Steve Murray
Consultant Cardiologist and Electrophysiologist
Freeman Hospital, Newcastle, UK

Ever since Michel Haissaguerre published his series demonstrating how triggers for atrial fibrillation (AF) often occurred within pulmonary veins (PVs), there has been a quest to develop the most efficient way of doing this. Strategies so far include a mesh ablation catheter (Bard), the use of advanced mapping systems, with and without concurrent use of 3D reconstructions from CT or MRI scas (e.g. CARTO & NAV-X) and recently the cryo-ablating balloon. The Medtronic Ablation Frontiers system has tackled the problem using multi-electrode catheters and perhaps more innovatively, a new generator system to deliver radio-frequency. The object of all these systems is to achieve PV isolation, with complete lines of ablation, whilst minimising complications. Since Haissaguerre first published, we have now seen complications related to thrombo-embolism and damage to bystander structures, such as the oesophagus, coronary arteries & the phrenic nerve. In this article we will report our experience of using the Medtronic Ablation Frontiers equipment and give our early results from two regional centres in the North of England.

The GENius (TM) Ablator

In simplistic terms, this may be thought of as 12 separate ablators within one box, all independently controlled in real-time by both a computer and the operator. This allows up to 6 electrode pairs to be individually managed during an RF delivery, with power being constantly titrated to achieve an optimum temperature; the temperature may be user-defined, although 60 degrees Centigrade is standard. As well as this, power may be delivered either bipolar or unipolar – that is, the energy cycles between electrodes and also between the electrodes and a back plate. This cycling of RF may set as a ratio, such as 4:1, 2:1, or 1:1. A cycle of 4:1 means that RF cycles 4 times as often between electrodes than to the back-plate, thus leading to less depth, yet a more contiguous lesion. This, for instance, would be a standard setting for ablating within the PV osteum. 1:1 is used for thicker structures, such as the inter-atrial septum. The design of the catheters and their electrodes allows for efficient lesion formation, but with lower powers, possibly leading to less collateral damage.

Ablation Catheters

PVAC:
To date, 4 different catheters are available. The PVAC (TM) is an off-centre circular catheter which can unwind to form a helical shape. There are 5 pairs of ablating electrodes mounted on the ring in a C-shaped configuration. The ablating electrodes are sat upon a nitinol ring, which is very soft, and allows excellent contact without fear of perforation. The ring shape may be unwound over a guidewire, and care must be taken not to damage the tip, of inadvertently shear-off the tip, and so it is recommended that the guidewire remains deployed at all times within the left atrium.

By manipulating the catheter within the osteum of the PVs it is possible to create several arcs of scar and so isolate the vein despite awkward anatomy, such as common trunks, etc. Again care must be taken not to simply insert the ring as far as it will go into the PV as this would often lead to RF delivery too deep within the vein, and so risk PV stenosis.

The PVAC is used with venography to guide placement and standard x-ray equipment; it has not been necessary to use an advanced mapping system, and indeed catheter registration is problematic with regard to an accurate representation on Nav-X.

MASC
This is a collapsible trefoil-shaped catheter which is deployed in low profile into the left atrium, and then opened and pulled back onto the septum. Each arm has 2 pairs of electrodes. It allows rapid burning of septal complex fractionated atrial electrograms (CFAEs) in persistent AF ablations. The septum is often a difficult structure to map and ablate, but this tool makes the job simple.

MAAC
This is an X shaped catheter which covers around 1cm squared of tissue; there are 4 pairs of electrodes, allowing burns to any combination of electrodes. Again, it is used to map and ablate the LA, looking for evidence of CFAEs. It can also be used to create a roof-line (and other lines) within the LA.

TVAC
This is linear, bi-curve catheter is used for creating linear ablations. It is currently being clinically evaluated. I have personally used it infrequently, although it has proved useful in cavo-tricuspid-isthmus ablation and creating roof lines in the LA. There are no published clinical data for this catheter as yet.

With all these catheters one can manage the burn in real time by delivering energy to only those electrodes which display a signal; during a burn, if contact/powers are poor, then those electrodes can be simply switched off, with energy continuing on the others unchanged. One drawback of the system is that signal interference from the duty-cycling prevents real-time signal display – hence burns are generally delivered for 30-60 seconds and then re-evaluated at the end. To date this has not resulted in any chars or perforations during RF, etc.

Clinical Results
Several centres from across Europe have published results for both chronic and paroxysmal AF; long term (ie. > 1 year) data is not available, but 6 month data from both James Cook Hospital and the Freeman Hospital demonstrate almost identical data. Freedom from AF reported clinically from patients, and checked with 7 day Holter recordings, demonstrate a success rate of over 80%.

Flouroscopy times were short (<30 mins) and complications rare.
Copies of studies presented at Heart Rhtym UK meeting 2008 may be found here, along with technical data : http://www.ablationfrontiers.com/posters-abstracts-OUS-US.htm

Summary:
This technology represents a new method for delivering RF and bespoke catheters to tackle ablation within the left atrium. Early clinical results from our own centre, and others across Europe demonstrate it is safe and effective, although longer term follow up data are awaited.

PVAC in left upper PV



MASC deployed on septum

 MAAC carrying out a coronary-sinus roof line
from within the LA

 

  TVAC linear catheter