The flow field near a venous needle in hemodialysis: A computational study

The flow field near a venous needle inhemodialysis: A computational study

David FULKER,1 Martin KANG,1 Anne SIMMONS,2 Tracie BARBER2

1Graduate School of Biomedical Engineering, University of New South Wales, Sydney, Australia; 2Schoolof Mechanical & Manufacturing Engineering, University of New South Wales, Sydney, Australia

AbstractThe vascular access used in hemodialysis can suffer from numerous complications, which may leadto failure of the access, patient morbidity, and significant costs. The flow field in the region of thevenous needle may be a source of damaging hemodynamics and hence adverse effects on thefistula. In this study, the venous needle flow has been considered, using three-dimensional com-putational methods. Four scenarios where the venous needle flow could potentially influencedialysis treatment outcome were identified and examined: Variation of the needle placement angle(10°, 20°, 30°), variation of the blood flow rate settings (200, 300, 400 mL/min), variation of theneedle depth (top, middle, bottom), and the inclusion of a back eye in the needle design. Thepresence of the needle has significant effect on the flow field, with different scenarios havingvarying influence. In general, wall shear stresses were elevated above normal physiological values,and increased presence of areas of low velocity and recirculation—indicating increased likelihoodof intimal hyperplasia development—were found. Computational results showed that the presenceof the venous needle in a hemodialysis fistula leads to abnormal and potentially damaging flowconditions and that optimization of needle parameters could aid in the reduction of vascular accesscomplications. Results indicate shallow needle angles and lower blood flow rates may minimizevessel damage.

Key words: Arteriovenous fistula, vascular access, hemodialysis, computational fluid dynamics(CFD)

INTRODUCTION

Arteriovenous fistulae (AVF) are the preferred vascularaccess in hemodialysis because of low rates of infection1

and good long-term performance in most patients.2

However, numerous complications can occur includingvenous stenosis,3 aneurysms at the anastomosis and areasof the vein wall4 and infections.5

Intimal hyperplasia (IH) results in thickening of thetunica intima of a blood vessel and can lead to stenosis inAVF. IH is believed to be caused by several different hemo-dynamic factors including low and high wall shear stress(WSS), oscillatory WSS, turbulence, and blood stasis,6–9

with WSS frequently used as a predictor of IH location.The development of adverse WSS conditions can be

affected by the direction of insertion of the venousneedle,10 the distance between the arterial and venousneedles,11 the angle of penetration of the needles,12 thenumber of side holes located on the needle and the geom-

Correspondence to: T. Barber, School of Mechanical &Manufacturing Engineering, University of New SouthWales, Sydney, NSW 2052, Australia. E-mail:[email protected] of interest statement: We have had no involve-ments that might raise the question of bias in the workreported or in the conclusions, implications, or opinionsstated.Financial disclosure: The authors declare no conflict offinancial interest.Grant or funding: None.

Hemodialysis International 2013; 17:602–611

© 2013 The AuthorsHemodialysis International © 2013 International Society for Hemodialysis

DOI:10.1111/hdi.12029602

etry of the needle.13,14 Red blood cell damage is highestnear the needle wall, where the velocities are low andshear stresses high.15 Turbulence created by the dialysisneedle has also been cited as a possible contributor to thedevelopment of IH.16,17

A number of complications can cause failure of vascularaccess in hemodialysis patients and a better understand-ing of the hemodynamics of the flow field around theneedles is required to design solutions that minimizevessel injury. Recently, computational models havebecome a feasible option for the investigation of the spe-cific hemodynamic conditions that may cause some ofthese complications. The purpose of the current study wasto examine the fluid dynamic effects of various aspects ofcannulation and to examine the subsequent effect on theWSS, an indicator of possible IH formation.

MATERIALS AND METHODS

The effect of venous needle placement angle (10, 20 and30 degrees), blood flow rate (200, 300 and 400 mL/min),needle depth (deep, central and top) and inclusion of aback eye in the needle were analyzed using a computa-tional model. The needle used in the study was modeledfrom samples obtained from Sydney Dialysis Centre(Crows Nest, NSW, Australia). The 15 gauge needles werescaled to have an inner diameter of 2.692 mm (D). Thevein was modeled as a straight cylinder with a diameter of7.4D and the length of the vein from the needle bore was56D, to allow for fully developed flow at the exit.

