Mechanisms of sudden cardiac death

Repolarization abnormalities and afterdepolarizations in a canine model of sudden cardiac death

Ventricular tachyarrhythmias are the most common cause of sudden cardiac death (SCD); a healed myocardial infarction increases the risk of SCD. We determined the contribution of specific repolarization abnormalities to ventricular tachyarrhythmias in a postinfarction model of SCD. For our methods, we used a postinfarction canine model of SCD, where an exercise and ischemia test was used to stratify animals as either susceptible (VF(+)) or resistant (VF(-)) to sustained ventricular tachyarrhythmias. Our results show no changes in global left ventricular contractility or volumes occurred after infarction. At 8-10 wk postmyocardial infarction, myocytes were isolated from the left ventricular midmyocardial wall and studied. In the VF(+) animals, myocyte action potential (AP) prolongation occurred at 50 and 90% repolarization (P < 0.05) and was associated with increased variability of AP duration and afterdepolarizations. Multiple repolarizing K(+) currents (I(Kr), I(to)) and inward I(K1) were also reduced (P < 0.05) in myocytes from VF(+) animals compared with control, noninfarcted dogs. In contrast, only I(to) was reduced in VF(-) myocytes compared with controls (P < 0.05). While afterdepolarizations were not elicited at baseline in myocytes from VF(-) animals, afterdepolarizations were consistently elicited after the addition of an I(Kr) blocker. In conclusion, the loss of repolarization reserve via reductions in multiple repolarizing currents in the VF(+) myocytes leads to AP prolongation, repolarization instability, and afterdepolarizations in myocytes from animals susceptible to SCD. These abnormalities may provide a substrate for initiation of postmyocardial infarction ventricular tachyarrhythmias.

Myocardial sympathetic innervation in patients with symptomatic coronary artery disease: follow-up after 1 year with neurostimulation

In both diabetic and nondiabetic patients, there is a loose correlation between coronary flow reserve (CFR) and sympathetic innervation in viable myocardial segments. The loose correlation implies that sympathetic innervation may be preserved even with major impairment of myocardial blood supply. In some patients, denervation is due to repetitive episodes of ischemia in areas with severely reduced CFR. We investigated the long-term effect of reduced CFR on myocardial sympathetic innervation in diabetic and nondiabetic patients with spinal cord stimulation.

METHODS

We analyzed 23 patients (10 diabetic and 13 nondiabetic) with coronary artery disease and without known cardiac autonomic neuropathy. At baseline, we determined quantitative myocardial blood flow using (13)N-ammonia PET, myocardial viability using (18)F-FDG PET, and cardiac innervation using (11)C-hydroxyephedrine (HED) PET. At the 1-y follow-up we measured CFR and (11)C-HED retention. During follow-up, no cardiac intervention was performed and no myocardial infarction occurred. In all patients, spinal cord stimulation was performed for relief of angina.

RESULTS

There was no significant difference in segmental (11)C-HED retention between baseline and follow-up in the whole patient group. In diabetic patients, as well as in segments with severely reduced CFR (<1.5), (11)C-HED retention showed a small but significant decrease (P<0.05). Linear regression of segmental (11)C-HED retention between baseline and follow-up was high (r(2)=0.81), confirming good reproducibility of the investigation on the one hand and little change in regional sympathetic innervation on the other hand.

CONCLUSION

In patients with stable chronic coronary artery disease, sympathetic innervation of the myocardium is almost unchanged in both diabetic and nondiabetic patients in a 1-y follow-up. In myocardial segments with severely altered blood supply, a small but significant decrease in (11)C-HED retention most probably reflects ischemic neuronal damage. The prognostic relevance of sympathetic denervation in viable myocardium still has to be determined.

A study on pathogenesis of sudden death syndrome in broiler chickens

Sudden death syndrome (SDS) in fast growing broiler chickens has been recognized as a patho-physiological entity for four decades, but its pathogenesis still remains unknown. More recent investigations provided evidence that link SDS to cardiac arrhythmia, but the mechanism triggering arrhythmogenesis and factors responsible for fatal outcome are poorly understood. In order to understand the chain of events leading to SDS in broilers, the present study focused on putative mechanisms that trigger arrhythmia and mechanisms that predispose the myocardium to fatal arrhythmia. Susceptibility of broilers to cardiac arrhythmia under stress conditions was evaluated using a simulated stress test with epinephrine. Detailed histopathological evaluation of the broiler heart was undertaken to identify structural features that may predispose the myocardium to fatal arrhythmia. The simulated stress challenge revealed that many broilers are highly susceptible to stress induced cardiac arrhythmia. In some broilers the stress challenge induced severe ventricular arrhythmia, and the life threatening nature of this arrhythmia was evidenced by the fact that several birds showing the most severe arrhythmic responses, died suddenly within several days after the stress challenge. Examination of hearts of broilers that died of SDS revealed microscopic lesions in the cardiomyocytes, and widespread changes in the sub-endocardial and mural His-Purkinje system (HPS). Immune staining for Caspase-3 confirmed that numerous Purkinje cells in the left ventricular myocardium from broiler chickens that died of SDS were undergoing apoptosis. The observed lesions suggest that the electrical stability of the myocardium was compromised. Taken together, our findings indicate that stress is a most likely trigger of cardiac arrhythmia in broilers, whereas the pathological changes seen in the myocardium and in the HPS in fast growing broilers provide a very conducive milieu for sustained ventricular arrhythmia. In cases where the electrical stability of the myocardium is compromised, even an episodic arrhythmic event may readily degenerate to catastrophic ventricular fibrillation and sudden death. We conclude that the combination of stress and changes in the cardiomyocytes and HPS are the key requisite features in the pathogenesis of SDS.

Women with dysrhythmia: a clinical challenge

There are identified differences in the electrophysiology structure and measurements in women and men. An understanding of these differences and of the increased incidence and prevalence of dysrhythmias in women, the differences in presentation, and the differences in risk factors for these dysrhythmias will help guide treatment decisions. As new knowledge is gained through research, practitioners can provide gender-specific care to women who have or are at increased risk of cardiac dysrhythmia.

