Possible mechanisms of sudden cardiac death in top athletes: a basic cardiac electrophysiological point of view

Progranulin, sICAM-1, and sVCAM-1 May Predict an Increased Risk for Ventricular Arrhythmias in Patients with Systemic Sclerosis

In systemic sclerosis (SSc), fibrosis of the myocardium along with ongoing autoimmune inflammation can alter the electric function of the cardiac myocytes, which may increase the risk for ventricular arrhythmias and sudden cardiac death. We analyzed the electrocardiographic (ECG) variables describing ventricular repolarization such as QT interval, QT dispersion (QTd), T wave peak-to-end interval (Tpe), and arrhythmogeneity index (AIX) of 26 patients with SSc and 36 healthy controls. Furthermore, echocardiographic and laboratory parameters were examined, with a focus on inflammatory proteins like C-reactive ptotein (CRP), soluble intracellular adhesion molecule-1 (sICAM-1), soluble vascular adhesion molecule-1 (sVCAM-1), and progranulin (PGRN). The CRP, sICAM-1, and sVCAM-1 levels were positively correlated with the length of the QT interval. Although the serum PGRN levels were not increased in the SSc group compared to the controls, in SSc patients, the PGRN levels were positively correlated with the QT interval and the AIX. According to our results, we conclude that there may be a potential association between autoimmune inflammation and the risk for ventricular arrhythmias in patients with SSc. We emphasize that the measurement of laboratory parameters of inflammatory activity including CRP, PGRN, sVCAM-1, and sICAM-1 could be helpful in the prediction of sudden cardiac death in patients with SSc.

[Analysis of sudden cardiac death in competitive athletes, possibilities for reducing cases]

INTRODUCTION

Sudden cardiac death in athletes is rare (0.5 to 1 per 100 000 athlete years), but sudden cardiac death in known athletes causes general shock.

OBJECTIVE

Our research aim was to collect and study as many sudden cardiac death cases as possible, judge the role of stress and look for ways to reduce fatal tragedies.

METHOD

From registers and newspaper articles found on the Internet, we collected 360 (including 14 women) athletes' sudden cardiac death cases where the sport, age and place of death (during training/competition/after) could be determined. From these, a single database has been prepared in order of the year of death. The cases were grouped and analyzed by sports. Based on our results and literature data, we made recommendations to reduce fatalities.

RESULTS AND CONCLUSION

There were more sudden cardiac deaths in competitions than in trainings (239 vs. 99), but tragedies also happened during warm-ups and chess without physical exertion, furthermore, there was no sudden cardiac death in the stakeless training of marathon/half marathon/triathlon athletes; all these prove the role of stress, so we recommend a psychological conversation before a high-stakes race. There were also a lot of sudden cardiac deaths (79/360) during team sports trainings, so we recommend reanimation readiness there as well. After training/competition, sudden cardiac death happened mainly in sports requiring high static effort, where post-competition monitoring is also recommended. Those who died in training were younger than those who died during the race (p<0.01), so young people should be monitored more closely for medical and (under)fitness. Marathon runners and triathletes were older than team athletes (p<0.005) and only died in competition, so for them a basic examination and an ECG within 1 month before competition are recommended. Conclusions drawn from literature data: sports medicine examination should be standardized and documented in an accessible way; since resuscitation started earlier and professionally is more effective, all competitors should receive reanimation training. Orv Hetil. 2023; 164(29): 1155-1163.

Cardiac electrophysiological remodeling associated with enhanced arrhythmia susceptibility in a canine model of elite exercise

The health benefits of regular physical exercise are well known. Even so, there is increasing evidence that the exercise regimes of elite athletes can evoke cardiac arrhythmias including ventricular fibrillation and even sudden cardiac death (SCD). The mechanism of exercise-induced arrhythmia and SCD is poorly understood. Here, we show that chronic training in a canine model (12 sedentary and 12 trained dogs) that mimics the regime of elite athletes induces electrophysiological remodeling (measured by ECG, patch-clamp, and immunocytochemical techniques) resulting in increases of both the trigger and the substrate for ventricular arrhythmias. Thus, 4 months sustained training lengthened ventricular repolarization (QTc: 237.1±3.4 ms vs. 213.6±2.8 ms, n=12; APD90: 472.8±29.6 ms vs. 370.1±32.7 ms, n=29 vs. 25), decreased transient outward potassium current (6.4±0.5 pA/pF vs. 8.8±0.9 pA/pF at 50 mV, n=54 vs. 42), and increased the short-term variability of repolarization (29.5±3.8 ms vs. 17.5±4.0 ms, n=27 vs. 18). Left ventricular fibrosis and HCN4 protein expression were also enhanced. These changes were associated with enhanced ectopic activity (number of escape beats from 0/hr to 29.7±20.3/hr) in vivo and arrhythmia susceptibility (elicited ventricular fibrillation: 3 of 10 sedentary dogs vs. 6 of 10 trained dogs). Our findings provide in vivo, cellular electrophysiological and molecular biological evidence for the enhanced susceptibility to ventricular arrhythmia in an experimental large animal model of endurance training.

