Cellular and pathophysiological mechanisms of ventricular arrhythmias in acute ischemia and infarction

Sudden Cardiac Death: A Review Focused on Cardiovascular Imaging

Sudden cardiac death (SCD) is defined as natural death due to cardiac causes, heralded by abrupt loss of consciousness within 1 h of the onset of acute symptoms; pre-existing heart disease may have been known to be present but the time and mode of death are unexpected. Prediction and prevention of SCD is an area of active investigation, but considerable challenges persist that limit the efficacy and cost-effectiveness of available methodologies. It was well-recognized that optimization of SCD risk stratification would require integration of multi-disciplinary efforts at the bench and bedside, with studies in the general population. This integration has yet to be effectively accomplished. There is also increasing awareness that more investigation needs to be directed toward the identification of early predictors of SCD. Significant advancements have recently occurred for risk prediction in the inherited channelopathies and other inherited conditions that predispose to SCD, but there is much to be accomplished in this regard for the more common complex phenotypes, such as SCD among patients with coronary artery disease. A multimodality imaging approach is actually the most important tool to provide comprehensive information on different pathophysiological mechanisms related to SCD.

Left ventricle radio-frequency ablation in the rat: a new model of heart failure due to myocardial infarction homogeneous in size and low in mortality

BACKGROUND

The purpose of the current study was to create a model of myocardial infarction (MI) that is homogeneous in size with a low immediate (24 hours) mortality.

METHODS AND RESULTS

Male and female rats (n = 256) underwent left ventricle (LV) ablation (Ab) by a radiofrequency current (1000 kHz; 12 watts for 12 seconds) to promote a MI. A transmural MI occurred in all rats. Post-Ab complex arrhythmias were frequent (atrioventricular block, ventricular tachycardia, and fibrillation), which rapidly and spontaneously reverted to sinus rhythm. Among 66 male rats, immediate mortality occurred in 7.5%. Small MI size dispersion was characterized by smaller variability following Ab (x +/- SD: 45 +/- 8%) when compared with coronary occlusion (Oc; 40 +/- 19%). The histopathologic evaluations identified lesions similar to those which occurred following Oc, with scarring complete at 4 weeks. The hemodynamic and Doppler echocardiograms showed comparable increases in LV dimension, end-diastolic pressure, and pulmonary water content 1 and 4 weeks post-MI. Papillary muscle mechanics 6 weeks post-MI had matched inotropic and lusitropic dysfunction.

CONCLUSIONS

LV Ab gave rise to a MI within a narrow size limit and with a low immediate mortality. LV Ab resulted in histopathologic evolution, ventricular dilation, and dysfunction, impairment in myocardial mechanics, and congestive outcome that reproduced a MI from Oc.

Intracoronary infusion of catecholamines causes focal arrhythmias in pigs

BACKGROUND

Acute ischemia causes myriad changes including increased catecholamines. We tested the hypothesis that elevated catecholamines alone are arrhythmogenic.

METHODS AND RESULTS

A 504 electrode sock was placed over both ventricles in six open-chest pigs. During control infusion of saline through a catheter in the left anterior descending coronary artery (LAD), no sustained arrhythmias occurred, and the refractory period estimated by the activation recovery interval (ARI) was 175 +/- 14 ms in the LAD bed below the catheter. After infusion of isoproterenol at 0.1 microg/kg/min through the catheter, the ARI in this bed was significantly reduced to 109 +/- 10 ms. A sharp gradient of refractoriness of 43 +/- 10 ms was at the border of the perfused bed. Sustained monomorphic ventricular tachycardia occurred after drug infusion in the perfused bed or near its boundary in all animals with a cycle length of 329 +/- 26 ms and a focal origin. The maximum slope of the ARI restitution curve at the focal origins of the tachyarrhythmias was always <1 (0.62 +/- 0.15). Similar results with a focal arrhythmia origin occurred in two additional pigs in which intramural mapping was performed with 36 plunge needle electrodes in the left ventricular perfused bed.

CONCLUSION

Regional elevation of a catecholamine, which is one of the alterations produced by acute ischemia, can by itself cause tachyarrhythmias. These arrhythmias are closely associated with a shortened refractory period and a large gradient of the spatial distribution of refractoriness but not with a steep restitution curve.

Phase 2 Reentry as a Trigger to Initiate Ventricular Fibrillation During Early Acute Myocardial Ischemia

Background— Phase 2 reentry caused by heterogeneous loss of the transient outward potassium current ( I to )–mediated epicardial action potential (AP) dome can produce a closely coupled R-on-T extrasystole leading to ventricular fibrillation (VF) under conditions of ST-segment elevation unrelated to ischemia. The present study examined the role of phase 2 reentry in the initiation of VF during early myocardial ischemia.

