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Microwave Ablation



                                           Microwave Ablation

                                                                                                                                                  a report by
                                                                                                                                    Adam E Saltman

                                                             Associate Professor of Surgery and Physiology, and Director of Research, Cardiothoracic Surgery,
                                                                                                                                 University of Massachusetts

Dr Adam E Saltman is an Associate          The most progress in the treatment of atrial                  The Flex 2 (and the newer Flex 4 and Flex 10
Professor of Surgery and Physiology
              and Director of Research,
                                           fibrillation has been made using ablative energy              devices) emits microwave radiation at 2.54GHz. It
         Cardiothoracic Surgery at the     sources. Seeking to replace the scissor and scalpel,          penetrates the tissue to excite water molecules,
University of Massachusetts. He also       surgeons have been using mostly hyperthermic                  creating dielectric heating by enhanced vibration and
      has an appointment as Adjunct
    Associate Professor of Biomedical      technologies to heat and destroy myocardial tissue            friction of the molecules. Studies on muscle tissue
         Engineering at the Worcester      (although the venerable cryothermy is still being used        phantoms have shown that the depth of penetration
   Polytechnic Institute. Dr Saltman’s
                                           by many and is a safe and fast technique with which           is governed by the generator power and the time of
        special interest is the surgical
   treatment of arrhythmias and the        to destroy atrial tissue). New instruments using              application (see Figure 2). It has been shown that
     pathophysiology underlying atrial     unipolar dry radiofrequency, unipolar irrigated               tissue radiated by microwave energy undergoes
   fibrillation, particularly new onset
 post-operative atrial fibrillation. He    radiofrequency, microwave, bipolar dry radio-                 coagulation necrosis and after a healing period is
 has authored or co-authored more          frequency, irrigated bipolar radiofrequency,                  replaced by non-conductive scar (see Figure 3).
 than 40 peer-reviewed articles and        ultrasound, and laser emissions have all been
             reviews and has presented
    numerous papers at national and        developed and assayed to differing extents. Each has          In contrast to other energy sources such as
      international meetings. He is a      its strengths and weaknesses but all represent a              radiofrequency, microwave does not heat tissue by
   fellow of the American College of
   Surgeons, the American College of
                                           significant improvement in speed and safety over              conduction. Therefore it does not require direct
Cardiology, the American College of        cutting with a knife or scissors and sewing the tissue        tissue contact in order to create a lesion. Microwave
 Chest Physicians and the American         together again. The remainder of this article examines        radiation, like any other energy source, does follow
     Heart Association (AHA). He also
   holds memberships in the Cardiac        microwave energy as one of those energy sources.              the rule of squares wherein energy density falls off
         Electrophysiology Society, the                                                                  with the square of the distance from the antenna.
    Society of Thoracic Surgeons, the
 Biomedical Engineering Society and
                                           Microwave energy has been used for several                    This means that the energy delivered to the tissue
      the North American Society for       decades to ablate tissues of all kinds. Extensive             will be less if the antenna is separated from its target
     Pacing and Electrophysiology. Dr      experience has been gained in using it to destroy             by an insulator such as air or fat. Although simulated
              Saltman attended Harvard
  University, receiving his AB degree      tumors of the uterus, breast, skeletal muscle, liver,         and in vitro studies have shown that power applied at
  in 1983. He subsequently received        and prostate. Some of the earliest interest in its use        the manufacturer’s recommended settings should
       his MD and PhD degrees from
                                           on the heart, however, can be attributed to work              produce a lesion between 7–9mm deep, even when
    Columbia University in 1990 and
             completed his general and     by Haines et al. and Wonnell et al., who tried to             there is blood flowing along the opposite surface of
   cardiothoracic surgical training at     ablate cardiac tissue using microwave-tipped                  the tissue, there has not yet been a method
           the New England Deaconess
                Hospital in Boston, MA.    catheters that were passed into the heart via the             developed by which the operator can determine that
                                           venous circulation.                                           a lesion is truly transmural in realtime. This is a
                                                                                                         technical problem that plagues all unipolar ablation
                                           These attempts were met with technical difficulties,          methods at this time (For a more detailed review of
                                           particularly cardiac chamber perforations, and so             the physics of microwave radiation the reader is
                                           progress in this area was slow initially. When the AFx        referred to Williams et al.).
                                           Corporation (Fremont, CA) designed and released
                                           the Flex 2 ‘Lynx’ device, focus was shifted from a            Methods of Application
                                           catheter-based, small diameter, forward-firing probe
                                           to a longer, rigid, topically applied device that emitted     The newer Flex 4 and Flex 10 probes have
                                           radiation over a 2cm length (see Figure 1). The linear        overcome some of the technical limitations of the
                                           lesion created by the Flex 2 could be made either             Lynx/Flex 2. Chiefly, the Flex 4 and 10 have
                                           inside the empty heart (endocardial) or outside the           flexible antenna guides and shafts that permit the
                                           full, beating heart (epicardial). Its efficacy and safety     operator to place them against heart muscle from
                                           were demonstrated during the MICRO-STAF and                   either within the heart (endocardial) or on its outer
                                           MICRO-PASS trials by Knaut and his co-workers,                surface (epicardial). Although the early experiences
                                           which have now enrolled more than 200 patients and            were all gathered using an endocardial approach, as
                                           have documented about an 80% cure rate of                     experience has grown with these devices, surgeons
                                           permanent fibrillation with no morbidity related to           have been creating lesions using both methods
98                                         the ablation procedure.                                       quite frequently.

