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Speckle-Tracking Echocardiography Elucidates the Effect of Pacing Site on Left Ventricular Synchronization in the Normal and Infarcted Rat Myocardium
1744 Methods: To identify common genetic factors modulating disease risk, we conducted a genome-wide association study on 312 individuals with BrS and 1115 ancestry-matched controls. Results: Two genomic loci displayed significant association. Replication testing on 2 independent case/control sets from Europe (598/855) and Japan (208/1016) confirmed both associations and revealed a third locus. The cumulative effect of the 3 loci on disease susceptibility was large, with an odds ratio of 21.5 in the presence of more than 4 risk alleles vs less than 2. Two of 3 loci had previously been shown to influence ECG conduction parameters in the general population. The third locus encompasses a transcription factor that has not previously been implicated in cardiac electrical function and arrhythmia. Functional studies in mice heterozygous knockout for this transcription factor identified differences in expression of Nav1.5 in addition to differences in conduction of the cardiac electrical impulse in the right ventricular outflow tract. Conclusions: Our findings (1) indicate that common genetic variation may have a strong impact on predisposition to BrS and (2) identify a new gene involved in the pathogenesis of the disease.
Heart Rhythm, Vol 10, No 11, November 2013 (AP), monophasic action potentials (MAP), and pseudo-ECGs in rabbit isolated ventricular myocytes and hearts. Results: In single myocytes, E4031 (1 mM) prolonged AP duration at 90% repolarization (APD90) from 206 ⫾ 13 to 362 ⫾ 36 (paired t-test, P o.001; Figure A). Inhibition of endogenous INaL by 1 mM TTX (IC50 ¼ 1.9 mM, n ¼ 25) or 1 mM GS967 (IC50 ¼ 0.5 mM, n ¼ 13) shortened APD90 from 212 ⫾ 19 ms to 175 ⫾ 12 ms (Figure B) and from 199 ⫾ 33 ms to 163 ⫾ 24 ms (Fig. C), respectively. In the continuous presence of 1 mM TTX (Figure B) or GS967 (Figure C), E4031 caused minimal prolongation of APD90. In isolated hearts, addition of E4031 (60 nM) prolonged MAPD90 from 179 ⫾ 5 ms to 256 ⫾ 13 ms (n ¼ 5, P o.001) and caused ectopic ventricular beats (EVBs) in 5 of 5 hearts. Pretreatment with 1 mM TTX or GS967 shortened MAPD90 from 180 ⫾ 2 ms to 174 ⫾ 2 ms (n ¼ 8, P o.05) and 171 ⫾ 6 ms to 169 ⫾ 3 ms (n ¼ 5, P o.05), respectively. In the continuous presence of 1 mM TTX or GS967, similar to single myocytes, E4031 caused minimal prolongation of MAPD90 and did not induce EVBs. Conclusions: Our data suggest that selective inhibition of endogenous INaL could stabilize cardiac repolarization and prevent arrhythmogenicity in LQT2-like conditions.
