Mean Action Potential Duration Research Articles

Sensitivity Analysis of Cardiac Alternans and Tachyarrhythmia to Ion Channel Conductance Using Population Modeling.

Action potential duration (APD) alternans, an alternating phenomenon between action potentials in cardiomyocytes, causes heart arrhythmia when the heart rate is high. However, some of the APD alternans observed in clinical trials occurs under slow heart rate conditions of 100 to 120 bpm, increasing the likelihood of heart arrhythmias such as atrial fibrillation. Advanced studies have identified the occurrence of this type of APD alternans in terms of electrophysiological ion channel currents in cells. However, they only identified physiological phenomena, such as action potential due to random changes in a particular ion channel's conductivity through ion models specializing in specific ion channel currents. In this study, we performed parameter sensitivity analysis via population modeling using a validated human ventricular physiology model to check the sensitivity of APD alternans to ion channel conductances. Through population modeling, we expressed the changes in alternans onset cycle length (AOCL) and mean APD in AOCL (AO meanAPD) according to the variations in ion channel conductance. Finally, we identified the ion channel that maximally affected the occurrence of APD alternans. AOCL and AO meanAPD were sensitive to changes in the plateau Ca2+ current. Accordingly, it was expected that APD alternans would be vulnerable to changes in intracellular calcium concentration.

Measures of repolarization variability predict ventricular arrhythmogenesis in heptanol-treated Langendorff-perfused mouse hearts.

BackgroundTime-domain and non-linear methods can be used to quantify beat-to-beat repolarization variability but whether measures of repolarization variability can predict ventricular arrhythmogenesis in mice have never been explored.MethodsLeft ventricular monophasic action potentials (MAPs) were recorded during constant right ventricular 8 ​Hz pacing in Langendorff-perfused mouse hearts, in the presence or absence of the gap junction and sodium channel inhibitor heptanol (0.1, 0.5, 1 or 2 ​mM).ResultsUnder control conditions, mean action potential duration (APD) was 39.4 ​± ​8.1 ​ms. Standard deviation (SD) of APDs was 0.3 ​± ​0.2 ​ms, coefficient of variation was 0.9 ​± ​0.8% and the root mean square (RMS) of successive differences in APDs was 0.15 ​± ​0.14 ​ms. Poincaré plots of APDn+1 against APDn revealed ellipsoid morphologies with a SD along the line-of-identity (SD2) to SD perpendicular to the line-of-identity (SD1) ratio of 4.6 ​± ​2.1. Approximate and sample entropy were 0.61 ​± ​0.12 and 0.76 ​± ​0.26, respectively. Detrended fluctuation analysis revealed short- and long-term fluctuation slopes of 1.49 ​± ​0.27 and 0.81 ​± ​0.36, respectively. Heptanol at 2 ​mM induced ventricular tachycardia in five out of six hearts. None of the above parameters were altered by heptanol during which reproducible electrical activity was observed (KW-ANOVA, P ​> ​0.05). Contrastingly, SD2/SD1 decreased to 2.03 ​± ​0.41, approximate and sample entropy increased to 0.82 ​± ​0.12 and 1.45 ​± ​0.34, and short-term fluctuation slope decreased to 0.82 ​± ​0.19 during the 20-s period preceding spontaneous ventricular tachy-arrhythmias (n ​= ​6, KW-ANOVA, P ​< ​0.05).ConclusionMeasures of repolarization variability, such as SD2/SD1, entropy, and fluctuation slope are altered preceding the occurrence of ventricular arrhythmogenesis in mouse hearts. Changes in these variables may allow detection of impending arrhythmias for early intervention.

Adrenergic supersensitivity and impaired neural control of cardiac electrophysiology following regional cardiac sympathetic nerve loss

Myocardial infarction (MI) can result in sympathetic nerve loss in the infarct region. However, the contribution of hypo-innervation to electrophysiological remodeling, independent from MI-induced ischemia and fibrosis, has not been comprehensively investigated. We present a novel mouse model of regional cardiac sympathetic hypo-innervation utilizing a targeted-toxin (dopamine beta-hydroxylase antibody conjugated to saporin, DBH-Sap), and measure resulting electrophysiological and Ca2+ handling dynamics. Five days post-surgery, sympathetic nerve density was reduced in the anterior left ventricular epicardium of DBH-Sap hearts compared to control. In Langendorff-perfused hearts, there were no differences in mean action potential duration (APD80) between groups; however, isoproterenol (ISO) significantly shortened APD80 in DBH-Sap but not control hearts, resulting in a significant increase in APD80 dispersion in the DBH-Sap group. ISO also produced spontaneous diastolic Ca2+ elevation in DBH-Sap but not control hearts. In innervated hearts, sympathetic nerve stimulation (SNS) increased heart rate to a lesser degree in DBH-Sap hearts compared to control. Additionally, SNS produced APD80 prolongation in the apex of control but not DBH-Sap hearts. These results suggest that hypo-innervated hearts have regional super-sensitivity to circulating adrenergic stimulation (ISO), while having blunted responses to SNS, providing important insight into the mechanisms of arrhythmogenesis following sympathetic nerve loss.

