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   4279篇
  免费   24篇
  国内免费   2篇
电工技术   23篇
综合类   6篇
化学工业   300篇
金属工艺   40篇
机械仪表   35篇
建筑科学   81篇
矿业工程   6篇
能源动力   74篇
轻工业   270篇
水利工程   22篇
石油天然气   3篇
无线电   236篇
一般工业技术   282篇
冶金工业   2699篇
原子能技术   23篇
自动化技术   205篇
  2023年   13篇
  2022年   16篇
  2021年   22篇
  2020年   15篇
  2019年   21篇
  2018年   26篇
  2017年   27篇
  2016年   39篇
  2015年   30篇
  2014年   26篇
  2013年   81篇
  2012年   53篇
  2011年   104篇
  2010年   67篇
  2009年   68篇
  2008年   90篇
  2007年   93篇
  2006年   98篇
  2005年   76篇
  2004年   57篇
  2003年   57篇
  2002年   68篇
  2001年   60篇
  2000年   36篇
  1999年   122篇
  1998年   870篇
  1997年   475篇
  1996年   286篇
  1995年   170篇
  1994年   146篇
  1993年   176篇
  1992年   54篇
  1991年   51篇
  1990年   57篇
  1989年   66篇
  1988年   44篇
  1987年   42篇
  1986年   46篇
  1985年   33篇
  1984年   26篇
  1983年   23篇
  1982年   27篇
  1981年   25篇
  1980年   33篇
  1979年   12篇
  1978年   17篇
  1977年   61篇
  1976年   119篇
  1975年   12篇
  1973年   10篇
排序方式: 共有4305条查询结果,搜索用时 0 毫秒
991.
OBJECTIVE: To determine whether laryngeal hemiplegia would increase transmural pulmonary artery pressure (TPAP). ANIMALS: 6 horses. DESIGN: Horses were studied under 5 conditions: control conditions, after induction of left laryngeal hemiplegia, during obstruction of the left nostril, after placement of an instrumented tracheostomy, and after placement of an open tracheostomy. Horses were evaluated after being given saline solution and after being given furosemide. PROCEDURES: Horses were exercised on a high speed treadmill, using a maximum speed of 13 m/s. During each exercise, airway pressures, airflow, esophageal and pulmonary artery pressures, and blood gas partial pressures were measured. RESULTS: When adjusted for horse, speed, and obstruction condition, mean TPAP (pulmonary artery pressure-esophageal pressure) and minimum TPAP were significantly lower after administration of furosemide than after administration of saline solution. In horses given saline solution, respiratory obstruction that increased intrapleural pressure significantly increased mean TPAP, and respiratory obstruction that decreased intrapleural pressure significantly decreased minimum TPAP. CONCLUSIONS: Changes in intrapleural pressure appear to play an important role in pulmonary artery pressure and TPAP. CLINICAL RELEVANCE: Because induction of laryngeal hemiplegia did not increase TPAP, laryngeal hemiplegia is unlikely to contribute to development of exercise-induced pulmonary hemorrhage.  相似文献   
992.
OBJECTIVE: Because survival time varies greatly, it is difficult for dementia patients to meet a key criterion for eligibility for the Medicare hospice benefit: a 6-month survival time. We have developed criteria for the Medicare hospice benefit that include the characteristics of advanced dementia and related medical complications. The purpose of the study was to determine survival time among dementia patients who met these criteria. Additionally, because the National Hospice Organization (NHO) developed its own guidelines while the study was in progress, we retrospectively examined the application of these guidelines to our sample. DESIGN: Two cohorts of hospice patients were studied longitudinally, each for 2 years. SETTING: Nine Midwestern hospice programs. PARTICIPANTS: Forty-seven patients were enrolled in home hospice and institutional hospice settings. MEASUREMENTS: Survival time consisted of the number of days between enrollment in the hospice program and death or the end of the study. Other measures included Activities of Daily Living, ratings of Appetite, Nourishment, and Mobility, Functional Assessment Staging (FAST), a Medical Complications Checklist, and a care plan concerning the use of medications for acute illness. RESULTS: Our hospice enrollment criteria predicted a median survival time of 4 months and a mean survival time of 6.9 months; 38% of patients survived for more than 6 months. FAST scores and Mobility ratings were significantly related to survival time. However, 41% could not be scored on the FAST as their disease progression was not ordinal. Among patients who could be scored on the FAST and who had reached Stage Seven C, their mean survival time was 3.2 months compared with 18 months among those who could be scored and had not reached this stage and 8.6 months among patients whose disease progression was not ordinal, P < .001. When the palliative care plans were examined, less aggressive care plans resulted in shorter survival times, P < .01. CONCLUSION: Our hospice enrollment criteria identified a group with a median survival time of 4 months and a mean survival time of 6.9 months. Using NHO criteria relying on the FAST allows the identification of a subgroup with very high mortality and a short time until death. Although the FAST can identify a subgroup of appropriate candidates for hospice, sole reliance on this measure might decrease access to hospice care for many dementia patients.  相似文献   
993.
