated in the neck of the femur to measure the axial and cross-sectional force components in response to external impact forces on the hip. The performance of the hip model was investigated in drop tests and validated against biomechanical data. In a first series of measurements, the shock absorption of 10 different hip protectors, including both energy-absorbing and energy-shunting systems, was analysed. To determine the importance of hip protector placement, each protector was tested in the correct anatomical alignment over the hip and anteriorly displaced by 3 cm. Considerable differences were found between individual hip protectors in their effectiveness to reduce impact forces on the femur. Position of the hip protector also influenced the forces applied to the femur.     

A new methodology for intra-breath control of mechanical ventilation

Recent improvements in digital control strategies are driving the implementation of innovative pulmonary ventilation modalities able to support the physician in the intensive care through a better management of the patient's respiratory pathologies. New control strategies, such as intra-breath ones, which guarantee successful patient-machine synchronization and dynamic adjustment to the patient, still require improvements in the control of the breathing process. Such improvements can be achieved by decreasing the response time of the actuation system driving the mechanical ventilators. This paper presents a new methodology of intra-breath control to generate traditional and high frequency ventilation techniques. The methodology is implemented through a custom-developed electro-mechanical system, which consists of on/off solenoid valves driven by a fast switching driver circuit. In vitro experimental trials show the system's ability to generate almost continuous flow-rate patterns in different shapes, a time resolution down to 20 ms, flow-rate resolution of 1L/min, and repeatability of 0.5L/min. Experimental results show that the proposed electro-mechanical system can be used as a stand-alone hardware solution for both intra-breath control techniques and high frequency ventilation.     

部分液体通气治疗家猪急性肺损伤的抗炎效应

目的：探讨部分液体通气治疗肺灌洗诱导的 急性肺损伤家猪模型时，是否具有抗炎作用。方法：16只健康家猪，采 用生理盐水肺内灌洗复制急性肺损伤模型，随机分为部分液体通气组及机械通气组给予不同 治疗，观察其肺脏湿/干比值及肺通透指数，观察其血浆、支气管肺泡灌洗液及肺组织匀浆 中TNF-α、MDA含量及SOD、MPO活性。结果：（1）部分液体通气组家猪 肺脏湿/干比值、肺通透指数及支气管肺泡灌洗液中白细胞计数明显低于机械通气组。（2） 肺组织MDA、MPO含量部分液体通气组明显低于机械通气组，但两组间SOD活性无明显差别。 （3）部分液体通气组支气管肺泡灌洗液及肺组织匀浆中TNF-α含量明显低于机械通气组。 结论：部分液体通气改善动物肺损伤指标，提示以氟碳化合物为呼吸媒 介的部分液体通气对肺灌洗诱导的急性肺损伤家猪具有抗炎效应。     

A miniature specimen mechanical testing technique scaled to articulating surface of polyethylene components for total joint arthroplasty

The small punch test was developed to investigate the mechanical behavior of polyethylene using miniature specimens (< 14 mg) measuring 0.5 mm in thickness and 6.4 mm in diameter. The objective of this study was to demonstrate the feasibility and reproducibility of the small punch test when applied to clinically relevant polyethylenes. Mechanical behavior was characterized during 66 tests performed on GUR4150HP and GUR4120 specimens following alternate sterilization methods and 4 weeks of accelerated aging at 80 degrees C. The small punch test was found to be highly reproducible with regard to characterizing the ductility, ultimate strength, and fracture resistance of sterilized and aged polyethylene. In the future, the small punch test can be used to directly measure mechanical properties near the articulating surface of retrieved components.     

AUTOPILOT-BT: a system for knowledge and model based mechanical ventilation.

A closed-loop system (AUTOPILOT-BT) for the control of mechanical ventilation was designed to: 1) autonomously achieve goals specified by the clinician, 2) optimize the ventilator settings with respect to the underlying disease and 3) automatically adapt to the individual properties and specific disease status of the patient. The current realization focuses on arterial oxygen saturation (SpO(2)), end-tidal CO(2) pressure (P(et)CO(2)), and positive end-expiratory pressure (PEEP) maximizing respiratory system compliance (C(rs)). The "AUTOPILOT-BT" incorporates two different knowledge sources: a fuzzy logic control reflecting expert knowledge and a mathematical model based system that provides individualized patient specific information. A first evaluation test with respect to desired end-tidal-CO(2)-level was accomplished using an experimental setup to simulate three different metabolic CO(2) production rates by means of a physical lung simulator. The outcome of ventilator settings made by the "AUTOPILOT-BT" system was compared to those produced by clinicians. The model based control system proved to be superior to the clinicians as well as to a pure fuzzy logic based control with respect to precision and required settling time into the optimal ventilation state.     

部分液体通气对猪急性肺损伤的影响

目的: 在肺灌洗诱导的急性肺损伤(ALI)家猪实施以氟碳(PFC)为媒介的部分液体通气(PLV), 观察其对肺气体交换及血液动力学的影响, 并比较PLV及气体通气两种方式对肺形态学的影响, 以探讨PLV改善肺气体交换的机制。方法: 12头体重为25-30 kg的健康雌猪,麻醉后经肺灌洗建立ALI模型。随机分为PLV组及CV(气体通气组)。对照组以呼吸机行常规机械通气, PLV组动物经气管导管肺内灌以15 mL/kg的PFC,然后以与对照组同样的呼吸参数行机械通气。每小时记录肺气体交换及血液动力学参数的变化。于实验结束后处死动物, 取肺标本送病理分析。 结果: PLV组肺气体交换明显改善,而血液动力学在整个实验过程中基本维持稳定。组织学检查: 光学显微镜下可见两组均为肺不张、肺气肿及肺过度膨胀交替出现,肺泡间质因水肿及细胞渗出而扩展。PLV组与CV组比较显示较轻微的肺不张、肺内炎性细胞渗出、肺出血及肺间质扩张。PLV组动物腹侧肺组织学损伤改变明显较背侧轻微。 结论: 在肺灌洗诱导的急性肺损伤幼猪, 以氟碳为媒介的部分液体通气可明显地改善肺气体交换,并无明显的血液动力学损伤。同时可见PLV相对于对照组明显减轻的肺组织学损伤及肺泡扩张, 证实PLV具一定的肺保护作用。     

Determinants of plasma copeptin: A systematic investigation in a pediatric mechanical ventilation model

Copeptin, the C-terminal part of the arginine vasopressin precursor peptide, holds promise as a diagnostic and prognostic plasma biomarker in various acute clinical conditions. Factors influencing copeptin response in the critical care setting are only partially established and have not been investigated systematically. Using an in vivo infant ventilation model (Wistar rats, 14 days old), we studied the influence of commonly occurring stressors in critically ill children. In unstressed ventilated rats basal median copeptin concentration was 22 pmol/L. In response to respiratory alkalosis copeptin increased 5-fold, while exposure to hypoxemia, high PEEP, hemorrhage, and psycho-emotional stress produced a more than 10-fold increase. Additionally, we did not find a direct association between copeptin and acidosis, hypercapnia, and hyperthermia. Clinicians working in the acute critical care setting should be aware of factors influencing copeptin plasma concentrations. Moreover, our results do have implications for animal studies in the field of stress research.