Thrombectomy is a mechanical interventional procedure by which a blood clot or thrombus is removed under image guidance using endovascular devices. Thrombectomy is most commonly used in acute cerebral ischemic stroke, although it is also a procedure used for clot removal in acute myocardial infarction (MI) and pulmonary embolism (PE). Mechanical thrombectomy utilizes various techniques. Most commonly, it uses different catheter-based therapies, including stent-retrieval, direct aspiration, or a combination of both.
The primary purpose of mechanical thrombectomy in ischemic cerebral stroke is rescuing the ischemic penumbra. The indications for mechanical thrombectomy have recently significantly increased, with recent literature proving its efficacy and superiority over intravenous (IV) thrombolytics. Some noteworthy trials are the DAWN, DEFUSE-3, CTP, and CRISP studies. The American Heart Association/American Stroke Association guidelines for the early management of acute ischemic stroke recommends the following criteria for endovascular mechanical thrombectomy therapy:
The DAWN and DEFUSE-3 trials demonstrated that mechanical thrombectomy improved clinical outcomes versus standard care alone in select patients with large vessel ischemic stroke presenting up to 24 hours after the start of symptoms. The current indications have extended the time of thrombectomy to 24 hours after the symptom onset.
Unlike in the case of cerebral stroke, recommendations for mechanical thrombectomy, specifically aspiration thrombectomy in cases of acute MI, remain doubtful. The TAPAS study (Thrombus Aspiration During Percutaneous Coronary Intervention in Acute Myocardial Infarction Study) suggested that routine thrombus aspiration improves microvascular reperfusion with improved outcomes at 30 days (the one-year follow up reported improved clinical outcomes) compared to traditional percutaneous coronary intervention (PCI) for a wide range of patients. However, the results from two subsequent studies, the TOTAL (ThrOmbecTomy with PCI vs. PCI ALone in patients with STEMI), and TASTE (Thrombus Aspiration in ST-Elevation myocardial infarction) trials indicated no significant improvement in clinical outcomes for acute MI patients undergoing aspiration thrombectomy.
The TOTAL trial compared PCI alone vs. PCI with thrombectomy in ST-elevation myocardial infarction 12 hours after MI onset and revealed an increased risk of stroke with routine thrombectomy in acute MI within 30 days of follow up. Researchers found similar findings when patients with a high thrombus burden. Based on these latter studies, the AHA downgraded the guideline recommendation for thrombectomy from class IIa to class III (routine TA is not recommended in acute MI cases before primary PCI).
The use of mechanical thrombectomy in PE remains under investigation; pivotal clinical trials have provided evidence to the safety and efficacy of mechanical thrombectomy in some patients. The FLARE (FlowTriever Pulmonary Embolectomy Clinical Study) study concluded that a mechanical thrombectomy mechanism appears safe and effective in persons with acute intermediate-risk PE. The current recommendations are for patients with absolute contraindications for systemic thrombolytics, prohibitive risk of bleeding, or lack of time (2 hours) for the systemic effect of thrombolytic agents.
An array of devices are used in thrombectomy. These include guide catheters, stent-retrievers, microcatheters, aspiration catheters, and aspiration pump systems.
Large-bore catheters suction out the thrombus using negative pressure aspiration. The selection of aspiration catheters includes a large variety of devices from different manufacturers. The selection of the catheter depends on the patient's anatomy and the location of the occlusion. For MI, it is uncertain whether one aspiration catheter provides any significant advantage over the others.
An aspiration pump machine or a 16 to 20 mL syringe serves to apply negative pressure to suction the thrombus.
These are small catheters advanced into a guiding catheter through a system where the catheter passes over the wire, which aids in the more complex and complete occlusions in the coronary and peripheral vasculature.
Stent Retrieval Tool
Solitaire utilizes a stent retrieval tool, by which an expandable stent is deployed, secures itself to the thrombus, and then can extract the clot. A systematic review revealed no significant difference in efficacy or safety between two different stent retrieval devices. Another stent retriever is the first device indicated for the treatment of symptom onset up to 24 hours.
Other Devices Used
Other innovative thrombectomy devices remove clots in large vessels through high-flow volume capped aspiration and self-expanding mesh discs. The device is indicated for use in the peripheral vasculature and pulmonary arteries. The FLARE trial provided evidence for the safety and efficacy of such devices in acute, intermediate-risk PE patients.
