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Notes: The Virtual Cell is a modeling tool that allows biologists and theorists alike to specify and simulate cell-biophysical models on arbitrarily complex geometries. The framework combines an intuitive, front-end graphical user interface that runs in a web browser, sophisticated server-side numerical algorithms, a database for storage of models and simulation results, and flexible visualization capabilities. In this paper, we present an overview of the capabilities of the Virtual Cell, and, for the first time, the detailed mathematical formulation used as the basis for spatial computations. We also present summaries of two rather typical modeling projects, in order to illustrate the principal capabilities of the Virtual Cell. © 2001 American Institute of Physics.

Notes: Purpose This study was designed to determine the effect of opacifiers used at different ratios on the color stability of pigmented A-2186 silicone maxillofacial elastomers and to evaluate the color spectrophotometrically before and after artificial aging.Materials and Methods Sixty experimental groups of elastomers were made using various concentrations (5%, 10%, and 15%) of 4 opacifiers (Georgia kaolin powder neutral, kaolin powder calcined, Artskin white, and dry pigment titanium white) with 1 of 5 dry earth cosmetic pigment groups (no pigment [control], red, yellow ochre, burnt sienna, and a mixture of all pigments). Five specimens of each elastomer were tested, for a total of 300 specimens. All specimens were placed in an aging chamber and artificially aged by exposure to light, water spray, fluctuating temperatures, and humidity. CIE L*a*b* values were measured by spectrophotometer. The color differences (ΔE*) at various exposure energies (150, 300, and 450 kJ/m2) were subjected to 4-way analysis of variance with repeated measures (super ANOVA). Mean values were compared with Tukey–Kramer intervals calculated at the 0.05 significance level.Results The trained human eye can detect color changes (ΔE*) greater than 1.0. Adding all pigments to any of the kaolin groups did not protect silicone A-2186 from color degradation over time. Mixing red pigment in all groups at all times drastically increased ΔE* values ranging from 0.1 to 1.3 up to 16.6 to 49.6. Yellow ochre had an effect only with 10% and 15% concentrations of kaolin powder calcined, increasing the value of ΔE* to greater than 1.0. Burnt sienna had an effect only with a 15% concentration of kaolin powder calcined, increasing the value of ΔE* at 300 and 450 kJ/m2 to greater than 11.0. At the 5% concentration, kaolin powder calcined had the smallest color changes, followed by, in order, dry pigment titanium white, Artskin white, and Georgia kaolin. At the 10% concentration, Artskin white had the smallest color changes, followed by, in order, dry pigment titanium white, kaolin powder calcined, and Georgia kaolin. At the 15% concentration, Artskin white again had the smallest color changes, followed by, in order, dry pigment titanium white, Georgia kaolin, and the kaolin powder calcined. The smallest color changes in each kaolin group were at the 10% concentration for Artskin white, dry pigment titanium white, and kaolin powder calcined and at the 5% concentration for Georgia kaolin.Conclusions Mixing dry earth cosmetic pigments with opacifiers did not protect silicone A-2186 from color degradation over time, especially in the case of red pigment. The group in which pigments were mixed with 10% Artskin white had the smallest color changes over time, followed by, in order, the groups in which pigments were mixed with 10% dry pigment titanium white, 10% kaolin powder calcined, and 5% Georgia kaolin. Red pigment had a significant effect on all opacifiers, especially Georgia kaolin and kaolin powder calcined. Yellow ochre and burnt sienna had an effect only on 15% kaolin powder calcined. Among all the pigment groups tested, yellow ochre remained the most color stable over time.

Notes: Objective: To perform blood gas analysis of the respiratory response to transdermal fentanyl in dogs which have experienced an open-chest surgical procedure.Design: Prospective trial.Setting: Veterinary Teaching Hospital Surgical Research and Student Laboratory.Intervention: Dogs were purchased for a surgical laboratory. Students performed a cranial abdominal exploratory and diaphragmatic hernia repair. Sixteen dogs were divided into 2 groups. Dogs received transdermal fentanyl (group F), using an average dose of 4.8 µg/kg/hr, applied to the caudal-lateral abdomen 22 hours before surgery, or intravenous buprenorphine (group B; 0.02 mg/kg) given 1 hour prior to anesthetic induction and every 6 hours postoperatively. All dogs received intravenous acepromazine (0.05 mg/kg) preoperatively and every 6 hours postoperatively. Dogs were instrumented with carotid artery catheters.Measurements and main results: Arterial blood gas values were analyzed every 2 hours postoperatively. Plasma fentanyl levels were analyzed every 4 hours postoperatively. The mean carbon dioxide tension (PCO2) did not exceed 45 mmHg in either group. The range in mean PCO2 levels was 32.9 (± 3.4)−38.1 (± 3.9) in group B and 34.7 (± 3.25)−43.6 (± 5.5) in group F. At 2 time points, the mean PCO2 was significantly lower in group B compared with normal levels in group F. Hypoxemia occurred in both the groups. The range in mean oxygen tension (PO2) was 76.5 (± 18.3)−91.1 (± 16.3) in group B and 76.0 (± 10.8)−96.6 (± 7.6) in group F. There was no significant difference in PO2 between groups. Levels of fentanyl considered to be analgesic were maintained for the postoperative period.Conclusions: The use of a relatively high dose of transdermal fentanyl did not induce postoperative hypoventilation as evidenced by serial arterial blood gas analysis in this model.

