Variety Of Arrhythmias Research Articles

Spatiotemporal Transitions in Cardiac Neuronal Co-Cultures

A variety of arrhythmias have been attributed to formation of large scale patterns of excitation such as formation and break-up of spiral waves. We investigate the relationship between neuronal activity and evolution of macroscopic spatiotemporal dynamics. We also aim to understand the role of target-derived trophic nerve growth factors (NGF) on the formation and function of synapses between sympathetic neurons and heart tissue.Hearts were isolated from neonatal rat pups, ventricular myocytes were isolated, cultured and after 24h, stellate ganglia were isolated from litter mates enzymatically digested, and neurons were plated on top on myocyte dishes. We used dye-free imaging techniques using to record wave pattern and beat-rate (in the presence/absence of NGF).For the first time, we report wave speeds in cardiac neuron co-cultures. Our results show that (i) basal speed of co-cultures is significantly faster than myocyte only cultures (p<0.001); (ii) nicotine causes an altered behaviour in wave patterns; (iii) application of nicotine to co-culture causes a transient increase in contraction rate; (iv) the spatial organization of cells in myocyte monocultures is significantly altered by the addition of NGF.Our observations may be relevant to understanding how neuronal activity modulates macroscopic cardiac function. The finding that NGF modulates the development and patterning of myocytes raises the question of how NGF and their receptors interact to lead to long term changes in cardiac and synaptic function.

H558R polymorphism in SCN5A is associated with Keshan disease and QRS prolongation in Keshan disease patients.

Keshan disease (KSD), a potentially fatal cardiomyopathy, has very high incidence in some selenium-poor regions of China. KSD may be accompanied with a variety of arrhythmia, which is associated with mutations in the gene coding for cardiac voltage-gated sodium channel (SCN5A). The molecular mechanism of KSD is still largely obscure. We aimed to determine the association between the H558R polymorphism of SCN5A and KSD. We recruited 71 patients with KSD and 80 geographical region-matched control subjects in our study. Vital sign and electrocardiographic (ECG) measurements were performed for heart rate, systolic pressure, diastolic pressure, PR interval, QT interval, QRS duration, ST-T changes and complete right bundle branch block (CRBBB), and H558R polymorphism was genotyped using the polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) method and sequencing. A significant association was found between the H558R polymorphism of exon 12 and KSD. Allele C carriers had a decreased risk for KSD with an odds ratio of 0.332 [95% confidence interval (CI), 0.160-0.692] as well as for QRS prolongation in KSD patients with an odds ratio of 0.089 (95%CI, 0.022-0.361). Our results provide support to the association between H558R polymorphism and the decreased risk for KSD. H558R polymorphism might increase susceptibility to KSD, and SCN5A containing the polymorphism might be a predisposing gene for QRS prolongation.

Ventricular arrhythmias in patients of atrial fibrillation treated with flecainide: A case report

Purpose: Flecainide is a class 1C antiarrhythmic drug, especially used for the management of supraventricular arrhythmia. Flecainide also has a recognized proarrhythmic effect in all age groups of adult patients treated for ventricular tachycardia. It is used to treat a variety of cardiac arrhythmias including paroxysmal fibrillation, Paroxysmal Supraventricular tachycardia and ventricular tachycardia. Flecainide works by regulating the flow of sodium in the heart, causing prolongation of the cardiac action potential. The proarrhythmic effects however noted are not widely reported. Case report: We report a case of paroxysmal atrial fibrillation with structurally normal heart who was treated with oral Flecainide. Despite subjective improvement and no adverse events [QTc prolongation] a repeat holter detected him to have multiple short non sustained ventricular arrhythmias. Results: Development of ventricular arrhythmias, salvos &non sustained ventricular tachycardia after a month of initiation of oral Flecainide detected by 24 hours ECG holter lead to discontinuation of Flecainide and subsequent early electro physiological studies and successful ablation. Conclusion: Initiation of oral Flecainide in a case of atrial fibrillation with subjective improvement and regular ECG monitoring, no QTc prolongation can still lead to development of dangerous ventricular arrhythmias. A cautious approach and thorough investigations and follow up are recommended.

