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   38642篇
  国内免费   789篇
  完全免费   5671篇
  综合类   45102篇
  2019年   4篇
  2018年   58篇
  2017年   34篇
  2016年   104篇
  2015年   39篇
  2014年   110篇
  2013年   117篇
  2012年   821篇
  2011年   1189篇
  2010年   592篇
  2009年   247篇
  2008年   1086篇
  2007年   1320篇
  2006年   2014篇
  2005年   1383篇
  2004年   1290篇
  2003年   1134篇
  2002年   1104篇
  2001年   1535篇
  2000年   1708篇
  1999年   1268篇
  1998年   754篇
  1997年   347篇
  1996年   293篇
  1995年   282篇
  1994年   294篇
  1993年   341篇
  1992年   939篇
  1991年   1076篇
  1990年   1128篇
  1989年   1186篇
  1988年   1282篇
  1987年   1407篇
  1986年   1281篇
  1985年   1357篇
  1984年   1028篇
  1983年   984篇
  1982年   545篇
  1981年   490篇
  1980年   319篇
  1979年   777篇
  1978年   605篇
  1977年   551篇
  1976年   411篇
  1975年   560篇
  1974年   795篇
  1973年   735篇
  1972年   702篇
  1971年   729篇
  1970年   658篇
  1969年   769篇
  1968年   727篇
  1967年   642篇
  1966年   619篇
  1965年   357篇
  1964年   115篇
  1963年   85篇
  1962年   86篇
  1961年   109篇
  1960年   98篇
  1959年   127篇
  1958年   186篇
  1957年   222篇
  1956年   189篇
  1955年   202篇
  1954年   163篇
  1953年   177篇
  1952年   175篇
  1951年   176篇
  1950年   139篇
  1949年   229篇
  1948年   254篇
  1947年   33篇
  1946年   5篇
  1945年   1篇
  1944年   2篇
  1943年   1篇
  1942年   1篇
  1941年   2篇
  1940年   1篇
  1939年   1篇
  1935年   1篇
  1934年   21篇
  1933年   31篇
  1932年   37篇
  1931年   28篇
  1930年   29篇
  1929年   7篇
  1928年   2篇
  1927年   34篇
  1926年   2篇
  1914年   4篇
排序方式: 共有45102条查询结果,搜索用时 78 毫秒
1.
CONTEXT: Inflammation is hypothesized to play a role in development of type 2 diabetes mellitus (DM); however, clinical data addressing this issue are limited. OBJECTIVE: To determine whether elevated levels of the inflammatory markers interleukin 6 (IL-6) and C-reactive protein (CRP) are associated with development of type 2 DM in healthy middle-aged women. DESIGN: Prospective, nested case-control study. SETTING: The Women's Health Study, an ongoing US primary prevention, randomized clinical trial initiated in 1992. PARTICIPANTS: From a nationwide cohort of 27 628 women free of diagnosed DM, cardiovascular disease, and cancer at baseline, 188 women who developed diagnosed DM over a 4-year follow-up period were defined as cases and matched by age and fasting status with 362 disease-free controls. MAIN OUTCOME MEASURES: Incidence of confirmed clinically diagnosed type 2 DM by baseline levels of IL-6 and CRP. RESULTS: Baseline levels of IL-6 (P<.001) and CRP (P<.001) were significantly higher among cases than among controls. The relative risks of future DM for women in the highest vs lowest quartile of these inflammatory markers were 7.5 for IL-6 (95% confidence interval [CI], 3.7-15.4) and 15.7 for CRP (95% CI, 6.5-37.9). Positive associations persisted after adjustment for body mass index, family history of diabetes, smoking, exercise, use of alcohol, and hormone replacement therapy; multivariate relative risks for the highest vs lowest quartiles were 2.3 for IL-6 (95% CI, 0.9-5.6; P for trend =.07) and 4.2 for CRP (95% CI, 1.5-12.0; P for trend =.001). Similar results were observed in analyses limited to women with a baseline hemoglobin A(1c) of 6.0% or less and after adjustment for fasting insulin level. CONCLUSIONS: Elevated levels of CRP and IL-6 predict the development of type 2 DM. These data support a possible role for inflammation in diabetogenesis.  相似文献
2.
