Cardioembolic Stroke in Atrial Fibrillation-Rationale for Preventive Closure of the Left Atrial Appendage

Boris Leithäuser,Jai-Wun Park 대한심장학회 2009 Korean Circulation Journal Vol.39 No.11

Atrial fibrillation is the most common cardiac arrhythmias, and a major cause of morbidity and mortality due to cardioembolic stroke. The left atrial appendage is the major site of thrombus formation in non-valvular atrial fibrillation. Loss of atrial systole in atrial fibrillation and increased relative risk of associated stroke point strongly toward a role for stasis of blood in left atrial thrombosis, although thrombus formation is multifactorial, and much more than blood flow irregularities are implicated. Oral anticoagulation with vitamin-K-antagonists is currently the most effective prophylaxis for stroke in atrial fibrillation. Unfortunately, this treatment is often contraindicated, particularly in the elderly, in whom risk of stroke is high. Moreover, given the risk of major bleeding, there is reason to be skeptical of the net benefit when warfarin is used in those patients. This work reviews the pathophysiology of cardioembolic stroke and critically spotlights the current status of preventive anticoagulation therapy. Various techniques to exclude the left atrial appendage from circulation were discussed as a considerable alternative for stroke prophylaxis. Atrial fibrillation is the most common cardiac arrhythmias, and a major cause of morbidity and mortality due to cardioembolic stroke. The left atrial appendage is the major site of thrombus formation in non-valvular atrial fibrillation. Loss of atrial systole in atrial fibrillation and increased relative risk of associated stroke point strongly toward a role for stasis of blood in left atrial thrombosis, although thrombus formation is multifactorial, and much more than blood flow irregularities are implicated. Oral anticoagulation with vitamin-K-antagonists is currently the most effective prophylaxis for stroke in atrial fibrillation. Unfortunately, this treatment is often contraindicated, particularly in the elderly, in whom risk of stroke is high. Moreover, given the risk of major bleeding, there is reason to be skeptical of the net benefit when warfarin is used in those patients. This work reviews the pathophysiology of cardioembolic stroke and critically spotlights the current status of preventive anticoagulation therapy. Various techniques to exclude the left atrial appendage from circulation were discussed as a considerable alternative for stroke prophylaxis.

Distributed Optimization of a Multisubchannel Ad Hoc Cognitive Radio Network

Leith, A.,Dong In Kim,Alouini, M-S,Zhiqiang Wu IEEE 2012 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY Vol.61 No.4

<P>In this paper, we study the distributed-duality-based optimization of a multisubchannel ad hoc cognitive radio network (CRN) that coexists with a multicell primary radio network (PRN). For radio resource allocation in multiuser orthogonal frequency-division multiplexing (MU-OFDM) systems, the orthogonal-access-based exclusive subchannel assignment (ESA) technique has been a popular method, but it is suboptimal in ad hoc networks, because nonorthogonal access between multiple secondary-user links by using shared subchannel assignment (SSA) can bring a higher weighted sum rate. We utilize the Lagrangian dual composition tool and design low-complexity near-optimal SSA resource allocation methods, assuming practical discrete-rate modulation and that the CRN-to-PRN interference constraint has to strictly be satisfied. However, available SSA methods for CRNs are either suboptimal or involve high complexity and suffer from slow convergence. To address this problem, we design fast-convergence SSA duality schemes and introduce several novel methods to increase the speed of convergence and to satisfy various system constraints with low complexity. For practical implementation in ad hoc CRNs, we design distributed-duality schemes that involve only a small number of CRN local information exchanges for dual update. The effects of many system parameters are presented through simulation results, which show that the near-optimal SSA duality scheme can perform significantly better than the suboptimal ESA duality and SSA-iterative waterfilling schemes and that the performance loss of the distributed schemes is small, compared with their centralized counterparts.</P>

Flexible Proportional-Rate Scheduling for OFDMA System

Leith, A.,Alouini, M-S,Dong In Kim,Xuemin Shen,Zhiqiang Wu IEEE 2013 IEEE transactions on mobile computing Vol.12 No.10

<P>In this paper, we study the sum-rate maximization algorithms for downlink and uplink orthogonal frequency division multiple access (OFDMA) systems under proportional-rate constraint (PRC) and minimum-rate constraint. We develop a low-complexity weighted channel signal-to-noise ratio (w-SNR)-based ranking scheme for user selection on each subchannel in OFDMA combined with waterfilling (WF) power allocation. Both offline and online optimization algorithms are developed to optimize the SNR weight vector to maximize the sum rate while satisfying several constraints, such as PRC. The offline weight optimization technique relies on the analytical throughput results developed in this paper, and the online weight adaptation method tracks the user rates and meets the PRC using a subgradient search. Furthermore, we introduce a novel SNR operating region test to enhance the multiuser diversity gain and the sum rate. The proposed schemes have a low complexity, which is linear to the numbers of users and subchannels. Simulation results verify the accuracy of the developed analytical rates and fairness formulas, and show that the proposed w-SNR schemes can achieve higher sum rates than several benchmark schemes that provide the PRC with either short-term or long-term fairness.</P>

Direct Chemical Vapor Deposition Synthesis of Phase-Pure Iron Pyrite (FeS₂) Thin Films

Samad, Leith,Cabá,n-Acevedo, Miguel,Shearer, Melinda J.,Park, Kwangsuk,Hamers, Robert J.,Jin, Song American Chemical Society 2015 Chemistry of materials Vol.27 No.8

<P>Resurgent interest in iron pyrite (FeS<SUB>2</SUB>) as an earth-abundant, nontoxic semiconductor for solar applications has resulted in many attempts to grow phase-pure thin films via chemical vapor deposition (CVD). However, all thin films grown via CVD or sulfidation to date have contained marcasite phase or other iron sulfide impurities. Here, we report the use of metallic cobalt pyrite (cattierite, CoS<SUB>2</SUB>) thin films as an ideal substrate leading to the first direct growth of phase-pure iron pyrite thin films via atmospheric pressure CVD. This synthesis was achieved by reacting FeCl<SUB>3</SUB> and ditert butyl disulfide (TBDS) at 400–450 °C. The products were confirmed as phase-pure iron pyrite using X-ray diffraction (XRD), Raman spectroscopy, and energy dispersive X-ray spectroscopy (EDS). In addition to phase-purity, the synthesis produced crystal domains >1 μm and a conformal coating 3–5 μm thick, which are attributed to the <2% lattice mismatch of the isostructural cattierite substrate. The surface was characterized by ultraviolet and X-ray photoelectron spectroscopy (UPS & XPS) and the electrical properties by electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis. The direct growth of a phase-pure iron pyrite film on a conductive substrate provides the most convenient configuration so far for potential solar cells.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/cmatex/2015/cmatex.2015.27.issue-8/acs.chemmater.5b00664/production/images/medium/cm-2015-00664c_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cm5b00664'>ACS Electronic Supporting Info</A></P>