Methods and Results— Regional myocardial ischemia was produced in an isolated, arterially perfused canine right ventricular wedge preparation. Transmembrane APs from 2 epicardial sites at each side of the ischemic border were simultaneously recorded together with measurements of extracellular potassium concentration ([K + ] o ) and a transmural ECG. Loss of the I to -mediated epicardial AP dome in the ischemic zone but not in the perfused tissue resulted in phase 2 reentry and associated R-on-T extrasystoles capable of initiating VF in 7 of 15 preparations during the first 3 to 9 minutes of myocardial ischemia, with marked ST-segment elevation and [K + ] o accumulation. The I to and phase 1 magnitude of epicardium contributed importantly to the onset of VF. Phase 1 magnitude and I to density at +30 mV in the group with phase 2 reentry–related R-on-T extrasystoles were 32.2±1.3 mV and 30.3±0.5 pA/pF (n=7), respectively, significantly greater than those (24.0±1.8 mV and 23.2±1.0 pA/pF) in the group without the extrasystoles (n=8, P <0.01).

Conclusions— Acute regional myocardial ischemia results in markedly heterogeneous loss of I to -mediated epicardial AP domes across the ischemic border, leading to phase 2 reentry. Phase 2 reentry can in turn produce an R-on-T extrasystole capable of initiating VF.

Intravenous administration of nifekalant hydrochloride for the prevention of ischemia-induced ventricular tachyarrhythmia in patients with renal failure undergoing hemodialysis

Excretion in the urine is an important pathway for the elimination of nifekalant hydrochloride (NIF), a novel class III antiarrhythmic agent. Three patients with renal failure were undergoing hemodialysis and receiving NIF for the prevention of ischemia-induced ventricular tachyarrhythmia. Because NIF is not dialyzed, dose adjustment at relatively low concentrations was required, with monitoring of the QT interval.

Outcomes and in-hospital treatment of out-of-hospital cardiac arrest patients resuscitated from ventricular fibrillation by early defibrillation

OBJECTIVE

To describe and evaluate the in-hospital treatment of ventricular arrhythmias and underlying structural heart disease in patients who survive ventricular fibrillation (VF) out-of-hospital cardiac arrest (OHCA) in a region with a high survival rate after hospital discharge.

PATIENTS AND METHODS

The study included all patients presenting in Olmsted County, Minnesota, who had experienced OHCA between November 1990 and December 2000 and who underwent defibrillation of VF by an emergency medical service system.

RESULTS

Of 200 patients who experienced VF arrest, 138 (69%) survived to hospital admission (7 died in the emergency department before admission), and 79 (40%) were discharged. Of patients who were discharged, 37 (47%) had a reversible cause of the arrest (perimyocardial infarction) and received treatment of the primary process. The other 42 patients who were discharged had ischemic coronary heart disease (CHD) (n=25), nonischemic CHD (n=10), or idiopathic VF (n=7). Four of the patients with CHD but no left ventricular dysfunction were treated with coronary artery bypass grafting or percutaneous coronary intervention alone. A total of 52 patients (66%) were candidates for electrophysiologic testing. Of these patients, 48 (92%) underwent electrophysiologic testing; of these patients, 10 received amiodarone alone, and 35 received an implantable cardioverter-defibrillator (ICD) (of whom 3 also received amiodarone). Patients who did not receive ICD therapy typically presented before 1998 with CHD and underwent coronary artery bypass grafting or percutaneous coronary intervention only. Of 79 patients who were discharged, 14 (18%) with an ICD have received subsequent shocks. Nineteen (24%) of 79 patients have died, 5 of a primary cardiac etiology (including 2 with repeated OHCA).

CONCLUSIONS

The VF OHCA survival rate is high in the setting of rapid defibrillation, with 40% of patients being discharged from the hospital. By the end of the 10-year study, more patients were receiving antiarrhythmic therapy, in particular ICD implantation, after hospital admission. Overall, the long-term survival in patients with VF OHCA is favorable.

Ischemic modulation of vulnerable period and the effects of pharmacological treatment of ischemia-induced arrhythmias: a simulation study