The performance of two EIAs (adsorption EIA and lipooligosaccharide [LOS] EIA) that detect antibodies to Haemophilus ducreyi was evaluated with serum specimens obtained from 163 patients (96 with genital ulcer disease [GUD]). Paired serum specimens (initial and follow-up) were obtained from 52 of the GUD patients. By use of initial serum specimens from 82 GUD patients whose etiologic agents for their ulcers had been identified, the adsorption EIA had a sensitivity and specificity for chancroid of 53% and 71%, while the LOS EIA had a sensitivity and specificity of 48% and 89%, respectively. Sensitivity and specificity of the adsorption EIA increased to 78% and 84%, respectively, when the results of follow-up serum specimens were used to calculate optimal performance. The proportion of patients testing positive for H. ducreyi who had anti-H. ducreyi IgG antibodies, as determined by adsorption EIA, increased with the duration of infection, thus limiting the role of EIAs in the diagnosis of chancroid.  相似文献   
994.
Larval flannelmouth sucker (Catostomus latipinnis) were exposed to arsenate, boron, copper, molybdenum, selenate, selenite, uranium, vanadium, and zinc singly, and to five mixtures of five to nine inorganics. The exposures were conducted in reconstituted water representative of the San Juan River near Shiprock, New Mexico. The mixtures simulated environmental ratios reported for sites along the San Juan River (San Juan River backwater, Fruitland marsh, Hogback East Drain, Mancos River, and McElmo Creek). The rank order of the individual inorganics, from most to least toxic, was: copper > zinc > vanadium > selenite > selenate > arsenate > uranium > boron > molybdenum. All five mixtures exhibited additive toxicity to flannelmouth sucker. In a limited number of tests, 44-day-old and 13-day-old larvae exhibited no difference in sensitivity to three mixtures. Copper was the major toxic component in four mixtures (San Juan backwater, Hogback East Drain, Mancos River, and McElmo Creek), whereas zinc was the major toxic component in the Fruitland marsh mixture, which did not contain copper. The Hogback East Drain was the most toxic mixture tested. Comparison of 96-h LC50 values with reported environmental water concentrations from the San Juan River revealed low hazard ratios for arsenic, boron, molybdenum, selenate, selenite, uranium, and vanadium, moderate hazard ratios for zinc and the Fruitland marsh mixture, and high hazard ratios for copper at three sites and four environmental mixtures representing a San Juan backwater, Hogback East Drain, Mancos River, and McElmo Creek. The high hazard ratios suggest that inorganic contaminants could adversely affect larval flannelmouth sucker in the San Juan River at four sites receiving elevated inorganics.  相似文献   
995.
BACKGROUND: This study was designed to better define the merits of the bileaflet and tilting-disc valves. METHODS: We prospectively randomized 156 patients (mean age, 59 years) to receive either the St. Jude (n = 80) or the Medtronic Hall (n = 76) mitral valve prosthesis between September 1986 and December 1997. The two groups were not significantly different with respect to preoperative New York Heart Association class, left ventricular ejection fraction, incidence of mitral stenosis or insufficiency, extent of coronary artery disease, completeness of revascularization, or cross-clamp or bypass time. RESULTS: The operative mortality (11.2% versus 13.1%, St. Jude versus Medtronic Hall, respectively) and late mortality (27% versus 22%, St. Jude versus Medtronic Hall, respectively) were not significantly different. Follow-up was complete in all hospital survivors with a mean of 60.7 months (range, 1 to 133 months). The analysis of 10-year actuarial survival and freedom from valve-related events demonstrated no significant differences between the cohorts. Freedom from reoperation was higher in the St. Jude group (p < 0.01). Comparisons of patient functional status and echocardiographic hemodynamic parameters obtained at the time of follow-up demonstrated no significant differences between the two prostheses. CONCLUSIONS: This study suggests that there is no difference between the St. Jude and Medtronic Hall prostheses with respect to late clinical performance or hemodynamic results and therefore does not support the preferential selection of either prosthesis.  相似文献   
996.
Myelodysplastic syndromes (MDS) are clonal hematopoietic disorders which begin in a pluripotential bone marrow (BM) stem cell. This early stem cell is believed to acquire a growth advantage over its neighbors as a result of an initial transforming event, the nature of which has remained obscure. In this paper, we propose that pathogens such as those belonging to the herpesvirus family of DNA viruses may play a role in the initial transformation of the stem cell. The case for cytomegalovirus (CMV) as a representative of this family of viruses is discussed at length and a molecular mechanism which may be involved in the oncogenic activity of CMV is proposed. No proof has been presented to implicate CMV directly in MDS, but circumstantial evidence which supports such a possibility is provided.  相似文献   
997.