Differences in ventricular vulnerability, serum enzyme activities, and electrolyte levels between male and female broilers

Sudden death syndrome (SDS) is one of the most serious diseases of fast-growing broilers. The incidence of SDS may result from a decrease in ventricular function. The purpose of this study was to explore the mechanism of sexual difference in the sensitivity of broilers to SDS by measuring their ventricular vulnerability, serum enzyme activities, and serum electrolyte levels. Results were as follows. 1) Ventricular fibrillation thresholds induced by injection of KCl and by electrical stimulus of male broilers were both significantly lower than those of female broilers (P < 0.05), suggesting that the ventricular vulnerability of male broilers was higher than that of female broilers. 2) Serum lactate dehydrogenase and creatine kinase activities of male broilers were significantly higher than those of female broilers (P < 0.01), but there was not a significant difference in serum aspartate aminotransferase activity between male and female broilers. 3) No significant difference was observed in serum electrolyte levels of potassium, sodium, and chloride between males and females. From these results, we concluded that there is a significant difference between males and females in their ventricular vulnerability and serum enzyme activities, which may result in a higher sensitivity of male broilers to injury of the myocardium by stress and may further result in a sexual difference in sensitivity to SDS.

Electrocardiographic predictors of sudden cardiac death

Sudden cardiac death (SCD) is widespread and the most serious of the cardiac diseases, accounting for over half of cardiovascular mortality in adults in the United States, and nearly 1 in 3 of these patients does not report symptoms of cardiac disease before the sudden death. Quantifying the left ventricular ejection fraction is currently the best way to risk-stratify patients for SCD and identify those who are most likely to benefit from the insertion of an implantable cardiac defibrillator (ICD). The strategy of systemically placing ICDs in patients at risk of SCD is expensive and leads to substantial psychological hardship. However, noninvasive electrocardiographic indices of depolarization and repolarization may better identify patients who are at an increased risk of SCD. Therefore, developing an approach to identify electrocardiographic changes associated with the highest risk of arrhythmic death could markedly improve patient selection for ICD therapy. This report describes electrocardiographic parameters that may be useful in identifying patients at risk of SCD. The state of the science currently suggests that it is unlikely that a single electrocardiographic parameter will predict SCD, but rather a risk stratification algorithm based on a combination of electrocardiographic parameters may yield the best result.

Serum transforming growth factor-beta1 as a risk stratifier of sudden cardiac death

Sudden cardiac death prematurely claims the lives of some 7 million each year worldwide. It occurs primarily in patients with an underlying structural cardiac abnormality, and regardless of the type of the underlying pathology (heart failure, dilated and hypertrophic cardiomyopathies, myocardial infarction and aging), death is almost always caused by ventricular tachycardia (VT) which rapidly degenerates to ventricular fibrillation (VF). Implantable cardioverter defibrillator is an effective but expensive therapy for preventing SCD, and finding a reasonably specific, sensitive and cost-effective risk stratification tool for patients at high risk of sudden cardiac death will have great clinical utility in preventing premature sudden cardiac death. Increased myocardial fibrosis has been shown to develop in a wide range of cardiac diseases all manifesting increased risk of VT and VF. Clinical and experimental studies attribute a major role for fibrosis in the initiation of VT, VF and sudden cardiac death. Transforming growth factor-beta1 (TGF-beta1) has been shown to promote myocardial tissue fibrosis and perhaps more importantly in cardiac conditions associated with increased myocardial fibrosis are shown to be positively correlated with increased serum levels of TGF-beta1. In the present hypothesis we suggest that monitoring the serum levels of TGF-beta1 may be a cost-effective risk stratifier to identify patients at high risk of sudden cardiac death caused by VT and VF.

Inhibition of the rapid component of the delayed rectifier potassium current in ventricular myocytes by angiotensin II via the AT1 receptor

BACKGROUND AND PURPOSE

There is increasing evidence that angiotensin II (Ang II) is associated with the occurrence of ventricular arrhythmias. However, little is known about the electrophysiological effects of Ang II on ventricular repolarization. The rapid component of the delayed rectifier K(+) current (I(Kr)) plays a critical role in cardiac repolarization. Hence, the aim of this study was to assess the effect of Ang II on I(Kr) in guinea-pig ventricular myocytes.

EXPERIMENTAL APPROACH

The whole-cell patch-clamp technique was used to record I(Kr) in native cardiocytes and in human embryonic kidney (HEK) 293 cells, co-transfected with human ether-a-go-go-related gene (hERG) encoding the alpha-subunit of I(Kr) and the human Ang II type 1 (AT(1)) receptor gene.

KEY RESULTS

Ang II decreased the amplitude of I(Kr) in a concentration-dependent manner with an IC(50) of 8.9 nM. Action potential durations at 50% (APD(50)) and 90% (APD(90)) repolarization were prolonged 20% and 16%, respectively by Ang II (100 nM). Ang II-induced inhibition of the I(Kr) was abolished by the AT(1) receptor blocker, losartan (1 muM). Ang II decreased hERG current in HEK293 cells and significantly delayed channel activation, deactivation and recovery from inactivation. Moreover, PKC inhibitors, stausporine and Bis-1, significantly attenuated Ang II-induced inhibition of I(Kr).

CONCLUSIONS AND IMPLICATIONS

Ang II produces an inhibitory effect on I(Kr)/hERG currents via AT(1) receptors linked to the PKC pathway in ventricular myocytes. This is a potential mechanism by which elevated levels of Ang II are involved in the occurrence of arrhythmias in cardiac hypertrophy and failure.

