OPENACCESS Citation: Klimek-Piotrowska W, Hoa MK, Koziej M, Hoa J, Piątek K, Tyrak K, et al. (2016) Clinical Anatomy of the Cavotricuspid Isthmus and Terminal Crest. PLoS ONE 11(9): e0163383. doi:10.1371/journal.pone.0163383 Editor: Markus M Bachschmid, Boston University, UNITED STATES Received: April 8, 2016 Accepted: September 6, 2016 Published: September 28, 2016 Copyright: © 2016 Klimek-Piotrowska et al. This is an open access article distributed under the terms of the Creative
Commons
Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: Data cannot be made publicly available because the family members of the donors from this study did not agree to the publication of results regarding their family members. Please contact corresponding author (mkh@onet.eu) to access the data for this study. Funding: The publication of this manuscript was supported by the Faculty of Medicine, Jagiellonian University Medical College KNOW (Leading National Research Centre 2012–2017) funds. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. RESEARCH ARTICLE Clinical Anatomy of the Cavotricuspid Isthmus and Terminal Crest Wieswa Klimek-Piotrowska1☯, Mateusz K. Hoa1☯*, Mateusz Koziej1, Jakub Hoa1, Katarzyna Piątek1, Kamil Tyrak1, Filip Bolecha2 1 Department of Anatomy, Jagiellonian University Medical College, Cracow, Poland, 2 Department of Forensic Medicine, Jagiellonian University Medical College, Cracow, Poland ☯These authors contributed equally to this work. * mkh@onet.eu Abstract The aim of this study was to provide useful information about the cavotricuspid isthmus (CTI) and surrounding areas morphology, which may help to plan CTI radio-frequency ablation. We examined 140 autopsied human hearts from Caucasian individuals of both sexes (29.3% females) with a mean age of 49.1±17.2 years. We macroscopically investigated the lower part of the right atrium, the CTI, the inferior vena cava ostium and the terminal crest. The paraseptal isthmus (18.5±4.0 mm) was significantly shorter than the central isthmus (p<0.0001), and the central isthmus (24.0±4.2 mm) was significantly shorter than the inferolateral isthmus (29.3±4.9 mm) (p<0.0001). Heart weight was positively correlated with all isthmus diameters. Three different sectors of CTI were distinguished: anterior, middle and posterior. The middle sector of the CTI presented a different morphology: trabeculae (N = 87; 62.1%), intertrabecular recesses (N = 35; 25.0%) and trabecular bridges (N = 18; 12.9%). A single sub-Eustachian recess was present in 48.6% of hearts (N = 68), and a double recess was present in 2.9% of hearts (N = 4) with mean depth = 5.6±1.8mm and diameter = 7.1±3.4mm. The morphology of the distal terminal crest was varied; 10 patterns of the distal terminal crest ramifications were noted. There were no statistically significant differences in any of the investigated CTI parameters between groups with different types of terminal crest ramifications. The presence of intertrabecular recesses (25.0%), trabecu-lar bridges (12.9%) and sub-Eustachian recesses (48.6%) within the CTI can make ablation more difficult. We have presented the macroscopic patterns of final ramifications of the terminal crest within the quadrilateral CTI area. Introduction Thecavotricuspidisthmus (CTI)is a partof the rightatriumlocatedbetweentheinferiorvena cava (IVC) ostiumand thetricuspidvalve. The CTIis a relatively newconceptthat was first introducedby Cosioetal. (1993);this regionof theheartplays an essentialrole inthe atrial fluttercircuit[1]. Sincethen,thissmall,quadrilateral-shapedarea of the rightatriumhas servedas a target for catheter-directedablation,whichhas becomethemethodof choicefor Competing Interests: The authors have declared that no competing interests exist. treating atrialflutter
[2].Despite
veryhighsuccessrates
and
almostno
complications
