Jan Nemec

University of Pittsburgh, Pittsburgh, Pennsylvania, United States

Are you Jan Nemec?

Claim your profile

Publications (37)159.37 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Backround: T-wave alternans (TWA) is associated with ventricular tachycardia (VT). Nonalternans repolarization variability (NARV) precedes VT in certain experimental models, but its link to clinical arrhythmia is unproven. This study was conducted to determine if NARV increases prior to ventricular tachycardia (VT) in patients with implantable defibrillators (ICDs). Methods: TWA and NARV were calculated from shock-channel electrograms preceding onset of VT or non-VT events in patients with an ICD. In each patient, presence of both a VT and a non-VT event with the same QRS morphology before the event was required. Mixed linear model was used for data analysis, using heart rate (HR) and the number of analyzed beats as covariates. Results: 568 events from 64 patients (males/females 51/13, 67±13 years) were analyzed. HR preceding non-VT events was higher than before VT events (RR interval 595±159 vs. 706 ±111 ms; p<0.0001). Both TWA and NARV increased with increasing HR (p<0.001). TWA decreased with increasing number of analyzed beats. When controlled for number of analyzed beats and HR, both TWA and NARV were higher before VT than before non-VT events (p<0.002 and p<0.0005, respectively). Conclusions: NARV is elevated prior to spontaneous VT onset. Both NARV and TWA increase with HR. The decrease of TWA with increasing number of analyzed beats may indicate contamination with NARV or noise when only a small number of beats is available for analysis. NARV might be useful for VT prediction in the future. This article is protected by copyright. All rights reserved.
    No preview · Article · Nov 2015 · Pacing and Clinical Electrophysiology
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Unstable (cyclical alternating pattern, or CAP) sleep is associated with surges of sympathetic nervous system activity, increased blood pressure and vasoconstriction, heightened baroreflex sensitivity, and unstable heart rhythm and breathing. In susceptible persons, CAP sleep provokes clinically significant events, including hypertensive crises, sleep-disordered breathing, and cardiac arrhythmias. Here we explore the neurophysiology of CAP sleep and its impact on cardiovascular and respiratory functions. We show that: (i) an increase in neurophysiological recovery rate can explain the emergence of slow, self-sustained, hypersynchronized A1 CAP-sleep pattern and its transition to the faster A2-A3 CAP-sleep patterns; (ii) in a two-dimensional, continuous model of cardiac tissue with heterogeneous action potential duration (APD) distribution, heart rate accelerations during CAP sleep may encounter incompletely recovered electrical excitability in cell clusters with longer APD. If the interaction between short cycle length and incomplete, spatially heterogeneous repolarization persists over multiple cycles, irregularities and asymmetry of depolarization front may accumulate and ultimately lead to a conduction block, retrograde conduction, breakup of activation waves, reentrant activity, and arrhythmias; and (iii) these modeling results are consistent with the nighttime data obtained from patients with structural heart disease (N=13) that show clusters of atrial and ventricular premature beats occurring during the periods of unstable heart rhythm and respiration that accompany CAP sleep. In these patients, CAP sleep is also accompanied by delayed adaptation of QT intervals and T-wave alternans. Copyright © 2015. Published by Elsevier Inc.
    Full-text · Article · Aug 2015 · Journal of electrocardiology

