A modeling study of position and orientation of hemodialysis needles and the impact on vascular access

Patients with chronic kidney disease (CKD) need renal replacement therapy (RRT) and the favored method is hemodialysis (HD). An arteriovenous fistula is the preferred choice of vascular access, with two metal needles used to transfer the blood with waste to the dialysis machine and return the blood without waste from the dialysis machine to the patient’s body. However, wounds on the veins can cause blood clots which if left untreated could be life threatening. The hemodialysis needles can cause vascular wall abnormalities. The position and orientation of these needles might cause intimal hyperplasia (IH) and finally lead to blood clots. This study aims to analyze the hemodynamic effects on the vascular endothelium in AVF vascular access. A 15G needle was placed inside a vein at angles of 20°, 40°, and 90° with normal insertion and flipped needle in an idealized cephalic vein with the bore of the needle centrally located, conforming to standard cannulation practice. The 3D model created by SolidWorks consists of shaft, back eye, bevel, and vein only; the vein and 15G needle were assembled together. The mathematical model of blood rheology in this paper used Carreau’s law in the fluid domain. It was imported into Ansys CFX for calculation, a finite volume based software, which was implemented to solve the governing equations of the blood flow. The result showed that if the venous needle was not towards the venous return, a vortex appeared at the vein both upstream and downstream due to the venous needle, resulting in wall shear stress at the vein increasing significantly, and the flipped bevel seems to cause higher wall shear stress. For antegrade positioning of the vein vascular access return in the model when normally inserted, it was found that only 1% of the flow out of the back eye and vortex occurred downstream of the vein. Needles placed at a higher angle can increase wall shear stress and pressure on the vein. The flipped bevel is not significant in terms of either pressure or wall shear stress. However, when the needle is placed at a greater angle, wall shear stress and pressure on the vein were also increased. The flipped bevel caused higher wall shear stress and pressure when increasing the needle angle. Jetting from the venous needle causing intimal hyperplasia (IH) leads to blood clots, pain on the vein, and ultimately arm pain. Thus, the best cannulation procedure is for the venous needle to be toward the venous return, at a lower angle, and with the tip of the needle on the middle of the vein. Furthermore, the back eye should not be used for the venous needle but should be used for the arterial needle.


Introduction
The kidneys are a necessary organ responsible for filtering waste products from the blood, maintaining mineral and PH balance, and controlling blood pressure in the body.When they are abnormal or are not able to work, this is known as kidney failure, and finally becomes chronic kidney disease (CKD), in which kidney function is gradually degraded over a period of time until it cannot filter waste products from the blood, possibly leading to serious complications such as swelling of the organ, anemia, increasing of potassium levels in the blood, and heart disease, etc.The glomerular filtration rate (GFR) is a measurement of the capability of kidney function.GFR 90-100 mL/min is considered as normal kidney function, whereas less than 15 mL/min indicates end-stage renal disease [1].In such cases, patients need renal replacement therapy (RRT), for which the favorite method is hemodialysis (HD) [2].Preparation for RRT is the connection of vessels (artery and vein) or the arteriovenous fistula (AVF).An arteriovenous fistula is the preferred choice of vascular access, with metal needles used to transfer blood to and from the dialyzer (artificial kidney).Usually, the blood in blood vessels contains red blood cells, white blood cells, platelets, and lymph flow.The walls of blood vessels are like vascular tubes.Whenever the blood vessel appears wounded, the blood will clot due to the clotting factor in blood.Thus, the wound can cause blood clots and more blood clotting, which if left untreated could be life threatening.Hemodialysis needles can cause vascular wall abnormalities, while the position and orientation of needle insertion might cause intimal hyperplasia (IH) and can lead to blood clots.Also, recirculating flow within the blood vessel can result in endothelial dysfunction [3,4,5].Choosing a cannulation technique along with properties of dialysis needles such as needle type, needle gauge, needle length, and back eye that are suitable for each patient can help reduce dialysis needle injuries [6].Needle direction and bevel position (up or down) are often considered components of the cannulation technique for hemodialysis, which will vary from clinic to clinic [7,8].Uston et al. studied bevel position in conjunction with anticoagulant to examine the coagulation time in hemodialysis patients.It was found that the coagulation time of needle bevel down was shorter than that of needle bevel up [9].David et al. investigated venous needle flow to examine the fluid dynamic effects of cannulation by considering needle angle, blood flow rate, needle depth, and back eye.Shallow needle angles and reduced blood flow rates may reduce vascular damage.The back eye of the needle may slightly reduce wall shear stress [10].David et al. also studied the hemodynamics around arterial and venous needles using computational fluid dynamics to determine the blood flow rate, needle position, and needle orientation.It was found that with the needle at a shallow angle, the blood flow rate is approximately 300 ml/min, and placing the needle tip away from the vein wall might help to reduce the risk of endothelial damage that leads to IH [11].Several cannulation techniques have been studied as mentioned earlier in order to obtain an appropriate cannulation technique that can effectively reduce the damage incurred by hemodialysis patients.This study aims to analyze the hemodynamic effects on the vascular endothelium in AVF vascular access considering the hemodialysis needle position and orientation in an idealized cannulation model, using computational fluid dynamics (CFD) and Ansys CFX for solutions.
The arterial venous fistula (AVF) needles or dialysis needles are shown in Table 1 [12].It depends on the volume flow rate to determine which can be used with the patient, where by a high volume flow rate is better than a low volume flow rate in hemodialysis.The patients with an AVF which is not large enough need to use gauge 18G, while a patient with a sufficiently large AVF can use 15G.Most Thai people use gauge 15G.