[3],ablationof the CTIcan beextremelydifficultinsomepatients withatypical anatomical conditions; theCTIanatomy is complex and associatedwitha significantinter-individualvariability[4–6]. Knowledgeof thedetailedanatomy of thisregioncan significantlyimprove thesafetyand successrate ofablation procedures.The morphologyand musculararchitectureof theCTIin thehuman heart,regardlessof its relevanceto clinicalpractice,is not yet fullyunderstood. Therehave beenonlya fewanatomical studieson thistopic, and only threeof themcomprehensively investigated the majorityof CTIdimensions[4, 5,7–11].Therefore,the aim of this study is to provide more informationabout the morphologyof theCTIand surroundingareas instructurallynormalhearts.We studiedthemacroscopicmusculararchitecture of thelowerrightatriumarea to provide anatomical detailsrelevant to clinicalpractice. Materials and Methods Study population Thisstudy wasconductedintheDepartmentofAnatomy, JagiellonianUniversity MedicalCollege(Cracow, Poland) and was approved by theBioethicalCommitteeof JagiellonianUniver-sity MedicalCollege(KBET/51/B/2013).In our study wepersonallycollectedheartsonly from deceasedpersonwhodidnot express objection,whenalive,and iffamilydidnot express objection.In accordancewithPolish LawourBioethicalCommitteewaivedtheneedfor writtenor verbalinformedconsent. Thesesamples were not procuredfrom a tissuebankordonation center. We studied140 autopsied human heartsfrom Caucasianindividualsof bothsexes(29.3% females)witha meanage of49.1±17.2years and an average bodymass index(BMI)of 27.7 ±6.1kg/m2 and a mean bodysurfacearea of 1.9±0.2m².We collectedthe heartsduringroutine forensic medicalautopsies performedin theDepartmentof Forensic Medicine,Jagiellonian University MedicalCollegefrom July 2013 untilNovember 2015.The primarycausesofdeath were:suicide,murders,trafficaccidentsand homeaccidents.The exclusion criteriaincluded severeanatomical defects,heartsurgeriesorheartgrafts,evidentseveremacroscopicpathologiesof theheartor vascularsystemfound duringautopsy (aneurysms,storage diseases),heart trauma and macroscopicsignsof cadaverdecomposition.None of the140 individualshad a historyof any typeof arrhythmia. Dissection and measurements Theheartswere dissectedtogetherwiththeproximal portionsof the great vessels:theascendingaorta,pulmonarytrunk,superiorvenacava, IVC, and allof thepulmonaryveins.We weighedtheheartsbeforefixationusingan electroniclaboratoryscalewitha precisionof 0.5 g (BSA-L Laboratory).Afterdissection,allof theheartswere fixedby immersionin10% paraformaldehyde solutionfor a maximum of twomonths untilthetimeof measurement [12]. The rightatrium wasopenedin a routineway usingan intercaval incisionextendingfrom theorificeof thesuperiorvenacava to the orificeof theIVC without sectioningthe orifices.If necessary, additionalcutswere madeto present theinvestigated area in a betterway. We obtainedlinearmeasurementsusingYATO electroniccalipers(YT–7201)preciseto0.03mm. Allof themeasurementswere madeby twoindependentresearchers in orderto reducebias.If themeasurementdifferencesbetweenthe researchersexceeded10%,bothmeasurementswere repeated. Themean of thetwomeasurementswas calculatedand approximated to a tenth of a decimalplace. The lower partof the rightatrium,CTI,IVC ostiumand terminalcrest(TC) were investigated. The CTIis thequadrilateral-shapedarea boundedmediallyby theparaseptalisthmus, laterally by theinferolateralisthmus,anteriorly by theseptaltricuspidleafletattachment and posteriorlyby the Eustachian valve and ridge.The followingmeasurementswere made(Fig1): • paraseptalisthmus(orseptalisthmus/Koch’s trianglebase)–widthoftheclosed-linesegment tangentialto theleftcontour of the coronarysinus boundedby thepoint whereit touchesthe tricuspidannulus anteriorly and by theleftendof theEustachian ridgeposteriorly; • centralisthmus(orinferioristhmus)–widthoftheclosed-linesegmentboundedbythepoint whereit