Endurance training-induced cardiac remodeling in a guinea pig athlete's heart model

Besides the health benefits of regular exercise, high-level training-above an optimal level-may have adverse effects. In this study, we investigated the effects of long-term vigorous training and its potentially detrimental structural-functional changes in a small animal athlete's heart model. Thirty-eight 4-month-old male guinea pigs were randomized into sedentary and exercised groups. The latter underwent a 15-week-long endurance-training program. To investigate the effects of the intense long-term exercise, in vivo (echocardiography, electrocardiography), ex vivo, and in vitro (histopathology, patch-clamp) measurements were performed. Following the training protocol, the exercised animals exhibited structural left ventricular enlargement and a significantly higher degree of myocardial fibrosis. Furthermore, resting bradycardia accompanied by elevated heart rate variability occurred, representing increased parasympathetic activity in the exercised hearts. The observed prolonged QTc intervals and increased repolarization variability parameters may raise the risk of electrical instability in exercised animals. Complex arrhythmias did not occur in either group, and there were no differences between the groups in ex vivo or cellular electrophysiological experiments. Accordingly, the high parasympathetic activity may promote impaired repolarization in conscious exercised animals. The detected structural-functional changes share similarities with the human athlete's heart; therefore, this model might be useful for investigations on cardiac remodeling.

Long-Term Endurance Exercise Training Alters Repolarization in a New Rabbit Athlete’s Heart Model

In the present study, the effect of long-term exercise training was investigated on myocardial morphological and functional remodeling and on proarrhythmic sensitivity in a rabbit athlete’s heart model. New-Zealand white rabbits were trained during a 12-week long treadmill running protocol and compared with their sedentary controls. At the end of the training protocol, echocardiography, in vivo and in vitro ECG recordings, proarrhythmic sensitivity with dofetilide (nM) were performed in isolated hearts, and action potential duration (APD) measurements at different potassium concentrations (4.5 and 2 mM) were made in the isolated papillary muscles. Expression levels of the slow component of delayed rectifier potassium current and fibrosis synthesis and degradation biomarkers were quantified. Echocardiography showed a significantly dilated left ventricle in the running rabbits. ECG PQ and RR intervals were significantly longer in the exercised group (79 ± 2 vs. 69 ± 2 ms and 325 ± 11 vs. 265 ± 6 ms, p &lt; 0.05, respectively). The in vivo heart rate variability (HRV) (SD of root mean square: 5.2 ± 1.4 ms vs. 1.4 ± 0.2 ms, p &lt; 0.05) and Tpeak-Tend variability were higher in the running rabbits. Bradycardia disappeared in the exercised group in vitro. Dofetilide tended to increase the QTc interval in a greater extent, and significantly increased the number of arrhythmic beats in the trained animals in vitro. APD was longer in the exercised group at a low potassium level. Real-time quantitative PCR (RT-qPCR) showed significantly greater messenger RNA expression of fibrotic biomarkers in the exercised group. Increased repolarization variability and higher arrhythmia incidences, lengthened APD at a low potassium level, increased fibrotic biomarker gene expressions may indicate higher sensitivity of the rabbit “athlete’s heart” to life-threatening arrhythmias.

Features of Metabolic Support of Physical Performance in Highly Trained Cross-Country Skiers of Different Qualifications during Physical Activity at Maximum Load

The purpose of our study was to identify the features of metabolic regulation in highly trained cross-country skiers of different qualifications at different stages of the maximum load test. We examined 124 highly trained cross-country skiers (male, ages 17–24). The group consisted of two subgroups based on their competition performance: 61 nonelite athletes (Group I) and 63 elite athletes (group II), who were current members of the national team of the Komi Republic and Russia. The bicycle ergometer test was performed by using the OxyconPro system (Erich Jaeger, Hoechberg, Germany). All the examined athletes performed the exercise test on a cycle ergometer “until exhaustion”. The results of our research indicate that the studied groups of athletes with high, but different levels of sports qualifications are a convenient model for studying the molecular mechanisms of adaptation to physical loads of maximum intensity. Athletes of higher qualifications reveal additional adaptive mechanisms of metabolic regulation, which is manifested in the independence of serum lactate indicators under conditions of submaximal and maximum power from maximal oxygen uptake, and they have an NO-dependent mechanism for regulating lactate levels during aerobic exercise, including work at the anaerobic threshold.

Arrhythmogenic Remodeling in the Failing Heart

Chronic heart failure is a clinical syndrome with multiple etiologies, associated with significant morbidity and mortality. Cardiac arrhythmias, including ventricular tachyarrhythmias and atrial fibrillation, are common in heart failure. A number of cardiac diseases including heart failure alter the expression and regulation of ion channels and transporters leading to arrhythmogenic electrical remodeling. Myocardial hypertrophy, fibrosis and scar formation are key elements of arrhythmogenic structural remodeling in heart failure. In this article, the mechanisms responsible for increased arrhythmia susceptibility as well as the underlying changes in ion channel, transporter expression and function as well as alterations in calcium handling in heart failure are discussed. Understanding the mechanisms of arrhythmogenic remodeling is key to improving arrhythmia management and the prevention of sudden cardiac death in patients with heart failure.

Sudden cardiac death in sports: could we save Pheidippides?

