Vascular access today

The number of patients with chronic kidney disease re-quiring renal replacement therapy has increased world-wide. The most common replacement therapy is hemo-dialysis （HD）. Vascular access （VA） has a key role for successful treatment. Despite the advances that have taken place in the feld of the HD procedure, few things have changed with regards to VA in recent years. Ar-teriovenous fstula （AVF）, polytetrafuoroethylene graft and the cuffed double lumen silicone catheter are the most common used for VA. In the long term, a number of complications may present and more than one VA is needed during the HD life. The most common com-plications for all of VA types are thrombosis, bleeding and infection, the most common cause of morbidity in these patients. It has been estimated that VA dysfunc-tion is responsible for 20% of all hospitalizations. The annual cost of placing and looking after dialysis VA in the United States exceeds 1 billion dollars per year. A good functional access is also vital in order to deliver adequate HD therapy. It seems that the native AVF that Brescia and Cimino described in 1966 still remains the frst choice for VA. The native forearm AVFs have the longest survival and require the fewest interventions. For this reason, the forearm AVF is the frst choice, fol-lowed by the upper-arm AVF, the arteriovenous graft and the cuffed central venous catheter is the final choice. In conclusion, VA remains the most importantissue for patients on HD and despite the technical im-provements, a number of problems and complications have to be resolved.

Percutaneous management of vascular access in transfemoral transcatheter aortic valve implantation

Transcatheter aortic valve implantation(TAVI) using stent-based bioprostheses has recently emerged as a promising alternative to surgical valve replacement in selected patients. The main route for TAVI is retrograde access from the femoral artery using large sheaths(16-24 F). Vascular access complications are a clinically relevant issue in TAVI procedures since they are reported to occur in up to one fourth of patients and are strongly associated with adverse outcomes. In the present paper, we review the different types of vascular access site complications associated with transfemoral TAVI. Moreover, we discuss the possible optimal management strategies with particular attention to the relevance of early diagnosis and prompt treatment using endovascular techniques.

Prospective Assessment of Catheter Migration in Implanted Vascular Access Devices for Adjuvant Colorectal Cancer Chemotherapy

Background: Chemotherapy in colorectal cancer is usually administered as continuous infusion of 5-fluorouracil, often in combination with oxaliplatin or irinotecan. Targeted drugs are most efficient and tolerable in conjunction with continuous infusion dosing. Implanted venous access devices (VAD) are the prerequisite for continuous infusion administration. The reported catheter migration frequency with VAD is 0% - 3.5%. The purpose of this case-control study was to evaluate the predisposing factors of catheter migration. Methods: We inserted VADs in 88 radically operated colorectal cancer patients randomized to adjuvant 48-hour-infusion chemotherapy repeated every 14 days, altogether 12 times over 24 weeks. Three out of 88 patients (3.4%) had a symptomatic catheter migration from the superior caval vein into the internal jugular vein. The fourth case had chemotherapy for osteosarcoma. These 4 cases were compared with 12 controls from the same 88 patient study population, matched for age, sex, body mass index (BMI), physical activity level and right subclavian insertion site. Tip position, port model, complications, catheter length and material was studied. The post insertion catheter tip position in the chest X-ray was numbered from 1 (in subclavia) to 8 (in atrium). Results: The four cases, all male, had a median position of 3 (range 3 - 4) and controls 6 (range 4 - 8), P = 0.004, median difference 3 (CI95% 1 - 5). At notification of migration the patients had experienced discomfort in the neck region starting 5 to15 days before at strenuous upper extremities activity with Valsalva maneuvers. Conclusion: Optimal catheter tip position when sitting is in the right atrium or low in the superior vena cava to avoid migration. Patients with VADs should avoid strenuous activity with Valsalva maneuvers.

Effect of Infrared Irradiation Combined with External Application Nursing on Maintenance of Vascular Access in Hemodialysis Patients

Objective:This paper mainly analyzes the effect of infrared radiation combined with external application nursing on maintenance of vascular access in hemodialysis patients.Methods:A total of 150 hemodialysis patients in our hospital from December 2019 to September 2020 were divided into the conventional group and the study group,75 cases in each group.The conventional group was given external application nursing alone,and the study group was given infrared radiation combined with external application nursing,and the effect on the maintenance of vascular access was counted.Results:After the intervention,the indexes of URR,kt/V and human albumin in the study group were higher than those in the routine group,and the indexes of β2-mg and MIS in the study group were lower than those in the routine group,P<0.05;The improvement of vascular elasticity and quality of life in the study group was higher than that in the conventional group,and the incidence of complications was lower than that in the conventional group,P<0.05.Conclusion:Infrared radiation combined with external application of nursing care in hemodialysis patients with vascular access maintenance effect is significant,can effectively improve vascular elasticity.

