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   1434篇
  免费   4篇
  国内免费   1篇
电工技术   2篇
化学工业   11篇
机械仪表   4篇
建筑科学   13篇
能源动力   1篇
轻工业   40篇
水利工程   1篇
石油天然气   1篇
无线电   183篇
一般工业技术   9篇
冶金工业   1162篇
自动化技术   12篇
  2021年   4篇
  2020年   1篇
  2019年   2篇
  2018年   1篇
  2017年   2篇
  2015年   3篇
  2014年   2篇
  2013年   7篇
  2012年   4篇
  2011年   11篇
  2010年   7篇
  2009年   22篇
  2008年   17篇
  2007年   24篇
  2006年   22篇
  2005年   27篇
  2004年   21篇
  2003年   13篇
  2002年   11篇
  2001年   9篇
  2000年   8篇
  1999年   40篇
  1998年   384篇
  1997年   215篇
  1996年   133篇
  1995年   87篇
  1994年   64篇
  1993年   76篇
  1992年   22篇
  1991年   27篇
  1990年   15篇
  1989年   20篇
  1988年   7篇
  1987年   20篇
  1986年   14篇
  1985年   10篇
  1984年   1篇
  1983年   2篇
  1982年   2篇
  1981年   8篇
  1980年   3篇
  1979年   1篇
  1978年   1篇
  1977年   20篇
  1976年   46篇
  1974年   1篇
  1973年   1篇
  1971年   1篇
排序方式: 共有1439条查询结果,搜索用时 31 毫秒
181.
The in vivo and in vitro effects of the insecticide deltamethrin (DM) on hepatic cytochrome P450 (Cyt P450) monooxygenase were examined in adult carp. The in vivo experiments were carried out with 0.2 microgram/l DM at 20 degrees C. The changes in the hepatic microsomal Cyt P450 content and the Cyt P450-dependent monooxygenase activities were studied in DM-treated fish. Although there were no changes in the Cyt P450 content during the exposure time, after treatment for 24 h all the investigated isoenzyme activities (para-nitrophenetole-O-deethylase, p-NPOD; aminopyrene-N-demethylase, APND; ethylmorphine-N-demethylase, EMND; 7-ethoxycoumarin-O-deethylase, ECOD; and ethoxyresorufin-O-deethylase, EROD) were significantly inhibited. After 72 h, all the activities were still lower than in the control animals. In vitro incubation of liver microsomes with DM led to a concentration-dependent decrease in total microsomal Cyt P450 content. A complete loss of Cyt P450 occurred after a 5-min incubation with 60 microM DM. The maximum in the difference spectra of microsomes was shifted to higher wavelength, showing the strong interaction of DM with Cyt P450. EROD and ECOD activities were inhibited by DM. The in vitro kinetic results on ECOD revealed that the inhibition was of non-competitive type, with K1 = 9.8 +/- 2.3 microM. This study indicates important biochemical effects of DM in fish liver, and suggests that exposure to DM may cause loss of the Cyt P450-dependent metabolism in fish.  相似文献   
182.
Purpose: Few studies have examined the relationship between directly measured oxygen uptake (VO2) and self-reported physical function (PF). The purpose of this study was: 1) to examine the relationship between peak V02 and PF and 2) to determine whether a threshold or cut point exist that distinguishes between individuals reporting required assistance in the performance of functional tasks (low PF) and those who report ability to perform tasks independently (high PF). Methods: Participants were 161 community-dwelling adults, ages 65-90, who had a baseline evaluation for a clinical trail that included measurement of peak V02 and PF consisted of a summary score combining scores from the Older Americans Resources and Services Multidimensional Functional Assessment Questionnaire, Nagi Disability Study. Rosow-Breslau Scale, Physical Function Scale of the Medical Outcomes Study, and the Falls Efficacy Scale. Decision tree, cubic spline, and logistic regression analyses explored these relationships with age, gender, education, race, body mass index, depression, and total number of chronic diseases included as important covariates. Results: Among all covariates examined, peak V02 was most strongly associated with (P = 0.004) with PF. There was not threshold effect. Decision tree analyses indicated that 18.3 mL.kg-1.min-1 was the optimal cut point distinguishing between low PF and High PG (P < 0.0001). Between-gender differences in PF (P = 0.002) were no longer significant when peak V02 was included in the PF model (P = 0.17). Conclusions: These data indicate that individuals with a V02 < 18 mL.kg-1min-1 report significant difficulty in the performance of daily tasks and that differences in peak V02 may explain, in part, why women report more impairment in PF.  相似文献   
183.
