out of the 10,807 patients enrolled for the original survey , access survival data were available for 7058 ( 65% ) patients .
these patients resided in portugal , the united kingdom , ireland , italy , turkey , romania , slovenia , poland , and spain .
the mean age was 63.515.0 years , 38.5% were female , 27.1% were diabetic , 90.6% had a native fistula , and 9.4% had a graft .
median dialysis vintage was 43.2 months ( minimum : 0.1 months ; maximum : 419.6 months ) .
access location was lower arm for 51.2% of patients . during the follow - up ,
prevalent needle sizes were 15 and 16  g for 63.7% and 32.2% of the patients , respectively ( 14  g : 2.7% 17  g : 1.4% ) . in spain ,
98% of patients were treated with 15-g needles , and in romania 75% of patients were treated with 16-g needles .
cannulation technique was area for 65.8% , rope - ladder for 28.2% , and buttonhole for 6% of patients , with some country preferences clearly visible : area technique was applied in as much as 77% of patients in romania , and rope - ladder was more common in poland than in the total study population ( 44% ) .
the direction of arterial puncture was antegrade for 57.3% of patients ; this was the preference for 99% of patients in poland .
the bevel orientation was upward for 70.2% of the patients , peaking in poland with 95% .
the practice of needle rotation after insertion was practiced for 42% of patients , with a much higher percentage in italy ( 82% ) .
the prevalent combination between arterial needle puncturing and bevel direction was antegrade with bevel upward ( 43.1% ) , followed by retrograde with bevel up ( 27.1% ) .
the proportion of the two other combinations , that is , antegrade and retrograde with bevel downward , was 14.2% and 15.6% , respectively .
the 15.6% with retrograde and bevel down were mainly treated in two countries ( spain and portugal ) .
median blood flow was 350400  ml / min . in italy and spain , 40% and 38% of patients
conversely , in slovenia and in poland 5455% of patients were treated with blood flows below 300  ml / min .
figure 3 shows the distribution of patients according to the prescribed needle size , blood flow , and venous pressure levels .
the primary outcome event ( i.e. , surgery for a new va during the follow - up period ) was observed in 1485 patients ( 21% ) .
univariate survival analysis revealed a significant benefit for access survival for patients who are younger , nondiabetic , male , have lower body mass index , do not take platelet antiaggregants , do not have heart failure , and are able to assist with compression .
a significant benefit was also seen for patients with fistula ( vs. graft ) , smaller needles , distal location of the access , and low venous pressure . with regard to cannulation technique ,
positive effects were observed for antegrade needle direction ( vs. retrograde ) , bevel up ( vs. down ) , nonalcohol - based disinfection , and application of local anesthesia .
although not statistically significant , a potential survival benefit was indicated for higher blood flow ( p=0.056 ) and buttonhole technique ( vs. rope - ladder and area , p=0.11 ) .
needle rotation did not affect the access survival ( p=0.81 ) , neither did access vintage ( age < 1 month before baseline vs. 1 month before baseline ; p=0.29 ) .
meier access survival curves according to blood flows , venous pressures , needle sizes , and cannulation techniques are presented in figure 4 . in a second step ,
after adjustment for age , gender , diabetes , va type , access location ( proximal vs. distal ) , dialysis vintage and heart failure , and incorporation of country differences , the use of a 16-g needle was associated with a significantly higher risk of access failure ( hazard ratio ( hr ) 1.21 ) compared with the use of a 15-g needle .
very few ( 1.4% ) patients were treated with the even smaller 17-g needles , but the direction of the results is the same , that is , increased hr for smaller needle size . using a blood flow of 300350 
ml / min as a reference , the hr tended to decrease as the blood flow increased . with regard to cannulation technique , both rope - ladder and buttonhole techniques performed significantly better than the area technique . considering antegrade with the bevel up as reference ,
the retrograde direction of the arterial needle with bevel down is associated with a significant increase of access failure risk of 18% .
all other options , that is , antegrade direction with bevel down or retrograde direction with bevel up , were not associated with a hr significantly different from 1.00 . with regard to venous pressure , using as reference the range between 100 and 150  mm  hg , the hrs increased proportionally to 1.4 , 1.87 , and 2.09 with the increase of venous pressure from 150 to 200  mm  hg , 200 to 300  mm  hg , and > 300  mm  hg , respectively ( all p0.008 ) . of note ,
venous pressures of > 300  mm  hg are extreme cases and were only recorded in 0.6% of the patients .
