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ª 2 0 1 8 B Y T H E AM E R I C A N C O L L E G E O F C A R D I O L O G Y F O UN DA T I O N

P U B L I S H E D B Y E L S E V I E R

STATE-OF-THE-ART REVIEW

Electroporation and its Relevance forCardiac Catheter Ablation

Fred H.M. Wittkampf, PHD,a René van Es, PHD,a Kars Neven, MD, PHDa,b,c

ABSTRACT

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Irreversible electroporation can be used as a nonthermal energy source to ablate tissue. Cardiac catheter ablation

by irreversible electroporation may be a safe and effective alternative for thermal ablation techniques such as

radiofrequency or cryoablation. Total applied current, not delivered power (watts), energy (joules), or voltage, is the

parameter that most directly relates to the local voltage gradient that causes electroporation. Electroporation can

be achieved with various modalities: direct current, alternating current, pulsed direct current, or any combination of

these. Experimental cardiac and noncardiac studies have demonstrated tissue specificity with survival of arteries and

nerves in large lesions. In addition, porcine data suggest that application inside a pulmonary vein does not lead to

pulmonary vein stenosis and that the esophagus is remarkably insensitive to electroporation. Therefore, irreversible

electroporation is a very promising technique for cardiac catheter ablation and especially for electrical pulmonary

vein isolation. (J Am Coll Cardiol EP 2018;4:977–86) © 2018 by the American College of Cardiology Foundation.

I n the last decade, we explored electroporationas a novel energy source for cardiac catheterablation. This project was triggered by the obser-

vation that internal cardioversion eliminated the leftatrial electrograms recorded via a cardioversioncatheter in the coronary sinus. As was discoveredafter starting this project, the same observation andeven lesion analysis had already reported by Wijffelset al. (1).

RELEVANT ELECTRICAL PARAMETERS

During cardiac stimulation, defibrillation, radio-frequency (RF) ablation, as well as during electropo-ration, an electrical voltage is applied between atleast 2 electrodes in contact with tissue. This resultsin a current flow that depends on the magnitudeof applied voltage and total electrical resistance.

N 2405-500X/$36.00

m the aDepartment of Cardiology, University Medical Center Utrecht,

ysiology, Alfried Krupp Krankenhaus, Essen, Germany; and the cWitten/H

onsultant for Abbott and St. Jude Medical; and is a coinventor of circular

commercial use. Dr. van Es is a coinventor of circular electroporation abla

. Neven is a consultant for Abbott.

authors attest they are in compliance with human studies committees

tutions and Food and Drug Administration guidelines, including patient co

JACC: Clinical Electrophysiology author instructions page.

nuscript received April 3, 2018; revised manuscript received June 6, 2018

The latter is affected by the size, shape, and surfacestructure of the electrodes, distance between elec-trodes, and the resistive properties of all structuresbetween electrodes.

Within the target tissue, the local effect of theapplication directly depends on the strength ofthe local electrical field. However, the relationshipbetween applied voltage and local field is rathercomplex. Simply dividing applied voltage by distancebetween electrodes yields 1 uniform value for theelectrical field strength v (V/cm) within all targetedtissue. Such calculation is only valid for the excep-tional case of a homogeneous current density in tissuesandwiched between 2 parallel plate electrodes.

With unipolar and bipolar catheter ablation, how-ever, the applied current spreads out from the appli-cation electrode(s). This creates a spatially dependentlocal current density i (A/cm2) through tissue that

https://doi.org/10.1016/j.jacep.2018.06.005

Utrecht, the Netherlands; bDepartment of Electro-

erdecke University, Witten, Germany. Dr. Wittkampf

electroporation ablation and receives royalties from

tion and receives royalties from its commercial use.

and animal welfare regulations of the authors’ in-

nsent where appropriate. For more information, visit

, accepted June 6, 2018.

ABBR EV I A T I ON S

AND ACRONYMS

DC = direct current

PV = pulmonary vein

RF = radiofrequency

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usually decreases with the square of distancefrom the electrode(s). The current densitycreates a local electrical field across targetedcells according Ohm law in 3-dimensionalspace: field strength v ¼ i � r, with r (U cm)being the local specific resistance. Local cur-

rent density and thus also field strength both relatelinearly to total ablation current: Everywhere inthe tissue, local current density and field strengthincreases by a factor of 2 when total ablation currentis doubled.