touchestheseptalleafletanteriorly and by the midpointof theEustachian ridgeposteriorly, parallelto theparaseptalisthmus; • inferolateralisthmus–widthoftheclosedlinesegmentboundedbytherightendoftheseptal leafletanteriorly and by theright endof theEustachian ridgeposteriorly, parallelto theparaseptaland centralisthmus; • thediameteroftheIVCostium; • theheightoftheEustachianvalve,measuredasthelengthbetweenthefreeedgeofthevalve and its attachment site totherightatrium; • thelengthoftheEustachianridgebetweentheparaseptalandinferolateralisthmus; • thetotallengthoftheseptaltricuspidleafletattachment; • theshortestlengthbetweentheparaseptalandinferolateralisthmus(CTIlength); • thethicknessoftheEustachianridge; • thedepthanddiametersofthesub-Eustachianrecesses; • thediameterofthecoronarysinusostium; • thediameteroftherightatrioventricularring. We distinguishedthreemorphologicalsectorswithintheCTI(betweenthecentraland inferolateralisthmus):anterior, middleand posterior. The antero-posteriordimensionsof the partsnotedabove at thelevelof thecentralisthmus weremeasured,and theirmorphology were evaluated. We alsocalculatedtheCTIarea. The presenceof thesub-Eustachianrecess (alsoknownas the sub-Thebesianrecessor thesinus of Keith) was assessed. We usedtransilluminationto macroscopicallyevaluatepatterns of the finalramificationsof thedistalTC musclefibersinto the lowerpartof therightatrium. Statistical analysis Thedata are presented as meanvalueswiththecorrespondingstandard deviationsorpercent-ages.WeperformedstatisticalanalyseswithSTATISTICA v12(StatSoftInc.,Tulsa,OK,USA). A p valueof lessthan 0.05was consideredto bestatisticallysignificant.TheShapiro-Wilk test wasusedto determineif thequantitative data were normallydistributed.To verifyhomogeneityofvariance,weperformedLevene’s test. WealsousedtheStudent’s t-testsand theMann-WhitneyUtests for statisticalcomparisons.We performedKruskal-Wallis one-way analysis of varianceto determinesignificantdifferencesinthe investigated CTIparametersbetween groupswith differenttypesof TC ramifications.Correlationcoefficientswere calculatedto measurethe statisticaldependencebetweenthemeasuredhearts’ parameters withscatter plots generatedfor selectedcases. Fig 1. Photograph of a cadaveric heart specimen showing the cavotricuspid isthmus area and a schematic view of the investigated heart region (mean ±standard deviations). AV–atrioventricular; CS–coronary sinus ostium; FO–fossa ovalis; IVC–inferior vena cava. doi:10.1371/journal.pone.0163383.g001 Results Themean heartweightwas 441.0 ± 119.1g. Table
1
and Fig
1
present theresults of allof the obtainedmeasurements.The paraseptalisthmus wassignificantlyshorterthan thecentralisth-mus (p<0.0001),and thecentralisthmus was significantlyshorterthan the inferolateralisth-mus (p<0.0001).The heartweightwascorrelatedwithallof theisthmus dimensions: paraseptal(r = 0.30;p = 0.001),central(r = 0.28;p = 0.002)and inferolateral(r =0.21; p =0.03).Furthermore,we foundthat a largercoronarysinus ostiumdiameterwascorrelated witha longer paraseptal(r = 0.25;p = 0.01),central(r = 0.20;p = 0.04)and inferolateral (r=0.23;p = 0.01)isthmus.Theparaseptalisthmus widthwas positivelycorrelated withage (r=0.3;p<0.001)and BMI(r = 0.2;p = 0.04).The inferolateralisthmus was positivelycorrelated withtheBMI(r = 0.2;p =0.04).TheCTIlengthwas positivelycorrelatedwith thediameterof therightatrioventricularring(r =0.25;p = 0.01).The CTIsurfacearea waspositively correlatedwithBMI(r = 0.21;p = 0.02),heartweight(r = 0.25;p = 0.007)and Eustachian valve height (r = 0.26;p = 0.02).Scatterplots ofage,BMIand heartweightfor selectedCTIdimensions are presented inFigs 2–4.Therewere nosignificantdifferencesinany of themeasured CTIdiametersbetweenthesexes(Table
2). We distinguishedthreedifferentsectorsof CTIbetweenthecentraland inferolateralisth- mus: anterior (smooth),middle(trabeculated)and posterior(membranous)(Table
1).