Antiarrhythmics: elimination and dosage considerations in hepatic impairment

Many drugs, including most antiarrhythmics (some of which are now of limited clinical use) are eliminated by the hepatic route. If liver function is impaired, it can be anticipated that hepatic clearance will be delayed, which can lead to more pronounced drug accumulation with multiple dosing. Consequently, the potential risks of adverse events could be increased, especially as antiarrhythmics have a narrow therapeutic index. The present review summarises the available pharmacokinetic data on the most popular antiarrhythmic drugs to identify the enzymes involved in the metabolism of the various agents and confirm whether liver disease affects their elimination. Despite long usage of some of these drugs (e.g. amiodarone, diltiazem, disopyramide, procainamide and quinidine), surprisingly few data are available in patients with liver disease, making it difficult to give recommendations for dosage adjustment. In contrast, for carvedilol, lidocaine (lignocaine), propafenone and verapamil, sufficient clinical studies have been performed. For these drugs, a marked decrease in systemic and/or oral clearance and significant prolongation of the elimination half-life have been documented, which should be counteracted by a 2- to 3-fold reduction of the dosage in patients with moderate to severe liver cirrhosis. For sotalol, disopyramide and procainamide, renal clearance contributes considerably to overall elimination, suggesting that dosage reductions are probably unnecessary in patients with liver disease as long as renal function is normal. The hepatically eliminated antiarrhythmics are metabolised mainly by different cytochrome P450 (CYP) isoenzymes (e.g. CYP3A4, CYP1A2, CYP2C9, CYP2D6) and partly also by conjugations. As the extent of impairment in clearance is in the same range for all of these agents, it could be assumed that they have a common vulnerability and that, consequently, hepatic dysfunction will affect CYP-mediated phase I pathways in a similar fashion. The severity of liver disease has been estimated clinically by the validated Pugh score, and functionally by calculation of the clearance of probe drugs (e.g. antipyrine). Both approaches can be helpful in estimating/predicting impairments in drug metabolism, including antiarrhythmics. In conclusion, hepatic impairment decreases the elimination of many antiarrhythmics to such an extent that dosage reductions are highly recommended in such populations, especially in patients with cirrhosis.

Electrical remodeling contributes to complex tachyarrhythmias in connexin43-deficient mouse hearts

Loss of connexin43 (Cx43) gap junction channels in the heart results in a marked increase in the incidence of spontaneous and inducible polymorphic ventricular tachyarrhythmias (PVTs). The mechanisms resulting in this phenotype remain unclear. We hypothesized that uncoupling promotes regional ion channel remodeling, thereby increasing electrical heterogeneity and facilitating the development of PVT. In isolated-perfused control hearts, programmed electrical stimulation elicited infrequent monomorphic ventricular tachyarrhythmias (MVT), and dominant frequencies (DFs) during MVT were similar in the right ventricle (RV) and left ventricle (LV). Moreover, conduction properties, action potential durations (APDs), and repolarizing current densities were similar in RV and LV myocytes. In contrast, PVT was common in Cx43 conditional knockout (OCKO) hearts, and arrhythmias were characterized by significantly higher DFs in the RV compared to the LV. APDs in OCKO myocytes were significantly shorter than those from chamber-matched controls, with RV OCKO myocytes being most affected. APD shortening was associated with higher levels of sustained current in myocytes from both chambers as well as higher levels of the inward rectifier current only in RV myocytes. Thus, alterations in cell-cell coupling lead to regional changes in potassium current expression, which in this case facilitates the development of reentrant arrhythmias. We propose a new mechanistic link between electrical uncoupling and ion channel remodeling. These findings may be relevant not only in cardiac tissue but also to other organ systems where gap junction remodeling is known to occur.

Generation and escape of local waves from the boundary of uncoupled cardiac tissue

We aim to understand the formation of abnormal waves of activity from myocardial regions with diminished cell-to-cell coupling. En route to this goal, we studied the behavior of a heterogeneous myocyte network in which a sharp coupling gradient was placed under conditions of increasing network automaticity. Experiments were conducted in monolayers of neonatal rat cardiomyocytes using heptanol and isoproterenol as means of altering cell-to-cell coupling and automaticity, respectively. Experimental findings were explained and expanded using a modified Beeler-Reuter numerical model. The data suggest that the combination of a heterogeneous substrate, a gradient of coupling, and an increase in oscillatory activity of individual cells creates a rich set of behaviors associated with self-generated spiral waves and ectopic sources. Spiral waves feature a flattened shape and a pin-unpin drift type of tip motion. These intercellular waves are action-potential based and can be visualized with either voltage or calcium transient measurements. A source/load mismatch on the interface between the boundary and well-coupled layers can lock wavefronts emanating from both ectopic sources and rotating waves within the inner layers of the coupling gradient. A numerical approach allowed us to explore how 1), the spatial distribution of cells, 2), the amplitude and dispersion of cell automaticity, and 3), the speed at which the coupling gradient moves in space affect wave behavior, including its escape into well-coupled tissue.

Spontaneous stellate ganglion nerve activity and ventricular arrhythmia in a canine model of sudden death

BACKGROUND

Little information is available on the temporal relationship between instantaneous sympathetic nerve activity and ventricular arrhythmia in ambulatory animals.

OBJECTIVE

The purpose of this study was to determine if increased sympathetic nerve activity precedes the onset of ventricular arrhythmia.

METHODS

Simultaneous continuous long-term recording of left stellate ganglion (LSG) nerve activity and electrocardiography was performed in eight dogs with nerve growth factor infusion to the LSG, atrioventricular block, and myocardial infarction (experimental group) and in six normal dogs (control group).

RESULTS

LSG nerve activity included low-amplitude burst discharge activity (LABDA) and high-amplitude spike discharge activity (HASDA). Both LABDA and HASDA accelerated heart rate. In the experimental group, most ventricular tachycardia (86.3%) and sudden cardiac death were preceded within 15 seconds by either LABDA or HASDA. The closer to onset of ventricular tachycardia, the higher the nerve activity. The majority of HASDA was followed immediately by either ventricular arrhythmia (21%) or QRS morphology changes (65%). HASDA occurred in a circadian pattern. HASDA occurred twice as often in the experimental group than in the control group. Electrical stimulation of LSG increased transmural heterogeneity of repolarization (Tpeak-end intervals) and induced either ventricular tachycardia or fibrillation in the experimental group but not in the control group. Immunohistochemical studies revealed increased synaptogenesis and nerve sprouting in the LSG in the experimental group.

CONCLUSION

Two distinct types of LSG nerve activity (HASDA and LABDA) are present in the LSG of ambulatory dogs. The majority of malignant ventricular arrhythmias are preceded by either HASDA or LABDA, with HASDA particularly arrhythmogenic.