Conversely, the relationship between appliedvoltage and local field strength across a targeted cellmembrane is rather complex. The resistance ofthe indifferent electrode is part of total ablationresistance. A higher indifferent electrode resistancecauses a smaller total applied current and thus asmaller local current density and field strength.Consequently, local field strength may differ despitea constant applied voltage. Using applied energy asablative parameter is even more erroneous. Energy isthe product of applied voltage, current, and durationof the application. Doubling of ablation currentand thus doubling of local current density and elec-trical field strength will require doubling of appliedvoltage and thus 4 times more energy! In otherwords, an increase in applied energy by a factorof 2 only increases local current density by a factor ofO2 ¼ 1.4.

With a given electrode configuration and tissuecontact, total applied current therefore is the param-eter that most directly relates to the strength of thelocal electrical field that may lead to electroporation(2–4). A future generator should therefore allow forcontrol of delivered current strength, not voltage orenergy.

ELECTROPORATION THRESHOLD

Electroporation current can be achieved with variousmodalities: direct current (DC); alternating current;pulsed DC; or a combination of these (5–10).Published data about the critical magnitude of therequired field strength for permanent electroporationrange from a few hundred millivolts across single cellmembranes to values in the 500 to 1,000 V/cm rangefor a suspension of living cells and tissue (11–16).Data of linear epicardial electroporation ablationwith a linear suction device allow for an estimationof the threshold current density for permanentelectroporation using a single monophasic applica-tion with a duration of a few milliseconds (17).The suction device ensures that all delivered currentwill be forced through myocardial tissue and the

half-circular cross-sectional shape of these lesionsallows estimation of the outer lesion surface areawhere current density must have reached lesionthreshold level. This calculation suggests a currentdensity threshold level of 1.7 A/cm2. For a specificmyocardial impedance of 158 U cm, as reported forhealthy sheep myocardium, this would correspond toan electrical field strength of 268 V/cm (18).

TISSUE SPECIFICITY

As explained, local current density creates an elec-trical field, expressed in volts per centimeter, and thelogical consequence is that the voltage across eachtissue cell will be proportional to its diametermeasured in the axial direction of the electricalcurrent. Cell size may thus be among the factors thatdetermine tissue specificity of electroporation (13).Cell membranes have a much higher electricalresistance than the extra- and intracellular fluid andtherefore the voltage across each cell is roughlydivided over the 2 membrane sections that areperpendicular to the direction of current flow. Boththese sections are thus exposed to approximatelyone-half the voltage gradient across the complete celland are the most likely sites where pores will becreated (8,19) (Figure 1).

The estimated permanent electroporation thres-hold of 268 V/cm is much lower than what has beenreported for other tissue types. However, our impulsewaveform also is very different from the pulse trainsthat have been applied in almost all noncardiacstudies. Therefore, these values cannot be used toexplain selective myocardial ablation.

BLOOD VESSELS

Various studies reported blood vessels to appearunaffected after electroporation ablation. Motivatedby the anticipated use electroporation for treatmentof tumors near large blood vessels, the carotid arteryhas been directly targeted in rats using a bipolarclamp around the artery. Histology, 4 weeks after theprocedure, showed that the connective matrix of theartery remained intact with no evidence of aneurysm,thrombus formation, or necrosis (20).

In a porcine study, histological analysis 14 daysafter pancreatic electroporation ablation revealedpatent vascular structures in targeted areas despitesignificant destruction of pancreatic tissue (21). Inthe same species, an electroporation study targetinglung tissue with 4 weeks survival demonstratedmild chronic inflammatory changes and hemosiderindeposition, but otherwise intact vessels without

FIGURE 1 Electroporated Red Blood Cell Membrane

Micrographs showing the structure of the membranes of red blood cells, frozen at different times (t) after the application of the porating

electrical pulse. (A) t ¼ 0.5 ms; (B) t ¼ 3 ms; (C) t ¼ 40 ms; (D) t ¼ 5 s; and (E) t ¼ 10 s. Original magnification: 60,000�. Reprinted with

permission from Chang and Reese (8).

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thrombosis or collapse (22). An experimental canineprostate ablation study demonstrated electropora-tion lesions with complete destruction of tissuedirectly up to the vessel wall, but preservation ofthe microvasculature. The study demonstrated the

absence of a heat sink effect, vessel destruction,or occlusion. Remarkably, nerves within the tar-geted area appeared to be intact and unaffected.Even ganglion cells showed no evidence of celldeath (16).

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NERVES

Multiple experimental studies investigated the long-term effect of direct high-power application onsciatic nerves. Acute damage with regeneration ofnerves within a few weeks after ablation suggestedthat electroporation may be a promising treatmenttool for ablation of tumors involving nerves (23–25).

