Modeling electical activity of the heart

Two are the basic goals of this study. The first goal is the development of an automatedsystem for the analysis of the monophasic action potentials. Monophasic action potentials(MAPs) are exrtacellular recordings that have been found to correlate sufficiently withintracellular action potentials (IAPs). There are many reports investigating the effects ofelectrophysiological and pharmacological interventions in the MAP of the intact heart. Thesystem consists of three stages: signal acquisition, feature extraction and statistical processing.The MAP features which are extracted with high accuracy are: MAP amplitude, actionpotential duration (APD) at all repolarization levels (10- 90%), the baseline and amplitude.The proposed system has been used for the analysis of MAPs from two series of experiments.The system performs well despite the morphological differences of the signals. The drugeffect on MAPs is recorded and useful input can de acquired for the myocardium condition.msec. The MAP signal is heavily contaminated by noise (power line interference and motionartifacts), which is removed by a combination of filters (notch, low, band and high pass). Dueto the power line interference at 50 Hz the digital notch filter has a transfer function. In thisstage the MAP signal features are extracted. The MAP morphology and the MAP usedfeatures for pigs and rats. The used threshold values are set and the signal is fed into thesystem. The algorithm can extract upstroke, the baseline, the plateau and the APDs. To makeeasier the analysis a window is used, whose length is adapted when an upstroke is found. Abaseline is acceptable when it compares well (using a threshold value) with the average of theprevious five baselines. The feature extraction ends when both plateau and APDs are detected.The second goal of this study was the investigation of the dynamical response of Zhangmodel. Physiological rhythms play an important role in life. Some of them are sustainedthroughout life but even a brief interruption leads to death. Variation of rhythms outside230normal limits, or appearance of new rhythms, where none existed before indicates disease.After a perturbation of the repetitive activity, the timing of subsequent repeats of the events isgenerally altered: the prior rhythm eventually reasserts itself, only with its phase reset relativeto an unperturbed control. The amount of the resetting depends both on the magnitude andmanner of the external stimulus and on the phase in the oscillation when the perturbationbegins. The topological approach offers new insights into the analysis of the resetting of thecardiac cells. In our work, we summarize the most important studies were the authors appliedthe PRT in cardiac models and we provide a brief description of the results that theypublished. Thereinafter, we present an extension of our work done in this field, where weinvestigate and characterize the effects of external stimulation on central and peripheral SANcells using the Zhang et al. model. In recent work, we have investigated the dynamic responseof the SAN to short external stimuli using the Zhang et al. model . The model equations aresolved twice for the central cell and for the peripheral cell.A short current pulse is applied to reset the spontaneous rhythmic activity of the singlesinoatrial node cell. Depending on the stimulus timing either a delay or an advance in theoccurrence of next action potential is produced. This resetting behavior is quantified in termsof PTCs for short electrical current pulses of varying amplitude which span the whole period.For low stimulus amplitudes the transition from advance to delay is smooth, while at higheramplitudes abrupt changes and discontinuities are observed in PTCs. Such discontinuitiesreveal critical stimuli, the application of which can result in annihilation of activity in centralSAN cells. The detailed analysis of the ionic mechanisms involved in its resetting behavior ofSAN cell models provides new insight into the dynamics and physiology of excitation of thesinoatrial node of the heartΣτην παρούσα διδακτορική διατριβή µελετάται η ηλεκτρική δραστηριότητα της καρδιάς µέσα από δύο διαφορετικές προσεγγίσεις. Μια πρώτη προσέγγιση είναι η ανάπτυξη ενός ολοκληρωµένου συστήµατος καταγραφής και αυτόµατης ανάλυσης µονοφασικών δυναµικών ενέργειας. Στο πλαίσιο της ανάπτυξης αυτού του συστήµατος µελετήθηκε η εξέλιξη της τεχνικής της καταγραφής των Μ?Ε, η σηµασία τους και οι κλινικές τους εφαρµογές. Προηγήθηκε µια ολοκληρωµένη παρουσίαση των βασικότερων γνώσεων στην ηλεκτροφυσιολογία, όπως το ερεθισµαταγωγό σύστηµα, η δηµιουργία των δυναµικών δράσεως, ο τρόπος εξέλιξης του ερεθίσµατος κ.α. Επίσης παρουσιάσθηκαν οι κυριότερες αρρυθµίες που δηµιουργούνται από δυσλειτουργίες του ερεθισµαταγωγού συστήµατος και από προβλήµατα στην διάδοση του ερεθίσµατος γενικά. Παρουσιάσθηκαν όλα τα προηγούµενα συστήµατα που έχουν αναπτυχθεί για την καταγραφή την αυτόµατη ανάλυση των Μ?Ε. Επίσης παρουσιάσθηκαν αναλυτικά τα χαρακτηριστικά των Μ?Ε, η µορφολογία τους και ο τρόπος υπολογισµού των χαρακτηριστικών τους. Το σύστηµα καταγραφής και ανάλυσης αποτελείται από τέσσερα υποσυστήµατα: το υποσύστηµα καταγραφής και ψηφιοποίησης των Μ?Ε, το υποσύστηµα προεπεξεργασίας των Μ?Ε, το υποσύστηµα αυτόµατη ανάλυσης των χαρακτηριστικών των Μ?Ε και το υποσύστηµα εξαγωγής των αποτελεσµάτων. Το κάθε υποσύστηµα παρουσιάσθηκε αναλυτικά. Το σύστηµα ελέγχθηκε για την αξιοπιστία του, τόσο στο στάδιο της καταγραφής των Μ?Ε όσο και στο στάδιο της αυτόµατης ανάλυσης των Μ?Ε. Τα αποτελέσµατα της. αυτόµατης ανάλυσης δεν ήταν δυνατόν να συγκριθούν άµεσα µε άλλες µεθόδους αλλά έµµεσα. Τα αποτελέσµατα έδειξαν ότι το σύστηµα καταγράφει και αναλύει αυτόµατα Μ?Ε µε παρά πολύ καλή ακρίβεια, σε πολύ µικρό χρονικό διάστηµα. Η δεύτερη προσέγγιση της ηλεκτρικής λειτουργίας έγινε µε την χρήση µαθηµατικών µοντέλων τα οποία προσοµοιώνουν την διέγερση (0D) και την διάδοση (1D) του ερεθίσµατος στα καρδιακά κύτταρα. Παρουσιάσθηκε η εξέλιξη των µοντέλων και η χρήση των µοντέλων µε την πάροδο των χρόνων. Για την µοντελοποίηση της διέγερσης του ερεθίσµατος χρησιµοποιήθηκε το µοντέλο του φλεβόκοµβου κουνελιού. Στο µοντέλο εφαρµόσθηκαν εξωτερικά ερεθίσµατα σε διάφορες χρονικές στιγµές του κύκλου (phase resetting technique) και µελετήθηκε η απόκριση του µοντέλου µε τις καµπύλες επαναφοράς της φάσης. Τα αποτελέσµατα έδειξαν ότι µόνο στα κεντρικά κύτταρα του φλεβόκοµβου παρατηρήθηκε το φαινόµενο της µόνιµης διακοπής της διέγερσης του ερεθίσµατος. Η χωρική επέκταση του µοντέλου, έδωσε την δυνατότητα να αναπτυχθεί ένα µονοδιάστατο µοντέλο κεντρικών και περιφερειακών κυττάρων του φλεβόκοµβου. Για την ανάπτυξη του µοντέλου χρησιµοποιήθηκαν οι διαστάσεις και οι αγωγιµότητες µεταξύ των κεντρικών και των περιφερειακών κυττάρων που έχουν δηµοσιευθεί στην βιβλιογραφία. Στο µονοδιάστατο µοντέλο εφαρµόσθηκαν εξωτερικά ερεθίσµατα και µελετήθηκαν τα αποτελέσµατα της απόκρισης του µοντέλου µέσα από τις καµπύλες επαναφοράς της φάσης