Hereditary diseases under the age of 35 are the most common underlying heart disease, leading to sudden cardiac death (SCD) in competitive sports, while in older people, atherosclerotic coronary artery disease (CAD) is the main cause. The following preventive measures are recommended: (a) The pre-participation cardiovascular screening, (b) the genetic testing, (c) the use of implantable cardioverter-defibrillator (ICD), (d) the prohibition of doping in sports, (e) the prevention of 'exercise-induced' cardiac complications, (f) the reduction of high-risk factors for CAD, and (g) the use of cardiopulmonary resuscitation. The cost-effectiveness of the electrocardiograms in the pre-participation screening programs remains questionable. Genetic testing is recommended in borderline cases and positive family history. Athletes with ICD can, under certain conditions, participate in competitive sports. Excessive endurance exercise appears to harm the endothelium, promotes inflammatory processes and leads to fibrosis in the myocardium, and calcium deposition in the coronary vessels. Cardiac arrest may be reversed if cardiopulmonary resuscitation is performed and a defibrillator is immediately used. Thus, equipping all fields with automatic external defibrillators are recommended.

Electrophysiological Effects of the Transient Receptor Potential Melastatin 4 Channel Inhibitor (4-Chloro-2-(2-chlorophenoxy)acetamido) Benzoic Acid (CBA) in Canine Left Ventricular Cardiomyocytes

Transient receptor potential melastatin 4 (TRPM4) plays an important role in many tissues, including pacemaker and conductive tissues of the heart, but much less is known about its electrophysiological role in ventricular myocytes. Our earlier results showed the lack of selectivity of 9-phenanthrol, so CBA ((4-chloro-2-(2-chlorophenoxy)acetamido) benzoic acid) was chosen as a new, potentially selective inhibitor. Goal: Our aim was to elucidate the effect and selectivity of CBA in canine left ventricular cardiomyocytes and to study the expression of TRPM4 in the canine heart. Experiments were carried out in enzymatically isolated canine left ventricular cardiomyocytes. Ionic currents were recorded with an action potential (AP) voltage-clamp technique in whole-cell configuration at 37 °C. An amount of 10 mM BAPTA was used in the pipette solution to exclude the potential activation of TRPM4 channels. AP was recorded with conventional sharp microelectrodes. CBA was used in 10 µM concentrations. Expression of TRPM4 protein in the heart was studied by Western blot. TRPM4 protein was expressed in the wall of all four chambers of the canine heart as well as in samples prepared from isolated left ventricular cells. CBA induced an approximately 9% reduction in AP duration measured at 75% and 90% of repolarization and decreased the short-term variability of APD₉₀. Moreover, AP amplitude was increased and the maximal rates of phase 0 and 1 were reduced by the drug. In AP clamp measurements, CBA-sensitive current contained a short, early outward and mainly a long, inward current. Transient outward potassium current (I_to) and late sodium current (I_Na,L) were reduced by approximately 20% and 47%, respectively, in the presence of CBA, while L-type calcium and inward rectifier potassium currents were not affected. These effects of CBA were largely reversible upon washout. Based on our results, the CBA induced reduction of phase-1 slope and the slight increase of AP amplitude could have been due to the inhibition of I_to. The tendency for AP shortening can be explained by the inhibition of inward currents seen in AP-clamp recordings during the plateau phase. This inward current reduced by CBA is possibly I_Na,L, therefore, CBA is not entirely selective for TRPM4 channels. As a consequence, similarly to 9-phenanthrol, it cannot be used to test the contribution of TRPM4 channels to cardiac electrophysiology in ventricular cells, or at least caution must be applied.

Muscarinic agonists inhibit the ATP-dependent potassium current and suppress the ventricle-Purkinje action potential dispersion

Activation of the parasympathetic nervous system has been reported to have an antiarrhythmic role during ischemia-reperfusion injury by decreasing the arrhythmia triggers. Furthermore, it was reported that the parasympathetic neurotransmitter acetylcholine is able to modulate the ATP-dependent potassium current (I K-ATP), a crucial current activated during hypoxia. However, the possible significance of this current modulation in the antiarrhythmic mechanism is not fully clarified. Action potentials were measured using the conventional microelectrode technique from canine left ventricular papillary muscle and free-running Purkinje fibers, under normal and hypoxic conditions. Ionic currents were measured using the whole-cell configuration of the patch-clamp method. Acetylcholine at 5 μmol/L did not influence the action potential duration (APD) either in Purkinje fibers or in papillary muscle preparations. In contrast, it significantly lengthened the APD and suppressed the Purkinje-ventricle APD dispersion when it was administered after 5 μmol/L pinacidil application. Carbachol at 3 μmol/L reduced the pinacidil-activated I K-ATP under voltage-clamp conditions. Acetylcholine lengthened the ventricular action potential under simulated ischemia condition. In this study, we found that acetylcholine inhibits the I K-ATP and thus suppresses the ventricle-Purkinje APD dispersion. We conclude that parasympathetic tone may reduce the arrhythmogenic substrate exerting a complex antiarrhythmic mechanism during hypoxic conditions.

Increased Ca2+ content of the sarcoplasmic reticulum provides arrhythmogenic trigger source in swimming-induced rat athlete's heart model

Sudden cardiac death among top athletes is very rare, however, it is 2–4 times more frequent than in the age-matched control population. In the present study, the electrophysiological consequences of long-term exercise training were investigated on Ca²⁺ homeostasis and ventricular repolarization, together with the underlying alterations of ion channel expression, in a rat athlete's heart model. 12-week swimming exercise-trained and control Wistar rats were used. Electrophysiological data were obtained by using ECG, patch clamp and fluorescent optical measurements. Protein and mRNA levels were determined by the Western immunoblot and qRT-PCR techniques. Animals in the trained group exhibited significantly lower resting heart rate, higher incidence of extrasystoles and spontaneous Ca²⁺ release events. The Ca²⁺ content of the sarcoplasmic reticulum (SR) and the Ca²⁺ transient amplitude were significantly larger in the trained group. Intensive physical training is associated with elevated SR Ca²⁺ content, which could be an important part of physiological cardiac adaptation mechanism to training. However, it may also sensitize the heart for the development of spontaneous Ca²⁺ release and extrasystoles. Training-associated remodeling may promote elevated incidence of life threatening arrhythmias in top athletes.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Combined effects of exercise training and high doses of anabolic steroids on cardiac autonomic modulation and ventricular repolarization properties in rats

