Indications
Patients with Chronic Kidney Disease have kidneys that aren't working. Therefore, they may require hemodialysis to clean the blood of its waste and extra fluid. They typically receive hemodialysis through either an arteriovenous (AV) fistula or graft, although some may receive treatment through a dialysis catheter. An AV fistula is created surgically when a vein, usually in the patient's arm, is connected surgically to an adjacent artery. An AV graft is created surgically when a graft (essentially a soft tube) is placed under the skin in such a way that one end of the graft is connected to an artery and the other end is connected to a vein. The purpose of both procedures is to create a conduit through which dialysis can be performed. In a patient with an AV fistula, that conduit is actually the vein downstream from the actual connection to the artery. In a patient with an AV graft, that conduit is the graft itself. Two needles are placed within the conduit so that blood can be removed, be passed through the dialysis machine, and then returned into the body.
Over time, the flow dynamics within an AV fistula or AV graft can change. This typically occurs because the high flow passing through the conduit and the native veins draining blood from the conduit is often higher than it is naturally. This can affect the lining of the outflow vein, which in turn can narrow the outflow vein and compromise flow. When this occurs, patients may present with increased pressures during dialysis, a prolongation of the amount of time it takes for the patient to stop bleeding after dialysis, or arm swelling. In time, this can lead to clot formation within the fistula or graft, which prevents the patient from receiving hemodialysis. This can be detected when no flow in the graft or fistula is appreciated. Patients experiencing these symptoms often undergo a dialysis fistulagram to study their AV dialysis graft or fistula in order to determine if there is a narrowing of a vein that is compromising flow through the system.
Procedural Details
The goal of an AV fistulagram is to inject contrast into an AV graft or fistula in order to visualize the connection to the artery, the conduit, and the outflow veins from the conduit through the heart. In order to perform this procedure, a small catheter needs to be placed inside the conduit. The skin over the graft or fistula is prepped and a sterile drape is placed over the planned access site. Once local anesthesia has been applied to the access site, a small catheter is placed into the graft or fistula. At this point, multiple injections of contrast are performed and x-ray images are obtained in order to assess the system.
In most patients, a narrowing of the vein can be detected during this procedure. When this is seen, the goal of the procedure is now to widen the vein so it can accommodate more blood flow which, in turn, should address the patient's symptoms. Angioplasty is the primary means by which a narrowing of the vein is treated. The angioplasty balloon is positioned across the area of narrowing and inflated. Patients may feel some slight discomfort when the balloon is inflated but this goes away once the balloon is removed. If an angioplasty does not sufficiently treat the area of narrowing, we can consider placing a stent within the vein. A stent is a tube made of reinforced wires that goes inside the vein and stays inside the vein in order to widen the area of narrowing. Over time, the inside of a stent can become narrowed as well, which is why we often reserve stent placement for patients treated with angioplasty and experiencing a return of the narrowing inside vein after a short amount of time.
If blood flow inside an AV graft or fistula is compromised sufficiently, the entire system can become clotted or thrombosed. When this occurs, blood is no longer flowing and patients are unable to undergo dialysis. Therefore, steps must be taken to restore flow to the graft or fistula. The "declotting" procedure performed by IR involves the use of specialized devices or drugs to remove clot from the AV graft or fistula. Once the clot has either been dissolved using medications or removed using these devices, and flow has been restored, it is often possible to identify an underlying area of narrowing that predisposed the patient to clotting. This area of narrowing is then treated using angioplasty or stent placement, restoring more optimum flow to the system.
Results
Several studies have been performed evaluating the results after procedures to optimize flow in dialysis grafts and fistulas. Angioplasty can be used successfully to treat venous disease in these patients, leading to clinical success in >95% of patients (Heye S, et al. 2014). However, the durability of this success is limited. For example, Rajan DK, et al (2003) has shown that angioplasty has limited success at treating patients with cephalic arch stenoses, with primary patency rates of 76%, 42%, and 23% at 3, 6, and 12 months. This is why patients undergoing angioplasty often have a recurrence of their symptoms and require repeat procedures within a few months.
Stents do have a role in the treatment of dialysis patients. In a 2016 review article by McLennan, the results of stent placement in dialysis grafts and fistulas were summarized. Chan MR, et al (2008) reported that stents increase flow and primary patency rates in AV grafts but did not improve patency rates when treating AV fistulas. Haskal, et al demonstrated success with using a covered Flair stent (CR Bard, Tempe, Arizona). He demonstrated an improvement in 6 month primary patency rates compared to angioplasty (51% vs. 23%). In addition, he showed that stent placement was associated with improvement in access circuit patency (32% vs. 20%) and freedom from repeat interventions (32% vs. 16%).