Identification of supraventricular tachycardia mechanisms with surface electrocardiograms using a convolutional neural network.

Background: It remains difficult to definitively distinguish supraventricular tachycardia (SVT) mechanisms using a 12-lead electrocardiogram (ECG) alone. Machine learning may identify visually imperceptible changes on 12-lead ECGs and may improve ability to determine SVT mechanisms. Objective: We sought to develop a convolutional neural network (CNN) that identifies the SVT mechanism according to the gold standard of SVT ablation and to compare CNN performance against experienced electrophysiologists among patients with atrioventricular nodal re-entrant tachycardia (AVNRT), atrioventricular reciprocating tachycardia (AVRT), and atrial tachycardia (AT). Methods: All patients with 12-lead surface ECG during sinus rhythm and SVT and had successful SVT ablation from 2013 to 2020 were included. A CNN was trained using data from 1505 surface ECGs that were split into 1287 training and 218 test ECG datasets. We compared the CNN performance against independent adjudication by 2 experienced cardiac electrophysiologists on the test dataset. Results: Our dataset comprised 1505 ECGs (368 AVNRT, 304 AVRT, 95 AT, and 738 sinus rhythm) from 725 patients. The CNN areas under the receiver-operating characteristic curve for AVNRT, AVRT, and AT were 0.909, 0.867, and 0.817, respectively. When fixing the specificity of the CNN to the electrophysiologist adjudicators' specificity, the CNN identified all SVT classes with higher sensitivity: (1) AVNRT (91.7% vs 65.9%), (2) AVRT (78.4% vs 63.6%), and (3) AT (61.5% vs 50.0%). Conclusion: A CNN can be trained to differentiate SVT mechanisms from surface 12-lead ECGs with high overall performance, achieving similar performance to experienced electrophysiologists at fixed specificities.

A heart-team hybrid approach for atrial fibrillation: a single-centre long-term clinical outcome cohort study.

OBJECTIVES: The Cox-maze IV is the gold standard for surgical ablation of atrial fibrillation (AF). A heart-team hybrid approach using selected epicardial thoracoscopic surgical ablations and completion endocardial ablations to replicate the Cox-maze IV lesion set has gained popularity and early results have been promising. We herein report our single-centre long-term clinical outcomes using the heart-team hybrid approach with 455 patients. METHODS: From 1 March 2013 to 1 July 2019, we prospectively collected data on all patients referred to our heart team for rhythm-control strategy for AF. Baseline characteristics, procedural complications and long-term freedom from AF (FFAF) both on and off anti-arrhythmic drug therapy were analysed. Ambulatory monitoring (>7 days) was obtained at 3 months and annually thereafter. RESULTS: Four hundred and fifty-five patients completed the hybrid approach. Four hundred and forty-five (97.8%) patients had non-paroxysmal AF (long-standing persistent AF n = 249, 54.7%; persistent AF n = 196, 43.1%; paroxysmal AF n = 10, 2.2%). Average duration of AF was 5.9 ± 6.1 years. Average left atrial diameter was 4.8 ± 0.8 cm. FFAF at 3, 12, 24 and 36 months was 92%, 87%, 81% and 72%, respectively. FFAF without the use of anti-arrhythmic medications was 75%, 81%, 76% and 66%. Any surgical complications occurred in 28 (6.1%) patients. CONCLUSIONS: A heart-team hybrid strategy for the treatment of AF is safe and effective. In a predominantly non-paroxysmal population with AF, at the 3-year follow-up, FFAF in patients on and off anti-arrhythmic drugs approaches that of patients who had the Cox-maze IV.

Gravimetric Quantification of Hydrophobic Fulvic Acids in Lignite Material Using Acetone.

