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PubMed2002

Molecular dynamics simulations of a double unit cell in a protein crystal: volume relaxation at constant pressure and correlation of motions between the two unit cells.

Eight molecular dynamics simulations of a double crystal unit cell of ubiquitin were performed to investigate the effects of simulating at constant pressure and of simulating two unit cells compared to a single unit cell. To examine the influence of different simulation conditions, the constant-pressure and constant-volume simulations were each performed with and without counterions and using two different treatments of the long-range electrostatic interactions (lattice-sum and reaction-field methods). The constant-pressure simulations were analyzed in terms of unit cell deformation and accompanying protein deformations. Energetic and structural properties of the proteins in the simulations of the double unit cell were compared to the results of previously reported one-unit-cell simulations. Correlation between the two unit cells was also investigated based on relative translational and rotational movements of the proteins and on dipole fluctuations. The box in the constant-pressure simulations is found to deform slowly to reach convergence only after 5-10 ns. This deformation does not result from a distortion in the structure of the proteins but rather from changes in protein packing within the unit cell. The results of the double-unit-cell simulations are closely similar to the results of the single-unit-cell simulations, and little motional correlation is found between the two unit cells.

Chlorides↗
PubMed2005

Construct and face validity of MIST-VR, Endotower, and CELTS: are we ready for skills assessment using simulators?

BACKGROUND: Video trainers may best offer visually realistic laparoscopic simulation, whereas virtual reality (VR) modules may best provide multidimensional objective measures of performance. This study compares the construct and face validity of three different laparoscopic simulators. METHODS: Subjects were voluntarily enrolled at the Learning Center during the 2004 SAGES annual meeting. Each subject completed two repetitions of a single task on each of three simulators, MIST-VR, Endotower, and CELTS; performance scores were automatically generated and recorded. Scores of individuals with various levels of experience were compared to determine construct validity for each simulator. Experience was defined according to four parameters: (a) PGY level, (b) fellowship training, (c) basic laparoscopic cases, and (d) advanced laparoscopic cases. Subjects rated each simulator regarding six face validity (realism of simulation) parameters using a 10-point Likert scale (10 = best rating) and participant scores were compared to previously established expert scores (proficiency goals for training). RESULTS: Ninety-one attendees completed the study. Construct validity was demonstrated for all three simulators; significant differences in scores were detected according to one parameter for MIST-VR, two parameters for Endotower, and all four parameters for CELTS. Face validity was rated as good to excellent for all three simulators (7.0 +/- 0.3 for MIST-VR, 7.9 +/- 0.3 for Endotower [p < 0.001 vs MIST-VR], and 8.7 +/- 0.1 for CELTS [p = 0.001 vs MIST-VR, p = 0.01 vs Endotower]); 6%, 0%, and 36% of "expert" participants obtained expert scores on MIST-VR, Endotower, and CELTS, respectively. CONCLUSIONS: All three simulators demonstrated significant construct and reasonable face validity. Although virtual reality holds great promise to expand the scope of laparoscopic simulation, current interfaces may limit their utility for assessment. Computer-enhanced video trainers may offer an improved interface while incorporating useful multidimensional metrics. Further work is needed to establish standards for appropriate skills assessment methods and performance levels using simulators.

Adult↗
PubMed2007

Training with simulation improves residents' endovascular procedure skills.

BACKGROUND: Endovascular procedure simulators are now commercially available and in use for physician training. The purpose of this study was to evaluate the role of simulation-based training in vascular surgery residencies. METHODS: Residents from vascular surgery programs in a five-state area were invited to participate in a series of 2-day endovascular training programs that used a high-fidelity endovascular procedure simulator (SimSuite; Medical Simulation Corporation, Denver, Colo), didactic instruction, computer-based training, and tabletop procedure demonstrations. The curriculum covered arteriography and intervention for treatment of aortoiliac, renal, and carotid artery disease. Nine residents participated, with one to three per training session. Each completed an average of 9.5 simulated endovascular cases. Performance on a standardized TransAtlantic Inter-Society Consensus B iliac angioplasty/stenting case was used to assess endovascular skills and knowledge at the beginning of the training program, and this was repeated at the completion of the training. Performance metrics were measured by the simulator, faculty observed trainees' performance of simulated cases, and trainees provided their evaluations of the usefulness of the simulation experiences. RESULTS: Endovascular procedural skills on the standardized iliac intervention case improved after completion of the training program. Compared with performance early on day 1, performance improved (P < or = .05; paired t test): total procedure time decreased 54%, volume of contrast decreased 44%, and fluoroscopy time decreased 48% (mean change from baseline). Selection of angioplasty balloon catheters and stents was improved, and the average number of catheters used and stents deployed decreased, although this did not reach statistical significance. Faculty observation allowed identification of shortcomings of knowledge and skills, including common problems with selection of catheter, balloon, and stent sizes; correct positioning of the sheath; and intraprocedural monitoring. Postcourse evaluations indicated support for the use of simulation in vascular surgery residents' endovascular training. CONCLUSIONS: Training with a simulator, incorporated into an individual or small group learning session, offers a means to learn and realistically practice endovascular procedures without direct risk to patients, with measurable improvements in key performance metrics. How simulation training affects subsequent clinical performance has yet to be established.