3.
正交设计直观分析法优化PCR条件   总被引:124,自引:0,他引:124  
Polymerase chain reaction (PCR) optimization is difficult but very important. It is hard to gain the optimal condition due to the confusion of too many factors influencing the experiment. We report a new method, orthogonal design-direct analysis, which has been confirmed to be scientific and easy to follow by many times of PCR.  相似文献
4.
Context  Severe acute respiratory syndrome (SARS) is an emerging infectious disease that first manifested in humans in China in November 2002 and has subsequently spread worldwide. Objectives  To describe the clinical characteristics and short-term outcomes of SARS in the first large group of patients in North America; to describe how these patients were treated and the variables associated with poor outcome. Design, Setting, and Patients  Retrospective case series involving 144 adult patients admitted to 10 academic and community hospitals in the greater Toronto, Ontario, area between March 7 and April 10, 2003, with a diagnosis of suspected or probable SARS. Patients were included if they had fever, a known exposure to SARS, and respiratory symptoms or infiltrates observed on chest radiograph. Patients were excluded if an alternative diagnosis was determined. Main Outcome Measures  Location of exposure to SARS; features of the history, physical examination, and laboratory tests at admission to the hospital; and 21-day outcomes such as death or intensive care unit (ICU) admission with or without mechanical ventilation. Results  Of the 144 patients, 111 (77%) were exposed to SARS in the hospital setting. Features of the clinical examination most commonly found in these patients at admission were self-reported fever (99%), documented elevated temperature (85%), nonproductive cough (69%), myalgia (49%), and dyspnea (42%). Common laboratory features included elevated lactate dehydrogenase (87%), hypocalcemia (60%), and lymphopenia (54%). Only 2% of patients had rhinorrhea. A total of 126 patients (88%) were treated with ribavirin, although its use was associated with significant toxicity, including hemolysis (in 76%) and decrease in hemoglobin of 2 g/dL (in 49%). Twenty-nine patients (20%) were admitted to the ICU with or without mechanical ventilation, and 8 patients died (21-day mortality, 6.5%; 95% confidence interval [CI], 1.9%-11.8%). Multivariable analysis showed that the presence of diabetes (relative risk [RR], 3.1; 95% CI, 1.4-7.2; P = .01) or other comorbid conditions (RR, 2.5; 95% CI, 1.1-5.8; P = .03) were independently associated with poor outcome (death, ICU admission, or mechanical ventilation). Conclusions  The majority of cases in the SARS outbreak in the greater Toronto area were related to hospital exposure. In the event that contact history becomes unreliable, several features of the clinical presentation will be useful in raising the suspicion of SARS. Although SARS is associated with significant morbidity and mortality, especially in patients with diabetes or other comorbid conditions, the vast majority (93.5%) of patients in our cohort survived.   相似文献
5.
