patients with complex congenital heart disease ( cchd ) with singleventricle physiology need to be finally palliated by fontantype operation.1 , 2 in fontan circulation , systemic venous blood flow is routed directly to the pulmonary circulation , totally bypassing the right ventricle , and the flow from the systemic veins through the bilateral lungs and back to the ventricle is entirely passive . because of this unique physiology , patients with fontan circulation are particularly intolerant of tachyarrhythmia with a short diastolic interval.3 atrial arrhythmia is the most common arrhythmia in such patients , and several studies have demonstrated that atrial tachyarrhythmia is associated with increased morbidity and mortality.3 , 4 , 5 in addition to atrial arrhythmia , supraventricular tachycardia ( svt ) with substrates that are unique in cchd patients can cause hemodynamic deterioration and fontan failure.3 , 6 
  catheter ablation is effective for eliminating svt ( ie , atrioventricular reentrant tachycardia [ avrt ] or atrioventricular nodal reentrant tachycardia [ avnrt ] ) in children without structural heart disease.7 in cchd patients , however , the location and characteristics of the atrioventricular node are often complex , and twin atrioventricular nodes ( tavn ) may occur in some patients.8 , 9 consequently , catheter ablation for svt in cchd patients is more challenging , particularly in small children . in this paper , we presented our experience to elucidate the electrophysiological characteristics and outcomes of catheter ablation in cchd patients . patients with singleventricle physiology ( defined as cchd patients ) who underwent an electrophysiology study ( eps ) for evaluation of svt at national taiwan university hospital during the period from 1995 to 2015 were enrolled . the study was approved by the institutional review board of our hospital , and the requirement of informed consent was waived . svt was defined as reentry arrhythmia involving atrioventricular nodal tissue ( including atrioventricular node and bundle of his conduction system ) or accessory pathways . we performed the epss under intravenous anesthesia with propofol , and antiarrhythmic agents were discontinued for 2  days , except for beta blockers . we then introduced 2 or 3 mapping catheters from the femoral veins or internal jugular vein , 1 quadripolar fixed catheter at the right atrium , and 1 multipolar fixed catheter ( steerable decapolar mapping catheter or epstar hisrv fixed catheter [ jcath ] ) at the right ventricle with or without 1 quadripolar fixed catheter to define the bundle of his area . for patients with limited vascular access , we used 1 quadripolar fixed catheter at the right atrium and 1 multipolar catheter to record the right ventricle and hisbundle signals . we mapped the bundle of his potential around the whole atrioventricular ring by using the fixed catheter or steerable ablation catheter . a basic eps was then performed using standard atrial and ventricular rapid pacing and extrastimulation . the 12lead ecg was verified at different pacing cycle lengths to detect a shift in qrs morphology . arrhythmia inducibility was determined in all patients . when arrhythmia could not be induced with the standard protocol for those with induced tachycardia , we introduced ( or exchanged when vascular access was limited ) an ablation catheter for mapping . tavn was defined if ( 1 ) at least 2 nonpreexcited qrs morphologies were recorded at 2 different sites , ( 2 ) hisbundle potential was recorded at 2 discrete sites , and ( 3 ) decremental atrioventricular conduction was defined at these 2 bundle of his areas.10 the major atrioventricular node was defined as the antegrade atrioventricular conduction at sinus rhythm , and the other atrioventricular nodes were defined as minor atrioventricular nodes . for the patients with an alternative or fusion ecg pattern during sinus rhythm , the major atrioventricular node was defined as the antegrade atrioventricular conduction during dominant svt . twin atrioventricular nodal reentrant tachycardia ( tavnrt ) was defined if 2 atrioventricular nodes were recorded after reentrant tachycardia induction with antegrade conduction through 1 atrioventricular node and retrograde conduction through the other atrioventricular node . junctional tachycardia was defined as narrow qrs tachycardia with the same morphology as atrial pacing but with dissociated atrioventricular conduction.11 
  after localization , radiofrequency energy was delivered through ablation catheters . for accessory pathway  related avrt , we targeted the accessory pathways ( va fusion or earliest retrograde a signal ) . for tavnrt , we targeted the retrogradeconducted minor atrioventricular node ( the earliest retrograde a signal ) . we used the temperature control mode with the highest temperature limit ranging from 60c to 65c and the highest energy given at 50  w. the therapeutic end point was noninducibility of tachycardia and elimination of the accessory pathway or minor atrioventricular node . patients received heparinization during the procedure and overnight after the eps , followed by aspirin at 3 to 5  mg / kg per day for 2 weeks . at followup , any cardiac symptoms , including palpitation , chest discomfort , and nearsyncope episodes , were recorded from the medical chart . all continuous variables were expressed as meansd , and categorical variables were expressed as number ( percentage ) . we performed the epss under intravenous