Endoscopy↗
PubMed1996

On simulating lipid bilayers with an applied surface tension: periodic boundary conditions and undulations.

As sketched in Fig. 1, a current molecular dynamics computer simulation of a lipid bilayer fails to capture significant features of the macroscopic system, including long wavelength undulations. Such fluctuations are intrinsically connected to the value of the macroscopic (or thermodynamic) surface tension (cf. Eqs. 1 and 9; for a related treatment, see Brochard et al., 1975, 1976). Consequently, the surface tension that might be evaluated in an MD simulation should not be expected to equal the surface tension obtained from macroscopic measurements. Put another way, the largest of the three simulations presented here contained over 16,000 atoms and required substantial computer time to complete, but modeled a system of only 36 lipids per side. From this perspective it is not surprising that the system is not at the thermodynamic limit. An important practical consequence of this effect is that simulations with fluctuating area should be carried out with a nonzero applied surface tension (gamma 0 of Fig. 2) even when the macroscopic tension is zero, or close to zero. Computer simulations at fixed surface area, which can explicitly determine pressure anisotropy at the molecular level, should ultimately lend insight into the value of gamma 0, including its dependence on lipid composition and other membrane components. As we have noted and will describe further in separate publications (Feller et al., 1996; Feller et al., manuscript in preparation), surface tensions obtained from simulations can be distorted by inadequate initial conditions and convergence, and are sensitive to potential energy functions, force truncation methods, and system size; it is not difficult, in fact, to tune terms in the potential energy function so as to yield surface tensions close to zero. This is why parameters should be tested extensively on simpler systems, for example, monolayers. The estimates of gamma 0 that we have presented here should be regarded as qualitative, and primarily underscore the assertion that the surface tension of a microscopically flat, simulation-sized patch is significantly greater than zero. As the simulation cell length increases, the surface tension that would be evaluated (or should be applied) decreases; in the limit of micrometer-sized simulation cells, gamma would approach zero or its appropriate thermodynamic value. The theories presented here also imply that the estimation of bilayer surface tension from monolayer data should take the degree of flatness into account. These conclusions are independent of the precise values of parameters such as bending constants. In conclusion, from the simulator's perspective, the question "What is the surface tension of a bilayer?" is better phrased as "What is the value of the applied surface tension necessary to simulate a particular experimental system with a given number of lipids?". As we have shown, the answer to the second question varies, but it should not be assumed a priori to equal zero.

Biophysical Phenomena↗
PubMed2004

Clinical evaluation of a computer simulated prediction model of contrast enhancement of the liver in spiral CT.

OBJECTIVE: A software program was developed simulating a compartmental model of blood circulation based on differential equations. The aim of this study was to compare software-simulated levels of hepatic enhancement with the true values in patients and to test how many patients reach the simulated hepatic enhancement level. METHODS: As software program the CT application software carebolus 2 (Siemens, Forchheim, Germany) was used. Hepatic contrast-enhancement curves were simulated prior to CT examinations to evaluate a patient specific time delay after contrast application. At the time delay, when the simulation curve showed an enhancement threshold of 40 Hounsfield Units (HU), the CT spiral scan was started applying 120 ml contrast media with 2 ml/s. The simulated curves were compared with the empiric curves of each patient. RESULTS: 25 of 28 patients (89%) achieved 40 HU. The mean enhancement of empiric patients curves was 46.32 +/- 11.9 HU, the mean simulated enhancement was 46.62 +/- 4.3 HU S.D. (P= 0.48). 4.4 values per patient liver could be compared with the simulation curve (122 points for 28 patients): 50% of the patient curves were within a range of 5 HU compared with the simulation curve. CONCLUSION: Software simulation of contrast enhancement curves of the liver is a feasible and valuable method to predict individual liver enhancement curves. Improvements concerning the integration of cardiovascular parameters and preexisting liver parenchymal diseases into the simulation software have to be arranged.

