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R Barth

Publications and source records attributed to R Barth.

At least 19 recordsLinked to original sources
PubMed2001

Enhanced CD4 reconstitution by grafting neonatal porcine tissue in alternative locations is associated with donor-specific tolerance and suppression of preexisting xenoreactive T cells.

BACKGROUND: Donor-specific xenograft tolerance can be achieved by grafting fetal porcine thymus tissue to thymectomized (ATX) mice treated with natural killer (NK) and T-cell-depleting monoclonal antibodies plus 3 Gy of total body irradiation (TBI). Grafting of neonatal, instead of fetal, thymus, along with neonatal pig spleen, leads to a lower level of mouse CD4 cell reconstitution, with less reliable tolerance induction. For a number of reasons, it would be advantageous to use neonatal rather than fetal pigs as donors. We therefore investigated the possibility that grafting larger amounts of neonatal porcine thymus tissue to different sites could allow improved outcomes to be achieved. MATERIALS AND METHODS: Multiple or single fragments of neonatal porcine thymus tissue were grafted with a splenic fragment to different sites (mediastinum, mesentery, and kidney capsule) of ATX B6 mice treated with T- and NK-cell-depleting antibodies and 3Gy TBI. Mice also received an intraperitoneal injection containing 1 x 10(7) donor splenocytes. Donor-specific skin graft tolerance was evaluated, and CD4 reconstitution and mouse anti-donor xenoantibodies were followed by flow cytometry. RESULTS: Peripheral repopulation of CD4+ cells occurred by 7 weeks after transplantation in mice grafted with four fragments of neonatal porcine tissue in either the mediastinum or the mesentery, but not in mice grafted under both kidney capsules with the same amount of tissue. The level of CD4 reconstitution correlated with skin graft tolerance and an absence of induced anti-donor xenoantibodies. Seventy-five percent of mice with >20% of CD4+ cells among peripheral blood lymphocytes (PBL) by 13 weeks posttransplantation accepted donor porcine skin, while rejecting either non-donor neonatal porcine or mouse BALB/c skin allografts. In contrast, only 29% of grafted mice with <20% CD4+ cells in the peripheral blood at 13 weeks accepted donor porcine skin. Grafted mice with poor reconstitution showed either low or high levels of anti-pig xenoantibodies of the IgM, IgG1, and IgG2a isotypes. Grafted mice with >20% CD4+ cells all had low levels of anti-pig xenoantibodies of these isotypes and displayed mixed lymphocyte reaction (MLR) tolerance to donor pig major histocompatibility complex (MHC), with responsiveness to allogeneic mouse stimulators. CONCLUSION: Grafting neonatal porcine thymus into either the mediastinum or mesentery provides earlier and more efficient reconstitution of the CD4 compartment than does grafting under the kidney capsule. Good CD4 reconstitution was associated with optimal donor-specific skin graft tolerance and avoidance of the anti-donor xenoantibody responses observed in mice with poor CD4 reconstitution. These results also suggest that there is a suppressive component to the porcine xenograft tolerance induced with this approach.

Animals↗
PubMed2001

Phospholipase C activity of Helicobacter pylori is not associated with the presence of the cagA gene.

BACKGROUND: Knowledge about the possible role of phospholipase C (PLC) activity of microbial pathogens in the development of disease is increasing. Recently attention has focused on investigating PLC activity elaborated by Helicobacter pylori, but the role of this enzyme in H. pylori pathogenesis is still unknown. The aim of this study was to correlate PLC-activity of H. pylori on the basis of the cagA status with the clinical diagnosis of the patients. MATERIALS AND METHODS: Helicobacter pylori was isolated from patients with gastritis (G; n = 38), duodenal ulcer (DU; n = 15), gastric ulcer (GU; n = 11) and gastric cancer (GC; n = 12). Polymerase chain reaction primers DZ3/R009 which amplified a 1350-bp fragment were used to detect the cagA gene. PLC activity was determined using p-nitrophenylphosphorylcholine as substrate. RESULTS: Of the strains, 60% were cagA(+) and 40% were cagA(-). All strains showed PLC activity (2.20 +/- 0.91 U mg(-1) protein). PLC activity showed no association with the cagA status: cagA(+) (2.21 +/- 1.03 U mg(-1) protein), cagA(-) (2.18 +/- 0.79 U mg(-1) protein). Patients with GU had the highest PLC activity (2.77 +/- 1.26 U mg(-1) protein) and patients with GC had the lowest activity (1.8 +/- 0.57 U mg(-1) protein). CONCLUSIONS: Although PLC activity was present in all strains tested, it may only have pathological importance in patients with GU. However, the extent of PLC activity was independent of the presence of the cagA gene.