Exercise-induced cardioprotection: endogenous mechanisms

It is now well established that exercise can result in cardioprotection against ischemia-reperfusion (I-R) injury; however, the adaptations within the heart that provide the protection are still in doubt. The cytoprotective proteins receiving the most attention to date are antioxidant enzymes and heat shock proteins. The extent of I-R injury is dependent on the interactions of several events, including energy depletion, metabolite accumulation, oxidant stress, and calcium overload. Adaptations that directly influence any of these could affect I-R outcome. Thus, the exercise-induced cardioprotective phenotype is likely to include additional cytoprotective proteins beyond antioxidant enzymes or heat shock proteins. In this review, we will consider evidence for some of these in the cytosol, mitochondria, and sarcolemma of the cardiomyocyte. We will not consider potentially important adaptations within vascular tissue or the autonomic nervous system. Results of recent studies support the hypothesis that exercise leads to cardioprotective adaptations that are unique from other forms of preconditioning against I-R injury.

Deficient ryanodine receptor S-nitrosylation increases sarcoplasmic reticulum calcium leak and arrhythmogenesis in cardiomyocytes

Altered Ca(2+) homeostasis is a salient feature of heart disease, where the calcium release channel ryanodine receptor (RyR) plays a major role. Accumulating data support the notion that neuronal nitric oxide synthase (NOS1) regulates the cardiac RyR via S-nitrosylation. We tested the hypothesis that NOS1 deficiency impairs RyR S-nitrosylation, leading to altered Ca(2+) homeostasis. Diastolic Ca(2+) levels are elevated in NOS1(-/-) and NOS1/NOS3(-/-) but not NOS3(-/-) myocytes compared with wild-type (WT), suggesting diastolic Ca(2+) leakage. Measured leak was increased in NOS1(-/-) and NOS1/NOS3(-/-) but not in NOS3(-/-) myocytes compared with WT. Importantly, NOS1(-/-) and NOS1/NOS3(-/-) myocytes also exhibited spontaneous calcium waves. Whereas the stoichiometry and binding of FK-binding protein 12.6 to RyR and the degree of RyR phosphorylation were not altered in NOS1(-/-) hearts, RyR2 S-nitrosylation was substantially decreased, and the level of thiol oxidation increased. Together, these findings demonstrate that NOS1 deficiency causes RyR2 hyponitrosylation, leading to diastolic Ca(2+) leak and a proarrhythmic phenotype. NOS1 dysregulation may be a proximate cause of key phenotypes associated with heart disease.

Mutation in glycerol-3-phosphate dehydrogenase 1 like gene (GPD1-L) decreases cardiac Na+ current and causes inherited arrhythmias

BACKGROUND

Brugada syndrome is a rare, autosomal-dominant, male-predominant form of idiopathic ventricular fibrillation characterized by a right bundle-branch block and ST elevation in the right precordial leads of the surface ECG. Mutations in the cardiac Na+ channel SCN5A on chromosome 3p21 cause approximately 20% of the cases of Brugada syndrome; most mutations decrease inward Na+ current, some by preventing trafficking of the channels to the surface membrane. We previously used positional cloning to identify a new locus on chromosome 3p24 in a large family with Brugada syndrome and excluded SCN5A as a candidate gene.

METHODS AND RESULTS

We used direct sequencing to identify a mutation (A280V) in a conserved amino acid of the glycerol-3-phosphate dehydrogenase 1-like (GPD1-L) gene. The mutation was present in all affected individuals and absent in >500 control subjects. GPD1-L RNA and protein are abundant in the heart. Compared with wild-type GPD1-L, coexpression of A280V GPD1-L with SCN5A in HEK cells reduced inward Na+ currents by approximately 50% (P<0.005). Wild-type GPD1-L localized near the cell surface to a greater extent than A280V GPD1-L. Coexpression of A280V GPD1-L with SCN5A reduced SCN5A cell surface expression by 31+/-5% (P=0.01).

CONCLUSIONS

GPD1-L is a novel gene that may affect trafficking of the cardiac Na+ channel to the cell surface. A GPD1-L mutation decreases SCN5A surface membrane expression, reduces inward Na+ current, and causes Brugada syndrome.

Intracellular Ca2+ oscillations, a potential pacemaking mechanism in early embryonic heart cells

Early (E9.5-E11.5) embryonic heart cells beat spontaneously, even though the adult pacemaking mechanisms are not yet fully established. Here we show that in isolated murine early embryonic cardiomyocytes periodic oscillations of cytosolic Ca(2+) occur and that these induce contractions. The Ca(2+) oscillations originate from the sarcoplasmic reticulum and are dependent on the IP(3) and the ryanodine receptor. The Ca(2+) oscillations activate the Na(+)-Ca(2+) exchanger, giving rise to subthreshold depolarizations of the membrane potential and/or action potentials. Although early embryonic heart cells are voltage-independent Ca(2+) oscillators, the generation of action potentials provides synchronization of the electrical and mechanical signals. Thus, Ca(2+) oscillations pace early embryonic heart cells and the ensuing activation of the Na(+)-Ca(2+) exchanger evokes small membrane depolarizations or action potentials.

Interleukin-18 promoter polymorphism associates with the occurrence of sudden cardiac death among Caucasian males: the Helsinki Sudden Death Study

OBJECTIVE

The increased plasma concentrations of pro-atherogenic and cardiomyocyte hypertrophic cytokine interleukin 18 (IL-18) predict mortality in patients with coronary heart disease (CHD) in addition to predicting the outcome of heart failure. The IL-18 gene has a functional -137G/C polymorphism (rs187238) in the promoter region. The C allele carriage is associated with attenuated IL-18 production. The effect of IL-18 genotype on SCD is unknown. We studied the association of the IL-18 gene -137G/C polymorphism with the occurrence of sudden cardiac death (SCD).

METHODS

Using the TaqMan 5' nuclease assay, we genotyped two independent consecutive and prospective autopsy series which were included in the Helsinki Sudden Death Study.

