text:
Competing Islamic Traditions in the Caucasus
Dobrosława Wiktor-Mach is PhD Candidate at the Institute of European Studies, Jagiellonian University, Cracow, Poland. The results of her anthropological research conducted among Muslims in the Georgian Pankisi gorge are reflected in the current paper.
The common dichotomized classification of Islam in the Caucasus ("traditional" versus "fundamentalist") does not take into account all major processes taking place in the region. The Sufi-Wahhabi discourse simplifies the social interactions between Muslims and suggests homogeneity of each of these categories. In this paper I would show how the term "Wahhabi" has been employed by the local community of Muslims who live in the Georgian Pankisi gorge to express their resistance towards new and radical ideas and practices. In this social conflict, Sufi brotherhoods assume the role of the defenders of traditional order, while the reformists attempt at changing not only religious but also social structures.
Keywords: Islam, Caucasus, Georgia, Sufism, Wahhabism, Pankisi
The Problems of the Sufi-Wahhabi Discourse
It is a common over-simplification to view post-Soviet Islam in the framework of a dichotomy between an age-old "traditional" Islam and the so-called "Wahhabism". Such a perspective consists of an opposition between the "liberal", "tolerant" Islam, espoused mainly by Sufism, and the "integrist", "backward", "terrorist" fundamentalism that poses a serious threat to the already unstable social and political situation in the Caucasus. Such labels, constantly employed by journalists, political analysts, and scholars are misleading for a number of reasons. Firstly, they imply homogeneity inside each of these groups, while even only Sufism embraces a diverse range of brotherhoods, spiritual practices, or political stances. Then, the historical developments such as the Sufi inspirations of the Chechen resistance movement to Russia's colonization in the nineteenth century are being ignored. Moreover, this view, supported by official authorities in the North Caucasian republics, does not take into consideration other groups of Muslims, one of them being the reform-oriented Chechens inspired by the global Islamic movements, who are far from engaging in terrorist activities. Classified as Wahhabists, they have been forced either to emigrate from Chechnya or to conceal their real identity. Lastly, there are Sunni Muslims who neither support the reformists nor practice Sufi rituals. These sorts of empirical cases are evidence of the ambivalence which question the sharp division between "traditional" and "Wahhabi" Islam.
This simple categorization—a pragmatic tool employed by the pro-Moscow Chechen politicians has also been accepted by the Muslims living in the Caucasus. The Russian rhetoric of the "Wahhabi threat" has been appropriated to underline the local resistance to new, alien, non-traditional ideas and practices. It is intended to summarize the attitude towards those Muslims who endeavor to deeply transform existing social order with the traditional authority of the elders, social norms of behavior, and Sufi religious practices.
I have conducted anthropological research among Muslims in the Georgian Pankisi gorge. Located in the immediate vicinity of the Georgian border with Chechnya, in the Akhmeta district, the Pankisi region is inhabited by the Kists—Georgians of Chechen descent and by the Chechen refugees who arrived there in a large number in the wake of the Second Chechen War. Both groups belong to a wider Vainakh group, which encompasses people of the North Caucasus who use the Nakh language.[1] Besides the Chechens who came directly from their territory, some Kists also visited Chechnya in the 1990s, mostly between the wars. Some of them had the intention to set up a new life there, but the violent events after August 1999 made them join the escaping Chechens and come back to Pankisi as refugees.
Looking particularly for the transformations of Muslim discursive traditions, I have also examined the social organization of a Muslim village, as well as specific economic, political and cultural contexts. In a brief analysis of the processes taking place in that particular place, I intend to throw some light on the general "big picture" of Islam in the Caucasus.
Specifically, I would like to show that the popular category of Wahhabism has been adopted by the Muslims themselves in order to designate the religious and social non-traditional practices and beliefs that appeared in the Caucasus in the 1990s and pose a serious threat to the existing norms, values, and customary law (adat). The strong connotations of the term Wahhabism are a useful discursive device to express the hostile attitude towards this phenomenon. In the community I have studied, however, the so-called "evil" – Wahhabi Islam – is not linked to terrorism. Moreover, I want to underline the inner diversity and historical dynamics of Islamic traditions in the Caucasus. The function of propagating Islamic revival was once (in the nineteenth century) linked with Sufism; nowadays it has been used by various reformist groups, who strongly oppose the Sufi tradition.
The Wahhabi Revolution
In Pankisi, the term Wahhabism emerged in the public discourse in the context of the Chechen wars. Suddenly, as the local people recall, in all Pankisi villages young bearded men and women in hijabs appeared. Besides a palpably distinct appearance, the newcomers also held beliefs about social life that were unacceptable for the majority of Pankisi traditional dwellers. The different stances towards Islamic law were another point of disagreement. Moreover, instead of joining Kist Muslims in prayers, the other Muslims built their own mosques ("Wahhabi mosques") headed by independent religious leaders.
The "Wahhabi revolution" in Pankisi was directed against traditional Islam of the Kists, influenced by Sufi practices as well as by syncretic rituals typical of multiethnic and multireligious milieux. However, following the principle that "Islam is the blueprint of a social order",[2] the newcomers embarked upon a project of introducing Sharia law, which governs basically all aspects of a Muslim's life. Although this attempt failed, the "Wahhabis" have not given up their propaganda and persist in criticizing traditional customs. Their radicalism and complete disregard for the traditional social order made the Sufi-oriented Kists and Chechens employ the label Wahhabi in regard to this phenomenon. The connotation is that it is a threat to the social stability of the community that cannot be ignored. What should be stressed again, in the Kists' views, is that these revolutionary ideas and practices are not linked to any kind of terrorism or political extremism. Indeed, at present, no political activism is publicly visible among the "newly pious" bearded men.
The accusations of terrorist activities, links with Al-Qaeda, and of the functioning of the training camps for jihadists in Pankisi were raised by the Russian authorities and led to Moscow's direct engagement in this Georgian region. In 2001, Russian military aircraft dropped a bomb in the gorge area. In fact, at the turn of the century, Pankisi villages became home to diverse groups. Alongside the Chechen and Kist refugees escaping from the war, in this Georgian region appeared also radical Muslims from many countries, either with an intention to take part in the Chechen "holy war" or just to take advantage of a general chaos and to spread the ideas of various reformist wings of Islam. A Japanese reporter Kosuke Tsuneoka depicted the activities of the Chechen warlord Ruslan Gelaev's group in the Pankisi gorge in 2001.[3] In his account of the situation in the region, Tsuneoka stressed that the extremists' influence on the local orders was significant to such an extent that the Pankisi elders consulted them on important issues.[4]
The missionary activities expanded and shortly the effects were widely visible. More local young people went abroad to study Islam, and after the return they joined the Muslim emissaries in their critique of the traditional Kist religious leaders. The reformers, unanimously classified by the Kist majority as Wahhabis, took an uncompromising stance in the dispute over the interpretation of Islam. They used to claim that the local people were not true Muslims.
These radical ideas labeled simply as Wahhabi, or sometimes Salafi, do not refer to the original religious ideology created by Muhammad ibn 'Abd al-Wahhab in the eighteenth century on the Arabian Peninsula. In the discourse of the Russian and pro-Russian Chechen and Dagestan authorities, these terms are exchangeable with "terrorists", "extremists", "Islamists", etc. However, a distinctive feature common to al-Wahhab's movement and to contemporary Caucasian reformers is the condemnation of Sufism.
The Socio-Religious Conflicts
When the war chaos diminished, most foreigners left Pankisi. Nevertheless, the ideology of "pure" Islam gained ground mostly among the unemployed, frustrated, and deprived of any perspectives Chechen and Kist youth. The slogans of following the example of the prophet Muhammad and turning to the sources of Islam—the Koran and the Hadith—have become attractive in the post-Soviet social reality. The reformists called for an instant and radical change of life promising not only benefits after death, but also their tangible support. The money from abroad enabled them to raise new mosques, madrasas and to send some young people abroad to study Islam in the Arab countries. Some Kists even claim that those visiting "Wahhabi mosques" could even expect allowance or benefits.
The socio-economic factors may not be the most significant, yet they constitute the context for the religious actions and contribute to the spread of radical attitudes. In addition to the general harsh situation of Georgian economy, Pankisi community experienced an arrival of refugees during the Russian-Chechen violent conflict. According to the UNHCR estimations, around 8,000 people from the Russian territory have sought safety in Georgia.[5] Although most of them have already left Georgia, a few hundred still live there. Helping them to get by has placed a financial burden on the Kists and worsened their already tough situation. In many cases they shared with the Chechens their homes and food without taking any money in return. The help of humanitarian organizations, although valuable, was only a drop in the ocean. At that time, an unprecedented rise in the number of crimes became evident.