The geometries were constructed using two drawingpackages; the needle and vein were drawn and assembledusing Catia V5 (Dassault Systems, Velizy-Villacoublay,France) and then imported as an IGES file into CreoElements (PTC, Needham, MA, USA) and the final geom-etry was imported into ANSYS 13.0 (ANSYS Inc., Canons-burg, PA, USA). A hybrid mesh technique was utilizedwithin the ANSYS CFX-Pre workbench with an unstruc-tured tetrahedral mesh forming the underlying structure.The unstructured mesh allowed the capturing of secondaryflows created from the back eye, time-dependent flowfeatures around the needle and mixing of the two streams.Inflation layers were placed upon the vein wall and fluidwithin the needle to adequately capture the laminar bound-ary layer. Face sizing was imposed upon the external needlewall, bevel and back eye and used to capture the flowleaving the back eye and any vortices that may form aroundthe needle body. The global sizing parameters were set tocontrol the number of nodes within the mesh and the sizingwas based upon a mesh independence test, which is dis-cussed later.

The walls of the vein were assumed to be rigid andsmooth. The fluid properties were based on a dynamicsimilarity to blood matched via Reynolds number:

ReVD VD

= =ρ

μ ν(1)

and the Womersley parameter:

αρωμ

ων

= =D D

2 2(2)

where r is the density of the fluid under consideration; V isthe average velocity of the fluid; D is the diameter of thepipe through which the fluid flows; m is the dynamic vis-cosity of the fluid; v is the kinematic viscosity of the fluid;and w is the angular frequency of oscillations in the fluid.

The corresponding Reynolds numbers are 924, 1385and 1847 for the flow rates 200, 300, and 400 mL/min,respectively. The Womersley number of the pulsatilevenous flow was 3.6. Laminar flow was assumed.18–21

Blood flow rates of 200, 300, and 400 mL/min wereconverted into a velocity based on the internal diameter ofthe needle. A transient analysis was used to incorporatethe pulsatile waveform, based on a study conducted byEne-lordache et al.22 for a patient with an end-to-endradio-cephalic fistula. A second-order backward Eulerscheme was imposed to discretize the transient waveform.The advection scheme was set to high resolution to furtheraid in discretizing the transient waveform and each timestep converged between zero and 500 iterations. This levelof convergence was found to provide an accurate solutionwithin a reasonable computational time.

Grid independence testing was based upon the casewith the needle at an angle of 20°, placed centrally in thevein and a needle flow rate of 300 mL/min was used. Theelement size and number of inflation layers was changedto determine the minimum number of elements in whichthe flow field can be accurately captured. A steady-statesimulation was imposed with the maximum flow of thepulsatile waveform used as the vein inlet flow, equating toa constant vein flow rate of 2460 mL/min. The studydemonstrated that 1.6 million elements were required togain an accurate representation of WSS.

RESULTS

Effect of venous needle placement angle

Figure 1 shows the vector flow fields along the centralaxis at the point of maximum venous flow for the three

Venous needle flow in hemodialysis

Hemodialysis International 2013; 17:602–611 603

different angles of penetration. The location where theneedle jet impacts the vein wall changes with needleangle. As the needle angle decreases, this point movesfurther downstream. For the smallest angle (10°), littledisturbance to the overall flow field is seen, with thevenous jet clearly visible in the vector field. Some smallentrainment into the back eye is observed, with the fluidtaking on an upwards component just aft of the back eye.The upper region of fluid is smoothly integrated into thejet flow, while the lower region of fluid experiences someminor disruption as it meets the jet. As the angle isincreased to 20°, back eye fluid entrainment increases,and this increases further for the the 30° case. For the twohigher angles, significant fluid flow disruption is observedwhen the lower region of fluid meets the jet. The swirlingflow resulting from this disturbance is evident in thepathlines for this angle. Also observed in the pathlines isthe large expansion of the jet after impact, compared withthe 10° case.