anesthesia with propofol , and antiarrhythmic agents were discontinued for 2  days , except for beta blockers . we then introduced 2 or 3 mapping catheters from the femoral veins or internal jugular vein , 1 quadripolar fixed catheter at the right atrium , and 1 multipolar fixed catheter ( steerable decapolar mapping catheter or epstar hisrv fixed catheter [ jcath ] ) at the right ventricle with or without 1 quadripolar fixed catheter to define the bundle of his area . for patients with limited vascular access , we used 1 quadripolar fixed catheter at the right atrium and 1 multipolar catheter to record the right ventricle and hisbundle signals . we mapped the bundle of his potential around the whole atrioventricular ring by using the fixed catheter or steerable ablation catheter . a basic eps was then performed using standard atrial and ventricular rapid pacing and extrastimulation . the 12lead ecg was verified at different pacing cycle lengths to detect a shift in qrs morphology . arrhythmia inducibility was determined in all patients . when arrhythmia could not be induced with the standard protocol , for those with induced tachycardia , we introduced ( or exchanged when vascular access was limited ) an ablation catheter for mapping . tavn was defined if ( 1 ) at least 2 nonpreexcited qrs morphologies were recorded at 2 different sites , ( 2 ) hisbundle potential was recorded at 2 discrete sites , and ( 3 ) decremental atrioventricular conduction was defined at these 2 bundle of his areas.10 the major atrioventricular node was defined as the antegrade atrioventricular conduction at sinus rhythm , and the other atrioventricular nodes were defined as minor atrioventricular nodes . for the patients with an alternative or fusion ecg pattern during sinus rhythm , the major atrioventricular node was defined as the antegrade atrioventricular conduction during dominant svt . twin atrioventricular nodal reentrant tachycardia ( tavnrt ) was defined if 2 atrioventricular nodes were recorded after reentrant tachycardia induction with antegrade conduction through 1 atrioventricular node and retrograde conduction through the other atrioventricular node . junctional tachycardia was defined as narrow qrs tachycardia with the same morphology as atrial pacing but with dissociated atrioventricular conduction.11 after localization , radiofrequency energy was delivered through ablation catheters . for accessory pathway  related avrt , we targeted the accessory pathways ( va fusion or earliest retrograde a signal ) . for tavnrt , we targeted the retrogradeconducted minor atrioventricular node ( the earliest retrograde a signal ) . we used the temperature control mode with the highest temperature limit ranging from 60c to 65c and the highest energy given at 50  w. the therapeutic end point was noninducibility of tachycardia and elimination of the accessory pathway or minor atrioventricular node . patients received heparinization during the procedure and overnight after the eps , followed by aspirin at 3 to 5  mg / kg per day for 2 weeks . at followup , any cardiac symptoms , including palpitation , chest discomfort , and nearsyncope episodes , were recorded from the medical chart . all continuous variables were expressed as meansd , and categorical variables were expressed as number ( percentage ) . we enrolled 30 cchd patients with singleventricle physiology ( 17 male , 13 female ) . we categorized these patients into 3 groups according to the underlying cardiac anomaly : right atrial isomerism ( rai ) , left atrial isomerism ( lai ) , and other cchds . all patients with rai and most with lai exhibited similar combinations of cardiac anomalies including a common atrioventricular canal and doubleoutlet right ventricle with or without left ventricular hypoplasia . the age at surgery has decreased gradually over the past 20  years because of better outcomes for early total cavopulmonary circulation surgery in recent reports.12 because transcatheter ablation is difficult after fontantype surgery ( the only route from the vena cava to the atrium after fontan surgery is fenestration of the conduit ) , we preferred performing an eps and transcatheter ablation before fontan surgery the mean age at the time of the eps was 7.1  years , and the mean body weight was 20.7  kg in our study population . cardiac anomalies of the 3 groups asd indicates atrial septal defect ; cchd , complex congenital heart disease ; cctga , congenitally corrected transposition of the great arteries ; lai , left atrial isomerism ; rai , right atrial isomerism ; tga , transposition of the great arteries ; vsd , ventricular septal defect . basic clinical characteristics of the 3 patient groups data are shown as meansd or number ( percentage ) except as noted . cchd indicates complex congenital heart disease ; eps , electrophysiology study ; lai , left atrial isomerism ; rai , right atrial isomerism ; svt , supraventricular tachycardia ; tcpc , total cavopulmonary circulation . only patients with clinical svt the indications for an eps included clinically confirmed svt ( with 12lead ecg confirmation ) in 13 patients ( 43.3% ; most presented with palpitation and dizziness or near syncope ) , clinical svt without 12lead ecg in 10 patients ( 33.3% ; mostly shown on the ecg monitor during hospitalization for surgery , cardiac catheterization , or other reasons ) , and preoperative assessment in 7 patients ( 23.3% ; including suspected tavn or abnormal atrioventricular node function at previous ecg