The radicalism of the reformist group in Pankisi manifests itself in rejection of hitherto accepted religious practices and social norms of the Kist community. As far as religion is concerned, the main points of controversy are the Sufi symbols and rituals. What for many Kists constituted the center of their practice and understanding of Islam, is currently being labeled as improper, erroneous, "un-Islamic". The reformist wing vehemently rejects such practices as zikr (ceremonial activity of the remembrance of God's name), saints veneration, or pilgrimages to ziyarats (tombs of sheikhs). Such rituals, the "Wahhabis" pointed out, are not part of an orthodox Islam, as no reference to them is made neither in the Koran nor in the Hadith. Furthermore, the critique carried on, Islam in the Caucasus has accommodated a range of influences, such as syncretic religious celebrations. Indeed, even nowadays there are older Kists who recollect their visits to the Alaverdi Cathedral dedicated to Saint George, an important place of the Georgian Orthodox Church in the Kakheti region. One elder woman from a Refugee Center in Duisi reminded herself of taking part in the important religious holidays when she was a child. Similar cases of common Christian-Muslim religious celebrations in the Central Caucasus are known from the ethnographic literature. The pagan rituals and Zoroastrianism also shaped the highlanders' religious life.[6]
Since the emergence of the Islamic renaissance in the late 1980s, most Christian and pagan elements of the Kist culture have been discarded by the Kists themselves. The Sufi tradition has nonetheless survived, becoming the most crucial target of the reformists' attacks. In fact, the Orthodox Muslims' arguments resemble those used by the Soviet authorities. Traditional Islam, also called "popular Islam" or "folk Islam" in the public discourse, is often seen as burdened with superstitions and relics. However in the view of the reformists, Sufism is more than that; it encompasses practices which were not mentioned by the Prophet Muhammad and his followers, and therefore are not Islamic. Sufi rituals and beliefs distract attention from the "real" Islam.
Rejection of the local traditions, manifested mostly through the religious sphere, may be also seen as a search for a new, more global identity. Reformist Islam—a world-wide phenomenon—accentuating the unity of the umma (Muslim community) and equality of all Muslims, proposes a more attractive frame of reference than the ritual practices of the elders. This homogenizing trend is being reinforced in the Caucasus, among others, by the increase in the knowledge of the Arabic language. The holy language of Islam is the gate to the holy scriptures, but also strengthens the sense of global Muslim unity.
The symbolic marker of the reformists' opposition to the Kist tradition is the appearance—beards in the case of men and veils tied under the chin and long-sleeved dresses, shirts or tunics worn by women. The majority of Kists seem to oppose especially the "Wahhabi" women's type of clothing and claim that it is "unnatural" and not prescribed by Islam. It should be noted that the headscarf itself has neutral meaning for most of the Kists and Chechen refugees. In fact, it is traditionally worn by married women living in Pankisi, and is treated as a sign of marital status rather than in religious terms.
The conventional approach to Chechnya identifies it as the land of Sufis. Historically, Sufi Islam is the earliest expression of Islam in the North Caucasus, where the Kists (at that time "normal" Chechens) had dwelled before their migration to Georgia in the nineteenth century. However, along with Sufism, canonical Sunni Islam of the Shafi'i or Hanafi theological schools (madhhabs) is also widespread in the Chechen society.[7] Unlike the usual understanding of Sufism as Islamic mysticism which involves following by an adept (murid) the path towards God, the contemporary Sufis in the Pankisi gorge seem to be more concerned with the rites recommended by Sufi orders. At present, there are no significant Sufi sheikhs or any other charismatic personalities that could "revive" the Sufi values and beliefs. The most significant people of the brotherhoods, tkhaamad (head) and turakh (deputy)[8] are more the leaders of the rituals than Sufi masters of a typical brotherhood.[9] Those who practice zikr refer to themselves as members of a brotherhood, without actually using the word "Sufism". In reality, zikr turns out to be the most significant element of the Kists' religious identity. This celebration in that community consists chiefly of the collective singing of religious formulas. It is followed by silent prayers, when the believer tries to establish a more personal relationship with God. The latter aspect is raised as an argument against the reformists. For the practitioners of zikr, the reformists are neglecting the personal relationship with God, putting too much emphasis on the correct execution of the rituals.
Imam Shamil and the Dynamics of Islam
In order to assess the role of the so-called Islamic revival in the post-Soviet space, it is valuable to explore not only the forms of religious practices and institutions, but also the historically changing socio-religious functions. Such a perspective also makes the dichotomized categories of moderate and peaceful traditional Islam and radical reformist Islam irrelevant, as it does not take into consideration the dynamics inside various traditions in Islam.
Let us start with the discussion of Imam Shamil and the Imamate he established in the nineteenth century. This charismatic Muslim leader is regarded as the most prominent symbol of the Islamic prosperity and of the resistance against Russians. The time of his rule is often described as "the golden age of Islam". The reference to the first Islamic revival is currently made in the Caucasus by diverse groups: secular nationalists, Islamic clergy, and leaders of Sufi brotherhoods, Islamists and reformists.[10] Although many Caucasian Muslims praise Shamil as a hero, the present interpretations of his actions and ideas vary to a large extent.
For the adherents of the Sufi tradition, the most crucial fact was that Imam Shamil was chiefly a Sufi sheikh and therefore a prominent Muslim leader. The distinct Sufi affiliation of the leader of the theocratic state in the North Caucasus is currently underlined. In fact, the most influential Sufi groups—the Naqshbandiyya and the Qadiriyya—made a weighty contribution to the spread of Islam among the mountain tribes in that region. Moreover, they influenced the "modern Islamic reformation, known as 'Neo-Sufism'".[11] Both these brotherhoods are nowadays active among Pankisi Muslims; the former is commonly referred to as the "Sheikh Efendi", the latter—"Kunta Hajji" brotherhood.
Recently, Islam in Pankisi has resumed old meanings and has become, as in Chechnya and Dagestan, a "factor of unification and consolidation".[12] This time, however, the Sufi-oriented Kists and Chechens appropriated the ideas of social protest against revolutionary changes propagated by the "Wahhabists". The social and religious networks and brotherhoods facilitate the spread of ideas of resistance against drastic changes in the local tradition. While in the nineteenth century the Sufi movement, led mainly by Imam Shamil, aimed at transforming the fundamental social and cultural principles of mountainous communities and challenging the old structures of authority, the contemporary Muslim brotherhoods take an opposite stance: it is the tradition passed on from generation to generation that should be preserved and protected.
Interestingly, Imam Shamil is also a celebrated by those Muslims with a reformist outlook. What they find attractive in the Imamate's heritage is the attempt at creating the theocratic state, even at the cost of traditional laws and customs. The Shamil's uncompromising attitude gains admiration among rebellious youth, looking for a real change in their life. Although the imam was indeed Sufi, he nonetheless put a strong emphasis on the need of the adherence to the Islamic law.
There is a need to reconsider the conventional and widespread conceptual approach to Islam in the Caucasus. The dichotomized perspective, though it seems to put some order to the chaotic social reality, does not lead to adequate understanding of the local processes and their constant dynamics. Particularly the diversity of Islamic practices and meanings attached to them call into question the usefulness of the sharp division between so-called "Sufis" and "Wahhabis". Exploring the real interactions inside each of this group and examining their discourses can lead to a more nuanced picture of Caucasian Islam. I have attempted to show that the category "Wahhabism" has been employed by the Muslims in Pankisi to express their protest against foreign, revolutionary ideas that appeared in the region in the context of the wars and political instability in Chechnya. The meaning of this alien ideology for the Vainakhs who support traditional rules in their communities differs however from that propagated by pro-Russian authorities in the Caucasian republics and by Russians. In the contemporary Pankisi region, Wahhabism is not associated with terrorism, but rather with socio-cultural radicalism that threatens the existing social order.
Lastly, it is worth remembering that the social role of Sufi movements changes. Although today they are regarded as the representatives of "peaceful", "moderate" Islam, in the past those same brotherhoods constituted the base for violent resistance to Russian conquest of the Caucasus and promoted the ideology of jihad. Moreover, it was a Sufi sheikh that called for a radical transformation of existing customs and for establishing the state ruled by Sharia law.
[1]Apart from Chechens and Kists, also Ingushs and Bats belong to the Vainakhs. Some interesting remarks about the historical development of this ethnic group can be found e.g. in Nunuev, Said-Khamzat Makhmudovich, "Ob Etnicheskoy Istorii Vaynakhov", in: Kh. V., Turkaev (ed.) "Kul'tura Chechni. Istoriya i sovremennyye problemy" (Moskva: Nayka, 2002), pp. 30–57.
[2]Gellner, Ernest, "Muslim Society", (Cambridge: Cambridge University Press, 1981), p. 1.
[3]"Japanese Journalist Tells a Story of Pankisi Hideout", Civil Georgia, March 06, 2003, http://www.civil.ge/eng/article.php? id=3300).
[4]"Ibid.
[5]A Report of the Human Rights Information and Documentation Center (HRIDC), Spring 2006, www.proasyl.de. It gives an account of the poor conditions of refugees' life.
[6]Kurtsikidze, Shorena; Chikovani, Vakhtang, "Georgia's Pankisi Gorge: An Ethnographic Survey", Berkeley Program in Soviet and Post-Soviet Studies. Working Paper Series, Spring 2002, p. 25.
[7]Vatchagaev, Mairbek, "The Kremlin's War on Islamic Education in the North Caucasus", in: North Caucasus Weekly, vol. 7(34), September 08, 2006.
[9]For a brief overview of Sufism see e.g. Makris, G.P., "Islam in the Middle East. A living Tradition", (Blackwell Publishing), pp. 142–154.
[10]Zelkina, Anna, "Jihad in the name of God: Shaykh Shamil as the religious leader of the Caucasus", in: Central Asian Survey, vol. 21: 3, pp. 249–264.
[11]Zelkina, Anna, "In Quest for God and Freedom", (London: Hurst and Company, 2000), p. 8.
[12]Sanikidze, George, "Islamic Resurgence in the Modern Caucasian Region: 'Global' and 'Local' in the Pankisi Gorge", in: Tomohiko Uyama (ed.) "Empire, Islam, and Politics in Central Eurasia", Slavic Eurasian Studies, no. 14, 2007, Slavic Research Center.