RESULTS

Of the 663 men, 359 (54.1%) had the wild-type GG-genotype, 261 (39.4%) were heterozygotes (CG) and 43 (6.5%) were CC homozygotes. Compared to the GG homozygotes, the C allele carriers (i.e. subjects having CC or CG genotypes) had a lower adjusted risk for SCD from any cause (odds ratio [OR] 0.49; 95% confidence interval [CI], 0.31-0.77, p=0.002), for SCD due to CHD (OR 0.51; 95% CI, 0.32-0.82, p=0.005), and for SCD caused by non-coronary heart diseases (OR 0.34; 95% CI 0.13-0.90, p=0.030).

CONCLUSION

IL-18 promoter -137G/C polymorphism, which regulates the expression of IL-18, is an important predictor of SCD from any cause as well as SCD in patients with and without underlying CHD.

Predicting Imminent Episodes of Ventricular Tachyarrhythmia Using Heart Rate

BACKGROUND

A reliable predictor of an imminent episode of ventricular tachyarrhythmia that could be incorporated in an implantable defibrillator capable of preventive therapy would have important clinical utility.

METHOD

A test set of 208 R-R records saved by defibrillators spanning a mean of 1.6 hours before sustained tachyarrhythmia were used to derive criteria that would improve the specificity of the previously identified monotonic heart rate acceleration predictor. Additional criteria were used, namely two such patterns need to occur within a period of 1.8 hour and the heart rate during these accelerations exceeds 86 bpm (700 ms). The specificity was tested using R-R records matched in duration from 26 control patients with defibrillators during normal periods.

RESULTS

The basic acceleration pattern was found during sinus rhythm in the 1.8-hour period prior to 83% of episodes of ventricular tachyarrhythmia. It was also found in 43% of the matched set of non-arrhythmic records, corresponding to a specificity of 57%. With the two extra requirement of multiplicity within 1.8 hour and peak heart rate, the sensitivity of the proposed predictor is reduced to 53%, but the specificity is increased to 91%, which corresponds to an average false positive rate of 0.8 event/day across the patient population.

CONCLUSION

A ventricular tachyarrhythmia predictor based on a pattern of heart rate acceleration has been proposed that can yield sensitivity from 53% to 69%, with specificity up to 91%. Instances of this predictor increase significantly prior to an episode of tachyarrhythmia.

Simulations of propagated mouse ventricular action potentials: effects of molecular heterogeneity

The molecular heterogeneity of repolarizing currents produces significant spatial heterogeneity and/or dispersion of repolarization in many mammalian cardiac tissues. Transgenic mice are prominent experimental models for the study of the molecular basis of repolarization and arrhythmias. However, it is debated whether the small mouse heart can sustain physiologically relevant heterogeneity of repolarization. We used a comprehensive model of the mouse action potential (AP) to predict how small a region of the cardiac tissue can maintain spatial gradients of repolarization due to differential expression of channels. Our simulations of a one-dimensional multicellular ring or cable predict that substantial gradients in repolarization and intracellular Ca(2+) concentration transients can be maintained through heterogeneity of expression of K(+) channels in distances of approximately 10 cells that are sufficient to block propagation. The abruptness of expression gradients and the site of stimulation can cause Ca(2+) transient oscillations and affect the stability of Ca(2+) dynamics and AP propagation. Two different mechanisms of instability of AP propagation in one-dimensional cable occur at fast pacing rates. Transitions from periodic activity to alternans or to irregular behavior were observed. Abrupt gradients of channel expression can cause alternans at slower pacing rates than gradual changes. Our simulations demonstrate the importance of incorporating realistic Ca(2+) dynamics and current densities into models of propagated AP. They also emphasize that microscopic aspects of tissue organization are important for predicting large-scale propagation phenomena. Finally, our results predict that the mouse heart should be able to sustain substantial molecularly based heterogeneity of repolarization.

Abrupt changes in FKBP12.6 and SERCA2a expression contribute to sudden occurrence of ventricular fibrillation on reperfusion and are prevented by CPU86017

AIM

The occurrence of ventricular fibrillation (VF) is dependent on the deterioration of channelopathy in the myocardium. It is interesting to investigate molecular changes in relation to abrupt appearance of VF on reperfusion. We aimed to study whether changes in the expression of FKBP12.6 and SERCA2a and the endothelin (ET) system on reperfusion against ischemia were related to the rapid occurrence of VF and whether CPU86017, a class III antiarrhythmic agent which blocks I(Kr), I(Ks), and I(Ca.L), suppressed VF by correcting the molecular changes on reperfusion.

METHODS

Cardiomyopathy (CM) was produced by 0.4 mg/kg sc L-thyroxin for 10 d in rats, and subjected to 10 min coronary artery ligation/reperfusion on d 11. Expressions of the Ca2+ handling and ET system and calcium transients were conducted and CPU86017 was injected (4 mg/kg, sc) on d 6-10.

RESULTS

A high incidence of VF was found on reperfusion of the rat CM hearts, but there was no VF before reperfusion. The elevation of diastolic calcium was significant in the CM myocytes and exhibited abnormality of the Ca2+ handling system. The rapid downregulation of mRNA and the protein expression of FKBP12.6 and SERCA2a were found on reperfusion in association with the upregulation of the expression of the endothelin-converting enzyme (ECE) and protein kinase A (PKA), in contrast, no change in the ryanodine type 2 receptor (RyR2), phospholamban (PLB), endothelin A receptor (ETAR), and iNOS was found. CPU86017 removed these changes and suppressed VF.

CONCLUSION

Abrupt changes in the expression of FKBP12.6, SERCA2a, PKA, and ECE on reperfusion against ischemia, which are responsible for the rapid occurrence of VF, have been observed. These changes are effectively prevented by CPU86017.

