首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   715篇
  免费   38篇
  国内免费   48篇
  2024年   3篇
  2023年   9篇
  2022年   14篇
  2021年   12篇
  2020年   26篇
  2019年   25篇
  2018年   27篇
  2017年   24篇
  2016年   36篇
  2015年   24篇
  2014年   23篇
  2013年   47篇
  2012年   24篇
  2011年   24篇
  2010年   18篇
  2009年   23篇
  2008年   25篇
  2007年   32篇
  2006年   26篇
  2005年   24篇
  2004年   29篇
  2003年   17篇
  2002年   23篇
  2001年   25篇
  2000年   19篇
  1999年   19篇
  1998年   15篇
  1997年   18篇
  1996年   11篇
  1995年   14篇
  1994年   18篇
  1993年   12篇
  1992年   16篇
  1991年   10篇
  1990年   7篇
  1989年   13篇
  1988年   7篇
  1987年   10篇
  1986年   12篇
  1985年   3篇
  1984年   3篇
  1983年   6篇
  1982年   10篇
  1981年   4篇
  1980年   2篇
  1979年   8篇
  1978年   1篇
  1977年   1篇
  1976年   1篇
  1973年   1篇
排序方式: 共有801条查询结果,搜索用时 31 毫秒
791.
In view of the increasing complexity of both cardiovascular implantable electronic devices (CIEDs) and patients in the current era, practice guidelines, by necessity, have become increasingly specific. This document is an expert consensus statement that has been developed to update and further delineate indications and management of CIEDs in pediatric patients, defined as ≤21 years of age, and is intended to focus primarily on the indications for CIEDs in the setting of specific disease categories. The document also highlights variations between previously published adult and pediatric CIED recommendations and provides rationale for underlying important differences. The document addresses some of the deterrents to CIED access in low- and middle-income countries and strategies to circumvent them. The document sections were divided up and drafted by the writing committee members according to their expertise. The recommendations represent the consensus opinion of the entire writing committee, graded by class of recommendation and level of evidence. Several questions addressed in this document either do not lend themselves to clinical trials or are rare disease entities, and in these instances recommendations are based on consensus expert opinion. Furthermore, specific recommendations, even when supported by substantial data, do not replace the need for clinical judgment and patient-specific decision-making. The recommendations were opened for public comment to Pediatric and Congenital Electrophysiology Society (PACES) members and underwent external review by the scientific and clinical document committee of the Heart Rhythm Society (HRS), the science advisory and coordinating committee of the American Heart Association (AHA), the American College of Cardiology (ACC), and the Association for European Paediatric and Congenital Cardiology (AEPC). The document received endorsement by all the collaborators and the Asia Pacific Heart Rhythm Society (APHRS), the Indian Heart Rhythm Society (IHRS), and the Latin American Heart Rhythm Society (LAHRS). This document is expected to provide support for clinicians and patients to allow for appropriate CIED use, appropriate CIED management, and appropriate CIED follow-up in pediatric patients.  相似文献   
792.
Alfalfa plants germinated and grown for 15 d in soil containing 80 mg Pb kg?1 were treated with ethylenediaminetetraacetic acid (EDTA) at 0.8 mM and indole-3-acetic acid-kinetin (IAA-KN) at 100 μM. Fifteen days after the treatment application, the concentration of lead (Pb), macronutrients, and micronutrients was determined using inductively coupled plasma/optical emission spectroscopy. The chlorophyll content and plant growth were also measured. Roots of plants exposed to Pb alone, Pb–EDTA, and Pb–EDTA-IAA-KN had 160, 140, and 150 mg Pb kg?1 DW, respectively. Pb was not detected in the stems of plants exposed to Pb alone; however, stems of plants treated with EDTA and EDTA–IAA-KN had 78 and 142 mg Pb kg?1 DW, respectively. While the Pb concentration in leaves of plants treated with EDTA and EDTA–IAA-KN was 92 and 127 mg kg?1 DW, respectively. In addition, EDTA and EDTA–IAA-KN significantly increased the translocation of zinc and manganese to leaves. The x-ray absorption spectroscopic studies demonstrated that Pb(II) was transported from roots to leaves without a change in the oxidation state.  相似文献   
793.