Flecainide Induced Ventricular Arrhythmias in Atrial Fibrillation

Purpose: Flecainide is a class 1C antiarrhythmic drug especially used for the management of supraventricular arrhythmia. Flecainide also has a recognized proarrhythmic effect in patients treated for ventricular tachycardia. It is used to treat a variety of cardiac arrhythmias including paroxysmal fibrillation, Paroxysmal Supraventricular tachycardia and ventricular tachycardia. Flecainide works by regulating the flow of sodium in the heart, causing prolongation of the cardiac action potential. The proarrhythmic effects however noted are not widely reported. Method: We report a case of paroxysmal atrial fibrillation with structurally normal heart who was treated with oral Flecainide. Despite subjective improvement and no adverse events [QTc prolongation] a repeat holter detected him to have multiple short non sustained ventricular arrhythmias. Results: Development of ventricular arrhythmias, salvos & non sustained ventricular tachycardia after a month of initiation of oral Flecainide detected by 24 hours ECG holter lead to discontinuation of Flecainide and subsequent early electro physiological studies and successful ablation. Conclusion: Initiation of oral Flecainide in a case of atrial fibrillation with subjective improvement and regular ECG monitoring, no QTc prolongation can still lead to development of dangerous ventricular arrhythmias. A cautious approach and thorough investigations and follow up are recommended.

Sinus Arrest and Asystole Caused by a Peripherally Inserted Central Catheter

Transient bradycardia in the critical care setting is frequently caused by hypoxemia or oropharyngeal manipulation. Central lines have been associated with a variety of cardiac arrhythmias, but sinus arrest and asystole have not been previously reported. A 38-year-old woman with multisystem organ failure had several episodes of prolonged sinus arrest, slow junctional escape rhythm, and periods of asystole lasting over 6 seconds. The cause of the repetitive bradyarrhythmia was evaluated by clinical observation including the response to parasympatholytic agents, by detailed analysis of rhythm strips, and review of cardiac imaging studies. The episodes of bradycardia did not coincide with orotracheal manipulation, were not prevented by escalating doses of glycopyrrolate, and were not accompanied by AV conduction disturbance as is frequently seen during a transient increase in vagal tone. Review of the patient's chest X-ray and chest CT revealed that the tip of a peripherally inserted central catheter migrated to the vicinity of the sinoatrial node. Removal of the catheter resulted in prompt resolution of the episodes of sinus arrest. This case demonstrates that migration of a peripherally inserted central catheter to the sinoatrial node can provoke prolonged sinus bradycardia, sinus arrest and asystole.

Decreased RyR2 refractoriness determines myocardial synchronization of aberrant Ca 2+ release in a genetic model of arrhythmia

Dysregulated intracellular Ca(2+) signaling is implicated in a variety of cardiac arrhythmias, including catecholaminergic polymorphic ventricular tachycardia. Spontaneous diastolic Ca(2+) release (DCR) can induce arrhythmogenic plasma membrane depolarizations, although the mechanism responsible for DCR synchronization among adjacent myocytes required for ectopic activity remains unclear. We investigated the synchronization mechanism(s) of DCR underlying untimely action potentials and diastolic contractions (DCs) in a catecholaminergic polymorphic ventricular tachycardia mouse model with a mutation in cardiac calsequestrin. We used a combination of different approaches including single ryanodine receptor channel recording, optical imaging (Ca(2+) and membrane potential), and contractile force measurements in ventricular myocytes and intact cardiac muscles. We demonstrate that DCR occurs in a temporally and spatially uniform manner in both myocytes and intact myocardial tissue isolated from cardiac calsequestrin mutation mice. Such synchronized DCR events give rise to triggered electrical activity that results in synchronous DCs in the myocardium. Importantly, we establish that synchronization of DCR is a result of a combination of abbreviated ryanodine receptor channel refractoriness and the preceding synchronous stimulated Ca(2+) release/reuptake dynamics. Our study reveals how aberrant DCR events can become synchronized in the intact myocardium, leading to triggered activity and the resultant DCs in the settings of a cardiac rhythm disorder.