Several studies have reported that high doses of synthetic anabolic androgenic steroids (AAS) can have serious negative effects on health, including the cardiovascular system. The aim of this study was to evaluate the combined effects of AAS and exercise training on ventricular repolarization and cardiac autonomic modulation in rats. Male Wistar rats were allocated into 4 groups: sedentary rats treated with vehicle, sedentary rats treated with nandrolone decanoate, swimming-trained rats treated with vehicle, and swimming-trained rats treated with nandrolone decanoate. Ventricular repolarization was evaluated by electrocardiographic analysis of QT interval and QT dispersion. Cardiac autonomic modulation was assessed by heart rate variability. Our results show that AAS increased QT interval and QT dispersion in sedentary rats treated with nandrolone decanoate as compared to sedentary rats treated with vehicle, indicating AAS-induced ventricular repolarization abnormalities. When rats treated with nandrolone decanoate were subjected to concomitant exercise training, ventricular repolarization was normalized. On the other hand, AAS-induced reduction in cardiac parasympathetic modulation was not prevented by exercise training. In conclusion, AAS produced cardiac autonomic dysfunction and ventricular repolarization disturbances in rats. Combining an exercise training protocol during the AAS treatment attenuated the ventricular repolarization abnormalities and did not prevent cardiac autonomic dysfunction.

Generalized Poincaré plots analysis of heart period dynamics in different physiological conditions: Trained vs. untrained men

Background

Recently we proposed a new method called generalized Poincaré plot (gPp) analysis which gave a new insight into the pattern of neurocaridac control. In this study we examined potential of gPp method to reveal changes in cardiac neural control in young athletes during three conditions: supine rest, running and relaxation, with respect to untrained subjects.

Methods

This method is based on the quantification of Pearson’s correlation coefficients r(j, k), between symmetrical (j = k) and asymmetrical summed j previous and k following RR intervals up to the 100th order (j,k≤100).

Results

Differences between groups were obtained at all levels of this analysis. The main result is the significant difference of NAI, normalized index of asymmetry, between groups in running, which was originated in different positions of local maxima of r(j, k). Compared with untrained subjects, these findings indicate modified neural control and altered intrinsic heart rate behavior in athletes which are related to some kind of memory mechanism between RR intervals.

Conclusion

Obtained results provide great potential of gPp method analysis in the recognition of changes in neurocardiac control in healthy subjects. Further studies are needed for identification of altered cardiac regulatory mechanisms whose background may be useful in the evaluation of genesis of athletes neurocardiovascular pathology.

Altered K+ current profiles underlie cardiac action potential shortening in hyperkalemia and β-adrenergic stimulation

Hyperkalemia is known to develop in various conditions including vigorous physical exercise. In the heart, hyperkalemia is associated with action potential (AP) shortening that was attributed to altered gating of K⁺ channels. However, it remains unknown how hyperkalemia changes the profiles of each K⁺ current under a cardiac AP. Therefore, we recorded the major K⁺ currents (inward rectifier K⁺ current, I_K1; rapid and slow delayed rectifier K⁺ currents, I_Kr and I_Ks, respectively) using AP-clamp in rabbit ventricular myocytes. As K⁺ may accumulate at rapid heart rates during sympathetic stimulation, we also examined the effect of isoproterenol on these K⁺ currents. We found that I_K1 was significantly increased in hyperkalemia, whereas the reduction of driving force for K⁺ efflux dominated over the altered channel gating in case of I_Kr and I_Ks. Overall, the markedly increased I_K1 in hyperkalemia overcame the relative decreases of I_Kr and I_Ks during AP, resulting in an increased net repolarizing current during AP phase 3. β-Adrenergic stimulation of I_Ks also provided further repolarizing power during sympathetic activation, although hyperkalemia limited I_Ks upregulation. These results indicate that facilitation of I_K1 in hyperkalemia and β-adrenergic stimulation of I_Ks represent important compensatory mechanisms against AP prolongation and arrhythmia susceptibility.

The impact of a 21-day ultra-endurance ride on the heart in young, adult and older adult recreational cyclists

BACKGROUND

This study assessed the acute effect of 21 days of challenging exercise on heart structure and function in recreationally active people across a range of age categories.

METHODS

15 recreationally active people completed a 21-day fundraising cycling ride (MADRIDE) over a distance of 3515 km. Twenty-four hour Holter electrocardiography and blood biochemistry analyses were performed before and after the MADRIDE.