WSS levels along the bottom of the vein for the threecases are shown in Figure 2. The results show a distinctspike where the needle jet impacts the wall. This spikereaches maximum values of 10.9, 25.0, and 41.8 Pa forneedle angles of 10°, 20°, and 30°, respectively. A higherneedle angle results in a higher WSS and the location ofthe maximum moves downstream with a smaller needleangle. The slope to the maximum for needle angles of 30°and 20° is sharp while the slope for a needle angle of 10°is more gradual. This indicates large spatial gradients forhigher needle angles. A recirculation zone is found behindthe needle jet with needle angles of 20° and 30°. Themagnitude of this recirculation zone is much larger whenthe needle angle is 30°.

Effect of needle flow rate

Figure 3 shows the velocity vector plot at the point ofmaximum venous flow for the three different blood flow

a

c

e

b

d

f

Figure 1 Pathlines, showing the flow field for the different needle placement angles. The pathlines represent the path ofparticles released from inside the needle and are colored by the local value of velocity. Velocity vectors are uniform length forclarity and colored by velocity. Depth is kept at a constant middle depth, flow rate is kept constant at 300 mL/min and a needleback eye is present in each case.

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rates. The needle jet is shown to hit the floor of the veinand continue along in this direction. In the case withlowest needle flow, the jet has significantly dissipatedwithin the vein approximately 100 mm downstream. Theneedle flow slightly increases the venous flow within thecenter of the vein, directly above the jet. This is mostclearly displayed in the case with highest needle flow. Inall three cases, the back eye flow is minimal and appearsto be an offshoot of the needle flow and occurs at the edgeclosest to the needle bevel. It is also evident that theincrease in needle flow has very small effect on theamount of flow from the back eye. Regions of recirculationare evident, downstream from the jet, at both maximumand minimum venous flow when the needle flow rate is400 mL/min. For the slowest needle flow rate, there are nodistinct regions of reduced velocity within the mainstreamvenous flow. This suggests that a lower needle flowreduces the effects of the jet on the mainstream flow. Inevery case, there is a distinct region of reduced velocitydirectly downstream from the needle insertion site. Thisregion is most prominent near the upper wall of the vein.This is due to the presence of the needle within the veinthat blocks the mainstream venous flow. A small recircu-lation zone also occurs behind the point where the needlejet impacts the bottom of the vein wall.

WSS distributions along the bottom of the vein for thethree cases are shown in Figure 4. The WSS along thefloor of the vein shows a dramatic spike where the needlejet impacts. The maximum WSS exerted on the bottom ofthe vein wall for needle flow rates of 200, 300 and400 mL/min are 12.7, 25.0, and 38.8 Pa, respectively. Themaximum values of WSS occur during minimum venousflow. Intuitively, an increase in needle flow rate results inan increase in WSS along the bottom of the vein. The WSSlevels remain elevated above their normal values at theoutlet. It is also clear that the spatial distribution betweenmaximum and minimum venous flow is almost identical.A preliminary spike in WSS shows the recirculation whichoccurs directly behind the needle jet.

Effect of needle depth

Figure 5 shows the velocity vector plot when the needledepth is varied. When the needle is placed at the bottomdepth, the jet impacts almost immediately against thevessel wall, and the pathlines show a rapid expansion ofthe jet diameter and some swirling downstream. Down-stream swirling is significantly reduced for the highestneedle placement. Vector plots for the top depth show thelower region of the vein fluid initially impacting the jetand experiencing a sharp change in direction. Furtheralong the jet, the main part of the fluid flow experiences amore gradual effect of interacting with the jet. Little effectfrom the inclusion of a back eye is seen in any of the threecases.