The Timing Statistics of Spontaneous Calcium Release in Cardiac Myocytes

A variety of cardiac arrhythmias are initiated by a focal excitation that disrupts the regular beating of the heart. In some cases it is known that these excitations are due to calcium (Ca) release from the sarcoplasmic reticulum (SR) via propagating subcellular Ca waves. However, it is not understood what are the physiological factors that determine the timing of these excitations at both the subcellular and tissue level. In this paper we apply analytic and numerical approaches to determine the timing statistics of spontaneous Ca release (SCR) in a simplified model of a cardiac myocyte. In particular, we compute the mean first passage time (MFPT) to SCR, in the case where SCR is initiated by spontaneous Ca sparks, and demonstrate that this quantity exhibits either an algebraic or exponential dependence on system parameters. Based on this analysis we identify the necessary requirements so that SCR occurs on a time scale comparable to the cardiac cycle. Finally, we study how SCR is synchronized across many cells in cardiac tissue, and identify a quantitative measure that determines the relative timing of SCR in an ensemble of cells. Using this approach we identify the physiological conditions so that cell-to-cell variations in the timing of SCR is small compared to the typical duration of an SCR event. We argue further that under these conditions inward currents due to SCR can summate and generate arrhythmogenic triggered excitations in cardiac tissue.

Etiology and clinical analysis of dilated cardiomyopathy in children

Objective To determine the etiology and clinical characteristics of children with dilated cardiomyopathy.Methods 112 children with DCM from January 2002 to December 2011 were studied.The age,cause and the clinical manifestations were analyzed.According to the children with heart function,they were divided into four groups,the incidence and type of arrhythmia were compared.Results In 112 cases of DCM in children,＜ 1 year old diagnosed 52 cases (46.42％),107 cases (95.53％) with heart failure,25 cases (22.32％) with chear causes,96 cases(85.71％) children with a variety of arrhythmias.According to cardiac function in Ⅰ,Ⅱ and Ⅲ and Ⅳ level,they were divided into four groups.The incidence of one kind of arrhythrnia were 20.00％,31.25％,36.17％ and 42.85％.The incidence with two or more kinds of arrhythmia was 0,18.75％,38.29％ and 50.00％ respectively.The incidence of malignant arrhythmia was 0,12.50％,40.42％and 67.85％ in different groups.There was significant differences among the groups of Ⅱ and Ⅲ and Ⅳ level (P ＜ 0 05).Conclusion Children with dilated cardiomyopathy disease younger than I year old had the highest incidence.The major causes were myocarditis,hereditary and congenital metabolic diseases.The incidence of arrhythmia in DCM were high and diverse,often exist for a variety of arrhythmia,and arrhythmia have closely relation with heart function.The embolism in children with DCM was very rare. Key words: Dilated cardiomyopathy; Etiology; Arrhythmia; Children

Percutaneous transhepatic access for catheter ablation of cardiac arrhythmias

Femoral venous access may be limited in certain patients undergoing electrophysiology (EP) study and ablation. The purpose of this study is to review a series of patients undergoing percutaneous transhepatic access to allow for ablation of cardiac arrhythmias. Six patients with a variety of cardiac arrhythmias and venous abnormalities underwent percutaneous transhepatic access. Under fluoroscopic and ultrasound guidance, a percutaneous needle was advanced into a hepatic vein and exchanged for a vascular sheath over a wire. Electrophysiology study and radiofrequency ablation was then performed. All tachycardias, including atrial tachycardia, atrial flutter, atrioventricular nodal tachycardia, and atrial fibrillation, were ablated. Procedural times ranged from 227 to 418 min. Fluoroscopy times ranged from 32 to 95 min. There were no complications. All six patients have been arrhythmia-free in follow-up (5-49 months, mean 23.1 months). Percutaneous transhepatic access is safe and feasible in patients with limited venous access who are undergoing EP study and ablation for a range of cardiac arrhythmias.

Nonfluoroscopic Catheter Ablation of Cardiac Arrhythmias in Adults: Feasibility, Safety, and Efficacy