First identified in the 1930s (Ferris et al., 1936 and Wiggers and Wegria, 1939), the concept of vulnerability applies perfectly to biological oscillators. We can safely say that vulnerability is an inherent property of any excitable media. The duration of vulnerable period (VP) (the time interval during which single stimuli can initiate self-sustaining propagation) is sensitive to medium properties and stimulus parameters (stimulus field, timing behind the conditioning wave, and stimulus amplitude). Apart from medium properties and stimulus characteristics, heart vulnerability is affected by any intervention targeting the excitatory and recovery process. Therefore, we can expect that any pathological condition disturbing heart excitation or tissue recovery will most probably alter the duration of VP. In this paper, we shall explore the implications of ischemia and one of the arrhythmia counteracting methods widely used in clinical practice-antiarrhythmic drugs--in changing the boundaries of VP. The Cardiac Arrhythmia Suppression Trial (CAST) studies, as well as classification based on functional characteristics, revealed the arrhythmogenic potential of both Class I and Class III agents, but failed to identify the proarrhythmic mechanisms. This study presents results from a mathematical model (Cimponeriu et al., 2001) of the ventricle based on Luo-Rudy cellular formulation Luo and Rudy, 1991) modified for studying the ischemic modulation of VP and the effects of pharmacological treatment of ischemia-induced arrhythmia. Simulations revealed the link between the cellular antiarrhythmic properties and the proarrhythmic effect at the multicellular level in the case of Na+ channel blockade. Na+ channel blockade delayed recovery of cellular excitability, but also introduced a nonuniform dispersion of refractoriness along the cardiac fiber that can serve as a substrate for initiating a new arrhythmia. Our initial analysis proved that fast unbinding rates are essential in reducing the proarrhythmic potential of Class I drugs. However, further investigations led us to believe that binding properties are equally important. An antiarrhythmic drug with high affinity for drug-channel complex formation elicits a higher level of blockade per time unit. Under this light, we hypothesize that even the modern, fast unbinding drugs are not necessarily safe.

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.

A theoretical analysis of acute ischemia and infarction using ECG reconstruction on a 2-D model of myocardium

We developed a two-dimensional ventricular tissue model in order to probe the determinants of electrocardiographic (ECG) morphology during acute and chronic ischemia. Hyperkalemia was simulated by step changes in [K+]out, while acidosis was induced by reducing Na+ and Ca2+ conductances. Hypoxia was introduced by its effect on potassium activity. During the initial moments of ischemia, ECG changes were characterized by increases in QRS amplitude and ST segment shortening, followed in the advanced phase by ST baseline elevation, T conformation changes, widening of the QRS and significant decreases in QRS amplitude in spite of an enlarged Q. During each phase, potential proarrhythmic mechanisms were investigated. The presence of unexcitable regions of simulated myocardial infarction led to polymorphic ECG. We also observed a nonuniform deflection of the ST segment from beat to beat. We used similar protocols to explore the responses of infarcted myocardium after impairment resolving. We found that despite irreversible uncoupling of the necrotic region, the restored normal ionic concentrations produced an isopotential ST segment and monomorphic ECG complexes, while an enlarged Q wave was still visible. In summary, these numerical experiments indicate the possibility to track in the ECG pathologic changes following the altered electrophysiology of the ischemic heart.

Initial experience with nifekalant hydrochloride (MS-551), a novel class III antiarrhythmic agent, in patients with acute extensive infarction and severe ventricular dysfunction

Nifekalant hydrocholoride, a novel class III antiarrhythmic agent, was used as the treatment in 4 patients with extensive anterior infarction and severe ventricular dysfunction. The malignant ventricular tachyarrhythmia was effectively suppressed at a relatively low dose, without compromising the hemodynamics, indicating that this potent K+ channel blocker has therapeutic potential for acute myocardial infarction.

[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.

Nonsustained ventricular tachycardia in the setting of acute myocardial infarction: tachycardia characteristics and their prognostic implications

BACKGROUND

Nonsustained ventricular tachycardia (NSVT) has significant prognostic implications in the setting of healing and healed myocardial infarction (MI), but only limited information is available on its importance in the setting of acute MI. We evaluated the prognostic significance of NSVT characteristics in the setting of acute MI.

METHODS AND RESULTS

A prospective database was used to identify 112 patients with NSVT within 72 hours of acute MI. A control group was identified matched for age, sex, type of MI, and thrombolytic treatment. Mean age was 64 to 65 years in the 2 groups with 71% to 72% men. Q-wave MI was noted in 52% to 53%, and thrombolytic therapy was administered to 31% to 32% of patients in each group. In-hospital ventricular fibrillation occurred more frequently in the NSVT group (9% versus 0% in the control group; P<0. 001), but total in-hospital (10% versus 4%) and follow-up mortality (10% versus 17%) did not differ between the 2 groups. With a Cox regression model, specific NSVT characteristics were predictive of mortality. The strongest predictor was time from presentation to occurrence of NSVT. Shortest RR interval during NSVT was also a univariate predictor of mortality. Multivariate analysis identified time from presentation to occurrence of NSVT as the strongest predictor of mortality (P<0.0001). The increased relative risk of NSVT was first significant when it occurred 13 hours from presentation and continued to increase as the time from presentation to occurrence of NSVT increased, plateauing at approximately 24 hours with a relative risk of 7.5.

CONCLUSIONS

Contrary to prevailing clinical opinion, NSVT that occurs in the setting of acute MI does have important prognostic significance. Specifically, the currently accepted notion that NSVT that occurs within 48 hours of acute MI has no prognostic significance needs to be adjusted. Although NSVT that occurs within the first several hours of presentation does not have an associated adverse prognosis, NSVT that occurs beyond the first several hours after presentation is associated with significant increases in relative risk.