Digitalis Promotes Ventricular Arrhythmias in Flecainide- and Ranolazine-Pretreated Hearts.

A post hoc analysis of the PALLAS trial suggested life-threatening interactions of digitalis and dronedarone. Thus, there is concern about an interplay between digitalis and other drugs that influence cardiac electrophysiology. We therefore investigated the interaction between digitalis and flecainide or ranolazine. Twenty-five rabbit hearts were Langendorff-perfused and treated with flecainide (2µM, 12 hearts) or ranolazine (10µM, 13 hearts). Infusion of flecainide prolonged mean action potential duration [APD90, from 153ms (interquartile range (IQR): 29.7ms) to 159ms (IQR: 24.9ms, p = 0.04)] and effective refractory period [ERP, 170ms (IQR: 40ms) vs. 200ms (IQR: 32.5ms, p < 0.01)]. Administration of ranolazine prolonged APD90 [144ms (IQR: 34.3ms)) vs. 157ms (IQR: 31.2ms, p < 0.01)] and ERP [180ms (IQR: 40ms) vs. 200ms (IQR: 30ms, p < 0.01)]. Additional infusion of the digitalis glycoside ouabain (0.2µM) abbreviated APD90 and ERP in both groups (flecainide: APD90: to 128ms (IQR: 19ms), ERP: to 170ms (IQR: 20ms), p < 0.01 each; ranolazine: APD90: to 141ms (IQR: 40ms), ERP: to 160ms (IQR: 30ms), p < 0.01 each). Ventricular vulnerability was assessed by a pacing protocol employing premature extra stimuli and burst stimulation. No proarrhythmic effect was observed with flecainide (1 vs. 3 episodes at baseline) or ranolazine (3 vs. 11 episodes at baseline). However, further infusion of ouabain had a proarrhythmic effect for both drugs (flecainide: 15 episodes, p = 0.04; ranolazine: 21 episodes, p = 0.09). Concomitant treatment of the sodium channel blockers flecainide or ranolazine with digitalis seems to be proarrhythmic. Abbreviation of repolarization and refractoriness that can facilitate re-entry was found as underlying mechanism.

Atrial fibrillation and electrophysiology in transgenic mice with cardiac-restricted overexpression of FKBP12.

Cardiomyocyte-restricted overexpression of FK506-binding protein 12 transgenic (αMyHC-FKBP12) mice develop spontaneous atrial fibrillation (AF). The aim of the present study is to explore the mechanisms underlying the occurrence of AF in αMyHC-FKBP12 mice. Spontaneous AF was documented by telemetry in vivo and Langendorff-perfused hearts of αMyHC-FKBP12 and littermate control mice in vitro. Atrial conduction velocity was evaluated by optical mapping. The patch-clamp technique was applied to determine the potentially altered electrophysiology in atrial myocytes. Channel protein expression levels were evaluated by Western blot analyses. Spontaneous AF was recorded in four of seven αMyHC-FKBP12 mice but in none of eight nontransgenic (NTG) controls. Atrial conduction velocity was significantly reduced in αMyHC-FKBP12 hearts compared with NTG hearts. Interestingly, the mean action potential duration at 50% but not 90% was significantly prolonged in αMyHC-FKBP12 atrial myocytes compared with their NTG counterparts. Consistent with decreased conduction velocity, average peak Na+ current ( INa) density was dramatically reduced and the INa inactivation curve was shifted by approximately +7 mV in αMyHC-FKBP12 atrial myocytes, whereas the activation and recovery curves were unaltered. The Nav1.5 expression level was significantly reduced in αMyHC-FKBP12 atria. Furthermore, we found increases in atrial Cav1.2 protein levels and peak L-type Ca2+ current density and increased levels of fibrosis in αMyHC-FKBP12 atria. In summary, cardiomyocyte-restricted overexpression of FKBP12 reduces the atrial Nav1.5 expression level and mean peak INa, which is associated with increased peak L-type Ca2+ current and interstitial fibrosis in atria. The combined electrophysiological and structural changes facilitated the development of local conduction block and altered action potential duration and spontaneous AF. NEW & NOTEWORTHY This study addresses a long-standing riddle regarding the role of FK506-binding protein 12 in cardiac physiology. The work provides further evidence that FK506-binding protein 12 is a critical component for regulating voltage-gated sodium current and in so doing has an important role in arrhythmogenic physiology, such as atrial fibrillation.