examination ) . overall , > 2 narrow qrs ecg patterns were common ( n=10 , 33.3% ) in the cchd patients , particularly those with rai and with other cchds . among these 10 patients , 4 ( 40% ) had fused qrs morphology ( figure  1a ) , and the other 6 ( 60% ) had 1 dominant ecg pattern , but another ecg pattern may occur occasionally . during the epss , more than half of the patients ( n=18 , 60% ) had tavn ( inclusive of 8 patients with 2 ecg patterns demonstrated only at eps ) , particularly the rai patients ( 72.2% ) ( figure  1b ) . the major atrioventricular node occurred more commonly at the posterior atrioventricular ring but also presented at the anterior atrioventricular ring ( figure  2 ) . in the 7 patients without heterotaxy syndrome , 3 ( 42.9% ) had tavn . of those , 2 had congenitally corrected transposition of the great arteries , and 1 had a doubleoutlet right ventricle with situs inversus , dextrocardia , atrial septal defect , large ventricular septal defect with unusual atrioventricular septal alignment , and pulmonary atresia . electrophysiological characteristics in the 3 patient groups data are shown as meansd or number ( percentage ) except as noted . av indicates atrioventricular ; avnrt , atrioventricular nodal reentrant tachycardia ; avrt , atrioventricular reentrant tachycardia ; cchd , complex congenital heart disease ; eps , electrophysiology study ; lai , left atrial isomerism ; rai , right atrial isomerism ; svt , supraventricular tachycardia ; tavnrt , twin atrioventricular nodal reentrant tachycardia . electrocardiogram of twin atrioventricular node . a , a patient with right atrial isomerism who had fused two nonpreexcited qrs morphologies with different axis at usual surface ecg recording . she was documented to have twin atrioventricular nodes ( tavn ) at later electrophysiology study ( eps ) . b , another patient with right atrial isomerism who had another qrs morphology ( arrow ) at the returning beat of atrial extrastimulation during eps , suggesting the existence of tavn . illustration of the distribution of atrioventricular ( av ) conduction tissue in our single ventricle disease patients . the major av nodes are mostly located in the posterior site and minor av nodes at the anterior site . however , some variations are observable . black circles indicate major av nodes , empty circles indicate minor av nodes , crosses indicate accessory pathways , and triangles indicate intrahisian sling connection or ectopic junctional focus . the wenckebach cycle length was prolonged in the lai group compared with the other 2 groups . one lai patient was implanted with a permanent pacemaker because of extreme bradycardia at age 7  years . svt could be induced in 21 of 30 patients ( 70% ) , and 2 patients ( 1 with rai and 1 with other cchd ) had concomitant atrial tachyarrhythmia . tavnrt accounted for 64.3% of svt in the rai patients ( figure  3 ) and 40% of svt in those with other cchds ( figure  4 ) . avrt was the second most common svt in the cchd patients and often was associated with multiple accessory pathways ( 3 of 6 , 50% ) . junctional tachycardia occurred in 3 patients ( 14.3% ) , 2 with tavn and another with a single atrioventricular node . avnrt occurred in 2 patients ( 9.5% ) ; one had typical avnrt ( and rai ) , and the other had atypical avnrt ( in transposition of the great artery with a large ventricular septal defect and pulmonary atresia ) . electrophysiology and ablation for twin atrioventricular nodal reentrant tachycardia in a patient with right atrial isomerism . ( a ) and ( c ) in the left panel , we can see antegrade his signal at the ablation catheter ( abld ) and located in the right posterior area ( arrow ) , which marked the location of antegrade major av node . ( b ) and ( d ) in the right panel , during tachycardia , we can see retrograde his signal at the ablation catheter ( abld ) , which was also at the nearby location of earliest retrograde a ( not shown in this tracing ) . the ablation catheter was at the right anterior av groove , which was the common site of minor av nodes . electrophysiology and ablation for twin atrioventricular nodal reentrant tachycardia in a patient with single ventricle disease . ( a ) and ( b ) this 9year old boy is a patient of crisscross heart , corrected transposition of the great artery , large ventricular septal defect , and pulmonary atresia . during svt , we observed an antegrade his signal at cs 3,4 catheter ( a , blank arrow ) and earliest retrograde a with a retrograde his signal at the abld catheter ( b , arrow ) . the antegrade atrioventricular ( av ) node is at the anterior av groove ( arrow ) , and the retrograde av node is at the posterior av groove ( blank arrow ) . radiofrequency ablation at the posterior site successfully eliminated the retrograde av node and terminated the twin av nodal reentrant tachycardia . transcatheter ablation through the limited catheter approach13 was performed in 14 of 21 svt patients ( 66.7% ) . the reasons for no ablation included limited vascular access in 3 patients , parental concern about the risk of atrioventricular block in 3 patients , and hemodynamic deterioration at the time of eps in 1 patient . because of young age and a limited catheter approach , 1 pattern of svt could be eliminated , but the other persisted ; therefore , the success rate of catheter ablation was 83.3% for both tavnrt and avrt . for catheter ablation of tavnrt , during radiofrequency energy application , junctional ectopic tachycardia