Many polluted sites are simultaneously contaminated with polycyclic aromatic hydrocarbons and heavy metals. In the present study, batch and continuous column experiments were performed utilizing self-composition soil to describe the sorption behavior of two contaminants: lead (Pb2+) and pyrene (PYR). Operational conditions such as contact time, bed depth, and flow rate were optimized. The effect of soil organic matter content on the process of adsorption of both contaminants was investigated. The presence of PYR in solution at neutral pH (6.0–7.5) decreased Pb2+ sorption. Similar behavior was observed for PYR in the presence of Pb2+ in solution. At room temperature, batch experimental data conducted as a function of contact time were analyzed using the Langmuir and Freundlich isotherms. Results revealed that Pb2+ sorption isotherms were fitted better by the Langmuir model and PYR sorption isotherms were fitted better by the Freundlich model. Column adsorption experiments were carried out at room temperature and under operating parameters (bed depth, flow rate, and initial contaminant concentration). Breakthrough curves were well fitted to the two-site first-order kinetic model with a sum of square errors less than 0.14. The Pb2+ adsorption kinetic data were processed also for the Thomas model with a good accuracy.  相似文献   
794.
A high defibrillation threshold occurs in approximately 6% of implants. The defibrillation threshold can be improved by addition of a defibrillation lead. However, the DF-4 high energy ICD header precludes the addition of a defibrillation lead. Here we report on use of a new high voltage adaptor/splitter that enables the addition of an extra defibrillation lead.  相似文献   
795.
Observed blood lead levels for young children from several communities are compared with blood lead levels predicted for those communities using the USEPA's Integrated Exposure Uptake Biokinetic (IEUBK) Model. In contrast to the compari sons described elsewhere, the blood lead levels observed in the communities con sidered here are not well represented by the model's predictions. The model's predictions for Midvale, UT; Sandy, UT; Cincinnati, OH; and a recent data set for Palmerton, PA, show considerable deviation from observation both for the geometric mean blood lead level and the percent of blood lead levels above 10?µg/dL. Various adjustments in the model to consider play area soils, site specific geometric standard deviations and the time children spend away from their homes do not substantially improve the comparisons to observation. It is difficult to predict a priori the data sets for which the model will yield adequate predictions. This reduces the value of the model for use in communities where blood lead measurements have not been made, and suggests that caution should be exercised when using the model to set soil lead cleanup levels or to predict the result of remediation.  相似文献   
796.
Summary I have investigated the effect of lead on the erythrocyte ghosts (Ca2+,Mg2+)-ATPase, with special attention to the role of calmodulin in this phenomena. Under regular incubation conditions, lead inhibits the enzyme with an IC50 of 6.0 µM. The presence of exogenously added calmodulin apparently does not change this inhibitory value. DTT added during the incubation period does not affect the inhibitory action of lead. However, when the membranes are preincubated with DTT, an important IC50 displacement is observed, either with or without added calmodulin. Since [125I]calmodulin binding to the membranes is enhanced when lead is used, the possibility of a lead/calmodulin complex that optimally stimulates the enzyme using lead concentrations between 1.0 and 10.0 µM, is suggested. Based on the experimental data, I propose two well defined actions of lead; first, an inhibitory action upon the ATPase above 1.0 µM lead, most probably related to essential sullphydryl groups in the enzyme; and second, a direct action of lead upon calmodulin at lead concentrations below 1.0 µM.A preliminary report has been presented at the 5th European Bioenergetics Conference. Aberystwyth, Wales. 20–26 July 1988. EBEC Reports. vol 5; p294 (1988).  相似文献   
797.