CARDIAC ELECTROPORATION

Cardiac damage in response to an applied electrical(defibrillation) field has been reported as early as 1962(6,26). DC catheter ablation for treatment of cardiacarrhythmias was introduced around 1980 (27,28).Then, full-power defibrillation energy of 200 to 360 Jwas applied via a relatively small distal electrode of astandard 6-F nonsteerable electrophysiological cath-eter. Such application always resulted in gas forma-tion, arcing, an explosion, and a pressure wave (29).When performed inside the coronary sinus, thepressure wave could lead to vessel rupture (30,31).Arcing, that even led to a melted “footprint” on theplatinum electrode with melting temperature near1,700�C, presumably also led to a short-lasting hightissue temperature rise (32).

Such high energy application created a highcurrent density and field strength in surroundingtissue. In those days, however, electroporation stillwas relatively unknown in the cardiology commu-nity. As side notes in a 1986 paper on experimentalDC catheter ablation, Hauer et al. (33) reported that“coronary arteries within the injured area wereremarkably spared.” This also has consistently beenobserved in our nonarcing experimental electropora-tion studies, which suggests that electroporationwas the main mechanism of lesion formation with“classical” DC catheter ablation.

CATHETER ABLATION USING

ELECTROPORATION

Development of a novel cardiac catheter ablationmethod not only requires an effective energy source,but also procedural safety.

In 2007, Lavee et al. (34) were the first whointentionally performed cardiac electroporationablation by using epiatrial bipolar direct currentapplications between 2 parallel linear electrodes.Their study demonstrated the ability to createnonthermal transmural lesions and electrical isola-tion of atrial appendages.

Our main goal was the development of a fasterand safer method for pulmonary vein (PV) isolation.We opted for a circular multielectrode catheter

because such catheter has important advantages: Witha single electrode, the current density around thiselectrode decreases with the square of distance. Cur-rent density around a circular electrode catheterhowever, decreases linearly with distance. This relatesto the surface area of a torus that linearly relates totorus thickness. Please note that this linear decreaseonly happens when all electrodes on the hoop deliverthe energy synchronously; not onlymust all electrodesbe “on” simultaneously, but in case of an alternatingcurrent, their phase must be equal as well. A lineardecay of current density very much increases currentdensity at some depth in target tissue as comparedwith unipolar RF ablation where current density de-creases with the square of distance. Moreover, elec-troporation depends on current density, whereas RFtissue heating depends on the square of current den-sity and thus decreases with the fourth power of dis-tance with unipolar RF ablation (35). These are 2reasons why circular electroporation application canhave a much greater penetration depth and lesion sizethan unipolar RF application. In addition, electrode-tissue contact will be much less critical with a lineardecay of electroporation current density than with afourth-power decay of direct resistive RF heating.Moreover, the large total electrode surface area of amultielectrode hoop catheter facilitates arc-freeapplication of electroporation current. Finally, theelectrical current delivered via a multielectrode hoopwill not flow to the center of the hoop, but predomi-nantly outwardwhere the target tissue, themyocardialsleeve, is located (36).

EFFICACY

After a first proof of concept for circular catheterablation by electroporation, lesion depth was inves-tigated in an in vivo porcine open thorax model usingan epicardial decapolar circular hoop covered underat least 10 mm of blood that simulated endocardialcircular catheter (10,36). The study demonstrated theability to create continuous circular and >5-mm deeplesions with single 200 J application lasting only afew milliseconds (Figure 2).

In this same study, we also attempted to investigatethe effect of multiple applications by the delivery of 5consecutive electroporation shocks at the same elec-trode position. Lesion depth and width were notsignificantly different from a single application (un-published data), but this negative outcome may relateto the long approximately 90-s interval between ap-plications, caused by limitations of the impulsegenerator and the time needed for registration of thevoltage and current waveforms of each application.

FIGURE 2 Section of a 3-Week-Old Left Ventricular

Epicardial Lesion

Histologic elastic von Gieson–stained section of a 3-week-old

porcine myocardial lesion after circular epicardial ablation.

Standard features of such lesions are their irregular outer contour

with root-like fibrozations along its edges and apparently

undamaged vessels. Sometimes epicardial nerves that appear

totally undamaged can be recognized.

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POTENTIAL COMPLICATIONS OF

CARDIAC ELECTROPORATION

CORONARY DAMAGE. The effect of electroporationablation on coronary arteries was investigated byhistological analysis of arteries that happened to bepresent in electroporation lesions, but also afterintentionally targeting main coronary arteries via aspecial epicardial circular catheter in the pericardialspace (17,37–39). Both studies demonstrated thatcoronary arteries remained free of significant damageamid huge transmural myocardial lesions, up to 3months after ablation.