This study evaluated the gravimetric quantification of fulvic components in lignite material. The current standard method considered only components soluble in both alkali and acid solutions and adsorpted to a hydrophobic resin (DAX-8) at pH 1, identified as hydrophobic fulvic acids (HFA). HFA were desorpted using 0.1 M sodium hydroxide (NaOH) and run through a cation exchange resin (IR-120) to remove Na ions. HFA were quantified gravimetrically, including ash corrections. Steps involving IR-120, however, resulted in a large volume of analytical solution and a significant time of analysis, which became a disadvantage of this method. A modification to this method was proposed by replacing NaOH with acetone to desorpt HFA from DAX-8. IR-120 was not required, and a smaller volume of analytical solution was produced. Gravimetric, Fourier transform infrared (FTIR), and ionic analyses were completed in this study. Both standard and modified methods resulted in the presence of hydrophobic neutrals within the quantification of HFA. Acetone at 50% strength produced HFA results corresponding to those of NaOH. An adequate regeneration of DAX-8 was achieved using the modified method but not with the standard method. The modified method produced higher ash contents, but they did not affect its accuracy. This method significantly reduced the overall analytical time. This study showed the potential of acetone as a desorpting agent in the gravimetric quantification of HFA in lignite material.

A novel pacing maneuver to verify the postpacing interval minus the tachycardia cycle length while adjusting for decremental conduction: Using "dual-chamber entrainment" for improved supraventricular tachycardia discrimination.

BACKGROUND: The postpacing interval (PPI) minus the tachycardia cycle length (TCL) is frequently used to investigate tachycardias. However, a variety of issues (eg, failure to entrain, decremental conduction, and oscillating TCLs) can make interpretation of the PPI-TCL challenging. OBJECTIVE: The purpose of this study was to investigate a novel maneuver to confirm the PPI-TCL value without using either the ventricular PPI or the TCL interval and to assess the ability of this maneuver to identify decremental conduction and differentiate supraventricular tachycardias. METHODS: We analyzed 77 intracardiac recordings from patients (age 25 ± 20 years; 40 female) who underwent catheter ablation of atrioventricular nodal reentrant tachycardia (AVNRT) or orthodromic reciprocating tachycardia (ORT) with a concealed pathway. We calculated the PPI-TCL, the AH-corrected PPI-TCL, and estimated the PPI-TCL using "dual-chamber entrainment" calculated as [PPIV - TCL = Stim(A→V) + Stim(V→A) - PPIA]. RESULTS: The PPI-TCL calculated by dual-chamber entrainment highly correlated with the observed and AH-corrected PPI-TCL (R2 = 0.79 and 0.96, respectively; P <.001]. A dual-chamber entrainment PPI-TCL value of 80 ms correctly differentiated all AVNRT from septal ORT cases, whereas the standard PPI-TCL and AH-corrected PPI-TCL methods were incorrect in 14% and 6% of cases, respectively. Dual-chamber entrainment identified 3 ± 10 ms of additional decremental conduction beyond AH prolongation, including 4 pathways with significant (>10 ms) decrement. CONCLUSION: Dual-chamber entrainment estimates the PPI-TCL value without using either the ventricular PPI or the TCL interval. This maneuver adjusts for all decremental conduction, including within concealed pathways, where a dual-chamber entrainment PPI-TCL value >80 ms favors AVNRT over ORT. This maneuver can be used to verify the observed PPI-TCL value in challenging cases.

The precise timing of tachycardia entrainment is determined by the postpacing interval, the tachycardia cycle length, and the pacing rate: Theoretical insights and practical applications.