798.
IntroductionCurrent guidelines advocate reviewing peri-procedural anticoagulation on individual case basis for transvenous lead extraction (TLE). We investigated the safety of TLE on uninterrupted warfarin with therapeutic INR.MethodsRetrospective registry of consecutive patients undergoing TLE on uninterrupted warfarin (Warfarin Group) across two centres. Age and sex matched controls not on anticoagulation (No-Warfarin Group) and undergoing TLE over the same time-period were included. Both groups were compared over one-year.Results121 TLEs over 18-months. 22 patients on uninterrupted anticoagulation were compared to 22 controls. Groups were well matched for baseline demographics other than INR. Warfarin group had mean INR of 2.2 ± 0.6 (range 2–3.5). Primary end point was procedural safety and efficacy. Amongst cases, 43/45 (96%) leads were removed in their entirety compared to 37/40 (93%) in controls (p = 0.66). In the cases, these included 44% defibrillator, 47% pace-sense and 9% CS leads of average duration 7yrs. There was no reported tamponade, haemothorax or procedural mortality in either group. One patient amongst cases required inotropic support while two patients amongst controls had device-site haematomas. No significant difference reported in Hb drop post-procedure or overall complication rate between the groups (p = 0.11,0.32). Cox regression showed a significant association between procedural success and device infection, number of leads extracted, serum creatinine (p = 0.03, 0.04, 0.02). Over a 1-year follow-up, there was lead displacement in one case and one control had infection of the re-implanted device.ConclusionTLE can be carried out safely in anticoagulated patients with therapeutic INRs. Larger multicentre studies are required to confirm these findings.  相似文献   
799.
Concentrations of Pb, Zn, Cd, Cu, Cr, Co, and Ni were determined in roadside topsoil collected from locations of varied vehicular traffic densities in the city of Ibadan, Nigeria, with a view to determining the level of contamination and the contribution of traffic density. Levels of Pb, Zn, Cd, and Cu were elevated above background concentrations measured in control areas. Average values (ppm) for all sample locations were Pb — 81±140; Zn — 48±37; Cd — 0.55±0.49; Cu — 17±17; Cr — 22.1±9.6; Co — 7.9±3.8; Ni — 10.5±9.7. Factors of accumulation of metals in roadsides relative to control sites were highest for Pb. Vehicular traffic was not an important source of chromium, cobalt and nickel, for which roadside concentrations were about those of the control sites. Metal concentrations were poorly correlated with traffic volumes. An average of about 60% of total soil concentration of the metals were determined to be held in bioavailable geochemical phases, of which the highest concentrations were mostly held in either the reducible or oxidizable phase. Levels of the metals in the topsoil were generally lower than the soil quality criteria of some developed countries.  相似文献   
800.
The distribution of total Pb in surface and subsurface soil horizons at an outdoor shooting range in southeastern Michigan was determined by single extraction elemental analysis (AAS and ICP‐AES). Significant Pb enrichment of the site's soils coincides closely with Pb vapor and particulate matter produced from shot shell primers and the downfall of Pb/Sb pellets associated with the recreational shooting of skeet and trap. Surface concentrations in these locations are 10 to 100 times greater than the background concentration found on adjacent properties. The distribution of Pb in the subsurface soil horizons corresponds to the distribution of Pb at the surface, which suggests the Pb is mobilizing and migrating downward through the vadose zone. This mobilization appears to be occurring despite the clay‐rich nature of the soils, and may be due to the transformation of metallic Pb into soluble Pb compounds of carbonate and sulfate: Both compounds appear to be present in crust material found coating many of the pellets found at the site. The downward migration of soluble Pb is a potential threat to groundwater that is present at the site at a depth of less than 1 m. The protection of surface water quality is also a concern because Pb pellets from the shooting range have been found in the bed sediments of a nearby stream.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号