The
middlesectorof the CTIpresenteda differentmorphology:trabeculae(N =87; 62.1%),inter trabecularrecesses(N = 35; 25.0%)and trabecularbridges(N = 18;12.9%)(Fig5). The mean ratio of the middlesectorwidthto thecentralisthmus widthwas0.3 ± 0.1.We foundthat the widthof theanterior sectorincreasedwithage (r = 0.26;p =0.009). Table 1. Results of obtained measurements. BMI–body mass index, CTI–cavotricuspid isthmus, N–number of samples, SD–standard deviation, Q1 and Q3—lower and upper quartiles doi:10.1371/journal.pone.0163383.t001 Fig 2. Scatter plots of age and (A) paraseptal isthmus; (B) central isthmus; (C) inferolateral isthmus; (D) cavotricuspid isthmus (CTI) length; (E) Eustachian ridge/valve length; (F) CTI area. Fig 3. Scatter plots of body mass index (BMI) and (A) paraseptal isthmus; (B) central isthmus; (C) inferolateral isthmus; (D) cavotricuspid isthmus (CTI) length; (E) Eustachian ridge/valve length; (F) CTI area. Fig 4. Scatter plots of heart weight and (A) paraseptal isthmus; (B) central isthmus; (C) inferolateral isthmus; (D) cavotricuspid isthmus (CTI) length; (E) Eustachian ridge/valve length; (F) CTI area. Table 2. Data distribution by sex–no statistically significant differences were observed except for the heart weight. doi:10.1371/journal.pone.0163383.t002 The Eustachian valve waspresent in70.7%of hearts(N = 99).Its height waspositivelycorrelated withthe widthof theinferolateralisthmus (r = 0.21;p =0.05).When the Eustachian valvewas present, thecentraland inferolateral isthmus were significantlylonger compared withspecimenswithout thevalve (p = 0.0.3and p = 0.006,respectively).Moreover, theinferolateralandparaseptalisthmuswidthwerepositivelycorrelatedwiththevalve’s height(r =0.27; p =0.02 and r =0.4;p = 0.001,respectively). The singlesub-Eustachianrecesswaspresent in48.6%of hearts(N =68), and the double recesswas present in2.9%of hearts(N =4) (Fig6).
Allof
therecesseswere
localizedbetween
theparaseptaland centralisthmus to the rightof thecoronarysinus ostiumorthecentralisth-mus traversing therecess.Theprominent Eustachian ridgeoccurredin47.9% of hearts (N= 67), and its thicknesswasnegatively correlatedwith theIVC ostiumdiameter(r = -0.27; p =0.04).The diameterof thesub-Eustachianrecesswassignificantlylargerin heartswithout theEustachian ridge(p = 0.01)and negatively correlatedwith thediameterof therightatrioventricularring(r = -0.31;p = 0.01).The depth of the sub-Eustachianrecesswaspositivelycorrelated withthe lengthof theEustachian ridgebetweentheparaseptaland inferolateral isthmus (r = 0.24;p =0.04). We foundthat themorphologyof thedistalTC presentedconsiderablevariability. Basedon transilluminationof thelower-right atrium walls,we were able to macroscopicallyclassifythe distalTC ramificationsinto thelower partof therightatrium.Thedivisionwasmadeon the basisof the main muscle bandcourse(Table
3). Smallermuscularbandsbranching fromthe major crest presented a non-uniformarchitecture and radiatedto theEustachian ridge,coronarysinus ostiumand vestibuleof thetricuspidvalve. Fig
7