Effectiveness of Ultrasound-Guided Arteriovenous Fistulas Cannulation in Difficult Hemodialysis Arteriovenous Access: A Meta-Analysis of Randomised Controlled Trials

Aim(s): Complex Arteriovenous fistula (AVF) poses challenges to cannulation. Ultrasound (US)-guided cannulation may promote successful cannulation and prevent AVF-related complications. Renal nurses performing US-guided cannulation may improve successful cannulation and reduce complications associated with unnecessary punctures. The study aims to conduct the meta-analyze to examine the effectiveness of renal nurse-performed US-guided cannulation to improve successful cannulation and reduce AVF-related complications in difficult AVF access. Design: A meta-analysis of randomised controlled trials. Methods: A systemic search was performed on electronic databases including CINAHL Plus, Web of Science, and PubMed from inception to October 2023. Risk ratios (RR) and standardized mean differences (SMD) were estimated using random-effect models for considerable homogeneity, and the Scottish Intercollegiate Guidelines Network (SIGN) methodology was adopted for critical appraisal. Results: Four RCTs were included. The results showed US-guided AVF cannulation had a significant effect in improving successful cannulation (RR: 0.19, 95% CI: 0.06 to 0.63, p = 0.007), and was favorable in reducing cannulation-associated complications (RR: 0.44, 95% CI: 0.10 to 1.93, p = 0.28), compared with blind needle cannulation. Conclusion: US-guided cannulation performed by renal nurses has the potential to improve successful cannulation, and fewer complications in hemodialysis patients with difficult arteriovenous access. Relevance to Clinical Practice: The results suggest the value of further training for renal nurses in US-guided cannulation, and broader implementation of US-guided cannulation to improve patient outcomes. Future studies could explore the optimal nursing training and longer-term benefits of US-guided cannulation by renal nurses in difficult AVF access. Patient or Public Contribution: No Patient or Public Contribution as this is a meta-analysis using the secondary data published in the RCTs.

A novel role of chitosan in reducing vascular access bleeding complications after transradial angiography

To investigate the safety and efficacy of chitosan ( Anscare,Daxon) in reducing vascular ac cess complications compared to a kind of radial compression device( TR-BAND,Terumo) . Method: We studied 128 patients who had undergone transradial angiography in our center from December 2009 to April 2010. Among them,64 patients were randomly treated with radial compression device ( TR-BAND,Terumo) when the procedure was over ( CD group) . The other 64 patients were dealt with chitosan ( CS group) . The compression time,the major and minor access site bleeding complications and the errhysis were observed. Results: There were no statistical differences in the baseline clinical characteristics of the patients between two groups. The compression time in CS group was significantly shorter than that of CD group ( P < 0. 001) . There were 12 patients suffering from minor access site bleeding while only 3 patients experienced these minor complications in CS group( 19% vs 4% ,P < 0. 05) . At the same time 20 patients had er- rhysis in CD group and 4 patients in CS group ( 31% vs 6% ,P < 0. 001) . Conclusion: Chitosan,compared to radial compression device ( TR-BAND,Terumo) ,can not only shorten the compression time,but also significantly reduce the rate of minor access site bleeding and puncture-site errhyis.