So, we went from being able to give each of our beloved children one gift each to having a tree filled with ornaments above a small mountain of presents. The kids were so surprised and thankful. It's so nice to have such good hearted children. Our daughter had such a wonderful surprise. She was blessed with two new 18 inch dolls,a bow and arrow set complete with a professional target block, The Hunger Games (both the second book and the DVD.) The boys enjoyed an assortment of remote controlled vehicles and each has a new wooden sword and shield (only two stabbings so far jk jk.) The baby is still being a real support waiting for batteries to be put into his new toys and I've just been happy to get some rest and an extra shower (or two) with my new sprite water filter!
Overall, we're calling it an impressive holiday. Next month the kids will get back to schooling and all the decorations will be up and in boxes in the garage. But, for now, we're happy leaving up the tree, Skyping with family and enjoying some extra calories here and there.


text:
Dejuan Hammond murder trial to move forward
Claire Galofaro
The Courier-Journal
Dejuan Hammond will stand trial next month on a murder charge, despite a series of revelations that prosecutors concealed evidence from the defense.
On Friday, Jefferson Circuit Judge Angela McCormick Bisig for the second time denied Hammond's request to scrap the murder indictment against him on the ground of prosecutorial malfeasance. She also for the second time acknowledged that a stack of police records was "wrongfully" withheld from the defense.
"While the court certainly understands and appreciates defense counsel's continued frustration and outrage ... the end result of dismissing criminal murder charges again seems an extreme sanction," the judge wrote.
Bisig chose instead to allow Hammond's defense attorneys, Ted Shouse and Annie O'Connell, to tell the jury at trial next month that prosecutors withheld evidence from them for years. She also allowed defense attorneys to inspect the original police file, to ensure no additional evidence is missing.
The case against Hammond, accused of hiring teenager Steven Pettway to murder 31-year-old Troya Sheckles because she reluctantly agreed to testify against his brother, has long been fraught by allegations of prosecutorial misconduct.
His murder trial in April collapsed when prosecutors revealed a police report, never before seen by the defense. In it, Hammond's girlfriend, Princess Bolin, denied knowing anything about the murder. Shouse and O'Connell charged that it was intentionally erased by former prosecutor Tom Van De Rostyne because it would have helped in Hammond's defense.
The prosecutors who inherited the case from Van De Rostyne, along with Commonwealth's Attorney Tom Wine, acknowledged that the report appeared to have been intentionally doctored.
But Bisig declined to toss Hammond's indictment, finding that the omission of the evidence was the "result of a mistake, sloppy file review and inadvertence," rather than deception.
This month, six weeks before his second trial is scheduled to begin, prosecutors revealed another 24 pages of previously unknown evidence.
Hammond repeated his request to halt the case. His attorneys charged that prosecutors buried the documents, again, because some could prove beneficial to him.
One page was the detective's summary of his meeting with a man named Gregory Jett, who lived at a homeless shelter and became a suspect in the murder after someone reported to police that they overheard him bragging about killing Sheckles. Prosecutors in 2011 turned over the recording of the detective's interview of Jett. In it, he provides himself with an alibi: he checked in at the shelter at 6:30, he says, and residents aren't allowed to leave after that.
But they did not submit the detective's report, in which he investigates Hammond's claims. An employee at the mission, unnamed in the report, said that tenants could come and go as they please after the 6:30 check-in.
Hammond's defense attorneys point to that as evidence that an initial suspect apparently lied to police.
Prosecutors countered that they gave the defense the interview years ago, properly putting them on notice that Jett was interrogated as an initial suspect.
Bisig wrote Friday that she did not believe the absence of that report – or any of the other 23 pages – would rob Hammond of a right to a fair trial.
"Similar to the court's previous ruling, the court finds the documents were wrongfully overlooked but not intentionally withheld from the defense," she wrote.
Shouse said Friday that he is prepared to go to trial. During the trial in March, Hammond claimed he was targeted by unscrupulous and overzealous prosecutors intent on building a case against him, regardless of the evidence.
"I appreciate that the judge recognizes our frustration, but I haven't seen anything to change my opinion of the games I think the commonwealth is playing in this case," he said.
Assistant Commonwealth's Attorney Elizabeth Jones Brown did not respond to a request for comment.
Meanwhile, Pettway, a year into his 55-year sentence for his murder conviction, recently asked a judge to grant him a new trial, based on the missing evidence that surfaced during Hammond's trial.
Reporter Claire Galofaro can be reached at (502) 582-7086. Follow her on Twitter at @clairegalofaro.

