The use of mechanical thrombectomy devices to treat thrombosed hemodialysis grafts

Thomas M. Vesely, MD

Dr. Vesely is Associate Professor of Radiology and Surgery at the Mallinckrodt Institute of Radiology at Washington University School of Medicine in St. Louis, MO.

Treatment of an occluded vascular access graft can be a challenging experience. Throughout the 1990s, there has been a plethora of published reports describing numerous percutaneous techniques for the treatment of thrombosed hemodialysis grafts. These techniques can be grouped into four general categories: 1) pharmacomechanical thrombolysis using urokinase, 2) pulsespray thrombolysis using heparinized saline, 3) balloon thrombectomy techniques, and 4) mechanical thrombectomy devices. No single method has been proven to be more efficacious than the others. There is reasonable evidence to support the belief that the long-term patency of a hemodialysis graft is independent of the method by hich he thrombus is removed, but is dependent upon identification and successful treatment of all significant stenoses and complete removal of the arterial plug.

A new type of endovascular tool, the mechanical hrombectomy device, has been developed to quickly, effectively, and safely remove thrombus from hemodialysis grafts. Mechanical Thrombectomy devices represent a new category of "tools of the trade". As such, most of us are still in the learning phase of how to best utilize these devices in our day-to-day practice. This article will review the techniques that interventional radiologists use for restoring blood flow to a thrombosed hemodialysis graft.

Mechanical thrombectomy devices are approved for use in hemodialysis grafts. Although interventionalists are beginning to utilize these devices for other applications, including treatment of thrombus in native veins and arteries, it is important to remember that mechanical thrombectomy devices have not yet received approval from the United States Food and Drug Administration (FDA) for these other applications.

There are now seven different mechanical thrombectomy devices which have been approved for use in polytetrafluoroethylene (PTFE) hemodialysis grafts (table 1). These include the "Clot Buster" Amplatz thrombectomy Device (ATD) (Microvena, White Bear Lake, MN) (figure 1); the AngioJet (Possis Medical, Minneapolis, MN) (figure 2); the Trerotola-Percutaneous Thrombectomy Device (PTD) (Arrow International, Reading, PA) (figure 3); the Cragg Thrombolytic Brush and the Castaneda Thrombolytic Brush (Micro Therapeutics, Inc., San Clemente, CA) (figure 4); the Gelbfish Endovac (Neovascular Technologies, Brooklyn, NY); the Oasis catheter (Boston Scientific Corp., Natick, MA) (figure 5); and the Hydrolyser catheter (Cordis Endovascular, Warren, NJ) (figure 6).

FIGURE 1. (A) The Amplatz Thrombectomy Device catheter and foot pedal. (B) The distal tip of the ATD, which contains the high-speed impeller for fragmenting thrombus, is shown here. (Images courtesy of Microvena, White Bear Lake, MN.)

FIGURE 2. The distal tip of the AngioJet catheter with its high-pressure saline jets. (Image courtesy of Possis Medical, Minneapolis, MN.)

FIGURE 3. The Trerotola-Percutaneous Thrombolytic Device (PTD) and handheld motor drive unit.(B) The distal tip of the PTD is a 9-mm diameter nitinol basket.

FIGURE 4. (A) The Cragg Thrombolytic Brush and motor drive unit. (B) The distal tips of the Castaneda Thrombolytic Brush (left) and the Cragg Thrombolytic (right) Brush catheters. (Images courtesy of Micro Therapeutics, Inc., San Clemente, CA.)

FIGURE 5. (A) The Oasis catheter. (B) The distal tip of the Oasis catheter over a guidewire.

FIGURE 6. The distal tip of the Hydrolyser catheter. (Image courtesy of Cordis Endovascular, Warren, NJ.)