The concentrations of endometrial proteins PP14 and CA-125 were measured in uterine flushings taken on days LH+10 and LH+12 (10 and 12 days after luteinizing hormone surge) of the menstrual cycle from 15 normal, fertile women and 49 women who suffered recurrent miscarriage. The concentration of PP14 was significantly lower in the flushings from the recurrent miscarriage patients than in those from fertile controls on both day LH+10 (median: 1300, range: 3-10 300 ng/ml versus median: 13 933, range: 2174-40 404 ng/ml; P < 0.01) and LH+12 (median: 1560, range: 820-12 100 ng/ml versus median: 14 047, range 1402-62 108 ng/ml; P < 0.05). Similarly concentrations of CA-125 were significantly lower in flushings from recurrent miscarriage women compared to controls on both day LH + 10 (median: 1555, range: 47-6710 U/ml versus median: 6385.5, range 2884-27 731 U/ml, P < 0.01) and LH+12 (median: 2892, range: 956-9974 U/ml versus median: 7127.5, range: 1591-21 343 U/ml; P < 0.05). In contrast there was no significant difference in the concentration of PP14 in plasma samples taken on the same days as the flushings from recurrent miscarriage patients and fertile controls. The concentrations of PP14 in uterine flushings obtained on day LH + 10 or LH + 12 from recurrent miscarriage women during a pre-pregnancy investigative cycle were significantly lower (P < 0.05) in patients who went on to miscarry (median: 1000, range: 9-2900 ng/ml) than those who went on to have a live birth (median: 1440, range: 4-12 100 ng/ml) during a subsequent pregnancy. In contrast there was no significant difference in uterine CA-125 or plasma PP14 concentrations between these two groups of recurrent miscarriage patients. The results suggest that measurements of uterine PP14 and CA-125 may be useful in the assessment of endometrial development in recurrent miscarriage patients and suggest the importance of PP14 in preparing the endometrium for embryo implantation. In addition pre-pregnancy uterine PP14 measurements may be useful in predicting subsequent pregnancy outcome.  相似文献   
184.
Cost-benefit and cost-effectiveness analyses (CEAs) are only now beginning to be used by business, government, and policymakers to evaluate various medical treatments. The evolution of why CEAs are being demanded is reviewed. To date, a formal CEA of obesity treatments has not been published. This article outlines how a CEA is performed, reviews data relevant to setting up a formal CEA of medical and surgical obesity treatments, and lists published reports that demonstrate the effectiveness of surgical obesity treatments. The general level of discrimination that society allows the obese to suffer also allows medical insurance companies, businesses, and government to not provide many obese Americans with obesity treatments that have established a level of effectiveness far surpassing many other forms of medical therapy. CEAs of obesity treatments, by themselves, cannot be expected to reverse this discrimination. This type of data, however, provides individual obese patients and their physicians with evidence to challenge policymakers' decisions, especially when cost-effective obesity treatments are excluded or placed at a lower priority than treatments with less proven effectiveness.  相似文献   
185.
186.
Serious infections caused by Staphylococcus aureus in HIV-infected patients have been reported. Contributing factors in the development of invasive S aureus infections include a high rate of skin and nasal colonization, frequent dermatologic disease, and the use of intravenous catheters. The authors report three cases of S aureus pericarditis in HIV-infected patients. While cases of viral, mycobacterial, and malignant pericardial effusions in HIV-infected patients have been reported, a review of the literature disclosed only three cases of bacterial pericarditis. Despite appropriate antibiotic therapy and drainage, a patient's condition may abruptly deteriorate and progress to tamponade. Early recognition of bacteremia and pericarditis and monitoring for cardiac tamponade, along with aggressive treatment, can result in a favorable outcome, but mortality remains high, particularly when S aureus is the causative agent.  相似文献   
187.
