 |
||||
| 2007 JASN IMPACT FACTOR 7.111 | HOME AUTHOR INFO EDITORIAL BOARD SUBSCRIBE FEEDBACK ALERTS HELP | |||
| CURRENT ISSUE | ARCHIVES | JASN Express | ONLINE SUBMISSION | |
|
||||
| 2007 JASN IMPACT FACTOR 7.111 | HOME AUTHOR INFO EDITORIAL BOARD SUBSCRIBE FEEDBACK ALERTS HELP | |||
| CURRENT ISSUE | ARCHIVES | JASN Express | ONLINE SUBMISSION | |
BRIEF COMMUNICATION |
*
Division of Nephrology, Hypertension and Transplantation, Department of
Medicine, University of Florida, Gainesville, Florida
Department of Radiology, University of Florida, Gainesville,
Florida.
Correspondence to Dr. Edward A. Ross, Director, End-Stage Renal Disease Program, Division of Nephrology, Hypertension and Transplantation, University of Florida, Box 100224, Gainesville, FL 32610-0224. Phone : 352-392-3756 ; Fax : 352-392-3581 ; E-mail : rossea{at}medicine.ufl.edu
Abstract
Abstract. The maintenance and longevity of hemodialysis vascular access remains one of the most problematic topics in the care of dialysis patients. Although much attention has focused on neointimal hyperplasia, the repetitive trauma to vessel walls by dialysis needles causes significant cumulative damage that has undergone little investigation. Commercial needles have beveled tips with intentional cutting surfaces to ease insertion. It was hypothesized that a pencil-point conical-shaped needle would cause less damage by taking advantage of the elasticity of native fistulae and produce an improved hole configuration in synthetic materials with minimal ability to stretch. A needle was subsequently designed with a removable pencil-point trocar and a side arm for the dialysis tubing. Once the trocar is removed, the blunt-ended cannula can be advanced or can be subject to inadvertent motion without causing damage to the luminal surface of the access. The new design as well as standard 15-gauge hemodialysis needles were tested on Gore-Tex® graft material and two bovine carotid artery preparations. Scanning electron microscopy was used to study the hole patterns. For all materials, the commercial needle holes had typical crescent shapes, and the cuts sliced sequentially through the various layers. For grafts, the new design caused a linear defect parallel to the axis of the graft that may preserve longitudinal strength. Interestingly, that tear line was nearly perpendicular to the linear hole in the thin polytetrafluoroethylene overwrap, which would be consistent with maintenance of hoop integrity. It is believed that these nonoverlapping defects would also improve hemostasis. The bovine specimens tested the importance of tissue stretching : Fresh carotid artery had experimental holes dramatically smaller than those from standard needles. In the denatured tissue, the experimental needle provided less benefit than that observed in fresh tissue, which is likely due to limited elasticity of the preserved artery. Improvement in needle design thus provides distinct advantages for native vessels and unique less traumatic holes in current synthetic materials. Pencil-point needle designs may be particularly applicable to the development of new elastomeric graft material.
The maintenance of hemodialysis vascular access has become one of the most important problems in the care of patients with end-stage renal disease. Much attention has focused on the superiority of native over synthetic materials (i.e., expanded polytetrafluoroethylene [ePTFE]), monitoring techniques, and interventions that may prolong their life span. In the particular case of synthetic grafts, it has been hoped that procedures which alleviate venous stenoses due to neointimal hyperplasia might be repeated as necessary to allow use for many years. Unfortunately, there has been relatively little investigation into the cumulative damage to the walls of the native or graft conduit by repeated needle sticks. The only commercially available dialysis needles have cutting surfaces, in that they have intentionally sharp edges at the beveled end of the thin-walled cannula. Cutting needles certainly facilitate insertion, but at the expense of significant damage to the wall and the sharp end remaining within the vessel during the entirety of the treatment. Advancing the needle or the inadvertent motion of the needle during the dialysis session can arc the sharp tip into the luminal walls and thereby cause further injury if not overt penetration through the other side. Charara et al. (1) clearly demonstrated the ultrastructure of the hole patterns and quantified the cross-sectional damaged surface area. By tallying the cut surfaces, they were able to project shredding of the graft as related to the needle size and available surface area of typical graft lengths.