The above mechanical thrombectomy devices can be divided into two categories based upon their mechanisms of action (table 2) ^1-3 . Recirculation-type devices create a hydrodynamic vortex, similar to a kitchen blender, which homogenizes the thrombus, converting it into a liquid slurry. The hydrodynamic vortex is created by either a powerful jet-spray of fluid (saline) or by a rotating high-speed micropropeller. Depending upon the specific device, the residual thrombus slurry may be aspirated and removed from the graft, or it may be allowed to embolize to the pulmonary arterial circulation. Non-recirculation-type devices, which include the Trerotola-PTD and the thrombolytic brush catheters (Cragg/Castaneda), utilize a rapidly spinning wire basket or plastic brush to mechanically breakup the thrombus into small fragments. Particulate thrombus which remains within the graft can then be aspirated or dissolved using urokinase.

There are no universally accepted methods for using these mechanical thrombectomy devices when treating a thrombosed hemodialysis graft. To the contrary, a wide variety of styles and techniques currently are utilized by interventional radiologists throughout the country. At this point in time, many interventional radiologists have used one, or maybe two, of the different mechanical thrombectomy devices, but very few radiologists are familiar with all seven of the FDA-approved devices. Hopefully, in the near future, clinical trials will compare these devices to determine if there are any significant differences in their performance. We may discover that each device has its own niche and is best utilized for certain specific situations.

Prior to performing a mechanical thrombectomy procedure, it is important to determine if the patient has a history of significant cardiac or pulmonary disease. Studies have revealed that fragments of thrombus can escape from the graft and travel to the lung as pulmonary emboli during the thrombectomy procedure. This can occur during a surgical thrombectomy as well but is more common when using endovascular techniques. Although most patients are able to tolerate these small pulmonary emboli without clinical sequelae, patients with significant pulmonary hypertension are at greater risk for acute cardiac decompensation. ⁴ Patients who have a history of right heart failure, pulmonary hypertension, or cardiac dysrhythmias also are not good candidates for mechanical thrombectomy procedures. These patients should undergo surgical thrombectomy, where venous out-flow can be occluded (clamped) to prevent any embolization of clot when the thrombus is removed from the graft.

The basic technique for performing a mechanical thrombectomy is similar to the method used for standard pulse spray thrombolysis. Intravenous sedation is frequently administered immediately prior to the procedure. Some radiologists also give a single dose of an intravenous antibiotic.

There are four basic steps to follow when treating a thrombosed hemodialysis graft using endovascular techniques: 1) an initial venogram is performed to evaluate the central and peripheral out-flow veins; 2) the thrombus is removed from the graft; 3) all significant stenoses are treated utilizing angioplasty, atherectomy, and/or a vascular stent; and 4) the arterial plug is dislodged. Each of these steps is further discussed below.

The initial needle puncture should be made in an antegrade direction into the arterial limb of the graft, approximately 3 cm from the arterial anastomosis. A standard angiographic catheter is then advanced into the graft over a guidewire. Leading with the guidewire, the catheter should be advanced through the venous anastomosis. A diagnostic venogram is performed to evaluate the graft and entire native venous out-flow, including that of the central veins. If a long segment stenosis (>7 cm), multiple sequential stenoses, or occlusions are identified in the native veins, the procedure is probably best terminated. It can be difficult to treat these extensive venous lesions, and even when they are successfully treated, the results usually are not durable. When possible, patients with these types of problematic lesions should undergo a surgical revision to extend the graft above the diffusely diseased segment. Occasionally, an entirely new graft may need to be created in a different location. More commonly, the diagnostic venogram reveals a single, focal stenosis at the venous anastomosis. This can be easily and effectively treated using angioplasty.

Although there are several different thrombectomy techniques, many interventional radiologists prefer to leave the venous anastomotic stenosis intact during thrombus removal. This stenosis serves to hold the thrombus within the graft, allowing it to be more readily removed, and prevents embolization of thrombus into the native venous and pulmonary circulation.