Adult↗
PubMed2005

Fatigue, sleepiness, and performance in simulated versus real driving conditions.

STUDY OBJECTIVES: To determine whether real-life driving would produce different effects from those obtained in a driving simulator on fatigue, performances and sleepiness. DESIGN: Cross-over study involving real driving (1200 km) or simulated driving after controlled habitual sleep (8 hours) or restricted sleep (2 hours). SETTING: Sleep laboratory and open French Highway. PARTICIPANTS: Twelve healthy men (mean age +/- SD = 21.1 +/- 1.6 years, range 19-24 years, mean yearly driving distance +/- SD = 6563 +/- 1950 miles) free of sleep disorders. MEASUREMENTS: Self-rated fatigue and sleepiness, simple reaction time before and after each session, number of inappropriate line crossings from the driving simulator and from video-recordings of real driving. RESULTS: Line crossings were more frequent in the driving simulator than in real driving (P < .001) and were increased by sleep deprivation in both conditions. Reaction times (10% slowest) were slower during simulated driving (P = .004) and sleep deprivation (P = .004). Subjects had higher sleepiness scores in the driving simulator (P = .016) and in the sleep restricted condition (P = .001). Fatigue increased over time (P = .011) and with sleep deprivation (P = .000) but was similar in both driving conditions. CONCLUSIONS: Fatigue can be equally studied in real and simulated environments but reaction time and self-evaluation of sleepiness are more affected in a simulated environment. Real driving and driving simulators are comparable for measuring line crossings but the effects are of higher amplitude in the simulated condition. Driving simulator may need to be calibrated against real driving in various condition.

Adult↗
PubMed1999

Fast digital simulation of spiking neural networks and neuromorphic integration with SPIKELAB.

We present a simulation environment called SPIKELAB which incorporates a simulator that is able to simulate large networks of spiking neurons using a distributed event driven simulation. Contrary to a time driven simulation, which is usually used to simulate spiking neural networks, our simulation needs less computational resources because of the low average activity of typical networks. The paper addresses the speed up using an event driven versus a time driven simulation and how large networks can be simulated by a distribution of the simulation using already available computing resources. It also presents a solution for the integration of digital or analogue neuromorphic circuits into the simulation process.

Action Potentials↗
PubMed2002

Using the human patient simulator to educate students of veterinary medicine.

INTRODUCTION: The human patient simulator has proved to be an effective educational device for teaching physicians and paramedical personnel. METHODOLOGY: To determine whether veterinary medicine students would benefit from similar educational sessions, 90 students each took a turn being the patient's clinician as real-life scenarios were played out on the simulator. The students induced and maintained anesthesia on their patient and monitored vital signs. Several critical events were presented for the students to diagnose and treat as they occurred. All students submitted a written evaluation of the course upon completion. The last 40 students were randomly divided into two groups of 20 students each. The students in Group I experienced the simulator before their clerkship examination, and those in Group II took the examination before their simulator experience. RESULTS: The students rapidly gained confidence in treating their simulated patient. This carried over to the clinical setting, where they appeared to be more confident when anesthetizing live patients. The simulator experience brought together much of the previous didactic material that they had been exposed to so they could appreciate its clinical relevance. The overwhelming response to the simulator experience was positive. The students in Group I had a significantly higher score on the clerkship examination dealing with concepts reviewed by simulation than those in Group II, who engaged in self-study instead of the simulation exercise (p < 0.001). CONCLUSION: We conclude that the human patient simulator was a valuable learning tool for students of veterinary medicine. It was exciting for the students to work with, made them deal with "real-life" scenarios, permitted them to learn without subjecting live patients to complications, enabled them to retrace their steps when their therapy did not correct the simulated patient's problems, and facilitated correlation of their basic science knowledge with clinical data, thus accelerating their ability to handle complex clinical problems in healthy and diseased patients.