APAMIN DOES NOT BLOCK MAJOR CARDIAC DEPOLARIZATION AND REPOLARIZATION CURRENTS C.C. Yu, T. Ai, P. Chen Krannert Institute of Cardiology, Indiana University, Indianapolis, IN. Background: Recent findings indicate that small-conductance Ca2þ-activated Kþ (SK) channels are abundantly present in atrial tissues and are upregulated in the diseased ventricles. Almost all studies used apamin, which is a specific SK channel blocker in neurons that does not block any other neural type of ion channels. However, the specificity of apamin in blocking SK currents in cardiac tissues remains unclear because apamin has been reported to block fetal L-type Ca2þ current in embryonic chick heart. The purpose of the present study was to test the hypothesis that apamin does not block any major cardiac type ion currents. Methods: We studied human embryonic kidney (HEK293) cells that either stably expressed human voltage-gated Naþ currents or were transiently transfected to express major human cardiac type Kþ and Ca2þ currents. Whole-cell patch-clamp techniques were used to determine ionic current densities before and after apamin administration. Results: Ca2þ currents (CACNA1CþCACNAB2B) in 4 cells (–17 ⫾ 4.9 pA/ pF) were not affected by apamin (500 nM) (–17 ⫾ 5.9 pA/pF, P ¼ NS) but were reduced by nifedipine (2 mM) to –5 ⫾ 4.0 pA/pF (P ¼ .008). Naþ currents (SCN5A) in 9 cells (–231 ⫾ 96 pA/pF) were not affected by apamin (500 nM) (–232 ⫾ 107 pA/pF, P ¼ NS) but were reduced by flecainide (100 mM) in a usedependent manner to 71 ⫾ 51 pA/pF (P ¼ o.001). IKs (KCNQ1þKCNE1) (25 ⫾ 9 pA/pF) was not blocked by apamin (500 nM) (n ¼ 8, 22 ⫾ 9 pA/pF, P ¼ NS) but was blocked by chromanol 293B (50 mM) to 5 ⫾ 2 pA/pF (n ¼ 4, P ¼ .01). IKr (KCNH2þKCNE2) (27 ⫾ 8 pA/pF) was not blocked by apamin (500 nM) (n ¼ 6, 26 ⫾ 6 pA/pF, P ¼ NS) but was blocked by E4031 (100 nM) to 13 ⫾ 4 pA/pF (n ¼ 6, P o.001). IK1 (KCNJ2) (–45 ⫾ 14 pA/pF) was not blocked by apamin (500 nM) (n ¼ 7, –46 ⫾ 17 pA/pF, P ¼ NS) but was blocked by CsCl (5 mM) to –16 ⫾ 5 pA/pF (n ¼ 7, P ¼ .004). Ito (KCND3) (526 ⫾ 115 pA/pF) was not blocked by apamin (500 nM) (n ¼ 2, 516 ⫾ 110 pA/pF, P ¼ NS) but was blocked by 4-aminopyridine (10 mM) to 235 pA/pF (n ¼ 1). Conclusions: Apamin does not block human Naþ current, L-type Ca2þ current, or other major Kþ currents in HEK293 cells. These findings indicate that apamin is a specific SK current blocker in human cardiac tissues. SELECTIVE INHIBITION OF ENDOGENOUS LATE Naþ CURRENT STABILIZES CARDIAC REPOLARIZATION AND PREVENTS ARRHYTHMOGENICITY N. El-Bizri, H. Li, G. Liu, S. Rajamani, L. Belardinelli Gilead Sciences, Fremont, CA. Background: Endogenous late Naþ current (INaL) is proarrhythmic in conditions where repolarization reserve is reduced. We tested the hypothesis that selective inhibition of endogenous INaL stabilizes cardiac repolarization and prevents arrhythmias in conditions where IKr is inhibited. Methods: E4031, an IKr blocker, was used to reduce repolarization reserve. We tested the effects of the INaL inhibitors tetrodotoxin (TTX) and GS458967 (GS967) in the absence and presence of E4031 on action potentials
SPECKLE-TRACKING ECHOCARDIOGRAPHY ELUCIDATES THE EFFECT OF PACING SITE ON LEFT VENTRICULAR SYNCHRONIZATION IN THE NORMAL AND INFARCTED RAT MYOCARDIUM W. Mulla1, M. Mor1, S. Elyagon1, H. Gabay1, S. Dror1, N. Liel-Cohen2, Y. Etzion1 1 Ben-Gurion University of the Negev, Beer-Sheva, Israel, 2Soroka University Medical Center, Beer-Sheva, Israel. Background: Right ventricular (RV) pacing impairs left ventricular (LV) mechanics. In contrast, biventricular (BIV) pacing can improve LV efficiency in the setting of electromechanical dysfunction, thereby constituting the rationale for cardiac resynchronization therapy (CRT). Critical issues such as correct patient selection and the molecular