Cardiac expression of skeletal muscle sodium channels increases longitudinal conduction velocity in the canine 1-week myocardial infarction.

BACKGROUND: Skeletal muscle sodium channel (Nav1.4) expression in border zone myocardium increases action potential upstroke velocity in depolarized isolated tissue. Because resting membrane potential in the 1-week canine infarct is reduced, we hypothesized that conduction velocity (CV) is greater in Nav1.4 dogs compared with in control dogs. OBJECTIVE: The purpose of this study was to measure CV in the infarct border zone border in dogs with and without Nav1.4 expression. METHODS: Adenovirus was injected in the infarct border zone in 34 dogs. The adenovirus incorporated the Nav1.4- and a green fluorescent protein (GFP) gene (Nav1.4 group, n = 16) or only GFP (n = 18). After 1 week, upstroke velocity and CV were measured by sequential microelectrode recordings at 4 and 7 mM [K(+)] in superfused epicardial slabs. High-density in vivo epicardial activation mapping was performed in a subgroup (8 Nav1.4, 6 GFP) at three to four locations in the border zone. Microscopy and antibody staining confirmed GFP or Nav1.4 expression. RESULTS: Infarct sizes were similar between groups (30.6% +/- 3% of left ventricle mass, mean +/- standard error of the mean). Longitudinal CV was greater in Nav1.4 than in GFP sites (58.5 +/- 1.8 vs. 53.3 +/- 1.2 cm/s, 20 and 15 sites, respectively; P <.05). Transverse CV was not different between the groups. In tissue slabs, dV/dt(max) was higher and CV was greater in Nav1.4 than in control at 7 mM [K(+)] (P <.05). Immunohistochemical Nav1.4 staining was seen at the longitudinal ends of the myocytes. CONCLUSION: Nav1.4 channels in myocardium surviving 1 week infarction increases longitudinal but not transverse CV, consistent with the increased dV/dt(max) and with the cellular localization of Nav1.4.

Anti- or profibrillatory effects of Na(+) channel blockade depend on the site of application relative to gradients in repolarization.

Sodium channel blockers are associated with arrhythmic sudden death, although they are considered antiarrhythmic agents. The mechanism of these opposing effects is unknown. We used a model of induction of ventricular fibrillation (VF) based on selective perfusion of the vascular beds of isolated porcine hearts (n = 8). One bed was perfused with sotalol (220 µM), the adjacent bed with pinacidil (80 µM), leading to repolarization heterogeneity (late repolarization in the sotalol-, early in the pinacidil-area). Premature stimulation from the area with the short action potential was performed. Epicardial activation/repolarization mapping was done. In three of the eight hearts VF was inducible prior to infusion of flecainide. In those hearts the Fibrillation Factor (FF), the interval between the earliest repolarization of the premature beat (S2) in the early repolarizing (pinacidil) domain, and the last S2-activation in the late repolarizing (sotalol) domain, was significantly shorter than in the hearts without VF (33 ± 22 vs 93 ± 11 ms, m ± SEM, p < 0.05). In the three hearts with VF flecainide was infused in the pinacidil domain after defibrillation. This led to shortening of the line of block, local delay of S2 activation and repolarization, an increase in FF and failure to induce VF. In the five hearts without VF, flecainide was subsequently infused in the sotalol domain. This led to a local delay of S2 activation, a shortening of FF (by 47 ± 3 ms) and successful induction of VF in three hearts. In the two remaining hearts FF did not decrease enough (maximally 13 ms) to allow re-entry. Sodium channel blockade applied to myocardium with a short refractory period is antifibrillatory whereas sodium channel blockade of myocardium with a long refractory period is profibrillatory. Our study provides a mechanistic basis for pro- and antiarrhythmic effects of sodium channel blockers in the absence of structural heart disease.

Dispersion of repolarization and arrhythmogenesis.

