Catheter tip-to-tissue contact force has, until now, been a subjective assessment by the operator using surrogate methods such as fluoroscopic visualization of the catheter tip, tactile feedback from catheter manipulation, and changes in electrograms and impedance measurements during ablation. These indirect measurements have been shown to be of limited value as a reliable predictor of actual catheter contact. One of the potential advantages of continuous contact force monitoring is an optimal initial placement of the ablation lesion, but also the ability to determine catheter stability in real time, therefore halting ineffective radiofrequency energy delivery. Optimal catheter tip-to-tissue contact force during radiofrequency (RF) application is instrumental to delivery of effective transmural ablation lesions.
The following is an interview with Dr. Moussa Mansour regarding force sensing catheters. Dr. Mansour served as a study author in both the SMART-AF and TOCCASTAR studies. The intent of the interview is to gain insights from one of the study authors as it pertains to force sensing catheters.
We encourage readers to reference the SMART-AF and TOCCASTAR studies for further details.
Do you foresee force sensing catheters becoming the standard of care catheter in atrial fibrillation (AF) ablations?
There are now two force sensing catheters approved for use in the United States for the performance of AF ablations. These are the TactiCath™ Quartz irrigated ablation catheter (St. Jude Medical) and the ThermoCool SmartTouch® catheter (Biosense Webster, Inc., a Johnson & Johnson company). They were approved based on the SMART-AF and TOCCASTAR studies. In addition to these trials, the two catheters have been used extensively in Europe. Both studies showed that if optimal force is achieved, the outcome of the procedure is significantly better. As a result, it is expected that the use of force sensing will significantly increase.
Any comments on force and time as it relates to catheter ablation?
SMART-AF showed that if you stay more than 80% of the time in your force range, long-term success rates can be improved at the one-year point. The force time integral (FTI) has limited value when used alone. It becomes a more important tool if it is linked to a stability indicator. The ability to achieve an adequate lesion depends on stable catheter-tissue contact. Future studies will prospectively define optimal FTIs for procedural success.
Is there a difference in quality of ablation when achieving an FTI of 300 gram seconds (gs) with either 30 grams of force for 10 seconds versus 10 grams of force for 30 seconds?
Probably not, but this issue has not yet been fully defined. Animal studies comparing 30 watts and 50 watts of power have shown that you can use more force with less power to achieve the same lesion depth. Therefore, force sensing catheters will allow operators to effectively titrate power delivery dependent on contact force.
What future improvements do you predict for force sensing catheters?
Systems may combine impedance drop, power, catheter stability, duration and force to come up with new measurements of lesion transmurality.
Is there a best practice for zeroing the SmartTouch catheter?
In the left atrium, the catheter is placed in the middle of the left atrium and can be confirmed by fluoroscopy and/or ICE.
What are typical scenarios where the force numbers are not accurate?
When in doubt, you should rezero. It takes very little effort to rezero. Studies have shown that even experienced operators do not always estimate accurately the amount of force they use In addition, there are times when the operators feel they may have low contact force, such as along the left atrial roof, when in point of fact, the contact force measurements suggest significant contact force. Force sensing catheters provide the practitioner an additional tool to assess in real time the adequacy of their tissue contact as measured by force.
Is there a force time integral you personally strive for when performing your AF ablations? Is there a minimum force level you personally strive for?
The force and FTI that I aim for depend on the location in the left atrium. In the posterior left atrium, most lesions are around 300 gs. Elsewhere, FTIs can be as high as 800-900 gs. I aim for a force range between 10-40 g,
What happens if you are unable to achieve the FTI of 300 gs and you achieved 200 gs?
I would attempt to reablate that region until achieving a minimum FTI of 300 gs. It would be inaccurate to think that you can simply go back to the area and perform an additional lesion with an FTI of 100 gs, and that the two lesions would be additive. One can argue that postoperative edema will be present from your initial ablation lesion, and you will need more ablation the second time to overcome the acute effects of postoperative edema. It is critical for the operator to spend time upfront ensuring adequate tissue contact and force before ablating.
Have you exclusively switched over to contact force catheters for AF ablations?
Certainly. Use of contact force measurements has opened my eyes to portions of the left atrium that can be more difficult to obtain consistent force readings above 10 grams, such as the left atrial ridge. There is evidence from previous studies that atrial ridges and muscular folds may decrease catheter stability. Because of force sensing catheters, I am able to more confidently achieve adequate tissue contact during ablation. ■
Rishi Anand, MD, FACC, FHRS is the Director of the Electrophysiology Lab at Holy Cross Hospital in Fort Lauderdale, Florida. Dr. Anand is also Chair of Payer Relations & Healthcare Economics, American College of Cardiology, Florida Chapter.
Moussa Mansour, MD is an Associate Professor in Medicine at Harvard Medical School, and Director of the Cardiac Electrophysiology Laboratory and Atrial Fibrillation Program at Massachusetts General Hospital in Boston, Massachusetts.
Disclosures: Dr. Anand has no conflicts of interest to report. Dr. Mansour reports research grants and consultancy from Biosense Webster and St. Jude Medical.