BACKGROUND: Previous observations have reported that the number of pacing stimuli required to entrain a tachycardia varies on the basis of arrhythmia type and location, but a quantitative formulation of the number needed to entrain (NNE) that unifies these observations has not been characterized. OBJECTIVE: We sought to investigate the relationship between the number of pacing stimulations, the tachycardia cycle length (TCL), the overdrive pacing cycle length (PCL), and the postpacing interval (PPI) to accurately estimate the timing of tachycardia entrainment. METHODS: First, we detailed a mathematical derivation unifying electrophysiological parameters with empirical confirmation in 2 patients undergoing catheter ablation of typical atrial flutter. Second, we validated our formula in 44 patients who underwent various catheter ablation procedures. For accuracy, we corrected for rate-related changes in conduction velocity. RESULTS: We derived the equations NNE = |(PPI - TCL)/(TCL - PCL)| + 1 and Tachycardia advancement = (NNE - 1) × (TCL - PCL) - (PPI - TCL), which state that the NNE and the amount of tachycardia advancement on the first resetting stimulation are determined using regularly measured intracardiac parameters. In the retrospective cohort, the observed PPI - TCL highly correlated with the predicted PPI - TCL (mean difference 5.8 ms; r = 0.97; P < .001), calculated as PPI - TCL = (NNE - 1) × (TCL - PCL) - tachycardia advancement. CONCLUSION: The number of pacing stimulations required to entrain a reentrant tachycardia is predictable at any PCL after correcting for cycle length-dependent changes in conduction velocity. This relationship unifies established empirically derived diagnostic and mapping criteria for supraventricular tachycardia and ventricular tachycardia. This relationship may help elucidate when antitachycardia pacing episodes are ineffective or proarrhythmic and could potentially serve as a theoretical basis to customize antitachycardia pacing settings for improved safety and effectiveness.

[The sentinel lymph node procedure also feasible in patients with recurrent breast cancer]. / Schildwachtklierprocedure: ook een mogelijkheid bij patiënten met een recidief van een mammacarcinoom.

The sentinel lymph node (SLN) concept has become a standard option for the diagnosis and treatment of patients with primary invasive breast cancer. The implementation of this SLN concept has created a new category of patients: those who had breast-conserving therapy without complete axillary lymph node dissection following a negative SLN biopsy. In cases of local relapse in the ipsilateral breast, questions arise on the lymphatic drainage of this new tumour. Such is also the case for patients who have been treated for ductal carcinoma in situ, who have had a previous mastectomy, or even after previous benign breast or axillary surgery. To date the literature on SLN biopsy in patients with recurrent breast cancer is scarce: only to publications dealing with 116 patients. It is concluded that a SLN procedure in recurrent disease is feasible and can possibly lead to the identification of specific or aberrant lymphatic drainages. This could then lead to useful changes being made to the multidisciplinary treatment strategy in selected patients following a successful SLN biopsy. Since there are still no proper guidelines for performing such a repeat SLN biopsy, it is necessary to reach consensus on this new indication in the short-term.

Early human experience with use of a deflectable fiberoptic endocardial visualization catheter to facilitate coronary sinus cannulation.

BACKGROUND: Despite improvements in cardiac resynchronization therapy (CRT) implantation techniques, a significant minority of CRT attempts are unsuccessful. Inability to cannulate the coronary sinus (CS) because of difficult anatomy is a major reason for unsuccessful CRT implantation. Direct visualization of intracardiac structures during the implant may facilitate access into the CS. The present study describes CRT implantation with the aid of an endocardial visualization catheter (EVC). METHODS: Fifty-eight consecutive patients (mean age 72 +/- 12 years; ejection fraction 26.2% +/- 7.0%; New York Heart Association [NYHA] class 2.9) underwent CRT implantation using a steerable fiberoptic EVC (Acumen Medical, Inc., Sunnyvale, CA). RESULTS: The EVC was able to visualize the CS ostium in all cases. The CS was successfully cannulated in 57 (98.3%) of 58 patients. The time from vascular access to CS visualization was 6 +/- 5 minutes, and the total time to CS access was 8 +/- 6 minutes. Successful left ventricle (LV) lead implantation was accomplished in 55 (94.8%) of 58 patients. Three patients who had a previous history of failed LV lead implantation were successfully implanted using the EVC. CONCLUSION: Fiberoptic imaging of intracardiac structures during CRT implantation may be performed rapidly in a wide range of patients with an EVC. The ability to visualize right atrial anatomy may aid CS access and LV lead implantation.

