Comparison of the effects of regional ischemia and hyperkalemia on the membrane action potentials of the in situ pig heart. Experimental Cardiology Group, University of North Carolina at Chapel Hill

The IK1/Kir2.1 channel agonist zacopride prevents and cures acute ischemic arrhythmias in the rat

Arrhythmogenesis in acute myocardial infarction (MI) is associated with depolarization of resting membraine potential (RMP) and decrease of inward rectifier potassium current (I_K1) in cardiomyocytes. However, clinical anti-arrhythmic agents that primarily act on RMP by enhancing the I_K1 channel are not currently available. We hypothesized that zacopride, a selective and moderate agonist of the I_K1/Kir2.1 channels, prevents and cures acute ischemic arrhythmias. To test this viewpoint, adult Sprague-Dawley (SD) rats were subjected to MI by ligating the left main coronary artery. The antiarrhythmic effects of zacopride (i.v. infusion) were observed in the settings of pre-treatment (zacopride given 3 min prior to coronary occlusion), post-treatment (zacopride given 3 min after coronary occlusion) and therapeutic treatment (zacopride given 30 s after the onset of the first sustained ventricular tachycardia (VT)/ventricular fibrillation (VF) post MI). In all the three treatment modes, zacopride (15 μg/kg) inhibited MI-induced ventricular tachyarrhythmias, as shown by significant decreases in the premature ventricular contraction (PVC) and the duration and incidence of VT or VF. In Langendorff perfused rat hearts, the antiarrhythmic effect of 1 μmol/L zacopride were reversed by 1 μmol/L BaCl₂, a blocker of I_K1 channel. Patch clamp results in freshly isolated rat ventricular myocytes indicated that zacopride activated the I_K1 channel and thereby reversed hypoxia-induced RMP depolarization and action potential duration (APD) prolongation. In addition, zacopride (1 μmol/L) suppressed hypoxia- or isoproterenol- induced delayed afterdepolarizations (DADs). In Kir2.x transfected Chinese hamster ovary (CHO) cells, zacopride activated the Kir2.1 homomeric channel but not the Kir2.2 or Kir2.3 channels. These results support our hypothesis that moderately enhancing I_K1/Kir2.1 currents as by zacopride rescues ischemia- and hypoxia- induced RMP depolarization, and thereby prevents and cures acute ischemic arrhythmias. This study brings a new viewpoint to antiarrhythmic theories and provides a promising target for the treatment of acute ischemic arrhythmias.

Electrophysiological properties of computational human ventricular cell action potential models under acute ischemic conditions

Acute myocardial ischemia is one of the main causes of sudden cardiac death. The mechanisms have been investigated primarily in experimental and computational studies using different animal species, but human studies remain scarce. In this study, we assess the ability of four human ventricular action potential models (ten Tusscher and Panfilov, 2006; Grandi et al., 2010; Carro et al., 2011; O'Hara et al., 2011) to simulate key electrophysiological consequences of acute myocardial ischemia in single cell and tissue simulations. We specifically focus on evaluating the effect of extracellular potassium concentration and activation of the ATP-sensitive inward-rectifying potassium current on action potential duration, post-repolarization refractoriness, and conduction velocity, as the most critical factors in determining reentry vulnerability during ischemia. Our results show that the Grandi and O'Hara models required modifications to reproduce expected ischemic changes, specifically modifying the intracellular potassium concentration in the Grandi model and the sodium current in the O'Hara model. With these modifications, the four human ventricular cell AP models analyzed in this study reproduce the electrophysiological alterations in repolarization, refractoriness, and conduction velocity caused by acute myocardial ischemia. However, quantitative differences are observed between the models and overall, the ten Tusscher and modified O'Hara models show closest agreement to experimental data.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

IKs protects from ventricular arrhythmia during cardiac ischemia and reperfusion in rabbits by preserving the repolarization reserve

Introduction

The function of the repolarization reserve in the prevention of ventricular arrhythmias during cardiac ischemia/reperfusion and the impact of ischemia on slowly activated delayed rectifier potassium current (I_Ks) channel subunit expression are not well understood.