Use RBF as a Sampling Method in Multistart Global Optimization Method

In this paper, a new sampling technique is proposed that can be used in the Multistart global optimization technique as well as techniques based on it. The new method takes a limited number of samples from the objective function and then uses them to train an Radial Basis Function (RBF) neural network. Subsequently, several samples were taken from the artificial neural network this time, and those with the smallest network value in them are used in the global optimization method. The proposed technique was applied to a wide range of objective functions from the relevant literature and the results were extremely promising

Use RBF as a Sampling Method in Multistart Global Optimization Method

In this paper, a new sampling technique is proposed that can be used in the Multistart global optimization technique as well as techniques based on it. The new method takes a limited number of samples from the objective function and then uses them to train an Radial Basis Function (RBF) neural network. Subsequently, several samples were taken from the artificial neural network this time, and those with the smallest network value in them are used in the global optimization method. The proposed technique was applied to a wide range of objective functions from the relevant literature and the results were extremely promising

NeuralMinimizer: A Novel Method for Global Optimization

The problem of finding the global minimum of multidimensional functions is often applied to a wide range of problems. An innovative method of finding the global minimum of multidimensional functions is presented here. This method first generates an approximation of the objective function using only a few real samples from it. These samples construct the approach using a machine learning model. Next, the required sampling is performed by the approximation function. Furthermore, the approach is improved on each sample by using found local minima as samples for the training set of the machine learning model. In addition, as a termination criterion, the proposed technique uses a widely used criterion from the relevant literature which in fact evaluates it after each execution of the local minimization. The proposed technique was applied to a number of well-known problems from the relevant literature, and the comparative results with respect to modern global minimization techniques are shown to be extremely promising