could be induced in 6 of 8 patients . the qrs morphology at surface ecg did not change after successful ablation of minor atrioventricular nodes in the patients with 1 dominant ecg pattern . for the 4 patients with fused qrs morphology at surface ecg , successful ablation was achieved in 2 , and qrs duration was not prolonged after ablation . after a mean followup duration of 6 years ( median 5.1 years , range 0.215.7 years ) after successful catheter ablation , 2 patients ( 16.7% ) exhibited svt recurrence . one patient who received transcatheter ablation through fenestration had the same arrhythmia recurrence 2.2  years later . the other patient , who had 3 accessory pathways ablated , exhibited svt recurrence 1  year after ablation . repeated epss showed another accessory pathway that was not detected at the initial eps and was successfully eliminated at the second ablation . we enrolled 30 cchd patients with singleventricle physiology ( 17 male , 13 female ) . we categorized these patients into 3 groups according to the underlying cardiac anomaly : right atrial isomerism ( rai ) , left atrial isomerism ( lai ) , and other cchds . all patients with rai and most with lai exhibited similar combinations of cardiac anomalies including a common atrioventricular canal and doubleoutlet right ventricle with or without left ventricular hypoplasia . the age at surgery has decreased gradually over the past 20  years because of better outcomes for early total cavopulmonary circulation surgery in recent reports.12 because transcatheter ablation is difficult after fontantype surgery ( the only route from the vena cava to the atrium after fontan surgery is fenestration of the conduit ) , we preferred performing an eps and transcatheter ablation before fontan surgery the mean age at the time of the eps was 7.1  years , and the mean body weight was 20.7  kg in our study population . cardiac anomalies of the 3 groups asd indicates atrial septal defect ; cchd , complex congenital heart disease ; cctga , congenitally corrected transposition of the great arteries ; lai , left atrial isomerism ; rai , right atrial isomerism ; tga , transposition of the great arteries ; vsd , ventricular septal defect . basic clinical characteristics of the 3 patient groups data are shown as meansd or number ( percentage ) except as noted . cchd indicates complex congenital heart disease ; eps , electrophysiology study ; lai , left atrial isomerism ; rai , right atrial isomerism ; svt , supraventricular tachycardia ; tcpc , total cavopulmonary circulation . only patients with clinical svt the indications for an eps included clinically confirmed svt ( with 12lead ecg confirmation ) in 13 patients ( 43.3% ; most presented with palpitation and dizziness or near syncope ) , clinical svt without 12lead ecg in 10 patients ( 33.3% ; mostly shown on the ecg monitor during hospitalization for surgery , cardiac catheterization , or other reasons ) , and preoperative assessment in 7 patients ( 23.3% ; including suspected tavn or abnormal atrioventricular node function at previous ecg examination ) . overall , > 2 narrow qrs ecg patterns were common ( n=10 , 33.3% ) in the cchd patients , particularly those with rai and with other cchds . among these 10 patients , 4 ( 40% ) had fused qrs morphology ( figure  1a ) , and the other 6 ( 60% ) had 1 dominant ecg pattern , but another ecg pattern may occur occasionally . during the epss , more than half of the patients ( n=18 , 60% ) had tavn ( inclusive of 8 patients with 2 ecg patterns demonstrated only at eps ) , particularly the rai patients ( 72.2% ) ( figure  1b ) . the major atrioventricular node occurred more commonly at the posterior atrioventricular ring but also presented at the anterior atrioventricular ring ( figure  2 ) . in the 7 patients without heterotaxy syndrome , 3 ( 42.9% ) had tavn . of those , 2 had congenitally corrected transposition of the great arteries , and 1 had a doubleoutlet right ventricle with situs inversus , dextrocardia , atrial septal defect , large ventricular septal defect with unusual atrioventricular septal alignment , and pulmonary atresia . electrophysiological characteristics in the 3 patient groups data are shown as meansd or number ( percentage ) except as noted . av indicates atrioventricular ; avnrt , atrioventricular nodal reentrant tachycardia ; avrt , atrioventricular reentrant tachycardia ; cchd , complex congenital heart disease ; eps , electrophysiology study ; lai , left atrial isomerism ; rai , right atrial isomerism ; svt , supraventricular tachycardia ; tavnrt , twin atrioventricular nodal reentrant tachycardia . a , a patient with right atrial isomerism who had fused two nonpreexcited qrs morphologies with different axis at usual surface ecg recording . she was documented to have twin atrioventricular nodes ( tavn ) at later electrophysiology study ( eps ) . b , another patient with right atrial isomerism who had another qrs morphology ( arrow ) at the returning beat of atrial extrastimulation during eps , suggesting the existence of tavn . illustration of the distribution of atrioventricular ( av ) conduction tissue in our single ventricle disease patients . the major av nodes are mostly located in the posterior site and minor av nodes at the anterior site . however , some variations are observable . black circles indicate major av nodes , empty circles indicate minor av nodes , crosses indicate accessory pathways , and triangles indicate intrahisian sling connection or ectopic