Mechanisms linking electrical alternans and clinical ventricular arrhythmia in human heart failure

Mechanisms of ventricular tachycardia (VT) and ventricular fibrillation (VF) in patients with heart failure (HF) are undefined. The purpose of this study was to elucidate VT/VF mechanisms in HF by using a computational-clinical approach. In 53 patients with HF and 18 control patients, we established the relationship between low-amplitude action potential voltage alternans (APV-ALT) during ventricular pacing at near-resting heart rates and VT/VF on long-term follow-up. Mechanisms underlying the transition of APV-ALT to VT/VF, which cannot be ascertained in patients, were dissected with multiscale human ventricular models based on human electrophysiological and magnetic resonance imaging data (control and HF). For patients with APV-ALT k-score >1.7, complex action potential duration (APD) oscillations (≥2.3% of mean APD), rather than APD alternans, most accurately predicted VT/VF during long-term follow-up (+82%; -90% predictive values). In the failing human ventricular models, abnormal sarcoplasmic reticulum (SR) calcium handling caused APV-ALT (>1 mV) during pacing with a cycle length of 550 ms, which transitioned into large magnitude (>100 ms) discordant repolarization time alternans (RT-ALT) at faster rates. This initiated VT/VF (cycle length <400 ms) by steepening apicobasal repolarization (189 ms/mm) until unidirectional conduction block and reentry. Complex APD oscillations resulted from nonstationary discordant RT-ALT. Restoring SR calcium to control levels was antiarrhythmic by terminating electrical alternans. APV-ALT and complex APD oscillations at near-resting heart rates in patients with HF are linked to arrhythmogenic discordant RT-ALT. This may enable novel physiologically tailored, bioengineered indices to improve VT/VF risk stratification, where SR calcium handling and spatial apicobasal repolarization are potential therapeutic targets.

Interactive effect of beta-adrenergic stimulation and mechanical stretch on low-frequency oscillations of ventricular action potential duration in humans

Ventricular repolarization dynamics are crucial to arrhythmogenesis. Low-frequency oscillations of repolarization have recently been reported in humans and the magnitude of these oscillations proposed to be a strong predictor of sudden cardiac death. Available evidence suggests a role of the sympathetic nervous system. We have used biophysically detailed models integrating ventricular electrophysiology, calcium dynamics, mechanics and β-adrenergic signaling to investigate the underlying mechanisms. The main results were: (1) Phasic beta-adrenergic stimulation (β-AS) at a Mayer wave frequency between 0.03 and 0.15Hz resulted in a gradual decrease of action potential (AP) duration (APD) with concomitant small APD oscillations. (2) After 3–4minutes of phasic β-AS, the mean APD adapted and oscillations of APD became apparent. (3) Phasic changes in haemodynamic loading at the same Mayer wave frequency (a known accompaniment of enhanced sympathetic nerve activity), simulated as variations in the sarcomere length, also induced APD oscillations. (4) The effect of phasic β-AS and haemodynamic loading on the magnitude of APD oscillations was synergistic. (5) The presence of calcium overload and reduced repolarization reserve further enhanced the magnitude of APD oscillations and was accompanied by afterdepolarizations and/or spontaneous APs. In conclusion, low-frequency oscillations of repolarization recently reported in humans were induced by phasic β-AS and phasic mechanical loading, which acted synergistically, and were greatly enhanced by disease-associated conditions, leading to arrhythmogenic events.

CT-1-CP-induced ventricular electrical remodeling in mice.