Catheter ablations are traditionally performed using fluoroscopic guidance, exposing both patients and medical staff to the risks of radiation. Nonfluoroscopic catheter ablation has been used successfully to treat limited types of arrhythmias in children, but whether this approach has broad application in adults is uncertain. The purpose of this study was to evaluate the feasibility, safety, and efficacy of fluoroless catheter ablation in adults being treated for a range of arrhythmias. Retrospective analysis was performed in 2 patient groups (both n = 60): (1) the nonfluoroscopy (NF) group consisting of consecutive adult patients, in which catheter positioning was accomplished exclusively with intracardiac electrograms (IE), electroanatomic mapping (EAM), and intracardiac echocardiography (ICE); and (2) the fluoroscopy (F) group, in which catheter positioning was additionally guided by fluoroscopy. The patients in the F group were selected to match the types of arrhythmias in the NF group. All ablation procedures were performed by one operator. The total procedure time did not differ between groups for any specific type of arrhythmia ablated. Acute procedural success was similar in both groups (NF, 59/60 [98%] and F, 60/60 [100%]). The complications were limited to a groin pseudoaneurysm in the NF group, and pericardial effusion and groin hematoma in the F group. Catheter ablations were efficiently and effectively performed in adults with a variety of arrhythmias using only IE, EAM, and ICE for catheter guidance. This nonfluoroscopic technique was feasible, posed no additional safety concerns, and should be readily implementable in most electrophysiology laboratories.

Sahaja yoga: A unique adjunctive approach for the management of cardiac arrhythmias?

Sahaja yoga (SY) is a unique meditative technique that has both spiritual and mundane aspects. Although SY has generally been regarded as a practice of spiritual and mental well-being, it is also closely associated with a variety of subtle influences on some organ systems. SY has been suggested to have some beneficial effects in a variety of psycho-somatic diseases [1]. Besides subtle effects throughout the whole body, sympatho-vagal regulation associated with SY technique has been proposed as the direct evidence of its beneficial effects on some organ systems including cardiovascular system etc. As describedbelow,SYpracticemaybe regarded as anadjunctiveoption for the management of a variety of cardiac arrhythmias primarily on the basis of its potential effects on the autonomic nervous system largely through unknown mechanisms. Autonomic imbalance including hyperactivation of sympathetic system has been known to be associated with the occurrence of life threatening conditions including acute coronary syndromes (ACS) and may directly trigger malign arrhythmic events in arrhythmia-prone subjects [2]. Hyperactivation of sympathetic system is also well known to prolong QT interval, and impair some indices of autonomic nervous system including heart rate variability (HRV) and heart rate turbulence (HRT) etc. thatmay serve asmarkers of arrhythmogenesis in susceptible subjects. Therefore, therapeutic strategies aiming at management of autonomic imbalance on topof conventional anti-arrhythmic therapy in arrhythmia-prone subjects have gained particular importance in the recent years. However, therapeutic options including beta blockers, sympathetic ganglion blockage etc. have limited value for the management of autonomic imbalance as these approaches generally

Ideas of TCM in treating viral myocarditis with arrhythmia

Viral myocarditis involving the cardiac conduction system is easily associated with a variety of arrhythmias. Negative inotropic effect and the potentially induced arrhythmia of antiarrhythmic drugs limit their application. Non-drug treatment with the body injuries and high medical costs holds patients back and leads them to take a conservative treatment. Traditional medical treatment of this disease has a long history and rich experience. Clinical and experimental studies show that traditional Chinese medicine is effective and has few side effects. This article discusses the ideas of treatments of arrhythmia, from three aspects of differentiating diseases and syndrome type, dispelling wind-evil and dredging collaterals and using results of pharmacological researches reasonably. Key words: Myocarditis; Arrhythmia; Treatment

Syndrome of Inappropriate Antidiuretic Hormone in Association with Amiodarone Therapy: A Case Report and Review of Literature

Background: Amiodarone is a class III antiarrhythmic agent that is widely used in the treatment of a variety of arrhythmias. Several different systemic side effects are reported after use of this medication. In this article, we report a case that had developed syndrome of inappropriate antidiuretic hormone (SIADH) after starting treatment with this agent. Case report: The patient is a 66-year-old male with past medical history of hypertension, hyperlipidemia, coronary artery disease, and class III New York Heart Association congestive heart failure who presented with monomorphic nonsustained ventricular tachycardia. A loading dose of amiodarone followed by maintenance dose was started. Baseline serum sodium of 138 mmol/L on admission decreased to 119 mmol/L by day 7, and a diagnosis of SIADH was made. The patient was not taking any other medication known to cause SIADH, nor had any such comorbidity to explain it. Serum sodium increased to 133 and 138 mmol/L, respectively, after 16 and 33 days from discontinuation of amiodarone. Conclusion: SIADH is a rare but serious side effect of amiodarone and practicing physicians should be aware of this complication, particularly after loading dose of the medication.