redpajama_set_name: RedPajamaCommonCrawl


text:
START_TESTS()

START_TEST("hhmm_get_difference")
	int difference;

	// no borrowing needed
	difference = hhmm_get_difference(1, 2359);
	ASSERT_EQUALS(difference, 1438);

	// need to borrow
	difference = hhmm_get_difference(31, 2315);
	ASSERT_EQUALS(difference, 1364);
END_TEST()

START_TEST("hhmm_add_minutes")
	// no addition
	HHMM result = hhmm_add_minutes(1927, 0);
	ASSERT_EQUALS(result, 1927);

	// no carry minutes
	result = hhmm_add_minutes(800, 30);
	ASSERT_EQUALS(result, 830);

	// carry minutes once (minumum edge case)
	result = hhmm_add_minutes(1230, 30);
	ASSERT_EQUALS(result, 1300);

	// carry minutes once (middling case)
	result = hhmm_add_minutes(1230, 45);
	ASSERT_EQUALS(result, 1315);

	// carry minutes twice (minumum edge case)
	result = hhmm_add_minutes(1230, 90);
	ASSERT_EQUALS(result, 1400);

	// carry minutes twice (middling case)
	result = hhmm_add_minutes(1230, 105);
	ASSERT_EQUALS(result, 1415);

	// carry minutes once (minumum edge case for minutes, maximum edge case for hours)
	result = hhmm_add_minutes(2330, 30);
	ASSERT_EQUALS(result, 2400);
END_TEST()

END_TESTS()

redpajama_set_name: RedPajamaGithub


text:
Virgin Atlantic chooses high-gain SATCOM antennas from Cobham for its fleet of seven Boeing 747 jetliners
CAPE TOWN, South Africa, 29 Nov. 2011. Avionics experts at Virgin Atlantic Airways Ltd. in Crawley, England, needed high-gain satellite communications (SATCOM) antennas for the airline's Boeing 747 cabin upgrade program, which will begin in 2012. They found their solution from Cobham plc Antenna Systems segment in Cape Town, South Africa. Virgin Atlantic will retrofit seven of its Boeing 747 jumbo passenger jets with the Cobham antenna, which will enable Inmarsat SwiftBroadband (SBB) SATCOM connectivity into the cockpit and cabin.
CAPE TOWN, South Africa, 29 Nov. 2011.Avionics experts at Virgin Atlantic Airways Ltd. in Crawley, England, needed high-gain satellite communications (SATCOM) antennas for aircraft communications in the airline's Boeing 747 cabin upgrade program, which will begin in 2012. They found their solution from Cobham plc Antenna Systems segment in Cape Town, South Africa.Virgin Atlantic will retrofit seven of its Boeing 747 jumbo passenger jets with the Cobham SATCOM antenna, which will enable Inmarsat SwiftBroadband (SBB) SATCOM connectivity into the cockpit and cabin.Cobham will install the SATCOM antennas in Virgin Atlantic 747-400 aircraft through an independent supplemental type certificate (STC), Cobham officials say. Other STCs are in progress to certify the Cobham antenna subsystem for other aircraft types.
Cobham SATCOM equipment has been certified for the Boeing 737, 747, 777 passenger jetliners, as well as for the Airbus A320 and A330/A340 families of aircraft. Certification is expected soon for the Airbus A400M and A380, Cobham officials say. Cobham antennas also have been selected for the Boeing 787 and Airbus A350 aircraft.
For more information contact Cobham Antenna Systems online at www.cobham.com, or Virgin Atlantic at www.virgin-atlantic.com.
EMS Defense & Space to supply high-bandwidth SATCOM antenna for military RC-12 reconnaissance aircraft
New Ka-band SATCOM antenna system tested by Boeing
Cobham to enhance its UHF SATCOM technologies offerings with acquisition of Trivec-Avant Corp..