Humans↗
PubMed2003

Validation of medical modeling & simulation training devices and systems.

For almost a decade, research has been conducted in many areas of science to develop medical simulation training devices and even comprehensive training systems. To propel the field, the Telemedicine and Advanced Technology Research Center (TATRC), an agency of the United States Army Medical Research Materiel Command (USAMRMC), has been managing a portfolio of research projects in the area of Medical Modeling and Simulation (MM&S) since 1999. Significant progress has made to identify and harness enabling technologies. Generally, these developments can be categorized in four areas: (1) PC-based interactive multimedia, (2) Digitally Enhanced Mannequins, (3) Virtual Workbench, or "part-task", simulators, and (4) Total Immersion Virtual Reality (TIVR). Many medical simulation-training systems have shown great potential to improve medical training, but the potential shown has been based largely on anecdotal feedback from informal user studies. Formal assessment is needed to determine the degree to which simulator(s) train medical skills and the degree to which skills learned on a simulator transfer to the practice of care. A robust methodology is required as a basis for these assessments. Several scientific workshops sponsored in 2001 focused on algorithm and metrics development in support of surgical simulation. Also in 2001, TATRC chartered a Simulation Working Group (SWG) to develop a robust methodology upon which to base an assessment of the effectiveness of simulation training devices and systems. After the terrorist attacks of September 11, 2001, attention was redirected for a period, and progress was delayed. In the summer of 2002, TATRC chartered a follow-on group called the Validation, Metrics and Simulation (VMAS) Committee. The poster will highlight and summarize the development of the methodology and identify validation studies to be conducted (supported by various funding sources and research programs). The interaction between TATRC and the National Capital Area Medical Simulation Center (NCAMSC) will be noted.

Computer Simulation↗
PubMed2004

Simulation-based training to improve acute care skills in medical undergraduates.

AIM: Acquisition of clinical skills by medical undergraduates can be problematic, especially in the context of medical emergencies. Simulation using computerised manikins may be an effective and ethical solution. We assessed the ability of undergraduates to manage medical emergencies, and evaluated simulation as an educational and assessment tool. METHOD: Medical undergraduates were assessed in standardised, highly contextualised simulations of medical emergencies using both checklists and global ratings, and were reassessed following a simulation-based educational intervention to measure learning effect. The scores for groups at different levels of training were compared to test construct validity of global ratings of simulator performance. We explored student perspectives of simulation through thematic analysis of questionnaire responses. RESULTS: Seventy-one students were studied. Final year students performed significantly better than fourth year students, but the ability to initiate management of medical emergencies was unsatisfactory in both groups. Performance improved significantly over the course of the simulation workshop. The learning processes in simulation-based education were perceived as more effective than traditional methods, and consistent with known principles of effective learning. Students felt the simulations were a reasonable measure of their abilities and 91% felt that such simulations should be included in their end-of-year assessment. CONCLUSION: Current medical undergraduate training does not ensure new graduates can intervene effectively in an emergency. Simulation-based workshops are effective and should be incorporated into the undergraduate curriculum both for education and assessment of competence in emergency management.

Clinical Competence↗
PubMedDate not supplied

Physical reality simulation for training of laparoscopists in the 21st century. A multispecialty, multi-institutional study.

BACKGROUND: Simulation is the most effective and safe way to train laparoscopic surgeons in an era of limited work hours, lack of funding, and increasing malpractice costs. However, the costs associated with the use of virtual reality simulators are significant, and although very technically sophisticated they still lack tactile feedback. We are proposing a physical reality simulator, the LTS 2000, as a reliable and effective alternative to virtual reality. This study was carried out to establish how reliably the simulator was able to differentiate between different levels of laparoscopic experience and to analyze the detection of skills improvement after simulation and clinical training. METHODS: This study was carried out, between July 2002 and August 2003, in the departments of Surgery and Obstetrics and Gynecology at 2 separate institutions. We enrolled 40 individuals in the study who had experience ranging from postgraduate year-1 to full-time faculty level. Five postgraduate year-3 residents were subsequently retested after rotating on clinical services, performing advanced laparoscopic procedures to assess whether the simulator was sensitive enough to detect improvements in laparoscopic skills at the intermediate level. Six tasks were included in the test, and they were scored for speed and precision with the McGill system. Two scores were obtained: a coordination score and a suturing score combined in a total score. Other variables analyzed were handedness, specialty, number of laparoscopic procedures performed, and hours spent on the simulator. RESULTS: Forty-five tests were performed. The number of subjects in each group based on level of experience was equally distributed. No difference occurred in scores between institutions, specialty, and right- or left-handed surgeons. A significant increase occurred in the coordination score and suturing score combined in the total score with increasing experience (P < 0.05) at each level. Furthermore, the simulator was sensitive enough to detect a significant difference in all 3 scores between subjects who had practiced with the simulator before being tested (P < 0.05). The scores of the 5 postgraduate year-3 participants doubled when tested, without reaching statistical significance due to the small sample size. CONCLUSIONS: Our study shows that the LTS 2000 reliably and reproducibly detects different levels of laparoscopic expertise and progression of the learning curve. LTS 2000 as a model of physical reality simulation should be considered a reliable alternative to virtual reality simulation.