effects of CRT are still elusive, and CRT models currently rely on large animals, which are expensive and cannot be manipulated genetically. Using a methodology for efficient implantation of cardiac electrodes in rodents, we explored the effect of pacing site on LV synchronization in rats. Methods: Two miniature bipolar hook electrodes were implanted in vivo in the rat heart as follows: Group A (n ¼ 6) right atrium (RA) and apical RV sites; group B (n ¼ 7) RV and posterobasal LV sites; and group C (n ¼ 7) similar implantation as in group B in combination with left coronary artery ligation. Electrodes were exteriorized through the back of the animal. Following postoperative recovery, two-dimensional transthoracic echocardiography was performed during pacing through the different electrodes. Segmental systolic circumferential strain (CS) analysis was used to evaluate LV dyssynchrony. Results: In group A, RA pacing compared to sinus rhythm had no effect on LV synchrony as measured by standard deviation (SD) of segmental time to peak CS, SD of time to peak CS, and average delay between opposing ventricular segments. In contrast, RV pacing induced marked dyssynchrony (P o.01 for all 3 parameters noted). In group B, LV or BIV pacing markedly improved LV dyssynchrony compared to RV pacing in normal rats. However, in rats with extensive myocardial infarction (group C), the effects of LV and BIV pacing were markedly attenuated. Nevertheless, analysis of the delay between opposing ventricular segments demonstrated improvement in 5 of 7 individual rats. Conclusions: In rats, LV and BIV pacing created favorable LV mechanics compared to RV pacing. These effects of pacing were less marked in the presence of extensive ischemic cardiomyopathy. Thus, rodent pacing
1745 appears to mimic important features seen in large mammals and may become a simple and reliable tool for pacing-related studies. LEADING REENTRY CIRCUIT(S) IN THE RIGHT ATRIAL FREE WALL UNDERLIES ADENOSINE-INDUCED ATRIAL FIBRILLATION IN CORONARY PERFUSED HUMAN HEARTS B.J. Hansen1, T.A. Csepe1, A. Kalyanasundaram1, G. Kamalov1, J. Zhao2, P.M.L. Janssen1, P.J. Mohler1, E.A. Davies1, R. Weiss1, V.V. Fedorov1 1 The Ohio State University, Columbus, OH, 2University of Auckland, Auckland, New Zealand. Background: Atrial fibrillation (AF) is the most common arrhythmia, yet the mechanism in human hearts remains to be elucidated. Clinical data connect adenosine to the induction of AF. We aimed to characterize the mechanism of adenosine-induced AF in explanted human hearts by using high-resolution optical and anatomic mapping. Methods: Explanted human hearts (n ¼ 12) with various chronic cardiac diseases were obtained immediately after cardioplegic arrest. Coronaryperfused atrial preparations of the right and left atrial appendages, free walls (RAFW, LAFW), intraatrial septum (IAS), and tricuspid and mitral valve rings were optically mapped from the epi- and/or endocardium simultaneously by two high-resolution CMOS cameras (100 100 pixels), positioned for whole atrial (95 95 mm) and specific (33 33 mm) fields of view. Activation, action potential duration (APD80%), and dominant frequency (DF) were analyzed during pacing and AF with correlation to atrial anatomy (histology). Results: At baseline, a stepwise pacing protocol found the functional refractory period (FRP) to be 252 ⫾ 80 ms, which was not short enough to induce AF. Adenosine (10–100 mM) led to spontaneous (n ¼ 1) and pacing-induced AF (n ¼ 5, 3/5 had clinical AF history) by shortening FRP to 183 ⫾ 61 ms (P ¼ .008 vs baseline) and heterogeneously shortening APD with maximum changes in RAFW. AF-susceptible hearts exhibited more pronounced RAFW APD shortening, from 244 ⫾ 55 ms to 177 ⫾ 45 vs nonsusceptible and from 294 ⫾ 37 ms to 252 ⫾ 38 ms (P ¼ .01) paced at 2 Hz. The