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   25654篇
  免费   2141篇
  国内免费   2093篇
  29888篇
  2024年   59篇
  2023年   244篇
  2022年   574篇
  2021年   958篇
  2020年   734篇
  2019年   879篇
  2018年   931篇
  2017年   757篇
  2016年   1109篇
  2015年   1574篇
  2014年   1848篇
  2013年   1987篇
  2012年   2303篇
  2011年   2156篇
  2010年   1458篇
  2009年   1189篇
  2008年   1493篇
  2007年   1325篇
  2006年   1229篇
  2005年   1098篇
  2004年   993篇
  2003年   908篇
  2002年   834篇
  2001年   435篇
  2000年   399篇
  1999年   326篇
  1998年   252篇
  1997年   213篇
  1996年   171篇
  1995年   125篇
  1994年   117篇
  1993年   86篇
  1992年   131篇
  1991年   107篇
  1990年   101篇
  1989年   89篇
  1988年   70篇
  1987年   80篇
  1986年   53篇
  1985年   48篇
  1984年   45篇
  1983年   29篇
  1982年   36篇
  1981年   21篇
  1980年   28篇
  1979年   37篇
  1978年   26篇
  1977年   25篇
  1973年   24篇
  1972年   24篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
191.
192.
A new design method of a broadband wide-angle metal-dielectric-metal plasmonic absorber is presented based on the cavity mode theory. The broadband absorption is implemented by filling a unit cell with multi-size square metal patches resonant at adjacent wavelengths, with the widths of the patches and thickness of the dielectric layer optimized with the presented method. A broadband plasmonic absorber working in the visible range is designed, the absorption of which is insensitive to the azimuth angle of incident field and keeps over 0.7 at incident angle up to 60° for p polarization and above 0.6 at up to 40° for s polarization.  相似文献   
193.
194.

Background

Atrial fibrillation (AF) is reported to be a less frequent cause of ischemic stroke in China than in Europe and North America, but it is not clear whether this is due to underestimation. Our aim was to define the true frequency of AF-associated stroke, to determine the yield of 6-day Holter ECG to detect AF in Chinese stroke patients, and to elucidate predictors of newly detected AF.

Methods

Patients with acute ischemic stroke or transient ischemic attack (TIA) were enrolled in a prospective, multicenter cohort study of 6-day Holter monitoring within 7 days after stroke onset at 20 sites in China between 2013 and 2015. Independent predictors of newly-detected AF were determined by multivariate analysis.

Results

Among 1511 patients with ischemic stroke and TIA (mean age 63 years, 33.1% women), 305 (20.2%) had either previously known (196, 13.0%) or AF newly-detected by electrocardiography (53, 3.5%) or by 6-day Holter monitoring (56/1262, 4.4%). A history of heart failure (OR?=?4.70, 95%CI, 1.64–13.5), advanced age (OR?=?1.06, 95%CI, 1.04–1.09), NIHSS at admission (OR?=?1.06, 95%CI, 1.02–1.10), blood high density lipoprotein (HDL) (OR?=?1.52, 95%CI, 1.09–2.13), together with blood triglycerides (OR?=?0.64, 95%CI, 0.45–0.91) were independently associated with newly-detected AF.

Conclusions

Contrary to previous reports, AF-associated stroke is frequent (20%) in China if systemically sought. Prolonged noninvasive cardiac rhythm monitoring importantly increases AF detection in patients with recent ischemic stroke and TIA in China. Advanced age, history of heart failure, and higher admission NIHSS and higher level of HDL were independent indicators of newly-detected AF.

Trial registration

NCT02156765 (June 5, 2014).
  相似文献   
195.

Background

The Supreme? laryngeal mask airway (SLMA) is a single-use LMA with double lumen design that allows separation of the respiratory and the alimentary tract, hence potentially reducing the gastric volume and risk of aspiration. The purpose of this prospective cohort study is to evaluate the the role of the SLMA as an airway technique for women undergoing category 2 and 3 Cesarean delivery under general anesthesia.

Methods

We recruited 584 parturients who underwent category 2 or 3 Cesarean delivery under general anesthesia, in which 193 parturients underwent category 2 and 391 parturients underwent category 3 Cesarean delivery. The primary outcome was insertion success rate at 1st attempt in SLMA insertion. The secondary outcomes included anaesthetic, obstetric outcomes and maternal side effects associated with airway device.

Results

The 1st attempt insertion success rate was 98.3%, while the overall insertion success rate was 100%. The mean (Standard deviation) time to effective ventilation was 15.6 (4.4) seconds. Orogastric tube insertion was successful at the 1st attempt in all parturients. There was no clinical evidence of aspiration or regurgitation. No episodes of hypoxemia, laryngospasm or bronchospasm were observed intra-operatively. The incidence of complications was low and with good maternal satisfaction reported.

