Clinically relevant computer model of cardiac rhythm and pacemaker/heart interaction

Modeling cardiac pacemaker malfunctions with the Virtual Heart Model

Implantable cardiac devices such as artificial pacemakers deliver therapies according to the timing information from the heart. Such devices work under the assumptions of perfect sensing, which are: (a) the pacemaker leads remain in place, and (b) the pacing therapy in one chamber (e.g. atrium) is insulated from the other chambers (e.g. ventricles). But there are common cases which violate these assumptions and the mechanisms for imperfect sensing cannot be captured by a simple signal generator. In this paper we use the Penn Virtual Heart Model (VHM) to investigate the spatial and temporal aspects of the electrical conduction system of the heart in a closed-loop with a pacemaker model. We utilize the spatial properties of the heart to model the sensing mechanism, and use clinical cases to show the validity of our sensing model. Such closed-loop evaluation of the pacemaker operation allows for functional testing of pacemaker software, the development of new algorithms for rhythm therapy and also serves as a tool for incoming cardiac electrophysiology fellows.

Using the Virtual Heart Model to validate the mode-switch pacemaker operation

Artificial pacemakers are one of the most widely-used implantable devices today, with millions implanted worldwide. The main purpose of an artificial pacemaker is to treat bradycardia, or slow heart beats, by pacing the atrium and ventricles at a faster rate. While the basic functionality of the device is fairly simple, there are many documented cases of death and injury due to device malfunctions. The frequency of malfunctions due to firmware problems will only increase as the pacemaker operations become more complex in an attempt to expand the use of the device. One reason these malfunctions arise is that there is currently no methodology for formal validation and verification of medical device software, as there are in the safety-critical domains of avionics and industrial control automation. We have developed a timed-automata based Virtual Heart Model (VHM) to act as platform for medical device software validation and verification. Through a case study involving multiple arrhythmias, this investigation shows how the VHM can be used with closed-loop operation of a pacemaker to validate the necessity and functionality of the complex mode-switch pacemaker operation. We demonstrate the correct pacemaker operation, to switch from one rhythm management mode to another, in patients with supraventricular tachycardias. (1).

Multi-formalism modelling and simulation: application to cardiac modelling

Cardiovascular modelling has been a major research subject for the last decade. Different cardiac models have been developed at a cellular level as well as at the whole organ level. Most of these models are defined by a comprehensive cellular modelling using continuous formalisms or by a tissue-level modelling often based on discrete formalisms. Nevertheless, both views still suffer from difficulties that reduce their clinical applications: the first approach requires heavy computational resources while the second one is not able to reproduce certain pathologies. This paper presents an original methodology trying to gather advantages from both approaches, by means of a hybrid model mixing discrete and continuous formalisms. This method has been applied to define a hybrid model of cardiac action potential propagation on a 2D grid of endocardial cells, combining cellular automata and a set of cells defined by the Beeler-Reuter model. For simulations under physiological and ischemic conditions, results show that the action potential propagation as well as electrogram reconstructions are consistent with clinical diagnosis. Finally, the advantage of the proposed approach is discussed within the frame of cardiac modelling and simulation.

A stochastic network model of the interaction between cardiac rhythm and artificial pacemaker

The electrical interaction between the heart and an artificial pacemaker is often complex. Because of the sophistication and diversity of dual-chamber device algorithms, even experienced cardiologists can have difficulty interpreting paced electrocardiograms (ECG's). In order to study heart-pacemaker interaction (HPI), a computer model of the cardiac conduction system has been developed which includes the effects of artificial pacemaker function and failure. The stochastic network model of cardiac conduction consists of five vertices, each representing a functional electrophysiologic element. Electrophysiologic multidimensional conditional probability functions determine the depolarization status of each vertex. The atrioventricular (AV) node is emulated using a mathematical model which includes the influence of past cycle lengths on AV nodal conduction time. Twenty-three classes of arrhythmias may be simulated and, for pacing simulation, one of 12 antibradycardia pacing modes may be chosen. Random effects of pacemaker malfunction including oversensing, undersensing, or failure-to-capture may be simulated through the use of probability distribution functions. This model should prove useful in the development of pacemaker algorithms, determining patient-specific pacemaker therapy, and predicting causes for apparent pacemaker malfunction. The model has been used in the development of an expert system to analyze paced ECG's for pacemaker function and malfunction.

Cardiac electrophysiological experiments in numero, Part III: Simulation of arrhythmias and pacing

This paper is the third and final part of a series of articles reviewing mathematical and computer models of the electrophysiological processes. This section reviews the arrhythmia simulation and discusses models of arrhythmogenic processes, fibrillation and defibrillation, and of heart-pacemaker interaction. The models of arrhythmogenesis are classified into three main sections: models of reentry and vortex reentry, models of myocardial electrotonic interactions, and models of macroreentrant supraventricular tachycardias. This final part of the review discusses the future potential of mathematical and computer models of different cardiac processes.