CONTEXT: Patients experience the highest rate of death and recurrent ischemic events during the early period after an acute coronary syndrome, but it is not known whether early initiation of treatment with a statin can reduce the occurrence of these early events. OBJECTIVE: To determine whether treatment with atorvastatin, 80 mg/d, initiated 24 to 96 hours after an acute coronary syndrome, reduces death and nonfatal ischemic events. DESIGN AND SETTING: A randomized, double-blind trial conducted from May 1997 to September 1999, with follow-up through 16 weeks at 122 clinical centers in Europe, North America, South Africa, and Australasia. PATIENTS: A total of 3086 adults aged 18 years or older with unstable angina or non-Q-wave acute myocardial infarction. INTERVENTIONS: Patients were stratified by center and randomly assigned to receive treatment with atorvastatin (80 mg/d) or matching placebo between 24 and 96 hours after hospital admission. MAIN OUTCOME MEASURES: Primary end point event defined as death, nonfatal acute myocardial infarction, cardiac arrest with resuscitation, or recurrent symptomatic myocardial ischemia with objective evidence and requiring emergency rehospitalization. RESULTS: A primary end point event occurred in 228 patients (14.8%) in the atorvastatin group and 269 patients (17.4%) in the placebo group (relative risk [RR], 0.84; 95% confidence interval [CI], 0.70-1.00; P =.048). There were no significant differences in risk of death, nonfatal myocardial infarction, or cardiac arrest between the atorvastatin group and the placebo group, although the atorvastatin group had a lower risk of symptomatic ischemia with objective evidence and requiring emergency rehospitalization (6.2% vs 8.4%; RR, 0.74; 95% CI, 0.57-0.95; P =.02). Likewise, there were no significant differences between the atorvastatin group and the placebo group in the incidence of secondary outcomes of coronary revascularization procedures, worsening heart failure, or worsening angina, although there were fewer strokes in the atorvastatin group than in the placebo group (12 vs 24 events; P =.045). In the atorvastatin group, mean low-density lipoprotein cholesterol level declined from 124 mg/dL (3.2 mmol/L) to 72 mg/dL (1.9 mmol/L). Abnormal liver transaminases (>3 times upper limit of normal) were more common in the atorvastatin group than in the placebo group (2.5% vs 0.6%; P<.001). CONCLUSION: For patients with acute coronary syndrome, lipid-lowering therapy with atorvastatin, 80 mg/d, reduces recurrent ischemic events in the first 16 weeks, mostly recurrent symptomatic ischemia requiring rehospitalization.  相似文献
6.
北京地区肺炎链球菌的耐药性及其分子流行病学调查   总被引:76,自引:4,他引:72  
目的 了解北京地区肺炎链球菌的耐药性以及青霉素耐药株的分子流行病学特征。方法 1997年9-11月采集北京市8所幼儿园985名儿童鼻咽拭子标本,分离培养肺炎链球菌,用Etest法和琼脂稀释法测定菌株对12种抗生素的最低抑菌浓度,用“荚膜肿胀”技术测血清分型,燕用盒式聚合酶链反应分析青霉素耐药株的亲缘关系。  相似文献
7.
Context.— Although cholesterol-reducing treatment has been shown to reduce fatal and nonfatal coronary disease in patients with coronary heart disease (CHD), it is unknown whether benefit from the reduction of low-density lipoprotein cholesterol (LDL-C) in patients without CHD extends to individuals with average serum cholesterol levels, women, and older persons. Objective.— To compare lovastatin with placebo for prevention of the first acute major coronary event in men and women without clinically evident atherosclerotic cardiovascular disease with average total cholesterol (TC) and LDL-C levels and below-average high-density lipoprotein cholesterol (HDL-C) levels. Design.— A randomized, double-blind, placebo-controlled trial. Setting.— Outpatient clinics in Texas. Participants.— A total of 5608 men and 997 women with average TC and LDL-C and below-average HDL-C (as characterized by lipid percentiles for an age- and sex-matched cohort without cardiovascular disease from the National Health and Nutrition Examination Survey [NHANES] III). Mean (SD) TC level was 5.71 (0.54) mmol/L (221 [21] mg/dL) (51st percentile), mean (SD) LDL-C level was 3.89 (0.43) mmol/L (150 [17] mg/dL) (60th percentile), mean (SD) HDL-C level was 0.94 (0.14) mmol/L (36 [5] mg/dL) for men and 1.03 (0.14) mmol/L (40 [5] mg/dL) for women (25th and 16th percentiles, respectively), and median (SD) triglyceride levels were 1.78 (0.86) mmol/L (158 [76] mg/dL) (63rd percentile). Intervention.