Methods and Results

The responses of monophasic action potential duration (MAPD) prolongation and triangulation were investigated following an L-768,673-induced blockade of I_Ks with or without ischemia/reperfusion in a rabbit model of left circumflex coronary artery occlusion/reperfusion. Ischemia/reperfusion and I_Ks blockade were found to significantly induce MAPD90 prolongation and increase triangulation at the epicardial zone at 45 min, 60 min, and 75 min after reperfusion, accompanied with an increase in premature ventricular beats (PVBs) during the same period. Additionally, I_Ks channel subunit expression was examined following transient ischemia or permanent infarction and changes in monophasic action potential (MAP) waveforms challenged by β-adrenergic stimulation were evaluated using a rabbit model of transient or chronic cardiac ischemia. The epicardial MAP in the peri-infarct zone of hearts subjected to infarction for 2 days exhibited increased triangulation under adrenergic stimulation. KCNQ1 protein, the α subunit of the I_Ks channel, was downregulated in the same group. Both findings were consistent with an increased incidence of PVBs.

Conclusion

Blockade of I_Ks caused MAP triangulation, which precipitated ventricular arrhythmias. Chronic ischemia increased the incidence of ventricular arrhythmias under adrenergic stimulation and was associated with increased MAP triangulation of the peri-infarct zone. Downregulation of KCNQ1 protein may be the underlying cause of these changes.

Developmental basis for electrophysiological heterogeneity in the ventricular and outflow tract myocardium as a substrate for life-threatening ventricular arrhythmias

Reentry is the main mechanism of life-threatening ventricular arrhythmias, including ventricular fibrillation and tachycardia. Its occurrence depends on the simultaneous presence of an arrhythmogenic substrate (a preexisting condition) and a "trigger," and is favored by electrophysiological heterogeneities. In the adult heart, electrophysiological heterogeneities of the ventricle exist along the apicobasal, left-right, and transmural axes. Also, conduction is preferentially slowed in the right ventricular outflow tract, especially during pharmacological sodium channel blockade. We propose that the origin of electrophysiological heterogeneities of the adult heart lies in early heart development. The heart is formed from several progenitor regions: the first heart field predominantly forms the left ventricle, whereas the second heart field forms the right ventricle and outflow tract. Furthermore, the embryonic outflow tract consists of slowly conducting tissue until it is incorporated into the ventricles and develops rapidly conducting properties. The subepicardial myocytes and subendocardial myocytes run distinctive gene programs from their formation onwards. This review discusses the hypothesis that electrophysiological heterogeneities in the adult heart result from persisting patterns in gene expression and function along the craniocaudal and epicardial-endocardial axes of the developing heart. Understanding the developmental origins of electrophysiological heterogeneity contributing to ventricular arrhythmias may give rise to new therapies.

Prolongation of the QTc interval is seen uniformly during early transmural ischemia

OBJECTIVES

In order to more clearly understand the electrocardiographic manifestations of early transmural ischemia, we studied electrocardiograms (ECGs) in patients undergoing balloon angioplasty.

BACKGROUND

Decisions regarding reperfusion strategies in patients with acute myocardial infarction rely largely on the presence of ST-segment elevation (STE) in the ECG, consequently with significant limitations. Studies of the "ischemic cascade" show that ST-segment changes occur well after the onset of wall motion abnormalities.

METHODS

We prospectively analyzed ECGs obtained at 20-s intervals in 74 patients undergoing elective balloon angioplasty. The ECGs were analyzed using 3 methodologies. In 74 patients, the ST-segment, the T-wave, and the QT-interval were analyzed using the MUSE (General Electric HC, Menomonee Falls, Wisconsin) automated system (MUSE). Fifty patients were also analyzed using the Interval Editor automated system (IE; General Electric HC). In 20 patients, measurements were made manually.

RESULTS

Transmural ischemia prolonged the QTc interval (using the Bazett's formula) in 100% of patients. In all 74 patients analyzed with MUSE, QTc interval prolonged from 423 +/- 25 ms to 455 +/- 34 ms (p < 0.001). In the 50 patients analyzed with IE, QTc interval prolonged in 50 of 50 (100%) patients (from 424 +/- 27 ms to 458 +/- 33 ms [p < 0.001]). Mean time to maximal QTc interval prolongation, changes in T-wave polarity, > or =1 mm STE, and ST-segment depression (STD) were 22, 24, 29, and 35 s, respectively. Although QTc interval prolonged in 100% of patients, T-wave changes, STE, and STD (> or =1 mm) occurred in 7%, 15%, and 7%, respectively.