BACKGROUND: The relation between induction of arrhythmias and dispersion of repolarization is not completely understood. OBJECTIVE: The purpose of this study was to study the relation between heterogeneity in repolarization and arrhythmogenesis under conditions of selective regional action potential prolongation and shortening. METHODS: Pig hearts were perfused in a Langendorff setup. The left anterior descending artery (LAD) was cannulated and perfused. Sotalol (220 microM) was infused in the aortic cannula, and pinacidil (20 microM) was infused through the LAD, causing a gradient in repolarization time between the two myocardial regions. Premature stimulation was performed from the LAD region. RESULTS: No transmural repolarization gradients developed after infusion of the drugs. High-density epicardial activation/repolarization mapping (176 unipolar electrodes, 2-mm interelectrode spacing) revealed a maximum repolarization gradient of approximately 120 ms over 14 mm. The critical parameter for differentiating between the occurrence of reentry and the mere occurrence of a line of activation block between the two myocardial regions (and no reentry) was not the magnitude of the repolarization gradient but the timing of arrival of the premature activation wave at the distal side of the line of activation block relative to the repolarization time of the premature beat proximal to the line of block. No spontaneous arrhythmias were observed despite the presence of the repolarization gradient. CONCLUSION: It is not the repolarization gradient but the restitution characteristics of the tissue with the shorter action potential, in combination with the time of arrival of the premature wavefront at the distal side of the line of block, that determines the occurrence of reentry.

The effect of enhanced gap junctional conductance on ventricular conduction in explanted hearts from patients with heart failure.

AIM: To investigate ventricular conduction and refractoriness before and after application of rotigaptide, an enhancer of gap junctional conductance, to explanted hearts of patients with heart failure (HF). METHODS AND RESULTS: In six explanted perfused hearts of patients with end-stage HF, activation/repolarization mapping was performed and refractory periods (RPs) and activation recovery intervals (ARIs) were measured before and after application of 50 nM rotigaptide. Rotigaptide caused a decrease of RP from 476 +/- 36 to 453 +/- 31 ms (P < 0.05), but did not change ARI-dispersion. During premature activation along the fibers rotigaptide decreased the minimal activation time (AT(min)) and maximal activation time (AT(max)) significantly from 35 +/- 12 to 24 +/- 9 and from 97 +/- 38 to 43 +/- 7 ms, respectively. Rotigaptide did not change AT(min) and AT(max) during activation perpendicular to the fiber direction. After application of rotigaptide conduction curves normalized in five/six recordings when activation was parallel, but destabilized in three/six hearts when activation was perpendicular to fiber direction. The destabilization was associated with local conduction delays rather than with facilitation of conduction. CONCLUSION: Rotigaptide applied to hearts of patients with end-stage HF shortened RPs normalized conduction curves and increased conduction parallel to fiber direction. However, in 50% of the hearts local slowing of conduction with destabilization of conduction (curves) occurs at sites close to the stimulation site, when activation is perpendicular to fiber direction.

Long-term cardiac memory in canine heart is associated with the evolution of a transmural repolarization gradient.

OBJECTIVE: The contribution of regional electrophysiologic heterogeneity to the T-wave changes of long-term cardiac memory (CM) is not known. We mapped activation and repolarization in dogs after induction of CM and in sham animals. METHODS AND RESULTS: CM was induced by three weeks of AV-sequential pacing at the anterior free wall of the left ventricle (LV), midway between apex and base in 5 dogs. In 4 sham controls a pacemaker was implanted but ventricular pacing was not performed. At 3 weeks, unipolar electrograms were recorded (98 epicardial, 120 intramural and endocardial electrodes) during atrial stimulation (cycle length 450 ms). Activation times (AT) and repolarization times (RT) were measured and activation recovery intervals (ARIs) calculated. CM was associated with 1) deeper T waves on ECG, with no change in QT interval; 2) longer activation time at the site of stimulation in CM (29.7+/-1.0, X+/-SEM) than sham (23.9+/-1.3 ms p<0.01); 3) an LV transmural gradient in repolarization time such that repolarization at the epicardium terminated 12.4+/-2.4 ms later than at the endocardium p<0.01), in contrast to no gradient in shams (2.7+/-4.2 ms); in memory dogs, the repolarization time gradient was greatest at sites around the pacing electrode varying from 13.1+/-2.3 ms to 25.5+/-3.8 ms; 4) more negative left ventricular potentials at the peak of the body surface T wave (-4.9+/-0.8 vs -2.2+/-0.4 mV; p<0.05) but no altered right ventricular epicardial T-wave potentials. ARIs did not differ between groups. Right ventricular activation was delayed but was not associated with altered repolarization because of compensatory shortening of the right ventricular ARIs. CONCLUSION: CM-induced T-wave changes are caused by evolution of transmural repolarization gradients manifested during atrial stimulation that are maximal near the site of ventricular pacing.