principal mechanism of AF episodes (48 seconds) was a leading reentrant circuit(s) anchored to the pectinate muscles and interstitial fibrosis strands in the areas of shortest APD during adenosine treatment (superior and middle RAFW). The atrial regions outside the leading reentrant circuit(s) had slower activation due to intermittent failure to respond 1:1. As such, DF during AF was fastest in the RAFW (8.7 ⫾ 2.8 Hz) vs 4.1 ⫾ 0.8 in the IAS/LAFW (P ¼ .003). DF patterns remained similar during each AF episode in the same heart. Conclusions: This study demonstrates that in the human heart, adenosine primarily induces RAFW reentrant AF with leading reentrant circuits (rotors) anchored to the regions of shortest APD and anatomic architecture. A CACNA1C MUTATION THAT CAUSES A SUBSET OF TIMOTHY SYNDROME PHENOTYPES CORRELATES J.A. Hennessey1, Y. Jiang1, J.D. Miller2, H.A. Stadt1, W. Patrick1, R. Pfeiffer3, C. Antzelevitch3, R. Kanter1, G.S. Pitt1 1 Duke University, Durham, NC, 2Virginia Tech Carilion School of Medicine, Roanoke, VA, 3Masonic Medical Research Laboratory, Utica, NY. Background: Timothy syndrome (TS) is a rare congenital long QT syndrome (LQTS) associated with extracardiac manifestations including craniofacial dysmorphia and dental abnormalities. The locus for TS is CACNA1C, which encodes the CaV1.2 L-type Ca2þ channel, for which canonical mutations lead to a decrease in voltage-dependent inactivation (VDI). However, a recent report of a patient with LQTS in isolation and a CACNA1C mutation that did not affect VDI raised the question whether altered VDI is necessary for extracardiac phenotypes. In a patient with a maternally inherited microdeletion with a chromosomal translocation who presented with LQTS and associated ventricular tachyarrhythmias (Figure A), a subset of TS phenotypes, and a skeletal myopathy not readily explained by the translocation, we sought to identify a causative mutation for the TS phenotypes. Methods: A candidate gene approach identified a mutation in CACNA1C that was absent in the mother. We performed electrophysiologic studies on the mutant and characterized CACNA1C expression in skeletal muscle with a mouse CACNA1C reporter line. Results: We identified a glycine to arginine mutation at position 1911 (G1911R) in CaV1.2. Functional studies revealed that G1911R increased
CaV1.2 channel availability (Figures B and C) and decreased VDI (Figure D). The CACNA1C reporter mouse showed no CaV1.2 expression in skeletal muscle. Conclusions: We describe a CACNA1C mutation that leads to a subset of TS phenotypes. In the context of a recently described CACNA1C mutation that does not affect VDI in an LQTS patient without extracardiac phenotypes, these data suggest that the extracardiac phenotypes seen in TS require effects on VDI.
NOVEL PARAMETERS TO IMPROVE QUANTIFICATION OF REPOLARIZATION RESERVE AND ARRHYTHMOGENESIS USING A DOFETILIDE CHALLENGE T.R.G. Stams, S.K.G. Winckels, A. Oros, R. Varkevisser, P. Oosterhoff, R. van der Nagel, H.D.M. Beekman, M.A. Vos University Medical Center Utrecht, Utrecht, Netherlands. Background: In the canine complete AV block (AVB) model with bradycardia, inducibility of drug-induced torsades de pointes (TdP) is usually the primary outcome parameter, whereas short-term variability (STV) of the left ventricular monophasic action potential duration (LV MAPD) is used as primary surrogate parameter. The dichotomous parameter inducibility is cumbersome because it requires Z3 TdP episodes and does not take into account (multiple) ectopic beats or severity of TdP episodes. In this study, we introduce arrhythmia score (AS) and T25, using dofetilide (D) to improve quantification of repolarization reserve. Methods: Experiments performed under general anesthesia in unremodeled, acute AVB dogs (aAVB, n ¼ 13) and 3 