CONCLUSIONS

The QTc interval prolongs in 100% of patients with early transmural ischemia. When compared with clinically accepted indexes of transmural ischemia (i.e., STD and STE [> or =1 mm]) it is the earliest ECG abnormality.

Class III effects of dofetilide and arrhythmias are modulated by [K+]o in an in vitro model of simulated-ischemia and reperfusion in guinea-pig ventricular myocardium

To evaluate class III effects of clinically relevant concentrations of dofetilide (5 and 10 nmol/l) and the effects of extracellular potassium [K+]o modulation of arrhythmias onset at the level of the "border zone," we used a previously reported in vitro model whereby normoxic and ischemic/reperfused zones were studied. Guinea-pig right ventricular strips (driven at 1 Hz at 36.5+/-0.5 degrees C) were superfused with Tyrode's solution in oxygenated (HCO3- 25 mmol/l, K+ 4 mmol/l, pH 7.35+/-0.05, glucose 5.5 mmol/l: normal zone) and ischemia-simulating conditions (HCO3- 9 mmol/l, pH 6.90+/-0.05, no oxygen and no glucose: altered zone) having either [K+]o 4 (n=20), 8 (n=20) or 12 (n=20) mmol/l. Action potentials in normal and altered zones were recorded simultaneously during 30 min of simulated-ischemia and after 30 min of reperfusion with oxygenated Tyrode's solution. Each preparation served as control for successive phases of dofetilide studies (at 5 and 10 nmol/l) and action potential values were normalized to those present at the beginning of the experiment. During simulated-ischemia, the higher the [K+]o the worse were action potential changes, although full recovery was seen upon 30 min of reperfusion in all [K+]o groups. A high incidence of ischemia/reperfusion arrhythmias was observed in 4 and 12 mmol/l [K+]o groups as opposed to a low incidence of arrhythmias in 8 mmol/l [K+]o group. Dofetilide at 5 and 10 nmol/l with all [K+]o explored: (i) exhibited class III effects, (ii) was effective (or neutral) against ventricular arrhythmias during both simulated-ischemia and reperfusion, and (iii) did not globally increase the dispersion of action potential durations between normal and altered zones. Different arrhythmogenic mechanisms are involved in this model at different [K+]o with 8 mmol/l providing relative protection. Class III effects of dofetilide are evident in the normal zone when in the ischemic-like zone [K+]o ranges from 4 to 12 mmol/l. Thus dofetilide did not increase dispersion of repolarization and had either an antiarrhythmic or a neutral effect during ischemia/reperfusion.

Dynamical and cellular electrophysiological mechanisms of ECG changes during ischaemia

The interpretation of normal and pathological electrocardiographic (ECG) patterns in terms of the underlying cellular and tissue electrophysiology is rudimentary, as the existing theories rely on geometrical aspects. We relate effects of sub-endocardial ischaemia on the ST-segment depression in ECG to patterns of transmural action potential propagation in a one-dimensional virtual ventricular wall. Our computational study exposes two electrophysiological mechanisms of ST depression: dynamic-predominantly positive spatial gradients in the membrane potential during abnormal repolarization of the wall, produced by action potential duration changes in the ischaemic region; and static-a negative spatial gradient of the resting membrane potential between the normal and ischaemic regions. Hyperkalaemia is the major contributor to both these mechanisms at the cellular level. These results complement simulations of the effects of cardiac geometry on the ECG, and dissect spatio-temporal and cellular electrophysiological mechanisms of ST depression seen in sub-endocardial ischaemia.

Coronary occlusion and reperfusion promote early afterdepolarizations and ventricular tachycardia in a canine tissue model of type 3 long QT syndrome