The chronic effects of carboxyl-terminal polypeptide of Cardiotrophin-1 (CT-1-CP) on ventricular electrical remodeling were investigated. CT-1-CP, which contains 16 amino acids in sequence of the C-terminal of Cardiotrophin-1, was selected and synthesized, and then administered to Kunming mice (aged 5 weeks) by intraperitoneal injection (500 ng·g⁻¹·day⁻¹) (4 groups, n=10 and female: male=1:1 in each group) for 1, 2, 3 and 4 weeks, respectively. The control group (n=10, female: male=1:1) was injected by physiological saline for 4 weeks. The epicardial monophasic action potential (MAP) was recorded by using a contact-type MAP electrode placed vertically on the left ventricular (LV) epicardium surface, and the electrocardiogram (ECG) signal in lead II was monitored synchronously. ECG intervals (RR, PR, QRS and QT) and the amplitude of MAP (Am), the maximum upstroke velocity (Vmax), as well as action potential durations (APDs) at different repolarization levels (APD30, APD50, APD70, and APD90) of MAP were determined and analyzed in detail. There were no significant differences in RR and P intervals between CT-1-CP-treated groups and control group, but the PR segment and the QRS complex were greater in the former than in the latter (F=2.681 and 5.462 respectively, P<0.05). Though QT interval and the corrected QT interval (QTc) were shorter in CT-1-CP-treated groups than in control group, the QT dispersion (QTd) of them was greater in the latter than in the former (F=3.090, P<0.05) and increased with the time. The ECG monitoring synchronously with the MAP showed that the compression of MAP electrode on the left ventricular epicardium induced performance similar to myocardium ischemia. As compared with those before chest-opening, the PR segment and QT intervals remained basically unchanged in control group, but prolonged significantly in all CT-1-CP-treated groups and the prolongation of QT intervals increased gradually along with the time of exposure to CT-1-CP. The QRS complex had no significant change in control group, one-week and three-week CT-1-CP-treated groups, but prolonged significantly in two-week and four-week CT-1-CP-treated groups. Interestingly, the QTd after chest-opening was significantly greater than that before chest-opening in control group (t=5.242, P<0.01), but decreased along with the time in CT-1-CP-treated groups. The mean MAP amplitude, Vmax and APD were greater in CT-1-CP-treated groups than those in control group, and became more obvious along with the time. The APD in four CT-1-CP-treat groups was prolonged mainly in middle to final repolarization phase. The difference among these groups became significant in middle phase (APD50) (F=6.076, P<0.01) and increased furthermore in late and final phases (APD70: F=10.054; APD90: F=18.691, P<0.01) along with the time of injection of CT-1-CP. The chronic action of CT-1-CP might induce the adapting alteration in cardiac conductivity and ventricular repolarization. The amplitude and the Vmax of the anterior LV epicardial MAP increased obviously, and the APD prolonged mainly in late and final phase of repolarization.

The role of dye affinity in optical measurements of Cai(2+) transients in cardiac muscle.

Previous experiments in cultures of neonatal rat myocytes demonstrated that the shape of Cai(2+) transients measured using high-affinity Ca(2+)-sensitive dyes may be misrepresented. The purpose of this study was to examine the role of dye affinity in Cai(2+) measurements in intact adult cardiac tissue by comparing optical recordings obtained with high- and low-affinity dyes. Experiments were carried out in porcine left ventricular (LV) wedge preparations stained locally by intramural injection via microcapillaries (diameter = 150 μm) with a low-affinity Ca(2+)-sensitive dye Fluo-4FF or Fluo-2LA (nominal Kd, ~7-10 μmol/l), high-affinity dye Rhod-2 (Kd = 0.57 μmol/l), and Fluo-4 or Fluo-2MA (Kd, ~0.4 μmol/l); in addition, tissue was stained with transmembrane potential (Vm)-sensitive dye RH-237. Optical recordings of Vm and Cai(2+) were made using optical fibers (diameter = 325 μm) glued with the microcapillaries. The durations of Cai(2+) transients measured at 50% level of recovery (CaD50) using high-affinity Fluo-4/Fluo-2MA dyes were up to ~81% longer than those measured with low-affinity Fluo-4FF/Fluo-2LA at long pacing cycle lengths (CL). In Fluo-4/Fluo-2MA measurements at long CLs, Cai(2+) transients often (~50% of cases) exhibited slow upstroke rise and extended plateau. In Rhod-2 measurements, CaD50 was moderately longer (up to ~35%) than in Fluo-4FF recordings, but Cai(2+) transient shapes were similar. In all series of measurements, mean action potential duration values were not significantly different (P > 0.05). The delays between Vm and Cai(2+) upstrokes were comparable for low- and high-affinity dyes (P > 0.05). In conclusion, measurements of Cai(2+) transient in ventricular myocardium are strongly affected by the affinity of Ca(2+) dyes. The high-affinity dyes may overestimate the duration and alter the shape of Cai(2+) transients.

Phlorizin prevents electrically-induced ventricular tachyarrhythmia during ischemia in langendorff-perfused guinea-pig hearts.