junctional focus . the wenckebach cycle length was prolonged in the lai group compared with the other 2 groups . one lai patient was implanted with a permanent pacemaker because of extreme bradycardia at age 7  years . svt could be induced in 21 of 30 patients ( 70% ) , and 2 patients ( 1 with rai and 1 with other cchd ) had concomitant atrial tachyarrhythmia . tavnrt accounted for 64.3% of svt in the rai patients ( figure  3 ) and 40% of svt in those with other cchds ( figure  4 ) . avrt was the second most common svt in the cchd patients and often was associated with multiple accessory pathways ( 3 of 6 , 50% ) . junctional tachycardia occurred in 3 patients ( 14.3% ) , 2 with tavn and another with a single atrioventricular node . avnrt occurred in 2 patients ( 9.5% ) ; one had typical avnrt ( and rai ) , and the other had atypical avnrt ( in transposition of the great artery with a large ventricular septal defect and pulmonary atresia ) . electrophysiology and ablation for twin atrioventricular nodal reentrant tachycardia in a patient with right atrial isomerism . ( a ) and ( c ) in the left panel , we can see antegrade his signal at the ablation catheter ( abld ) and located in the right posterior area ( arrow ) , which marked the location of antegrade major av node . ( b ) and ( d ) in the right panel , during tachycardia , we can see retrograde his signal at the ablation catheter ( abld ) , which was also at the nearby location of earliest retrograde a ( not shown in this tracing ) . the ablation catheter was at the right anterior av groove , which was the common site of minor av nodes . electrophysiology and ablation for twin atrioventricular nodal reentrant tachycardia in a patient with single ventricle disease . ( a ) and ( b ) this 9year old boy is a patient of crisscross heart , corrected transposition of the great artery , large ventricular septal defect , and pulmonary atresia . during svt , we observed an antegrade his signal at cs 3,4 catheter ( a , blank arrow ) and earliest retrograde a with a retrograde his signal at the abld catheter ( b , arrow ) . the antegrade atrioventricular ( av ) node is at the anterior av groove ( arrow ) , and the retrograde av node is at the posterior av groove ( blank arrow ) . radiofrequency ablation at the posterior site successfully eliminated the retrograde av node and terminated the twin av nodal reentrant tachycardia . transcatheter ablation through the limited catheter approach13 was performed in 14 of 21 svt patients ( 66.7% ) . the reasons for no ablation included limited vascular access in 3 patients , parental concern about the risk of atrioventricular block in 3 patients , and hemodynamic deterioration at the time of eps in 1 patient . because of young age and a limited catheter approach , 1 pattern of svt could be eliminated , but the other persisted ; therefore , the success rate of catheter ablation was 83.3% for both tavnrt and avrt . for catheter ablation of tavnrt , during radiofrequency energy application , junctional ectopic tachycardia could be induced in 6 of 8 patients . the qrs morphology at surface ecg did not change after successful ablation of minor atrioventricular nodes in the patients with 1 dominant ecg pattern . for the 4 patients with fused qrs morphology at surface ecg , successful ablation was achieved in 2 , and qrs duration was not prolonged after ablation . after a mean followup duration of 6 years ( median 5.1 years , range 0.215.7 years ) after successful catheter ablation , 2 patients ( 16.7% ) exhibited svt recurrence . one patient who received transcatheter ablation through fenestration had the same arrhythmia recurrence 2.2  years later . the other patient , who had 3 accessory pathways ablated , exhibited svt recurrence 1  year after ablation . repeated epss showed another accessory pathway that was not detected at the initial eps and was successfully eliminated at the second ablation . reports concerning epss and the ablation of complex svt in cchd patients with singleventricle physiology are limited . in the present study , tavnrt was common not only in the rai patients but also in those with other cchds . other forms of svt , namely , avrt from accessory pathways and avnrt , accounted for < 50% of svt cases . transcatheter ablation targeting the minor atrioventricular node for tavnrt , multiple accessory pathways for avrt , and a slow pathway for avnrt was effective and should be performed before fontan completion . after fontan surgery , the current 10year survival rate can reach 75% to 89%14 ; however , a certain percentage of patients still exhibit early fontan failure and even mortality . several risk factors have been proposed , and clinical arrhythmia has been shown to be one of the most crucial.3 common arrhythmias after fontan surgery include atrial arrhythmia , svt , and ventricular arrhythmia . many reports have discussed the impact of atrial arrhythmia on fontan circulation , and successful catheter ablation can improve the clinical outcome , although recurrence is a concern.4 , 5 nevertheless , studies concerning the presence of svt in cchd patients receiving fontantype surgery are scant . in cchd patients , because of the varied atrioventricular conduction system , various types of svt unrelated to the atrial scar from fontan surgery can occur . heterotaxy syndrome is the most common cchd associated with svt . in a metaanalysis of heterotaxy syndrome , the incidence of arrhythmia was shown to be > 70% by age 5  years.15 in another large cohort study ( n=257 ) , wu et  al found that svt occurred in 26% of patients . by age 10  years , > 50% of rai patients would have had > 1 episode of svt.16 in lai patients , although bradyarrhythmia caused by either sinus node or atrioventricular node dysfunction is a common conduction disturbance,17 svt remains a concern . miyazaki et  al found that supraventricular arrhythmia occurred in 23% of 83 lai patients ; although atrial arrhythmia was the most common arrhythmia mechanism , svt and junctional ectopic tachycardia accounted for 19% of arrhythmia cases.18 in patients who have other cchds with singleventricle physiology , relevant studies about the electrophysiological characteristics were lacking , with only some case reports published.19 , 20 , 21 
