Young Keun On

Sungkyunkwan University, Sŏul, Seoul, South Korea

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Publications (80)170.01 Total impact

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    ABSTRACT: An association between baroreflex sensitivity (BRS) and the response to tilt training has not been reported in patients with neurally mediated syncope (NMS). This study sought to investigate the role of BRS in predicting the response to tilt training in patients with NMS.
    Preview · Article · Jan 2016 · Yonsei Medical Journal
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    ABSTRACT: Background: Prolongation of corrected QT (QTc) interval reflects an increased risk of fatal arrhythmia and cardiac death in various populations. However, it is not clear whether the paced-QTc (p-QTc) interval is associated with new-onset left ventricular systolic dysfunction (new-LVSD) or cardiac death. Methods: In 491 consecutive patients (64±14years) with preserved LV ejection fraction (64±7%), the p-QTc interval was measured within 2weeks after PPM implantation. We assessed the rates of new-LVSD and cardiac death based on the degree of p-QTc interval. Results: During the follow-up period (78±51months), new-LVSD and cardiac death were identified in 53 (10.8%) and 26 (5.3%) patients, respectively. Patients with new-LVSD had more frequent atrioventricular block (P=0.041), a higher percentage of ventricular pacing (P=0.005), a longer p-QRS duration (P<0.001), and more prolonged p-QTc interval (P<0.001) compared to those without new-LVSD. There was a graded increase in the rates of new-LVSD (P<0.001) and cardiac death (P=0.001) from the patients in the lowest to those in the highest tertile of the p-QTc interval. Additionally, the incidence of cardiac death was significantly elevated especially in the patients with new-LVSD and wider p-QTc interval. In Cox regression analyses, the p-QTc interval was independently associated with new-LVSD and cardiac death even after adjusted with various relevant confounding factors. Conclusions: Prolonged p-QTc interval was closely associated with new-LVSD and cardiac death after PPM implantation in patients with preserved LV systolic function. The rate of cardiac death significantly increased especially in patients who showed more p-QTc widening along with new-LVSD.
    No preview · Article · Nov 2015 · International journal of cardiology
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    Preview · Article · Sep 2015
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    ABSTRACT: Background and objectives: Frequent ventricular premature complexes (VPCs) increase the risk of cardiomyopathy (CMP). However, most data regarding VPCs have been obtained from Western population and in-hospital patient-based studies. The objective of this study was to define the clinical characteristics and features of idiopathic VPCs in the Korean population. Subjects and methods: We investigated subjects undergoing transthoracic echocardiography and documented VPC burdens >1% by Holter monitoring in an outpatient clinic at Samsung Medical Center, Korea. We analyzed demographic and clinical features and the nature of the VPCs by electrocardiography (ECG). Results: A total of 666 patients were registered. Mean age was 54.7±16.8 years, and 365 (54.8%) patients were female. Typical VPC-related symptoms, such as palpitation and a dropped beat, were observed in 394 (59.2%) patients. Some patients received beta-blockers (n=95; 14.3%) and anti-arrhythmic agents (n=14; 2.1%). The ECG analysis was performed in 405 patients; 322 (79.5%) exhibited left bundle branch block (LBBB) and 347 (85.8%) exhibited an inferior axis. The precordial R-wave transition was predominantly distributed over V3 in 230 patients (56.6%). Thirty-one patients (4.5%) were diagnosed with VPC-induced CMP. Conclusion: The incidence of frequent VPCs was slightly higher in females, and palpitation was the most frequent complaint. The most common ECG features were LBBB, inferior axis, and late precordial R-wave transition.