© Fitts et al.; Licensee Bentham Open.
Surgically-created blood conduits used for chronic hemodialysis, including native arteriovenous fistulas (AVFs) and synthetic AV grafts (AVGs), are the lifeline for kidney failure patients. Unfortunately, each has its own limitations; AVFs often fail to mature to become useful for dialysis and AVGs often fail due to stenosis as a result of neointimal hyperplasia, which preferentially forms at the graft-venous anastomosis. No clinical therapies are currently available to significantly promote AVF maturation or prevent neointimal hyperplasia in AVGs. Central to devising strategies to solve these problems is a complete mechanistic understanding of the pathophysiological processes. The pathology of arteriovenous access problems is likely multi-factorial. This review focuses on the roles of fluid-wall shear stress (WSS) and endothelial cells (ECs). In arteriovenous access, shunting of arterial blood flow directly into the vein drastically alters the hemodynamics in the vein. These hemodynamic changes are likely major contributors to non-maturation of an AVF vein and/or formation of neointimal hyperplasia at the venous anastomosis of an AVG. ECs separate blood from other vascular wall cells and also influence the phenotype of these other cells. In arteriovenous access, the responses of ECs to aberrant WSS may subsequently lead to AVF non-maturation and/or AVG stenosis. This review provides an overview of the methods for characterizing blood flow and calculating WSS in arteriovenous access and discusses EC responses to arteriovenous hemodynamics. This review also discusses the role of WSS in the pathology of arteriovenous access, as well as confounding factors that modulate the impact of WSS.
Keywords: Arteriovenous fistula, arteriovenous grafts, endothelial cells, hemodialysis vascular access, hemodynamics, shear stress..
Two types of long-term hemodialysis vascular access exist: the arteriovenous (AV) fistula and AV graft. For patients with chronic kidney disease (CKD), long-term maintenance hemodialysis may be required as the disease progresses and kidney fails. Chronic hemodialysis vascular access is achieved through either the surgical creation of a native AV fistula (AVF) or the implantation of a synthetic AV graft (AVG) made of expanded Teflon® (polytetrafluoro-ethylene) . In October 2010, 57% and 20% of chronic hemodialysis patients in the U.S. use AVFs and AVGs, respectively , and the rest use catheters. Without long-term vascular access, the only safe, clinically used method to obtain sufficient blood flow rate for hemodialysis is through a central venous catheter with its inherent risks. Although AVFs and AVGs are the lifeline for patients who require maintenance hemodialysis, each has its limitations: AVFs often fail to mature to become useful for dialysis and AVGs often fail due to stenosis as a result of neointimal hyperplasia (NH), which preferentially forms at the graft-venous anastomosis. Currently no clinical therapies are available to significantly improve the problems of AVF non-maturation or prevent NH in AVG. The pathology of AV access problems is likely multi-factorial, and this review focuses on the roles of fluid-wall shear stress (WSS) and endothelial cells. In AV access, shunting of arterial blood flow directly into the vein alters the hemodynamics in the vein drastically. These hemodynamic changes are likely major contributors to non-maturation of an AVF vein and/or formation of NH at the venous anastomosis of an AVG. Endothelial cells line the inner surface of the blood vessel walls, separating blood from the other vascular wall cells and also influencing the phenotype of these other cells. In AV access, endothelial cells are directly exposed to aberrant WSS and their responses to aberrant WSS may subsequently lead to AVF non-maturation and/or AVG stenosis.
The relationship between aberrant WSS and undesired vascular remodeling is well-established in the arterial circulation, but not yet in the vein, particularly in the AV conduit setting. Studies have shown that local hemodynamics clearly contribute to atherosclerosis [3-5]. Unidirectional, laminar WSS activates mechanotransduction pathways that lead to quiescent endothelial phenotype, release of nitric oxide, and down-regulated expression of pro-proliferative and pro-inflammatory genes [4, 6-9]. Thus, arterial regions where unidirectional, laminar WSS exists are relatively free of the formation of atherosclerosis or arterial NH [5, 10, 11]. In contrast, disturbed flow or oscillatory WSS, examples of complex blood flow, activates mechanotransduction pathways that lead to decreased release of nitric oxide and up-regulated expression of pro-inflammatory, pro-proliferative, and pro-thrombotic genes [4, 7-9, 12, 13]. Complex blood flow tends to exist at areas of stenosis or vascular bifurcation, and these locations have increased presence of atherosclerosis and arterial NH [3, 5, 10, 11, 14]. Venous NH, which could lead to stenosis and even occlusion in an AV conduit, is considered to be a primary cause of AVF and AVG failure. Limited information exists about the impacts of WSS on AVFs and AVGs where aberrant hemodynamics occurs, despite extensive research on these concepts in arterial NH and atherosclerotic vascular disease.
To provide an overview of the major clinical problems for chronic hemodialysis vascular access.
To discuss the role of fluid-wall shear stress in the pathology of arteriovenous access failure.
To provide an overview of methods for characterizing blood flow and calculating shear stress in arteriovenous access.
To discuss hemodynamics in arteriovenous access.
To provide an overview of endothelial cell responses to physiological and arteriovenous hemodynamics.
To discuss confounding factors modulating the impact of shear stress on venous remodeling.
To consider additional relevant factors beyond shear stress and endothelial cells.
The AVF is the preferred method of long-term hemodialysis vascular access, because it uses native blood vessels to create an AV conduit, could last for many years, and has few complications when compared to synthetic AVGs. However, AVFs may take months (approximately 3-6 months) to mature to become usable, and up to 60% of AVFs fail to mature . The AVG, made of synthetic expanded polytetrafluoroethylene (ePTFE), provides sufficient blood flow rate needed for dialysis within weeks (approximately 2 weeks) after implantation, but is inherently subject to complications related to foreign body response. The central venous catheter (CVC) is only recommended for short-term use due to high rates of infection, and is used as a transitional access for patients who have received AVF creation or AVG implantation surgery but need dialysis before the AV conduits become ready. The tunneled CVC has a subcutaneous cuff that could potentially render it suitable for long-term use (months to years), but this is not a standard practice due to the many complications associated with CVCs . All three types of vascular access produce aberrant flow conditions that could lead to neointimal hyperplasia, thrombosis, stenosis and eventual occlusion. The focus of this review is on AVFs and AVGs.
AVFs are usually created in the forearm between the radial artery and cephalic vein or in the upper arm between the brachial artery and the cephalic or basilic vein. The KDOQI guideline for pre-surgical arterial diameter is >2.0 mm . The KDOQI guidelines of the post-surgical, mature AVF vein criteria are venous diameter >6.0 mm and flow rates >600 ml/min with a depth beneath the skin surface of approximately 0.6 mm . AVFs are the preferred mode of long-term vascular access because once they mature and become functional, they have the least complications. In the U.S., primary patency rates in AVFs are approximately 56% at 1 year and 40% at 2 years , and intervention rates in AVFs are 0.06-0.57 interventions/year . The main problem of AVFs is that many fail to mature and become clinically usable . The etiology of failure can be separated into acute failure (such as thrombosis) and chronic non-maturation. Non-maturation, defined as an inability to achieve clinically necessary blood flow rate during two needle hemodialysis within two to four months after AVF creation surgery, is the primary cause of AVF failure, with recent studies reporting 20-50%  and even 60% non-maturation in a multicenter randomized trial that had 1284 patients . Several studies have investigated the specific blood flow rates and the structural changes of blood vessels that are associated with AVF non-maturation. For example, a few studies reported that the insufficient dilation of blood vessels and the formation of NH are responsible for at least some AVF non-maturation [21-24]. Krishnamoorthy et al. reported that vascular stenosis often occurred in the venous segments of porcine AVFs that did not adequately mature , and a recent clinical study by Allon et al. showed that post-operative stenosis is associated with AVF non-maturation .
