Distal Embolic Protection:A routine part of interventional procedures in the near futureCath Lab Digest talks with Donald S.

Distal Embolic Protection:A routine part of interventional procedures in the near futureCath Lab Digest talks with Donald S.
Author(s): 

Professor of Medicine, Harvard Medical School, Director, Center for Integration of Medicine and Innovative Technology, Brigham and Women's Hospital, Boston, Massachusetts

Why do you think that distal embolic protection has only relatively recently become a topic of interest in invasive cardiology?

In 1977, before Gruentzig did his first catheter-based angioplasties during bypass surgery, he evaluated for arterial debris by collecting the effluent from an artery dilated during surgery, and looking for embolic particles (none were seen). Clinically, when you look at the literature up through the mid-1990s, the only reports of embolization are in patients with thrombotic lesions, where a chunk of clot breaks off, embolizes and blocks a distal large branch on the angiogram. There was nothing compelling people to think about distal embolization or protection against such embolization. There were, however, some unexplained problems. The two biggest examples are:

1. Elevations of CK after otherwise successful procedures;

2. The no-reflow phenomenon, where antegrade flow in the coronary is reduced and patients have chest pain with ST-segment elevation even though the large vessel is open.

In 1990, we were one of the first groups to suggest that no-reflow might be due to spasm not of the big arteries but of the small vessels.1 We recommended treatment with intracoronary calcium channel blockers like verapamil, diltiazem, and cardipine, which most of the time seemed to improve the flow. However, these drugs didn’t really work in a preventative way. You could give them before an intervention but you might still get no-reflow.

Next, there arose a great deal of interest in the glyco-protein IIb/IIIa receptor blockers to prevent platelet adhesion, with the belief that maybe platelet clumping was causing no-reflow. However, at least in saphenous vein graft interventions, the randomized trials of GP IIb/IIIa inhibitors did not show a reduction in adverse events in saphenous vein grafts.

Finally, the first of the distal protection devices, the PercuSurge GuardWire (actually developed with the hope of preventing stroke during carotid interventions), underwent testing in vein grafts in the SAFER trial.2 The trial showed that embolic debris was being recovered and demonstrated a dramatic reduction in the incidence of CK elevations and no-reflow in saphenous vein graft intervention. There was a 42% reduction in CK elevations, from 16 to 9, and more than a 50% reduction in no-reflow, if I remember correctly, from 8.3 to 3.3.

The SAFER trial really was the turning point where people went from saying, Embolization doesn’t occur why should I worry about it? to understanding that in fact it occurs very frequently, in almost every case with saphenous vein grafts, and protecting against embolization reduces these otherwise unexplained adverse events.

What are some of the ongoing challenges of creating an embolic protection device?
There are 3 classes of device:

1. Distal occlusion. The PercuSurge GuardWire utilizes distal occlusion, which temporarily blocks the flow of blood while the intervention is done. Any debris trapped in a stagnant column of blood can be aspirated before the distal occlusion is released.

2. Distal filters

3. Proximal occlusion. This type of device interrupts the flow of blood upstream of the blockage.

There are pros and cons of each type of device, and I think one of the unsolved questions is which class of device and within that, which device is going to be the best, in terms of deliverability, maintaining flow, recovery of debris, and so forth.

The second big question is, how far can you generalize this model beyond saphenous vein grafts? We chose vein grafts as the initial testing ground, because they had a very high complication rate, and there’s a lot of viable debris. Regardless, whenever people have looked at intervention in other atherosclerotic vessels, be they carotids or renals, people have also found debris, and have shown that embolic protection reduces complications. The same thing probably should hold true for native coronary intervention, because people have certainly shown that debris is released, particularly in acute myocardial infarction (MI) interventions, and probably also in vulnerable plaque/unstable angina-type interventions.

We’re still struggling with demonstrating the clinical benefit of embolic protection. The EMERALD trial, which was just reported at the American College of Cardiology meeting by Gregg Stone, was the trial of the PercuSurge GuardWire in acute MI interventions. Even though the pilot data suggested that it improved outcomes, that was not demonstrated in this big, randomized trial.

My view is that embolization is a common in fact, one could almost say ubiquitous complication of dilating atherosclerotic stenoses in any vessel. The end organ being perfused by the vessel is sensitive to such embolization, which is certainly the case in the brain, heart, and probably the kidneys as well. The incidence of adverse outcomes would be reduced by embolic protection. I believe we’re headed for a situation where embolic protection becomes more of a routine part of intervention, the same way stenting is a routine part of intervention.

Do you feel that embolic protection is still important even with direct stenting?

Oh, yes. You know, we’ve looked at that in the SAFER trial. It’s true that people who got direct stented had a lower incidence of CK elevation, but patients weren’t randomized to direct stenting. When you look at the patients who did get direct stented, they tended to have simpler lesions. Also, patients where direct stenting was tried and failed, were then predilated. Anybody with hard lesions to fix ended up in the predilated group. That explains some of the baseline apparent benefit of direct stenting it was used in simpler lesions. However, there was significant further reduction in adverse events utilizing the combination of embolic protection, even with direct stenting.

