Vascular access is the single most critical issue in patients with end-stage renal disease (ESRD) undergoing hemodialysis. It is obviously required for the procedure to take place, and it determines the adequacy of dialysis and, thus, a better patient outcome. The importance of a well-functioning and safe access can hardly be overemphasized.
Historically, the initial Scribner shunt with Teflon tubing made hemodialysis a reality, but it was associated with frequent complications.
The next advance in vascular access occurred in 1966 with the Brescia-Cimino arteriovenous fistula (AVF). The AVF made it feasible to safely provide dialysis with fewer access-related complications. In the earliest stages of hemodialysis, an AVF was the only access.
As the dialysis procedure expanded, though, the prosthetic access most commonly known as the polytetrafluoroethylene (PTFE) graft was developed.
The major advantages of the arteriovenous graft (AVG) were its readiness for use soon after placement and its ability to be used in patients without veins suitable for an AVF. The AVG soon became a preferred method in many surgical practices, a trend some have blamed on the reimbursement system, which favored graft use up until recently.
The double-lumen catheter was developed at about the same time as the graft, and it enabled dialysis to begin at the earliest, without having to wait for an AVF or AVG to mature. Originally intended as a temporary measure to provide an emergency or urgent dialysis access, the catheter became the access of choice for many patients.
There are significant disadvantages to the use of central venous catheters (CVCs) and AVGs, though. To encourage the preferred and safer mode of vascular access, namely the AVF, the Centers for Medicare & Medicaid Services (CMS) launched the Fistula First National Vascular Access Improvement Initiative, which recommends a 66% rate of AVF use.
The hallmarks of an optimal vascular access are good blood flow, few complications, and long survival. The AVF fulfills many of these goals, but the incidence of CVC use in dialysis units across the United States is still surprisingly high. Both patient- and physician-related factors play a role.
In incident dialysis patients in the United States, CVC use ranges from 56% to 71%,1,2 and in a national random sample of patients who started dialysis with a catheter, 56% still had that access 90 days after dialysis initiation.3
Among all vascular access options, however, the central venous catheter is associated with the most complications, including poorer clearance; frequent clotting; infection, such as bacteremia and endocarditis; septic shock; epidural abscess; and septic arthritis. There is a high frequency of central stenosis of the vein, which may preclude the placement of an AVF or AVG later. Nephrologists agree that a CVC is at best a short-term access and should be avoided for any longer period of time.
It is unclear why high incidence of catheter use persists despite evidence-based practice guidelines recommending the contrary. There may be several reasons, such as limited access to medical care, limited patient education, failure of AVF to mature, and delayed referral to the nephrologist.
In our own center, which has a population of 150 patients, at least 25% of patients still have a CVC as their access, despite a well-coordinated effort to reduce catheter use.
One of the main reasons for the persistence of catheters at our center is that roughly 60% of patients initiating dialysis here come with a CVC, reflecting the pre-ESRD care they received.
Only about 6% of our patients have no other means for dialysis access because several attempts to establish an AVF have failed, and only about 5%, despite all efforts, have refused to get an access and continue to be dialyzed via catheter. The remaining CVC patients are awaiting placement of a maturing fistula.
In reducing the incidence of CVC use, one critical factor is early referral to a nephrologist, who can then plan on providing the optimal care to these patients and preparing them for renal replacement therapy.
Issues with AVGs
The AVG has been in use for the last several years, and until 2000 it was the most common vascular access in dialysis patients in the United States.
It has the advantages of a shorter time to maturation, good blood flow, and suitability for patients with poor veins. There also had been a disproportionate reimbursement advantage to graft use.
It is widely believed that survival and complication rates with AVGs are much higher than with AVFs. A review of the literature supports better survival with an AVF over an AVG, but the evidence is not very impressive, and several studies have not shown any better patency with an AVF. Prospective studies are very few.
In a recent single-center study, the primary patency for an AVG was 39% at one year and 26% at two years, compared with AVF primary patency rates of 44% and 37% for similar periods.4
In terms of secondary patency, which describes the proportion of accesses that require a procedure in order to function, the rates were 71% and 63% for an AVG and 75% and 72% for an AVF at one and two years, respectively. There were no statistically significant differences.
The results may be confounded by the fact that they came from a Veterans Affairs (VA) patient population, but they may be true of the general dialysis population as well.
Nevertheless, AVGs are associated with more complications, mostly related to clotting and a higher incidence of infection compared with AVFs.
It is also speculated that prosthetic grafts in dialysis patients are associated with higher inflammatory responses. In our own ongoing investigations, we have been unable to confirm that (unpublished).
When an AVF Isn't Possible
While an arteriovenous fistula is the clear choice for almost all patients needing vascular access, a significant number of them are unable to get one because of poor veins or other factors. What is the best vascular choice in these patients?
CVCs have the advantage of ease of placement, but their complication rate is unacceptably high, so AVGs should be used instead. It is also believed that an AVG should be placed only five to six weeks before the anticipated initiation of dialysis because of shortened and limited patency compared with an AVF, but prospective studies are needed to ascertain optimal timing of graft placement.
Optimizing AVF Use
When an AVF is possible, on the other hand, it should be placed well before the anticipated start of dialysis, as about two months will be required for it to fully mature in most patients.
As a fistula will remain patent for a long time if properly done, it can be placed earlier than an AVG, probably when dialysis is anticipated to be required in six months.
In anticipating dialysis initiation, one should be guided by the progression of renal failure rather than any particular glomerular filtration rate (GFR). Patients progress at different rates, and a few may never require renal replacement therapy.
Most CKD patients are treated by a primary care physician, and educating these physicians about early referral to a nephrologist is crucial to success. A large majority of patients continue to be referred to a nephrologist within one month or less of requiring dialysis, and, under those circumstances, the only option may be to start treatment with a catheter. As discussed earlier, that is one factor behind the rate of catheter use.
Internists and primary care physicians should be assured that early referral to a nephrologist does not mean loss of the patient but rather the opportunity to have a joint strategy for optimal treatment.
Also, the preservation of a fistula or graft is crucial. The National Kidney Foundation Kidney Disease Outcomes Quality Initiative (NKF KDOQI) Clinical Practice Guidelines for Vascular Access beautifully suggest that the access examination take place in three steps: look, touch, and listen.
This recommendation should be followed at every unit for every single patient before dialysis is initiated. Its importance should be emphasized to members of the dialysis staff, and training should be provided.
The guidelines also recommend periodic surveillance of the access by techniques such as Doppler ultrasound, ultrasound dilution, and in-line dialysance. These techniques are probably useful in anticipating the problems that may occur in the near future so a preemptive correction can be made, but a recent meta-analysis and systematic review failed to show a clear advantage for any one technique over the others.5
Vascular access is vital to the well-being of a hemodialysis patient. It requires early coordination between primary care physicians and nephrologists and, once a patient is on dialysis, a well-coordinated and proactive approach to the surveillance and preservation of access. A dedicated dialysis team should work in close cooperation with vascular surgeons and interventional radiologists or interventional nephrologists.