groups of AVB dogs with chronic bradycardia but differences in remodeling due to alterations in chronic activation pattern were analyzed: spontaneous idioventricular rhythm (IVR, n ¼ 19), more physiologic high-septal pacing (HSP, n ¼ 10), and right ventricular apex pacing (RVA, n ¼ 8). AS was calculated as 1 þ mean number of beats of the 3 most severe arrhythmias after D (0.025 mg/kg/5 min), with scores of 50, 75, and 100, for 1, 2, and Z3 cardioversions, respectively. T25 was defined as the time required to obtain 25-ms increase of LV MAPD after D. Results: For groups aAVB, IVR, HSP and RVA, TdP inducibility was 0%, 74% (P o.001 vs aAVB), 30%, and 75% (P o.01 vs aAVB), respectively, with a similar pattern in AS: 3.8 ⫾ 6.5 (mean ⫾ SD), 41 ⫾ 28 (P o.001 vs aAVB), 14 ⫾ 22 (P o.05 vs IVR), and 28 ⫾ 24. Serial analysis in aAVB dogs revealed that AS was increased by D (from 1.3 ⫾ 0.4 to 4.3 ⫾ 7.4; n ¼ 10; P o.05), although 3 or more TdPs were not induced in any dog. T25 values were 2.1 ⫾ 0.4 minutes 1.4 ⫾ 0.4 minutes (P o.001 vs aAVB), 1.9 ⫾ 0.4 minutes (P o.01 vs IVR), and 1.5 ⫾ 0.3 minutes (P o.01 vs aAVB), respectively. Unexpectedly, STV before or after D was not significantly higher in any group vs aAVB. ”Predicting” inducibility after pooling all data
Heart Rhythm, Vol 10, No 11, November 2013 (AP), monophasic action potentials (MAP), and pseudo-ECGs in rabbit isolated ventricular myocytes and hearts. Results: In single myocytes, E4031 (1 mM) prolonged AP duration at 90% repolarization (APD90) from 206 ⫾ 13 to 362 ⫾ 36 (paired t-test, P o.001; Figure A). Inhibition of endogenous INaL by 1 mM TTX (IC50 ¼ 1.9 mM, n ¼ 25) or 1 mM GS967 (IC50 ¼ 0.5 mM, n ¼ 13) shortened APD90 from 212 ⫾ 19 ms to 175 ⫾ 12 ms (Figure B) and from 199 ⫾ 33 ms to 163 ⫾ 24 ms (Fig. C), respectively. In the continuous presence of 1 mM TTX (Figure B) or GS967 (Figure C), E4031 caused minimal prolongation of APD90. In isolated hearts, addition of E4031 (60 nM) prolonged MAPD90 from 179 ⫾ 5 ms to 256 ⫾ 13 ms (n ¼ 5, P o.001) and caused ectopic ventricular beats (EVBs) in 5 of 5 hearts. Pretreatment with 1 mM TTX or GS967 shortened MAPD90 from 180 ⫾ 2 ms to 174 ⫾ 2 ms (n ¼ 8, P o.05) and 171 ⫾ 6 ms to 169 ⫾ 3 ms (n ¼ 5, P o.05), respectively. In the continuous presence of 1 mM TTX or GS967, similar to single myocytes, E4031 caused minimal prolongation of MAPD90 and did not induce EVBs. Conclusions: Our data suggest that selective inhibition of endogenous INaL could stabilize cardiac repolarization and prevent arrhythmogenicity in LQT2-like conditions.
APAMIN DOES NOT BLOCK MAJOR CARDIAC DEPOLARIZATION AND REPOLARIZATION CURRENTS C.C. Yu, T. Ai, P. Chen Krannert Institute of Cardiology, Indiana University, Indianapolis, IN. Background: Recent findings indicate that small-conductance Ca2þ-activated Kþ (SK) channels are abundantly present in atrial tissues and are upregulated in the diseased ventricles. Almost all studies used apamin, which is a specific SK channel blocker in neurons that does not block any other neural type of ion channels. However, the specificity of apamin in blocking SK currents in cardiac tissues remains unclear because apamin has been reported to block fetal L-type Ca2þ current in embryonic chick heart. The purpose of the present study was to test the hypothesis that apamin does not block any major cardiac type ion currents. Methods: We studied human embryonic kidney (HEK293) cells that either stably expressed human voltage-gated Naþ currents or were transiently transfected to express major human cardiac type Kþ and Ca2þ currents. Whole-cell patch-clamp techniques were used to determine ionic current densities before and after apamin administration. Results: Ca2þ currents (CACNA1CþCACNAB2B) in 4 cells (–17 ⫾ 4.9 pA/ pF) were not affected by apamin (500 nM) (–17 ⫾ 5.9 pA/pF, P ¼ NS) but were reduced by nifedipine (2 mM) to –5 ⫾ 4.0 pA/pF (P ¼ .008). Naþ currents (SCN5A) in 9 cells (–231 ⫾ 96 pA/pF) were not affected by apamin (500 nM) (–232 ⫾ 107 pA/pF, P ¼ NS) but were reduced by flecainide (100 mM) in a usedependent manner to 71 ⫾ 51 pA/pF (P ¼ o.001). IKs (KCNQ1þKCNE1) (25 ⫾ 9 pA/pF) was not blocked by apamin (500 nM) (n ¼ 8, 22 ⫾ 9 pA/pF, P ¼ NS) but was blocked by chromanol 293B (50 mM) to 5 ⫾ 2 pA/pF (n ¼ 4, P ¼ .01). IKr (KCNH2þKCNE2) (27 ⫾ 8 pA/pF) was not blocked by apamin (500 nM) (n ¼ 6, 26 ⫾ 6 pA/pF, P ¼ NS) but was blocked by E4031 (100 nM) to 13 ⫾ 4 pA/pF (n ¼ 6, P o.001). IK1 (KCNJ2) (–45 ⫾ 14 pA/pF) was not blocked by apamin (500 nM) (n ¼ 7, –46 ⫾ 17 pA/pF, P ¼ NS) but was blocked by CsCl (5 mM) to –16 ⫾ 5 pA/pF (n ¼ 7, P ¼ .004). Ito (KCND3) (526 ⫾ 115 pA/pF) was not blocked by apamin (500 nM) (n ¼ 2, 516 ⫾ 110 pA/pF, P ¼ NS) but was blocked by 4-aminopyridine (10 mM) to 235 pA/pF (n ¼ 1). Conclusions: Apamin does not block human Naþ current, L-type Ca2þ current, or other major Kþ currents in HEK293 cells. These findings indicate that apamin is a specific SK current blocker in human cardiac tissues. SELECTIVE INHIBITION OF ENDOGENOUS LATE Naþ CURRENT STABILIZES CARDIAC REPOLARIZATION AND PREVENTS ARRHYTHMOGENICITY N. El-Bizri, H. Li, G. Liu, S. Rajamani, L. Belardinelli Gilead Sciences, Fremont, CA. Background: Endogenous late Naþ current (INaL) is proarrhythmic in conditions where repolarization reserve is reduced. We tested the hypothesis that selective inhibition of endogenous INaL stabilizes cardiac repolarization and prevents arrhythmias in conditions where IKr is inhibited. Methods: E4031, an IKr blocker, was used to reduce repolarization reserve. We tested the effects of the INaL inhibitors tetrodotoxin (TTX) and GS458967 (GS967) in the absence and presence of E4031 on action potentials
SPECKLE-TRACKING ECHOCARDIOGRAPHY ELUCIDATES THE EFFECT OF PACING SITE ON LEFT VENTRICULAR SYNCHRONIZATION IN THE NORMAL AND INFARCTED RAT MYOCARDIUM W. Mulla1, M. Mor1, S. Elyagon1, H. Gabay1, S. Dror1, N. Liel-Cohen2, Y. Etzion1 1 Ben-Gurion University of the Negev, Beer-Sheva, Israel, 2Soroka University Medical Center, Beer-Sheva, Israel. Background: Right ventricular (RV) pacing impairs left ventricular (LV) mechanics. In contrast, biventricular (BIV) pacing can improve LV efficiency in the setting of electromechanical dysfunction, thereby constituting the rationale for cardiac resynchronization therapy (CRT). Critical issues such as correct patient selection and the molecular effects of CRT are still elusive, and CRT models currently rely on large animals, which are expensive and cannot be manipulated genetically. Using a methodology for efficient implantation of cardiac electrodes in rodents, we explored the effect of pacing site on LV synchronization in rats. Methods: Two miniature bipolar hook electrodes were implanted in vivo in the rat heart as follows: Group A (n ¼ 6) right atrium (RA) and apical RV sites; group B (n ¼ 7) RV and posterobasal LV sites; and group C (n ¼ 7) similar implantation as in group B in combination with left coronary artery ligation. Electrodes were exteriorized through the back of the animal. Following postoperative recovery, two-dimensional transthoracic echocardiography was performed during pacing through the different electrodes. Segmental systolic circumferential strain (CS) analysis was used to evaluate LV dyssynchrony. Results: In group A, RA pacing compared to sinus rhythm had no effect on LV synchrony as measured by standard deviation (SD) of segmental time to peak CS, SD of time to peak CS, and average delay between opposing ventricular segments. In contrast, RV pacing induced marked dyssynchrony (P o.01 for all 3 parameters noted). In group B, LV or BIV pacing markedly improved LV dyssynchrony compared to RV pacing in normal rats. However, in rats with extensive myocardial infarction (group C), the effects of LV and BIV pacing were markedly attenuated. Nevertheless, analysis of the delay between opposing ventricular segments demonstrated improvement in 5 of 7 individual rats. Conclusions: In rats, LV and BIV pacing created favorable LV mechanics compared to RV pacing. These effects of pacing were less marked in the presence of extensive ischemic cardiomyopathy. Thus, rodent pacing
