Part one: Acute and occlusive thrombosis
In addition to rupture and bleeding, aneurysmal fistulas are prone to thrombosis which occurs in two forms: (1) acute and occlusive, or (2) indolent and laminated. Laminated clots were previously presented in a Michigan Vascular Access Facebook post (Sept. 27th, 2016)
“Acutely clotted aneurysms”, image #1
This patient had a renal transplant, and is not using the fistula, which is large, tortuous, complicated by outflow stenosis, and now clotted. Although it may be advantageous to keep a backup fistula functional just in case of transplant failure, in this patient the transplant had a good prognosis, the fistula was problematic and would most likely require multiple interventions, so the decision was made to sacrifice the fistula.
“Acutely clotted aneurysms”, image #2, with occlusive clot
This cross sectional utrasound image shows a large dilated fistula with a large thrombus. Unlike a typical PTFE graft, a thrombosed fistula can contain a significiant amount of clot, which, if released into the venous circulation, would embolize into the lung, and which could cause an acute compromise of right heart function. The options are to do an open thrombectomy to reduce the clot burden, or to do an extended chemical thrombolysis, or combine chemical and mechanical thrombolysis to reduce the risk. Some practitioners prefer not to tackle these cases at all. Frequently, one of our patients with a large fistula will check into another hospital for another problem and thrombose their fistula – no thrombolysis is performed, and the patient is discharged with a femoral catheter and instructions to come to my office.
“Acutely clotted aneurysm”, image #3 with occlusive clot
This specimen is clot from a thrombsed aneurysmal fistula removed during an open thrombectomy. Ths is just as large as the biggest clots from the lower extremities and pelvic veins. It is obvious that releasing this type and amount of clot would likely be fatal. In addition, it is unlikely that any amount of chemical or mechanical thrombolysis would be effective in making a purely percutanous procedure practical. Therefore, a cut-down on the fistula with open extraction of the majority of the clot is normaly required. Because most access centers are not prepared for an open procedure of this kind, the patient will normally get a catheter, and be sent back to the surgeon.
Acute and occlusive thrombosis
An aneurysmal fistula is prone to clot for the same reasons that any AV shunt clots: elevated venous pressures or insufficient inflow leading to access flow below 600 cc/min, and in some cases, hypercoagulability. Since aneurysmal changes occur most often in the setting of outflow stenosis, and since slowing of flow may not be always appreciated (“it had a good pulse”), aneurysmal fistulas will on occasion clot. At that juncture the fistula may become firm, erythematous, and tender (image #1). It is not unusual for the reddened and tender fistula to become regarded as infected, even in the absence of an elevated white count, temperature, or positive culture. Therer may be a concern about thrombolysis releasing septic emboli into the circulation. In fact, these are seldom infected, and thrombus is frequently inflammatory (e.g., like a thrombosed hemorrhoid).
Unlike the typical graft declot (percutanous thrombolysis), a large aneurysmal fistula can be hazardous and difficult due to the excessive clot burden (image #2). The volume of clot in a typical graft has been estimated at five to seven cc. Even if the clot cannot be extracted from a graft, that small volume of macerated clot released into the pulmonary circuation during a percutaneous thrombolysis rarely causes the patient any difficulty.
The situation is different with a clotted aneurysmal fistula. The clot burden can be substantial, and it may be unwise to release so much clot into the central circulation(Image #3). In this situation, an open thrombectomy reduces the clot burden, and revision of the access with aneurysmoplasty can be done at the same time.
After open extraction of the clot from an aneurysmal fistula, a Fogarty balloon is used to extract clot from the outflow, and then hopefully from the arterial end. When all the clot that can be extracted openly is removed, the fistulotomy or aneurysmoplasty is closed and the inflow control released. A good vibration and flow may be obtained, but more frequently the access is not running well. An imaging and interventional procedure is indicated.
The fistula is punctured and a sheath placed, then contrast is injected for imaging and further endovacular intervention. Very often, an inflow, intraccess or outflow stenosis is seen that most likely caused the thrombosis, and of course this must be addressed or the access is likely to rethrombose. Inflow, outflow, or intraaccess venoplasty may be required, and sometimes stent placement.
If good flow is restored, the procedure is uncomplicated, and with a normal potassium level, the patients are normally discharged to followup with their dialysis center.
Somethimes residual clot burden is noted even after everything else has been done, and frequently such patients are kept overnight for a heparin drip. Dialysis is done in the hospital the next day, and the patient re-examined. If everything has gone well, the patient may be discharged to the dialysis unit. Antiplatelet agents or anticoagulation may be prescribed. If persisitent dysfunction is detected, an immediate or short-term return to the OR may be recommended.
Laminated clot is usually non obstuctive. The patient and dialysis unit may not be aware it is in the fistula. The typical history is of difficulty cannulating, firmness of the fistula, frequent infiltration, pulling stringy clots, and the usual complaints that go along with elevated pressures and poor flows. The access may be pulsatile, or have a reasonable thrill, but areas of firmness in the fistula are evident. Ultrasound shows an eccentric clot in the fistula, which may have striations showing that it was laid down in layers …… more to follow in next blog.