Adenocarcinoma↗
PubMed2000

Emission pattern of high-energy pions: A new probe for the early phase of heavy-Ion collisions

The emission pattern of charged pions has been measured in Au+Au collisions at 1 GeV/nucleon incident energy. In peripheral collisions and at target rapidities, high-energy pions are emitted preferentially towards the target spectator matter. In contrast, low-energy pions are emitted predominantly in the opposite direction. The corresponding azimuthal anisotropy is explained by the interaction of pions with projectile and target spectator matter. This interaction with the spectator matter causes an effective shadowing which varies with time during the reaction. Our observations show that high-energy pions stem from the early stage of the collision whereas low-energy pions freeze out later.

Journal Article↗
PubMed2000

Impact of vaccine financing on vaccinations delivered by health department clinics.

OBJECTIVES: This study measured the number of childhood vaccinations delivered at health department clinics (HDCs) before and after changes in vaccine financing in 1994, and it assessed the impact of changes in financing on HDC operations. METHODS: We measured the number of vaccination doses administered annually at all 57 HDCs in New York State between 1991 and 1996, before and after the financing changes. Interviews of HDC personnel assessed the impact of financing changes. A secondary study measured trends in Pennsylvania and California. RESULTS: HDC vaccinations for preschool children in New York State declined slightly prior to the financing changes (6%-8% between 1991 and 1993) but declined markedly thereafter (53%-56% between 1993 and 1996). According to nearly two thirds of New York State's HDCs, the primary cause for this decline was the vaccine-financing changes. HDC vaccinations for preschool children in Pennsylvania declined by 12% between 1991 and 1993 and by 56% between 1993 and 1997. HDC vaccinations for polio-containing vaccines in California declined by 31% between 1993 and 1997. CONCLUSIONS: Substantially fewer vaccinations have been administered at HDCs since changes in vaccine financing, thereby keeping preschool children in their primary care medical homes.

Adolescent↗
PubMed2000

Evaluation of New York State's Child Health Plus: methods.

BACKGROUND: The State Children's Health Insurance Program (SCHIP) is the largest public investment in child health care in 30 years, targeting 11 million uninsured children, yet little is known about the impact of health insurance on uninsured children. In 1991 New York State implemented Child Health Plus (CHPlus), a health insurance program that was a prototype for SCHIP. A study was designed to measure the association between CHPlus and access to care, utilization of services, and quality of care. METHODS: The setting was a 6-county region in upstate New York (population 1 million) around and including the city of Rochester. A before-and-during design was used to compare children's health care for the year before they enrolled in CHPlus versus the first year during CHPlus, for 1828 children (ages 0-6.99 years at enrollment) who enrolled between November 1, 1991 and August 1, 1993. An additional study involved 187 children 2 to 12.99 years old who had asthma. Parents were interviewed to assess demographic characteristics, sources of health care, experience with CHPlus, and impact of CHPlus on their children's quality of care and health status. Medical charts were reviewed to measure utilization and quality of care, for 1730 children 0 to 6.99 years and 169 children who had asthma. Charts were reviewed at all primary care offices and at the 12 emergency departments and 6 public health department clinics in the region. CHPlus claims files were analyzed to determine costs during CHPlus and to impute costs before CHPlus from utilization data. ANALYSES: Logistic regression and Poisson regression were used to compare the means of dependent measures with and without CHPlus coverage, while controlling for age, prior insurance type, and gap in insurance coverage before CHPlus. CONCLUSIONS: This study developed and implemented methods to evaluate the association between enrollment in a health insurance program and children's health care. These methods may also be useful for evaluations of SCHIP.