Impact of central venous port implantation method and access choice on outcomes

BACKGROUND Although the number of patients who need central venous ports for permanent vascular access is increasing,there is still no“gold standard”for the implantation technique.AIM To identify the implantation technique that should be favored.METHODS Two hundred central venous port-implanted patients in a tertiary hospital were retrospectively evaluated.Patients were assigned into two groups according to the access method.The first group comprised patients whose jugular veins were used,and the second group comprised patients whose subclavian veins were used.Groups were evaluated regarding age,sex,application side,primary diagnosis,active follow-up period in the hospital,chemotherapy agents administered,number of complications,and the Clavien-Dindo severity score.The distribution of the variables was tested with the Kolmogorov-Smirnov test and the Mann-Whitney U test.Theχ^(2) test was used to analyze the variables.RESULTS There was no statistically significant difference between the groups regarding age,sex,side,number of chemotherapy drugs,and duration of port usage(P>0.05).Only 2 patients in group 1 had complications,whereas in group 2 we observed 19 patients with complications(P<0.05).No port occlusion was found in group 1,but the catheters of 4 patients were occluded in group 2.One port was infected in group 1 compared to three infected ports in group 2.Two port ruptures,two pneumothorax,one revision due to a mechanical problem,one tachyarrhythmia during implantation,and four suture line problems were also recorded in group 2 patients.We also showed that it would be sufficient to evaluate and wash ports once every 2 mo.CONCLUSION Our results robustly confirm that the jugular vein route is safer than the subclavian vein approach for central venous port implantation.

Korean single-center experience with femoral access closure using the ExoSeal device

AIM To report 17-mo experience of femoral artery puncture site closure during angiographic procedures using Exo Seal vascular closure devices(VCDs).METHODS Between November 2015 and April 2017, we performed 179 diagnostic and interventional angiographic procedures via a common femoral arterial access.The Exo Seal VCD was used at the puncture site to achieve hemostasis in 125 patients.We evaluated the technical and procedural success rates, the complications, and the factors affecting the hemostasis time of the ExoSeal VCDs.RESULTS Technical and procedural successes were achieved in 176 cases(98.0%) and 128 cases(71.5%), respectively.Device failure occurred in 3(1.7%) cases.In 1 case(0.6%) a small hematoma developed, but there were no major complications.Among the hemostasis-relevant variables, a history of drinking alcohol, low platelet(PLT) count, and high prothrombin time-international normalized ratio(commonly known as PT-INR) values were the statistically significant predictors of the need for longer manual compression(MC).There was no difference in the success rates between the repeat and single Exo Seal procedure groups, and repeated use of the ExoSeal did not affect hemostasis time.CONCLUSION The ExoSeal VCD effectively achieves hemostasis, with few complications.Longer light MC may be needed with alcohol drinkers, low PLT count, and high PT-INR values.

Brachial access technique for aortoiliac stenting revisited

We report a modified technique to perform iliac artery stenting through the brachial artery access. A 6F Brite tip sheath (Cordis, Jonhson & Jonhson Medical, Miami Lakes, FL, USA) is inserted into either brachial artery and a standard 4F Judkins Right diagnostic catheter was inserted over a 260 cm 0.038” Terumo Stiff wire (Terumo Corp, Tokyo, Japan) through the sheath. The catheter is navigated down to the aortic bifurcation, and after selecting the common iliac artery ostium, the wire is navigated through the lesion and advanced to the ipsilateral superficial femoral arteries. The catheter should be then moved forward over the wires beyond the lesion and the Terumo guidewire is replaced by two 0.038” 260 cm Supracor wires (Boston Scientific Corporation, San Jose, CA, USA). In order to facilitate advancement of the stent without risk of dislodgement as well as to check the position with low contrast dose injection, a 6 F (or 7F if large stent is selected) 90cm Shuttle Flexor introducer long sheath (Cook Group, Bloomington, IN, USA) should be advanced over the Supracor wire until it reaches the common iliac artery ostium. A road-map technique can be used to check the ostium position in order to properly deploy the selected stent. This technique promises to be safe and effective offering more support than guiding catheter technique; moreover it reduces the stress on the arterial vessel at the subclavian site and enables a stiff balloon or stent catheter to be advanced even through a very elongated and calcified aorta without the risk of stent dislodgement.