Although long QT syndrome (LQTS) and coronary occlusion-reperfusion (O/R) are arrhythmogenic, they affect ventricular action potential duration (APD) differently. In contrast to the prolonged APD in LQTS, ischemia abbreviates APD after a transient prolongation. Thus we hypothesized that the dynamic interactive effects of ischemia and LQTS on APD and its dispersion would affect ventricular arrhythmogenicity. We mapped transmural distribution of action potentials in 6 groups of 10 isolated wedges of canine ventricular walls: LQTS-O/R, LQTS only, and O/R only, with separate groups for pacing cycle lengths (PCL) of 1,000 and 2,000 ms. We created type 3 LQTS with anemone toxin (ATX) II followed >30 min later by arterial occlusion (40 min) and reperfusion (>100 min). Arterial occlusion initially (first 4 min) prolonged and then shortened APD. Early afterdepolarizations (EADs) occurred during the initial 4 min of occlusion in 4 of the 10 LQTS-O/R wedges at PCL of 2,000 ms but not in the other groups. Reperfusion restored APD in the O/R-only groups but caused APD to overshoot its original duration, indicating depressed repolarization reserve, in the LQTS-O/R group. Reperfusion increased the dispersion of APDs and initiated ventricular tachycardia-fibrillation in 7 of 10 and 6 of 10 LQTS-O/R wedges and in 2 of 10 and 1 of 10 O/R-only wedges at PCLs of 1,000 and 2,000 ms, respectively. The LQTS-only wedges exhibited neither EADs nor ventricular tachycardia. We conclude that coronary O/R increased the arrhythmogenicity of LQTS via cumulative prolongation of APD, increase in repolarization dispersion, and suppression of repolarization reserve.

Effects of barium and 5-hydroxydecanoate on the electrophysiologic response to acute regional ischemia and reperfusion in rat hearts

The aim of this work was to investigate the role of the inward rectifying (K1) and the sarcolemmal ATP-sensitive K+ (K-ATP) channels in the electrical response to regional ischemia and the subsequent development of ventricular tachyarrhythmias on reflow (RA). Surface electrograms (ECG) and the transmembrane potential from subepicardial left ventricular cells were recorded in spontaneously beating rat hearts perfused with buffer alone (controls) or exposed to 100 microM BaCl2 or 100 microM 5-hydroxydecanoate (5-HD) to block either K1 or K-ATP channels respectively. After 20 min of equilibration and 10 min of control recordings, the left anterior descending coronary artery was occluded for 10 min. This was followed by reperfusion. The effects of regional ischemia as well as those of reperfusion (10 min) were recorded throughout. In the three groups, ischemia induced a modest decrease in heart rate and a sharp reduction in resting potential within 3 min. The latter as well as the accompanying depression of propagated electrical activity were enhanced by Ba2+. A partial recovery of the resting potential was observed in all groups during the last 2 min of coronary occlusion. Concomitantly, a slight reduction in the action potential duration was found in the control hearts. This effect was blocked by 5-HD. Under Barium the action potential duration increased by a factor of 3 and its ischemic variations were minimized. Severe sustained ventricular tachyarrhythmias developed on reflow in the controls and in the 5-HD exposed hearts. Barium limited the duration of arrhythmic episodes to a few seconds. Our data indicate that the initial electrical effects of ischemia are unrelated to activation of ATP sensitive K+ channels and that gK1 dominates the K+ membrane conductance at this stage. Furthermore, they show that action potential lengthening limits the duration of arrhythmic episodes triggered by reperfusion. This suggests that electrical heterogeneity plays an important role in the perpetuation of reperfusion arrhythmias.

Regulation of inward rectifier K+ channels by shift of intracellular pH dependence

The mechanistic link between mitochondrial metabolism and inward rectifier K+ channel activity was investigated by studying the effects of a mitochondrial inhibitor, carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) on inward rectifiers of the Kir2 subfamily expressed in Xenopus oocytes, using two-electrode voltage-clamp, patch-clamp, and intracellular pH recording. FCCP inhibited Kir2.2 and Kir2.3 currents and decreased intracellular pH, but the pH change was too small to account for the inhibitory effect by itself. However, pre-incubation of oocytes with imidazole prevented both the pH decrease and the inhibition of Kir2.2 and Kir2.3 currents by FCCP. The pH dependence of Kir2.2 was shifted to higher pH in membrane patches from FCCP-treated oocytes compared to control oocytes. Therefore, the inhibition of Kir2.2 by FCCP may involve a combination of intracellular acidification and a shift in the intracellular pH dependence of these channels. To investigate the sensitivity of heteromeric channels to FCCP, we studied its effect on currents expressed by heteromeric tandem dimer constructs. While Kir2.1 homomeric channels were insensitive to FCCP, both Kir2.1-Kir2.2 and Kir2.1-Kir2.3 heterotetrameric channels were inhibited. These data support the notion that mitochondrial dysfunction causes inhibition of heteromeric inward rectifier K+ channels. The reduction of inward rectifier K+ channel activity observed in heart failure and ischemia may result from the mitochondrial dysfunction that occurs in these conditions.