We hypothesized that new less traumatic needle designs would be of benefit, especially for synthetic graft materials, because they would not have the reparative ability of native vessels. Interestingly, fine needles with conical ends and sharp tips have been available for interventional radiology purposes for decades. Because in that setting there was the clinical impression of less vessel injury (with minimal bleeding on insertion and removal), we devised a blunt-ended thin-walled needle with a removable pencil-point trocar for hemodialysis purposes. We believed that there would be elastic stretching of the hole in native vessels and an improved hole configuration in synthetic materials. The new design was tested on the Gore-Tex® product as an example of ePTFE material with minimal ability to stretch. Two bovine carotid artery preparations were chosen to specifically address the issue of elasticity : fresh artery compared to commercially available "modified" tissue with its elastic quality attenuated by the manufacturing process. Ultrastructural analysis and quantification of the various hole patterns demonstrated superiority of the experimental design over that of standard commercial beveled cutting needles.
Materials and Methods
Materials
We designed the new needle (Figure
1) to have a trocar that once removed, through a self-sealing
diaphragm, would allow blood to flow through a side port into the polyvinyl
chloride dialysis tubing. If so desired, a groove could be placed along the
axis of the trocar that would permit "flashback" of blood to
indicate entry into the vessel lumen. The cannula itself is thin-walled and
has a blunt end so that damage to the vessel's luminal surfaces by needle
motion would be minimized. For the purpose of the ultrastructural studies, we
manufactured mock needles consisting of solid stainless steel shafts ground in
a conical (pencil-point) shape to sharp points. The rods were purchased in a
15-gauge diameter, and this size was confirmed by measurement with a
micrometer. The commercial comparison needles were of the same diameter and
were made for hemodialysis (15-gauge ; Terumo Corp., Elkton, MD).
|
The needles were tested on three types of material : Gore-Tex®, commercial modified bovine carotid artery marketed as dialysis conduit, and freshly harvested bovine carotid artery. The Gore-Tex® graft material (W. L. Gore & Associates, Flagstaff, AZ) was 6 mm in diameter and was punctured by the needles at a 45° angle while being distended by pressurized tap water. The conduits were subsequently cut longitudinally to lay the surfaces flat for microscopy. It was important to perform the hole punctures before cutting through the wrapping layer of PTFE because that offset layer needed to be intact to maintain its structural role. This was not an issue for the bovine vessels. The commerical modified bovine heterograft material (Artegraft, Morristown, NJ) was supplied in a sterilizing solution consisting of 1% propylene oxide in 40% aqueous ethyl alcohol. After puncture, the segments were preserved in 70% alcohol solution until further study. The fresh bovine carotid artery was harvested within minutes after slaughter (courtesy of the University of Florida School of Veterinary Medicine), transported in a physiologic buffered solution, and studied immediately. Subsequent to needle puncture both the commercial and fresh bovine tissue were handled similarly. The arteries were immersed in a 1% glutaraldehyde solution in Tyrode's buffer for 2 to 4 h at room temperature and then overnight at 4°C. The tissue was then rinsed in Tyrode's buffer, and approximately 1.5-cm2 samples of the areas that were pierced by the needles were mounted on rigid supports. The samples were dehydrated in a graded series of acetone and dried by heating to the critical point in CO2 in a SAMDRI 790 critical point dryer (Tousimis Research Corp., Rockville, MD).
Microscopy
Arterial and Gore-Tex® samples were mounted on aluminum
stubs and sputter-coated with palladium-gold in an SEM Coating Unit E5100
(Polaron Instruments, Line Lexington, PA). Samples were observed with an
International Scientific Instruments model DS 130C scanning electron
microscope (Topcon Technologies, Paramus, NJ). A series of photomicrographs
was obtained at identical magnifications to compare hole patterns with the
experimental and commercial needles. For the arterial samples, the layer of
intima was then teased off with fine-pointed forceps, and new images were
obtained of the underlying hole pattern. The surface areas and perimeters of
the needle holes were determined using a Dage 72S CCD video camera (Michigan
City, IN) and MicroComputer Imaging Device software (Imaging Research, St.
Catharines, Ontario, Canada). This was only possible for the PTFE images,
since a single plane for measurement of the entire hole perimeter was not
consistently present in the bovine samples regardless of whether the intima
was intact. Quantification was performed in triplicate at two puncture holes
by each type of needle.
Results
Ease of Cannulation
Compared with the commercial cutting needles, the experimental devices were
subjectively only slightly more difficult to insert into
Gore-Tex® material. The modified bovine material provided
somewhat more resistance to pencil-point needle puncture, but at a level
easily acceptable for patient use. The fresh carotid artery material, however,
was quite difficult to puncture with the experimental device, consistent with
it lacking the cutting surfaces.