1745 appears to mimic important features seen in large mammals and may become a simple and reliable tool for pacing-related studies. LEADING REENTRY CIRCUIT(S) IN THE RIGHT ATRIAL FREE WALL UNDERLIES ADENOSINE-INDUCED ATRIAL FIBRILLATION IN CORONARY PERFUSED HUMAN HEARTS B.J. Hansen1, T.A. Csepe1, A. Kalyanasundaram1, G. Kamalov1, J. Zhao2, P.M.L. Janssen1, P.J. Mohler1, E.A. Davies1, R. Weiss1, V.V. Fedorov1 1 The Ohio State University, Columbus, OH, 2University of Auckland, Auckland, New Zealand. Background: Atrial fibrillation (AF) is the most common arrhythmia, yet the mechanism in human hearts remains to be elucidated. Clinical data connect adenosine to the induction of AF. We aimed to characterize the mechanism of adenosine-induced AF in explanted human hearts by using high-resolution optical and anatomic mapping. Methods: Explanted human hearts (n ¼ 12) with various chronic cardiac diseases were obtained immediately after cardioplegic arrest. Coronaryperfused atrial preparations of the right and left atrial appendages, free walls (RAFW, LAFW), intraatrial septum (IAS), and tricuspid and mitral valve rings were optically mapped from the epi- and/or endocardium simultaneously by two high-resolution CMOS cameras (100 100 pixels), positioned for whole atrial (95 95 mm) and specific (33 33 mm) fields of view. Activation, action potential duration (APD80%), and dominant frequency (DF) were analyzed during pacing and AF with correlation to atrial anatomy (histology). Results: At baseline, a stepwise pacing protocol found the functional refractory period (FRP) to be 252 ⫾ 80 ms, which was not short enough to induce AF. Adenosine (10–100 mM) led to spontaneous (n ¼ 1) and pacing-induced AF (n ¼ 5, 3/5 had clinical AF history) by shortening FRP to 183 ⫾ 61 ms (P ¼ .008 vs baseline) and heterogeneously shortening APD with maximum changes in RAFW. AF-susceptible hearts exhibited more pronounced RAFW APD shortening, from 244 ⫾ 55 ms to 177 ⫾ 45 vs nonsusceptible and from 294 ⫾ 37 ms to 252 ⫾ 38 ms (P ¼ .01) paced at 2 Hz. The principal mechanism of AF episodes (48 seconds) was a leading reentrant circuit(s) anchored to the pectinate muscles and interstitial fibrosis strands in the areas of shortest APD during adenosine treatment (superior and middle RAFW). The atrial regions outside the leading reentrant circuit(s) had slower activation due to intermittent failure to respond 1:1. As such, DF during AF was fastest in the RAFW (8.7 ⫾ 2.8 Hz) vs 4.1 ⫾ 0.8 in the IAS/LAFW (P ¼ .003). DF patterns remained similar during each AF episode in the same heart. Conclusions: This study demonstrates that in the human heart, adenosine primarily induces RAFW reentrant AF with leading reentrant circuits (rotors) anchored to the regions of shortest APD and anatomic architecture. A CACNA1C MUTATION THAT CAUSES A SUBSET OF TIMOTHY SYNDROME PHENOTYPES CORRELATES J.A. Hennessey1, Y. Jiang1, J.D. Miller2, H.A. Stadt1, W. Patrick1, R. Pfeiffer3, C. Antzelevitch3, R. Kanter1, G.S. Pitt1 1 Duke University, Durham, NC, 2Virginia Tech Carilion School of Medicine, Roanoke, VA, 3Masonic Medical Research Laboratory, Utica, NY. Background: Timothy syndrome (TS) is a rare congenital long QT syndrome (LQTS) associated with extracardiac manifestations including craniofacial dysmorphia and dental