Dimensions Gauge
Diameter × Length (mm × mm) In the process of hemodialysis, vascular access is essential using two AVF metal needles inserted into the vein.One needle transfers waste blood from the patient to the dialysis machine (arterial needle) and the other returns clean blood from the dialysis machine to the patient (venous needle) for performing efficient hemodialysis as shown in Figure 1    The metal needle selection for cannulation practice depends on blood flow requirements and the size of the patient's vein as shown in Figure 3 [16].The specific gauge of the needles used for cannulation should always be prescribed by the nephrologist in order to ensure that an adequate blood flow rate is achieved for the proper delivery of the dialysis prescription.

Materials and methods
For the mathematical model of blood rheology in this paper, Carreau's law is used in the fluid domain.The rheological model demonstrates the behavior of the fluid that depends on the relationship between the deviatoric stress tensor and the strain rate tensor.In the domain with low shear rates and complex shear thinning blood rheology, the mathematical model can be expressed as Equation 1: Mathematical models such as Equation 1 are commonly known as Newtonian models, but unlike Newtonian models, due to the viscosity not being constant, it depends on a function of the shear rate that can be written as Equation 2: Carreau's law governs a type of generalized Newtonian fluid and it is one of the most widely used rheologic models for blood, expressed by following Equation 3. In this paper a 15G needle was placed at an angle of 20, 40º and 90º in an idealized cephalic vein with the bore of the needle centrally located, conforming to standard cannulation practice [15].The 3D model was created by SolidWorks, consisting of a shaft, back eye, bevel, and vein only.The vein and 15G needle were assembled together for each case as shown in Figures 5 and 6, which are different in their blood flow direction.The flow rate of the needle is 400 mL/min with a Reynolds number of 1862, and the flow rate of the vein from Doppler ultrasound is 1,575 cc per minute, with an actual vein diameter giving a Reynolds number of 326.The created models were imported into Ansys CFX for calculation.This finite volume-based software was implemented to solve the governing equations of the blood flow.The key process output variables (KPOV), or responses, were velocity, wall shear stress in veins, and pressure.The velocity and the mass flow rate were applied at the vein and the needle, respectively.

Results
In this investigation, 15G venous needles were placed at different angles to assess the epithelial damage using numerical modeling.This modeling study found that in the case of flip flow, the venous needle was not toward the venous return, and the wall shear stress at the vein was increased significantly.The flipped bevel seemed to have higher wall shear stress even though the needle was placed at the center of the vein and at an angle of 20 degrees.The upstream vein reinforcement leads to high wall shear stress.There is a higher vortex both upstream and downstream for the flipped needle, with smaller angles leading to a greater vortex.Both the normally inserted and flipped bevel yielded the same trend.This is the reason that insertion of the venous needle not toward the venous return is not preferred.Also, needles placed at a greater angle can damage the endothelial layer as shown in Figure 7.For antegrade positioning of vein vascular access, the venous needle point was toward the venous return, so the model was normally inserted and it was found that only 1% of the flow out of the back eye and vortex occurred downstream of the vein.The needle placed at a higher angle is to increase wall shear stress and pressure on the vein as shown in Figure 8.As for the antegrade positioning of vein vascular access by flipped bevel, there is no flow out of the back eye, and the flipped bevel is not significant in terms of either pressure or wall shear stress.But when needle placement angle increased wall shear stress, the pressure on the vein also increased.The flipped bevel brought about higher shear stress and pressure when increasing the needle angle.There was a vortex which appeared downstream of the vein as shown in Figure 9.