Phlorizin is a type of flavonoids and has a peroxynitrite scavenging effect. This study aimed to elucidate the effects of phlorizin on ischemia-induced ventricular tachyarrhythmia (VT). Optical signals from the epicardial surface of the ventricle or left ventricular end diastolic pressure (LVEDP) were recorded during acute global ischemia in 42 Langendorff-perfused guinea pig hearts. Experiments were performed in the control condition and in the presence of phlorizin or N-2-mercaptopropionylglycine (2-MPG), a peroxynitrite scavenger, respectively. Mean action potential duration at 20 min of ischemia did not differ among the three interventions. Impulse conduction time-dependently slowed during 20 min of ischemia in the control. Phlorizin but not 2-MPG improved the ischemic conduction slowing at 15 and 20 min of ischemia. Programmed stimulation induced VT at 20 min of ischemia in the control and in the presence of 2-MPG but not in the presence of phlorizin (p<0.05). LVEDP was increased during 30 min of ischemia in the control and in the presence of 2-MPG but not in the presence of phlorizin. These results indicate that phlorizin prevents VT through the improvement of impulse conduction slowing during ischemia. Phlorizin may be more useful for ischemia-induced VT than 2-MPG.

Engraftment of human embryonic stem cell derived cardiomyocytes improves conduction in an arrhythmogenic in vitro model

In this study, we characterized the electrophysiological benefits of engrafting human embryonic stem cell-derived cardiomyocytes (hESC-CMs) in a model of arrhythmogenic cardiac tissue. Using transforming growth factor-β treated monolayers of neonatal rat ventricular cells (NRVCs), which retain several key aspects of the healing infarct such as an excess of contractile myofibroblasts and slowed, heterogeneous conduction, we assessed the ability of hESC-CMs to improve conduction and prevent arrhythmias. Cells from beating embryoid bodies (hESC-CMs) can form functional monolayers which beat spontaneously and can be electrically stimulated, with mean action potential duration of 275 ± 36 ms and conduction velocity (CV) of 10.6 ± 4.2 cm/s (n = 3). These cells, or cells from non-beating embryoid bodies (hEBCs) were added to anisotropic, NRVC monolayers. Immunostaining demonstrated hESC-CM survival and engraftment, and dye transfer assays confirmed functional coupling between hESC-CMs and NRVCs. Conduction velocities significantly increased in anisotropic NRVC monolayers after engraftment of hESC-CMs (13.4 ± 0.9 cm/s, n = 35 vs. 30.1 ± 3.2 cm/s, n = 20 in the longitudinal direction and 4.3 ± 0.3 cm/s vs. 9.3 ± 0.9 cm/s in the transverse direction), but decreased to even lower values after engraftment of non-cardiac hEBCs (to 10.6 ± 1.3 cm/s and 3.1 ± 0.5 cm/s, n = 11, respectively). Furthermore, reentrant wave vulnerability in NRVC monolayers decreased by 20% after engraftment of hESC-CMs, but did not change with engraftment of hEBCs. Finally, the culture of hESC-CMs in transwell inserts, which prevents juxtacrine interactions, or engraftment with connexin43-silenced hESC-CMs provided no functional improvement to NRVC monolayers. These results demonstrate that hESC-CMs can reverse the slowing of conduction velocity, reduce the incidence of reentry, and augment impaired electrical propagation via gap junction coupling to host cardiomyocytes in this arrhythmogenic in vitro model.

Nicorandil normalizes prolonged repolarisation in the first transgenic rabbit model with Long-QT syndrome 1 both in vitro and in vivo

Transgenic rabbits expressing loss-of-function pore mutants of the human gene KCNQ1 (K vLQT1-Y315S) have a Long QT-Syndrome 1 (LQT1) phenotype. We evaluated for the first time the effect of nicorandil, an opener of ATP-sensitive potassium channels, and of isoproterenol on cardiac action potential duration and heart rate dependent dispersion of repolarisation in transgenic LQT1 rabbits. In vivo LQT1 and littermate control were subjected to transvenous electrophysiological studies; in vitro monophasic action potentials were recorded from explanted Langendorff-perfused hearts. In vivo ventricular effective refractory periods (VERP) at the right ventricular base were significantly prolonged in LQT1 as compared to littermate control, resulting in a more pronounced VERP dispersion in LQT1. This difference in VERP dispersion between LQT1 and littermate control disappeared after infusion of nicorandil. In vitro, mean action potential durations (APD 75 and APD 90) of LQT1 were significantly prolonged compared to littermate control at baseline. Nicorandil decreased APD 75 and APD 90 in LQT1 and littermate control at all stimulated heart rates. After adding nicorandil, the APD 90 at all hearts rates and the APD 75 at high heart rates were no longer different. Dispersion of repolarisation (∆APD 75 and ∆APD 90) was heart rate dependently decreased after nicorandil at all tested stimulation cycle lengths only in LQT1. We demonstrated phenotypic differences of LQT1 and littermate control in vivo and in vitro. Nicorandil 20 μmol/l improved repolarisation abnormalities and heterogeneities in transgenic LQT1 rabbits.

