心腔导管电消融治疗顽固性室上性心动过速

对17例抗心动过速药物治疗无效的顽固性室上性心动过速的患者,经使用心内导管电消融房室交界区,采取R波同步放电200～400J电能2～1次后,其中4例在电消融术后未安置长期心脏起搏器,另13例电消融术后形成完全性房室传导阻滞,安置了长期性心胜起搏器。临床实践证实了使用心腔导管电消融术对治疗顽固性室上性心动过速,可收到较好的效果。     

Safer signalized junction design and self-optimizing control

The UK Transport Research Laboratory has long had a Worldwide reputation for contributions to the field of traffic signal control, especially as originators of the TRANSYT and SCOOT signal coordination methods. This article describes some less widely known work. Accident risk at urban junctions and on road links between them is related to a wide variety of factors including: traffic and pedestrian flows, signal control parameters, geometry, and other layout features. Comprehensive studies have derived, and are continuing to derive, quantitative risk relationships for use in off-line software. Those for individual signalized cross-roads have already been incorporated in the TRL's program OSCADY to assist junction design. Now, an area-wide safety model incorporating the CONTRAM traffic assignment program is being actively developed to provide a tool for evaluation of network traffic management schemes. For on-line signal control at individual junctions, the MOVA system has been developed to provide a delay-minimizing or capacity-maximizing control logic as appropriate. ‘Before and after’ comparisons with the UK's previous fully vehicle-actuated signal system are presented, including both delay and safety aspects.     

Nutrient and organic matter patterns across the Mackenzie River, estuary and shelf during the seasonal recession of sea-ice

Suspended material, nutrients and organic matter in Mackenzie River water were tracked along a 300 km transect from Inuvik (Northwest Territories, Canada), across the estuarine salinity gradient in Kugmallit Bay, to offshore marine stations on the adjacent Mackenzie Shelf. All particulates measured (SPM, POC, PN, PP) declined by 87–95% across the salinity gradient and levels were generally below conservative mixing. Organic carbon content of suspended material decreased from 3.1% in the river to 1.7% in shelf surface waters while particulate C:N concurrently decreased from 17.1 to 8.6. Nitrate and silicate concentrations declined by more than 90% across the salinity gradient, with nitrate concentrations often below the conservative mixing line. Phosphate concentrations increased from 0.03 μmol/L in the river to 0.27 μmol/L over shelf waters, thereby changing the inorganic nutrient regime downstream from P to N limitation. Dissolved organic carbon decreased conservatively offshore while dissolved organic N and P persisted at high levels in the Mackenzie plume relative to river water, increasing 2.7 and 25.3 times respectively. A deep chlorophyll-a maximum was observed at two offshore stations and showed increases in most nutrients, particulates and organic matter relative to the rest of the water column. During river passage through the Mackenzie estuary, particulate matter, dissolved organic carbon and inorganic nutrients showed sedimentation, dilution and biological uptake patterns common to other arctic and non-arctic estuaries. Alternatively, inorganic content of particles increased offshore and dissolved organic N and P increased substantially over the shelf, reaching concentrations among the highest reported for the Arctic Ocean. These observations are consistent with the presence of a remnant ice-constrained (‘stamukhi’) lake from the freshet period and a slow flushing river plume constrained by sea-ice in close proximity to shore. Nutrient limitation in surface shelf waters during the ARDEX cruise contributed to the striking deep chlorophyll-a maximum at 21 m where phytoplankton communities congregated at the margin of nutrient-rich deep ocean waters.     

Maritime transport and international seaborne trade

Drawing upon over four decades of the United Nations Conference on Trade and Development (UNCTAD) Review of Maritime Transport, this article considers selected key trends that have shaped the maritime transportation of today. Over the review period, a fundamental shift in the geography of trade has been observed, with developing countries increasingly driving growth in the world economy and seaborne trade, and becoming important players in global value chains. The specialization of countries, including in developing regions, in the provision of the maritime goods and services for which they have established a comparative advantage constitutes another major trend, a move away from previous patterns where developing countries were typically users of maritime transport services. The article also observes that bilateral freight sharing agreements between countries which were prominent in 1970s have largely lost their practical relevance, leading to a further liberalization of markets and resulting in overall relatively lower freight costs.     

Improving the performance of rail yards through dynamic reassignments of empty cars

This paper considers the problem of reducing the time that empty cars spend in classification yards of rail systems operating under real-time information and automated schedule-adjustment technologies. The proposed methodology performs dynamic reassignments of empty cars through a fast and efficient solution procedure based on the assignment algorithm. The procedure has been tested on real-life data from one of the major railroads in North America. Computational results show that the procedure runs fast and yields savings in the time that the empty cars spend in the yard.