  in the present study , the most common svt mechanism in cchd patients was tavnrt , followed by avrt , junctional tachycardia , and avnrt . tavn was previously suggested to be associated with rai.9 , 22 , 23 on the basis of pathological findings , dickinson et  al suggested that paired atrioventricular nodes with a sling of conduction tissue are common in patients with heterotaxy and cchds.9 in the present study , the incidence of tavn in rai patients was as high as 72.2% , and tavnrt accounted for 64.3% of svt cases in the rai patients . in addition , tavnrt was not uncommon in the patients with lai and other cchds . in the present study , 2 lai patients with tavn physiology shared similar cardiac anomalies including a common atrioventricular canal and doubleoutlet right ventricle and pulmonary stenosis . in 3 patients with other cchds with tavn , cardiac anomalies varied from crisscross atrioventricular valves to corrected transposition of the great arteries . two of these patients had rai , and the remaining 5 had either a common atrioventricular canal or crisscross atrioventricular valves.10 these results suggest that tavn is not unique in rai patients and also can occur in patients with lai and other cchds , particularly those with abnormal atrioventricular connection . previous studies of catheter ablation of tavnrt are very limited.10 , 16 in the present study , both the success rate ( 10 of 12 , 83.3% ) and the recurrence rate ( 1 or 10 , 10% ) were acceptable . consequently , catheter ablation was effective for complex svt in cchd patients . most of our patients with tavn ( 77.8% ) present with 1 dominant ecg pattern at resting ecg . the qrs morphology at surface ecg did not change after successful ablation of minor atrioventricular nodes in these patients . for the other 4 patients with fused qrs morphology at surface ecg , although cardiac synchronization might be a concern after ablation in these patients , in our limited experience , it did not cause dyssynchronization after successful ablation . in addition to tavnrt , another important arrhythmia mechanism related to tavn is junctional tachycardia . monckeberg was the first to identify sling conduction tissue between 2 atrioventricular nodes , which suggested that this tissue may be a reentry circuit.9 , 24 takeuchi et  al further confirmed the presence of an intrahisian sling through detailed mapping.25 wu et  al and miyazaki et  al17 , 18 showed junctional tachycardia in certain lai patients . in the present study , junctional tachycardia , which has narrow qrs tachycardia with dissociated va conduction , was present in 3 patients2 with rai and 1 with lai . in the rai patients with junctional tachycardia , both had tavn , and both could be induced by atrial extrastimulation ; therefore , reentry through an intrahisian sling may be the mechanism of this unusual type of junctional tachycardia . catheter ablation was not further performed in these patients because of the high risk of complete atrioventricular block , which may aggravate hemodynamic deterioration in patients who have undergone fontan surgery , even after pacemaker implantation.26 new ablation tools , such as the cryoablation technique , may be helpful because of their reversibility and can be valuable for treating this junctional tachycardia.27 , 28 in addition , for cchd with tavnrt with nearby paired atrioventricular nodes , avnrt without normal anatomical guidance , and accessory pathways close to a single atrioventricular node , cryoablation can also be useful.29 
  avrt is also a key mechanism of svt in cchd patients . in a large cohort study , chetaille et  al presented the outcome of catheter ablation in 83 patients with congenital heart disease . multiple accessory pathways were present in 20% of patients , and the ablation success rate in patients without ebstein 's anomaly was 69% , with a recurrence rate of 19% . in their cohort , 17 patients had singleventricle cchd , including 7 patients with heterotaxy syndrome30 ; however , that study did not further discuss those with singleventricle physiology . in the present study , multiple accessory pathways , perhaps as many as 4 , are very common and account for 50% of avrt cases . nevertheless , catheter ablation remains effective , with a success rate of 83.3% and a recurrence rate of 20% ( 1 in 5 ) . some patients are young when they undergo an eps ; some have vessel occlusion resulting from previous interventions , and some have completed fontan surgery with only vascular access to the atrium as fenestration . in 3 of our patients , catheter ablation new mapping catheters , such as 2f multipolar catheters , have been shown to be helpful . we performed the eps by using two 2f catheters from the internal