After the venogram, a vascular sheath is inserted into the graft. This provides access for the mechanical thrombectomy device and angioplasty balloon catheters. In addition, the sideport of the vascular sheath can be used to infuse dilute x-ray contrast material into the graft, which often helps to delineate the thrombus within the graft. This allows visualization of the thrombus during the thrombectomy procedure. Before inserting the mechanical thrombectomy device, heparin (2000 to 5000 units) is slowly infused into the graft. This prevents the formation of new thrombus during the procedure. The thrombectomy device is then inserted through the sheath into the graft. It is activated and slowly advanced under fluoroscopic observation. The thrombectomy device can then be appropriately positioned to fragment the thrombus. Generally, it takes only 2 to 5 minutes to completely remove the thrombus from the graft.

After removing the thrombus from the venous limb of the graft, the thrombus in the proximal arterial limb, which is trapped behind the vascular sheath, will then need to be removed. In order to reach the arterial limb of the graft, a second retrograde puncture is performed into the venous limb of the graft. Another vascular sheath is inserted, and the mechanical thrombectomy device is advanced into the proximal segment of the arterial limb. Occasionally, the vascular sheath in the arterial limb will need to be temporarily removed over a guidewire to facilitate thrombus removal from this segment of the graft.

At this point, there are two final steps to perform: angioplasty of the venous stenosis and dislodgement of the arterial plug. As previously mentioned, the most common location for a stenosis is at or just beyond the venous anastomosis. These lesions can be very resistant to dilatation, often requiring the use of high pressure (20 atm) angioplasty balloons for effective dilatation.

After the venous stenoses have been successfully treated, attention is directed to the "arterial plug" within the graft adjacent to the arterial anastomosis. The arterial plug consists of densely packed red blood cells and fibrin which are formed into a hard, bullet-shaped plug; this is often adherent to the graft wall. ^5-6 A Fogarty thrombectomy balloon can be used to dislodge this arterial plug.

Some interventional radiologists favor dislodging the arterial plug before performing the venous angioplasty. In this method, the venous stenosis traps the arterial plug within the graft, allowing it to be fragmented by the thrombectomy device. Other radiologists prefer to perform angioplasty on the venous anastomosis stenosis first. Opening the venous outflow channel allows the arterial plug to quickly exit the graft. This technique prevents the plug from getting caught up within the graft or at a venous stenosis, minimizing the risk of graft rethrombosis.

Studies have shown that the volume of the arterial plug is small (0.3 ml), representing minimal risk if it is allowed to embolize to the lung. ⁵

After the arterial plug has been dislodged, there should be brisk blood flow within the graft. A final fistulogram should then be performed to verify that all of the thrombus has been removed and the venous stenoses have been effectively dilated.

High flow vascular sheaths can be used if the patient is to undergo hemodialysis immediately following the procedure. Different types of high flow hemodialysis sheaths are available from several companies. The sidearm of a sheath has high blood flow capabilities, and can be directly connected to thehemodialysis machine. This type of sheath is advantageous for two reasons: 1) The radiologist does not have to hold compression on the graft puncture sites to obtain hemostasis, saving time for both the radiologist and patient; and 2) the patient does not have to be re-cannulated with needles for hemodialysis treatment. The nurse can directly connect the sheaths to the hemodialysis machine.

If the patient does not need hemodialysis immediately following the thrombectomy procedure, the radiologist may then use a purse string suture to close the vascular access sites. ⁷ This suture technique can substantially reduce the time needed to achieve hemostasis, particularly when large diameter vascular access sheaths have been used. The stitch is removed 2 to 3 days after the thrombectomy procedure, usually at the time of the next hemodialysis treatment.

Conclusion

In summary, an interventional radiologist has a wide variety of tools and techniques for treating thrombosed hemodialysis grafts. When compared to more traditional methods, the use of mechanical thrombectomy devices can substantially decrease the time required to perform thrombectomy procedures. No doubt, these new "tools of the trade" will also prove useful for other endovascular applications as well.     AR