Efficacy of chloroquine + sulfadoxine--pyrimethamine, mefloquine + artesunate and artemether + lumefantrine combination therapies to treat Plasmodium falciparum malaria in the Chittagong Hill Tracts, Bangladesh.

Bangladesh faces growing levels of Plasmodium falciparum resistance to chloroquine (CQ) and sulfadoxine-pyrimethamine (SP). Alternative antimalarial therapies, particularly combination regimens, need to be considered. Therefore, the efficacy of three antimalarial combination therapies was assessed in Chittagong Hill Tracts. A total of 364 P. falciparum patients were recruited and randomly assigned to either CQ + SP, mefloquine + artesunate (MQ + AS) or lumefantrine + artemether (Coartem). Results showed that CQ + SP therapy was less effective than the two artemisinin-based combination therapies. The day 42 PCR-corrected efficacy rate was 62.4% for CQ + SP, 100% for MQ + AS and 97.1% for Coartem. Failures occurred at a shorter interval after CQ + SP treatment than after Coartem. The artemisinin-based therapies effectively prevented development of gametocytes, whereas CQ + SP did not. All three therapies were well tolerated, although reports of mild complaints during treatment appeared higher with MQ + AS. We conclude that CQ + SP is not a viable option for replacing CQ monotherapy as first-line P. falciparum treatment in this area of Bangladesh. A change to artemisinin-based combination therapy is recommended. Both Coartem and MQ + AS appear to be good options, effective in curing P. falciparum malaria and in preventing recrudescences following treatment.

[Neuroleptic malignant syndrome in users of risperidone]. / Het maligne neurolepticasyndroom bij gebruik van risperidon.

Neuroleptic malignant syndrome is a potentially life-threatening complication of neuroleptic therapy. Three nursing-home residents who suffered from multiple somatic disorders and dementia, two men aged 78 and 76 years respectively and a woman aged 72 years, developed neuroleptic malignant syndrome following the use of risperidone. The first two patients presented with severe muscular pain. Initially, none of them exhibited high fever and so it is debatable whether the diagnosis 'neuroleptic malignant syndrome' can be excluded in the absence of fever. A review of the literature indicates that the syndrome can easily be mistaken for an infection. Patients with extrapyramidal side effects as a result of earlier use of neuroleptics seem to be more vulnerable to developing neuroleptic malignant syndrome. Patients using neuroleptics who become ill with an altered level of consciousness, sweating and muscular rigidity, should at least have their blood tested for the level of creatine phosphokinase activity. Furthermore, pain might be an initial symptom. Treatment of the neuroleptic malignant syndrome failed in all three patients, and they died.

Effects of cardiac resynchronization on disease progression in patients with left ventricular systolic dysfunction, an indication for an implantable cardioverter-defibrillator, and mildly symptomatic chronic heart failure.

BACKGROUND: The effects of cardiac resynchronization therapy (CRT) in patients with mildly symptomatic heart failure have not been fully elucidated. METHODS AND RESULTS: The Multicenter InSync ICD Randomized Clinical Evaluation II (MIRACLE ICD II) was a randomized, double-blind, parallel-controlled clinical trial of CRT in NYHA class II heart failure patients on optimal medical therapy with a left ventricular (LV) ejection fraction < or =35%, a QRS > or =130 ms, and a class I indication for an ICD. One hundred eighty-six patients were randomized: 101 to the control group (ICD activated, CRT off) and 85 to the CRT group (ICD activated, CRT on). End points included peak VO2, VE/CO2, NYHA class, quality of life, 6-minute walk distance, LV volumes and ejection fraction, and composite clinical response. Compared with the control group at 6 months, no significant improvement was noted in peak VO2, yet there were significant improvements in ventricular remodeling indexes, specifically LV diastolic and systolic volumes (P=0.04 and P=0.01, respectively), and LV ejection fraction (P=0.02). CRT patients showed statistically significant improvement in VE/CO2 (P=0.01), NYHA class (P=0.05), and clinical composite response (P=0.01). No significant differences were noted in 6-minute walk distance or quality of life scores. CONCLUSIONS: In patients with mild heart failure symptoms on optimal medical therapy with a wide QRS complex and an ICD indication, CRT did not alter exercise capacity but did result in significant improvement in cardiac structure and function and composite clinical response over 6 months.