Gore-Tex® Microscopy Findings
The commercial cutting needles produced a hole pattern consistently similar
to that reported previously
(1). As illustrated in
Figure 2a, there was a
crescent-shapped hole with small tear marks on each side in some views. In
addition, there was a small flap corresponding to the location of the
elliptical opening at the end of the needle. The needle thus sliced through
the thin PTFE overwrap and then into the underlying main structure of the
conduit, thereby producing an identical corresponding hole pattern in both
layers. Because the cutting surface was in the shape of a crescent, the arc
sliced through the main layer of the graft in a configuration that would
disrupt both its longitudinal and circumferential integrity. This was in
marked contrast to the experimental needle. Because the PTFE had negligible
elasticity, the conical point spread and then tore the layers of the material
in their structurally weakest directions. Because the thin overwrap is applied
circumferentially at an approximate 45° angle to the longitudinal conduit,
the tear patterns of the two layers are in different directions and do not
overlap. As depicted in Figure
3, the holes appear grossly to be straight slashes, running nearly
perpendicular to the axis of the conduit for the thin overlay and parallel to
the axis for the main underlying tubular graft.
Figure 2b demonstrates the
latter linear puncture pattern, which does not appear to disrupt the
longitudinal integrity. Higher magnification in
Figure 2c shows the node
pattern of the extruded PTFE substructure and allows an appreciation of the
plane of node disruption at the edge of the hole. Quantification of the hole
sizes revealed similar measurements for the experimental and commercial
devices (mean of 1.4 versus 1.2 mm2 for surface area, and
6.8 versus 5.1 mm for the perimeters). However, the effective surface
area of the standard needle's crescent defect may have been much higher
because of the potentially limited structural value of the unsupported
flap.
|
|
Modified Bovine Carotid Artery Heterograft Microscopy Findings
Because the serosal surface of the bovine product was so obscured by
connective tissue, images were limited to views obtained from the intimal
surface. The standard beveled cutting needles demonstrated their typical
crescent-shaped hole patterns (Figure
4, panel a with the intima and panel b without it), which were
again markedly different from those produced by the experimental device. The
pencil-point needle caused rather irregular hole patterns that appeared
different in the intima and media layers, suggesting varying degrees of
elastic stretch before the nearly linear tearing of the tissue. Notably, the
linear direction of the hole in the intima was nearly perpendicular to that in
the media. The result of these nonoverlapping tear patterns was that there was
only a small opening traversing both layers
(Figure 4c), which was much
smaller than that caused by the commercial needle
(Figure 4a). Removing the
intima revealed the previously obscured defect in the media
(Figure 4d).
|
Fresh Bovine Carotid Artery Microscopy Findings
Figure 5a demonstrates the
crescent configuration of the hole produced by the commercial needles with the
intima intact. Two examples of the dramatically smaller holes produced by the
experimental design are shown in Figure 5,
b and c. Interestingly, the experimental holes were so small that
it was difficult finding them without the use of a field microscope to
properly dissect the tissue for electron microscopy. The striking difference
in hole size persisted after the intima was removed (two examples of the
commercial needle cut in Figure 6, a and
b, and the experimental hole in
Figure 6c).
|
|
Discussion
Gross anatomic study of excised hemodialysis vascular accesses demonstrates the cumulative damage to the native or synthetic vessel walls caused by repetitive needle punctures (2). Appreciation of the progressive weakening of the graft wall and ultimate development of pseudoaneurysms has also been of help in the design of new PTFE materials. Proprietary modifications of ePTFE have sought to address circumferential (or hoop) strength, although there are limited data in support of any particular material design. For example, the Gore-Tex® access product has a thin PTFE film wrapped around the main tubular conduit. Although the overlay may provide support, it is subject to the same needle trauma and may be lifted off from the underlying structure at the site of cannulation. Charara et al. (1) reported on the pattern of damage to ePTFE caused by current designs of commercial beveled edge (cutting) hemodialysis access needles. We confirmed their observation that there is a crescent-shaped slice with a contralateral tongue. With an angled needle insertion, the circular shaft would cause an elliptical hole distortion. Then, on removal of the needle, the displaced flap could theoretically recoil into place. The flap, along with any minor inherent elastic recoil to the material, would decrease the defect's size and perhaps improve hemostasis. Careful rotation of 15-gauge needle sites to make use of the entire length of a 25-cm graft could theoretically allow up to 5 yr half-life of available access material ; however, this would be dramatically decreased by the reality of not using (or not being able to use) the entire surface, pseudoaneurysm formation, stenoses leading to low flow or thrombosis, and synergistic weakening by unintentional contiguous connecting holes.
Although beveled cutting needles are appealing because of the ease of insertion, other designs are possible. Small pencil-point needles, typically 19-gauge size, were popular with some radiologists in the 1970s for arterial and venous radiocontrast media injections. Although there were no convincing data, the impression was that there was less vessel trauma as manifest by little bleeding after needle removal. The conical design has largely been supplanted by the easier-to-use beveled needles, with hemorrhage being minimized by the use of much smaller diameters.