A synthetic AVG is surgically created usually in the forearm between the radial artery and cephalic vein. One of the clinical criteria for AVGs in the U.S. is a blood flow rate to the hemodialysis machine of >350 ml/min . When compared to AVFs, the critical problem of AVGs is their low primary patency rate, approximately 38% at 1 year and 25% at 2 years, as reported in a clinical study of 1574 patients . In 90% of AVG patency failure cases, acute failure due to thrombosis is attributed to stenosis caused by NH . In a study of 90 subjects, NH in 58% of AVG patients was seen at the graft-venous anastomosis or within 1 cm up- or downstream of the venous anastomosis; in contrast, only 4% of AVG patients had NH at the graft-arterial anastomosis and 19% in the peripheral basilic vein, with the rest primarily in upstream proximal veins . Currently, stenosis in AVGs is largely treated with balloon angioplasty, but this procedure induces restenosis due to vascular wall injury. A clinical trial showed that the use of balloon angioplasty plus placement of a stent graft is superior to balloon angioplasty alone for the treatment of stenotic dialysis grafts .
The mechanisms behind AVF non-maturation and/or AVG NH are not yet completely understood, but likely result from aberrant fluid-wall shear stress affecting vascular remodeling and endothelial cell functions, which are discussed below.
Major hemodynamic stresses exerted on vascular wall cells. Pressure and stress are defined as force per unit area. 'Shear' stress, such as fluid-wall shear stress (WSS), acts parallel to the area and 'normal' stress, such as blood pressure, acts perpendicularly to the area. Cells in the wall also experience circumferential wall stress. This figure was initially published in Kidney International by Nature Publishing Group .
Magnetic resonance imaging (MRI)-to-computational fluid dynamics (CFD) pipeline. The pipeline to obtain the blood vessel lumen geometry serving as the structural boundary for CFD is shown in Panels A-D. Panels E-F show the pipeline for extracting blood flow rates to be used as CFD flow boundary conditions. (A) Contrast-free black-blood MRI was performed to image lumen geometry. An example DICOM image shows an axial cross-section of the arm (arrow indicates the lumen of an AVF vein). (B) Segmentation of the AVF vein and artery was performed on the same example DICOM image (AVF vein lumen filled green). (C) The segmented lumen from several slices resulted in a 3D reconstruction and (D) were meshed volumetrically. (E) Contrast-free cine phase-contrast MRI was performed over a cardiac cycle over a forearm AVF, and (F) blood flow rates during the cardiac cycle were extracted from the MRI data. (G) A CFD-derived wall shear stress (WSS) profile where the feeding artery is the vessel on the bottom and the fistula vein is the curved vessel branching off of the feeding artery. Reprinted from Journal of Biomechanics, Y He, C Terry, C Nguyen, S Berceli, Y Shiu, A Cheung, "Serial analysis of lumen geometry and hemodynamics in human arteriovenous fistula for hemodialysis using magnetic resonance imaging and computational fluid dynamics," 46:165-169. Copyright (2013), with permission from Elsevier .
WSS profiles in an AVF of a hemodialysis patient. Contour plots of the WSS values averaged over a cardiac cycle for MRI scans at (A) 4, (B) 5, and (C) 7 months (mos) after AVF creation. The highest WSS values were found at the anastomosis (arrow) and the region of stenosis (arrowhead) of the AVF vein at all 3 time points. However, AVF WSS decreased from 4 to 7 mos. Note that the color scale bar was adjusted for peak WSS at each time point and the anastomosis and artery were maintained at the same angle of view for all 3 time points. However, the AVF vein deformed over time, obscuring the view of the accessory vein at 7 mos. In these images, the feeding artery is to the right of the anastomosis, the distal artery is to the left of the anastomosis, and the fistula vein is branching off of the artery at the anastomosis and the accessory vein branching off of the fistula vein just above the stenosis. Reprinted from Journal of Biomechanics, Y He, C Terry, C Nguyen, S Berceli, Y Shiu, A Cheung, "Serial analysis of lumen geometry and hemodynamics in human arteriovenous fistula for hemodialysis using magnetic resonance imaging and computational fluid dynamics," 46:165-169. Copyright (2013), with permission from Elsevier .
where Q is the blood's volumetric flow rate and r is the inner radius of the blood vessel . Thus, WSS depends on a variety of factors. For example, WSS increases with higher blood viscosity, velocity and volumetric flow rate, and also increases as the radius of the blood vessel decreases .
Large arteries have physiological WSS values of 10-70 dyn/cm2, while physiological WSS levels in large veins are 1-5 dyn/cm2 [32, 34]. AVF creation and AVG implantation with subsequent shunting of arterial flow into the vein results in a drastic increase in WSS on the venous wall. While there is a wealth of information in the literature regarding the effect of WSS on arterial wall remodeling and arterial wall cell function, such information is not yet available for the vein. For example, in arteries, atherosclerosis preferentially occurs in regions of low and/or oscillatory WSS , whereas in relatively straight arteries where blood flow is usually laminar, chronically increased blood flow and WSS result in chronically enlarged lumen diameter [35, 36]. These adaptive responses imply that the vessel area adjusts to return the WSS levels to the initial values [37, 38]. Whether a similar relationship exists between the WSS and lumen diameter in the vein is yet to be verified by experimentation. There are a few reports linking WSS to venous remodeling in the AV access setting, and they are discussed below.
Several published findings support a role of blood flow in AVF maturation and/or non-maturation . For example, pre-surgical blood flow has been correlated with subsequent successful maturation, with a clinical study finding AVF venous blood flow significantly lower in the non-maturing group (n = 10) vs the mature group (n=43) (450 ± 214 vs 814 ± 348 ml/min, p = 0.003) . Among the several types of hemodynamic stresses exerted on the vascular wall, WSS appears to have the greatest impact on vascular responses. In a study of AVF hemodynamics by Ene-Iordache and Remuzzi , both low and high WSS values have been associated with stenosis formation and therefore, are implicated in AVF non-maturation. However, in this report the WSS value was calculated for a large portion of the AVF vein . It is important to note that WSS is strongly dependent on local wall curvature and physiological flow patterns, thus, ideally it would be better to consider the relationship between stenosis and WSS at a higher spatial resolution.
NH is the primary cause of AVG failure and is defined by proliferation of smooth muscle cells (SMCs) and fibroblasts, formation of microvessels, and matrix deposition . The venous anastomosis is the main location of NH in AVGs . In an animal study using a porcine model with an AVG implanted between the carotid artery and jugular vein, it was found that a rapid neo-intimal response occurred at the venous anastomosis along with a 50% loss of patency at 8 weeks . Although low WSS levels (<3 dyn/cm2) have been reported to increase NH in a canine model of interpositional artery graft made of ePTFE , high WSS levels (>50 dyn/cm2) have been reported to increase NH in a porcine model of arteriovenous graft made of ePTFE . These reports, however, used roughly estimated WSS. In an iliac artery to ipsilateral iliac vein porcine AVG model, Misra et al. calculated WSS by using Poiseuille's law (Equation  above) and found that the average WSS value in the venous anastomosis normalized at day 3 post graft-implantation surgery, then increased by day 7 and day 14 while the mean vessel lumen area decreased as a result of NH formation at day 14 .
Hemodynamic parameters such as wall shear stress (WSS) and oscillatory shear index (OSI, which describes the change in WSS values and blood flow direction during a cardiac cycle) cannot be measured directly, as they are functions of the local velocity gradient near/at the blood vessel wall (see Equation  above). The lumen size and flow rate are needed for calculating WSS. In previous studies, several methods of geometric reconstruction have been used (reviewed by Taylor and Steinman ), including ultrasound, X-ray computed tomography, and magnetic resonance imaging (MRI). Local blood flow rates have been measured using Doppler ultrasound and phase-contrast MRI . The pressure, flow rate, and WSS that have been reported in the literature for human are summarized in Table 1. This table also includes strain, which can be used to estimate circumferential wall stress (Fig. 1). Pig hemodynamics and vessel size are similar to human and have been used extensively in AVF and AVG research . The pressure, flow rate, and WSS that have been reported in the literature for pigs are summarized in Table 2. As shown in both tables, most of the WSS values reported in the literature were calculated using Poiseuille's law (Equation ) and the flow rate and lumen size that were measured at specific locations. In order to obtain a detailed spatial profile of WSS throughout the entire lumen of an AVF or AVG, methods that can give 3D lumen geometry need to be used; ideally, these methods should acquire geometry and flow information simultaneously.
Hemodynamics in human arteriovenous fistula (AVF), arteriovenous graft (AVG), arterial bypass graft (ABG), artery and vein.
Notes: (a) Unless otherwise specified, time in parenthesis is post-surgery. (b) n/a: not available. (c) CFD: computational fluid dynamics.