Conclusions

The SLMA could be an alternative effective airway in category 2 and 3 parturients emergency Cesarean Delivery under general anesthesia in a carefully-selected obstetric population.

Trial registration

Clinical Trials Registration: Clinicaltrials.gov Registration NCT02026882. Registered on December 31, 2013.
  相似文献   
196.
This study assessed the effects of combined chromium picolinate (CP) and conjugated linoleic acid (CLA) supplementation on energy restriction and exercise-induced changes in body composition, glucose metabolism, lipid lipoprotein profile and blood pressure in overweight, premenopausal women. For 12 weeks, 35 women [age 36+/-1 years (mean+/-S.E.M.); BMI 28.0+/-0.5 kg/m2] were counseled to consume a 2092 kJ/day (500 kcal/day) energy deficit diet and performed 30 min of moderate-intensity walking or jogging 5 days/week. The women were randomly assigned to ingest either CP-CLA [400 mug chromium (Cr), 1.8 g CLA in 2.4 g tonalin oil, n=19] or placebo (<0.1 microg Cr, 2.4 g canola oil, n=16). Compared to baseline, urinary Cr excretion increased 22-fold, plasma CLA isomer 18:2 (c9,t11) content increased 79% and plasma CLA isomer 18:2 (t10,c12) became detectable in CP-CLA and were unchanged in Placebo. Over time, body weight decreased 3.5+/-0.5% (CP-CLA -2.6+/-0.5; placebo -2.5+/-0.5 kg) and fat mass decreased 8.9+/-1.3% (CP-CLA -2.7+/-0.5, placebo -2.4+/-0.5 kg), with no differences in responses between groups. Fasting blood hemoglobin A1c, plasma glucose and insulin, a homeostatic assessment of insulin resistance, serum total cholesterol (CHOL), high-density lipoprotein cholesterol (HDL), low-density lipoprotein cholesterol, triacylglycerol (TG), CHOL/HDL ratio, TG/HDL ratio and sitting systolic and diastolic blood pressures were not changed over time or influenced by CP-CLA. The use of a combined CP and CLA supplement for 3 months does not affect diet- and exercise-induced changes in weight and body composition or improve indexes of metabolic and cardiovascular health in young overweight women.  相似文献   
197.
Helicid is a traditional Chinese medicine used to treat headache and insomnia with definite effects. To facilitate pharmacokinetic studies of helicid in man, a sensitive and specific LC-MS/MS method for the quantitative detection of helicid in human plasma was developed and validated. The method involved the addition of bergeninum as the internal standard (IS), protein precipitation, HPLC separation, and quantification by MS/MS system using negative electrospray ionization in the multiple reaction monitoring mode (MRM). The precursor→product ion transitions were monitored at m/z 282.8→120.9 for helicid and m/z 326.9→192.2 for the IS, respectively. The lower limit of quantification (LLOQ) was 0.2 μg/L. The calibration curves for helicid was linear over a concentration range of 0.2-20 μg/L. The intra- and inter-batch analyses of QC samples at 0.4, 2, 20 μg/L indicated good precision (%R.S.D. between 2.69 and 5.47%) and accuracy (between 96.15 and 105.05%). The helicid was stable in human plasma stored at room temperature for at least 24h, 4°C for at least 24h, -20°C for at least 1 month, and for routine three freeze-thaw cycles. This accurate and specific assay provides a useful method for evaluating the pharmacokinetic profile of helicid in humans.  相似文献   
198.
Chromatin plays a fundamental role in eukaryotic genomic regulation, and the increasing awareness of the importance of epigenetic processes in human health and disease emphasizes the need for understanding the structure and function of the nucleosome. Recent advances in chromatin structural studies, including the first structures of nucleosomes containing the Widom 601 sequence and the structure of a chromatin protein-nucleosome assembly, have provided new insight into stretching of nucleosomal DNA, nucleosome positioning, binding of metal ions, drugs and therapeutic candidates to nucleosomes, and nucleosome recognition by nuclear proteins. These discoveries ensure promising future prospects for unravelling structural attributes of chromatin.  相似文献   
199.
A phytochemical study on the roots of Pongamia pinnata afforded five new isoflavone and isoflavanone derivatives (15), including two previously undescribed phenylisoflavones possessing an 1,2-oxetane ring, along with 21 known compounds (626) among which compound 18 is the first time to be isolated from nature. The structures of the isolated compounds were determined on the basis of 1D, 2D NMR, and HRESIMS. The absolute configurations of the compounds were assigned via analysis of the specific rotations and circular dichroism (CD) spectra, as well as by comparison of the calculated and experimental electronic circular dichroism (ECD) data. All the isolated compounds were evaluated for their inhibitory effects on NO production in LPS-stimulated BV-2 microglial cells. Twelve compounds exhibited different levels of inhibitory effects against NO production, and compound 1 showed the best activity with an IC50 value at 9.0?μM.  相似文献   
200.
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