in addition , a venous pressure of < 100  mm  hg was associated with a significantly higher hr of 1.51 . to investigate this further
, we also looked for interaction effects between blood flow and venous pressure , as well as between arterial and venous pressures ; no significant associations were found .
finally , the use of a tourniquet and not applying any pressure at the time of cannulation were associated with hrs of 1.30 and 1.25 ( p<0.008 and < 0.02 ) , respectively , compared with exertion of arm compression by the patient at the time of cannulation ( labeled  patient assistance ' in table 1 ) .
in summary , this study revealed that area cannulation technique , albeit being identified as the most commonly used technique in this population of over 7000 patients , was inferior to rope - ladder and to buttonhole for maintenance of va functionality . with regard to the effect of needle and bevel direction ,
the combination of antegrade positioning of the arterial needle with bevel - up orientation was significantly associated with better access survival than retrograde positioning with bevel down . the use of larger needles tended to favor access patency , with 15  g being superior to 16 or 17  g. the application of arm pressure by the patient at the time of cannulation had a favorable effect on access longevity compared with not applying pressure or using a tourniquet .
results pertaining to the type and location of the access and the technical parameters ( i.e. , blood flow and venous pressure ) were as follows : there was an increased risk for access failure for grafts vs. fistulas , proximal location vs. distal , right arm vs. left arm , blood flows below 300  ml / min vs. those in the range of 300350  ml / min , and for the presence of a venous pressure > 150  mm  hg vs. pressures between 100 and 150  mm  hg .
tissue reparative processes triggered by cannulation procedures may cause enlargement of the fistula and the formation of aneurisms and scars that , in turn , can favor the development of stenotic lesions and ultimately impact fistula survival .
repetitive punctures at the va site cause vessel wall defects that are initially filled by thrombi before finally healing . of the three cannulation techniques ,
the buttonhole approach has the theoretical advantage of limiting the process of dilatation and fibrosis because the thrombus is displaced while being formed , favoring the formation of a cylindrical scar from the subcutaneous and vessel wall tissues .
the rope - ladder technique may have the initial advantage of favoring progressive maturation along the entire length of the fistula , but it requires fistula with sufficiently long segments suitable for cannulation .
the area puncture technique weakens the fistula wall and is associated with the least favorable consequences , that is , localized dilation , disruption of the vessel wall , and subsequent development of ( pseudo)aneurysms and strictures . despite this and the fact that area cannulation has been discouraged for over two decades , it was disheartening to observe that this was the predominant practice in almost two - thirds of patients . according to the ebpg and the clinical practice guidelines for va
, the rope - ladder technique should be used for cannulation of grafts . specifically , according to the latter ,
this study showed a 22% lower risk for va failure in those patients whose va was cannulated with the buttonhole technique as opposed to area , confirming the results of a recently published randomized controlled clinical trial .
although the buttonhole technique is associated with good results , one should also take into consideration that it is a practice performed in centers with highly trained personnel that work with strict protocols and that it may also be used for fistulas with only short segments available for cannulation . in our study
, this practice is used in 22 centers , mainly in portugal , turkey , the united kingdom , and italy .
research questions that arise from current guidelines address the effectiveness of structured cannulation training , increased remuneration for expert cannulators , and whether self - cannulation can lead to better outcomes . indeed , as buttonhole cannulation requires the designation of a reference nurse , especially for the initial 46 weeks , it is likely that this technique benefits from its association with centers offering the necessary training ( i.e. , centers capable of stemming the increased organizational effort and assigning the right cannulator to the right patient ) .
in addition , once the tunnel is created , cannulation can be performed directly by patients .
however , irrespective of the influence of cannulator training and center organizational issues , the underlying question to be addressed , optimally in a well - designed clinical study , is which cannulation techniques can be recommended to ensure long - term va functionality .
this study showed that retrograde direction of arterial needle with bevel down is associated with the least favorable outcome .
this is consistent with the findings of woodson and shapiro who reported that retrograde puncturing may be associated for an increased risk of hematoma formation , possibly owing to the related venous return of the blood ( i.e. , retrograde filling ) .
antegrade puncturing , on the other hand , may be considered fistula - protective by the same reasoning , that is , tract closure through flow force .