    Preview · Article · Sep 2015 · Korean Circulation Journal
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    ABSTRACT: Apical hypertrophic cardiomyopathy (ApHCM) is thought to have a favourable clinical outcome, compared with other types of HCM. We sought to investigate the clinical and anatomical differences in cardiovascular imaging between ApHCM and non-ApHCM. A total of 350 patients diagnosed with HCM underwent cardiovascular magnetic resonance (CMR) and echocardiography. All enrolled subjects were prospectively followed up for adverse clinical outcomes. Eighty-five patients were classified as ApHCM. On CMR, the amount and proportion of late gadolinium enhancement (LGE) as well as left ventricular volumetric parameters were evaluated. Echocardiographic evaluations included diastolic function and global strain. Patients with ApHCM were less likely to present with history of syncope and have less frequency of family history of sudden cardiac death than those with non-ApHCM. Functional class was also more favourable in ApHCM [frequency of New York Heart Association (NYHA) class I; 89.4 vs. 66.8%, P < 0.001]. LGE was less frequently detected (87.1 vs. 93.9%, P = 0.04), and the amount of LGE was significantly smaller in ApHCM (7.0 ± 6.0 vs. 14.6 ± 10.5%, P < 0.001). The E/e' level and left atrial volume index were also lower in ApHCM patients (all P < 0.001). During follow-up, a composite of adverse clinical events including cardiac death, admission for heart failure, and cerebrovascular accident was higher in patients with ApHCM than those with non-ApHCM (P = 0.01). ApHCM showed a relatively small burden of myocardial fibrosis and less severe diastolic dysfunction and subsequently more favourable clinical manifestations in comparison with other HCMs. This may be one explanation of why most patients with ApHCM show a benign course of disease compared with non-ApHCM. Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2015. For permissions please email: journals.permissions@oup.com.
    No preview · Article · Aug 2015 · European Heart Journal Cardiovascular Imaging
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    ABSTRACT: Thoracoscopic ablation for lone atrial fibrillation (AF) has evolved rapidly in the past decade. We investigated the electrophysiologic results and midterm durability of totally thoracoscopic ablation in patients with lone persistent AF. Seventy-nine consecutive patients with paroxysmal AF (8 patients, 10.1%), persistent AF (17 patients, 21.5%), and long-standing persistent AF (54 patients, 68.3%) were prospectively enrolled. Thoracoscopic ablation consisted of a bilateral closed-chest approach to performing pulmonary isolation (a box lesion), ganglionated plexus ablation, division of the Marshall ligament, and left atrial auricle resection. An electrophysiologic study was performed 5 days after the surgical procedure in 61 patients (77%). Freedom from AF was assessed with electrocardiograms or Holter monitoring every 3 months, with a mean follow-up of 12.1 (maximum, 28) months. No deaths or conversion to cardiopulmonary bypass occurred. During electrophysiologic study, 28 residual pulmonary vein potentials were observed in 15 patients (19%). Out of a total of 28 gaps, 20 (71%) were located in the superior and inferior ridges of pulmonary veins. Six gaps (21%) were detected in the carina of pulmonary veins. The mitral isthmus was ablated in 2 patients (7%). Freedom from AF at 2 years was 92.6 ± 3.3%. Freedom from cardiac-related events at 2 years was 74.7 ± 6.0%. Cox regression analysis demonstrated that the predictors of atrial arrhythmias were old age, hypertension, and left atrial volume index. Thoracoscopic ablation followed by electrophysiologic confirmation was safe and provided excellent midterm durability in patients with AF. However, the incidence of residual potentials around the pulmonary veins was not negligible. Copyright © 2015 The Society of Thoracic Surgeons. Published by Elsevier Inc. All rights reserved.
    No preview · Article · Jul 2015 · The Annals of thoracic surgery
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    ABSTRACT: Little is known about the long-term prognosis of or predictors for the different clinical types of atrial fibrillation (AF) in Korean populations. The aim of this study was to validate a risk stratification to assess the probability of AF progression from paroxysmal AF (PAF) to persistent AF (PeAF) or permanent AF. A total of 434 patients with PAF were consecutively enrolled (mean age; 71.7 ± 10.7 yr, 60.6% male). PeAF was defined as episodes that are sustained > 7 days and not self-terminating, while permanent AF was defined as an ongoing long-term episode. Atrial arrhythmia during follow-up was defined as atrial premature complex, atrial tachycardia, and atrial flutter. During a mean follow-up of 72.7 ± 58.3 months, 168 patients (38.7%) with PAF progressed to PeAF or permanent AF. The mean annual AF progression was 10.7% per year. In univariate analysis, age at diagnosis, body mass index, atrial arrhythmia during follow-up, left ventricular ejection fraction, concentric left ventricular hypertrophy, left atrial diameter (LAD), and severe mitral regurgitation (MR) were significantly associated with AF progression. In multivariate analysis, age at diagnosis (P = 0.009), atrial arrhythmia during follow-up (P = 0.015), LAD (P = 0.002) and MR grade (P = 0.026) were independent risk factors for AF progression. Patients with younger age at diagnosis, atrial arrhythmia during follow-up, larger left atrial chamber size, and severe MR grade are more likely to progress to PeAF or permanent AF, suggesting more intensive medical therapy with close clinical follow-up would be required in those patients.