Haloperidol increases expression of the inositol 1,4,5-trisphosphate receptors in rat cardiac atria, but not in ventricles

Numerous ligands of sigma receptors are known to prolong the QT interval and therefore cause a variety of arrhythmias. High affinity binding sites for the prototypical sigma ligand haloperidol were found in membranes of cardiac myocytes from adult rats. Activation of sigma 1 receptor leads to a release of calcium from the endoplasmic reticulum that follows increased synthesis of inositol 1,4,5-trisphosphate (IP3). We studied the effect of long-term haloperidol treatment on the expression of sigma 1 receptors, IP3 receptors of type 1 and 2 in the individual parts of the rat heart, in isolated rat cardiomyocytes and in PC12 cells. We have found that prolonged treatment with haloperidol significantly increased mRNA levels of sigma 1 receptors in both atria and ventricles. Sigma 1 receptor's mRNA was increased also in isolated cardiomyocytes. Haloperidol treatment affects the expression of IP3 receptors of type 1 and 2 in cardiac atria, but not in cardiac ventricles. We observed increase in IP3 receptors in differentiated PC12 cells, but not in isolated cardiomyocytes. We propose that this increase might participate in triggering cardiac arrhythmias during haloperidol treatment, which has to be further verified.

Chaotic based reconstructed phase space features for detecting ventricular fibrillation

Among the variety of cardiac arrhythmias, ventricular fibrillation (VF) and ventricular tachycardia (VT) are life-threatening; thus, accurate classification of these arrhythmias is a crucial task for cardiologists. Nevertheless, VT and VF signals are very similar in the time domain and accurate distinguishing these signals with naked eyes in some cases is impossible. In this paper, a novel self-similarity image-based scheme is introduced to classify the underlying information of VT, VF and normal electrocardiogram (ECG) signals. In this study, VT, VF and normal ECG signals are selected from CCU of the Royal Infirmary of Edinburgh and MIT-BIH datasets. According to the time delay method, signal samples can be assigned to state variables and a trajectory can be achieved. To extract the proposed self-similarity feature, first, two different trajectories from each signal trial are drawn according to two different delay time values. The two-dimensional state space of each trial trajectory is considered as an image. Therefore, two trajectory images are produced for each signal. Number of visited pixels in the first image is determined and is subtracted from that of the second image as the self-similarity feature of that signal. Moreover, another scheme is proposed to have a better estimation of self-similarity in which the logical AND operator is applied to both images (matrices) of each ECG trial. The third proposed criterion is similar to box counting method by this difference that each pixel is assigned a weight according to the trajectory density at that point and finally visited weighted pixels are counted. To classify VF from VT and normal ECG, a threshold is determined through the cross validation phase under the Receiver Operating Characteristic (ROC) criterion. To assess the proposed methods, the mentioned signals are classified using the-state-of-art chaotic features such as correlation dimension, the largest Lyapunov exponent and Approximate Entropy (ApEn). Experimental results indicate superiority of the proposed method in classifying the VT, VF and normal ECG signals compared to present traditional schemes. In addition, computational complexity of the introduced methods is very low and can be implemented in real-time applications.

Apparent desensitization of the effects of sigma receptor ligand haloperidol in isolated rat and guinea pig hearts after chronic treatmentThis article is one of a selection of papers published in a special issue on Advances in Cardiovascular Research.

The supposed role of cardiac sigma receptors is fine tuning of contractility. Sigma receptors affect several ionic channels and hence their signaling is reflected by the electrophysiological properties of the heart. Numerous ligands of sigma receptors are known to prolong the QT interval and therefore cause a variety of arrhythmias, including severe ones. The effects of the prototypical sigma ligand haloperidol have been studied extensively in humans as well as in various animal models, primarily after acute administration. We examined the incidence of arrhythmias, changes in heart rate, and prolongation of QT interval in isolated Langendorff-perfused rat and guinea pig hearts after they were exposed to nanomolar concentrations of haloperidol. Hearts from both untreated (acute) and pretreated (chronic) animals were investigated. While QT prolongation and arrhythmias due to haloperidol administration were observed in untreated rat and guinea pig hearts, arrhythmias were completely prevented in both species of chronically treated animals. In treated guinea pigs, the results were generally less convincing. Since the hearts were exposed to nanomolar concentration of haloperidol, we conclude that our data may be explained by desensitization of sigma receptors.