Alternans in Genetically Modified Langendorff-Perfused Murine Hearts Modeling Catecholaminergic Polymorphic Ventricular Tachycardia

The relationship between alternans and arrhythmogenicity was studied in genetically modified murine hearts modeling catecholaminergic polymorphic ventricular tachycardia (CPVT) during Langendorff perfusion, before and after treatment with catecholamines and a β-adrenergic antagonist. Heterozygous (RyR2p/s) and homozygous (RyR2s/s) RyR2-P2328S hearts, and wild-type (WT) controls, were studied before and after treatment with epinephrine (100 nM and 1 μM) and propranolol (100 nM). Monophasic action potential recordings demonstrated significantly greater incidences of arrhythmia in RyR2p/s and RyR2s/s hearts as compared to WTs. Arrhythmogenicity in RyR2s/s hearts was associated with alternans, particularly at short baseline cycle lengths. Both phenomena were significantly accentuated by treatment with epinephrine and significantly diminished by treatment with propranolol, in full agreement with clinical expectations. These changes took place, however, despite an absence of changes in mean action potential durations, ventricular effective refractory periods or restitution curve characteristics. Furthermore pooled data from all hearts in which arrhythmia occurred demonstrated significantly greater alternans magnitudes, but similar restitution curve slopes, to hearts that did not demonstrate arrhythmia. These findings thus further validate the RyR2-P2328S murine heart as a model for human CPVT, confirming an alternans phenotype in common with murine genetic models of the Brugada syndrome and the congenital long-QT syndrome type 3. In contrast to these latter similarities, however, this report demonstrates the dissociation of alternans from changes in the properties of restitution curves for the first time in a murine model of a human arrhythmic syndrome.

Action Potential Duration Modulation by Activation Sequence in Rat Vs. Pig Myocardium

Dispersion of the cardiac action potential duration (APD) is known to influence the susceptibility of cardiac tissue to arrhythmias. Several experimental studies have revealed that APD can be modulated by the activation sequence. Our lab has recently shown strong correlations between APD and activation time (AT) in hearts from small rodents. However, a recent computational study indicated that the magnitude of such APD modulation may not be consistent across species. Therefore, the present study sought to compare experimentally APD modulation by activation sequence in rat and pig. Optical imaging using the voltage-sensitive dye Di-4-ANEPPS was performed in Langendorff perfused rat hearts (n=4) and coronary perfused pig left ventricular slabs (n=5). The left ventricular mid-free wall was paced at 6Hz (rat) and 2Hz (pig), close to their intrinsic heart rate, and optical action potentials were acquired for 5s. The mean APD near the pacing site (at 4ms AT) was 44.9 ± 9.4 ms in rat and 142.2 ± 11.3 ms in pig. A significant decrease of APD was revealed at larger AT in rat (37.2 ± 9.5 ms at 10ms AT, P0.05). Plotting APD as a function of AT revealed a linear correlation of APD with AT. Slope analysis revealed a decreasing trend in rats (mean slope = −0.79 ± 0.26) whilst pigs showed no such modulation of APD (mean slope = 0.29 ± 0.22) Heterogeneity, defined as the APD covariance over the whole field of view, was 0.10 in rat and 0.05 in pig (P<0.05). In conclusion, APD can be strongly modulated by the activation sequence in hearts from small rodents whilst this modulation is absent in pig myocardium. This study emphasises the importance of APD heterogeneity induced by activation sequence and differences between species.

Increased cycle length during long-duration ventricular fibrillation is caused by decreased upstroke velocity as well as prolonged refractoriness

Cycle length (CL) increases as ventricular fibrillation (VF) progresses. The purpose of this study was to test the hypotheses that increased CL is due to increased diastolic interval (DI), not increased action potential duration (APD), and that the DI increase is not solely due to increased postrepolarization refractoriness. In 10 swine, VF was recorded for 20 minutes using a floating microelectrode through a hole in a 504-electrode epicardial plaque. Mean APD, DI, action potential amplitude (APA), maximum change in voltage during the AP upstroke (V(max)), and CL were calculated from the floating microelectrode recordings each minute of VF. The refractory period was estimated from the minimum DI (DI(min)). In two animals, rapid pacing was performed to gauge refractoriness. As VF progressed, CL, DI, and DI(min) increased (P <.05), whereas APD, V(max), and APA decreased (P <.05). At 20 minutes, DI(min) was not different from mean DI at VF onset. Pacing captured, but 53% of paced wavefronts blocked within the plaque. Increasing CL in VF is due to increased DI and not APD, which shortens. The increase in DI(min) over time is much less than the increase in mean DI, indicating that the myocardium is excitable during much of the DI. This finding, along with the ability to pace at a CL shorter than the native VF CL and the poor paced wavefront propagation, suggests that the increase in DI is due not only to increased postrepolarization refractoriness but also to poor wavefront propagation because of decreased APA and V(max) secondary to global ischemia caused by VF.