Adult↗
PubMed2004

The learning curve for a colonoscopy simulator in the absence of any feedback: no feedback, no learning.

BACKGROUND: The hypothesis of this study is that working on the simulator without a structured feedback does not change performance; hence, any effects shown after structured feedback would amount to useful learning of the procedure. The aim was to investigate the learning curve for the HT Immersion Medical Colonoscopy Simulator without any structured feedback. This could then be potentially applied to validate the learning curve on the simulator when structured feedback is provided. There are no previous studies on this matter. METHODS: Candidates were asked to perform colonoscopy on the HT Immersion Medical Colonoscopy Simulator. Modules 3 and 4 were used at random. In total, each candidate was asked to perform five consecutive virtual colonoscopies on the same module. These five episodes were collectively referred to as one trial. A time result of 3,600 sec (1 h) was used to denote perforation. No guidance or feedback was given to candidates before, during, or after each procedure. A total of 26 postgraduate doctors were recruited, including nine research fellows, five preregistration house officers, six specialist registrars, and six consultants. Fourteen candidates recorded five attempts each (i.e., one trial each) on the same module of the colonoscopy simulator (14 trials over 70 episodes). Another 12 candidates recorded five attempts (i.e., one trial each) on two modules of the colonoscopy simulator (24 trials over 120 episodes). Hence, 190 episodes were recorded in total, representing 38 trials. RESULTS: There was no improvement in performance on the simulator from first attempt to the fifth in the absence of feedback. If there was any initial gain in any measurable outcome, this was lost in subsequent attempts indicating lack of learning. The outcomes measured included time taken to complete the test, percentage of the mucosa visualized, depth of the instrument inserted, and the path length used. The results were statistically significant for all outcomes. CONCLUSIONS: This study demonstrates that in the absence of feedback, it is not possible to improve performance on the HT Immersion Medical Colonoscopy Simulator. Thus, there is no learning curve for the machine. The information from this study is vital for using the simulators in training and assessment because any improvement in learning curves shown after training on simulators can be presumed to be due to learning the procedure and not the simulator.

Clinical Competence↗
PubMed2003

Procedures for high precision setup verification and correction of lung cancer patients using CT-simulation and digitally reconstructed radiographs (DRR).

PURPOSE: In a recent study, large systematic setup errors were detected in patients with lung cancer when a conventional simulation procedure was used to define and mark the treatment isocenter. In the present study, we describe a procedure to omit the session at a conventional simulator to remove simulation errors entirely. Isocenter definition and verification was performed at a computed tomography (CT) simulator, and digitally reconstructed radiographs (DRRs) were used for setup verification and correction at the treatment unit. METHODS AND MATERIALS: A CT simulation protocol was developed, in which radiopaque markers were used to verify the coincidence of the isocenter marked on the patients' skin with the isocenter defined in the planning CT scan. This protocol was evaluated for 20 patients. Subsequently, electronic portal images were acquired at the treatment unit. The three-dimensional setup error was established from a template match of the appropriate anatomy visible in two orthogonal beams with the corresponding anatomy in DRRs. An offline setup correction protocol was applied to reduce systematic setup errors. RESULTS: For all patients, the skin marks defined the planning CT scan isocenter to within +/- 1.5 mm in each of the three main directions. Random setup errors at the treatment unit were 1.8, 2.0, and 1.9 mm (1 SD) for the lateral (x), the superior-inferior (y), and the anterior-posterior (z) directions, respectively. With the use of the correction protocol, the systematic errors for x, y, and z were 1.5, 1.5, and 1.3 mm (1 SD). CONCLUSIONS: Because the distributions of treatment setup errors measured against DRRs obtained in our CT simulation were equal to previously obtained distributions measured against simulator films, conventional simulation can be omitted and DRRs are well-suited for setup verification. By adopting our CT simulation procedure, the large systematic simulation setup errors, which remain hidden if a conventional simulation is performed, can be avoided.