A simple time domain algorithm for the detection of ventricular fibrillation in electrocardiogram

Ventricular fibrillation (VF) is the most serious variety of arrhythmia which requires quick and accurate detection to save lives. In this paper, we propose a new time domain algorithm, called threshold crossing sample count (TCSC), which is an improved version of the threshold crossing interval (TCI) algorithm for VF detection. The algorithm is based on an important feature of the VF signal which relies on the random behavior of the electrical heart vector. By two simple operations: comparison and count, the technique calculates an effective measure which is used to separate life-threatening VF from other heart rhythms. For assessment of the performance of the algorithm, the method is applied on the complete MIT-BIH arrhythmia and CU databases, and a promising good performance is observed. Seven other classical and new VF detection algorithms, including TCI, have been simulated and comparative performance results in terms of different quality parameters are presented. The TCSC algorithm yields the highest value of the area under the receiver operating characteristic curve (AUC). The new algorithm shows strong potential to be applied in clinical applications for faster and accurate detection of VF.

Detection of ventricular fibrillation using empirical mode decomposition and Bayes decision theory

Ventricular fibrillation (VF) is the most serious variety of arrhythmia which requires quick and accurate detection to save lives. In this paper, we propose an empirical mode decomposition (EMD) based algorithm for VF detection. The intrinsic mode functions (IMFs) of VF are orthogonal whereas the lower order IMFs of normal sinus rhythm (NSR) are not. The orthogonality indices derived from the first three consecutive intrinsic mode functions (IMFs) of NSR and VF are used for their discrimination. The proposed technique is applied to the MIT-BIH arrhythmia database. The accuracy of detection of VF is 99.70% for a window length of 3 s. This early estimate of VF may be useful in emergency cases where defibrillators are to be applied. Comparative results with the existing methods in terms of quality parameters and integrated receiver operating characteristic (IROC) are presented.

Understanding drug-induced torsades de pointes: a genetic stance

Drugs may produce a variety of arrhythmias, but drug-induced QT prolongation and the risk of the polymorphic ventricular tachycardia torsades de pointes (drug-induced long QT syndrome) has garnered the most attention. The wide array of drugs with potential for QT prolongation, the correspondingly large number of patients exposed to such drugs, the difficulty in predicting an individual's risk, and the potentially fatal outcome, make drug-induced long QT syndrome an important public health problem for clinicians, researchers, drug development programs, and regulatory agencies. This review focuses on the genetic risk factors and mechanisms underlying QT prolongation and proarrhythmia. The post-genomic era hints at an improved understanding (and prediction) of how the gene–environment interaction produces this particular adverse drug response.

Experience of robotic catheter ablation in humans using a novel remotely steerable catheter sheath

BackgroundA novel remotely controlled steerable guide catheter has been developed to enable precise manipulation and stable positioning of any eight French (Fr) or smaller electrophysiological catheter within the heart for the purposes of mapping and ablation.ObjectiveTo report our initial experience using this system for remotely performing catheter ablation in humans.MethodsConsecutive patients attending for routine ablation were recruited. Various conventional diagnostic catheters were inserted through the left femoral vein in preparation for treating an accessory pathway (n = 1), atrial flutter (n = 2) and atrial fibrillation (n = 7). The steerable guide catheter was inserted into the right femoral vein through which various irrigated and non-irrigated tip ablation catheters were used. Conventional endpoints of loss of pathway conduction, bidirectional cavotricuspid isthmus block and four pulmonary vein isolation were used to determine acute procedural success.ResultsTen patients underwent remote catheter ablation using conventional and/or 3D non-fluoroscopic mapping technologies. All procedural endpoints were achieved using the robotic control system without manual manipulation of the ablation catheter. There was no major complication. A radiation dosimeter positioned next to the operator 2.7 m away from the X-ray source showed negligible exposure despite a mean cumulative dose area product of 7,281.4 cGycm2 for all ten ablation procedures.ConclusionsSafe and clinically effective remote navigation of ablation catheters can be achieved using a novel remotely controlled steerable guide catheter in a variety of arrhythmias. The system is compatible with current mapping and ablation technologies Remote navigation substantially reduces radiation exposure to the operator.Electronic supplementary materialThe online version of this article (doi:10.1007/s10840-007-9184-z) contains supplementary material, which is available to authorized users.