presents schemesof all of the Fig 5. Three different types of muscular arrangement in the middle sector of the cavotricuspid isthmus. The anatomical position of samples has not been maintained during the taking photos to get a better picture of cavotricuspid isthmus sectors. (A) trabeculae (N = 87; 62.1%); (B) intertrabecular recesses (N = 35; 25.0%); (C) trabecular bridges (N = 18; 12.9%); *–electrocardiological catheter. doi:10.1371/journal.pone.0163383.g005 establishedTC types.There wereno statisticallysignificantdifferencesin any of theinvestigatedCTIparametersbetweengroups withdifferent typesof TC ramifications. Discussion Therehave only beenthreeanatomical studiesthat have comprehensively investigatedthe majorityof CTIdimensions[4, 6,7].Pejkovic and Krajnc as wellas Gamietal. have investigatedpreciselytheCTIregion,but theseauthors didnot measurethemajor CTIdimensions [9,13]. Table
4
presents a comparisonourresults withthoseof previousstudies.A fundamentaldifferenceconcernsthe widthof thecentralisthmus.Cabreraetal. showed that thecentral isthmus is theshortestof allisthmus dimensions;it likely represents thebestof allpossible Fig 6. Double sub-Eustachian recess (R1 and R2). CS–coronary sinus ostium; ER–Eustachian ridge. doi:10.1371/journal.pone.0163383.g006 Table 3. Patterns of the final ramifications of the terminal crest muscle fibers into the lower part of the right atrium. Type N % Description A Thick muscular bundle from the distal crest to the vestibule of the tricuspid valve B Many thinner bundles radiated from the distal crest beyond the CTI (to the right) C Bundles of the distal crest radiated in fan-like fashion, obliquely in the CTI area D 10 7.1% Two thick muscular bundles from distal crest to the vestibule of the tricuspid valve and to the Eustachian ridge E 9 6.4% Thick bundle extended obliquely through CTI to the coronary sinus ostium, below the Eustachian ridge/valve F 8 5.7% Two thick muscular bundles from distal crest to the vestibule of the tricuspid valve going through and beyond the CTI G 7 5.0% Two thick muscular bundles from distal crest to the vestibule of the tricuspid valve and to the coronary sinus ostium CTI–cavotricuspid isthmus, N-number of samples doi:10.1371/journal.pone.0163383.t003 ablation sites [4].We cannot confirmthisfinding.We found that the paraseptalisthmus was significantlyshorterthan the centralisthmus.Ourresults, however, are consistent withimagingstudy observationsthat were obtainedusingmulti-detectorrow computed tomography; theseobservationsrevealedthat the centralisthmus wasalsothe shortest(Table
4) [6]. Althoughnot theshortest,theinferolateraland centralisthmusesare consideredto bebetter placesfor ablation [14]. Ablation at theparaseptalisthmus is associatedwitha significantriskof atrioventricularnodeinjuryand conductionblock.Moreover, ablation difficultiesare not typicallycausedby anatomical width;theyare instead causedby tissuethickness,pouchesor muscu-lar bridgesor trabeculae[15]. The paraseptalregionhas the thickestmuscularcontent among the CTI[4].Therefore, despite its shortestwidth,it is not a preferable target for ablation. The TC plays an important role intypicalatrialflutter. It provides a barrierto conduction transverselyacross it. The transverseconductionblockoftheTC occursmore likely inthick bundles.Ontheotherhand, fast conductionvelocitiescan beobservedinthelongitudinal directionof theTC (anisotropy) [16].The proximal and intercaval courseof theTC is quite universalin allhearts[8].The distalTC (finalramifications)and its non-uniformpattern seemstoplay a role inthe propagation of impulsesand has an impacton the successrate of CTIablation [17].In thisstudy, wepresent theprimarypatterns of TC finalramifications withintheCTI(Table