A low-velocity region is observed near the insertionsite when the needle tip is in the center and bottom ofthe vein, because of the presence of the needle divertingthe mainstream venous flow. As expected, the size of thelow-velocity region increases as the needle penetratesdeeper within the vein. Figure 6 show the WSS distribu-tion along the bottom of the vein for the three cases. Themaximum WSS occurs during minimum venous flow,when the effects of the jet within the vein are greatest.The maximum WSS for needle locations near the top,middle, and bottom of the vein are 16.4, 25.0, and45.8 Pa, respectively. WSS increases dramatically as theneedle moves closer to the vein wall and the effect of themainstream venous flow on the needle jet is less whenthe needle is closer to the vein wall. The distance to thelocal maximum is also greater when the needle is closerto the top of the vein. This displays the effect of themainstream venous flow in dissipating the effects of theneedle jet. The minimum WSS for the three needledepths are 0.03, 0.05, and 0.1 Pa and occur near theinlet. Furthermore, a recirculation region is evident

Figure 2 Wall shear stress (WSS; Pa) along the bottom of thevein for varying needle angle, at the maximum venous flow.The x-axis is the distance from the needle insertion, in termsof needle diameter (D). The dashed line represents the WSSdistribution for 10°, the dotted line for 20° and the solid linefor 30°. Depth is kept at a constant middle depth, flow rate iskept constant at 300 mL/min and a needle back eye is presentin each case.



behind the needle jet when the needle tip is at thebottom and middle of the vein.

Effect of needle back eye

The effect of the back eye was analyzed by comparing twosituations one with a back eye and one without a back eye.Both cases were run for the situation of 20° angle,300 mL/min flowrate and the middle depth. Only a smallamount of flow came through the back eye. In both cases,the absence of the back eye does not result in any dramaticchanges of the flow field. Interestingly, the flow appears tobe entrained into the back eye at the edge furthest fromthe needle tip. A high-pressure point is exerted on theleading edge of the back eye, causing the flow withinthe needle to shoot off the leading edge of the back eye.The low pressure region in front of the needle resultsin the entrainment of the venous flow into the high pres-sure region within the needle.

The WSS along the bottom of the vein varied slightlybetween the two needles. The maximum WSS for a needlewithout back eye is 25.6 Pa, whereas the maximum WSSfor a needle with back eye is 25.0 Pa. This suggests thatthe flow through the back eye reduces the velocity withinthe needle jet slightly. As such, a needle with a back eyemay result in a slightly lower WSS along the bottom of thevein.

DISCUSSION

The physiological WSS reported for healthy arteries andveins typically falls within the range of 1–7 Pa23,24 and0.1–0.6 Pa,23 respectively. Seven days after the creation ofan AVF, Remuzzi et al.19 measured WSS in the artery to befrom 5.5 to 11 Pa over the cardiac cycle. Values outsidethese ranges have been shown to initiate the cascade ofevents that lead to intimal thickening.6,25,26 Red blood cells

a

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e

b

d

f

Figure 3 Pathlines, showing the flow field for the different needle flow rates. The pathlines represent the path of particlesreleased from inside the needle and are colored by the local value of velocity. Velocity vectors are uniform length for clarity andcolored by velocity. Depth is kept at a constant middle depth, angle is kept constant at 20° and a needle back eye is present ineach case.

Fulker et al.


can suffer extensive damage at 150 Pa,27and the endothe-lial layer of a blood vessel will be stripped hindering theability of the wall to initiate clotting if WSS exceeds100 Pa.28 WSS values above 40 Pa have been shown todeform the cells in the endothelial layer and may result inerosion.29 In addition, to the effect of the local maximumWSS, another factor which must be taken into consider-ation is the time in which the endothelial cells or bloodcells can withstand WSS outside of the normal physiologi-cal range.

Table 1 summarizes the maximum and minimumvalues of WSS recorded for the models tested in this study.It is evident that a maximum WSS far exceeding thenormal physiological value is observed in every case, andthis occurs where the needle jet impacts the bottom of thevein. In no case did the WSS reach levels where theendothelial layer or red blood cells would become severelydamaged, however as the length of a dialysis session istypically, 4–6 hours, these results suggest that the endot-helial layer may suffer damage. Fry30 reported that theendothelial layer suffered damage within one hour ofexposure when the WSS was greater than 35 Pa. It is alsoevident in every case that the WSS falls below the normalphysiological range, providing the conditions for IH to

develop. In particular, the minimum values listed inTable 1 fall within the category specified as extremely lowWSS suggesting that the rate of IH formation may behigher than normal.

