During vascular surgery or endovascular treatment of vessels including thrombectomy, atherectomy, balloon angioplasty, and/or stent deployment, debris such as plaque and blood clots can move from the treatment site through a vein or artery and compromise the flow of blood at a location removed from the treatment site. In particular, various protection systems have been developed to prevent such debris from embolizing in the vessel. Distal protection devices include filters and occlusive devices (e.g., balloons) placed distally of the treatment site. Proximal protection devices include filters and occlusive devices placed proximally of the treatment site. In the case of filters, emboli collect within or on the filter. The filter with captured emboli is typically collapsed into a recovery catheter and the catheter withdrawn from the patient's body.
The size or number of emboli that must be retained by the filter in order to prevent clinically undesirable sequaelae is unknown. This uncertainty adds to the complexity of designing a filter with the appropriate characteristics. Small particles might pass through the filter pores and lodge downstream in tissues where they may cause tissue ischemia or tissue necrosis. In the heart, blood can be drawn and measurements can be made to track enzyme levels and determine myocardial damage. However, in the brain there is no easy and inexpensive method to evaluate the effect of a shower of emboli. Within the downstream tissue bed, there is a statistical component to the consequences of an embolus. For example, a 100 micron particle may lodge in a part of the brain where few adverse consequences are detected clinically, or it can lodge in a retinal artery, resulting in blindness in one eye. Therefore, it may be necessary to adjust the filter characteristics to suit the region of emboli filtration. A smaller pore size filter may be needed if protecting the brain than protecting the heart or kidney.
Embolic protection filters permit the passage of blood while retaining emboli that are larger than the pore size of the filter. Filter meshes are commonly made by incorporating holes in a polymer film, by interweaving filaments, or by producing interconnected porosity in a sheet of material (e.g., foam). It is difficult to make an embolic protection filter with the appropriate combination of pore size, pore area, embolic capacity, patency, mechanical strength, low collapsed or retracted profile, and recovery characteristics. Embolic filters made from polymer films commonly have a narrow range of pore sizes but suffer from a low percent open area because there is a limit to how closely the holes can be placed. Too little spacing between holes can result in a weak film that tears upon filter recovery. Foams tend to be bulky, thereby compromising the collapsed profile, and they have low strength.
Interwoven meshes such as braids have the advantage of a pore area which is a high percentage of the total mesh area, excellent strength, and good flexibility, but tend to be made and used in ways that result in a wide range of pore sizes. A wide range of pore sizes is undesirable for a number of reasons. Patency is influenced by pore size. Theoretically, blood can be sheared as it flows through the pore, particularly at the edges of the pore opening. Shearing of blood can activate platelets and initiate a cascade of events that cause blood clotting. When filters are used in the bloodstream, it is common for thrombus to form in the vicinity of the smallest pores and no thrombus to form in the vicinity of the largest pores. Flow through the filter is thereby reduced because part of the filter becomes occluded. In addition, while some filters have a reasonable average pore size, a wide range of pore sizes in these filters may allow large particles to pass through the large pores during either the capture or recovery phase.
A need in the art remains for an embolic protection filter having pores which are both small in size and which do not vary in size beyond an acceptable range.