RESULTS

Incidence of cardiac arrhythmia was higher after MADRIDE (OR: 5.93; 95% CI: 5.68-6.19), with increases in both ventricular arrhythmias (OR: 9.90; 95% CI: 9.27-10.57) and supraventricular arrhythmias (OR: 3.09; 95% CI: 2.91-3.29). Adults (OR: 11.45; 95% CI: 7.41-17.69) and older adults (OR: 10.42 95% CI 9.83-11.05) were approximately 10 times more likely to experience arrhythmias after the MADRIDE. Whereas, young participants experienced 18% less cardiac arrhythmias after MADRIDE (OR: 0.82; 95% CI: 0.75-0.90). Aortic valve max velocity was reduced (MD: -0.12 m/s; 95% CI: -0.19-0.05 m/s) and mitral valve deceleration time was slower (MD: -28.91 m/s; 95% CI: -50.97-6.84 m/s) after MADRIDE. Other structural and functional characteristics along with heart rate variability were not different after MADRIDE.

CONCLUSIONS

Multi-day challenging exercise increased the incidence of both supraventricular and ventricular arrhythmias in active adults and older adults. Increases in arrhythmia rates after MADRIDE occurred without changes in cardiac structure and autonomic control. Further exploration is necessary to identify the causes of exercise-induced cardiac arrhythmia in adult and older adults.

Definition of hidden drug cardiotoxicity: paradigm change in cardiac safety testing and its clinical implications

Unexpected cardiac adverse effects are the leading causes of discontinuation of clinical trials and withdrawal of drugs from the market. Since the original observations in the mid-90s, it has been well established that cardiovascular risk factors and comorbidities (such as ageing, hyperlipidaemia, and diabetes) and their medications (e.g. nitrate tolerance, adenosine triphosphate-dependent potassium inhibitor antidiabetic drugs, statins, etc.) may interfere with cardiac ischaemic tolerance and endogenous cardioprotective signalling pathways. Indeed drugs may exert unwanted effects on the diseased and treated heart that is hidden in the healthy myocardium. Hidden cardiotoxic effects may be due to (i) drug-induced enhancement of deleterious signalling due to ischaemia/reperfusion injury and/or the presence of risk factors and/or (ii) inhibition of cardioprotective survival signalling pathways, both of which may lead to ischaemia-related cell death and/or pro-arrhythmic effects. This led to a novel concept of ‘hidden cardiotoxicity’, defined as cardiotoxity of a drug that manifests only in the diseased heart with e.g. ischaemia/reperfusion injury and/or in the presence of its major comorbidities. Little is known on the mechanism of hidden cardiotoxocity, moreover, hidden cardiotoxicity cannot be revealed by the routinely used non-clinical cardiac safety testing methods on healthy animals or tissues. Therefore, here, we emphasize the need for development of novel cardiac safety testing platform involving combined experimental models of cardiac diseases (especially myocardial ischaemia/reperfusion and ischaemic conditioning) in the presence and absence of major cardiovascular comorbidities and/or cotreatments.

Comprehensive review of cardiovascular toxicity of drugs and related agents

Cardiovascular diseases are a leading cause of morbidity and mortality in most developed countries of the world. Pharmaceuticals, illicit drugs, and toxins can significantly contribute to the overall cardiovascular burden and thus deserve attention. The present article is a systematic overview of drugs that may induce distinct cardiovascular toxicity. The compounds are classified into agents that have significant effects on the heart, blood vessels, or both. The mechanism(s) of toxic action are discussed and treatment modalities are briefly mentioned in relevant cases. Due to the large number of clinically relevant compounds discussed, this article could be of interest to a broad audience including pharmacologists and toxicologists, pharmacists, physicians, and medicinal chemists. Particular emphasis is given to clinically relevant topics including the cardiovascular toxicity of illicit sympathomimetic drugs (e.g., cocaine, amphetamines, cathinones), drugs that prolong the QT interval, antidysrhythmic drugs, digoxin and other cardioactive steroids, beta-blockers, calcium channel blockers, female hormones, nonsteroidal anti-inflammatory, and anticancer compounds encompassing anthracyclines and novel targeted therapy interfering with the HER2 or the vascular endothelial growth factor pathway.

Beat-to-beat variability of cardiac action potential duration: underlying mechanism and clinical implications

Beat-to-beat variability of cardiac action potential duration (short-term variability, SV) is a common feature of various cardiac preparations, including the human heart. Although it is believed to be one of the best arrhythmia predictors, the underlying mechanisms are not fully understood at present. The magnitude of SV is basically determined by the intensity of cell-to-cell coupling in multicellular preparations and by the duration of the action potential (APD). To compensate for the APD-dependent nature of SV, the concept of relative SV (RSV) has been introduced by normalizing the changes of SV to the concomitant changes in APD. RSV is reduced by I_Ca, I_Kr, and I_Ks while increased by I_Na, suggesting that ion currents involved in the negative feedback regulation of APD tend to keep RSV at a low level. RSV is also influenced by intracellular calcium concentration and tissue redox potential. The clinical implications of APD variability is discussed in detail.