These results indicate that IH may occur in regions ofrecirculation. A key low-velocity region identified in allcases was near the needle insertion site. If IH developsnear the needle insertion site, it may slow the develop-ment of a buttonhole, if this technique has been adopted.It has been shown that platelet adhesion, aggregation andthe permeability of the endothelial layer increase whensubjected to low WSS.18 Furthermore, dead cells andother matter may accumulate within regions of recircula-tion increasing the risk of infection at the insertion site.Furthermore, these areas coincide with regions of recircu-lation which occur due to the needle jet. The recirculationregions downstream of the needle will vary in time due tothe pulsatile waveform and the prevalence of IH mayincrease due to these temporal WSS gradients.

The effect of the needle jet was most prominent whenthe venous flow was at minimum levels, demonstratingthat the mainstream venous flow plays a role in dissipatingthe needle jet before it impacts the vein wall. However, asthe difference between the maximum and minimumvenous flowrates was small, the needle jet was prominent150 mm downstream of the needle insertion in all cases.This result indicates that the endothelial layer along thebottom of the vein may be subjected to elevated WSS at asignificant distance downstream from the insertion site.

The development of recirculation zones downstream ofthe needle resulted in low WSS along the top of the vein.The prominence of the recirculation zone was greatestwhen the needle flow was at the highest levels. A degree ofswirling was also shown to initiate after the point of jetimpact and force the jet up the walls of the vein. This

Figure 4 Wall shear stress (WSS; Pa) along the bottom of thevein for varying needle flow rate, at the maximum venousflow. The x-axis is the distance from the needle insertion, interms of needle diameter (D). The dashed line represents theWSS distribution for 200 mL/min, the dotted line for300 mL/min and the solid line for 400 mL/min. Depth iskept at a constant middle depth, angle is kept constant at 20°and a needle back eye is present in each case.

Table 1 Summary of wall shear stress recorded

Case Parameter Max. WSS (Pa) Min. WSS (Pa)

Flow 200 mL/min 12.8 <0.01300 mL/min 25.0 <0.01400 mL/min 38.8 <0.01

Angle 10° 10.9 <0.0120° 25.0 <0.0130° 41.8 <0.01

Depth Top 16.4 <0.01Middle 25.0 <0.01Bottom 45.8 <0.01

Back eye Back eye 25.0 <0.01No back eye 25.6 <0.01

WSS = wall shear stress.



indicates that swirling is present throughout the vein. Italso indicates that a significant degree of mixing betweenthe two streams occurs. At the boundary of these twoflows, a high level of shear force ensues, which leads toinstability and the development of turbulent struc-tures.31,32 The turbulent nature of the flow could lead tosevere complications as suggested by Davies et al.33 whonoted that endothelial turnover had a greater response tolow levels of WSS in turbulent flow than higher levels ofWSS in laminar flow. In addition, it has also been shownthat disturbed flow causes an increase in endothelial cellturnover.34 Low WSS coincided with these regions ofrecirculation which supports the hypothesis that intimalthickening may occur on the top of the vein.

In this study, the highest WSS values along the bottomof the vein were recorded when the needle angle was 30°.In addition, the highest levels of WSS in the recirculation

behind the needle jet also occurred when the needle anglewas 30°. No recirculation occurred behind the point ofimpact when the needle angle was 10° as the shallowangle and longer distance to impact allowed the venousflow to dissipate the strength of the jet. Hence, the spatialgradients behind the needle jet are minimized when ashallow needle angle is implemented. The pathologicalimplications associated with a high anastomotic angle isthe formation of aneurysms and thinning of the endothe-lial layer on the bottom of the vein.35 The size of thelow-velocity region near the needle insertion siteincreased when the needle angle increased due to a greateramount of flow separation around the solid body at higherneedle angles. Hence, more intimal thickening maydevelop near the insertion site, leaving this region at agreater risk to pathological complications when the needleangle is high.

a

c

e

b

d

f

Figure 5 Pathlines, showing the flow field for the different needle placement depths. The pathlines represent the path ofparticles released from inside the needle and are colored by the local value of velocity. Velocity vectors are uniform length forclarity and colored by velocity. Angle is kept constant at 20°, flow rate is kept constant at 300 mL/min and a needle back eyeis present in each case.