Hemodynamics in porcine arteriovenous fistula (AVF), arteriovenous graft (AVG), artery and vein.
Notes: (a) Time in parenthesis is post-surgery. (b) n/a: not available. (c) CFD: computational fluid dynamics.
Computational fluid dynamics (CFD) is a method of numerical simulation that can be used to determine blood flow fields in anatomically accurate vessels. Direct measurement of velocity at high resolution in the entire AV conduit lumen is difficult, due to the complex geometry and high blood flow rates in those regions. Image-based CFD simulations in blood vessels were first used to study hemodynamics in atherosclerotic carotid [61, 62], coronary arteries , and abdominal aortic aneurysms . These are some of the most clinically relevant and well-studied image-based CFD simulations in large arteries. Image-based CFD simulations in both AVFs and AVGs have been increasing in recent years, with AVF studies focusing primarily on fistula maturation, and AVG studies on NH development. In AVGs, image-based CFD studies include canine  and porcine models . In these previous studies, hemodynamic values are averaged over a large area, even though WSS is dependent on local wall geometry. For this reason, there is a need to develop higher-resolution CFD studies in order to fully explore the hemodynamics of both AVF and AVG.
He et al. developed an MRI-CFD protocol that uses state-of-the-art techniques to characterize blood flow and calculate WSS in human AVF (Fig. 2) ; this protocol has also been used in AVG . The protocol by He et al. uses contrast-free MRI, black-blood MRI for lumen geometry and cine phase-contrast MRI for flow measurement. These techniques have several advantages over the alternatives: (a) simultaneous measurement of both luminal geometry and blood flow rate, allowing better registration of geometry and flow than using other methods that obtain this information separately (e.g., computed tomography for geometry and ultrasound for flow rate); (b) higher resolution (0.25 x 0.25 x 2 mm for black-blood, 0.7 x 0.7 x 3 mm for phase-contrast) and a larger field of view (180 x 180 mm2 for black-blood and 180 x 112 mm2 for phase-contrast images) capable of spanning the entire AVF or AVG lumen, as compared to ultrasound; (c) MRI does not involve ionizing radiation as in computed tomography, so the patients will not be subjected to ionizing radiation. This is particularly important for patients with compromised kidney function [68, 69]. The paper by He et al. was the first to provide a comprehensive, detailed profile of WSS of a dialysis AVF (Fig. 3).
remodeling in the artery and vein, though the vein has received more attention in AVF research [72, 73]. Normally in a physiological setting, in response to a sudden increase in flow rate, the vein and artery would dilate to maintain a baseline range of WSS levels (which is 1-5 dyn/cm2 for vein and 10-70 dyn/cm2 for artery [32, 34]); the flow-mediated dilation is via endothelial-mediated responses such as the release of nitric oxide and other endothelium-dependent vasodilators [20, 22, 25, 34, 70]. In the arterial circulation, low WSS has been linked to atherosclerotic lesion formation and increase in intima-media thickening , whereas high WSS levels are considered to result in the survival of the endothelium and decreased intima-media thickening. However, it is important to note that in arteries, extremely high WSS levels around 1000 dyn/cm2 can usually be attributed to stenosis and thrombosis [75, 76].
It has been proposed that high WSS levels (approximately 25 dyn/cm2 in the vein ) result in large vessel diameters and lack of NH, whereas low and/or oscillatory WSS levels (<10 dyn/cm2 in the vein ) result in small vessel diameters that are attributed to insufficient vascular dilation and neointimal hyperplasia . However, decreased WSS levels over time have been found to result in vein dilation in a pig AVF model . It is important to note that in these previous studies, the reported WSS values were usually estimated for a large region and the WSS values and lumen size were determined at the same time. To better understand any causal relationship between early WSS and later lumen area changes, ideally we should know WSS and lumen area at a higher spatial resolution in a longitudinal study. As mentioned above, He et al. developed an MRI-CFD protocol that uses state-of-the-art techniques to characterize blood flow and calculate WSS in human AVF. The detailed WSS profiles within the AVF of a dialysis patient at several time points after AVF surgery are shown in Fig. (3). This figure allows us to appreciate the heterogeneity of WSS, how WSS spans a large range of values in the AVF lumen, and how WSS changes over time.
Synthetic vascular grafts made of expanded Teflon® are used in several other applications than hemodialysis access, including peripheral artery bypass and coronary artery bypass. Like AVG, these synthetic arterial bypass grafts (ABGs) also suffer from NH, which often occur at the downstream (i.e., distal) graft-vessel anastomosis . In a clinical study of surveillance of infrainguinal lower limb grafts, stenosis tended to form at the distal anastomosis of the prosthetic graft . It is important to note that synthetic ABGs, particularly those in peripheral arteries, tend to have much lower flow rates than AVGs (200-300 ml/min in synthetic ABGs as compared to 1000-2000 ml/min in AVGs) and therefore, WSS tends to be lower in synthetic ABGs (approx. 20 dyn/cm2in synthetic ABGs as compared to approx. 60 dyn/cm2 in AVGs) . Synthetic ABGs also have less occurrence of turbulent flow than AVGs. Cuffed graft designs have been used to improve anastomotic hemodynamics, and have been shown to be more effective at preventing hyperplasia in synthetic ABGs, but have not been extensively researched in the extremely high WSS environment of AVGs [77, 79].
As mentioned above, low WSS (<3 dyn/cm2) has been reported to increase NH development in a canine model of interpositional artery graft , whereas local elevated WSS (>50 dyn/cm2) has been reported to increase NH development in a porcine AVG model . Previous studies regarding the impact of WSS on NH development have focused on large regions such as the venous anastomosis and have not yet investigated the longitudinal (over the time course of NH development) impact of local hemodynamics or other features of disturbed flow (such as OSI). For example, in Binns et al. , WSS in the canine model was obtained by estimation from Poiseuille's law (Equation ) using flow rates measured with an electromagnetic flow meter and luminal size obtained by histology. In Misra et al. , WSS in the porcine model was also obtained by estimation from Poiseuille's law using velocity measured at discrete 2D slices using cine phase-contrast MRI and luminal area obtained by 3D magnetic resonance angiography and histology. We have used the MRI-CFD approach by He et al. mentioned above  to characterize WSS in a porcine AVG in a more comprehensive manner .
The mechanobiology of endothelial cells (ECs) has been extensively studied in the context of arterial function, both in homeostasis and dysfunction. Much of the pioneering work on mechanotransduction of blood flow in ECs was done in the context of atherosclerosis (see comprehensive reviews by Davies  and Malek et al. ). Atherosclerotic lesions (whose histological features are accumulation of calcium and fatty materials) tend to develop at locations of large arterial wall curvature near vascular branching points , with correspondingly complex flow as compared to a straight conduit arterial section where high and laminar WSS of 10-70 dyn/cm2 occurs. In arteries, low and oscillatory WSS (±0-4 dyn/cm2) has been linked to the localization and formation of atherosclerotic lesions [14, 74, 82, 83]. For this reason, it has been proposed that low and oscillatory WSS at the graft-vessel junction may lead to EC dysfunction and subsequent NH formation at this location in ABG and also AVG [77, 84]. However, it is important to note that the histological feature of neointimal hyperplasic lesions is very different from that of atherosclerotic lesions. NH primarily consists of highly proliferating vascular smooth muscle cells(SMCs) and myofibroblasts, as well as the presence of neovessels and inflammatory cells . In addition, there exists an asymmetric pattern of development of NH in ABGs and AVGs. In ABGs, NH tends to develop at the downstream (i.e., distal) graft-arterial anastomosis, rather than the upstream (i.e., proximal) graft-arterial anastomosis . In AVGs, NH is much more prevalent at the downstream venous anastomosis than the upstream arterial anastomosis .
Some studies have suggested a number of means by which the biology of venous stenosis could be influenced by WSS [25, 71]. Optimal venous endothelial function has been associated with moderate and high WSS levels; additionally, reduced expression of adhesion molecules, increased expression of endothelial nitric oxide synthase, and reduction in oxidative stress have also been attributed to moderate and high WSS levels, though the authors did not indicate specific WSS values . Conversely, low WSS levels have been associated with endothelial dysfunction, reduced production of nitric oxide, increase in oxidative stress, increase in neointima formation, and a tendency toward vasoconstriction in the vein [25, 85]. In AVF- and AVG-specific settings, it is important to note that venous EC response to WSS is not always identical to arterial EC response.