Carcinoma, Non-Small-Cell Lung↗
PubMed2005

Molecular modeling on the binary blend compatibility of poly(vinyl alcohol) and poly(methyl methacrylate): an atomistic simulation and thermodynamic approach.

Computer simulations play an important role in designing new polymers as well as in predicting properties of existing polymers. In this paper, the blend compatibility of poly(vinyl alcohol) (PVA) with poly(methyl methacrylate) (PMMA) was studied over the wide range of compositions allowed by the atomistic and mesoscopic simulation methods. The Flory-Huggins interaction parameter, chi, of the blends computed using the atomistic simulation confirmed the blend compatibility for compositions containing >60 wt % of PVA. This observation was further supported by differential scanning calorimetric experiments. Solubility parameters of the polymers obtained from the simulation procedure were in good agreement with those of the literature data. Simulation results were further supported by the spectral and solution property measurements. From the atomistic simulations, chi versus concentration plots were constructed, which showed trends similar to those experimentally measured melting temperature versus concentration. The chi values for the blends, which satisfied the criteria of miscibility of two polymers by the atomistic simulation, agreed quite well with the solubility criteria related to order parameters calculated from the mesoscopic simulation. Kinetics of phase separation was examined via density profiles calculated using the MesoDyn approach for incompatible blends. The length and time scales spanned by these simulations were found to be relevant to the real application scales. The free energy computed in the mesoscopic simulation for blends reached equilibrium, particularly when the simulation was performed at a higher time step, indicating the stability of the blend system at certain compositions.

Journal Article↗
PubMed2000

Use of a single-tree simulation model to predict effects of ozone and drought on growth of a white fir tree.

A physiologically based, single-tree simulation model, TREGRO, was parameterized with existing phenological, allometric, and growth data and used to predict effects of ozone and drought on growth of a 53-year-old white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.) tree following a 3-year model simulation. Multiple experimental simulations were conducted to assess the individual and interactive effects of ozone (O(3)) exposure and drought on growth of white fir. The effects of O(3) were imposed as reductions in carbon (C) assimilation of 0, 2.5, 5, 10, and 20%. Drought was imposed as 0, 10, 25, and 50% reductions in total annual precipitation. The results of the simulations were compared with the effects of O(3) on white fir seedlings grown in the presence and absence of ozone in open-top chambers and with a field survey of white fir trees subjected to a gradient of O(3). In the O(3) simulations, an O(3)-induced reduction in C assimilation of 2.5% reduced total tree biomass and branch total nonstructural carbohydrate (TNC) content by < 7%. Although quantifiable in simulation experiments, such small reductions would probably not be detectable in the field. Results from both an open-top chamber experiment and a field survey indicated that reductions in C assimilation of white fir growing in elevated O(3) were much greater than 2.5%, but were not statistically different from control values. A simulated O(3) reduction in C assimilation of >/= 10% reduced total tree biomass by 7% and branch TNC by 55%. Results from the field survey indicated that branch elongation was reduced in response to increased O(3) concentration, corroborating the simulated response of reduced C allocation to the branches of white fir. Although simulated reductions in total annual precipitation of >/= 25% reduced final tree biomass, the simulated reductions also reduced O(3) uptake and therefore reduced the O(3) response of white fir. However, a combination of low amounts of O(3) (2.5% reduction in C assimilation) and drought (25% reduction in annual precipitation) synergistically reduced C gain of white fir more than either stress individually. Our simulations predict that moderate drought (no more than a 25% reduction in total annual precipitation) may not ameliorate the response of white fir to O(3) and that moderate amounts of atmospheric O(3) and drought could be more detrimental to white fir than either stress singly.

Journal Article↗
PubMed2004

A method for simulation of foveal vision during wear of corrective lenses.

