Kalyanam Shivkumar

This interview was conducted to understand the clinical implications of the Navistar™ DS 8 mm Tip Navigation and Ablation Catheter — the first catheter specifically designed to treat atrial flutter.

Dr. Shivkumar's arrhythmia unit at the University of California, Los Angeles (UCLA) is one of the leading electrophysiology centers in the nation, with a distinguished track record of pioneering work in cardiac ablation. As one of the Clinical Investigators involved in the clinical study leading to FDA approval of the Navistar™ DS 8 mm Catheter for ablation of atrial flutter, Dr. Shivkumar has extensive hands-on experience with the device. Therefore, when Biosense Webster, Inc. (BWI) decided to seek objective feedback on the performance of the Navistar™ DS Catheter, Dr. Shivkumar was the logical choice. The following is the condensation of an interview with Dr. Shivkumar at UCLA on March 24, 2003.

As an Investigator in our clinical study, you spent a great deal of time becoming familiar with the Navistar™ DS 8 mm Tip Catheter, did you not?
Yes, indeed. Our whole team was interested in trying this catheter under a variety of conditions. We wanted to understand how much we could do with this new technology.

What ablation catheters were you using to ablate atrial flutter before the Navistar™ DS 8 mm Tip Catheter became available?
We used the Navistar™ 4 mm Tip Catheter, and a variety of other 4 mm catheters.

Are there particular types of cases where you tend to use the Navistar™ 8 mm Tip Catheter rather than a 4 mm catheter?
I've used an 8 mm tip catheter as the catheter of choice when dealing with patients who are referred to us for recurrent flutter or failed flutter ablations — primarily because you can achieve a more extensive lesion. We've tended to use it for patients who have had surgical scars in the atria, such as patients who have congenital heart disease — primarily Fontan, Mustard, and Senning. It's also a catheter that I prefer for patients who have right free-wall accessory pathways. It's a location where lesion formation and heating is a problem, and the 8 mm tip worked well for our group.

You mentioned using the 8 mm catheter for patients who have been referred to you for failed flutter ablations. Would you also consider the Navistar™ DS 8 mm Tip Catheter your catheter of choice for a patient who presents for the first time with flutter?
Yes, it is the catheter we predominantly use for atrial flutter. The 8 mm tip catheter is an effective approach. If the 4 mm tip catheter does not achieve the desired endpoint, you would go to the 8 mm tip. So why not use it from the beginning?

Figure 1

RF ablation of the TCV-IVC "Flutter" Isthmus.

Where do you see the greatest value for the 8 mm tip catheter compared with the standard 4 mm tip?
I see the greatest benefit in cases of previously failed ablations, difficult anatomy, and substrates such as patients who have thick muscle bundles in the tricuspid valve IVC isthmus. It is also beneficial in cases where achieving bi-directional block across the isthmus with a 4 mm tip catheter tends to be a problem — that is where we prefer to use the 8 mm tip catheter. The Navistar™ DS 8 mm Tip Catheter is helpful in ablating difficult areas such as the right atrium isthmus.

Why does the 8 mm tip catheter allow you to deal with difficult anatomy and substrates better than a 4 mm tip catheter?
Because larger lesions can be created. Also, you have access to a generator that can have increased power output. The end result is formation of a deeper, bigger lesion.

How important is tip orientation to the substrate with the 8 mm catheter?
Intuitively, it would be ideal to keep the catheter parallel to the tissue.

Do you use the 8 mm in parallel to achieve a longer lesion to move quickly through your case, or do you prefer using the catheter in perpendicular orientation to the substrate to achieve an even deeper lesion?
I try to orient the catheter parallel to the tissue. But this is not always possible because of anatomical variants. For instance, ablating within a pouch requires a perpendicular orientation of the catheter tip.

Figure 2

Carto™ XP System map shows pre-ablation flutter.

Traditionally, clinicians have targeted an ablation temperature of 50 ºC or more. With the Navistar™ DS 8 mm Catheter, local blood flow has a differential cooling effect on the catheter tip. Do you find the lower temperature acceptable for your patients?
That's an interesting question. There are times when we have seen target temperatures less than what we are used to seeing with 4 mm tip catheters, but we are still achieving the same end result. There has been no prospective validation of this, however. Ideally, we still try to achieve temperatures somewhere in the range of 50 degrees.