Asthma↗
PubMed2000

Evaluation of New York State's Child Health Plus: access, utilization, quality of health care, and health status.

BACKGROUND: The recently enacted State Children's Health Insurance Program (SCHIP) is modeled after New York State's Child Health Plus (CHPlus) program. Since 1991, CHPlus has provided health insurance to children 0 to 13 years old whose annual family income was below 222% of the federal poverty level and who were ineligible for Medicaid or did not have equivalent health insurance coverage. CHPlus covered the costs for ambulatory, emergency, and specialty care, and prescriptions, but not inpatient services. OBJECTIVES: To assess the change associated with CHPlus regarding 1) access to health care; 2) utilization of ambulatory, inpatient, and emergency services; 3) quality of health care; and 4) health status. SETTING: Six western New York State counties (including the city of Rochester). SUBJECTS: Children (0-6.99 years old) enrolled for at least 9 consecutive months in CHPlus. METHODS: The design was a before-and-after study, comparing individual-level outcomes for the 12 months immediately before CHPlus enrollment and the 12 months immediately after enrollment in CHPlus. Parent telephone interviews and medical chart reviews conducted 12 months after enrollment to gather information. Subjects' primary care charts were located by using interview information; emergency department (ED) charts were identified by searching patient records at all 12 EDs serving children in the study; and health department charts were identified by searching patient records at the 6 county health department clinics. Logistic regression and Poisson regression were used to compare the means of dependent measures with and without CHPlus coverage, while controlling for age, prior insurance type, and gap in insurance coverage before CHPlus. RESULTS: Complete data were obtained for 1730 children. Coverage by CHPlus was associated with a significant improvement in access to care as measured by the proportion of children reported as having a usual source of care (preventive care: +1.9% improvement during CHPlus and sick care: +2. 7%). CHPlus was associated, among children 1 to 5 years old, with a significant increase in utilization of preventive care (+.23 visits/child/year) and sick care (+.91 visits/child/year) but no measurable change in utilization of specialty, emergency, or inpatient care. CHPlus was also associated, among children 1 to 5 years old, with significantly higher immunization rates (up-to-date for immunizations: 76% vs 71%), and screening rates for anemia (+11% increased proportion screened/year), lead (+9%), vision (+11%), and hearing (+7%). For 25% of the children, a parent reported that their child's health was improved as a result of having CHPlus. CONCLUSION: After enrollment in CHPlus, access to and utilization of primary care increased, continuity of care improved, and many quality of care measures were improved while utilization of emergency and specialty care did not change. Many parents reported improved health status of their child as a result of enrollment in CHPlus. Implication. This evaluation suggests that SCHIP programs are likely to improve access to, quality of, and participation in primary care significantly and may not be associated with significant changes in specialty or emergency care.

Child↗
PubMed2000

["Reading-your-baby lessons" for parents of excessively crying infants--the concept of "guided parent-infant training sessions"].

New findings indicate that persistent crying (infantile "colic") may be closely interlinked with regulatory difficulties in the organization of sleep and waking states and in sustaining prolonged episodes of uninterrupted sleep. It has been shown that preverbal communication plays a significant role in the regulation of infant states and in the prevention of infantile persistent crying. However, parents are usually unaware of these explanations, and tend to attribute the crying to inappropriate causes or have no understanding at all about why their child cries so much. Therefore, the baby's signals are not answered in an effective way. In this article "guided parent-infant-training sessions", a new treatment method for parents with excessively crying babies, will be introduced. It is the goal of this approach to help parents to improve the "reading" of their child's signals and to act according to the baby's needs. In order to provide an opportunity for training, parents are given their appointment at a time, when it is most likely for the child to cry. In the context of a therapeutic relationship they may be experiencing for the first time calming a baby they previously thought to be inconsolable.

Adult↗
PubMed1999

Scattering of primary care: doctor switching and utilization of health care by children on fee-for-service Medicaid.