— Lovastatin (20-40 mg daily) or placebo in addition to a low–saturated fat, low-cholesterol diet. Main Outcome Measures.— First acute major coronary event defined as fatal or nonfatal myocardial infarction, unstable angina, or sudden cardiac death. Results.— After an average follow-up of 5.2 years, lovastatin reduced the incidence of first acute major coronary events (183 vs 116 first events; relative risk [RR], 0.63; 95% confidence interval [CI], 0.50-0.79; P<.001), myocardial infarction (95 vs 57 myocardial infarctions; RR, 0.60; 95% CI, 0.43-0.83; P=.002), unstable angina (87 vs 60 first unstable angina events; RR, 0.68; 95% CI, 0.49-0.95; P=.02), coronary revascularization procedures (157 vs 106 procedures; RR, 0.67; 95% CI, 0.52-0.85; P=.001), coronary events (215 vs 163 coronary events; RR, 0.75; 95% CI, 0.61-0.92; P=.006), and cardiovascular events (255 vs 194 cardiovascular events; RR, 0.75; 95% CI, 0.62-0.91; P=.003). Lovastatin (20-40 mg daily) reduced LDL-C by 25% to 2.96 mmol/L (115 mg/dL) and increased HDL-C by 6% to 1.02 mmol/L (39 mg/dL). There were no clinically relevant differences in safety parameters between treatment groups. Conclusions.— Lovastatin reduces the risk for the first acute major coronary event in men and women with average TC and LDL-C levels and below-average HDL-C levels. These findings support the inclusion of HDL-C in risk-factor assessment, confirm the benefit of LDL-C reduction to a target goal, and suggest the need for reassessment of the National Cholesterol Education Program guidelines regarding pharmacological intervention.   相似文献
8.
Context  Traditionally, stent thrombosis has been regarded as a complication of percutaneous coronary interventions during the first 30 postprocedural days. However, delayed endothelialization associated with the implantation of drug-eluting stents may extend the risk of thrombosis beyond 30 days. Data are limited regarding the risks and the impact of this phenomenon outside clinical trials. Objective  To evaluate the incidence, predictors, and clinical outcome of stent thrombosis after implantation of sirolimus-eluting and paclitaxel-eluting stents in routine clinical practice. Design, Setting, and Patients  Prospective observational cohort study conducted at 1 academic hospital and 2 community hospitals in Germany and Italy. A total of 2229 consecutive patients underwent successful implantation of sirolimus-eluting (1062 patients, 1996 lesions, 2272 stents) or paclitaxel-eluting (1167 patients, 1801 lesions, 2223 stents) stents between April 2002 and January 2004. Interventions  Implantation of a drug-eluting stent (sirolimus or paclitaxel). All patients were pretreated with ticlopidine or clopidogrel and aspirin. Aspirin was continued indefinitely and clopidogrel or ticlopidine for at least 3 months after sirolimus-eluting and for at least 6 months after paclitaxel-eluting stent implantation. Main Outcome Measures  Subacute thrombosis (from procedure end through 30 days), late thrombosis (>30 days), and cumulative stent thrombosis. Results  At 9-month follow-up, 29 patients (1.3%) had stent thrombosis (9 [0.8%] with sirolimus and 20 [1.7%] with paclitaxel; P = .09). Fourteen patients had subacute thrombosis (0.6%) and 15 patients had late thrombosis (0.7%). Among these 29 patients, 13 died (case fatality rate, 45%). Independent predictors of stent thrombosis were premature antiplatelet therapy discontinuation (hazard ratio [HR],  89.78; 95% CI, 29.90-269.60; P<.001), renal failure (HR,  6.49; 95% CI, 2.60-16.15; P<.001), bifurcation lesions (HR,  6.42; 95% CI, 2.93-14.07; P<.001), diabetes (HR,  3.71; 95% CI, 1.74-7.89; P = .001), and a lower ejection fraction (HR,  1.09; 95% CI, 1.05-1.36; P<.001 for each 10% decrease). Conclusions  The cumulative incidence of stent thrombosis 9 months after successful drug-eluting stent implantation in consecutive "real-world" patients was substantially higher than the rate reported in clinical trials. Premature antiplatelet therapy discontinuation, renal failure, bifurcation lesions, diabetes, and low ejection fraction were identified as predictors of thrombotic events.   相似文献
9.