Prediction of COVID-19 Cases Using Constructed Features by Grammatical Evolution

A widely used method that constructs features with the incorporation of so-called grammatical evolution is proposed here to predict the COVID-19 cases as well as the mortality rate. The method creates new artificial features from the original ones using a genetic algorithm and is guided by BNF grammar. After the artificial features are generated, the original data set is modified based on these features, an artificial neural network is applied to the modified data, and the results are reported. From the comparative experiments done, it is clear that feature construction has an advantage over other machine-learning methods for predicting pandemic elements

Hypertension in Dialysis Patients: Diagnostic Approaches and Evaluation of Epidemiology

Whereas hypertension is an established cardiovascular risk factor in the general population, the contribution of increased blood pressure (BP) to the huge burden of cardiovascular morbidity and mortality in patients receiving dialysis continues to be debated. In a large part, this controversy is attributable to particular difficulties in the accurate diagnosis of hypertension. The reverse epidemiology of hypertension in dialysis patients is based on evidence from large cohort studies showing that routine predialysis or postdialysis BP measurements exhibit a U-shaped or J-shaped association with cardiovascular or all-cause mortality. However, substantial evidence supports the notion that home or ambulatory BP measurements are superior to dialysis-unit BP recordings in diagnosing hypertension, in detecting evidence of target-organ damage and in prognosticating the all-cause death risk. In the first part of this article, we explore the accuracy of different methods of BP measurement in diagnosing hypertension among patients on dialysis. In the second part, we describe how the epidemiology of hypertension is modified when the assessment of BP is based on dialysis-unit versus home or ambulatory recordings

Endothelin B-receptors and sympathetic activation: Impact on ventricular arrhythmogenesis during acute myocardial infarction

AbstractAimsWe investigated the role of endothelin-B receptors on sympathetic activation originating from the adrenal gland or from the myocardium and its impact on arrhythmogenesis during acute myocardial infarction.Main methodsWe studied two groups of rats (n=120, 284±2g), namely wild-type and ETB-deficient. Myocardial infarction was induced by permanent ligation of the left coronary artery and ventricular tachyarrhythmias were evaluated from continuous electrocardiographic recordings. Sympathetic activation, measured by indices of heart rate variability, was evaluated after adrenalectomy or catecholamine depletion induced by reserpine. Acute left ventricular failure was assessed by total animal activity.Key findingsAdrenalectomy decreased the total duration of tachyarrhythmias in ETB-deficient rats, but their incidence remained higher, compared to wild-type rats. After reserpine, heart rate variability indices and tachyarrhythmias were similar in the two groups during the initial, ischaemic phase. During evolving infarction, tachyarrhythmia duration was longer in ETB-deficient rats, despite lower sympathetic activation. Heart rate was lower in ETB-deficient rats throughout the 24-hour observation period, whereas activity was comparable in the two groups.SignificanceEndothelin-B receptors modulate sympathetic activation during acute myocardial infarction not only in the ventricular myocardium, but also in the adrenal gland. Sympathetic activation markedly increases early-phase ventricular tachyarrhythmias, but other mechanisms involving the endothelin system underlie delayed arrhythmogenesis

Beat-to-Beat P-Wave Analysis Outperforms Conventional P-Wave Indices in Identifying Patients with a History of Paroxysmal Atrial Fibrillation during Sinus Rhythm

Early identification of patients at risk for paroxysmal atrial fibrillation (PAF) is essential to attain optimal treatment and a favorable prognosis. We compared the performance of a beat-to-beat (B2B) P-wave analysis with that of standard P-wave indices (SPWIs) in identifying patients prone to PAF. To this end, 12-lead ECG and 10 min vectorcardiogram (VCG) recordings were obtained from 33 consecutive, antiarrhythmic therapy naïve patients, with a short history of low burden PAF, and from 56 age- and sex-matched individuals with no AF history. For both groups, SPWIs were calculated, while the VCG recordings were analyzed on a B2B basis, and the P-waves were classified to a primary or secondary morphology. Wavelet transform was used to further analyze P-wave signals of main morphology. Univariate analysis revealed that none of the SPWIs performed acceptably in PAF detection, while five B2B features reached an AUC above 0.7. Moreover, multivariate logistic regression analysis was used to develop two classifiers—one based on B2B analysis derived features and one using only SPWIs. The B2B classifier was found to be superior to SPWIs classifier; B2B AUC: 0.849 (0.754–0.917) vs. SPWIs AUC: 0.721 (0.613–0.813), p value: 0.041. Therefore, in the studied population, the proposed B2B P-wave analysis outperforms SPWIs in detecting patients with PAF while in sinus rhythm. This can be used in further clinical trials regarding the prognosis of such patients