OBJECTIVE: To determine whether children on fee-for-service Medicaid who switch primary care doctors use less health care and are less up to date with preventive care visits than children who do not switch primary care doctors. DESIGN: Retrospective cohort study using Medicaid claims data. SETTING: 51,027 children enrolled on Medicaid in Monroe County, New York. PATIENTS: 14,187 children enrolled continuously on fee-for-service Medicaid between January 1992 and December 1994. MAIN OUTCOME MEASURES: Utilization of primary care, emergency department (ED) services, and specialty care and proportion up to date with preventive care visits according to American Academy of Pediatrics guidelines. RESULTS: During the 2-year study period, 22% of children switched primary care doctors. Compared with children who did not switch primary care doctors, those who switched had more primary care visits (4.7 vs. 3.2 visits/year, P < .01), age-adjusted preventive care visits (1.2 vs. 1.0 visits/year), ED visits (0.72 vs. 0.47 visits/year, P < .01), and specialist visits (0.99 vs. 0.31, P < .01). On multivariate analysis, doctor switching was associated with increased odds of being up to date with preventive care visits (odds ratio [OR] = 1.7; 95% confidence interval [CI] 1.3 to 2.1). However, on multivariate analysis stratified by age, the association was significant only for older children (ages 11 to 14). Altogether, 68% of all children and 44% of infants less than 1 year old made the recommended number of preventive care visits during the study period. CONCLUSIONS: All groups of children received less preventive care than recommended by the American Academy of Pediatrics. Children who switched primary care doctors had higher utilization of health care, including primary care, ED, and specialty care. Contrary to expectations, they were more likely to be up to date with preventive care visits. The heavy utilization of health services by doctor switchers indicates that this subgroup of children on Medicaid may not be at risk for poor access to health care, but additional research is needed to determine whether the quality of care is related to doctor switching.

Adolescent↗
PubMed1999

A randomized study of tracking with outreach and provider prompting to improve immunization coverage and primary care.

OBJECTIVE: To compare and measure the effects and cost-effectiveness of two interventions designed to raise immunization rates. SETTINGS: Nine primary care sites serving impoverished and middle-class children. SUBJECTS: Complete birth cohorts (ages 0 to 12 months; n = 3015) from these sites. INTERVENTIONS: Two 18-month duration interventions: 1) tracking with outreach [tracking/outreach] to bring underimmunized children to their primary care provider office, and 2) a primary care provider office policy change to identify and reduce missed immunization opportunities (prompting). DESIGN: Randomized, controlled trial, randomizing within sites using a two-by-two factorial design. Subjects were allocated to one of four study groups: control, prompting only, tracking/outreach only, and combined prompting with tracking/outreach. Outcomes were obtained by blinded chart abstraction. MEASURES: Immunization status for age; number of days of delay in immunization; primary care utilization; and rates of screening for occult disease. RESULTS: Out of 3015 subjects, 274 subjects (9%) transferred out of the participating sites or had incomplete charts and were excluded. The 2741 (91%) remaining subjects were assessed. At baseline, study groups did not differ in age, gender, insurance type, or immunization status. Of the remaining subjects, 63% received Medicaid. Final series-complete immunization coverage levels were: control, 74%; prompting-only, 76%; tracking/outreach-only 95%; and combined tracking/outreach with prompting, 95%. Analysis of variance showed that: 1) tracking/outreach increased immunization rates 20 percentage points; 2) tracking/outreach decreased mean immunization delay 63 days; 3) tracking/outreach increased mean health supervision visits 0.44 visits per child; 4) tracking/outreach increased mean anemia screening 0.17 screenings per child and mean lead screenings 0.12 screenings per child; 5) impact of tracking/outreach was greatest for uninsured and impoverished patients; and 6) the prompting intervention had no impact on the studied outcomes, and its failure was caused by inconsistent use of prompts and failure to vaccinate ill children when prompted. Using tracking/outreach, the cost per additional child fully immunized was $474. Each $1000 spent on the tracking/outreach intervention resulted in: 2.1 additional fully vaccinated children and 668 fewer child-days of delayed immunization; 4.6 additional health supervision visits and 5.9 additional other visits to the primary care provider; and 1.8 additional anemia screenings and 1.3 additional lead screenings. CONCLUSIONS: Outreach directed toward children not up-to-date on immunizations improves not only immunization status, but also health supervision visit attendance and screening rates. The cost per additional child immunized was high, but should be interpreted in view of the spillover benefits that accompanied improved immunization. Effective means to improve coverage by reducing missed immunization opportunities still need to be identified.