                                                                                     BUSINESS BRIEFING: GLOBAL SURGERY – FUTURE DIRECTIONS 2005

Figure 1: The first microwave ablation device, the Flex 2 or Lynx. A rigid          Whereas the endocardial approach offers the
antenna, 2cm long, was attached to a flexible shaft. The antenna was                advantage of direct visualization and access to all
shielded so microwaves were emitted unidirectionally as controlled by the           intra-atrial structures, it does add to cardiopulmonary
operator.                                                                           bypass time, cross-clamp ischemia time, and
                                                                                    sometimes is plagued by difficulties in visualization of
                                                                                    the intracardiac targets. The epicardial approach offers
                                                                                    the advantage of beating heart access and therefore
                                                                                    applicability to any patient without opening the heart.
                                                                                    There is also the theoretical advantage to ablating
                                                                                    inward toward the intracardiac blood pool, rather
                                                                                    than toward juxtaposed structures, greatly increasing
                                                                                    the safety margin of the ablation. Epicardial ablation
                                                                                    does take longer for each lesion, however, and also
                                                                                    does not appear to penetrate as well. Also, not all
                                                                                    structures can be accessed directly from an epicardial
                                                                                    approach, such as the right atrial isthmus and the left
                                                                                    atrial region near the mitral valve apparatus.

                                                                                    There are some unique properties about the Flex
                                                                                    10, however, that allow the operator to position it
                                                                                    around the PVs without opening the chest.
                                                                                    Because the ablating element can be positioned
                                                                                    over any of 10 different locations along its sheath,
                                                                                    it is not necessary to move and reposition the
                                                                                    device between energy applications. Because the
                                                                                    antenna is shielded, energy is emitted in only one
                                                                                    direction, protecting collateral structures from
                                                                                    injury. Finally, because the device is 9mm in
                                                                                    diameter, it has recently been used to perform
                                                                                    ablation through the closed chest using
Figure 2: The depth of lesion penetration is governed by the time for which         thoracoscopic techniques.
the microwave energy is applied. These curves were taken during ablation
with the Flex 10 device with energy applied at 65 watts and differing times. A      Endoscopic Ablation of AF
2cm long lesion is created 9mm deep when power is applied for 90 seconds, as
recommended by the manufacturer.                                                    Since 2002, the Cardiothoracic Surgery at the
                                                                                    University of Massachusetts has performed the
   40                                                                               completely endoscopic ablation of AF on 26 patients.
                                                                                    Most of these patients (67%) presented with
   35                                                                               paroxysmal AF, with an average duration of 79
                                                                                    months. They were an average of 60 years old (range
                                                                                    39–82) and 67% were male. Half had undergone
   30                                                                               multiple cardioversions and 46% had failed a minimum
                                                                                    of two attempts at controlling their arrhythmia with
                                                                                    anti-arrhythmic drugs. Their left atria were an average
                                                                                    of 49mm in diameter (range 27–100) and their average
                                                                                    ejection fraction was 50% (range 10–65). None had
   20                                                                               had prior chest or heart surgery.