Mechanisms of Preventive Effect of Nicorandil on Ischaemia‐Induced Ventricular Tachyarrhythmia in Isolated Arterially Perfused Canine Left Ventricular Wedges

Whether nicorandil is effective at preventing ventricular tachyarrhythmia (VT) during acute myocardial ischaemia is still controversial. We examined effects of nicorandil on the induction of VT during acute myocardial ischaemia. Optical action potentials were recorded from the entire transmural wall of arterially perfused canine left ventricular wedges. Ischaemia was produced by arterial occlusion for 20 min. During endocardial pacing, nicorandil shortened mean action potential duration (APD) in the transmural wall before ischaemia and further shortened it during ischaemia without increasing dispersion of APD. HMR1098, a selective blocker of sarcolemmal ATP-sensitive K(+) channels, inhibited the shortening of APD by nicorandil before and during ischaemia. Ischaemia decreased transmural conduction velocity (CV). Nicorandil partially restored CV to a similar extent in the absence and presence of HMR1098. In contrast, HMR1098 did not suppress the ischaemic conduction slowing in the absence of nicorandil. Nicorandil suppressed the increased dispersion of local CV during ischaemia. Isochrone maps on the initiation of VT showed that reentry in the transmural surface resulted from the excitation of the epicardial region of transmural surface. Nicorandil significantly increased the size of non-excited area in the epicardial region of the transmural wall, thereby significantly reducing the incidence of VT induced during ischaemia. HMR1098 inhibited this effect of nicorandil. These results suggest that nicorandil prevents VT during acute global ischaemia primarily by augmenting the inactivation of epicardial muscle through the activation of sarcolemmal K(ATP) channels.

Temporal variability of repolarization in rat ventricular myocytes paced with time‐varying frequencies

Adaptation of action potential duration (APD) to pacing cycle length (CL) has been previously characterized in isolated cardiomyocytes for sudden changes in constant CL and for pre-/postmature stimuli following constant pacing trains. However, random fluctuations characterize both physiological sinus rhythm (up to 10% of mean CL) and intrinsic beat-to-beat APD at constant pacing rate. We analysed the beat-to-beat sensitivity of each APD to the preceding CL during constant-sudden, random or linearly changing pacing trains in single patch clamped rat left ventricular myocytes, in the absence of the autonomic and electrotonic effects that modulate rate dependency in the intact heart. Beat-to-beat variability of APD at -60 mV (APD(-60 mV)), quantified as S.D. over 10-beat sequences, increased with corresponding mean APD. When measured as coefficient of variability (CV), APD(-60 mV) variability was inversely proportional to pacing frequency (from 1.2% at 5 Hz to 3.2% at 0.2 Hz). It was increased, at a basic CL (BCL) of 250 ms, by 55% by the L-type calcium current (I(CaL)) blocker nifedipine, and decreased by 23% by the transient-outward potassium current (I(to)) blocker 4-aminopyridine. Variability of APD at BCL of 250 ms prevented the detection of random changes of CL smaller than approximately 5%. Ten per cent random changes in CL were detected as a 40% increase in CV of APD and tended to correlate with it (r = 0.43). Block of I(CaL) depressed this correlation (r = 0.23), whereas block of I(to) significantly increased it (r = 0.67); this was similar with linearly changing CL ramps (ranging +/-10% and +/-20% of 250 ms). We conclude that beat-to-beat APD variability, a major determinant of the propensity for development of arrhythmia in the heart, is present in isolated myocytes, where it is dependent on mean APD and pacing rate. Action potential duration shows a beat-to-beat positive correlation with preceding randomly/linearly changing CL, which can be pharmacologically modulated.