abnormalities. The locus for TS is CACNA1C, which encodes the CaV1.2 L-type Ca2þ channel, for which canonical mutations lead to a decrease in voltage-dependent inactivation (VDI). However, a recent report of a patient with LQTS in isolation and a CACNA1C mutation that did not affect VDI raised the question whether altered VDI is necessary for extracardiac phenotypes. In a patient with a maternally inherited microdeletion with a chromosomal translocation who presented with LQTS and associated ventricular tachyarrhythmias (Figure A), a subset of TS phenotypes, and a skeletal myopathy not readily explained by the translocation, we sought to identify a causative mutation for the TS phenotypes. Methods: A candidate gene approach identified a mutation in CACNA1C that was absent in the mother. We performed electrophysiologic studies on the mutant and characterized CACNA1C expression in skeletal muscle with a mouse CACNA1C reporter line. Results: We identified a glycine to arginine mutation at position 1911 (G1911R) in CaV1.2. Functional studies revealed that G1911R increased
CaV1.2 channel availability (Figures B and C) and decreased VDI (Figure D). The CACNA1C reporter mouse showed no CaV1.2 expression in skeletal muscle. Conclusions: We describe a CACNA1C mutation that leads to a subset of TS phenotypes. In the context of a recently described CACNA1C mutation that does not affect VDI in an LQTS patient without extracardiac phenotypes, these data suggest that the extracardiac phenotypes seen in TS require effects on VDI.
NOVEL PARAMETERS TO IMPROVE QUANTIFICATION OF REPOLARIZATION RESERVE AND ARRHYTHMOGENESIS USING A DOFETILIDE CHALLENGE T.R.G. Stams, S.K.G. Winckels, A. Oros, R. Varkevisser, P. Oosterhoff, R. van der Nagel, H.D.M. Beekman, M.A. Vos University Medical Center Utrecht, Utrecht, Netherlands. Background: In the canine complete AV block (AVB) model with bradycardia, inducibility of drug-induced torsades de pointes (TdP) is usually the primary outcome parameter, whereas short-term variability (STV) of the left ventricular monophasic action potential duration (LV MAPD) is used as primary surrogate parameter. The dichotomous parameter inducibility is cumbersome because it requires Z3 TdP episodes and does not take into account (multiple) ectopic beats or severity of TdP episodes. In this study, we introduce arrhythmia score (AS) and T25, using dofetilide (D) to improve quantification of repolarization reserve. Methods: Experiments performed under general anesthesia in unremodeled, acute AVB dogs (aAVB, n ¼ 13) and 3 groups of AVB dogs with chronic bradycardia but differences in remodeling due to alterations in chronic activation pattern were analyzed: spontaneous idioventricular rhythm (IVR, n ¼ 19), more physiologic high-septal pacing (HSP, n ¼ 10), and right ventricular apex pacing (RVA, n ¼ 8). AS was calculated as 1 þ mean number of beats of the 3 most severe arrhythmias after D (0.025 mg/kg/5 min), with scores of 50, 75, and 100, for 1, 2, and Z3 cardioversions, respectively. T25 was defined as the time required to obtain 25-ms increase of LV MAPD after D. Results: For groups aAVB, IVR, HSP and RVA, TdP inducibility was 0%, 74% (P o.001 vs aAVB), 30%, and 75% (P o.01 vs aAVB), respectively, with a similar pattern in AS: 3.8 ⫾ 6.5 (mean ⫾ SD), 41 ⫾ 28 (P o.001 vs aAVB), 14 ⫾ 22 (P o.05 vs IVR), and 28 ⫾ 24. Serial analysis in aAVB dogs revealed that AS was increased by D (from 1.3 ⫾ 0.4 to 4.3 ⫾ 7.4; n ¼ 10; P o.05), although 3 or more TdPs were not induced in any dog. T25 values were 2.1 ⫾ 0.4 minutes 1.4 ⫾ 0.4 minutes (P o.001 vs aAVB), 1.9 ⫾ 0.4 minutes (P o.01 vs IVR), and 1.5 ⫾ 0.3 minutes (P o.01 vs aAVB), respectively. Unexpectedly, STV before or after D was not significantly higher in any group vs aAVB. ”Predicting” inducibility after pooling all data