西藏地区血液净化质量控制单位单中心维持性血液透析患者血管通路现况分析

目的 调查西藏地区血液净化质量控制单位血管通路现况。方法 以2021年12月─2022年12月在西藏自治区人民医院血液净化中心进行维持性血液透析(maintenance hemodialysis,MHD)的患者为研究对象。结果 共纳入194例患者,使用临时静脉置管、带隧道和涤纶套导管(tunnel-cuffed catheter,TCC)、移植物动静脉内瘘(arteriovenous graft,AVG)和自体动静脉内瘘(arteriovenous fistula,AVF)作为首次血管通路的比例分别为34.0%、45.4%、1.0%和19.6%。24.2%患者在透析前完成造瘘。提前造瘘和未提前造瘘的患者比,男性(χ^(2)=3.919,P=0.048)和大学及以上学历(χ^(2)=17.875,P<0.001)更多;和未提前造瘘组相比,提前造瘘组患者透析后以AVF或AVG为血管通路的比例更高(χ^(2)=4.860,P=0.029)。在MHD患者中,以AVF、AVG、TCC为血管通路的比例分别为86.6%、2.6%、10.8%。和使用AVF或AVG的患者相比,使用TCC为血管通路的患者年龄更大(t=2.039,P=0.043),原发性肾小球肾炎更少(χ^(2)=4.213,P=0.040)。透析开始血管通路为临时中心静脉导管的患者(n=66)和其它血管通路患者(n=128)相比,有更高的比例以TCC作为MHD血管通路(χ^(2)=8.157,P=0.007)。结论 AVF使用率符合指南要求,TCC使用率略高于指南要求,需要技术提高和加强教育。

即穿型人工血管在血管通路中的应用效果及并发症分析

目的 研究即穿型人工血管在血管通路中的应用效果及并发症。方法 选取从2019年2月—2022年2月于资阳市人民医院接受人工血管内瘘手术的83例患者为研究对象。将流出道静脉直径>4mm分为即穿型人工血管组(n=41),流出道静脉直径3~4 mm分为膨体聚四氟乙烯(expanded polytetrafluoroethylen,ePTFE)人工血管组(n=42),分别选用即穿型人工血管、ePTFE人工血管进行造瘘手术。对所有患者均实施为期18个月的随访,比较2组手术后通畅率,治疗效果,手术后并发症,手术后生存率。结果 即穿型人工血管组与e PTFE人工血管组手术后6、12、18个月初级通畅率(t=1.437、0.609、0.595,P=0.231、0.435、0.441)及次级通畅率(t=2.108、0.149、0.310,P=0.147、0.699、0.578)对比无统计学差异。即穿型人工血管组首次穿刺时间、手术后透析导管移除时间均短于ePTFE人工血管组(t=78.386、21.491,均P<0.001);2组手术后透析血流量及人工血管使用率对比均不明显(t/χ^(2)=0.214、0.988,P=0.831、0.320)。即穿型人工血管组手术后肿胀、血栓形成发生率均低于ePTFE人工血管组(χ^(2)=24.418、7.540,P<0.001、0.006)。2组手术后6、12、18个月患者生存率对比均不明显(χ^(2)=1.037、0.239、0.183,P=0.309、0.625、0.668)。结论 即穿型人工血管应用于血管通路中可实现早期穿刺,通畅率高,且并发症发生风险较低,值得推广应用。

血管通路小组护理对血液透析患者应对方式、自我管理行为的影响

目的对血液透析患者行血管通路小组护理的可行性、影响进行探讨。方法参与研究的80例血液透析患者为我院于2023年1月—2023年12月所收治,按随机数字表法分成两组,即对照组(给予常规护理,n=40)、观察组(给予血管通路小组护理,n=40)。以负面心理、应对方式、自我管理行为为观察指标评价两组效果。结果各组焦虑自评量表(SAS)、抑郁自评量表(SDS)评分在干预后均下降,且观察组低于对照组(P<0.05);各组回避、屈服评分在干预后均下降,面对评分均上升,且观察组回避、屈服评分低于对照组,面对评分高于对照组(P<0.05);干预后,观察组成年人健康自我管理能力测评量表(AHSMSRS)评分高于对照组(P<0.05)。结论对血液透析患者行血管通路小组护理,可改善患者心理状态以及应对方式,提升患者的自我管理行为。