  • No preview · Article · Mar 2015 · Journal of the American College of Cardiology
  • [Show abstract] [Hide abstract]
    ABSTRACT: Dofetilide is a class III antiarrhythmic agent approved for the maintenance of sinus rhythm in patients with persistent atrial fibrillation (AF). The goal of this study was to determine if chemical cardioversion (CCV) suggests a greater sensitivity to dofetilide and, therefore, portends a higher risk of proarrhythmia. We analyzed 99 consecutive patients with persistent AF who were loaded on dofetilide before cardioversion. CCV occurred after 2 ± 1.5 doses of dofetilide in 46 patients whereas electrical cardioversion (ECV) was required in the remaining 53 patients after 4.7 ± 1.3 doses. During index hospitalization, there were higher rates of dofetilide discontinuation because of QT prolongation or torsades de pointes (TdP) in the CCV group compared with the ECV group (24% vs 2%, p = 0.001). All patients with CCV requiring drug discontinuation converted after a single dose of dofetilide. Additionally, all 3 patients with TdP were in the CCV group. Furthermore, 15 of the 21 patients with CCV (71%) who converted after the first dose of dofetilide developed significant QT prolongation, requiring dose adjustment or discontinuation of drug. Among patients discharged on drug, AF recurrence and drug discontinuation rates were similar between groups at 2-year follow-up. In patients hospitalized for initiation of dofetilide, CCV occurs in almost 50% and is associated with higher rates of pathologic QT prolongation and TdP compared with those who require ECV. Once discharged on dofetilide, safety and efficacy is similar in both groups. In conclusion, patients with CCV may require closer monitoring for proarrhythmia.
    No preview · Article · May 2013 · The American journal of cardiology
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Introduction and Objectives: Bradycardia prolongs action potential durations (APD-adaptation), enhances dispersion-of-repolarization (DOR) and promotes tachyarrhythmias. Yet, the mechanisms responsible for enhanced DOR and tachyarrhythmias remain largely unexplored. Methods: Ca(2+) transients (CaiT) and APs were measured optically from Langendorff rabbit hearts at high (150x150µm(2)) or low (1.5x1.5cm(2)) magnification while pacing at a physiological (120-beats/min) or a slow heart rate (SHR=50-beats/min). Western blots and pharmacological interventions were used to elucidate the regional effects of bradycardia. Results: Bradycardia (SHR=50-beats/min) increased APDs gradually (time-constant τf→s =48±9.2s) and caused a secondary Ca(2+) release (SCR) from the sarcoplasmic reticulum (SR) during AP plateaus, occurring at the base on average 184.4±9.7 ms after the CaiT upstroke. In subcellular imaging, SCRs were temporally synchronous and spatially homogeneous within myocytes. In diastole, SHR elicited variable asynchronous SR Ca(2+) release events leading to subcellular Ca(2+)-waves, detectable only at high-magnification. SCR was regionally heterogeneous, correlated with APD prolongation (p<0.01, n=5), enhanced DOR (r=0.9277±0.03, n=7) and was gradually reversed by pacing at 120-beats/min along with APD shortening (p<0.05, n=5). A stabilizer of leaky ryanodine receptors (RyR2), K201 (1µM) suppressed SCR and reduced APD at the base, thereby reducing DOR (p<0.02, n=5). Ventricular ectopy induced by bradycardia (n=5/15) was suppressed by K201. Western blots revealed spatial differences of Cav1.2α, NCX1, Nav1.5 and RERG (but not RyR2 or SERCA2a) that correlate with the SCR distribution and explain the molecular basis for SCR heterogeneities. Conclusions: Acute bradycardia elicits synchronized subcellular SCRs of sufficient magnitude to overcome the 'source-sink' mismatch and to promote afterdepolarizations and tachyarrhythmias.
    Preview · Article · Jan 2013 · AJP Heart and Circulatory Physiology
  • [Show abstract] [Hide abstract]
    ABSTRACT: Background: The Riata implantable cardioverter-defibrillator lead was recalled by the Food and Drug Administration because of an increased rate of failure associated with cable externalization. Because of its mechanical separation, the Riata lead may be challenging to extract. We therefore examined the experience with Riata lead extractions at our institution. Methods: All patients implanted with the Riata lead underwent detailed review of their electronic medical records and operative notes. Procedural complications were ascertained by reviewing the medical records up to 30 days after Riata extraction. Results: From a total of 627 patients implanted with the Riata lead at our institution, 20 patients (age at time of extraction, 57 ± 11 years; 85 % men; lead dwell time, 48 ± 27 months) underwent lead extraction. Extraction was successful in 19 of 20 (95 %) patients and required the use of laser-powered sheaths in 18 (90 %) patients. Over a 30-day follow-up period, 1 of 20 (5 %) patients had a minor procedure-related complication consisting of a new pericardial effusion that did not require drainage. Conclusions: Extraction of the Riata lead seems to be successful and safe and frequently requires the use of powered sheaths.
    No preview · Article · Nov 2012 · Journal of Interventional Cardiac Electrophysiology
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Food and Drug Administration recently issued a class I recall of the St. Jude Medical Riata implantable cardioverter-defibrillator lead presumably because of increased risk of electric failure and mechanical separation via inside-out abrasion. We sought to examine the incidence and time dependence of inside-out abrasion in asymptomatic patients implanted with the Riata lead. Asymptomatic patients implanted with the Riata lead at our institution were offered voluntary fluoroscopic screening in 3 views. Electric testing of the Riata lead with provocative isometric muscle contraction was performed at the time of fluoroscopic screening. Of the 245 patients undergoing fluoroscopic screening, 53 (21.6%) patients showed clear evidence of lead separation. Of these externalized leads, 0%, 13%, and 26% had a dwell time of <3 years, 3 to 5 years, and >5 years, respectively (P=0.037). Externalized leads had a significantly pronounced decrease in R-wave amplitude (-1.7±2.9 mV versus +0.35±2.5 mV; P<0.001), and more patients with externalized leads had ≥25% decrease in R-wave amplitude from baseline (28.0% versus 8.1%; P=0.018). One patient with externalization exhibited new noise on near-field electrogram. The Riata lead exhibits time-dependent high rates of cable externalization exceeding 20% at >5 years of dwell time. Externalized leads are associated with a more pronounced decrease in R-wave amplitude, which may be an early marker of future electric failure. The use of fluoroscopic and electric screening of asymptomatic patients with the Riata lead remains controversial in the management of patients affected by the recent Food and Drug Administration recall.
    No preview · Article · Jul 2012 · Circulation Arrhythmia and Electrophysiology
  • Source
    Jong J. Kim · Jan Nemec · Guy Salama