Study for relevance of the acute myocardial ischemia to arrhythmia by the optical mapping method

Ventricular arrhythmias are commonly observed in patients with acute coronary occlusion and ischemia. The purpose of the present study is to determine ischemic electrophysiological effects and their role in ischemia-induced arrhythmia. Optical mapping of the membrane potential with voltage-sensitive dyes was used in the study. The mapping was performed with di-4-ANEPPS in Langendorff-perfused rabbit hearts. The excitation-contraction decoupler 2,3-butanedione monoxime was used to suppress motion artifacts caused by contraction of the heart. The acute global ischemia was developed by a rapid reduction of the flow rate. The experiments revealed that ischemic tissues were characterized by an obvious reduction in action potential duration and action potential upstroke, slower conduction velocity (CV) and the property of post-repolarization refractoriness. Moreover, the magnitude of CV reduced both in control and ischemia when the pacing cycle length was short. CV reduction was even early in ischemia, resulting in a broader curve during ischemia. Moreover, the dominant frequency of ventricular tachycardia/ventricular fibrillation (VT/VF) in ischemia was less than that in control, implying a decreasing tendency of VT/VF frequency for low excitability. Therefore, combined with our previous simulation study, the dynamic changes of CV and longer refractory period were suggested to play an important role in the ischemia-related arrhythmia. Low excitability in ischemic tissue was the fundamental mechanism in it.

CPU86017: a novel Class III antiarrhythmic agent with multiple actions at ion channels

CPU86017 is a novel Class III antiarrhythmic agent derived from berberine and with an improved pharmacological profile, solubility and bioavailability. It is active in suppressing arrhythmias in several animal models. The ED(50) of CPU86017 for suppressing ischemia/reperfusion arrhythmias in rats was 0.22 mg/kg against 2.23 mg/kg for lidocaine. CPU86017 is about 10-fold more potent than lidocaine. It blocks I(K(R.tail)), I(K(S)), and I(Ca(L)) currents with IC(50) values of 25, 14.4, and 11.5 microM, respectively. The plasma t(1/2) of CPU86017, i.v. bolus, in rabbits and dogs is approximately 90 min. The effective plasma levels of CPU86017 in rabbits required to delay the appearance of oubain-induced ventricular arrhythmias is in the range of 0.13-0.31 microg/mL. Higher levels of the drug are required to eliminate ventricular arrhythmias produced by two-stage ligation of the coronary artery in anesthetized dogs. Drug levels in myocardium are much higher than in plasma. CPU80617 has an antioxidant effect that is likely to contribute to its antiarrhythmic activity. The abnormal expression of the ryanodine receptor type 2 (RyR2) and of FKBP12.6 is reduced by CPU80617 during its ventricular tachyarrhythmia-suppressing action. CPU86017 appears to be a promising antiarrhythmic agent with a cardioprotective action. It can be expected to protect from malignant arrhythmias and sudden cardiac death by suppressing molecular events caused by channelopathies.

Implications of genetic testing for sudden cardiac death syndrome

The completion of the Human Genome Project in 2003 is shifting the focus of modern health care from disease management based on clinical signs to genomic-based treatment and prevention. Nurses at all levels of practice are going to increasingly come under pressure to deliver evidence-based, competent care to families undergoing genetic testing for hereditary conditions (Skirton and Barnes, 2005). A new chapter added to the National Service Framework for Coronary Heart Disease in 2005 raised awareness of the genetic basis of up to 400 sudden cardiac deaths that occur every year in the UK (Department of Health, 2005). This article addresses some of the clinical and ethical implications for nurses caring for families who may be at risk of an inherited sudden cardiac death syndrome. Nursing practice implications are discussed, concluding that more research is needed to explore how family members cope with genetic information which will shape the provision of future genetic healthcare.

Lipid-lowering therapy with statins, a new approach to antiarrhythmic therapy

Inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (statins) are the most effective and best-tolerated drugs to treat elevated levels of low-density lipoprotein cholesterol (LDL-C). In addition, they exhibit other effects unrelated to their lipid lowering effects (pleiotropic actions). In recent years, experimental and clinical evidence demonstrates that statins exert antiarrhythmic properties, reducing the recurrences of supraventricular and life-threatening ventricular arrhythmias both in patients with and without coronary artery disease (CAD). Thus, statins may constitute a novel therapeutic approach to cardiac arrhythmias. This article reviews the antiarrhythmic properties of statins as well as the possible mechanisms involved, including the lowering of LDL-C levels, the improvement of endothelial dysfunction and autonomic function, the stabilization of the atherosclerotic plaques, the antioxidant, antiinflammatory, antithrombotic and cardioprotective properties and the modulation of transmembrane ion fluxes.

Altered Na+ channels promote pause-induced spontaneous diastolic activity in long QT syndrome type 3 myocytes

Long QT syndrome (LQTS) type 3 (LQT3), typified by the DeltaKPQ mutation (LQT3 mutation in which amino acid residues 1505 to 1507 [KPQ] are deleted), is caused by increased sodium entry during the action potential plateau resulting from mutation-altered inactivation of the Na(v)1.5 channel. Although rare, LQT3 is the most lethal of common LQTS variants. Here we tested the hypothesis that cellular electrical dysfunction, caused not only by action potential prolongation but also by mutation-altered Na(+) entry, distinguishes LQT3 from other LQTS variants and may contribute to its distinct lethality. We compared cellular electrical activity in myocytes isolated from mice heterozygous for the DeltaKPQ mutation (DeltaKPQ) and myocytes from wild-type littermates. Current-clamp pause protocols induced rate-dependent spontaneous diastolic activity (delayed after depolarizations) in 6 of 7 DeltaKPQ, but no wild-type, myocytes (n=11) tested. Voltage-clamp pause protocols that independently control depolarization duration and interpulse interval identified a distinct contribution of both depolarization duration and mutant Na(+) channel activity to the generation of Ca(i)(2+)-dependent diastolic transient inward current. This was found at rates and depolarization durations relevant both to the mouse model and to LQT3 patients. Flecainide, which preferentially inhibits mutation-altered late Na(+) current and is used to treat LQT3 patients, suppresses transient inward current formation in voltage-clamped DeltaKPQ myocytes. Our results demonstrate a marked contribution of mutation-altered Na(+) entry to the incidence of pause-dependent spontaneous diastolic activity in DeltaKPQ myocytes and suggest that altered Na(+) entry may contribute to the elevated lethality of LQT3 versus other LQTS variants.