PV STENOSIS. After focal trigger ablation and ostialPV isolation using RF energy in humans, as originallydescribed by Haïssaguerre et al. (40), PV stenosis mayoccur in a relatively large percentage of patients(40–44). Segmental and wide area circumferential PVantrum isolation reduced this incidence significantly,but it still may be associated with the risk of PV nar-rowing (45–47).

The effect of electroporation ablation on develop-ment of PV stenosis was studied by comparing anoverdose of 10 circular 200 J electroporation applica-tions deep inside porcine PV with sequential circularRF applications. In PV ablated with electroporation,angiographic narrowing was absent after 3 months’follow-up, in contrast to PV in which RF ablation hadbeen performed (48). The study also demonstratedthat PV treated with RF had intimal proliferation,

necrotic myocardium, proliferation of the elastic lam-ina, and large amounts of scar tissue surrounding themyocardial sleeve, whereas electroporation-ablatedPV only showed minor intimal proliferation.

ESOPHAGEAL FISTULAS. A rare, but devastatingcomplication of thermal left atrial catheter ablation isan atrial-esophageal fistula (49–51). This complicationcan be caused by all presently used thermal ablationmodalities (52–57). This complication probably iscaused by the combination of a thermal lesion in theesophageal wall and periesophageal nerve injury,leading to gastric acid regurgitation and etching ofthe lesion (58–63).

To investigate the acute and chronic effects of irre-versible electroporation on the esophagus, full powerlinear electroporation ablation has been performeddirectly onto the outer porcine esophageal wall (64).Such ablation caused harmless, self-limiting, vesicleson the nonkeratinizing squamous epithelium at theablation site. At 7 days’ follow-up, the epithelium hadcompletely normalized. This normalization persistedafter 60 days’ follow-up. There were no signs of ul-ceration or other adverse reactions. Microscopy after 2and 60 days’ follow-up showed shallow non-transmural ablation lesions, proving that the deliveredenergy reached its target area. The epithelial layer af-ter 60 days’ follow-up showed intact epithelium andan intact extracellular matrix, proving that the archi-tecture of the esophageal tissue remained chronicallyintact. Experimental data thus suggest that irrevers-ible electroporation is the first ablation technique thatcan create deep myocardial ablation lesions withoutthe risk of severely damaging the esophagus, acutelyor chronically.

PHRENIC NERVE DAMAGE. Another potentialcomplication of thermal catheter ablation is phrenicnerve injury (65–67). Right-sided phrenic nervedamage can occur with catheter ablation in or aroundthe right superior PV or in the right atrium (68–74).Also with epicardial ablation, left phrenic nerveinjury has been described (75,76).

This complication can be avoided by identificationof the course of the phrenic nerve by pace-mapping orby continuous monitoring of phrenic nerve functionduring catheter ablation (77,78). The consequence ofthese precautions may be that the electrophysiologistis forced to refrain fromablation to avoid phrenic nervedamage. As described earlier, various studies hadalready suggested a low sensitivity of nerves for elec-troporation. However, given the unknown effect of thedifferent application mode and current density, theright phrenic nerve was targeted using a circular 200 Japplication with good tissue contact inside the

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superior caval vein. Sometimes, the phrenic nerve wastransiently affected, but normal function was demon-strated by capture-pacing from the superior caval vein30 min and 3 months after the application (79).

Sparing of nerves, arteries, and esophageal musclesuggests a tissue specificity that happens to be ad-vantageous for selective myocardial electroporationablation, but the reason for this specificity still is notclear. With axons, only part of their membrane will beexposed to the electroporation field and that sectionmay be too small to be lethal.

LIMITATIONS OF CARDIAC

ELECTROPORATION ABLATION

GAS FORMATION. DC current causes electrolysis andgas formation that, at least theoretically, may lead toneurological damage (29,80,81). Previous researchlearned that standard procedures, such as transseptalpuncture, sheath and catheter introduction in the leftatrium, sheath flushing, and RF ablation, also lead tothe release of gas (82). An early in vitro study in bloodhad reported the highest gas production with anodalapplications, but these data were obtained witharcing shocks (29). In a recent, still unpublished,study that we performed in 9 animals, 8 to 16 circular200 J ablations were delivered in the ascending aorta.Neurological magnetic resonance imaging analysisbefore and at 1 and 5 days after ablation, followed byhistological neurological investigation on day 5demonstrated the absence of any pathologicalchanges in neurological tissue. However, gas forma-tion remains a point of concern with endocardialapplication. Therefore, a novel energy source thatmay eliminate not only electrolysis, but also skeletalmuscle contractions is currently in development.