    Preview · Article · Apr 2011 · Journal of the American College of Cardiology
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 52-year-old patient presented with palpitation and well tolerated monomorphic ventricular tachycardia. He had normal echocardiogram and coronary angiogram 3 months prior to presentation. Surface EKG revealed regular wide-complex tachycardia with right bundle branch block morphology and right inferior axis. In conjunction with recent negative cardiac evaluation, this suggested idiopathic focal ventricular tachycardia from anterolateral basal left ventricle. CARTO based activation mapping confirmed the presence of VT focus in that area. Radiofrequency ablation at the site of perfect pacemap resulted in a partial suppression of the focus. Echocardiogram was subsequently performed because of progressive dyspnea. It revealed asymmetrical thickening of posterolateral left ventricle, with delayed enhancement on contrast magnetic resonance imaging. Fine needle aspiration of abdominal fat stained with Congo red confirmed the diagnosis of systemic AL amyloidosis due to IgG lambda-light chain deposition. Consequently, the patient underwent placement of implantable defibrillator and hematopoetic stem cell transplantation. He remains in excellent functional status 18 months after presentation.
    Full-text · Article · Mar 2010 · Indian pacing and electrophysiology journal
  • Source
    Jan Nemec · Jong Kim · Bethanny Gabris · Guy Salama