We hypothesized that pencil-point needles would have distinct advantages in the hemodialysis setting. In the case of native fistulae, we hoped for significant stretching rather than tearing of the vessel with subsequent elastic recoil and only a small defect. With synthetic grafts (apart from a small component of elastic stretch), the underlying filament and node substructure could theoretically lead to tears along the the lines of preexisting weakness that could be in different directions in the separate layers. Our findings support a dramatic difference in hole patterns that is consistent with this concept. Interestingly, the manner in which the graft material tore was not what we had anticipated. The ePTFE tubing is extruded in a manner which creates circumferential rings of dense material that have a characteristic periodicity along the length of the conduit. These dense rings, or nodes, are connected by fine filaments and result in the alternating band/filament/band/filament repeating substructure that comprises the long tubular conduit wall. We had originally considered the possibility that because of only minimal stretching ability, the conical needle tip would cause tearing of the very fine longitudinal filaments. This would thereby allow separation of the dense rings and give the gross appearance of a slice perpendicular to the length of the tube. The observed hole pattern was opposite to this, namely a grossly linear defect parallel to the long axis of the conduit, which is consistent with rupture of the circumferential nodes and separation of adjacent fine filaments. Higher magnification of the ePTFE substructure away from the needle site provides an explanation for this phenomenon. Figure 2d demonstrates that in the manufacturing process, the nominally circumferential rings of material are often discontinuous or of irregular width, and these gaps or weaknesses would be the foci of the tear.
In the case of Gore-Tex®, the shape of the experimental needle hole was dramatically different from that of the typical standard needle's crescent slice. This makes it difficult to measure and interpret the finding of similar experimental hole perimeters and surface areas for the new and old devices. Because the flap of material contralateral to the crescent cut would be expected to have lost its hoop and longitudinal strength, the effective defect size may have been much larger than that measured by morphometry. We believe that much more important was the observation that the pencil-point needle hole in the main tubular Gore-Tex conduit had a different stretch-tear pattern than that in the overwrap. Thus, the holes in the layers did not coincide as they do with the cutting needle. The linear tears were at a nearly 90° angle, so that the openings did not overlap, and this may infer an additional hemostatic advantage to the new design (Figure 3). For the bovine materials, the ultrastructural images were consistent with our hypothesis that the greatest advantage of the new needle was that of stretching elastic material. The fresh tissue showed the most benefit, as opposed to the "modified" artery, which had been chosen because the preservation process denatured the tissue and attenuated its elasticity. The elastic recoil from the fresh vessel was so profound that the holes were difficult to find for microscopic sectioning and viewing. The finding of smaller holes by the experimental needle thus supported our hypothesis that stretching of the tissue by the conical point would be followed by elastic recoil upon needle removal and hence less permanent damage to the conduit. This is also consistent with the clinical observations of excellent radiology outcomes when used for arterial puncture as noted above. Interestingly, there was an unexpected benefit in the stretch-tear pattern of the layers of the denatured bovine material. Analogous to the synthetic graft layers, the linear tears in the bovine intima and media did not fully overlap because they were at angles to each other, and this would be consistent with improved hemostasis after needle removal.
Additional studies will be of importance in finalizing the design of the needle. The optimum length of the cannula as well as the ideal angle of the side arm will need to be determined so as to minimize resistance and maximize blood flow. In addition, although we do not anticipate that the walls of native fistula would be problematic with regard to the ease of needle insertion (having been somewhat difficult with fresh bovine carotid artery), this has yet to be determined for arterialized veins.
The benefit of noncutting needles may also be of importance for newer graft materials currently under design or just recently marketed. An appreciation of how structurally weak planes within synthetic materials determine tear patterns could help in the fabrication of better conduits. Of greater relevance would be the pursuit of elastomeric materials with maximum stretch and minimal tear characteristics that would be superb candidates for this pencil-point needle approach. There also is the need for additional studies of the pencil-point design's effect on native shunts, including the ultrastructural analysis of hole patterns in arterialized veins of native fistulae in animal models after single and repetitive punctures. In summary, these preliminary gross and ultrastructural findings support the further investigation of pencil-point needle designs to minimize trauma to native and synthetic hemodialysis vascular accesses, which may be an important factor in improving their longevity.
Footnotes
This work was presented in part at the 32nd Annual Meeting of the American Society of Nephrology, November 1-8, 1999, Miami Beach, Florida.
References
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
HOME
CURRENT ISSUE
ARCHIVES
JASN Express
ONLINE SUBMISSION
AUTHOR INFO
EDITORIAL BOARD SUBSCRIBE FEEDBACK ALERTS HELP |