基层医院维持性血液透析患者血管通路使用现状调查

目的 调查基层医院维持性血管透析患者血管通路的使用现状。方法 采用多中心横断面研究,收集2023年3月—8月盐城市大丰人民医院、安徽省明光人民医院和常州市溧阳人民医院等15家基层医院血液透析中心维持性血液透析患者的临床资料。结果 共筛查符合标准的维持性血液透析患者650例,其中男性342例(52.62%),女性308例(47.38%),中位透析龄5.48(2.00,8.00)年。首次血液透析治疗血管通路类型主要为无隧道和涤纶套的透析导管291例(48.10%)和自体动静脉内瘘271例(44.79%),目前使用的血管通路类型主要为自体动静脉内瘘518例(85.62%),在自体动静脉内瘘维持性血液透析患者中绳梯穿刺执行率为19.30%(100例),区域穿刺412例(79.54%),在移植物动静脉内瘘绳梯穿刺29例(90.63%),血管狭窄是血管通路的主要并发症,在自体动静脉内瘘和移植物动静脉内瘘中分别为177例(34.17%)和10例(31.25%)。结论 基层医院血液透析中心维持性血液透析患者的血管通路的临床使用现状符合临床预期,但需加强相关操作规范的落实及人员培训管理工作,以早期发现血管通路功能障碍以采取积极有效干预措施。

基于血管重构的纤细动脉自体动静脉内瘘成熟影响因素分析

目的探讨纤细动脉建立自体动静脉内瘘(arteriovenous fistula,AVF)后成熟率及通畅率,并基于血管重构理论分析影响AVF成熟的相关因素。方法选择手术前桡动脉内径≤1.5 mm的AVF患者,超声测量手术前桡动脉及头静脉血管内径,手术后1月测量头静脉内径、肱动脉内径及血流量,评估成熟并随访12个月,分析纤细血管建立AVF的预后。结果56例患者纳入研究,手术前平均桡动脉内径(1.26±0.28)mm,束臂后平均头静脉内径(2.72±0.53)mm,最细桡动脉内径0.5 mm。40例(71.43%)患者AVF未经干预自然发育成熟,其余16例患者中,13例(23.21%)经腔内手术或外科重建干预后成熟。回归模型显示:手术前桡动脉内径是AVF成熟预测的独立相关因素,预测AVF成熟的最佳临界值为桡动脉内径>1.35 mm。56例患者平均随访(11.39±5.81)月,手术后3月,6月,12月初级通畅率分别为73.08%,68.75%,65.71%;3月,6月,12月次级通畅率分别为100%,97.92%,100%。结论手术前桡动脉内径≤1.5mm的纤细桡动脉构建的AVF可具有较高的成熟率及远期通畅率,桡动脉内径1.35 mm以上有较大概率自然发育成熟。

血液透析血管通路并发症及其防治

维持性血液透析是终末期肾脏病患者常用的肾脏替代治疗方法,功能良好的血管通路是保证透析顺利进行的前提。然而,血管通路相关并发症仍是临床面临的重要问题,包括出血和(或)血肿、感染、血栓形成、动脉瘤或假性动脉瘤、窃血综合征等。本文对血管通路相关并发症的发生率、病因及防治策略等进行综述,以期对临床工作提供帮助。

ICU成人患者静脉血管通路装置评估与选择的最佳证据总结

目的系统检索、评价并整合ICU成人患者静脉血管通路装置评估与选择的最佳证据。方法依据PIPOST确立循证问题,按照“6S”金字塔模型系统检索相关数据库中ICU成人患者血管通路装置评估与适宜性选择的相关决策、指南、专家共识、证据总结、推荐实践、系统评价及原始研究等。检索时间为2018年6月1日至2023年6月1日。由2名经过循证护理培训的研究者对纳入文献独立进行文献质量评价与证据提取。结果共纳入14篇文献,其中临床决策3篇,证据总结1篇,指南4篇,专家共识4篇,随机对照试验2篇。共提取9个主题总结出34条最佳证据。结论ICU成人患者血管通路装置评估与选择的最佳证据总结可以为医护人员临床决策提供借鉴,但临床情境复杂多样,应根据患者病情变化及其自身情况适当调整选择策略。