                                                                                    All procedures were performed with the Flex 10
   15                                                                               device. Briefly, the patient was brought to the
                                                                                    operating room and general anesthesia was
                                                                     Flow : 41pm
   10                                                                               induced. A double-lumen endotracheal tube was
                                                                             2 mm
                                                                                    used to permit separate lung ventilation as required
                                                                             4 mm
                                                                             6 mm
                                                                                    during the procedure. The patient was positioned
    5                                                                               supine with the arms out at 90º to the side. The
                                                                                    anterior and lateral chest, abdomen and groins were
    0                                                                               sterilely prepped and draped.
        0      20         40              60             80    100       120
                               Duration of ablation (secs)                          The right lung was then deflated and three access

                                                                     BUSINESS BRIEFING: GLOBAL SURGERY – FUTURE DIRECTIONS 2005
                                                                                            Microwave Ablation

ports into the right hemithorax created. Using 5mm           Figure 3: A photomicrograph of sheep pulmonary
cameras and operating instruments, the right                 veins 30 days after ablation with a Flex 10 device at
pericardium was visualized and opened longitudinally         90 seconds and 65 watts power. The region
parallel and about 2cm anterior to the phrenic nerve.        indicated by the arrow is completely replaced by
This permitted visualization of the aorta, the superior      fibrotic scar; the lesion is completely transmural.
vena cava, the right atrium, the inferior vena cava,
and the right PVs.

In order to pass the Flex 10 around the back of the
heart and encircle the PVs, it was necessary to access
and instrument the transverse and oblique sinuses.
As the entrance to each is guarded by a fold of
pericardium under the superior and inferior vena
cava, these folds were taken down using gentle
blunt dissection with a 5mm instrument. Each sinus
was then instrumented with a thin rubber catheter
as a guide. Once the guide catheters were placed,
the instruments were removed from the right side
and the right lung reinflated.
                                                             length of stay was four days, although 47% stayed
The left lung was then deflated and a mirror-image           three days or less.
pattern of access ports created. Again, using 5mm
instruments, the left side of the pericardium was            There was a 67% incidence of AF immediately after
opened posterior to the phrenic nerve and the left           surgery and during the recuperative phase.
atrial appendage visualized. The two guide catheters         Therefore all patients were maintained on their anti-
were then retrieved under direct vision and their tips       arrhythmic drugs (usually amiodarone) and their
delivered outside the chest where they were sutured          anti-coagulation for at least three months. Follow-
to each other. Traction on the right-sided ends of the       up was obtained at one, three, six, nine, and 12
catheters brought the tips back into the chest and           months after surgery and the medications were
now the PVs were completely encircled.                       stopped if there was no AF for at least a three-month
                                                             consecutive period.
The Flex 10 device was then attached to the
transverse sinus catheter and delivered around the           The success rate of achieving sinus rhythm continued
PVs by placing traction on the oblique sinus catheter.       to improve over time. By six and nine months
Once position and orientation were confirmed by              follow-up, 100% of patients were in either a paced or
direct visualization from both the right and left sides,     sinus rhythm.
an ablation ‘box’ was created by applying microwave
energy at each of the ten separate locations for 90          Conclusion
seconds at 65 watts. Lesion creation was confirmed
by inspection of the areas that were visible.                Microwave ablation is a familiar and mature
                                                             technology. It has been used thousands of times to
In order to prevent any possibility of failure due to left   ablate cardiac tissue and is safe and easy to handle.
atrial flutter, a lesion is created along the back wall of   Recent innovations in delivery vehicles have
the left atrium connecting the PV ‘box’ to the left          produced a device that is long enough to
atrial appendage. This lesion was performed in lieu of       completely surround all of the pulmonary veins at
the mitral annular lesion of the Maze-III operations,        once as well as thin enough to be introduced
as it is not yet possible to perform a mitral lesion from    through a 10mm operating port. This has allowed
the epicardium without likely injuring the coronary          the development of a new procedure using
sinus and/or circumflex coronary artery.                     microwave ablation to treat AF in a safe and
                                                             effective manner without opening the patient’s
Once the appendage lesion was created, the                   chest, arresting the heart or making cardiac
appendage was amputated with an endoscopic                   incisions. This type of procedure will surely
stapler. A small chest tube was placed in either             enhance patient and referring physician acceptance
pleural space and the lungs re-expanded. The                 of a surgical approach to the treatment of AF, as
instruments were withdrawn and the wounds                    well as make surgeons more comfortable and more
closed and bandages applied. All patients were               willing to offer such a therapy. Such treatments are
awoken in the operating room and transferred to              aticipated to be used early for patients just
the intensive care unit for observation. The chest           presenting with AF, rather than subjecting them to
tubes were removed later that day. The median                drug trials and life-long anticoagulation. ■            101


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