jugular vein through a glenn shunt to the pulmonary artery and to the right atrium and right ventricle in a patient with bilateral femoral vein occlusion ( figure  5 ) . these small catheters can provide more precise mapping when vascular access is limited in cchd patients . electrophysiology study using limited catheter approach with smallsized catheters in a patient with bilateral femoral vein occlusion . a 7yearold boy with pulmonary atresia with intact ventricular septum receiving a rightventricular outflow tract patch and rightsite bidirectional glenn shunt . a , right ventriculogram showed a hypoplastic right ventricle with inlet and trabecular bipartile hypoplasia . b , we used a guiding catheter and two 2f multipolar catheters for right atrial and right ventricle signal sensing , pacing , and mapping . because his accessory pathway was located at the left atrioventricular groove , we used a retrograde transaortic approach and successfully eliminated the accessory pathway . after fontan surgery , the current 10year survival rate can reach 75% to 89%14 ; however , a certain percentage of patients still exhibit early fontan failure and even mortality . several risk factors have been proposed , and clinical arrhythmia has been shown to be one of the most crucial.3 common arrhythmias after fontan surgery include atrial arrhythmia , svt , and ventricular arrhythmia . many reports have discussed the impact of atrial arrhythmia on fontan circulation , and successful catheter ablation can improve the clinical outcome , although recurrence is a concern.4 , 5 nevertheless , studies concerning the presence of svt in cchd patients receiving fontantype surgery are scant . in cchd patients , because of the varied atrioventricular conduction system , various types of svt unrelated to the atrial scar from fontan surgery can occur . heterotaxy syndrome is the most common cchd associated with svt . in a metaanalysis of heterotaxy syndrome , the incidence of arrhythmia was shown to be > 70% by age 5  years.15 in another large cohort study ( n=257 ) , wu et  al found that svt occurred in 26% of patients . by age 10  years , > 50% of rai patients would have had > 1 episode of svt.16 in lai patients , although bradyarrhythmia caused by either sinus node or atrioventricular node dysfunction is a common conduction disturbance,17 svt remains a concern . miyazaki et  al found that supraventricular arrhythmia occurred in 23% of 83 lai patients ; although atrial arrhythmia was the most common arrhythmia mechanism , svt and junctional ectopic tachycardia accounted for 19% of arrhythmia cases.18 in patients who have other cchds with singleventricle physiology , relevant studies about the electrophysiological characteristics were lacking , with only some case reports published.19 , 20 , 21 in the present study , the most common svt mechanism in cchd patients was tavnrt , followed by avrt , junctional tachycardia , and avnrt . tavn was previously suggested to be associated with rai.9 , 22 , 23 on the basis of pathological findings , dickinson et  al suggested that paired atrioventricular nodes with a sling of conduction tissue are common in patients with heterotaxy and cchds.9 in the present study , the incidence of tavn in rai patients was as high as 72.2% , and tavnrt accounted for 64.3% of svt cases in the rai patients . in addition , tavnrt was not uncommon in the patients with lai and other cchds . in the present study , 2 lai patients with tavn physiology shared similar cardiac anomalies including a common atrioventricular canal and doubleoutlet right ventricle and pulmonary stenosis . in 3 patients with other cchds with tavn , cardiac anomalies varied from crisscross atrioventricular valves to corrected transposition of the great arteries . two of these patients had rai , and the remaining 5 had either a common atrioventricular canal or crisscross atrioventricular valves.10 these results suggest that tavn is not unique in rai patients and also can occur in patients with lai and other cchds , particularly those with abnormal atrioventricular connection . previous studies of catheter ablation of tavnrt are very limited.10 , 16 in the present study , both the success rate ( 10 of 12 , 83.3% ) and the recurrence rate ( 1 or 10 , 10% ) were acceptable . consequently , catheter ablation was effective for complex svt in cchd patients . most of our patients with tavn ( 77.8% ) present with 1 dominant ecg pattern at resting ecg . the qrs morphology at surface ecg did not change after successful ablation of minor atrioventricular nodes in these patients . for the other 4 patients with fused qrs morphology at surface ecg although cardiac synchronization might be a concern after ablation in these patients , in our limited experience , it did not cause dyssynchronization after successful ablation . in addition to tavnrt , another important arrhythmia mechanism related to tavn is junctional tachycardia . monckeberg was the first to identify sling conduction tissue between 2 atrioventricular nodes , which suggested that this tissue may be a reentry circuit.9 , 24 takeuchi et  al further confirmed the presence of an intrahisian sling through detailed mapping.25 wu et  al and miyazaki et  al17 , 18 showed junctional tachycardia in certain lai patients . in the present study , junctional tachycardia , which has narrow qrs tachycardia with dissociated va conduction , was present in 3 patients2 with rai and 1 with lai . in the rai patients with junctional tachycardia , both had