therefore , retrograde direction of the arterial needle is more likely to be associated with a higher risk for aneurism . despite recommendations by kdoqi to rotate the needle during insertion , the univariate analysis performed here found no evidence of any benefit of this practice . on the contrary ,
the authors share the opinion of many cannulators that the 180 rotation of the needle is unnecessary and may constitute an additional trauma to the va .
further studies are needed to clarify whether rotation of the va needle during cannulation should be recommended or not .
there are a number of possible reasons for the association of the higher failure risk with smaller needle sizes . while increased trauma and prolonged bleeding time are generally associated with the use of large needles , the use of small needles at the same blood flow results in a higher speed of the blood returning to the vasculature , possibly damaging the intima of the avfs .
for example , at an operative blood flow of 350  ml / min , the maximum speed of the injected blood will be 8.79  m / s with a 17-g needle and 5.80  m / s with a 15-g needle ( presented by ralf jungmann at vascular access coursestockholm , 1112 october 2012 , stockholm , sweden ) .
furthermore , the shear forces created by returning blood can have a role in inflammation and stenosis formation .
stenotic fistula and graft lesions are associated with the induction of the expression of profibrotic cytokines , local inflammation , and neointimal proliferation .
however , we can not exclude that this association may be a consequence of bias by indication .
needles of smaller inner dimension are generally prescribed not only for a new va but also for problematic avfs , that is , those likely to fail in the following months .
therefore , it is difficult to derive a conclusion from this association , but on the basis of figure 3 , 17-g needles are clearly linked to blood flow levels below 300  ml / min and , on the contrary , 14-g needles are mainly prescribed to patients with 350400  ml / min or greater blood flows .
it is also of interest to underline that higher venous pressure is mainly associated with the 16-g needles , which have a wider distribution of different blood flows .
measurement of venous pressure during dialysis is currently used as a surveillance tool within the dialysis session , and not as a standard monitoring strategy .
this study showed a significant and proportionally increasing risk of va failure with venous pressures higher than 150  mm  hg .
an increased hr was also detected for venous pressure below 100  mm  hg .
as shown in figure 3 , an association between needle size , blood flow , and venous pressure is indicated in that for needle gauges 15 , 16 , and 17 low venous pressures appear to be associated with low blood flows . such an association could be an indication of stenosis in the artery .
venous pressure is crucially dependent on the characteristics of the needle ( e.g. , the needle gauge , the length of the metallic portion , and the length and the thickness of the needle shaft ) , which vary among manufacturers . in this network , at the time of the study , the vast majority of the needles ( 85% ) were from a single producer and the length of the needle was 25  mm .
the unexpectedly high hr associated with a venous pressure of under 100  mm  hg compared with 150200  mm  hg should motivate reflection on the currently accepted limits .
one could consider integration of venous pressure monitoring into an algorithm for the detection of increased risk of access failure .
this study has certain limitations over and beyond those inherent to observational studies , for example , that residual confounding can not be completely ruled out .
being a retrospective study , patient data for those patients on dialysis before admission to the nephrocare clinic were not collectable , and thus robust information on the number of prior vaes , on their respective lengths , and on first cannulation was not available .
particularly , the missing information on the length of the va , its depth , and the access flow constitute a major weakness because a particular cannulation technique could have been chosen on the basis of what is possible with the given access characteristics .
in addition , the length of the access can influence the way in which the needles are placed . despite these missing data ,
we feel that this study has its merits , as it shows that traditional local practices have a significant influence on procedures exercised .
a further limitation is that the va practice was surveyed in april 2009 and was assumed not to have been changed during the follow - up ( 31 march 2012 ) .
however , as nursing practices in this field are strongly related to the clinic culture and experience , we have reason to believe that it is should not constitute a significant bias . of course
, some cannulation particulars , such as needle size and arterial blood flow , may vary over time , in that smaller needle sizes and low blood flow rate are used for initial access use and that large needles are taken for mature accesses .
however , we feel that the model selected here is also justified because it is an explanatory model , based on the association of baseline characteristics with access survival .
other limitations are that we had follow - up of 65% of the patients and that most countries were in europe ( owing to deployment of the electronic reporting system ) .
as reported , an association between clinical practice patterns and country has been detected , and consequently not all different practices were covered by our model .