Specific electromechanical responses of cardiomyocytes to individual and combined components of ischemia

The main factors of myocardial ischemia are hypoxia, substrate deprivation, acidosis, and high extracellular potassium concentration ([K+]e), but the influence of each of these factors has not yet been evaluated in a cardiomyocyte (CM) culture system. Electromechanical responses to the individual and combined components of ischemia were studied in CM cultured from newborn rat ventricles. Action potentials (APs) were recorded using glass microelectrodes and contractions were monitored photometrically. Glucose-free hypoxia initially reduced AP duration, amplitude, and rate and altered excitation-contraction coupling, but AP upstroke velocity (Vmax) remained unaffected. Early afterdepolarizations appeared, leading to bursts of high-rate triggered impulses before the complete arrest of electromechanical activity after 120 min. Acidosis reduced Vmax whereas AP amplitude and rate were moderately decreased. Combining acidosis and substrate-free hypoxia also decreased Vmax but attenuated the effects of substrate-free hypoxia on APs and delayed the cessation of the electrical activity (180 min). Raising [K+]e reduced the maximal diastolic potential and Vmax. Total ischemia (substrate deletion, hypoxia, acidosis, and high [K+]e) decreased AP amplitude and Vmax without changing AP duration. Moreover, delayed afterdepolarizations appeared, initiating triggered activity. Ultimately, 120 min of total ischemia blocked APs and contractions. To conclude, glucose-free hypoxia caused severe functional defects, acidosis delayed the changes induced by substrate-free hypoxia, and total ischemia induced specific dysfunctions differing from those caused by the former conditions. Heart-cell cultures thus represent a valuable tool to scrutinize the individual and combined components of ischemia on CMs.

Mechanisms of shock-induced arrhythmogenesis during acute global ischemia

Little is known about the mechanisms of vulnerability and defibrillation under ischemic conditions. We investigated these mechanisms in 18 Langendorff-perfused rabbit hearts during 75% reduced-flow ischemia. Electrical activity was optically mapped from the anterior epicardium during right ventricular shocks applied at various phases of the cardiac cycle while the excitation-contraction decoupler 2,3-butanedione monoxime (BDM; 15 mM) was used to suppress motion artifacts caused by contraction of the heart. During ischemia, vulnerable window width increased [from 30-90% of the action potential duration (APD) in the control to -10 to 100% of the APD in ischemia]. Moreover, arrhythmia severity increased along with the reduction of APD (176 +/- 9 ms in control and 129 +/- 26 ms in ischemia, P < 0.01) and increased dispersion of repolarization (45 +/- 17 ms in control and 73 +/- 28 ms in ischemia, P < 0.01). Shock-induced virtual electrode polarization was preserved. Depolarizing (contrary to hyperpolarizing) response time constants increased. Virtual electrode-induced wavefronts of excitation had much more tortuous pathways leading to wavefront fractionation. Defibrillation failure at all shock strengths was observed in four hearts. Optical mapping revealed that the shock extinguished the arrhythmia; however, the arrhythmia self-originated after an isoelectric window of 339 +/- 189 ms. In conclusion, in most cases, virtual electrode-induced phase singularity (VEIPS) was responsible for shock-induced arrhythmogenesis during acute global ischemia. Enhancement of arrhythmogenesis was associated with an increased dispersion of repolarization and altered deexcitation. In four hearts, arrhythmogenesis could not be explained by VEIPS.