Ca2+-activated Cl- current is antiarrhythmic by reducing both spatial and temporal heterogeneity of cardiac repolarization

The role of Ca²⁺-activated Cl^- current (I_Cl(Ca)) in cardiac arrhythmias is still controversial. It can generate delayed afterdepolarizations in Ca²⁺-overloaded cells while in other studies incidence of early afterdepolarization (EAD) was reduced by I_Cl(Ca). Therefore our goal was to examine the role of I_Cl(Ca) in spatial and temporal heterogeneity of cardiac repolarization and EAD formation. Experiments were performed on isolated canine cardiomyocytes originating from various regions of the left ventricle; subepicardial, midmyocardial and subendocardial cells, as well as apical and basal cells of the midmyocardium. I_Cl(Ca) was blocked by 0.5mmol/L 9-anthracene carboxylic acid (9-AC). Action potential (AP) changes were tested with sharp microelectrode recording. Whole-cell 9-AC-sensitive current was measured with either square pulse voltage-clamp or AP voltage-clamp (APVC). Protein expression of TMEM16A and Bestrophin-3, ion channel proteins mediating I_Cl(Ca), was detected by Western blot. 9-AC reduced phase-1 repolarization in every tested cell. 9-AC also increased AP duration in a reverse rate-dependent manner in all cell types except for subepicardial cells. Neither I_Cl(Ca) density recorded with square pulses nor the normalized expressions of TMEM16A and Bestrophin-3 proteins differed significantly among the examined groups of cells. The early outward component of I_Cl(Ca) was significantly larger in subepicardial than in subendocardial cells in APVC setting. Applying a typical subepicardial AP as a command pulse resulted in a significantly larger early outward component in both subepicardial and subendocardial cells, compared to experiments when a typical subendocardial AP was applied. Inhibiting I_Cl(Ca) by 9-AC generated EADs at low stimulation rates and their incidence increased upon beta-adrenergic stimulation. 9-AC increased the short-term variability of repolarization also. We suggest a protective role for I_Cl(Ca) against risk of arrhythmias by reducing spatial and temporal heterogeneity of cardiac repolarization and EAD formation.

Hypothesised mechanisms of swimming-related death: a systematic review

BACKGROUND

Recent reports from triathlon and competitive open-water swimming indicate that these events have higher rates of death compared with other forms of endurance sport. The potential causal mechanism for swimming-related death is unclear.

OBJECTIVE

To examine available studies on the hypothesised mechanisms of swimming-related death to determine the most likely aetiologies.

MATERIAL AND METHODS

MEDLINE, EMBASE and the Cochrane Database of Systematic Reviews (1950 to present) were searched, yielding 1950 potential results, which after title and citation reviews were reduced to 83 possible reports. Studies included discussed mechanisms of death during swimming in humans, and were Level 4 evidence or higher.

RESULTS

A total of 17 studies (366 total swimmers) were included for further analysis: 5 investigating hyperthermia/hypothermia, 7 examining cardiac mechanisms and responses, and 5 determining the presence of pulmonary edema. The studies provide inconsistent and limited-quality or disease-oriented evidence that make definitive conclusions difficult.

CONCLUSIONS

The available evidence is limited but may suggest that cardiac arrhythmias are the most likely aetiology of swimming-related death. While symptoms of pulmonary edema may occur during swimming, current evidence does not support swimming-induced pulmonary edema as a frequent cause of swimming-related death, nor is there evidence to link hypothermia or hyperthermia as a causal mechanism. Further higher level studies are needed.

Increased Short-Term Beat-To-Beat Variability of QT Interval in Patients with Acromegaly

Cardiovascular diseases, including ventricular arrhythmias are responsible for increased mortality in patients with acromegaly. Acromegaly may cause repolarization abnormalities such as QT prolongation and impairment of repolarization reserve enhancing liability to arrhythmia. The aim of this study was to determine the short-term beat-to-beat QT variability in patients with acromegaly. Thirty acromegalic patients (23 women and 7 men, mean age±SD: 55.7±10.4 years) were compared with age- and sex-matched volunteers (mean age 51.3±7.6 years). Cardiac repolarization parameters including frequency corrected QT interval, PQ and QRS intervals, duration of terminal part of T waves (T_peak-T_end) and short-term variability of QT interval were evaluated. All acromegalic patients and controls underwent transthoracic echocardiographic examination. Autonomic function was assessed by means of five standard cardiovascular reflex tests. Comparison of the two groups revealed no significant differences in the conventional ECG parameters of repolarization (QT: 401.1±30.6 ms vs 389.3±16.5 ms, corrected QT interval: 430.1±18.6 ms vs 425.6±17.3 ms, QT dispersion: 38.2±13.2 ms vs 36.6±10.2 ms; acromegaly vs control, respectively). However, short-term beat-to-beat QT variability was significantly increased in acromegalic patients (4.23±1.03 ms vs 3.02±0.80, P<0.0001). There were significant differences between the two groups in the echocardiographic dimensions (left ventricular end diastolic diameter: 52.6±5.4 mm vs 48.0±3.9 mm, left ventricular end systolic diameter: 32.3±5.2 mm vs 29.1±4.4 mm, interventricular septum: 11.1±2.2 mm vs 8.8±0.7 mm, posterior wall of left ventricle: 10.8±1.4 mm vs 8.9±0.7 mm, P<0.05, respectively). Short-term beat-to-beat QT variability was elevated in patients with acromegaly in spite of unchanged conventional parameters of ventricular repolarization. This enhanced temporal QT variability may be an early indicator of increased liability to arrhythmia.