Efficacy of a microwave antenna for ablation of the tricuspid valve--inferior vena cava isthmus in dogs as a treatment for type 1 atrial flutter.

INTRODUCTION: Radiofrequency catheter ablation of the tricuspid valve-inferior vena cava (TV-IVC) isthmus for treatment of atrial flutter (AFL), may in some cases require a large number of energy applications and a long procedure and fluoroscopy time. AIMS OF STUDY: Therefore, we studied the safety and efficacy of a 4 cm long microwave antenna mounted on a steerable 9Fr catheter for linear ablation of the TV-IVC isthmus. METHODS: In 6 anesthetized dogs, multi-electrode catheters were positioned in the coronary sinus (decapolar), at the His bundle (quadripolar) and around the TV annulus (decapolar) for pacing and recording atrial activation sequences before and after ablation. The microwave antenna was then positioned across the TV-IVC isthmus from the TV annulus (identified by equal A and V potentials) to the inferior vena cava with slight traction on the catheter to ensure adequate endocardial contact. Microwave energy was then applied at a fixed power for 120 seconds during each ablation attempt. Ablation was repeated until bi-directional isthmus block was demonstrated during pacing from the coronary sinus ostium and low lateral right atrium, respectively. RESULTS: Linear microwave ablation of the TV-IVC isthmus was completed in all ten dogs using a total of 2.6 +/- 1.17 energy applications per dog. Power was applied in a range of 45-50 watts. There were no acute procedural complications. Bi-directional TV-IVC isthmus block was achieved in all ten dogs, as demonstrated by a strictly descending activation wavefront in the ipsilateral atrial wall, during pacing from the CSO and LLRA respectively. In addition, after ablation conduction time to the LLRA during pacing from the CSO increased from 52 +/- 16.62 before to 87 +/- 12.74 msec (p <.05), and to the CSO during pacing from the LLRA from 51 +/- 12.43 before to 79.50 +/- 9.85 msec (p <.05). Gross and histological examination of the TV-IVC isthmus after ablation revealed continuous transmural lesions, ranging from 3-5 mm in width, spanning the entire TV-IVC isthmus in all ten dogs. CONCLUSIONS: (1) Microwave ablation of the TV-IVC isthmus was safe and effective in this study. (2) Ablation of the entire width and thickness of the TV-IVC isthmus can be rapidly achieved using a long microwave antenna in a fixed trans-isthmus position.

Significance of inducible ventricular flutter/fibrillation in risk stratification in patients with coronary artery disease.

BACKGROUND: Although inducible ventricular fibrillation (VF) has been used as an indication for prophylactic implantation of cardioverter-defibrillators (ICDs) in patients with coronary artery disease (CAD), the significance of inducible VF remains controversial. METHODS: Among 364 CAD patients who underwent electrophysiologic (EP) study for risk stratification, 23 patients, 12 without any history of VF or cardiac arrest (group A) and 11 with previously documented VF or cardiac arrest (group B), exhibited inducible ventricular flutter (VFL) or VF and subsequently underwent ICD implantation. Additionally, 11 CAD patients without previous VF or cardiac arrest, who had no inducible ventricular tachyarrhythmias but received an ICD, were included for comparison (group C). RESULTS: During 2 years of follow-up, 1 (8%), 5 (45%), and 1 (9%) patients had appropriate ICD shocks in groups A, B, and C, respectively. The survival free from appropriate ICD shocks was significantly lower in group B compared to groups A and C (p<0.05). There were no significant differences in age, sex, ejection fraction (EF), or induction protocol between groups A and B or between groups A and C. CONCLUSIONS: In CAD patients with inducible VFL/VF, patients without any history of VF or cardiac arrest had significantly lower incidence of appropriate ICD shocks when compared to those with such clinical events. Conversely, in CAD patients without any history of VF or cardiac arrest, incidence of appropriate ICD shocks was similar regardless of inducible VFL/VF. Inducible VFL/VF is therefore not useful as an indication for prophylactic ICD implantation in this patient population.