Dispersion of repolarization in canine ventricle and the electrocardiographic T wave: Tp-e interval does not reflect transmural dispersion.

BACKGROUND: The concept that the interval between the peak (T(peak)) and the end (T(end)) of the T wave (T(p-e)) is a measure of transmural dispersion of repolarization time is widely accepted but has not been tested rigorously by transmural mapping of the intact heart. OBJECTIVES: The purpose of this study was to test the relationship of T(p-e) to transmural dispersion of repolarization by correlating local repolarization times at endocardial, midmural, and epicardial sites in the left and right ventricles with the T wave of the ECG. METHODS: Local activation times, activation-recovery intervals, and repolarization times were measured at 98 epicardial sites and up to 120 midmural and endocardial sites in eight open-chest dogs. In four of the dogs, long-term cardiac memory was induced by 3 weeks of ventricular pacing at 130 bpm because previous data suggest that, in this setting, delayed epicardial repolarization increases transmural dispersion. The other four dogs were sham operated. RESULTS: In sham dogs, T(p-e) was 41 +/- 2.2 ms (X +/- SEM), whereas the transmural dispersion of repolarization time was 2.7 +/- 4.2 ms (not significant between endocardium and epicardium). Cardiac memory was associated with evolution of a transmural gradient of 14.5 +/- 1.9 ms (P <.02), with epicardium repolarizing later than endocardium. The corresponding T(p-e) was 43 +/- 2.3 ms (not different from sham). In combined sham and memory dogs, T(p-e) intervals did not correlate with transmural dispersion of repolarization times. In contrast, dispersion of repolarization of the whole heart (measured as the difference between the earliest and the latest moment of repolarization from all left and right ventricular, endocardial, intramural, and epicardial recording sites) did correlate with T(p-e) (P <.0005, r = 0.98), although the latter underestimated total repolarization time by approximately 35%. The explanation for this finding is that parts of the heart fully repolarize before the moment of T(peak). CONCLUSION: T(p-e) does not correlate with transmural dispersion of repolarization but is an index of total dispersion of repolarization.

Dietary n-3 fatty acids promote arrhythmias during acute regional myocardial ischemia in isolated pig hearts.

OBJECTIVE: Dietary supplementation with fish oil-derived n-3 fatty acids reduces mortality in patients with myocardial infarction, but may have adverse effects in angina patients. The underlying electrophysiologic mechanisms are poorly understood. We studied the arrhythmias and the electrophysiologic changes during regional ischemia in hearts from pigs fed a diet rich in fish oil. METHODS: Pigs received diets rich in fish oil, in sunflower oil, or a control diet for 8 weeks. Hearts were isolated and perfused. Ischemia was created by occluding the left anterior descending artery. Diastolic stimulation threshold, refractory period, conduction velocity, activation recovery intervals and the maximum downstroke velocity of 176 electrograms were measured in the ischemic zone. Spontaneous arrhythmias during 75 min of regional ischemia were counted. RESULTS: More episodes of spontaneous ischemia-induced sustained ventricular tachycardia and ventricular fibrillation occurred in the fish oil and sunflower oil group than in the control group. More inexcitable myocardium was present in the ischemic zone in the group fed fish oil or sunflower oil than in the control group after 20 min of ischemia. After 40 min of ischemia, more block occurred in the control group than in the other groups. The downstroke velocity of the electrograms in the ischemic border zone was lower in the fish oil group and sunflower oil group than in the control after 20 min. CONCLUSIONS: A diet rich in fish oil results in proarrhythmia compared to a control diet during regional ischemia in pigs. Myocardial excitability is reduced in the fish oil and sunflower oil group during the early phase of arrhythmogenesis. In the late phase of arrhythmogenesis, excitability is more reduced in the control group than in the fish oil and sunflower oil group.