OBJECTIVES: This study investigated the ability of on-line vectorcardiography (VCG) to detect myocardial ischaemia as assessed by dobutamine stress echocardiography after an episode of unstable coronary artery disease. DESIGN: Dobutamine stress test (incrementally infused doses of 5, 10, 20, 30 and 40 microg kg(-1) per minute) was performed during simultaneous VCG monitoring. QRS vector difference (QRS-VD), ST vector magnitude (ST-VM), ST change vector magnitude (STC-VM) and T change vector magnitude (TC-VM) were monitored. A new wall motion abnormality or worsening of abnormal wall motion was regarded as indicating a positive dobutamine stress test. SUBJECTS: Fifty-five patients with unstable angina pectoris and 16 patients with an acute non-Q-wave myocardial infarction were studied 5-8 days after admission. RESULTS: A positive dobutamine stress test was observed in 37 patients (52%). The sensitivity, specificity and total accuracy of identifying patients with a positive dobutamine stress test by VCG were: 38%, 91% and 63% for QRS-VD; 59%, 88% and 73% for ST-VM; 84%, 79% and 82% for STC-VM; and 86%, 23% and 56% for TC-VM. Patients with a positive dobutamine stress test showed higher maximum VCG values than patients with a negative dobutamine stress test (QRS-VD, P < 0.01; ST-VM, P < 0.001; STC-VM, P < 0.001). The maximum VCG values were related to the number of segments showing new or worsening wall motion abnormality (QRS-VD: r=0.41, P < 0.001; ST-VM: r=0.44, P < 0.001; STC-VM: r=0.57, P < 0.001). CONCLUSION: STC-VM and ST-VM seem to be the best VCG variables for detection of myocardial ischaemia as assessed by dobutamine stress echocardiography.  相似文献   
998.
Although coronary vasospasm can contribute to the development of unstable angina, the definite diagnostic method has not been established. The purpose of this study was to determine if ergonovine echocardiography (detection of regional wall motion abnormality during bedside ergonovine challenge) after angiographic confirmation of insignificant fixed disease would be useful and safe in detecting coronary vasospasm in patients with unstable angina. After control of chest pain with medications in patients admitted to the coronary care unit under the tentative diagnosis of unstable angina, diagnostic coronary angiography was performed. All patients with normal or insignificant fixed disease underwent ergonovine echocardiography after discontinuation of medications for 4+/-1 days. Among 208 consecutive patients enrolled for this study, 75% (156 of 208) showed significant fixed disease in the angiography. Ergonovine echocardiography was performed in 52 patients with insignificant disease, and coronary vasospasm was documented in 33 (63%, 33 of 52). No serious procedure-related arrhythmia or myocardial infarction occurred. Esophageal motility disorder and hypertrophic cardiomyopathy were diagnosed in 6 and 3 patients, respectively. Chest pain of undetermined etiology was the final diagnosis at discharge in 10 patients (5%, 10 of 208); among them chest pain redeveloped in 2 patients, and repeated ergonovine echocardiography revealed positive results. Our data suggest that among patients with the clinical presentation of unstable angina, coronary vasospasm is the main cause of myocardial ischemia in a considerable number of patients with a normal or near-normal angiogram, and ergonovine echocardiography after confirmation of absence of significant fixed disease is useful and safe for noninvasive diagnosis of coronary vasospasm in this setting.  相似文献   
999.
The potential for transfer of Escherichia coli O157:H7 from contaminated ground beef to grinding equipment and the inactivation of attached cells during cleaning and sanitizing was examined. Chub-packed ground beef with lean:fat ratios of 75:25, 80:20 or 90:10 was inoculated with 6 log CFU/g or 2 log CFU/g E. coli O157:H7 strain FRIK 910. Samples were consecutively ground in a Hobart meat grinder with stainless steel (SS) chips (1 cm2) glued to the auger housing. Chips were harvested after grinding, detergent washing with or without manual scrubbing and rinsing, sanitizing in a chlorine or peroxyacetic acid sanitizer, and overnight storage. Survival of E. coli O157:H7 was evaluated both by plate count and enrichment in trypticase soy broth. Approximately 3 to 4 log CFU/cm2 were attached to the SS after grinding with all three fat contents. After washing and sanitizing in a chlorine or peroxyacetic acid sanitizer, viable bacteria were infrequently recovered by plate count. Enrichment of chips resulted in a higher survival rate with both sanitizing treatments, indicating that cell numbers below the limit of detection (5 CFU/cm2) or potentially injured organisms remained on the surface. Manual scrubbing during the washing step reduced the recovery rate. The scrubbing step also increased the number of passing scores assigned using an ATP bioluminescence assay of total residual soil on the chips sanitized in chlorine. The overall results indicate that plate counts alone may not be a reliable indicator of sanitation efficacy and may be validated by enrichment assay.  相似文献   
1000.
In vitro fertilization and embryo transfer, embryo cryo-preservation, multiple pregnancies, embryo reduction, embryo and gamete donation: each of these questions deserves an articulate ethical and medical discussion. In Italy until this moment we do not have a legislation for the regulation of medically assisted reproduction. One of the most suitable means to monitoring large scale initiatives for medically assisted procreation is through registries that collect standardized information on all treatment cycles performed. In 1992, the Italian national registry on medically assisted procreation was established at the Istituto Superiore di Sanità (ISS, the Italian National Health Institute). The Registry collects data sent voluntarily by centers to the ISS for each cycle of treatment started and for each outcome. The data collected can be used to conduct an adequate statistical-epidemiological analysis and to better evaluate results of specific techniques.  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