    Preview · Article · Mar 2010 · Journal of the American College of Cardiology
  • [Show abstract] [Hide abstract]
    ABSTRACT: The recently published Ventricular Arrhythmia Suppression Trial (VAST) found no effect of rate-smoothing (RS) algorithm on frequency of ventricular tachycardia (VT) episodes in patients with implantable defibrillator. A similar recent trial reported an opposite result. In order to determine possible reasons for the discrepancy between the trials and achieve better understanding of events preceding VT onset, we analyzed stored device electrograms preceding 162 VT episodes from 50 VAST trial patients with dual-chamber devices. In this analysis, short-long sequences were more common prior to polymorphic VTs than before monomorphic VTs. The proportion of VT episodes preceded by short-long sequences was lower during randomization to RS ON (5.3% vs 31.3%, P < 0.001). For patients with multiple episodes of monomorphic VT, there was higher interpatient than intrapatient variability in preceding RR intervals. When adjusting for this similarity of RR interval sequences preceding VT onset in individual patients, the difference in proportion short-long sequences between RS ON and RS OFF programming was no longer significant. Episodes of VT were preceded by stereotypic, patient-specific sequences of RR intervals in several VAST trial patients. RS reduced the percentage of VTs preceded by short-long sequences, but did not change overall VT incidence.
    No preview · Article · Dec 2008 · Journal of Cardiovascular Electrophysiology
  • [Show abstract] [Hide abstract]
    ABSTRACT: National trends in chronic lead extractions from cardiac rhythm management (CRM) devices are on the rise. The main objective of the study was to identify the predictors of complications of chronic lead extractions. All patients who underwent endovascular chronic CRM lead extraction at our institution between 2002 and 2008 were included in this analysis. Demographic data as well as details of the extraction procedure and its complications within the ensuing 30 days were collected on all patients. Data of 212 consecutive patients (456 leads, age = 65 +/- 17 years, men 75%, left ventricular ejection fraction = 36 +/- 16%, coronary artery disease 80%, defibrillators 49%) were analyzed. There were a total of 26 (11.8%) complications in 25 patients including 9 (4.2%) major complications (death 1, hemothorax 4, pneumothorax 2, tamponade 1, stroke 1) and 17 (8.0%) minor complications. Independent predictors of any complications included a higher number of explanted right ventricular leads (HR = 3.51, P = 0.013). Explantation of ICD as opposed to a pacemaker device showed a strong trend toward significance (HR = 2.57, P = 0.053). An elevated white blood cell count also predicted major complications (HR = 1.52, P = 0.005). Predictors of complications after lead extraction procedures include a higher number of extracted leads and the presence of defibrillator as opposed to pacemaker leads. A new paradigm of the removal of all leads not connected to a device may, therefore, reduce the risk of complications from lead extraction procedures and deserves to be tested prospectively.
    No preview · Article · Oct 2008 · Journal of Cardiovascular Electrophysiology
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a case of a patient with lymphoma in an ICD pocket in the setting of posttransplant immune suppression. Infection of the ICD system was suspected and the correct diagnosis was established by biopsy.
    No preview · Article · Jul 2008 · Pacing and Clinical Electrophysiology
  • Jan Nemec · Steven Heifetz
    [Show abstract] [Hide abstract]
    ABSTRACT: A patient with a sporadic heart-hand syndrome, which includes thumb hypoplasia, septum primum atrial septal defect, and cleft mitral valve is described. During attempted placement of a pacemaker lead, persistence of left superior and inferior vena cava was found in addition to the right-sided caval veins. This corresponds to persistence of left-sided supracardinal vein present during fetal development.
    No preview · Article · Jun 2008 · Congenital Heart Disease
  • [Show abstract] [Hide abstract]
    ABSTRACT: Advisories for implanted devices present a challenging management problem since no specific guidelines exist for device replacement under these circumstances. Since the rate and severity of complications is an important factor in the decision-making, we sought to review our experience with replacement of devices under advisory. Medical records of patients with devices under advisory were reviewed. A total of 237 patients (age 68+/-13 years, men 71%, implantable cardioverter-defibrillator (ICD) 87%) underwent device replacement in response to advisories (Medtronic Inc. 43% and Guidant Inc. 57%) at our institution between February 2005 and June 2006. The mean time from original device implantation to replacement was 31+/-16 months (3-73 months). During a mean follow-up of 198+/-103 days, there was a 5.5% overall rate of complications related to the procedure. Major complications requiring re-operation affected 2.1% of patients. There were no deaths associated with device replacement. Device failure prior to replacement was documented in 1.7% of patients, with syncope occurring in one patient. A history of diabetes mellitus (23%), peripheral vascular disease (4%), obstructive lung disease (7%), end-stage renal disease (2%), or use of anticoagulation (44%) did not predict the occurrence of complications after advisory device replacement. This single center experience shows a lower major complication rate from replacement of devices under advisory than previously reported. Experience from this center and others may be useful in guiding future management of patients in the setting of device advisories.