3,Fig
7). Conductionin theCTIcoursespreferentiallyalongthicker bundlesthat couldbecomethetargetsfor ablation. The main pattern represents one thickbundleof thedistalcrest terminatingin thevestibuleof thetricuspidvalve(26%),followedby many thinnerbundlesradiatingfrom thedistalcrest eitherbeyondtheCTI(16%)or obliquely inthe CTIarea ina fan-likefashion (14%).From our pointof view, themostinteresting types are thoseinwhichtheTC continues its courseas a thickmuscularbundlethroughtheCTI area (typesD, E,G,I).The TC finalbundles,accordingtotheso-called“muscle bundle”hypothesisfor CTIconduction,canserveas preferential conductionpathways [15]. Ablation of significantlythickerTC inlinealCTIablation may necessitateincreasedradio-frequency energyand bethecauseofproceduralfailure. Ontheotherhand,thepunctualablation of distal CTmay result intheinterruptionof pathologicalconduction.Clinicalstudiesdevotedtothe role of differenttypesof TC ramificationsare clearly required. Fig 7. Schemes of the final ramifications of the distal terminal crest muscle fibers (orange) into the lower part of the right atrium (types A–I; see Table 3). Smaller bands branching from the major crest were not presented with the exception of types B and C in which the main muscle band is absent. CSO–coronary sinus ostium; CTI–cavotricuspid isthmus; EuchV–Eustachian valve; FO–fossa ovalis; IVC–inferior vena cava; SL–septal leaflet; SVC–superior vena cava. doi:10.1371/journal.pone.0163383.g007 The sizeof theEustachian valveand ridgemay alsoaffectnegatively CTIablation [18]. Theseparametersmay hindercatheteraccessto theregionslocatedanterior to theridgeand valve. Also, theEustachian ridge,due to the presenceof musclefibers[5], is able toconduct electricalimpulsesand represents the site ofconductiongaps that are difficultto ablate. In thesecases,only thecomplete abolitionof theEustachian ridgeusingmore powerfulablation cathetersresults incomplete bidirectionalisthmus block[19].We have demonstratedthe con- tinuationof theTC in theEustachian ridgeinnearly 10%of allcases(typeDand I),whichcan promote thepropagation of impulses. Sub-Eustachian recessesare commonfindingswitha prevalencereaching80%[4].The presenceof sub-Eustachianrecessessignificantlyprolongs ablation timeand is associatedwith a higherriskof complications and a lower rate of success[20].Theserecessesare always found slightly to therightof thecoronarysinus ostiumand are neverfound inthe lateral thirdof the CTI.As a result, theablation linemay beperformedmore laterally toavoid recesses[13].On theotherhand, wecan findintertrabecularrecesses(25%) and trabecularbridges(13%) lat- erallyto thesub-Eustachianrecesseslocatedin themiddleCTIsector. Theirtransverse arrangement inrelation to the long CTIaxis (perpendicularto theseptalleafletattachment Table 4. Comparison between results of the present study and those conducted previously (mean ±standard deviation). Present study Cabrera et al. (1998) Cabrera et al. (2005) Saremi et al. (2008) [7] [4] [6] Sub-Eustachian recces diameter 7.1±3.4 - 14.0±3.0 7.3 ±2.3 [mm] Sub-Eustachian recces depth 5.6±1.8 - 2.9±1.2 7.7±2.6 [mm] Coronary sinus ostium diameter 9.4±2.7 9.5±2.0 - 9.1±2.0 [mm] CTI anterior sector dimension 10.1±2.7 12.0±2.0 - - [mm] *middiastole (70% of Cardiac Cycle) CTI–cavotricuspid isthmus; MDR-CT–multi-detector row computed tomography, N–number of samples