    Preview · Article · Jul 2015 · Journal of Korean Medical Science
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    ABSTRACT: Frequent ventricular premature complex (VPC) is one of the most common arrhythmia syndromes. Symptoms observed frequently with this arrhythmia syndrome remain limited. We sought to identify predictors of VPC-related symptoms by analyzing demographic information, VPC burden, and VPC surface electrocardiogram characteristics. We prospectively enrolled 109 patients with idiopathic outflow tract VPCs (63 males, 49±16 years old). They were divided into Group A (n=30, without VPC-related symptoms of palpitations or "dropped beats") and Group B (n=79, with VPC-related symptoms). Measured parameters were sinus and VPC QRS width, coupling interval (CI) between the previous sinus beat and VPC, CI ratio (%, CI/sinus cycle length), post-VPC CI and CI ratio, and VPC amplitude. Both groups had similar age (p=0.22), daily VPC burden (p=0.15), and VPC site of origin (p=0.36). The VPC CI ratio was higher in Group B (60±15%) than in Group A (49±22%) (p=0.01). VPC-related symptoms are associated with a higher VPC CI ratio (>50%). The physiologic basis for these results deserves further study.
    Preview · Article · Jul 2015 · Korean Circulation Journal
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    ABSTRACT: Most patients with idiopathic ventricular premature depolarizations (VPDs) complain of symptoms related to this arrhythmia, but some patients are asymptomatic even with a high VPD burden. Our understanding of the relationship between symptoms and cardiomyopathy related to this arrhythmia remains limited. We evaluated 801 subjects (381 men; mean age, 55 ± 17 years) who visited our outpatient clinic. All subjects were diagnosed with frequent VPDs (1% or >1000 beats/day). The patients were divided into two groups according to the presence or absence of typical VPD symptoms (palpitations or skipped beats during VPDs): symptomatic patients (n = 455) and asymptomatic patients (n = 346). Clinical and electrocardiogram parameters were compared between these two groups. In the symptomatic group, palpitations were the most frequent symptom (91%). Daily VPD burden (P = 0.90) and electrocardiogram parameters (P>0.05) did not differ significantly between groups. The incidence of frequent VPDs with left ventricular dysfunction was significantly higher in the asymptomatic group (symptomatic patients, 3.0%; asymptomatic patients, 10.5%; P < 0.001). The absence of typical VPD-related symptoms may be a risk factor for cardiomyopathy and be associated with an adverse outcome. Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2015. For permissions please email: journals.permissions@oup.com.
    No preview · Article · May 2015 · Europace
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    ABSTRACT: High ventricular premature depolarization (VPD) burden is associated with left ventricular (LV) dysfunction that typically resolves after successful ablation. Some patients, however, have persistent LV dysfunction, even after successful radiofrequency (RF) ablation. Identifying factors associated with irreversibility of LV cardiomyopathy (CMP) may help predict clinical outcome.Methods and Results:Patients with frequent VPD (>10%/day) who underwent successful VPD suppression were divided into 2 groups according to transthoracic echocardiography (TTE) before and after suppression: group A (n=38) had depressed LV function that normalized after VPD suppression; group B (n=19) had depressed LV function before and after suppression. Of 57 patients (43 men; mean age, 54±15 years), RF ablation was performed in 39. Clinical, electrocardiographic, and TTE parameters were compared between groups. LV end-diastolic dimension (LVEDD; group A vs. B: 54±5 mm vs. 60±10 mm, P=0.01), end-systolic dimension (group A vs. B: 42±6 mm vs. 48±11 mm, P=0.01) before VPD suppression differed significantly between groups. Pre-suppression LVEDD was ≤66 mm in all reversible-CMP patients. LVEDD >66 mm predicted irreversible CMP with 50% sensitivity, 100% specificity, 100% positive predictive value, and 81% negative predictive value. LVEDD was a good predictor of irreversible LV CMP with frequent VPD, with 50% sensitivity and 100% specificity.