This brings up another question that needs to be addressed, namely, should embolic protection (let’s say in saphenous vein grafts) be used selectively? Can you identify lesions that are going to embolize well enough to say, okay, here’s a case where I will use embolic protection, or on the other hand, to say, this lesion really looks pretty good and I’m not going to use embolic protection?

It turned out that we really couldn’t identify any lesions that were so benign that they didn’t have MACE events for that class of lesion, or that events were not reduced by embolic protection. When I speak about this, I say it’s like wearing your seatbelt. You don’t say, well, I’m only going a couple miles, or the weather looks alright, or I’m a really careful driver, so I’m not going to put my seatbelt on today. Basically, you get in the car and you put on your seatbelt. I think that the same approach needs to be taken with embolic protection in saphenous vein graft intervention. The lesions are so unpredictable, and there’s not much you can do about the events, when they happen. If you have a device that’s easy to use and effective in preventing these events, the best approach is to use it in every case.

Can you talk about your involvement with the distal filter-type Rubicon Filter System?

I’ve been working with Rubicon for the last couple of years as an advisor. One of the challenges of the filter devices has been that virtually all of them are held in their collapsed state by an outer sheath as they’re placed across the lesion. The outer sheath means that the device profile, or diameter, is larger, and the device is somewhat stiffer than the PercuSurge GuardWire, which essentially is a balloon on a guardwire. The profile of the current-generation guardwires is between 26 and 29 thousandths of an inch. Most of the filters, with their delivery sheath in place, have a current profile of very close to 3F, which is 40 thousandths of an inch, so most filters are somewhat stiffer than a guardwire.

The Rubicon Filter is unique because it uses a different mechanism to keep the filter closed during delivery. The mechanism is a very thin envelope around the collapsed filter, which is released by pulling a microwire that runs inside the guidewire. The deflated profile of the device is very comparable to the PercuSurge device. The flexibility is very much like a guidewire in terms of crossing a lesion, and the filter can be released by pulling the internal wire, which has really been the main advantage.

The Rubicon Filter is in clinical practice in Europe; it’s been able to go places that no other filter device could navigate through tighter lesions and through greater tortuosity, acting really as a guidewire.

What is going on with the planned Rule-SVG trial?

I think it’s still at the FDA; it’s awaiting initiation in the second quarter of 2004.

There are trials going on in Europe right now?

Yes, the European regulatory path is always much easier. Rubicon is expecting their CE mark in Europe in 2004. The device is really being used in vein grafts and carotids. There’s been some acute MI trials as well and some small registries in Europe. It’s been behaving very well in terms of deliverability.

What type of learning curve does it have?

I think it’s probably the simplest of the embolic protection devices, because there’s really no set up and the guidewire-like device is placed across the lesion as any bare wire would be. Then the center ripcord is pulled to deploy the filter, and the rest of the procedure, in terms of stent delivery, takes place as it ordinarily would. A separate retrieval sheath is used, to recollapse and remove the filter. I think it is probably the most intuitive and simple of the devices.

Do we have any idea (no doubt it varies by device) what percentage of particles get missed by an embolic protection device?

Well, there are two issues to address in terms of missed particles. One issue is particles that go through the holes in the filter, and the other is particles that bypass the device completely, because the device is not completely opposed to the vessel wall.

That would be an operator issue?

Well, it may be a device limitation, or it may be an inability of the device to handle particular features of the lesion. There are devices that tend to pull away from the vessel wall if they’re in a curved segment of the vessel (as compared to other devices that stay opposed to the wall and prevent particles from going past). The EPI FilterWire3, is the only approved filter right now in the U.S., behaves very well. But in the trial with the original or EX design, operators had to be careful to look at the deployed filter in two views, in order to be sure it was opposed to the wall in curved vessels. If it wasn’t opposed to the wall, there were more complications, and that was addressed in operator training. Now Boston Scientific has a second-generation device that’s awaiting approval, called the FilterWire EZ, that tends to be more automatically well-opposed to the wall, a key feature in any distal filter system. The second mechanism of debris loss involves debris passing through the filter pores. This depends somewhat on the size of the pores, which in most devices is about 100 microns. Some of the devices don’t use plastic sheaths with holes, but use nitinol mesh instead. That tends to let some bigger particles through, up to 150 microns in size. The good news is that even the filters with 100-micron holes tend to trap some of the smaller particles as well. Anyway, it turns out that smaller particles are much better tolerated by the distal end organ than are large particles, so those particles that do get through the filters don’t seem to cause much in the way of clinical problems.

References: 

1. Piana RN, Paik GY, Moscucci M, Cohen DJ, Gibson CM, Kugelmass AD, Carrozza Jr. JP, Kuntz RE, Baim DS. Incidence and treatment of no-reflow after percutaneous coronary intervention. Circulation 1994;89: 2514-2518.

2. Baim DS, Wahr D, George B, et al. Randomized trial of a distal embolic protection device during percutaneous intervention of saphenous vein aorto-coronary bypass grafts. Circulation 2002;105(11):1285-1290.

3. Stone G, Rogers C, Hermiller J, et al. Randomized Comparison of Distal Protection With a Filter-Based Catheter and a Balloon Occlusion and Aspiration System During Percutaneous Intervention of Diseased Saphenous Vein Aorto-Coronary Bypass Grafts. Circulation 2003; 108: 548-553.


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