doi:10.1371/journal.pone.0163383.t004 point) may contributetoenter thecatheterbetweenthe tissuebridgeand the thinatrialwall. Thegaps betweenthebridgesand theproperatrialwallmay negatively influencetheablation, and more energymay berequiredto reach theisthmus block. The majority of anatomical obstaclesare unfortunately detectedonly at thetimeof CTI ablation,whichsignificantlyprolongs theproceduretimeand reducesthesuccessrate. Meanwhile,evena vagueassessment of CTIarea contributes significantsafetyand benefitsto preproceduralablationbyidentifyingunfavorableanatomy. TheCTIcanbeimagingusingcomputedtomography, magneticresonanceimaging,rightatriumangiography, and transesophageal,transthoracicor intracardiacechocardiography[18, 19]. In particular, theuseof non-invasive, preproceduraltransthoracicechocardiographyis stronglyrecommendedfor patients sinceit proves theusefulnessfor predictingoutcome of isthmus-dependentatrialflutter abla-tion[18]. The primarylimitationofourstudy isthat allof themeasurementsweremadeonautopsied, structurallynormalheartspecimensthat had beenfixedinformaldehyde. Thisfixingmight have resultedin someslightchanges inthesizeand shape of thehearts.However, theuseof 10%paraformaldehyde didnot causesignificantchanges in the dimensionsof theatrialtissue; thedimensionsof fixedheartsare similarto thosethat are unfixed[12].We cannot alsosay anything aboutthebehaviorand dimensionchanges of the CTIregionwithinthecardiaccycle. Ourinvestigationwas a purely anatomical study without directelectrophysiologicalcorrelates; nevertheless,it provides informationrelevant to clinicalpractice. Acknowledgments Thepublicationof thismanuscript wassupportedby theFaculty of Medicine,JagiellonianUniversity MedicalCollegeKNOW (LeadingNational ResearchCentre2012–2017) funds.The fundershad no role instudy design,data collectionand analysis,decisionto publish,orpreparation of the manuscript. Author Contributions Conceivedand designedtheexperiments:WKPMKH. Performedtheexperiments:WKP MKHMKJHKP KT FB. Analyzedthedata: WKP MKHMK JHKP KT FB. Contributedreagents/materials/analysistools: WKP MKH. Wrote thepaper:WKP MKHMK JH KP KT FB. References 1. Cosio FG, Lo´pez-GilM,Goicolea A, ArribasF,BarrosoJL. Radiofrequencyablation oftheinferior vena cava-tricuspid valve isthmus in common atrial flutter. Am J Cardiol. 1993; 71(8):705–9. PMID: 8447269. 2. Wellens HJ. Catheter ablation for cardiac arrhythmias. N Engl J Med. 2004; 351(12):1172–4. doi: 10. 1056/NEJMp048197 PMID: 15371574. 3. Filgueiras-Rama D, ArceluzM, Castrejo´n S, EstradaA,FigueroaJ,Ortega-Molina M,etal. What is behind radiofrequency delivery at the cavo-tricuspid isthmus? Arch Cardiol Mex. 2014; 84(1):51–2. doi: 10.1016/j.acmx.2013.07.007 PMID: 24598353. 4. CabreraJA,Sa´nchez-QuintanaD,Farre´ J,RubioJM,HoSY.Theinferiorrightatrialisthmus:further architectural insights for current and coming ablation technologies. J Cardiovasc Electrophysiol. 2005; 16(4):402–8. doi: 10.1046/j.1540-8167.2005.40709.x PMID: 15828885. 5. Waki K, Saito T, Becker AE. Right atrial flutter isthmus revisited: normal anatomy favors nonuniform anisotropic conduction. J Cardiovasc Electrophysiol. 2000; 11(1):90–4. PMID: 10695468. 6. Saremi F, Pourzand L, Krishnan S, Ashikyan O, Gurudevan SV, Narula J, et al. Right atrial cavotricuspid isthmus: anatomic characterization with multi-detector row CT. Radiology. 2008; 247(3):658–68. doi: 10.1148/radiol.2473070819 PMID: 18487534. 