    Preview · Article · May 2015 · Circulation Journal
  • Y-E Kim · H I Woo · Y K On · J S Kim · S-Y Lee
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    ABSTRACT: Vitamin K intake is considered as a controllable contributor to warfarin sensitivity. It is restricted in warfarin-treated patients. However, little study has assessed the vitamin K status in warfarin-treated patients. We directly measured plasma vitamin K in warfarin-treated patients and evaluated its effect on anticoagulation. A total of 302 plasma vitamin K concentrations were assessed using high-performance liquid chromatography for 203 outpatients with atrial fibrillation under warfarin treatment. Clinical and laboratory information including warfarin dosage, plasma warfarin concentrations, prothrombin time international normalized ratio (PT INR) and CYP2C9/VKORC1 genotypes was reviewed retrospectively. The anticoagulation stability (intra-individual variability, frequency of PT INR tests and complications) was investigated in 163 patients with long-term warfarin therapy. Plasma vitamin K was measured in 40 healthy subjects and in 40 patients before and after initial warfarin treatment. Vitamin K concentrations were significantly decreased after the initiation of warfarin treatment (before treatment: 1.72 ng/ml; after treatment: 0.59 ng/ml, P<0.05). There was a large inter-individual variability in vitamin K levels (0.2-4.2 ng/ml) in warfarin-treated patients. PT INR was more frequently checked in patients with low plasma vitamin K levels than in those with high vitamin K levels (9.5 times/year vs 7.5 times/year, P=0.029). Two patients with gross hematuria showed very low vitamin K levels (<0.4 ng/ml). We found high inter- and intra-individual variability in vitamin K concentration in warfarin-treated patients. Low vitamin K concentration in warfarin-treated patients suggested excessive dietary restriction. Plasma vitamin K measurement would be helpful for dietary control and anticoagulation stability.European Journal of Clinical Nutrition advance online publication, 1 April 2015; doi:10.1038/ejcn.2015.41.
    No preview · Article · Apr 2015 · European journal of clinical nutrition
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    ABSTRACT: Atrial fibrillation (AF) is the most common heart arrhythmia. Untreated AF incurs a considerable burden of stroke and associated healthcare costs. Asians have AF risk factors similar to Caucasians and a similarly increased risk of AF-related stroke; however, with a vast and rapidly ageing population, Asia bears a disproportionately large disease burden. Urgent action is warranted to avert this potential health crisis. Antithrombotic therapy with oral anticoagulants is the most effective means of preventing stroke in AF and is a particular priority in Asia given the increasing disease burden. However, AF in Asia remains undertreated. Conventional oral anticoagulation with warfarin is problematic in Asia due to suboptimal control and a propensity among Asians to warfarin-induced intracranial haemorrhage. Partly due to concerns about intracranial haemorrhage, there are considerable gaps between AF treatment guidelines and clinical practice in Asia, in particular overuse of antiplatelet agents and underuse of anticoagulants. Compared with warfarin, new direct thrombin inhibitors and Factor Xa inhibitors are non-inferior in preventing stroke and significantly reduce the risk of life-threatening bleeding, particularly intracranial bleeding. These agents may therefore provide an appropriate alternative to warfarin in Asian patients. There is considerable scope to improve stroke prevention in AF in Asia. Key priorities include: early detection of AF and identification of asymptomatic patients; assessment of stroke and bleeding risk for all AF patients; evidence-based pharmacotherapy with direct-acting oral anticoagulant agents or vitamin K antagonists for AF patients at risk of stroke; controlling hypertension; and awareness-raising, education and outreach among both physicians and patients. Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.