Effects of acute global low-flow ischemia on triggered arrhythmias in d-sotalol-induced long Q-T intervals in perfused rabbit hearts

Little information is available on how acute ischemia modifies the electrophysiologic substrate associated with long Q-T interval conditions. We studied the effects of low-flow ischemia (10 min at 5.0 ml/min followed by 10 min of 2.5 ml/min) in Langendorff perfused rabbit hearts during control and in hearts 20 min after the addition to the perfusate of 92 microM d-sotalol, which reliably produced triggered activity. Epicardial electrograms, a left ventricular endocardial monophasic action potential (MAP), and simulated X and Y lead electrocardiograms were used to characterize myocardial activation and recovery during ventricular pacing. In the control hearts, conduction velocity as indicated by the mean epicardial activation time accelerated for most of the period of ischemia (maximum decrease of -9.4 +/- 7.9%). The mean activation-recovery interval, MAP duration, and Q-T interval were moderately decreased (-4.9 +/- 8.6%, -7.5 +/- 4.4%, and -4.6 +/- 2.3%, respectively). The mean standard deviation of the activation-recovery interval (epicardial heterogeneity of recovery) was increased by 34.6 +/- 23.4%. d-Sotalol had no effect on conduction but prolonged myocardial recovery time, increased heterogeneity, and produced triggered arrhythmias in all hearts. Within 2 min of ischemia triggered activity was eliminated. With d-sotalol, ischemia slowed conduction and produced relatively larger decreases in the activation-recovery interval, MAP duration, and Q-T interval (-11.8 +/- 10.3%, -13.9 +/- 12.0%, and -15.8 +/- 11.2%). The increased epicardial heterogeneity seen with d-sotalol was attenuated by ischemia. Thus ischemia superimposed on long Q-T conditions had antiarrhythmic as well as arrhythmogenic effects.

Changes in ventricular repolarization during percutaneous transluminal coronary angioplasty in humans assessed by QT interval, QT dispersion and T vector loop morphology

OBJECTIVES

Based on clinical, epidemiological, and experimental data, transient cardiac ischaemia is one of the major triggering factors of malignant ventricular arrhythmia. According to animal studies, increased dispersion of repolarization is of pathophysiological relevance in this context. Therefore we explored the impact of myocardial ischaemia during single vessel coronary angioplasty on the change in ventricular repolarization, measured by QT and JT intervals and their dispersion in the 12-lead electrocardiogram. We also assessed a novel method, the 3-dimensional T vector loop, to find out whether it was sensitive to changes in ventricular repolarization during ischaemia, and whether there was any correlation with changes in the dispersion of the QT and/or JT intervals.

DESIGN

This study was prospective with consecutive patients. Only patients in sinus rhythm and without bundle branch block were included.

SETTING

All coronary angioplasties were performed at Norrlands University Hospital, Umeå. The analysis of the material was performed at the Karolinska Hospital, Stockholm.

SUBJECTS

Twenty-nine consecutive patients went through 30 elective one-vessel percutaneous transluminal coronary angioplasty (PTCA) procedures. PTCA was performed in 10 stenoses of the left anterior descending, 10 of the left circumflex, and 10 of the right coronary artery.

INTERVENTIONS

A 12-lead electrocardiogram was recorded continuously as part of routine monitoring of the patient during PTCA and the T vector loop was calculated from the simultaneously recorded. X, Y, Z leads.

MAIN OUTCOME MEASURES

Repolarization was assessed by the QRS, QT and JT intervals as well as by the T vector loop parameters (Tarea, Tavplan, and Teigenv) before and at the end of the first occlusion during PTCA.

RESULTS

PTCA, with an average occlusion time of 171 +/- 60 s (mean +/- SD), induced ischaemia on the 12-lead electrocardiogram in 73% of cases. The overall response for the 30 procedures was a significantly increased dispersion of ventricular repolarization, both corrected and uncorrected for heart rate. QT dispersion increased by, on average, 19% from 74 +/- 35 to 88 +/- 36 ms, QTc dispersion by 27% from 71 +/- 39 to 90 +/- 42 ms, and JTc dispersion by 19% from 78 +/- 32 to 94 +/- 43 ms (P < 0.05). The T vector loop became more circular and bulgy during occlusion (all three parameters changed by between 33% and 59%). There was a significant correlation between changes in one of the T vector loop parameters (Teigenv), and changes in JT and QT dispersion in the left anterior descending group.

CONCLUSIONS

Transient ischaemia during PTCA induced significant changes in ventricular repolarization, especially during occlusion of the left anterior descending artery and resulted in a significant increase in both QT and QTc dispersion. The degree of QT dispersion was such that several patients were at risk of ventricular arrhythmia, if a proper triggering extrasystole had occurred. In addition, and as an original observation, the 3-dimensional T vector loop morphology seemed even more sensitive to coronary occlusion than QT dispersion.