Determinants of sudden cardiac death in patients with persistent atrial fibrillation in the rate control versus electrical cardioversion (RACE) study

This report evaluated the correlates of sudden cardiac and nonsudden cardiac death in patients with persistent atrial fibrillation randomized to rate or rhythm control in the RAte Control vs Electrical cardioversion (RACE) study. Sudden cardiac death was observed in 16 patients, 8 patients in each group. Previous myocardial infarction resulted in a 4.9-fold increased risk of sudden death (95% confidence interval 1.8 to 13.2). The use of beta blockers showed their protective nature (hazard ratio 0.2, 95% confidence interval 0.05 to 0.9). The randomized treatment strategy, heart rhythm during follow-up, use of antiarrhythmic drugs, and number of stroke risk factors were not associated with sudden cardiac death. In conclusion, the treatment of underlying disease, rather than the heart rhythm, seems essential to prevent mortality.

Modeling cardiac ischemia

Myocardial ischemia is one of the main causes of sudden cardiac death, with 80% of victims suffering from coronary heart disease. In acute myocardial ischemia, the obstruction of coronary flow leads to the interruption of oxygen flow, glucose, and washout in the affected tissue. Cellular metabolism is impaired and severe electrophysiological changes in ionic currents and concentrations ensue, which favor the development of lethal cardiac arrhythmias such as ventricular fibrillation. Due to the burden imposed by ischemia in our societies, a large body of research has attempted to unravel the mechanisms of initiation, sustenance, and termination of cardiac arrhythmias in acute ischemia, but the rapidity and complexity of ischemia-induced changes as well as the limitations in current experimental techniques have hampered evaluation of ischemia-induced alterations in cardiac electrical activity and understanding of the underlying mechanisms. Over the last decade, computer simulations have demonstrated the ability to provide insight, with high spatiotemporal resolution, into ischemic abnormalities in cardiac electrophysiological behavior from the ionic channel to the whole organ. This article aims to review and summarize the results of these studies and to emphasize the role of computer simulations in improving the understanding of ischemia-related arrhythmias and how to efficiently terminate them.

Radiofrequency catheter ablation and nerve growth factor concentration in humans

BACKGROUND

In animal models, expression of nerve growth factor (NGF) is increased after necrotic myocardial injury. Whether radiofrequency (RF) catheter ablation increases NGF expression in humans is unclear.

OBJECTIVES

The purpose of this study was to determine NGF concentrations in the aorta, coronary sinus, and peripheral veins before and after RF ablation in patients.

METHODS

We sampled blood from aorta and either great cardiac vein (group 1, N = 18) or proximal (group 2, N = 20) coronary sinus before and after RF ablation. In group 3 (N = 21), peripheral venous blood was sampled before and after RF ablation and then up to postoperative day 7. In group 4 (N = 10), we sampled peripheral venous blood during diagnostic electrophysiologic study. The NGF concentration was determined by enzyme-linked immunosorbent assay. Transcardiac NGF concentration was the difference in NGF concentrations between coronary sinus and aorta.

RESULTS

There was no change in transcardiac NGF concentrations in groups 1 and 2. In group 3, the NGF level did not change significantly from before the procedure (17.10 +/- 15.80 ng/mL) to immediately after the procedure (14.46 +/- 10.36 ng/mL). However, NGF levels increased significantly to 31.24 +/- 19.82 ng/mL (N = 21, P <.0001) on postoperative day 1, 26.23 +/- 16.89 ng/mL (N = 20, P <.001) on postoperative day 2, and 22.01 +/- 11.35 ng/mL (N = 16, P = .003) on postoperative day 3. NGF concentrations did not change significantly in group 4.

CONCLUSION

RF ablation did not result in a detectable increase of transcardiac NGF concentration immediately after the procedure. However, the systemic NGF concentration increased significantly on postoperative days 1 to 3, suggesting that RF ablation resulted in increased NGF expression.

Mechanisms of the acute ischemia-induced arrhythmogenesis – A simulation study

The underlying ionic mechanisms of ischemic-induced arrhythmia were studied by the computer simulation method. To approximate the real situation, ischemic cells were simulated by considering the three major component conditions of acute ischemia (elevated extracellular K(+) concentration, acidosis and anoxia) at the level of ionic currents and ionic concentrations, and a round ischemic zone was introduced into a homogeneous healthy sheet to avoid sharp angle of the ischemic tissue. The constructed models were solved using the operator splitting and adaptive time step methods, and the perturbation finite difference (PFD) scheme was first used to integrate the partial differential equations (PDEs) in the model. The numerical experiments showed that the action potential durations (APDs) of ischemic cells did not exhibited rate adaptation characteristic, resulting in flattening of the APD restitution curve. With reduction of sodium channel availability and long recovery of excitability, refractory period of the ischemic tissue was significantly prolonged, and could no longer be considered as same as APD. Slope of the conduction velocity (CV) restitution curve increased both in normal and ischemic region when pacing cycle length (PCL) was short, and refractory period dispersion increased with shortening of PCL as well. Therefore, dynamic changes of CV and dispersion of refractory period rather than APD were suggested to be the fundamental mechanisms of arrhythmia in regional ischemic myocardium.

Sudden death secondary to cardiac arrhythmias: mechanisms and treatment strategies

PURPOSE OF REVIEW

This article reviews mechanisms and available therapeutic options for arrhythmias leading to sudden cardiac death in patients with coronary artery disease.

RECENT FINDINGS

Intensive efforts have led to a better understanding of the pathophysiology and various treatments of sudden cardiac death. Antiarrhythmic medications have not demonstrated a survival benefit. Beta-adrenergic blocking agents have been revalidated in recent studies to improve survival and reduce risk of sudden cardiac death in patients with myocardial infarction. Angiotensin-converting enzyme inhibitors and aldosterone antagonists should also be used in these patients. Data from randomized trials demonstrate significant survival benefit with an implantable cardioverter-defibrillator and indications have expanded. Patients with established ischemic cardiomyopathy do not require electrophysiologic studies for induction of tachyarrhythmias based on these trials. One recent trial did not demonstrate mortality reduction with implantable defibrillators in patients with recent myocardial infarction. Devices may not provide survival benefit in patients with advanced New York Heart Association class IV heart failure.

SUMMARY

The incidence of arrhythmia-related sudden death in the general population remains relatively high. Better risk stratification tools are needed to identify high-risk patients in the general population and in those with known coronary disease and to exclude low-risk patients.