Arrhythmias in athletes: evidence-based strategies and challenges for diagnosis, management, and sports eligibility

Assessment and management of cardiac rhythm disorders in athletes is particularly challenging. An accurate diagnosis and optimal risk-stratification are often limited because of substantial phenotypic overlap between pathological entities and adaptive cardiovascular responses that normally occur in athletes. An accurate diagnosis, however, is particularly important in this population, as 2 competing risks need to be cautiously balanced: the risk of under-diagnosis of an arrhythmogenic substrate that may trigger life-threatening events versus the risk of over-diagnosis that may result in an athlete's improper disqualification. Accordingly, the management of arrhythmias in athletes may pose therapeutic dilemmas, and often differs substantially compared with the general population. In this review, we present the most frequently observed arrhythmias in athletes and briefly discuss their pathophysiologic substrate. We further propose diagnostic and therapeutic strategies based upon current guidelines, official recommendations, and emerging evidence from relevant clinical investigations. We focus particularly on disparities in current guidelines regarding the management of certain rhythm disorders, as these areas of uncertainty may reflect the challenging nature of these disorders and may indicate the need for individualized approaches in every-day clinical practice. A better understanding of the normal electrophysiological responses to chronic exercise, and of the pathophysiological basis and the true clinical significance of arrhythmias in athletes, may enhance decision-making, and may allow for management strategies which more prudently weigh the risk-to-benefit ratio of each approach.

Noninvasive cardiac screening in young athletes with ventricular arrhythmias

The aim of this study was to analyze using noninvasive cardiac examinations a series of young athletes discovered to have ventricular arrhythmias (VAs) during the preparticipation screening program for competitive sports. One hundred forty-five athletes (mean age 17 ± 5 years) were evaluated. The study protocol included electrocardiography (ECG), exercise testing, 2-dimensional and Doppler echocardiography, 24-hour Holter monitoring, signal-averaged ECG, and in selected cases contrast-enhanced cardiac magnetic resonance imaging. Results of ECG were normal in most athletes (85%). VAs were initially detected prevalently during exercise testing (85%) and in the remaining cases on ECG and Holter monitoring. Premature ventricular complexes disappeared during exercise in 56% of subjects. Premature ventricular complexes during Holter monitoring averaged 4,700 per day, predominantly monomorphic (88%), single, and/or in couplets (79%). The most important echocardiographic findings were mitral valve prolapse in 29 patients (20%), congenital heart disease in 4 (3%), and right ventricular regional kinetic abnormalities in 5 (3.5%). On cardiac magnetic resonance imaging, right ventricular regional kinetic abnormalities were detected in 9 of 30 athletes and were diagnostic of arrhythmogenic right ventricular cardiomyopathy in only 1 athlete. Overall, 30% of athletes were judged to have potentially dangerous VAs. In asymptomatic athletes with prevalently normal ECG, most VAs can be identified by adding an exercise test during preparticipation screening. In conclusion, cardiac screening with noninvasive examinations remains a fundamental tool for the identification of a possible pathologic substrate and for the characterization of electrical instability.

Diclofenac prolongs repolarization in ventricular muscle with impaired repolarization reserve

BACKGROUND

The aim of the present work was to characterize the electrophysiological effects of the non-steroidal anti-inflammatory drug diclofenac and to study the possible proarrhythmic potency of the drug in ventricular muscle.

METHODS

Ion currents were recorded using voltage clamp technique in canine single ventricular cells and action potentials were obtained from canine ventricular preparations using microelectrodes. The proarrhythmic potency of the drug was investigated in an anaesthetized rabbit proarrhythmia model.

RESULTS

Action potentials were slightly lengthened in ventricular muscle but were shortened in Purkinje fibers by diclofenac (20 µM). The maximum upstroke velocity was decreased in both preparations. Larger repolarization prolongation was observed when repolarization reserve was impaired by previous BaCl(2) application. Diclofenac (3 mg/kg) did not prolong while dofetilide (25 µg/kg) significantly lengthened the QT(c) interval in anaesthetized rabbits. The addition of diclofenac following reduction of repolarization reserve by dofetilide further prolonged QT(c). Diclofenac alone did not induce Torsades de Pointes ventricular tachycardia (TdP) while TdP incidence following dofetilide was 20%. However, the combination of diclofenac and dofetilide significantly increased TdP incidence (62%). In single ventricular cells diclofenac (30 µM) decreased the amplitude of rapid (I(Kr)) and slow (I(Ks)) delayed rectifier currents thereby attenuating repolarization reserve. L-type calcium current (I(Ca)) was slightly diminished, but the transient outward (I(to)) and inward rectifier (I(K1)) potassium currents were not influenced.

CONCLUSIONS

Diclofenac at therapeutic concentrations and even at high dose does not prolong repolarization markedly and does not increase the risk of arrhythmia in normal heart. However, high dose diclofenac treatment may lengthen repolarization and enhance proarrhythmic risk in hearts with reduced repolarization reserve.

[Molecular and genetic background of sudden cardiac death]

Despite recent findings on the functional, structural and genetic background of sudden cardiac death, the incidence is still relatively high in the entire population. A thorough knowledge on susceptibility, as well as pathophysiology behind the development of malignant arrhythmias will help us to identify individuals at risk and prevent sudden cardiac death. This article presents a review of the current literature on the role of altered intracellular Ca2+ handling, acute myocardial ischaemia, cardiac autonomic innervation, renin-angiotensin-aldosterone system, monogenic and complex heritability in the pathogenesis of sudden cardiac death.

Sudden cardiac death in low- and middle-income countries

Cardiovascular disease, and the incidence of sudden cardiac death (SCD), will increase significantly in low- and middle-income countries (LMIC). Thus, SCD threatens to become a global public health problem. We present a summary of the current research that has investigated the epidemiology of SCD in LMIC. Few studies of SCD in LMIC exist, and they are of variable methodological quality. Risk factors for SCD are described, taking into account recent global burden of disease and risk factor statistics. We describe 1 proposal for a community-based, prospective, multiple-source methodology for SCD monitoring and surveillance that can be implemented in LMIC. Further research into the epidemiology of SCD in LMIC, using standardized methodology, would allow investigators and policy makers to determine the regions, communities, and individuals most at need for SCD prevention. Focusing on SCD and its prevention in LMIC should be a priority for the global health community.