Clinical experience of contrast venography guided axillary vein puncture in biventricular pacing.

BACKGROUND AND OBJECTIVES: Biventricular pacing is a new modality of treatment for patients with drug-refractory heart failure and ECG evidence of dyssynchronous ventricular contraction. A third lead is required to be placed in a coronary vein through the coronary sinus for left ventricular pacing, in addition to the atrial and right ventricular leads. Subclavian crush phenomenon is associated with lead placement using subclavian puncture. Cephalic venous cutdown, free of this complication, provides vascular access which is probably too small for multiple leads placement in biventricular pacing. Contrast venography guided axillary vein puncture (AP) is an alternative approach. This study reports on clinical experience of this technique in biventricular pacing. METHODS: AP is the method of choice for biventricular pacing in Princess Margaret Hospital and Stanford University Medical Center. Patients' clinical characteristics, effectiveness and safety of the technique were analysed. RESULTS: AP was performed in 35 patients, 29 male and six female with mean age 57.1 +/- 14.7 years from 1 January 2000 to 30 June 2001. Six patients had biventricular pacing alone and 29 patients had biventricular implantable cardioverter defibrillator (ICD) implanted. Twelve (34.3%) patients have ischaemic cardiomyopathy, 22 (62.9%) patients have dilated cardiomyopathy and one (2.9%) patient has hypertrophic cardiomyopathy. Six (17.1%) patients were in NYHA Class II heart failure, 21 (60.0%) patients were in Class III and eight (22.9%) patients were in Class IV. The mean EF was 23.1 +/- 7.6%. AP was attempted for placement of all three leads in all patients. AP was successful in 34 (97.1%) patients. The single failure was due to small size of the axillary vein. There were no AP, lead or contrast-related complications with a follow up of 12 months. CONCLUSIONS: AP is both effective and safe for biventricular pacing. It may be the method of choice in biventricular pacing.

Limited predictive value of inducible sustained ventricular tachycardia for future occurrence of spontaneous ventricular tachycardia in patients with coronary artery disease and relatively preserved cardiac function.

To evaluate the significance of inducible sustained ventricular tachycardia (VT) in patients with coronary artery disease and relatively preserved cardiac function, 33 patients who met the following criteria were studied; documented nonsustained VT but no history of life-threatening arrhythmia, inducible sustained VT at electrophysiologic study, and implantation of a cardioverter-defibrillator. Eighteen patients developed clinical sustained VT within 2 years. By univariate analysis, left ventricular ejection fraction (EF) and the cycle length of induced VT were associated with clinical VT occurrence. By multivariate analysis, however, EF was the only independent predictor. Among 23 patients with EF 40% (P <.01). In coronary artery disease patients with relatively preserved EF, the incidence of clinical VT is considerably low even though sustained VT is inducible. Inducible VT is therefore not appropriate for risk stratification in this patient population.

Combined cardiac resynchronization and implantable cardioversion defibrillation in advanced chronic heart failure: the MIRACLE ICD Trial.