PURPOSE: To determine the causes of visual impairment in a population-based group of visually impaired preterm children. METHODS: Ophthalmological examination and magnetic resonance imaging or computed tomography of the brain were performed in all visually impaired preterm children born 1989-95 in V?rmland. RESULTS: Ten of 18 children had periventricular leukomalacia affecting the optic radiation, six had other lesions or malformations in the posterior visual pathways/cortex, but no child had visually impairing retinopathy of prematurity. CONCLUSION: We conclude that cerebral lesions or malformations are common causes of the visual functional deficit in visually impaired children born preterm. Brain damage should be suspected in a prematurely born child who presents with either of the signs: fixation difficulties, strabismus or nystagmus.  相似文献   
188.
INTRODUCTION: Previous mapping studies in patients with typical atrial flutter have demonstrated the crista terminalis to be a posterior barrier of the reentrant circuit forming a line of block. However, the functional role of the crista terminalis in patients with or without a history of atrial flutter is not well known. The aim of this study was to determine whether the conduction properties of the crista terminalis are different between patients with and those without a history of atrial flutter. METHODS AND RESULTS: The study population consisted of 12 patients with clinically documented atrial flutter (group 1) and 12 patients with paroxysmal supraventricular tachycardia as well as induced atrial flutter (group 2). A 7-French, 20-pole, deflectable Halo catheter was positioned around the tricuspid annulus. A 7-French, 20-pole Crista catheter was placed along the crista terminalis identified by the recording of double potentials with opposite activation sequences during typical atrial flutter. After sinus rhythm was restored, pacing from the low posterior right atrium near the crista terminalis was performed at multiple cycle length to 2:1 atrial capture. No double potentials were recorded along the crista terminalis during sinus rhythm in both groups. In group 1, the longest pacing cycle length that resulted in a line of block with double potentials along the crista terminalis was 638 +/- 119 msec. After infusion of propranolol, it was prolonged to 832 +/- 93 msec without change of the interdeflection intervals of double potentials. In group 2, the longest pacing cycle length that resulted in a line of block with double potentials along the crista terminalis was 214 +/- 23 msec. After infusion of procainamide, it was prolonged to 306 +/- 36 msec with increase of interdeflection interval of double potentials. CONCLUSION: The crista terminalis forms a line of transverse conduction block during typical atrial flutter. Poor transverse conduction property in the crista terminalis may be the requisite substrate for clinical occurrence of typical atrial flutter.  相似文献   
189.
We compared the diaphragmatic electromyographic (EMG) recordings from 32 patients with known neuromuscular disease and respiratory symptoms (23 neuropathies, 9 myopathies) to recordings from 23 normal subjects. Turns analysis of 219-ms sections, or epochs, of the EMG demonstrated a significant overlap between diagnostic groups, although some epochs from neuromuscular patients were significantly different from normal. Empirical rules were derived to infer neuropathic and myopathic involvement of the diaphragmatic EMG.  相似文献   
190.
Primary intracranial malignant lymphoma is rate in clinic. But the incidence tends to be increasing recently. In its relatively short course, children and the slightly more woman headache vomiting IICP (increased intracranial pressure) and corresponding neurofunctional disorders due to various location of the tumors are the main clinical manifestation. Even high density which obviously increased after contrast and peripheral edema may be seen in CT scanning. MRI examination shows lowereal signals in T1 phase. But enhanced signals in T2 phase. Histologically the tumor mass is grayish in color often with a blussed borders (those located in brain surface often have rough borders) to the naked eye. Undermicroscope tumor cell may be seen around blood vessels with decreased cytosol, pathologic rayokinesis and somtimes focal hemorrhage necrosis. All patients in the 40 cases of our study group were connfirmed by surgical excision and patholog examination. Three patients died within half a year after resection. The others showed a survival period ranging from 4 to 20 months after postoperative irradiation solely or combined chemotherapy. Nine patients failed in total resection. Patients undergone postoperative irradiation but without following chemotherapy, only survived 4 to 9.7 months. Thus it can be seen that prolongation of the survival period of primary intracranial malignant lymphoma depends upon possibly early excision combined irradiation and effective chemotherapy.  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