Notes: Effects of Azimilide on Reentrant Circuits. Introduction: Azimilide blocks the slow (IKs) and fast (IKr) components of the delayed rectifier potassium channel. It also has blocking effects on sodium (INa) and calcium currents (ICaL). Its effects on reentrant circuits in infarct border zones causing ventricular tachyarrhythmias are unknown. Methods and Results: Activation in reentrant circuits causing sustained ventricular tachycardia (SVT) and the initial polymorphic tachycardia that leads to ventricular fibrillation (VF) was mapped in the epicardial border zone (EBZ) of 4-day-old canine infarcts. Azimilide prolonged the effective refractory period (ERP) in both normal myocardium and EBZ, but reverse use-dependence in EBZ was prominent. Azimilide abolished SVT initiation by programmed electrical stimulation by prolonging the ERP at the site of stimulation either in normal or EBZ, preventing the occurrence of early premature impulses and the formation of lines of block in the EBZ necessary for formation of reentrant circuits. Azimilide prevented VF initiation by programmed electrical stimulation by causing conduction block of reentrant impulses in the EBZ during the initial beats of rapid polymorphic ventricular tachycardia, despite the reverse use-dependent effects on ERP. Conclusion: Azimilide has antiarrhythmic effects to prevent reentry causing SVT and VF in a canine infarct model.

Notes: AC Cardioversion in a Canine Slice. Introduction: A heart in fibrillation can be entrained by long-lasting alternating current (AC) stimuli, leading to defibrillation. To investigate the role entrainment plays in defibrillation, computer simulations of AC cardioversion in a three-dimensional slice of the canine heart were performed. Methods and Results: A bidomain finite element model of a 1-mm thick slice across the ventricles of a canine heart was used to simulate termination of transmural reentry with AC shocks. Cardioversion defibrillation thresholds (DFTs) were determined for 200-msec (L) AC shocks at varying frequencies. At the DFT, the entire tissue is entrained by the AC shock. DFT decreases as the frequency of the long-lasting AC shock increases. We hypothesize that this decrease is due to the short period of the high-frequency AC waveform, leaving strong virtual electrode polarization (VEP) after the shock ends. To test this hypothesis, the end-shock VEP were compared for different frequencies, demonstrating stronger polarization as frequency increased. To examine whether entrainment by the long-lasting AC shock contributes to the VEP at the end of the shock, additional simulations were conducted using single-period (Z) AC waveforms. Z waveform DFTs were higher than L waveform DFTs; the Z waveform VEP was weaker than the L waveform VEP at the same frequency. This indicates that entrainment contributes to the development of stronger VEP and, thus, to lower DFT at high frequencies. Conclusion: This study offers for the first time a mechanistic insight into cardioversion with long-lasting AC shocks.

Notes: Since the complexity, morbidity, and costs of coronary interventions are increased when coronary thrombus is present, identification of the cause of an angiographic filling defect is potentially important. We present a case report and review our experience with a flow artifact that mimicked thrombus (“pseudothrombus”) in the setting of a severe proximal stenosis in the left circumflex coronary artery.

Notes: The purpose of this article is to introduce virtual reality technology as a potential tool for personnel selection in organizations. We describe virtual reality technology, its hardware and software requirements, and some current applications. Then we propose specific types of job knowledge, skills, abilities, and other characteristics (KSAOs) particularly suitable to being assessed using virtual reality technology. We emphasize KSAOs that hold the greatest promise in terms of yielding greater validity than more commonly used selection techniques. We hope the present article will stimulate and guide future empirical research on the potential of virtual reality technology as a personnel selection tool.

Notes: Abstract The field of computational cell biology has emerged within the past 5 years because of the need to apply disciplined computational approaches to build and test complex hypotheses on the interacting structural, physical, and chemical features that underlie intracellular processes. To meet this need, newly developed software tools allow cell biologists and biophysicists to build models and generate simulations from them. The construction of general-purpose computational approaches is especially challenging if the spatial complexity of cellular systems is to be explicitly treated. This review surveys some of the existing efforts in this field with special emphasis on a system being developed in the authors' laboratory, Virtual Cell. The theories behind both stochastic and deterministic simulations are discussed. Examples of respective applications to cell biological problems in RNA trafficking and neuronal calcium dynamics are provided to illustrate these ideas.

Notes: We evaluate dielectric tuning on nanosecond time scales in ferroelectric Ba0.3Sr0.7TiO3 thin films by measuring nonlinear harmonic generation at a fundamental frequency of 3 GHz. We compare the form of the distributed nonlinear capacitance per unit length C(Vrf) extracted from a simple model of harmonic generation in coplanar waveguide transmission line structures with the nonlinear capacitance C(Vdc) measured using a dc bias voltage, and obtain excellent agreement for temperatures in the range 235–295 K. This demonstrated agreement implies that full dielectric tuning can be expected in these ferroelectric thin films on nanosecond time scales, and also demonstrates that detrimental high-frequency nonlinear effects in device structures can be accurately predicted based on dc biased measurements. © 2002 American Institute of Physics.