however , according to the results of this analysis , each country has a combination of practices that positively and negatively influence the va survival .
for example , in romania , positive influences were the puncture direction being antegrade ( 82% ) , bevel orientation being predominantly upward ( 95% ) , and needle not rotated ( 84% ) ; negative associations were the use of area technique ( 77% ) , preferred needle size ( 75% with 16  g ) , and the use of blood flows < 300  ml / min ( 47% ) .
for this reason , intracountry correlations were considered using a sandwich estimator in the multivariate model . to assess the influence of individual center practices
, we also performed a sensitivity analysis by applying the sandwich estimator at the center level .
there were only negligible differences to the results obtained with the original model at the country level , raising our confidence that there is no severe confounding of the model by center practice effects .
given the relevant impact of the investigated variables on the survival of the va , itself a key driver of hemodialysis patient survival , we believe it is time to organize a large - scale randomized clinical trial to facilitate the formulation of practical and comprehensive cannulation practice guidelines . as the associations between practice patterns and va survival reported here are mainly related to national procedures and only partially related to actual patient limitations , they offer some promising indications for improving clinical practice .
in april 2009 , a cross - sectional survey was conducted in 171 dialysis units located in europe , the middle east , and africa to collect details on va cannulation practices on a clinic by clinic level .
all patients who were on double - needle hemodialysis or online hemodiafiltration during the week of the survey were selected for analysis , as long as a fistula or graft was used for va , survey data were complete , and follow - up data were available in our clinical database .
primary outcome was time until the first surgical access intervention resulting in the generation of a new access ( i.e. , as opposed to any surgical intervention done just for revision , thrombectomy , etc . , or any endovascular intervention ) .
patients were censored for transplantation , death , loss of follow - up , or end of the follow - up period ( 31 march 2012 ) .
information on cannulation retrieved from the survey comprised fistula type and location , cannulation technique , needle size , needle and bevel direction , needle rotation , blood flow , arterial and venous pressure , use of disinfectants , use of local anesthesia , and application of arm compression at the time of cannulation . to adjust for individual patient characteristics ,
the following information was extracted from the clinical database : patient age , gender and body mass index , prevalence of diabetes , and the use of ace inhibitors , platelet antiaggregants , and anticoagulants .
in addition , the median blood flow prescription was documented at a center level at the time of the survey . for univariate analysis , kaplan 
meier curves were calculated and comparisons were performed using the log - rank test . by combining univariate results with medical and statistical experience , a set of variables for multivariable analysis was determined .
in particular , specific interaction terms ( e.g. , bevel vs. arterial needle direction ) were defined for statistical examination , and decisions were made regarding their inclusion or omission in the cox model depending on their significance or collinearity , respectively .
a final cox model based on these variables was calculated , using the sandwich estimator to account for within - country correlation .
step by step , the final model was reduced , setting a p - value of 0.1 for variable inclusion .
all analyses were performed with sas v9.2 ( sas institute , cary , nc ) .
 hemodialysis patient survival is dependent on the availability of a reliable vascular access . in clinical practice , 
 procedures for vascular access cannulation vary from clinic to clinic . 
 we investigated the impact of cannulation technique on arteriovenous fistula and graft survival . 
 based on an april 2009 cross - sectional survey of vascular access cannulation practices in 171 dialysis units , a cohort of patients with corresponding vascular access survival information was selected for follow - up ending march 2012 . of the 10,807 patients enrolled in the original survey , access survival data were available for 7058 patients from nine countries . 
 of these , 90.6% had an arteriovenous fistula and 9.4% arteriovenous graft . 
 access needling was by area technique for 65.8% , rope - ladder for 28.2% , and buttonhole for 6% . 
 the most common direction of puncture was antegrade with bevel up ( 43.1% ) . 
 a cox regression model was applied , adjusted for within - country effects , and defining as events the need for creation of a new vascular access . 
 area cannulation was associated with a significantly higher risk of access failure than rope - ladder or buttonhole . 
 retrograde direction of the arterial needle with bevel down was also associated with an increased failure risk . 
 patient application of pressure during cannulation appeared more favorable for vascular access longevity than not applying pressure or using a tourniquet . 
 the higher risk of failure associated with venous pressures under 100 or over 150  mm  hg should open a discussion on limits currently considered acceptable . 
RESULTS
DISCUSSION
MATERIALS AND METHODS