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M R Wolfson

Publications and source records attributed to M R Wolfson.

115 records · Page 7Linked to original sources
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Quantitative structure-activity relationships of perfluorinated hetero-hydrocarbons as potential respiratory media. Application to oxygen solubility, partition coefficient, viscosity, vapor pressure, and density.

It has been extensively reported that liquid-assisted ventilation, using inert perfluorocarbon liquids (PFCs), can reduce interfacial surface tension and allow for improved ventilation at decreased alveolar pressures. PFCs are bioinert, minimally absorbed, and have no deleterious histologic, cellular, or biochemical effects when used as respiratory media. Although several types of PFCs have been characterized, a select few are considered to be compatible with life. Compatibility is often related to the physicochemical profile inherent to the PFC liquids. It is essential that certain physical properties such as respiratory gas solubility, vapor pressure, density, viscosity, and tissue permeability be within a narrow, acceptable range for a PFC to be considered as a possible candidate for respiratory media. The current study sought to characterize the physicochemical profile of commercially available PFCs. This was accomplished by creating a method for accurate, rapid prediction of a host of unknown physical characteristics of PFCs. The physicochemical properties of 16 perfluorinated hetero-hydrocarbons were catalogued from the literature. The input data were categorized into three major groups: empiric properties, geometric indices, and quantum mechanical descriptors, to generate a database. Algorithms were then developed, one for each dependent variable (FUNCTION), including oxygen solubility, partition coefficient (logP), vapor pressure, viscosity, and density, that related the values of these physical properties of potential breathable PFC liquids to the parameters listed in the database. The general form of the algorithm can be written as follows: FUNCTION = sigma (CiPi/magnitude of Pi) + constant; where the FUNCTIONS are oxygen solubility, logP, vapor pressure, viscosity, and density. Ci is a coefficient that weights the relative contribution of each parameter. Each independent parameter, Pi, was normalized by the average value of the parameter used in the analysis, magnitude of Pi. Residual analysis demonstrated validity with all five equations. This method is expected to assist in the prediction of physical properties of PFC liquids with acceptable accuracy, such that PFC production and selection from currently available liquids can be optimized for each liquid ventilation application.

Absorption↗
PubMedDate not supplied

Liquid ventilation: clinical experiences.

Liquid breathing has been in the medical literature for nearly 80 years and has been proposed as a means of improving gas exchange in patients with acute respiratory failure since the 1970s. There are many potential clinical applications of perfluorochemical (PFC) liquids that span many specialties in medicine. The ability to lower surface tension directed the initial clinical focus on neonatal therapy in the treatment of respiratory distress syndrome. The first clinical trial of PFC ventilation was performed in neonates in 1989. Additional trials using LiquiVent, a medical-grade PFC liquid, were initiated in 1993 in infants, children, and adults with severe respiratory distress. Based on the results of several studies during this time, it was concluded that the technique of liquid ventilation seemed to be safe, improve lung function, and recruit lung volume in patients from these various populations. More than 100 patients from preterm neonates to elderly patients with respiratory distress have been studied. In general, patients experience improvement in lung function and oxygenation, without adverse events. The results of such trials are encouraging and suggest the feasibility of this technique in the neonate with severe respiratory failure requiring respiratory support. Limitations of the technique include a limited understanding of how it works and its effectiveness in different patient populations. It has also become evident that there is a need for additional diagnostic and therapeutic instrumentation to expedite its implementation in the intensive care environment. This article discusses clinical experiences with liquid ventilation and highlights the technical needs to facilitate its implementation in respiratory care management.

Adolescent↗
PubMedDate not supplied

Multifactorial analysis of exchanger efficiency and liquid conservation during perfluorochemical liquid-assisted ventilation.

Liquid-assisted ventilation (LAV) of the lung with perfluorochemical (PFC) requires a method of oxygenating and removing CO2 from the liquid. Current PFC LAV techniques consist of total liquid ventilation, PFC lavage, and partial liquid ventilation. Because PFC liquid is volatile, it may be lost from the lung or ventilator circuit in the expired gas. This study evaluated the efficiencies of two types of exchangers (spray bubbler and membrane oxygenator) with respect to CO2 elimination from the PFC liquid and prevention of the loss of PFC liquid. A multifactorial analysis of exchanger efficiency was performed with respect to liquid conservation and CO2 removal. PFC losses and relative efficiencies of two types of exchangers to eliminate CO2 from expired PFC liquid were evaluated, along with two types of PFC liquids. Gas (100% O2 at 4 and 8 L/min) and PFC liquid were circulated countercurrently through the exchangers (oxygenator and bubbler) through a temperature-controlled (25 degrees or 37 degrees C) open circuit. To evaluate effectiveness of CO2 elimination, an exchanger efficiency index (EEI) for CO2 was calculated applying mass-transfer theory to characterize gas transport down a concentration gradient where EEI equals: [PPFCCO2 out--PPFCCO2 in]/PgasCO2 in--PPFCCO2 in]. Rate of PFC loss from the circuit was calculated from mixed expired gas samples using a thermal detector analyzer. EEI and PFC loss rate were analyzed with respect to gas: PFC liquid flow ratios (analogous to the V/Q ratio). The results showed that 1) PFC loss rate and exchanger efficiency to remove CO2 increased with increasing gas: PFC liquid flow rates independent of the type of exchanger or PFC liquid; 2) PFC loss rate at any gas or liquid flow rate was greater for the bubbler than for the oxygenator; 3) the oxygenator was more efficient than the bubbler with respect to CO2 elimination; 4) although PFC loss rate increased with temperature and vapor pressure, there was little difference in the EEIs for the temperatures studied. These results 1) identify exchanger requirements necessary to maintain effective CO2 elimination up to four times normal CO2 loading conditions during LAV; 2) suggest that using a membrane oxygenator as the gas exchanger, in concert with stringent fluid temperature control, improves PFC liquid conservation and CO2 elimination relative to bubbler exchanger configurations; 3) highlight the importance of matching the exchanger type to the physiocochemical properties of the specific PFC liquid to optimize CO2 elimination while reducing PFC liquid loss by minimizing gas relative to PFC liquid flow rates. Because PFC liquid loss occurs with all current means of oxygenating and removing CO2, this study raises the importance of developing alternative, bulk-gas-flow-independent, means to recondition PFC liquids.