六味地黄汤加减对血液透析肝肾阴虚型血小板减少症患者微炎症状态、血管通路功能及血清GPⅡb、PMPs和PAIg的影响

目的:观察六味地黄汤加减对血液透析肝肾阴虚型血小板减少症患者微炎症状态、血管通路功能及血清血小板膜糖蛋白(GPⅡb)、血小板微颗粒(PMPs)和血小板相关免疫球蛋白(PAIg)水平的影响。方法:选择2020年4月至2023年4月该院收治的血液透析肝肾阴虚型血小板减少症患者120例,根据随机数字表法分为观察组及对照组。对照组60例患者给予常规西医治疗,观察组60例患者在对照组的基础上给予六味地黄汤加减治疗。治疗前后检测两组患者Th17/Treg细胞比例、PAIg、GPⅡb、PMPs、高迁移率族蛋白B1(HMGB1)、血小板活化因子(PAF)、白细胞介素10(IL-10)、血栓调节蛋白(TM)、血小板生成素(TPO)、P-选择素(CD62P)、脂蛋白相关磷脂酶A2(Lp-PLA2)和血小板计数(PLT)水平,并计算血小板压积(PCT)、血红蛋白、总胆红素水平,比较两组患者的临床疗效及不良反应发生情况。结果:本研究无脱落病例。治疗后,观察组患者的Th17/Treg、HMGB1、Lp-PLA2、PAF、CD62P、TM、GPⅡb、PMPs、PAIg、TPO、白细胞总数和总胆红素水平较对照组明显降低,IL-10、PLT、PCT和血红蛋白水平较对照组明显升高,差异均有统计学意义(P<0.05)。观察组患者的总有效率为96.67%(58/60),较对照组的86.67%(52/60)明显更高,差异有统计学意义(P<0.05)。治疗期间,两组患者均无明显不良反应产生。结论:六味地黄汤加减治疗血液透析肝肾阴虚型血小板减少症患者,可调节患者机体免疫,改善微炎症状态及血管通路功能,降低GPⅡb、PMPs和PAIg水平,改善血小板水平,调节白细胞总数、总胆红素和血红蛋白水平,提高临床疗效,安全性高。

血液透析病人血管通路决策过程的质性研究

目的:探讨血液透析病人的血管通路决策过程,了解医护人员如何鼓励病人参与共享决策。方法:于2022年4月—8月对10例血液透析病人和11名医护人员进行半结构访谈,借助Nvivo软件整理访谈资料,采用科宾和施特劳斯扎根理论的分析方法。结果:血液透析病人在确定接受血液透析治疗后,经历了接受多方面信息阶段、形成初步感知阶段和淡化信息阶段或逐步深入感知阶段的不断反思的过程,病人在血液透析治疗前和治疗后会对血管通路的选择重新审视,医护人员鼓励病人早期参与血管通路共享决策,将血管通路的最终决策权交给病人。结论:血液透析病人的血管通路决策过程是一个不断反思的过程,医护人员可针对病人的不同阶段的特点提供血管通路相关信息,从而促进病人更好地参与血管通路共享决策。

河南省血液透析血管通路协调员岗位设置的调查研究

目的调查河南省血液透析血管通路(vascular access,VA)协调员的设置情况及影响因素。方法2022年12月采用便利抽样法,运用自行设计的问卷,对河南省各级医疗机构的226家血液净化中心的基本情况和VA协调员设置相关情况进行问卷调查。结果226家血液净化中心只有11.06%的中心设置有透析通路协调员专职岗位,且工作内容较为局限;护理管理者认为透析通路协调员应至少具备5年血液净化专科工作经验,本科及以上学历以及主管护师及以上的职称,取得血液净化专科护士证书;多元logistic回归分析显示:VA团队建立、人力资源配置及开展通路相关培训是VA协调员岗位设置的影响因素。结论本研究结果显示河南省VA协调员岗位设置不足,未来需要完善该岗位职能并从优化人力配置、加强通路相关培训及成立通路管理小组,促进该岗位的发展。

一种新型氧合器的设计、数值仿真与实验研究

目的:设计一种新型氧合器,以解决目前体外膜肺氧合设备存在的跨膜压差大、血氧交换效率低、易发血栓等问题。方法:首先,将氧合器血液通路流场主体设置为扁平圆柱体。其次,对血液通路拓扑结构进行三维建模,并将整个流场切分为血液入口段及入口缓冲区、热交换区、血氧交换区、出口缓冲区及血液出口段。最后,采用ANSYS FLUENT软件进行仿真分析和样机制作实验,对比设计的氧合器和Quadrox氧合器的性能。结果:仿真计算表明,设计的氧合器整体性能与临床常用氧合器处于同等水平,且在跨膜压差、血氧交换和流量均匀性方面更优。实验结果表明,设计的氧合器跨膜压差和血氧交换率优于Quadrox氧合器。结论:设计的氧合器跨膜压差小、变温效果好、血氧交换效率高、血栓发生概率低,具有一定的优越性。