Mechanisms Underlying the Effects of the Pyrethroid Tefluthrin on Action Potential Duration in Isolated Rat Ventricular Myocytes

Due to increased global use, acute exposures to pyrethroid insecticides in humans are of clinical concern. Pyrethroids have a primary mode of action that involves interference with the inactivation of Na+ currents (I(Na)) in excitable cells, which may include cardiac myocytes. To investigate the possible cardiac toxicity of these agents, we have examined the effects of a type-1 pyrethroid, tefluthrin, on isolated rat ventricular myocytes. Under whole-cell current-clamp, tefluthrin prolonged the mean action potential duration at 90% repolarization (APD90) by 216 +/- 34% in 19 myocytes isolated from 14 hearts. About one-third of this prolongation was apparently due to persistent I(Na), with the balance associated with spontaneous cytosolic Ca2+ waves, and Na+-Ca2+ exchange. In some action potentials, tefluthrin also activated early after-depolarizations (EADs). Using a selected EAD-containing action potential clamp, we observed that EADs could evoke a Cd2+-sensitive membrane current (I(EAD)) that triggered secondary sarcoplasmic reticulum (SR) Ca2+ release. The notion that EADs could stimulate Ca2+ current was strengthened by the persistence of I(EAD) in myocytes exposed to extracellular Li+ and Sr2+ ions, used to minimize Na+-Ca2+ exchange and SR Ca2+ release, respectively. Tefluthrin inhibited I(EAD) by approximately 10%. Together, our results support an arrhythmogenic model whereby tefluthrin exposure stimulated Na+ influx, provoking cellular Ca2+ overload by reverse Na+-Ca2+ exchange. During Ca2+ waves, forward Na+-Ca2+ exchange prolonged the action potential markedly and kindled EADs by permitting the reactivation of Ca2+ current. Similar mechanisms may be involved in pyrethroid toxicity in vivo, and also in type 3 long QT syndrome, wherein Na+ channel mutations prolong I(Na).

Genesis of the P wave: Atrial signals as generated by the equivalent double layer source model

To assess the effectiveness of the equivalent surface source model in the simulation of atrial signals as observed in ECG leads. P waves were extracted from 64-lead ECGs recorded in healthy subjects. The geometries of torso, lungs, heart, and blood cavities of a healthy subject, derived from magnetic resonance imaging, were used to position a detailed, thick-walled 3D model of the atria consisting of a set of 800,000 units representing the activity of all atrial myocytes. The ion-kinetics of the units was based on the formulation of Courtemanche et al. The simulated transmembrane potentials following a normal sinus beat, as well as those during atrial fibrillation, were projected on the 1297 nodes of the surface encapsulating all atrial myocytes (endocardium and epicardium). The transmembrane potentials at these nodes formed the source strengths of the elements of the equivalent generator, which were used to compute body surface potentials. After invoking slight adaptations of the timing of depolarization of the transmembrane potentials, the simulated signals during the P wave closely corresponded to recorded ones. The correspondence during the entire PR interval improved markedly after the inclusion of early repolarization effects in the interval between the end of the P wave and onset of QRS. This demanded a shortening of the mean action potential duration generated by the Courtemanche model. The simulated ECGs related to atrial fibrillation demonstrated the characteristic features of those clinically observed. The equivalent double layer is a useful source model for the genesis of atrial signals observed on the thorax. The interval from the end of the P wave to onset of QRS is not iso-electric. The Courtemanche model of the ion-kinetics of atrial cells needs to be adapted when applied to represent the activity of healthy, 'common' atrial myocytes.

Recording Atrial Monophasic Action Potentials Using Standard Pacemaker Leads:

AF leads to electrophysiological changes, but it is not known if similar alterations also appear before the onset of the first episode of AF because invasive electrophysiological studies are not justified in otherwise symptom-free patients. To address this question requires a safe method of obtaining atrial electrophysiological parameters at no extra risk or discomfort for the patient. The aim of this study was to test if recording of monophasic action potentials (MAPs) is feasible during pacemaker implantation. The study included 22 patients undergoing pacemaker implantation for symptomatic bradycardia without any history of AF. Using a custommade amplifier and a minor modification of the routine procedure for intraoperatively measured P waves, atrial electrograms could be recorded using a standard active pacemaker lead. MAP-like electrograms were obtained in 15 patients. MAP amplitude was 2.6 +/- 0.3 mV, mean action potential duration was 316 +/- 12 ms at a spontaneous heart rate of 67.2 +/- 3.2 beats/min. MAP duration was decreased when atria were stimulated at shorter cycle lengths (249 +/- 12 ms at 150 beats/min, P <0.05 vs sinus rhythm). In about two thirds of patients undergoing pacemaker implantation, recording of MAP-like electrograms was feasible with only minor modification of the atrial electrogram recording technique. The method should allow screening patients for electrophysiological alterations even before the onset of AF.