CONTEXT: Cardiac resynchronization therapy (CRT) through biventricular pacing is an effective treatment for heart failure (HF) with a wide QRS; however, the outcomes of patients requiring CRT and implantable cardioverter defibrillator (ICD) therapy are unknown. OBJECTIVE: To examine the efficacy and safety of combined CRT and ICD therapy in patients with New York Heart Association (NYHA) class III or IV congestive HF despite appropriate medical management. DESIGN, SETTING, AND PARTICIPANTS: Randomized, double-blind, parallel-controlled trial conducted from October 1, 1999, to August 31, 2001, of 369 patients with left ventricular ejection fraction of 35% or less, QRS duration of 130 ms, at high risk of life-threatening ventricular arrhythmias, and in NYHA class III (n = 328) or IV (n = 41) despite optimized medical treatment. INTERVENTIONS: Of 369 randomized patients who received devices with combined CRT and ICD capabilities, 182 were controls (ICD activated, CRT off) and 187 were in the CRT group (ICD activated, CRT on). MAIN OUTCOME MEASURES: The primary double-blind study end points were changes between baseline and 6 months in quality of life, functional class, and distance covered during a 6-minute walk. Additional outcome measures included changes in exercise capacity, plasma neurohormones, left ventricular function, and overall HF status. Survival, incidence of ventricular arrhythmias, and rates of hospitalization were also compared. RESULTS: At 6 months, patients assigned to CRT had a greater improvement in median (95% confidence interval) quality of life score (-17.5 [-21 to -14] vs -11.0 [-16 to -7], P =.02) and functional class (-1 [-1 to -1] vs 0 [-1 to 0], P =.007) than controls but were no different in the change in distance walked in 6 minutes (55 m [44-79] vs 53 m [43-75], P =.36). Peak oxygen consumption increased by 1.1 mL/kg per minute (0.7-1.6) in the CRT group vs 0.1 mL/kg per minute (-0.1 to 0.8) in controls (P =.04), although treadmill exercise duration increased by 56 seconds (30-79) in the CRT group and decreased by 11 seconds (-55 to 12) in controls (P<.001). No significant differences were observed in changes in left ventricular size or function, overall HF status, survival, and rates of hospitalization. No proarrhythmia was observed and arrhythmia termination capabilities were not impaired. CONCLUSIONS: Cardiac resynchronization improved quality of life, functional status, and exercise capacity in patients with moderate to severe HF, a wide QRS interval, and life-threatening arrhythmias. These improvements occurred in the context of underlying appropriate medical management without proarrhythmia or compromised ICD function.

Unidirectional conduction block at cavotricuspid isthmus created by radiofrequency catheter ablation in patients with typical atrial flutter.

INTRODUCTION: Although unidirectional conduction block at the cavotricuspid isthmus can be created by radiofrequency ablation for atrial flutter, its underlying mechanism has not been elucidated. METHODS AND RESULTS: Twenty-seven patients (22 men and 5 women; mean age 59 +/- 11 years) who met the following criteria were studied: (1) bidirectional isthmus conduction demonstrable at baseline; (2) at least one linear lesion attempted on the cavotricuspid isthmus with radiofrequency catheter ablation; and (3) conduction times at anterolateral and posteromedial portions of the isthmus measured for both clockwise and counterclockwise directions before the ablation procedure. Unidirectional conduction block was observed before achieving bidirectional block in 9 patients (group I); the remaining 18 patients did not exhibit unidirectional conduction block (group II). All unidirectional conduction blocks were demonstrated in the counterclockwise direction. Anterolateral isthmus conduction time in group I was significantly longer than that in group II in both directions. However, there were no significant differences in posteromedial isthmus conduction time between groups I and II in either direction. Anterolateral isthmus conduction time was significantly longer than posteromedial conduction time in group I but not in group II. CONCLUSION: There were significant differences in conduction properties at the cavotricuspid isthmus between patients who developed unidirectional conduction block and those who did not. Our results support the notion that anisotropy contributes to the genesis of unidirectional conduction block at the cavotricuspid isthmus during the radiofrequency ablation procedure.