PURPOSE: The aim was to simulate the visual appearance of images viewed through corrective lenses having known, arbitrary types and amounts of monochromatic aberration, so that the visual effect of changing the design parameters of the lens could be explored. METHODS: We first calculate the optical response of the eye and any corrective lens using a numerical model eye. We then use this response as a filter, which we convolve with a selected original (unaberrated) image, to obtain an initial simulated retinal image. This image is then deconvolved by a second filter, which is calculated as the optical response of the eye of the observer who views the final image displayed on a video monitor. The originality of our approach to visual simulation is to take the aberrational characteristics of the observer's eye into account in the calculation. We validated our simulation by comparing images degraded by simulated dioptric blur with real defocused images seen through corresponding optical lenses. RESULTS: When using a small (2.5 mm) pupil size and a "typical" observer wavefront aberration model, there was a close resemblance between optical and simulated blurs. Although it was not necessary to consider the measured aberrations of the subject when simulating vision with a small pupil size, this requirement could not be ignored when vision through a larger pupil was simulated. With a 5.7-mm pupil diameter, use of Shack-Hartmann measurements of the ocular aberrations of the individual observers rather than "typical" levels of aberrations for the entire population gave excellent agreement between the effects of simulated and real defocus blur in monochromatic and polychromatic light. A Bland-Altman analysis of the differences between matching simulated and real blurs for a 5.7-mm pupil in polychromatic light with the model including allowance for individual measured aberrations gave mean differences close to zero and 95% confidence limits of about +/-0.25 D over a defocus range of -2.00 to +2.00 D. CONCLUSION: The simulation technique can be expected to be a useful tool to evaluate the potential performance of an eye that wears various designs of corrective lens.

Adult↗
PubMed1989

The role of mucus sol phase in clearance by simulated cough.

Using a simulated cough machine, we analyzed the effect of adding tensio-active liquids as sol phase simulant on the clearance of gel mucus simulant by cough. Polysaccharides crosslinked with sodium tetraborate were used at different concentration as gel mucus simulant. A drop of gel mucus simulant was deposited either directly on the model trachea or on a sol phase layer simulant (2% sodium dodecyl sulfate in water). The clearance of the mucus simulants was quantified by observing the movement of marker particles in the gel layer. The viscoelastic properties of gel mucus simulants were determined by using a viscoelastometer (SEFAM). The adhesive properties were analyzed by means of the platinum ring technique. The wettability of the mucus simulants was quantified by the automatic measurement of the contact angle of the drop of gel on the model trachea. We found that the addition of a sol phase significantly decreased by about 50% the adhesivity and wettability of the gel mucus simulants. This decrease was associated with a marked enhancement of cough clearance, whatever the viscoelastic properties of the gel mucus simulants. These results suggest that the sol phase is essential in bronchial respiratory mucus clearance by the cough mechanism.

Adhesiveness↗
PubMedDate not supplied

Validity of the baltimore therapeutic equipment work simulator in the measurement of lifting endurance in healthy men.

OBJECTIVE: To examine the criterion validity of the Baltimore Therapeutic Equipment (BTE) work simulator by comparing endurance time, oxygen uptake (VO2), and heart rate measured during real and simulated lifting tasks and to derive a regression equation for predicting actual lifting endurance from measurements on the work simulator. METHOD: Twenty healthy male volunteers repetitively lifted and lowered a load of 40 lb using the BTE work simulator and actual weights at a laboratory workstation. Postures, location, and frequency of lifts were kept constant. Endurance (defined as the time taken for the rating of perceived exertion to increase 2 units on the Borg scale) was measured under both conditions. VO2 and heart rate were also recorded, using standard physiological procedures. RESULTS: The mean values for endurance time, steady-state VO2, and heart rate were significantly different between the real and simulated tasks (p < .05). Correlation of endurance time between the two tasks was significant (r = .71, p < .05). Step wise regression analysis resulted in the following equation for predicting real endurance from simulated time measurements: predicted real time = .34 simulated time + 3.29; r = .71; SE = 1.00 min. CONCLUSION: The BTE work simulator tends to overestimate real lifting endurance performance in healthy men. The lower physiological stress during the simulated task suggests a significant difference between the real and simulated loads. Occupational therapists should exercise caution when using the results of the BTE work simulator during functional capacity evaluations.

Adolescent↗