7. Cabrera JA, Sanchez-Quintana D, Ho SY, Medina A, Anderson RH. The architecture of the atrial mus-culature between the orifice of the inferior caval vein and the tricuspid valve: the anatomy of the isth-mus. J Cardiovasc Electrophysiol. 1998; 9(11):1186–95. PMID: 9835263. 8. Sa´nchez-QuintanaD,AndersonRH,CabreraJA,ClimentV,MartinR,Farre´ J,etal.Theterminal crest: morphological features relevant to electrophysiology. Heart. 2002; 88(4):406–11. PMID: 12231604; PubMed Central PMCID: PMCPMC1767383. 9. PejkovićB, Krajnc I. Anatomical peculiarities of the cavo-tricuspid isthmus in the human heart. Wien Klin Wochenschr. 2006; 118 Suppl 2:43–7. doi: 10.1007/s00508-006-0544-y PMID: 16817043. 10. Kozwski D, Hreczecha J, Skwarek M, Piwko G, Kosiński A, Gawrysiak M, et al. Diameters of the cavo-sinus-tricuspid area in relation to type I atrial flutter. Folia Morphol (Warsz). 2003; 62(2):133–42. PMID: 12866674. 11. Wang K, Ho SY, Gibson DG, Anderson RH. Architecture of atrial musculature in humans. Br Heart J. 1995; 73(6):559–65. PMID: 7626357; PubMed Central PMCID: PMCPMC483920. 12. Hoa M, Klimek-Piotrowska W, Koziej M, Piątek K, Hoa J. Influence of different fixation protocols on the preservation and dimensions of cardiac tissue. Journal of Anatomy. 2016 Mar 31. doi: 10.1111/joa. 12469 13. Gami AS, Edwards WD, Lachman N, Friedman PA, Talreja D, Munger TM, et al. Electrophysiological anatomy of typical atrial flutter: the posterior boundary and causes for difficulty with ablation. J Cardiovasc Electrophysiol. 2010; 21(2):144–9. doi: 10.1111/j.1540-8167.2009.01607.x PMID: 19804553. 14. Anselme F, Klug D, Scanu P, Poty H, Lacroix D, Kacet S, et al. Randomized comparison of two targets in typical atrial flutter ablation. Am J Cardiol. 2000; 85(11):1302–7. PMID: 10831944. 15. Schernthaner C, Haidinger B, Brandt MC, Kraus J, Danmayr F, Hoppe UC, et al. Influence of cavotricuspid isthmus length on total radiofrequency energy to cure right atrial flutter. Kardiol Pol. 2015. doi: 10.5603/KP.a2015.0159 PMID: 26305365. 16. Saffitz JE, Kanter HL, Green KG, Tolley TK, Beyer EC. Tissue-specific determinants of anisotropic conduction velocity in canine atrial and ventricular myocardium. Circ Res. 1994; 74(6):1065–70. PMID: 8187276. 17. Lin YJ, Tai CT, Liu TY, Higa S, Lee PC, Huang JL, et al. Electrophysiological mechanisms and catheter ablation of complex atrial arrhythmias from crista terminalis:. Pacing Clin Electrophysiol. 2004; 27 (9):1231–9. doi: 10.1111/j.1540-8159.2004.00614.x PMID: 15461713. 18. Chen JY, Lin KH, Liou YM, Chang KC, Huang SK. Usefulness of pre-procedure cavotricuspid isthmus imaging by modified transthoracic echocardiography for predicting outcome of isthmus-dependent atrial flutter ablation. J Am Soc Echocardiogr. 2011; 24(10):1148–55. doi: 10.1016/j.echo.2011.06.007 PMID: 21764555. 19. Scaglione M, Caponi D, Di Donna P, Riccardi R, Bocchiardo M, Azzaro G, et al. Typical atrial flutter ablation outcome: correlation with isthmus anatomy using intracardiac echo 3D reconstruction. Euro-pace. 2004; 6(5):407–17. doi: 10.1016/j.eupc.2004.05.008 PMID: 15294265. 20. Da CostaA, FaureE, The´veninJ, Messier M,BernardS, Abdel K,etal.Effectof isthmusanatomyand ablation catheter on radiofrequency catheter ablation of the cavotricuspid isthmus. Circulation. 2004; 110(9):1030–5. doi: 10.1161/01.CIR.0000139845.40818.75 PMID: 15326078.