Cardiac ventricular repolarization reserve: a principle for understanding drug-related proarrhythmic risk

Cardiac repolarization abnormalities can be caused by a wide range of cardiac and non-cardiac compounds and may lead to the development of life-threatening Torsades de Pointes (TdP) ventricular arrhythmias. Drug-induced torsades de pointes is associated with unexpected and unexplained sudden cardiac deaths resulting in the withdrawal of several compounds in the past. To better understand the mechanism of such unexpected sudden cardiac deaths, the concept of repolarization reserve has recently emerged. According to this concept, pharmacological, congenital or acquired impairment of one type of transmembrane ion channel does not necessarily result in excessive repolarization changes because other repolarizing currents can take over and compensate. In this review, the major factors contributing to repolarization reserve are discussed in the context of their clinical significance in physiological and pathophysiological conditions including drug administration, genetic defects, heart failure, diabetes mellitus, gender, renal failure, hypokalaemia, hypothyroidism and athletes' sudden deaths. In addition, pharmacological support of repolarization reserve as a possible therapeutic option is discussed. Some methods for the quantitative estimation of repolarization reserve are also recommended. It is concluded that repolarization reserve should be considered by safety pharmacologists to better understand, predict and prevent previously unexplained drug-induced sudden cardiac deaths.

Is the 'athlete's heart' arrhythmogenic? Implications for sudden cardiac death

Whether the ventricular hypertrophic response to athletic training can predispose to fatal ventricular dysrhythmias via mechanisms similar to that of pathological hypertrophy is controversial. This review examines current information regarding the metabolic and electrophysiological differences between the myocardial hypertrophy of heart disease and that associated with athletic training. In animal studies, the biochemical and metabolic profile of physiological hypertrophy from exercise training can largely be differentiated from that of pathological hypertrophy, but it is not clear if the former might represent an early stage in the spectrum of the latter. Information as to whether the electrical remodelling of the athlete's heart mimics that of patients with heart disease, and therefore serves as a substrate for ventricular dysrhythmias, is conflicting. If ventricular remodelling associated with athletic training can trigger fatal dysrhythmias, such cases are extraordinarily rare and thereby impossible to investigate by any standard experimental approach. Greater insight into this issue may come from a better understanding of the electrical responses to both acute bouts of exercise and chronic training in young athletes.

Increased short-term variability of the QT interval in professional soccer players: possible implications for arrhythmia prediction

Background

Sudden cardiac death in competitive athletes is rare but it is significantly more frequent than in the normal population. The exact cause is seldom established and is mostly attributed to ventricular fibrillation. Myocardial hypertrophy and slow heart rate, both characteristic changes in top athletes in response to physical conditioning, could be associated with increased propensity for ventricular arrhythmias. We investigated conventional ECG parameters and temporal short-term beat-to-beat variability of repolarization (STV_QT), a presumptive novel parameter for arrhythmia prediction, in professional soccer players.

Methods

Five-minute 12-lead electrocardiograms were recorded from professional soccer players (n = 76, all males, age 22.0±0.61 years) and age-matched healthy volunteers who do not participate in competitive sports (n = 76, all males, age 22.0±0.54 years). The ECGs were digitized and evaluated off-line. The temporal instability of beat-to-beat heart rate and repolarization were characterized by the calculation of short-term variability of the RR and QT intervals.

Results

Heart rate was significantly lower in professional soccer players at rest (61±1.2 vs. 72±1.5/min in controls). The QT interval was prolonged in players at rest (419±3.1 vs. 390±3.6 in controls, p<0.001). QTc was significantly longer in players compared to controls calculated with Fridericia and Hodges correction formulas. Importantly, STV_QT was significantly higher in players both at rest and immediately after the game compared to controls (4.8±0.14 and 4.3±0.14 vs. 3.5±0.10 ms, both p<0.001, respectively).

Conclusions

STV_QT is significantly higher in professional soccer players compared to age-matched controls, however, further studies are needed to relate this finding to increased arrhythmia propensity in this population.

Sudden cardiac death athletes: a systematic review

Previous events evidence that sudden cardiac death (SCD) in athletes is still a reality and it keeps challenging cardiologists. Considering the importance of SCD in athletes and the requisite for an update of this matter, we endeavored to describe SCD in athletes. The Medline (via PubMed) and SciELO databases were searched using the subject keywords "sudden death, athletes and mortality". The incidence of SCD is expected at one case for each 200,000 young athletes per year. Overall it is resulted of complex dealings of factors such as arrhythmogenic substrate, regulator and triggers factors. In great part of deaths caused by heart disease in athletes younger than 35 years old investigations evidence cardiac congenital abnormalities. Athletes above 35 years old possibly die due to impairments of coronary heart disease, frequently caused by atherosclerosis. Myocardial ischemia and myocardial infarction are responsible for the most cases of SCD above this age (80%). Pre-participatory athletes' evaluation helps to recognize situations that may put the athlete's life in risk including cardiovascular diseases. In summary, cardiologic examinations of athletes' pre-competition routine is an important way to minimize the risk of SCD.