Monophasic action potentials and activation recovery intervals as measures of ventricular action potential duration: experimental evidence to resolve some controversies.

BACKGROUND: Activation recovery intervals (ARIs) and monophasic action potential (MAP) duration are used as measures of action potential duration in beating hearts. However, controversies exist concerning the correct way to record MAPs or calculate ARIs. We have addressed these issues experimentally. OBJECTIVES: To experimentally address the controversies concerning the correct way to record MAPs or calculate ARIs. METHODS: Left ventricular local electrograms were recorded in isolated pig hearts with an exploring electrode grid, with a KCl reference electrode on the left ventricular myocardium, the aortic root, or the left atrium. Local activation was determined from calculated Laplacian electrograms. RESULTS: With the KCl electrode on the aortic root, local electrograms represented local activation. However, with the KCl electrode on the myocardium remote from the exploring electrode, a combined electrogram emerged consisting of local activation recorded from the grid and remote activation recorded from the reference electrode. The remote, inverted monophasic component did not show propagation and did not correlate with the Laplacian complex. When the KCl electrode was placed on the atrium during AV block, remote atrial monophasic components were completely dissociated from local, ventricular deflections. At left ventricular sites with a positive T wave, the Laplacian signal showed that the end of the T wave was caused by remote repolarization. During cooling-induced regional action potential prolongation, the T wave became negative, whereby the positive flank of the T wave remained correlated with repolarization (recorded with a MAP at the same site). CONCLUSIONS: MAPs are recorded from the depolarizing electrode. In both negative and positive T waves, the moment of maximum dV/dt corresponds to local repolarization.

Right ventricular fibrosis and conduction delay in a patient with clinical signs of Brugada syndrome: a combined electrophysiological, genetic, histopathologic, and computational study.

BACKGROUND: The mechanism of ECG changes and arrhythmogenesis in Brugada syndrome (BS) patients is unknown. METHODS AND RESULTS: A BS patient without clinically detected cardiac structural abnormalities underwent cardiac transplantation for intolerable numbers of implantable cardioverter/defibrillator discharges. The patient's explanted heart was studied electrophysiologically and histopathologically. Whole-cell currents were measured in HEK293 cells expressing wild-type or mutated sodium channels from the patient. The right ventricular outflow tract (RVOT) endocardium showed activation slowing and was the origin of ventricular fibrillation without a transmural repolarization gradient. Conduction restitution was abnormal in the RVOT but normal in the left ventricle. Right ventricular hypertrophy and fibrosis with epicardial fatty infiltration were present. HEK293 cells expressing a G1935S mutation in the gene encoding the cardiac sodium channel exhibited enhanced slow inactivation compared with wild-type channels. Computer simulations demonstrated that conduction slowing in the RVOT might have been the cause of the ECG changes. CONCLUSIONS: In this patient with BS, conduction slowing based on interstitial fibrosis, but not transmural repolarization differences, caused the ECG signs and was the origin of ventricular fibrillation.

Conduction slowing by the gap junctional uncoupler carbenoxolone.

BACKGROUND: Cellular electrical coupling is essential for normal propagation of the cardiac action potential, whereas reduced electrical coupling is associated with arrhythmias. Known cellular uncoupling agents have severe side effects on membrane ionic currents. We investigated the effect of carbenoxolone on cellular electrical coupling, membrane ionic currents, and atrial and ventricular conduction. METHODS AND RESULTS: In isolated rabbit left ventricular and right atrial myocytes, carbenoxolone (50 micromol/l) had no effect on action potential characteristics. Calcium, potassium, and sodium currents remained unchanged. Dual current clamp experiments on poorly coupled cell pairs revealed a 21+/-3% decrease in coupling conductance by carbenoxolone (mean+/-S.E.M., n=4, p<0.05). High-density activation mapping was performed in intact rabbit atrium and ventricle during Langendorff perfusion of the heart. The amplitude of the Laplacian of the electrograms, a measure of coupling current in intact hearts, decreased from 1.45+/-0.66 to 0.75+/-0.51 microA/mm(3) (mean+/-SD, n=32, p<0.05) after 15 min of carbenoxolone. Carbenoxolone reversibly decreased longitudinal and transversal conduction velocity from 66+/-15 to 49+/-16 cm/s and from 50+/-14 to 35+/-15 cm/s in ventricle, respectively (mean+/-SD, n=5, both p<0.05). In atrium, longitudinal and transversal conduction velocity decreased from 80+/-29 to 60+/-16 cm/s and from 49+/-10 to 38+/-10 cm/s (mean+/-SD, n=8, both p<0.05). CONCLUSIONS: Carbenoxolone-induced uncoupling causes atrial and ventricular conduction slowing without affecting cardiac membrane currents. Activation delay is larger in poorly coupled cells.