    No preview · Article · Mar 2015 · International journal of cardiology
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    ABSTRACT: An implantable cardioverter-defibrillator (ICD) is the only proven effective therapeutic strategy for patients with Brugada syndrome (BS). However, it is controversial whether the device should be replaced even in patients who had never experienced appropriate ICD therapy until the time of generator replacement. This was a nationwide, multicenter retrospective study that enrolled patients who were diagnosed with BS and had an ICD implantation between January 1998 and April 2014. Appropriate ICD therapies administered for ventricular tachyarrhythmia were evaluated during follow-up. A total of 117 patients (age 43±12years, male 115 [98.3%]) were enrolled, and the mean follow-up duration was 6.0±4.1years. Thirty-seven (31.6%) patients had experienced appropriate ICD therapy during follow-up. Of all patients, 46 underwent replacement of the device. After the first generator replacement, the incidence of appropriate ICD therapy remained as high as 65.2% in patients who previously experienced appropriate ICD therapy before generator replacement. In 30 patients who did not experience any cardiac events until the first generator change, two (8.7%) had an episode of appropriate ICD therapy afterwards. No episode of ICD therapy before generator replacement could not guarantee a safe clinical course. ICD generator replacement should be considered even in patients without ICD therapy before. Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.
    No preview · Article · Mar 2015 · International journal of cardiology

  • No preview · Article · Mar 2015 · International journal of cardiology
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    ABSTRACT: Statin pretreatment in patients undergoing cardiac surgery is understood to prevent postoperative atrial fibrillation (AF). However, this is based on observational and limited randomized trial evidence, resulting in uncertainty about any genuine anti-arrhythmic benefits of these agents in this setting. We therefore aimed to quantify precisely the association between statin pretreatment and postoperative AF among patients undergoing cardiac surgery. A detailed search of MEDLINE and PubMed databases (1st January 1996 to 31st July 2012) was conducted, followed by a review of the reference lists of published studies and correspondence with trial investigators to obtain individual-participant data for meta-analysis. Evidence was combined across prospective, randomized clinical trials that compared the risk of postoperative AF among individuals randomized to statin pretreatment or placebo/control medication before elective cardiac surgery. Postoperative AF was defined as episodes of AF lasting ≥5 min. Overall, 1105 participants from 11 trials were included; of them, 552 received statin therapy preoperatively. Postoperative AF occurred in 19% of these participants when compared with 36% of those not treated with statins (odds ratio 0.41, 95% confidence interval 0.31-0.54, P < 0.00001, using a random-effects model). Atrial fibrillation prevention by statin pretreatment was consistent across different subgroups. Short-term statin pretreatment may reduce the risk of postoperative AF among patients undergoing cardiac surgery. Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2015. For permissions please email: journals.permissions@oup.com.
    No preview · Article · Mar 2015 · Europace
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    ABSTRACT: In patients with nonvalvular atrial fibrillation (AF), the risk of stroke varies considerably according to individual clinical status. The CHA2DS2-VASc score is better than the CHADS2 score for identifying truly lower risk patients with AF. With the advent of novel oral anticoagulants (NOACs), the strategy for antithrombotic therapy has undergone significant changes due to its superior efficacy, safety and convenience compared with warfarin. Furthermore, new aspects of antithrombotic therapy and risk assessment of stroke have been revealed: the efficacy of stroke prevention with aspirin is weak, while the risk of major bleeding is not significantly different from that of oral anticoagulant (OAC) therapy, especially in the elderly. Reflecting these pivotal aspects, previous guidelines have been updated in recent years by overseas societies and associations. The Korean Heart Rhythm Society has summarized the new evidence and updated recommendations for stroke prevention of patients with nonvalvular AF. First of all, antithrombotic therapy must be considered carefully and incorporate the clinical characteristics and circumstances of each individual patient, especially with regards to balancing the benefits of stroke prevention with the risk of bleeding, recommending the CHA2DS2-VASc score rather than the CHADS2 score for assessing the risk of stroke, and employing the HAS-BLED score to validate bleeding risk. In patients with truly low risk (lone AF, CHA2DS2-VASc score of 0), no antithrombotic therapy is recommended, whereas OAC therapy, including warfarin (international normalized ratio 2-3) or NOACs, is recommended for patients with a CHA2DS2-VASc score ≥2 unless contraindicated. In patients with a CHA2DS2-VASc score of 1, OAC therapy should be preferentially considered, but depending on bleeding risk or patient preferences, antiplatelet therapy or no therapy could be permitted.
    Full-text · Article · Jan 2015 · Korean Circulation Journal
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    ABSTRACT: A case of a fistula running from the pulmonary vein to the esophagus after a staged hybrid procedure combining total thoracoscopic ablation and percutaneous radiofrequency catheter ablation has not been reported previously. We describe such a case in a 37-year-old man who was successfully treated by surgery.