(N-3) long-chain polyunsaturated fatty acids prolong survival following myocardial infarction in rats

Many clinical studies report that (n-3) PUFAs decrease the incidence of sudden death in patients with coronary artery disease after myocardial infarction (MI). However, the mechanisms for the beneficial effects of (n-3) PUFAs are unknown. The objectives of the present study were to confirm the findings from clinical trials using an animal model of MI in which dietary intake could be closely controlled and to utilize the model to investigate molecular mechanisms for the beneficial effects of (n-3) PUFAs. Male rats were subjected to coronary ligation to induce MI and were randomly assigned to diets high in (n-6) (58% of lipid) or (n-3) (28% of lipid) PUFAs for 6 mo. A diet high in (n-3) PUFAs was associated with an improvement in 6-mo survival (89.2% vs. 64.9%, P = 0.013) compared with rats consuming a diet high in (n-6) PUFAs (n = 37/group). In a separate study (n = 5 rats/diet group), the (n-3) PUFA diet decreased the (n-6):(n-3) PUFA ratio in plasma (0.6 +/- 0.1 vs. 7.9 +/- 1.8, P < 0.05) and cardiac tissue (0.9 +/- 0.1 vs. 11.8 +/- 1.6, P < 0.05) of rats fed for 4 wk. The increased survival in the (n-3) diet group was associated with decreased cardiac activities of protein kinase A and calcium calmodulin-dependent kinase II by 33-38% (P < 0.05) and a 28% decrease (P < 0.05) in phosphorylation (activation) of the ryanodine receptor calcium release channel. Based upon our results, we speculate that decreased activities of protein kinases induced by diets high in (n-3) PUFAs are associated with a decrease in sudden death after MI in rats.

Spatial distribution of nerve sprouting after myocardial infarction in mice

BACKGROUND

Myocardial infarction (MI) elicits nerve sprouting.

OBJECTIVES

The purpose of this study was to determine the spatial distribution of nerve sprouting and neurotrophic gene expression after MI.

METHODS

We created MI in mice by coronary artery ligation. The hearts were removed 3 hours to 2 months after MI and examined for nerve fiber density and neurotrophic factor gene expression using Affymetrix microarray and mRNA analyses.

RESULTS

The density of nerve fibers immunopositive for growth-associated protein (GAP)-43 was the highest 3 hours after MI both in the peri-infarct area and in the area remote to infarct, resulting in sympathetic (but not parasympathetic) hyperinnervation in the ventricles. The GAP-43-positive nerve fiber density of myocardium was greater in the outer transverse loop than in the inner vertical loop. The differences between these two myocardial loops peaked within 3 hours after MI and persisted for 2 months afterward. Gene expression of nerve growth factor, insulin-like growth factor, leukemia inhibitory factor, transforming growth factor-beta(3), and interleukin-1alpha was increased up to 2 months after MI compared with normal control. Expression of these growth factors was more pronounced and persistent in the peri-infarct area than in the remote area.

CONCLUSION

MI induces sympathetic nerve sprouting in both peri-infarct and remote areas, more in the outer transverse loop. Selective up-regulation of nerve growth factor, insulin-like growth factor, leukemia inhibitory factor, transforming growth factor-beta(3), and interleukin-1alpha occurred in the peri-infarct area and, to a lesser extent, in the remote area.

High-precision guidance of ablation catheters to arrhythmic sites using electrocardiographic signals

Hemodynamically unstable ventricular arrhythmias are frequently untreatable with radio-frequency ablation due to the difficulty of rapidly and accurately localizing the site of origin of an arrhythmia with current technologies. We demonstrate a new catheter guidance method that will direct the tip of an ablation catheter to the site of origin of an arrhythmia and reduce the time needed to locate the site such that a patient need only be maintained in the arrhythmia for a few beats. The algorithm, based on a single-equivalent moving dipole (SEMD) model, is used to identify the bioelectric dipole corresponding to a site of origin of an arrhythmia. If a current dipole is produced at the ablation catheter tip, the tip position may also be calculated using this algorithm, and the catheter can be guided towards the site of origin of the arrhythmia. We present a method to compensate for the effect of systematic non-idealities, such as boundary effects, on the accuracy of this algorithm. In simulations, this method is able to guide the catheter tip to within 1.5 mm of the arrhythmic site at any location within the model torso with almost 100% success and with a realistic number of movements of the ablation catheter. These results suggest that this method has great potential to direct radio-frequency ablation procedures, especially in the significant patient population that is currently untreatable.

Circadian variations of stellate ganglion nerve activity in ambulatory dogs

BACKGROUND

The presence of circadian variations in sympathetic outflow from the stellate ganglia is unclear.

OBJECTIVES

The purpose of this study was to continuously record stellate ganglion nerve activity (SGNA) in ambulatory dogs.

METHODS

We performed continuous 24-hour left (N = 3) or bilateral (N = 3) SGNA recordings in normal ambulatory dogs using implanted Data Sciences International transmitters. We also performed simultaneous ECG recording (n = 5) or simultaneous ECG and blood pressure recordings (n = 1).

RESULTS

The total duration of continuous ambulatory recording averaged 41.5 +/- 16.6 days. Five dogs had persistent stable recording, and one dog developed hardware malfunction in week 3. SGNA was followed immediately (<1 second) by heart rate and blood pressure elevation and a reduced standard deviation of consecutive activation cycle length (SDNN) from 236 +/- 93 ms to 121 +/- 51 ms (P = 0.007). Heart rate correlated significantly with SGNA. When there was a sudden increase of SGNA, the sudden increase occurred bilaterally in 90% of the episodes. Both heart rate and SGNA showed statistically significant (P <.01) circadian variation. Nadolol (20 mg/day for 5 days) reduced average heart rate from 99 +/- 8 bpm at baseline to 88 +/- 9 bpm (N = 6, P = .001) but did not significantly alter SGNA. Immunohistochemical staining of the stellate ganglia showed tyrosine hydroxylase-positive ganglion cells and nerves at the recording site.

CONCLUSION

There is a circadian variation in sympathetic outflow from canine stellate ganglia. Circadian variation of SGNA is an important cause of circadian variations of cardiac sympathetic tone.