Increased pulse pressure and risk of heart failure in the elderly   总被引:73,自引:3,他引:70  
CONTEXT: Arterial stiffness increases with age. Thus, pulse pressure, an index of arterial stiffening, may predict congestive heart failure (CHF) in the elderly. OBJECTIVE: To study prospectively the association between pulse pressure and risk of CHF. DESIGN: Prospective cohort study. SETTING: The community-based East Boston Senior Health Project, East Boston, Mass. PATIENTS: A total of 1621 men and women (mean [SD] age, 77.9 [5.0] years) free of CHF who had blood pressure measurements taken in 1988-1989 and were followed up for 3.8 years. MAIN OUTCOME MEASURE: Incidence of CHF as ascertained by hospital discharge diagnosis (n = 208) and death certificates (n = 13). RESULTS: After controlling for age, sex, mean arterial pressure, history of coronary heart disease, diabetes mellitus, atrial fibrillation, valvular heart disease, and antihypertensive medication use, pulse pressure was an independent predictor of CHF. For each 10-mm Hg elevation in pulse pressure, there was a 14% increase in risk of CHF (95% confidence interval, 1.05-1.24; P = .003). Those in the highest tertile of pulse pressure (>67 mm Hg) had a 55% increased risk of CHF (P=.02) compared with those in the lowest (<54 mm Hg). Pulse pressure was more predictive than systolic blood pressure alone and was independent of diastolic blood pressure. CONCLUSION: Pulse pressure, an easily measurable correlate of pulsatile hemodynamic load, is an independent predictor of risk of CHF in this elderly cohort.  相似文献
10.
CONTEXT: Plasma levels of the inflammatory biomarker C-reactive protein (CRP) predict cardiovascular risk, and retrospective studies suggest that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) may lower CRP in a manner largely independent of low-density lipoprotein cholesterol (LDL-C). However, prospective trial data directly evaluating this anti-inflammatory effect of statins are not available. OBJECTIVE: To test the hypothesis that pravastatin has anti-inflammatory effects as evidenced by CRP reduction. DESIGN, SETTING, AND PARTICIPANTS: Community-based, prospective, randomized, double-blind trial including 1702 men and women with no prior history of cardiovascular disease (primary prevention cohort) and open-label study including 1182 patients with known cardiovascular disease (secondary prevention cohort) who provided at least baseline and 12-week blood samples. The study was conducted in US office-based practices from February to December 2000. INTERVENTIONS: Participants in the double-blind primary prevention trial were randomly assigned to receive 40 mg/d of pravastatin (n = 865) or placebo (n = 837) for 24 weeks. Participants in the secondary prevention cohort received 40 mg/d of open-label pravastatin for 24 weeks. MAIN OUTCOME MEASURE: Change in CRP levels from baseline to 24 weeks. RESULTS: In the primary prevention trial, compared with placebo, pravastatin reduced median CRP levels by 16.9% (P<.001) at 24 weeks, reflecting a decrease of 0.02 mg/dL in the pravastatin group while no change in CRP levels was observed in the placebo group. This effect was seen as early as 12 weeks (median reduction in CRP with pravastatin, 14.7%; P<.001) and was present among all prespecified subgroups according to sex, age, smoking status, body mass index, baseline lipid levels, presence of diabetes, and use of aspirin or hormone replacement therapy. No significant association was observed between baseline CRP and baseline LDL-C levels, end-of-study CRP and end-of-study LDL-C levels, or change in CRP and change in LDL-C levels over time. In linear regression analyses, the only significant predictors of change in CRP on a log scale were randomized pravastatin allocation and baseline CRP levels (P<.001 for both). Similar reductions in CRP levels were observed at 12 weeks (-14.3%) and 24 weeks (-13.1%) in the secondary prevention cohort treated with pravastatin (P<.005 for both). CONCLUSIONS: In this prospective trial, pravastatin reduced CRP levels at both 12 and 24 weeks in a largely LDL-C-independent manner. These data provide evidence that statins may have anti-inflammatory effects in addition to lipid-lowering effects.  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