    Full-text · Article · Dec 2014 · Korean Journal of Thoracic and Cardiovascular Surgery
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    ABSTRACT: We applied cardiac resynchronization therapy (CRT) for desynchronized heart failure patients. We evaluated clinical outcomes including morbidity, mortality, and echocardiographic parameters in 47 patients with implanted CRT in Korea from October 2005 to May 2013. The combined outcomes of hospitalization from heart failure, heart transplantation and death were the primary end point. Median follow-up period was 17.5 months. The primary outcomes listed above occurred in 10 (21.3%) patients. Two patients (4.3%) died after CRT and 8 (17%) patients were hospitalized for recurrent heart failure. Among patients hospitalized for heart failure, 2 (4.3%) patients underwent heart transplantation. The overall free rate of heart failure requiring hospitalization was 90.1% (95% CI, 0.81-0.99) over one year and 69.4% (95% CI, 0.47-0.91) over 3 yr. We observed improvement of the New York Heart Association classification (3.1±0.5 to 1.7±0.4), decreases in QRS duration (169.1 to 146.9 ms), decreases in left ventricular (LV) end-diastolic (255.0 to 220.1 mL) and end-systolic (194.4 to 159.4 mL) volume and increases in LV ejection fraction (22.5% to 31.1%) at 6 months after CRT. CRT improved symptoms and echocardiographic parameters in a relatively short period, resulting in low mortality and a decrease in hospitalization due to heart failure.
    Preview · Article · Dec 2014 · Journal of Korean Medical Science
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    ABSTRACT: Purpose: Recurrent syncope leads to poor functional status and psychiatric impairment in patients with syncope. The aim of the study was to prospectively analyze the risk factors attributed to syncope recurrence. Materials and methods: Between 2009 and 2010, 289 patients with all cause of syncope visited our institution. Syncope recurrences were followed for 1 year by telephone interview every 3 months. Results: We diagnosed 181 (63%) patients with neurally mediated syncope (NMS), 39 (13%) with orthostatic hypotension, 34 (12%) with cardiac syncope, and 35 (12%) with unexplained syncope. During the 1-year follow-up period, 19 (6.6%) patients suffered recurrent syncope. Kaplan-Meier curves showed that recurrent syncope was observed more often in patients with unexplained syncope compared with those with NMS (p < 0.01), and also observed more often in patients with six previous syncopal episodes compared to those with fewer episodes (p = 0.02). Cox regression analysis showed that the recurrence of syncope was significantly associated with more than six previous syncopal episodes (HR 5.38, 95% CI 1.17-24.71, p = 0.03) and that there was a tendency for association between recurrence of syncope and unexplained syncope (HR 6.13, 95% CI 0.96-39.1, p = 0.05). Conclusions: Patients with previous frequent syncopal episodes or unexplained syncope had higher rates of syncope recurrence during the follow up period.
    No preview · Article · Nov 2014 · Cardiology
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    ABSTRACT: Purpose The selection of the optimal right ventricular (RV) pacing site remains unclear. We hypothesized that a normal paced QRS axis would provide a physiological ventricular activation and lead to a better long-term outcome. Methods We evaluated 187 patients who underwent a permanent pacemaker implantation and were dependent on RV pacing. The pacing sites were classified as the apex and non-apex according to the chest radiography. A paced QRS axis was defined as that between −30° and 90°. Preservation of the left ventricular (LV) systolic function was defined as that with a
    No preview · Article · Nov 2014 · Journal of Interventional Cardiac Electrophysiology

Publication Stats

519 Citations
170.01 Total Impact Points

Institutions

  • 2008-2015
    • Sungkyunkwan University
      • • Samsung Medical Center
      • • School of Medicine
      Sŏul, Seoul, South Korea
  • 2007-2015
    • Samsung Medical Center
      • Department of Cardiology
      Sŏul, Seoul, South Korea
  • 2010
    • Indiana University-Purdue University Indianapolis
      Indianapolis, Indiana, United States
  • 2002-2004
    • Soonchunhyang University
      • College of Medicine
      Onyang, Chungcheongnam-do, South Korea
  • 2000-2001
    • Seoul National University Hospital
      • Department of Internal Medicine
      Seoul, Seoul, South Korea