    No preview · Article · Apr 2008 · International journal of cardiology
  • Jan Nemec · Win-Kuang Shen
    [Show abstract] [Hide abstract]
    ABSTRACT: The placement of intracardiac catheters to assess the risk of arrhythmia dates back nearly 40 years. The discovery by Wellens et al. in 1972 that clinical ventricular tachycardia (VT) in patients with prior myocardial infarction (MI) could be initiated by programmed ventricular stimulation (PVS) generated real enthusiasm about the technique. In the 1980s, many cardiologists would have labeled PVS the most precise and perhaps the most important tool for risk stratifi cation of patients at risk for sudden cardiac death (SCD). Selection of antiarrhythmic treatment on the basis of suppression of VT inducibility was reasonably believed to represent a scientific approach to management of life-threatening arrhythmias.
    No preview · Article · Jan 2008
  • [Show abstract] [Hide abstract]
    ABSTRACT: A patient with advanced ischemic cardiomyopathy underwent implantation of a vagal stimulator in an attempt to control recurrent drug refractory ventricular arrhythmia. Electrical storm was exacerbated after the implant and continued after neurostimulation was discontinued. The report aims to provide a cautionary note to application of vagal stimulation for control of cardiac arrhythmia.
    No preview · Article · Jan 2008 · Clinical Autonomic Research
  • Jan Nemec
    [Show abstract] [Hide abstract]
    ABSTRACT: A case of a patient with runaway implantable cardioverter defibrillator (ICD) due to radiation therapy of a lung cancer is reported. This manifested as poorly tolerated wide complex tachycardia due to inappropriate rapid ventricular pacing, The event terminated with polymorphic VT, which inhibited pacing and ceased spontaneously before ICD discharge. The likely cause was corruption of device random access memory by ionizing radiation.
    No preview · Article · Jun 2007 · Pacing and Clinical Electrophysiology
  • [Show abstract] [Hide abstract]
    ABSTRACT: The muscarinic-gated atrial potassium channel IKACh has been well characterized functionally, and has been an excellent model system for studying G protein/effector interactions. Complementary DNAs encoding the composite subunits of IKACh have been identified, allowing direct probing of structural and functional features of the channel. Here, we highlight recent approaches taken in our laboratory to determine the oligomeric structure of native cardiac IKACh, the mechanism of activation of IKACh by G proteins, and the relevance of IKACh to cardiac physiology.
    No preview · Article · Feb 2006 · Annals of the New York Academy of Sciences
  • [Show abstract] [Hide abstract]
    ABSTRACT: The heart rate dependence of QT interval duration is abnormal in patients with congenital long QT syndrome. Patients with LQT1 have a defective I(Ks) current, a major determinant of QT response to heart rate. We studied the heart rate dependence of QT interval duration in different long QT syndrome genotypes and control subjects using computerized QT measurements obtained from Holter recordings. The dependence of QT duration on heart rate is steeper in long QT syndrome than in control subjects (0.347 +/- 0.263 vs 0.162 +/- 0.083 at heart rate 100 beats/min; P < 0.05). In addition, QT interval is significantly longer in LQT2 and LQT3 than in LQT1 patients at slow (533 +/- 23 ms vs 468 +/- 30 ms at heart rate 60 beats/min; P < 0.0001) but not at rapid heart rate. The heart rate dependence of QT interval is steeper in LQT2 and LQT3 than in LQT1 (0.623 +/- 0.245 vs 0.19 +/- 0.079 at heart rate 100 beats/min; P < 0.05). For a given heart rate, the QT intervals vary more in LQT2 and LQT3 than in LQT1 patients (25.98 +/- 11.18 ms vs 14.39 +/- 1.55 ms; P < 0.01). Individual long QT syndrome genotypes differ with respect to QT interval dependence on heart rate. These differences may relate to the propensity of LQT2 and LQT3 patients to develop arrhythmias during bradycardia.
    No preview · Article · Jun 2004 · Journal of Cardiovascular Electrophysiology

Publication Stats

912 Citations
159.37 Total Impact Points


  • 2007-2015
    • University of Pittsburgh
      Pittsburgh, Pennsylvania, United States
  • 2003-2004
    • Charles University in Prague
      • 1st Faculty of Medicine
      Praha, Praha, Czech Republic
  • 1999-2003
    • Mayo Clinic - Rochester
      • Department of Cardiovascular Diseases
      Rochester, Minnesota, United States
    • Mayo Foundation for Medical Education and Research
      • Division of Cardiovascular Diseases
      Рочестер, Michigan, United States
  • 1998-1999
    • Harvard Medical School
      • Department of Neurobiology
      Boston, Massachusetts, United States