Carbon Dioxide↗
PubMedDate not supplied

Software for real-time control of a tidal liquid ventilator.

The purpose of this project was to develop and test computer software and control algorithms designed to operate a tidal liquid ventilator. The tests were executed on a 90-MHz Pentium PC with 16 MB RAM and a prototype liquid ventilator. The software was designed using Microsoft Visual C++ (Ver. 5.0) and the Microsoft Foundation Classes. It uses a graphic user interface, is multithreaded, runs in real time, and has a built-in simulator that facilitates user education in liquid-ventilation principles. The operator can use the software to specify ventilation parameters such as the frequency of ventilation, the tidal volume, and the inspiratory-expiratory time ratio. Commands are implemented via control of the pump speed and by setting the position of two two-way solenoid-controlled valves. Data for use in monitoring and control are gathered by analog-to-digital conversion. Control strategies are implemented to maintain lung volumes and airway pressures within desired ranges, according to limits set by the operator. Also, the software allows the operator to define the shape of the flow pulse during inspiration and expiration, and to optimize perfluorochemical liquid transfer while minimizing airway pressures and maintaining the desired tidal volume. The operator can stop flow during inspiration and expiration to measure alveolar pressures. At the end of expiration, the software stores all user commands and 30 ventilation parameters into an Excel spreadsheet for later review and analysis. Use of these software and control algorithms affords user-friendly operation of a tidal liquid ventilator while providing precise control of ventilation parameters.

Algorithms↗
PubMedDate not supplied

On-line techniques for perfluorochemical vapor sampling and measurement.

The authors developed a compact gas sampling and perfluorochemical (PFC) measuring system for use in total and partial liquid ventilation systems, based on a precision two-thermistor thermal detector (TD). They describe the sensitivity and linearity of their on-line method for PFC analysis of expired gases and show how it may be used in partial liquid ventilation studies for determining PFC saturation and loss. Gas is sampled for a short time from a breathing circuit through a heated tube at a selectable point in the breathing cycle. Inspiration is sensed by a pressure transducer. The sample of gas is pulled into the heated (48 degrees C) thermistor chamber by suction and held there while the cooling effect of the vapor changes the thermistor temperature. Dry air in another chamber affects a second thermistor, and the difference of these responses is amplified. The raw signal is corrected for the effects of varying O2 levels by a fuel cell. This signal is sampled and held and displayed on a front panel display. Calibration is performed in percentage saturation at 37 degrees C using the PFC in use at that temperature, or another standard such as O2. In-vitro testing showed a linear response in the thermal detector device (R2 = 0.99) over the range of vapor pressures tested (0-14) mmHg) and was reproducible to within 3%. When electronically corrected for changes in O2 concentration, there was less than a 2% change in PFC saturation. The TD responses to CO2 (R2 = 0.99) and water vapor (R2 = 1.0) were linear and approximately equal and opposite over the normal operating ranges of expired gases. In-vivo results in rabbits showed a significant (R2 = 0.73; p < 0.01) correlation between the auto-sampler and manual collection modes for determination of PFC in expired gas.

Animals↗
PubMedDate not supplied

A perfluorochemical loss/restoration (L/R) system for tidal liquid ventilation.

Tidal liquid ventilation is the transport of dissolved respiratory gases via volume exchange of perfluorochemical (PFC) liquid to and from the PFC-filled lung. All gas-liquid surface tension is eliminated, increasing compliance and providing lung protection due to lower inflation pressures. Tidal liquid ventilation is achieved by cycling fluid from a reservoir to and from the lung by a ventilator. Current approaches are microprocessor-based with feedback control. During inspiration, warmed oxygenated PFC liquid is pumped from a fluid reservoir/gas exchanger into the lung. PFC fluid is conserved by condensing (60-80% efficiency) vapor in the expired gas. A feedback-control system was developed to automatically replace PFC lost due to condenser inefficiency. This loss/restoration (L/R) system consists of a PFC-vapor thermal detector (+/- 2.5%), pneumatics, amplifiers, a gas flow detector (+/- 1%), a PFC pump (+/- 5%), and a controller. Gravimetric studies of perflubron loss from a flask due to evaporation were compared with experimental L/R results and found to be within +/- 1.4%. In addition, when L/R studies were conducted with a previously reported liquid ventilation system over a four-hour period, the L/R system maintained system perflubron volume to within +/- 1% of prime volume and 11.5% of replacement volume, and the difference between experimental PFC loss and that of the L/R system was 1.8 mL/hr. These studies suggest that the PFC L/R system may have significant economic (appropriate dosing for PFC loss) as well as physiologic (maintenance of PFC inventory in the lungs and liquid ventilator) impact on liquid ventilation procedures.

Calibration↗