Analysis of Variance↗
PubMed1999

[Parent-infant-program "Menschenskind"--concept and outcomes of the first three years].

Families with young children often experience high levels of difficulty and disturbance. Based on new knowledge of infant research, specific treatment approaches have been developed. The focus of these new approaches is the parent-infant relationship. To illustrate such a relationship-based treatment approach, the parent-infant programme "MenschensKind" will be introduced. Of the 263 families treated between October 1993 and December 1996, 72% had infants in the first year of life. Sleeping problems, excessive crying and explicit difficulties in the parent-infant relationship were the main reasons for referral. Treatment approaches included: developmental guidance, guided parent-infant training sessions (a new method for parents with excessively crying infants 0-4 months) and parent-infant-psychotherapy. Treatment rarely exceeded ten sessions. A proportion of the families recontacted the programme at a later stage, so serial brief sessions became a common method of treatment. The presented findings indicate a high need for a national network of specialised parent-infant programmes in Germany.

Crisis Intervention↗
PubMed1998

B-cell reconstitution and xenoreactive anti-pig natural antibody production in severe combined immunodeficient mice reconstituted with immunocompetent B cells from varying sources.

BACKGROUND: Little is known about the B-cell subsets that produce xenoreactive natural antibodies (NAb). This study was undertaken to investigate the potential role of varying B-cell populations in anti-pig NAb production in mice. METHODS: Severe combined immunodeficient (scid) mice were reconstituted with bone marrow or splenic or peritoneal B cells from immunocompetent mice. B-cell reconstitution and anti-pig NAb were evaluated by flow cytometric analysis. RESULTS: Adult marrow failed to reconstitute normal numbers of CD5+ B1a cells, but fully reconstituted CD5- Mac1- B2 and CD5- Mac1+ B1b cell populations in scid mice. Recipients of peritoneal B cells showed poor reconstitution of B2 cells, and an overshoot of B1 cells in the peritoneal cavity. Although B2 cells predominate in the adult spleen, splenic B cells from immunocompetent mice preferentially reconstituted B cells, including B1 cells, in the peritoneal cavity, but did not reconstitute splenic B2 cells. Therefore, neither adult marrow, splenocytes nor peritoneal cells can fully reconstitute scid mice with all B-cell subpopulations. Nevertheless, serum anti-pig NAb in marrow-reconstituted mice recovered to normal levels by 3 weeks, and were maintained for at least 30 weeks. Serum NAb in scid mice receiving peritoneal B cells reached normal levels by 4-7 weeks after transfer. However, NAb in sera of scid mice receiving splenic B cells took longer (>25 weeks) to reach normal levels. CONCLUSIONS: These results indicate that adult marrow-derived B cells can efficiently produce anti-pig NAb, and that peritoneal B cells have greater NAb-producing ability than splenic B cells or their immediate progeny.

Animals↗
PubMed1998

Boron neutron capture therapy: principles and potential.

This book on the therapeutic applications of neutrons and high-LET radiations in cancer therapy would not have been complete without a review of the present situation of boron neutron capture therapy (BNCT) and a discussion of its future perspectives. BNCT is a special type of high-LET radiation therapy that attempts to achieve a selectivity at the cellular level. The rationale is to incorporate boron atoms selectively in the cancer cells and then bombard those atoms with thermal neutrons to produce a neutron capture reaction and subsequent decay that emits alpha and lithium particles. The efficiency of the technique depends upon achieving selective incorporation of the boron atoms in the cancer cells and not (or to a lesser extent) in the normal cells. The present status and future directions are described, with emphasis on boron carriers (drugs) and their delivery, as well as physical and treatment planning aspects.