Regional hyperkalemia increases ventricular defibrillation energy requirements: role of electrical heterogeneity in defibrillation

INTRODUCTION

Increased spatial electrical heterogeneity has been associated with impaired defibrillation efficacy. The current study investigated the relationship between electrical heterogeneity and defibrillation efficacy by manipulating spatial electrical heterogeneity.

METHODS AND RESULTS

We increased spatial electrical heterogeneity by infusing potassium chloride (2 to 4 mEq/hour) or placebo in the left anterior descending artery in 13 pentobarbital anesthetized swine. Electrophysiologic measurements at five myocardial sites and defibrillation energy requirement (DER) values were determined at baseline and during regional hyperkalemia (n = 7) or placebo (n = 6). Regional potassium infusion was titrated to a 20% reduction in action potential duration in the perfused region. Regional hyperkalemia increased biphasic DER values by 87% (P = 0.02), whereas infusion of placebo did not alter defibrillation efficacy. Regional hyperkalemia decreased myocardial repolarization and refractoriness in the perfused region by 21% (P < 0.001) and 18% (P = 0.01), respectively. However, regional hyperkalemia increased ventricular fibrillation cycle length (VFCL) by 39% (P = 0.008). Consequently, dispersions of repolarization, refractoriness, and VFCL were significantly increased by 169%, 92%, and 200%, respectively. Regional hyperkalemia also increased ventricular conduction time to the perfused region by 54% (P = 0.006), indicating conduction velocity dispersion, while not affecting local pacing threshold or local voltage gradient.

CONCLUSION

Regional hyperkalemia increased DER values. Regional hyperkalemia likely impairs defibrillation by increasing myocardial electrical heterogeneity, which supports the theory that electrical heterogeneity promotes nonuniform propagation of early postshock activations, thereby inhibiting defibrillation.

High uniformity of left and right ventricular repolarization dynamics induced by an abrupt decrease in pacing cycle length in a dog is not affected by left ventricular ischemia

INTRODUCTION

After an abrupt increase in heart rate, action potential duration (APD) will shorten. To assess the effect of ischemia on APD shortening dynamics, we compared right ventricular (RV) and left ventricular (LV) APD shortening induced by an abrupt decrease in pacing cycle length (PCL) during control and LV ischemia.

METHODS AND RESULTS

In eight anesthetized AV block dogs, endocardial LV and RV APD were determined simultaneously after an abrupt PCL decrease from 800 to 350 msec. Measurements were repeated during left anterior descending coronary artery (LAD) occlusion. During control, LV and RV APD shortened 97 +/- 27 and 71 +/- 14 msec, respectively (P < 0.05). Shortening was pronounced in a short initial phase and gradual in the longer secondary phase. Linear regression analysis revealed very high uniformity of LV and RV APD shortening dynamics (r2 = 0.96 +/- 0.01). During repeated LAD occlusion, ischemia induced a gradual LV APD shortening from 314 +/- 25 msec to a new steady-state value of 251 +/- 23 msec, whereas RV APD remained stable at 289 +/- 28 msec. The additional PCL decrease resulted in LV and RV APD shortening of 72 +/- 8 and 68 +/- 15 msec, respectively, with the same high uniformity of shortening dynamics as seen during control (r2 = 0.94 +/- 0.03).

CONCLUSION

There is a pronounced difference in APD shortening dynamics induced by an abrupt decrease in PCL compared with ischemia. LV shortening dynamics induced by a decrease in PCL are not affected by LV ischemia, preserving a high interventricular uniformity of repolarization dynamics.

[Sudden death (II). Myocardial ischemia and ventricular arrhythmias in experimental models: triggering mechanisms]

Metabolic and electrolytic alterations generated in the acute ischemic myocardium, such as an increase in extracellular potassium or acidosis, are responsible for the occurrence of ventricular arrhythmias. In the first 5-10 minutes following coronary occlusion, reentry seems to have an important role, although not in the next 15 minutes. If the patient survives, a subacute arrhythmia period appears, 6 to 72 hours after the onset of ischemia, probably due to abnormal automaticity in the surviving Purkinje fibers. Finally, reentry in the epicardial border zone is the most likely mechanism for chronic arrhythmias. In this review we focus on the studies dealing with the mechanisms of ischemia-induced arrhythmias, with special reference to those conducted in experimental models.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.