Conclusion
This study analyzed the epithelial damage of vein vascular access when inserting needles by varying the angle in flipped and normal insertion using numerical modeling.The result from the modeling study revealed that needles placed at higher angles cause more damage to the endothelial layer, since high wall shear stress in excess of 40 Pa can damage the endothelial layer [20].Hemodialysis needles can exert high shear forces, and create a vortex downstream in the vein within the vein blood vessels, which can result in endothelial dysfunction and injury to the vein leading to arm pain [21].The tip of the needle near the vein wall might introduce severe damage to the vein vessel due to high wall shear stress and its sharpness.The result also found that if the venous needle was not toward the venous return, a vortex appeared at the vein both upstream and downstream due to the venous needle, resulting in the wall shear stress at the vein increasing significantly.The flipped bevel seemed to generate higher wall shear stress.
For the antegrade positioning of the vein vascular access return in the model when normally inserted, it was revealed that only 1% of the flow out of the back eye and vortex occurred downstream of the vein, and that placing the needle at a higher angle serves to increase wall shear stress and pressure on the vein.The flipped bevel was not significant in terms of increasing either pressure or wall shear stress when compared to normally inserted.However, when the needle was placed at a greater angle, wall shear stress and pressure on the vein also increased.The flipped bevel brought about higher shear stress and pressure when increasing the needle angle.Jetting from the venous needle causing intimal hyperplasia (IH) leads to blood clots, pain on the vein, and ultimately pain in the arm.Thus, the venous needle should point to the venous return and a small angle for the end of the needle on the middle of the vein is the best way.Moreover, the back eye should not be used for the venous needle but should be used for the arterial needle.The recommendation of this paper, for the patient with veins of 2 cm in diameter, the nephrologist can place the venous needle at 20 degrees.However, in practice, patients with high weight have small veins which are deep in the skin, and it is not possible to place the venous needle at 20 degrees or less, so the nephrologist needs to place the venous needle at 90 degrees or less, but reduce the flow rate and use 18G instead of 15G, which leads to lower treatment performance for RRT patients.
[13, 14].The standard cannulation for vascular access for RRT involves two cannulation procedures [15].The terms "antegrade" and "retrograde" are used to describe the direction of the arterial needle.Antegrade cannulation has the arterial needle pointing in the direction of the blood flow, that is, toward the venous limb.Retrograde cannulation has the arterial needle pointing toward the arterial anastomosis.The venous needle must always point toward the venous return.The arterial needle, on the other hand, may point in either direction as shown in Figure 2 [8].

Figure 3 .
Figure 3. Diagram recommendation using gauge and blood flow.

Figure 4 .
Figure 4. Basic structure of a hemodialysis needle.

Figure 7 .Figure 7 .
Figure 7.The 15G venous needle was placed an angle of (a) 20 (b) 40 and (c) 90 with flipped bevel of needle with the venous needle not toward the venous return (flip flow direction).

Figure 8 .
Figure 8.The 15G venous needle with antegrade positioning of the vein vascular access was placed at an angle of 20 normally inserted with the venous needle toward the venous return.

Figure 9 .
Figure 9.The 15G venous needle with antegrade positioning of the vein vascular access was placed at an angle of 20 flipped bevel with the venous needle toward the venous return.

TABLE 2 .
[19]the infinite shear rate viscosity  ∞ , so a Carreau fluid behaves as a Newtonian fluid again with viscosity.The rheologic models for the blood material properties of Carreau's Law can be seen in Table2.Material properties of Carreau's Law for CFX[19].
All parameters are material coefficients of Carreau fluid.The models in Equation 3 are non-Newtonian models in CFX and the parameters  and  control the transition region [18].At low shear rates (̇≪ 1  ) a Carreau fluid behaves as a Newtonian fluid with viscosity  0 .At intermediate shear rates (̇≥ 1  ), a Carreau fluid behaves as a power-law fluid.At high shear rates, the power-law depends on