Boron Neutron Capture Therapy↗
PubMed1997

Antiserum to activity-dependent neurotrophic factor produces neuronal cell death in CNS cultures: immunological and biological specificity.

Activity-dependent neurotrophic factor (ADNF) is a glia-derived protein that is neuroprotective at femtomolar concentrations. ADNF is released from astroglia after treatment with 0.1 nM vasoactive intestinal peptide (VIP). To further assess the biological role of ADNF, antiserum was produced following sequential injections of purified ADNF into mice. Anti-ADNF ascites fluid (1:10,000) decreased neuronal survival by 45-55% in comparison to untreated cultures or those treated with control ascites. The neuronal death after anti-ADNF treatment was observed in cultures derived from the spinal cord, hippocampus or cerebral cortex at similar IC50's. Using a terminal deoxynucleotidyl transferase in situ assay to estimate apoptosis in cerebral cortical cultures, anti-ADNF was shown to produce a 70% increase in the number of labeled cells in comparison to controls. In spinal cord cultures, anti-ADNF treatment produced a 20% decrease in choline acetyltransferase activity in comparison to controls. Neuronal cell death produced by the antiserum to ADNF was prevented in cultures co-treated with purified ADNF or ADNF-15, an active peptide derived from the parent ADNF. In vitro binding between the anti-ADNF and ADNF-15 was demonstrated with size exclusion chromatography. Comparative studies with other growth factors (insulin-like growth factor-1, platelet-derived growth factor, nerve growth factor, epidermal growth factor, ciliary neurotrophic growth factor, and neurotrophin-3) demonstrated that only ADNF prevented neuronal cell death associated with electrical blockade. These investigations indicated that an ADNF-like substance was present in cultures derived from multiple locations in the central nervous system and that ADNF-15 exhibited both neuroprotection and immunogenicity. ADNF appears to be both a regulator of activity-dependent neuronal survival and a neuroprotectant.

Amino Acid Sequence↗
PubMed1996

Reducing missed opportunities for immunizations. Easier said than done.

BACKGROUND: Missed opportunities for immunizations are associated with underimmunization of preschool-age children. Practice policies limiting immunizations to scheduled preventive visits and guidelines requiring legal guardians to sign consent forms for vaccinations are 2 factors contributing to missed opportunities. However, methods to change these policies have not been sufficiently evaluated. OBJECTIVE: To measure the effectiveness of (1) changing practice policies to incorporate the new national standard to screen and vaccinate eligible children at all office visits and (2) eliminating legal guardian signature requirements. DESIGN: A randomized controlled trial of 2 interventions: (1) changing practice policy and routine to have office nurses screen for immunization status at all visits, attach immunization reminder cards to medical charts for eligible patients, and have providers vaccinate eligible children ("no missed opportunities" intervention) and (2) changing practice guidelines to allow vaccinations without a legal guardian's signature. The first intervention was performed at both sites; the second only at the neighborhood health center (NHC). SETTING: A Pediatric Continuity Clinic in a teaching hospital (hereafter referred to as Clinic), and an NHC. PATIENTS: Enrolled in the trial were 1005 Clinic patients and 983 NHC patients, 0 to 2 years of age. MAIN OUTCOME MEASURES: Missed opportunity rates, immunization rates, and rates of preventive services. RESULTS: Eliminating the requirement for a legal guardian's signature had no effect on any of the outcome measures. The no missed opportunities intervention was partially effective. Study patients had slightly fewer missed opportunities than control patients at each site: (0.60 vs 0.90 per patient per year at the Clinic, P = .01; 1.1 vs 1.3 per patient per year at the NHC, P = .02). For study group patients, immunization reminder cards were attached to medical charts in only one third of vaccine-eligible visits; when attached, they markedly increased vaccination by providers (odds ratio for vaccinating at a visit was 6.9 comparing visits when immunization reminder cards were attached vs not attached). However, at the end of the study, immunization rates were similar for study and control groups at each site. The number of undervaccinated days was slightly lower for the no missed opportunities study group at the Clinic than for the control group (56 days vs 77 days, P < .001), but they were similar for both groups at the NHC. There were no differences in rates of preventive visits or screening tests between study and control groups. CONCLUSIONS: The interventions evaluated to reduce missed opportunities did not increase immunization rates. The key problem was failure to screen for immunization status at all visits. More effective interventions will be needed to overcome barriers within busy primary care practices to substantially reduce missed opportunities.