Using veins from patient AVFs, Jones et al.  found that 80-90% of fistula vein ECs showed strong staining for endothelin-1 as compared to control saphenous veins. Furthermore, when compared to those in the control vein, the majority of fistula vein ECs were smaller but elongated with several endothelial mitoses, newly divided cells, and hyperchromatic nuclei [86, 87]. Matrix metalloproteinase (MMP)-2 and MMP-9 were overexpressed at the site of neointima in a rat AVF model  and a rabbit AV shunt model . These results indicate that MMPs play a critical role in neointimal lesion development in AVF veins. Wang et al.  used a uremic AVF mouse model and showed that increased transforming growth factor (TGF)-β1 activates Notch signaling in ECs of AVFs; accelerated neointima formation and ultimately AVF failure occur following the activated Notch signaling.
Studies by Sho et al. were focused on arterial remodeling . Using a rabbit left common carotid artery-left jugular vein AVF model, Sho et al. found that arterial EC density increased from 2814 ± 183/mm2 under pre-surgery physio-logical conditions to 4917 ± 501/mm2 after being exposed to high flow conditions for 3 days after AVF creation . The gene expression of vascular endothelial growth factor (VEGF) was upregulated 13-fold 1 day after AVF creation, proceeding endothelial cell proliferation. The gene expression of VEGF and VEGF receptor-2 was upregulated 25-fold and 33-fold, respectively, 3 days after AVF creation. A strong detection of VEGF protein was seen in ECs from 1 to 3 days after AVF creation. These data suggest that ECs contribute to arterial remodeling.
Both patients receiving AVGs and porcine AVG models have increased levels of hypoxia-inducible factor 1-α (HIF-1α) expression in locations of restenosis [92, 93]. In stenotic vein samples obtained from 6 AVG patients upon graft failure, a large range of protein factors associated with HIF-1α were upregulated within 6 cm of the graft-venous anastomosis, including HIF-1α itself (1.8-fold vs control vein), macrophage migration inhibition factor (8.6-fold), MMP-2 (2.8-fold) and MMP-9 (2.5-fold), and tissue inhibitor of MMPs-1 (TIMP-1) (8.9-fold) . In a porcine model of AVG, many of these factors were increased in both the vein and also artery adjoining the graft, including MMP-2 and -9 [94, 95], and TIMP-1 and -2 . In another study using porcine AVG, significant MMP-9 and HIF-1α expression was seen at the venous anastomosis, coincident with the development of NH . Thus, HIF-1 α and MMPs seem to be critical in the formation of NH in AVG.
As mentioned above, synthetic ePTFE grafts are also used as ABGs, which also suffer from NH formation. In a baboon model of arterial-arterial grafting, an ePTFE graft was placed between the ascending aorta and iliac artery (n=15) to investigate the expression of bone morphogenetic proteins 2, 4, and 5 (BMP-2, -4, and -5). BMP-2 mRNA expression reached a maximum of 1.6-fold increase (n=5, p<0.05) at Day 1, while BMP-4 and -5 expression reached a maximum of 1.73- and 1.32-fold, respectively, at Day 7 (n=5, p<0.05) . Additionally, extracts from the neointima demonstrated a 2-fold increase in BMP-4 protein by Western blot, and BMP-4 binding receptors BMPR-IA, -IB, and -II stained positive by immunohistochemistry in the neointima. However, it is not yet known whether BMP is involved in the formation of NH in AVGs.
CKD patients often have factors other than kidney failure that may affect their vasculature, and it is possible that individual patients' blood vessels may respond differently to the same hemodynamic stresses. This could be attributed to heterogeneity in the vascular wall biology, including endothelial function and clinical factors such as age, diabetes and stages of CKD. For example, it is known that diabetes leads to increased arterial stiffness and therefore, may compromise the ability of the feeding artery to expand following an AVF creation surgery. Thus, blood vessels from diabetic CKD and non-diabetic CKD patients may react to the same arteriovneous flow differently.
Endothelial dysfunction is especially significant in the end-stage renal disease (ESRD) population due to the negative impacts of uremia and oxidative stress on the endothelium . Indeed, endothelium-dependent vasodilation has been found to be markedly impaired in chronic hemodialysis patients . Vascular dysfunction due to chronic uremia in CKD patients can be delineated into four categories, reviewed by Brunet P et al. : (a) increased rate of atherosclerosis; (b) increased arterial stiffness; (c) increased formation of vascular calcifications; and (d) impaired vascular repair and increased NH formation. Uremia is associated with a range of elevated toxins, including guanidines, advanced glycation end products (AGE), p-cresyl sulfate, platelet diadenosine polyphosphates, and indoxyl sulfate . These toxins are associated with increases in arterial stiffness, calcification, impaired vessel wound healing responses, and neointimal hyperplasia . Increased asymmetric dimethyarginine (ADMA) and AGE leads to a decrease in nitric oxide bioavalibility, leading to impaired dilatory function of arteries, as well as impaired signaling for nitric oxide-related processes such as wound healing . Thus, blood vessels from CKD patients may react to the same arteriovneous flow differently, depending on the endothelial damages by uremia.
Although this review is focus on ECs, it is important to note that, when the endothelial layer becomes stripped away, luminal blood flow can exert forces directly on SMCs. Surgical trauma such as AV fistula creation and AV grafting can contribute to such denudation of the endothelial layer. The destruction of the endothelial monolayer can also be attributed to high shear stress (>400 dyn/cm2) . SMC proliferation is a key feature of NH, with the vast majority of neointimal cells being SMCs (77.0 ± 12.0% in a clinical study by Rekhter et al. ) [21, 102]. SMCs are responsive to WSS (see a comprehensive review by Shi and Tarbell ). Neointimal hyperplasia may be a result of upregulation of certain mechanotransduction pathways in SMCs induced by elevated levels of WSS. Sho et al. demonstrated that in a rabbit carotid AVF model of flow-induced arterial remodeling, SMCs contribute to arterial remodeling with SMC proliferation and migration occurring after EC proliferation . The role of SMCs in flow-induced venous remodeling in the AV access setting remains to be explored.
In addition to WSS, ECs and SMCs are also exposed to CWS (Fig. 2). Studies by Price et al. and Pries et al. [104, 105] suggested that WSS and CWS dictate recruitment and differentiation of SMCs. Van Gieson et al.  observed that increased pressure led to increased diameter and wall strain in microvessels, which subsequently led to increased CWS, and they hypothesized that the differentiation of immature SMCs was catalyzed by the local mechanical stimulus of increased wall strain. AVG implantation with subsequent shunting of arterial flow into the vein results in an increase in CWS at the venous wall. Substantial research has been conducted to investigate the effect of CWS on arterial remodeling and cell function, but not for the vein. Increased blood pressure, wall tension, and/or CWS have been shown to induce arterial wall thickening, particularly in the medial layer of arteries . Studies have reported a relationship between varicosity development and increased blood pressure in the vein [107-112]. Currently few reports linking CWS and pressure to venous NH formation exist.
Although this review is focused on AVFs and AVGs, aberrant blood flow also occurs in central venous catheters (CVCs) which are inserted into a central vein such as the vena cava, internal or external jugular, or femoral vein to serve as a temporary access to a patient's blood stream for hemodialysis . Ideally, CVCs are as wide and short as feasible for the patient . This is due to less blood flow resistance the larger the diameter and the shorter the length of the catheter. The recommended blood flow rate for a CVC is 300 ml/min, with pre-pump arterial pressure less than 250 mmHg . Failure to do so can lead to thrombosis and stenosis, as well as external fibrin sheath formation . Aberrant flow-induced thrombosis in CVC is beyond the scope of this review.
AVF non-maturation and AVG stenosis are important problems and yet currently no clinical therapies are available to significantly promote AVF maturation or prevent neointimal hyperplasia in AVGs. This review has discussed the roles of fluid-wall shear stress and endothelial cells on AV access problems and provided an overview of the methods for characterizing shear stress in AV access. In the coming years, investigations should aim at understanding how different vascular wall cells (i.e., endothelial cells and smooth muscle cells) integrate various hemodynamics stresses (i.e., pressure, circumferential wall stress, fluid-wall shear stress), as well as confounding factors that modulate the impact of these hemodynamic stresses on cells, thereby creating a coherent theory of the contributions of aberrant hemodynamic stresses to AV access failure.
The funding sources are NIH grants R01DK088777 and R01HL67646. The authors thank Mr. Jason Tey for his help in preparing this review.
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