tavn , and both could be induced by atrial extrastimulation ; therefore , reentry through an intrahisian sling may be the mechanism of this unusual type of junctional tachycardia . catheter ablation was not further performed in these patients because of the high risk of complete atrioventricular block , which may aggravate hemodynamic deterioration in patients who have undergone fontan surgery , even after pacemaker implantation.26 new ablation tools , such as the cryoablation technique , may be helpful because of their reversibility and can be valuable for treating this junctional tachycardia.27 , 28 in addition , for cchd with tavnrt with nearby paired atrioventricular nodes , avnrt without normal anatomical guidance , and accessory pathways close to a single atrioventricular node , cryoablation can also be useful.29 avrt is also a key mechanism of svt in cchd patients . in a large cohort study , chetaille et  al presented the outcome of catheter ablation in 83 patients with congenital heart disease . multiple accessory pathways were present in 20% of patients , and the ablation success rate in patients without ebstein 's anomaly was 69% , with a recurrence rate of 19% . in their cohort , 17 patients had singleventricle cchd , including 7 patients with heterotaxy syndrome30 ; however , that study did not further discuss those with singleventricle physiology . in the present study , avrt accounted for 28.6% of svt cases in the cchd patients . multiple accessory pathways , perhaps as many as 4 , are very common and account for 50% of avrt cases . nevertheless , catheter ablation remains effective , with a success rate of 83.3% and a recurrence rate of 20% ( 1 in 5 ) . some patients are young when they undergo an eps ; some have vessel occlusion resulting from previous interventions , and some have completed fontan surgery with only vascular access to the atrium as fenestration . in 3 of our patients , catheter ablation new mapping catheters , such as 2f multipolar catheters , have been shown to be helpful . we performed the eps by using two 2f catheters from the internal jugular vein through a glenn shunt to the pulmonary artery and to the right atrium and right ventricle in a patient with bilateral femoral vein occlusion ( figure  5 ) . these small catheters can provide more precise mapping when vascular access is limited in cchd patients . electrophysiology study using limited catheter approach with smallsized catheters in a patient with bilateral femoral vein occlusion . a 7yearold boy with pulmonary atresia with intact ventricular septum receiving a rightventricular outflow tract patch and rightsite bidirectional glenn shunt . a , right ventriculogram showed a hypoplastic right ventricle with inlet and trabecular bipartile hypoplasia . b , we used a guiding catheter and two 2f multipolar catheters for right atrial and right ventricle signal sensing , pacing , and mapping . because his accessory pathway was located at the left atrioventricular groove , we used a retrograde transaortic approach and successfully eliminated the accessory pathway . epss in 30 cchd patients revealed that tavn was common not only in the rai patients but also in those with other singleventricle cchds . transcatheter ablation of the minor atrioventricular node for tavnrt , multiple accessory pathways for avrt , and a slow pathway for avnrt are effective approaches . ablation feasibility is usually limited by vascular access and the risk of atrioventricular block , which may be overcome by using a new mapping catheter and ablation tools .
backgroundsupraventricular tachycardia ( svt ) is common in complex congenital heart disease ( cchd ) patients with singleventricle physiology and may cause hemodynamic deterioration . 
 we reported the outcomes of catheter ablation for such complex svt in these singleventricle cchd patients.methods and resultspatients with singleventricle physiology ( defined as cchd patients ) who received electrophysiology studies and catheter ablation between 1995 and 2015 were studied . 
 we enrolled 30 cchd patients ( 18 with right atrial isomerism , 5 with left atrial isomerism , and 7 with other cchds ; 17 male , 13 female ) . 
 the age of onset of clinical svt was 6.7 years ( 4.7 years ) . 
 electrophysiology studies and ablation were performed at age 7.1 years ( 3.9 years ) ; body weight was 20.7 kg ( 10.0 kg ) . 
 twin atrioventricular nodes were present in 60% of patients ( right atrial isomerism , 72.2% ; left atrial isomerism , 40% ; other cchds , 42.9% ) . 
 manifested preexcitation was noted in 10% of patients . 
 svt was induced in 21 patients . 
 twin atrioventricular nodal reentrant tachycardia was the most common ( 57.1% ) , followed by atrioventricular reentrant tachycardia ( 28.6% ) , junctional tachycardia ( 14.3% ) , and atrioventricular nodal reentrant tachycardia ( 9.5% ) . 
 multiple arrhythmias were common ( 33.3% ) , particularly in patients with atrioventricular reentrant tachycardia ( 50% ) . 
 ablation successfully eliminated svt in 12 of 14 patients ( 85.7% ) , with a recurrence rate of 16.7% during 6  years of followup.conclusionstranscatheter ablation of complex svt substrates , including minor atrioventricular node of twin atrioventricular nodal reentrant tachycardia , accessory pathways of atrioventricular reentrant tachycardia , and a slow pathway of atrioventricular nodal reentrant tachycardia , is effective in cchd patients . 
 the limitations are limited vascular access and the risk of atrioventricular block .