Over the wire systems are used to remove a significant clot burden from large diameter vessels and involve functions such as coring out clot from vein walls. Such systems are composed of a catheter and a sheath. The catheter consists of a coring element and a collecting bag. Once the catheter is advanced to the occlusion level, it is further advanced through the clot and is unsheathed and expanded. Upon retraction of the catheter, the coring element releases the clot from the vessel walls, while the proximal collecting bag serves to prevent the spread of any embolic clot material.
Shared principles guide the procedure of mechanical thrombectomy; however, certain modifications are made based on the type of intervention performed. The procedure begins with the insertion of a balloon-guided catheter commonly through a groin puncture. It is advanced until it reaches the thrombus. A guidewire is then advanced through the thrombus, and a microcatheter is passed over the guidewire through the thrombus. The guidewire is withdrawn, while the stent is deployed. The retrievable stent opens up, and its projections grip the thrombus, effectively securing it to the stent. Contrast is injected through the initial balloon catheter to observe flow distal to the removed occlusion. The balloon is then inflated to temporarily restrict flow as the stent and microcatheter are removed slowly through the guide catheter, with concurrent aspiration. An angiogram is performed to check for the complete removal of the thrombus.
The ADAPT (A Direct Aspiration First Pass Technique) is an alternate technique that uses direct aspiration with a large-bore catheter as a first pass step in removing the thrombus. This novel technique demonstrates the ongoing development and adaptations of mechanical thrombectomy. First, a large guide catheter is advanced to the thrombus' level, passing over a microwire and microcatheter. Next, aspiration is applied to the system using an aspiration pump or a syringe. Lack of flow means contact with the thrombus. The large bore catheter is advanced slightly for 1 to 2 mm to encircle the thrombus. Aspiration is again applied. Evidence of thrombus retrieval is through cessation of flow in the system. If the thrombus is not retrieved, repeat attempts and utilization of adjuncts such as a stent retriever are quickly implemented to remove the thrombus.
Symptomatic intracerebral hemorrhage is a potential adverse event of mechanical thrombectomy resulting from instrument manipulation. Despite this complication, the risk of symptomatic intracerebral bleeding is not significantly different from the medical standard of care.
Although these acute complications are not common, they indicate a poor prognosis.
The use of mechanical thrombectomy is considered the standard of care for large vessel occlusion. The MR CLEAN trial (Multicenter Randomized Clinical Trial of Endovascular Treatment for Acute Ischemic Stroke in the Netherlands) published in 2015 was a large multicenter randomized trial that compared patients who underwent intraarterial treatment plus usual care with those who underwent standard medical treatment.
Intraarterial intervention, including mechanical thrombectomy, resulted in a higher rate (13.5%) of functional independence in acute ischemic stroke than in the control group, supporting the safety and efficacy of an intraarterial intervention. However, subsequent trials have shown that intracranial hemorrhage's risk due to intraoperative complications remains a concern. Improvements in health-related quality of life and cognitive function are additional outcomes observed in long term follow-up for patients suffering an ischemic stroke.
Interprofessional communication and parallel workflows are critical parts of stroke treatment and the safe and timely use of mechanical thrombectomy in stroke treatment. The length of time to successful revascularization is a vital metric in determining a successful outcome for the patient. The ESCAPE clinical trial (Endovascular Treatment for Small Core and Anterior Circulation Proximal Occlusion with Emphasis on Minimizing CT to Recanalization Times) demonstrated that improved workflow could improve both functional outcomes and mortality. [Level 1]
While the neuro interventionalist is responsible for performing the thrombectomy, technologists, nursing staff, procedure room assistants play a critical role in ensuring promptness and success of the procedure. The treatment team consists of an anesthesiologist, at least one neuro interventionalist, a technologist, and a radiology nurse. The parallel workflow model ensures that several of the roles carried out by these individuals occur simultaneously.
An example of a parallel workflow is as follows: device preparation by the neuro interventionalist occurs at the same time the groin puncture site is sterile prepared, and the technologist drapes the patient. The nurse simultaneously retrieves bags of heparinized saline. The technologist or neuro interventionalist helps in the connection of the bags. The anesthesiologist would, at the same time, work to administer conscious sedation or general anesthesia to the patient. Implementation of this improved parallel workflow, the primary use of conscious sedation (whenever possible over general anesthesia), and the stroke cart availability has shown to produce a reduction of 29 minutes from room entry to reperfusion compared to a process where steps take place in succession. [Level 2]
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