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The size of the downstream recirculation increasedwhen the needle tip was closer to the top of the vein.Variations in depth between the arterial and venous dialy-sis needles may aid in avoiding placing the venous needlewithin a recirculation zone caused by the arterial needle.36

The needle jet was more prominent downstream whenthe needle tip was closer to the top of the vein. Thepresence of the needle jet on the side of the vein was alsohigher under the same conditions. When the needle iscloser to the bottom, a greater WSS is encountered at thepoint of jet impact. However, even when the needle wasnear the top of the vein, the WSS was above the physi-ological range. Therefore, the flow characteristics appearto lend themselves to intimal thinning along the bottom ofthe vein and intimal thickening along the top of the vein,regardless of depth.

Recirculation zones can result in a local deposit of par-ticles, which can aid in the formation of a stenosis.37

Recirculation zones were identified behind the impactpoint when the needle was located in the middle andbottom of the vein. This recirculation zone did not movein time or space; however, the magnitude was slightlyhigher when the needle was at the bottom of the vein.Conversely, no recirculation was identified when theneedle was closer to the top, as the effects of the needle

flow are reduced by the mainstream venous flow. Hence,the spatial gradients behind the needle jet are minimizedwhen the needle tip is closer to the top of the vein andkeeping the needle near the top of the vein may reduce therisk of IH development. Furthermore, the low-velocityregion at the site of insertion was larger when the needlewas closer to the bottom, as a greater surface area ispresented to the venous flow, which enhances flow sepa-ration around the needle. For the geometries examined, itwas observed that there was little flow passing through theback eye. A reason for inclusion of a back eye is to preventthe arterial needle suctioning to the vein wall38 and furtherstudies are planned to investigate the function and effectof the back eye in an arterial needle.

In this study, blood was modeled as a Newtonian fluid.This will decrease the magnitude of the results obtainedfor WSS as it has been shown that the incorporation ofnon-Newtonian fluid properties in computational analysiscan result in increases in WSS values of up to 10%.39,40 Afurther assumption is that the walls of the vein wereassumed rigid. In reality, the diameter of the vein willchange within the pulsatile cycle and it has been shownthat WSS values are reduced by up to 25% when a dis-tensible wall is used instead of a rigid wall.41 However,veins undergo an arterialization process after AVF creationwhere the vessel walls thicken in response to increasedpressure and flow20 and the effects of the assumption arebelieved to be minimal.

CONCLUSIONS

Vascular access is a crucial component in hemodialysistreatment, but long-term success rates are variable. Abnor-mal flow features caused by the placement of the venousneedle were identified, which can be associated with vas-cular complications including IH, stenosis development,thrombosis, aneurysm and infection. Flow rate throughthe venous needle, insertion angle, and depth were variedto identify an optimum position. The effect of the back eyewas also evaluated.

It was found that high levels of WSS occur along thebottom and sides of the vein. These are significantly abovethe normal physiological range quoted in the literature.These levels may cause an elevated endothelial turnover,endothelial erosion and platelet activation. Additionally,regions of low WSS were identified along the top of thevein and behind the needle jet, some of which fell belowthe level that has been shown to cause an increase in therate of intimal thickening.

The useful life of a vascular access can be extended byminimizing the chance of vascular complications, and

Figure 6 Wall shear stress (WSS; Pa) along the bottom of thevein for varying needle depth, at the maximum venous flow.The x-axis is the distance from the needle insertion, in termsof needle diameter (D). The dashed line represents the WSSdistribution for the bottom depth, the dotted line for themiddle depth and the solid line for the top depth. Angle iskept constant at 20°, flow rate is kept constant at 300 mL/min and a needle back eye is present in each case.



computational fluid dynamics has been shown to be aviable means of analyzing the effect the venous needle canhave on the hemodynamics within the vein. The results ofthis study suggest that optimization of these needleparameters could aid in the reduction of vascular accesscomplications. Early results indicate that inserting thevenous needle at a low angle as shallow as possible inthe vessel, inserting the arterial needle centrally within thevessel away from the recirculation zone caused by thevenous needle and using lower blood flow rates mayminimize vessel damage.

Manuscript received 20 October 2012; revised 16 January2013.

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The flow field near a venous needle in hemodialysis: A computational study

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