When you say the 8 mm tip catheter creates bigger lesions, is it because the extra cooling enables you to deliver greater power to the tissue and therefore create a deeper lesion?
I think the fact that you can achieve cooling allows you to deliver more resistive heating. You're optimizing ablation. It is likely that blood itself cools the tip instead of you having a cooling system either internal or external to the catheter tip. There are no in-depth studies to verify this theory, but that is the most likely reason. Also, you can use more power through a circuit with a bigger tip compared to a smaller tip.

So the 8 mm, compared to the 4 mm, allows you to deliver higher power?
Yes, it delivers higher power at an optimum temperature.

Are there specific things that you can do with an 8 mm tip that you cannot do or would be too difficult to do with the 4 mm?
When you encounter any thick muscle bundles in patients with isthmus-dependent flutter, or when you are dealing with a post-surgical scarred atrium for difficult accessory pathway ablations. Places where lesions are harder to create — such as the right free wall — a Navistar™ DS Catheter with an 8 mm tip is indeed helpful. There are many cases where we had to go to an 8 mm tip after starting out with a 4 mm tip — even for flutter, where we couldn't get bi-directional block.

What are the reasons the 4 mm can't perform at the same level in these situations?
Limited power. The tip can heat up too fast, resulting in lesions not adequate to achieve block.

Figure 3

ECG demonstrates termination of flutter.

The Stockert 70 RF Generator permits delivery of up to 70 watts of power when using the Navistar™ DS 8 mm Tip Catheter. How does that change the way you approach procedures?
That changes things a lot because there are times when you are not able to achieve loss of local electrograms during RF. And if the power is what is limiting the degree of energy, you can increase the power.

Table 1

Intraprocedural Data. Clinical summary of multicenter study on Navistar™ DS 8 mm Tip Navigation and Ablation Catheter.*

How often do you find yourself using that higher energy capacity?
We use it periodically in approximately 50 or 60% of flutter cases.

As one of the endpoints of the study, we looked at the power and the mean was 52.5 ± 10.4. I would have thought it would be a much higher number.
The way the study protocol was designed, you have to start off at or below 50 watts. If you could not achieve your endpoint, then you dial up the power. I think it was very wisely designed. We do the same thing in our unit. You start at 50 watts, see where you are, and then see if you have to go up. We find that with the Navistar™ DS 8 mm Tip Catheter, as in the study, we typically achieve our endpoint at relatively low power levels.

How often do you find yourself up in the 65 to 70 watt range?
I would say that in approximately 30% or 40% of cases, especially previously failed ablations, we have to dial it up to the higher power levels. This is often because we have encountered a thick muscle bundle.

Do you find that the 8 mm tip catheter helps you minimize the need to go back and touch up your initial lines?
That is a very good point. What we have observed is that a carefully placed tricuspid valve IVC line as the first line achieves the endpoint 70–80% of the time. That is important, because you really don't want to keep doing a lot of touch-ups. You want to get your first line very carefully done, because if you don't, you have edema as well as other problems. This tends to limit the efficacy of subsequent RF delivery during the case. I think using the 8 mm tip catheter has been beneficial in that setting.

Are you doing fewer re-burns because you're covering a larger area, you're getting deeper lesions, or for some other reason?
Usually the reason you do a re-burn is because you pulled back too much with a 4 mm catheter. When you use a 4 mm catheter, you should pull back what appears to be a small bit each time instead of just dragging it. When you drag, that's when you get holes in your line. However, if you do it very slowly, you get a contiguous lesion.

It would seem intuitive that if you get effective lesions with fewer burns, you would reach your endpoint faster and hence save time overall, including time under fluoro. Does your experience indicate that this might be true?
Intuitively, yes. It would tend to make procedure time shorter. People have never compared this head-to-head, so again, we do not have enough data to make a definitive statement. However, I wouldn’t be surprised at all if that were true.

With the longer tip, are you doing fewer burns with a longer energy delivery cycle or are the burns, in fact, more effective?
It's definitely not a case of longer burn times. Actually, in the same amount of time you're simply getting a more effective burn. The catheter just has to be there long enough to achieve its electrical endpoint, and an 8 mm tip catheter achieves a more effective burn. Our experience suggests that it has to be there for a lesser time to achieve the burn. That's why the procedure time can come down.

What is your overall impression of the Navistar™ DS 8 mm Tip Catheter?
It's a great technology for ablation of atrial flutter, especially previously failed ablations, and intuitively, this technology should be beneficial in a variety of indications. As with any new product, this gives the physician an opportunity to train his entire team on an exciting new technology with an excellent safety record.