Mechanical effects on arrhythmogenesis: from pipette to patient.

Mechanical stimuli delivered to the precordium can, if strong enough and timed at the beginning of the T-wave, induce ventricular premature beats or runs of ventricular tachycardia and even fibrillation. On the other hand, there are reports that a properly timed "chest thump" can terminate ventricular tachycardia, or can act as pacemaker stimuli during an episode of asystole. It is likely that in these cases mechanical energy is translated to an electrical stimulus. There are more subtle ways in which mechanical stimuli, mediated by stretch, can exert electrophysiological effects, and the most common name to describe these effects is mechanoelectrical feedback. Most studies have concentrated on acute stretch or dilatation, while the effects of chronic stretch, which may clinically be more important, are difficult to evaluate since they are accompanied by other factors, such as hypertrophy, heart failure, fibrosis, neurohumeral disturbances, and electrolyte abnormalities, all of which have arrhythmogenic effects. There are a number of ion channels that are activated following stretch. Stretch during diastole usually leads to a depolarization, resembling a delayed afterdepolarization, which may reach threshold and initiate a ventricular premature beat. Stretch during systole usually shortens the action potential, but action potential prolongation, resulting in early afterdepolarizations has been described as well. The arrhythmias during acute myocardial ischaemia occur in two phases: the 1A phase between 2 and 10 min following coronary artery occlusion, and the 1B phase between 18 and 30 min. Experiments will be described, indicating that the ventricular premature beats of the 1B phase, which may induce ventricular fibrillation, are caused by stretch of the border between ischaemic and normal myocardium. Briefly, 1B arrhythmias are much less frequent in the isolated perfused heart than in the heart in situ, but in working, ejecting isolated hearts, the number of 1B arrhythmias is similar to those in the in situ heart. The ventricular premature beats have a focal origin at the border, and they occur more often after a pause-induced potentiated contraction.

Origin of ischemia-induced phase 1b ventricular arrhythmias in pig hearts.

OBJECTIVES: The goal of this study was to establish the role of ventricular filling on the 1b phase of ischemia-induced arrhythmias. BACKGROUND: Ischemia-induced ventricular arrhythmias occur in two phases. The mechanism of the initiation of delayed (1b) arrhythmias is unknown. The 1b arrhythmias (15 to 60 min of ischemia) are abundant in in situ hearts but scarce in isolated perfused hearts (with drained ventricles). METHODS: Left ventricular (LV) epicardial mapping (11 x 11 matrix, 5 mm interelectrode distance) of the initiation of delayed arrhythmias was performed in open-chested pigs (group A, n = 7) and isolated pig hearts without (group B, n = 8) and with a filled intraventricular balloon (group C, n = 5). RESULTS: There were no differences in ischemic zone size between groups. The ischemia-induced rise in tissue impedance was similar in groups A and B. Arrhythmias were less frequent and less severe in group B than in groups A or C, with no differences between groups A and C. An epicardial focal origin was detected in 26% of all first beats, significantly more from the ischemic border than from elsewhere. During a pacing protocol with a long pause (a separate group of four isolated hearts with a balloon), more premature beats occurred in the first postpause interval than in any other interval. CONCLUSIONS: In isolated hearts 1b arrhythmias were less frequent and less severe than in working preparations. Focal activity was documented in 26% of arrhythmias and emerged from the ischemic border. Postpause contractile potentiation was associated with more arrhythmias. Our study suggests that the initiation of ischemia-induced 1b arrhythmias is related to LV wall stress.