Female↗
PubMed1996

Molecular dynamics simulations of silicon-fluorine etching.

Molecular dynamics simulations of the reactions between gaseous fluorine atoms and (SiFx)n adsorbates on the Si(100) - (2 x 1) surface are performed using the SW potential and compared to simulations with the WWC reparameterization of the SW potential. Theoretical and experimental work has demonstrated that the reactive fluorosilyl layer during silicon-fluorine etching is composed of tower-like adspecies of SiF, SiF2 and SiF3 groups. The objective of the simulations is to determine how the chemical composition, mechanism of formation, and energy distribution of the etched gas-phase products depend on the identity of the reacting adsorbate, the incident kinetic energy, and the parameterization of the potential energy function. Three reactions are simulated: F(g) + SiF3(a), F(g) + SiF2-SiF3(a), and F(g) + SiF2-SiF2-SiF3(a). SiF4 is the major product and Si2F6 and Si3F8 are minor products. In Si2F6 and Si3F8, the silicon-fluorine bond that is formed is stronger than the silicon-silicon bond in the molecule and, therefore, the majority of these products have enough energy to dissociate and will fragment before reaching the detector. An SN2-like mechanism is the primary mechanism responsible for the formation of SiF4, Si2F6, and Si3F8. In addition, at higher energies, the simulations have discovered a previously unknown mechanism for the formation of SiF4, which involves an insertion between a silicon-silicon bond. The results of the simulations with the two potentials differ quite substantially in their prediction of the reactivity of the adsorbates. The SW potential predicts a 2- to 3-eV lower energy threshold for reaction and a much higher reaction cross-section, especially for the SiF4 product. These results are explained in terms of the differences in the potential energy functions used to describe the silicon-fluorine interactions. In addition, the results are compared to experimental data on silicon-fluorine etching.

Computer Graphics↗
PubMed1995

Sensitization to mite allergens is a risk factor for early and late onset of asthma and for persistence of asthmatic signs in children.

BACKGROUND: To describe the natural history of asthma between the ages of 7 and 10 years and to analyze risk factors for prevalences, as well as new onset of asthma-like symptoms, a longitudinal study of 1812 children was conducted. METHODS: In four surveys, each 1 year apart, four asthma-like symptoms and several hypothetical risk factors were ascertained through standardized questionnaires. Sensitization to seven common inhalant allergens was measured by skin prick testing. Exposure to mite allergens (Der p I, Der f I) was assessed by measuring the antigen concentrations in the dust of each child's mattress. Occurrence of more than one asthma-like symptom closely related to the practioner's diagnoses of bronchial asthma and recurrent wheezy bronchitis was used as the outcome variable. RESULTS: After an initial prevalence of 14.5%, new onset of symptoms in children unaffected at the beginning was reported in 7.2% during the 3 years. Of the factors explaining prevalence and persistence of asthma-like symptoms (sensitization to mite allergens and animal danders, history of hay fever and eczema, low gestational age, male gender, parental atopy), only sensitization to mite allergens (odds ratio = 2.3, 95% confidence interval = 1.1-4.7) and parental atopy (odds ratio = 2.1, 95% confidence interval = 1.2-3.7) were also significantly associated with new onset. In a relatively small number of sensitized subjects with new onset of symptoms (n = 31), mite antigen concentration did not appear to be associated with incidence of symptoms. CONCLUSION: Sensitization to mite allergens antedated the onset of asthma-like symptoms, and no strong effect of allergen exposure on clinical development could be found. Thus the primary focus should be on preventing sensitization to mite allergens by implementing avoidance measures in infancy or at early school age in order to reduce the onset of asthma at a later stage.

Allergens↗