PERSISTENCE OF PV ISOLATION. Experimentalporcine studies with a follow-up duration of 3months after electroporation ablation demonstratedcompletely fibrotic lesions, suggesting their perma-nent character (17,38,39,48,79). However, we alsoobserved acute myocardial stunning that hadrecovered after 3 weeks (see Figure 4 in reference35). Insufficient electrode tissue contact may lead toshallow and discontinuous lesions that may not berecognized acutely by myocardial stunning (83). Thismay become a major pitfall of electroporation abla-tion and lead to recurrences.

OTHER POTENTIAL CARDIAC APPLICATIONS

EPICARDIAL LEFT VENTRICULAR ABLATION. Currently,epicardial catheter ablation may have an insufficient

penetration depth to reach the arrhythmogenic sub-strate and also is impeded by presence of coronaryarteries and the phrenic nerve (75,84–87). The pres-ervation of these structures during electroporationablation may pave the way for epicardial electropo-ration ablation. Experimental epicardial catheterelectroporation application demonstrated huge andoften transmural left ventricular lesions by using asmall, 12-mm diameter multielectrode epicardialhoop catheter (34,37–39). In part, these lesions maybe explained by the absence of current leakage to theblood pool and also by good electrode-tissue contactin the limited pericardial space by end-diastolic R-wave triggered application. In the clinical setting,however, epicardial fat may limit lesion formation,and this aspect still needs to be investigatedexperimentally.

ATRIAL MYOCARDIAL ABLATION. Atrial lesion for-mation after cardioversion via a coronary sinuscatheter, the width of lesions observed after porcinePV ostial electroporation ablation, and successfulepicardial atrial appendage ablation suggest thatatrial tissue can successfully be ablated by electro-poration (1,10,34). The absence of esophageal damageafter direct full power targeting suggests thatepicardial catheter ablation of the posterior left atrialwall is a viable option.

FUTURE PERSPECTIVES

Data collected so far suggest that electroporation is apromising tool for electrical PV isolation and myocar-dial ablation (Central Illustration). However, this alsohas been said about other technologies and we shouldexercise due caution as the safety and efficacy ofelectroporation is investigated. One important aspectthat has received little attention so far is the energysource. Most studies have used pulse trains, whereasour team used a single capacitive discharge from astandard monophasic defibrillator. Both are effective,but power sources should be compared and possiblyoptimized while preserving the nonthermal and se-lective character of the applications (5,34,36,81,88).

The catheter needs to be optimized too, not onlythe distal shape and electrode configuration, butspecial measures will be required for sufficientelectrical isolation. With monopolar application, thehigh voltages applied may cause catheter isolationbreakdown by arcing through the outer catheterisolation. With bipolar application, the very thinelectrical isolation between wires inside thecatheter may very easily fail (83). Both potentialproblems will require specially designed catheters

CENTRAL ILLUSTRATION Endocardial and Epicardial Targeting

Wittkampf, F.H.M. et al. J Am Coll Cardiol EP. 2018;4(8):977–86.

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and qualification requirements that not yet havebeen specified.

Before starting clinical studies on epicardial elec-troporation ablation for ventricular arrhythmias,

the effect of epicardial fat and tissue fibrosis shouldbe investigated.

Finally, electrode tissue contact will remain animportant parameter that affects the efficacy of the

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electroporation application. It still is not clear howtissue contact affects lesion size. Pressure sensors foreach electrode of a multielectrode catheter probablyis a major technical challenge, but an electricalmethod may be feasible and hopefully would facili-tate optimal catheter positioning (89).

CONCLUSIONS

Numerous experimental cardiac and noncardiacstudies demonstrate that most if not all unwantedside effects and complications of thermal catheterablation are absent with electroporation ablation.Blood vessels and nerves are spared in the midst oflarge electroporation lesions and an overdose of

ablations deep inside PV does not lead to PV stenosis.Full power application directly on the porcineesophagus only causes superficial lesions. This sug-gests that electroporation is a very promising energysource for cardiac ablation with reduced risk of pe-ripheral damage; similar claims have been made forother technologies in the past. We should exercisedue caution as the safety and efficacy of electropo-ration is investigated.

ADDRESS FOR CORRESPONDENCE: Dr. FredWittkampf, Department of Cardiology, UniversityMedical Center Utrecht, Kloosterlaan 23, 3749AJ LageVuursche, the Netherlands. E-mail: fredwittkampf@gmail.com.

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KEY WORDS atrial fibrillation, catheterablation, electroporation