Computer simulation of overdrive pacing during atrioventricular reentrant tachycardia

The study used a computer model of cardiac excitation to reproduce atrioventricular (AV) reentrant tachycardia and to evaluate the possibility of its termination by overdrive burst pacing. The model simulated activation waves radiating along a one-dimensional circular pathway, the portions of which represented the atrial, AV nodal, His-Purkinje, ventricular, and bypass parts of the tachycardia circuit. The pathway consisted of 289 elements. Only depolarised and resting states of elements were modelled. Differential refractoriness and conduction velocity for each element and the cycle length dependence of AV nodal decremental conduction were introduced. The experiments with the model examined the ability of overdrive 'on-circuit' pacing to terminate the tachycardia in order to determine the relevance of: (a) the coupling interval of the first beat in the burst; (b) the cycle length of the burst; (c) the number of stimuli in the burst; (d) His-Purkinje refractoriness; and (e) the degree of AV nodal decremental conduction. The results suggested that: (A) the general impression of a regular recovery wave and of a regular excitable window moving uniformly along the macro-reentrant circular path is incorrect; (B) the use of overdrive bursts of several stimuli with a short coupling interval has unpredictable effects; (C) the use of faster bursts with a cycle length only slightly shorter than the tachycardia cycle length is more safe (with respect to tachycardia reinitiation) and for certain combinations of the coupling interval and cycle length, prolonged bursts do not reinitiate the tachycardia; (D) the likelihood of tachycardia termination is increased by prolonging the refractoriness of the tachycardia circuit and by reducing AV nodal decremental conduction.

Termination of macro-reentrant tachycardia by a single extrastimulus delivered during the 'effective' refractory period: a computer modeled 'case report'

A computer model of cardiac excitation sequences was used to reproduce atrioventricular (AV) reentrant tachycardia (AVRT) and its termination by a single 'on-circuit' extrastimulus. The model simulated activation waves revolving along a one-dimensional circular pathway, the portions of which represented the atrial, AV nodal, His-Purkinje, ventricular, and accessory pathway sections of the tachycardia circuit. The modeled pathway was composed of 289 elements. The model distinguished only the depolarised and resting states of constituent elements, but introduced differential refractoriness and conduction velocity for each element. These values approximated the natural situation established in a patient suffering from AVRT associated with the right bundle branch block. The results of the study suggest that: (A) the usual impression of a regular recovery wave and of a regular excitable window moving uniformly along the macro-reentrant circular path is incorrect; (B) during the tachycardia, islands of repolarized cells appear which are surrounded by tissue that is still refractory; (C) an extrastimulus which captures the island of early repolarized tissue may cause an excitation restricted to a small part of the myocardium but the local refractoriness following such an extrastimulus may be sufficient to terminate the tachycardia.

Compensating conduction times as a mechanism of alternating reentry tachycardia: computer modelling experiments

This article concerns a currently reported hypothesis explaining the alternations of conduction time during intra atrioventricular (AV) nodal reentrant tachycardia (AVNRT). This hypothesis supposes simple mutual influence of sequences of intranodal conduction during reentry tachycardia that use the same circuit in different tachycardia cycles. It has been suggested that delayed conduction prolongs the recovery time of the circuit, thus making it possible to transmit the excitation wave with a faster speed in the next tachycardia loop; this less delayed loop shortens the recovery interval of the pathway and results in subsequently delayed conduction of the following tachycardia cycle. A computer model simulating intra-AV nodal reentrant conduction was used to imitate and prove this hypotheses. The results are negative, showing that the suggested mechanism is too simplistic and that either the pathophysiologic background of the AVNRT cycle length oscillations must be different or additional phenomena must be considered to improve the hypothesis.

Computer modeling of cardiac rhythm disturbances and heart-pacemaker interaction

Different computer models have been developed in order to study various aspects of cardiac electrophysiological processes. These models can be classified according to many parameters and also in respect to their application areas. One group of the models is devoted to computer simulation of cardiac rhythm disturbances and to reproduction of interactions between the heart and an artificial pulse generator. This report overviews the recent models of arrhythmias and heart-pacemaker interaction. Special attention is paid to (1) functioning of fundamental model elements, (2) structure of the heart model, (3) pacemaker models, and (4) forms of results offered by simulation experiments. Existing models are classified and compared. To illustrate the medical capability of rhythm and pacemaker models, three computational experiments are presented with model atrioventricular reentry tachycardia and reentry tachycardia mediated by a DDD pacemaker. Future development and utilization of arrhythmia and pacemaker models are briefly discussed.

Modification of the DDD pacing mode to prevent junctional reentry tachycardia: computer modelling experiments

This paper examines the possibility of using short atrioventricular (AV) delay dual chamber pacing to prevent junctional reentry tachycardia mediated by an accessory pathway or by an intra-AV nodal circuit. For this purpose, a clinically realistic computer simulation model of cardiac rhythm and heart-pacemaker interactions has been used. The computational experiments compared the actions of two pacemaker models: (A) a clinically realistic DDD mode operating with quasi-Wenckebach prolongation of the AV delay; and (B) a new modification of the DDD mode introducing independent counters for the atrial and ventricular refractory periods of the heart, and the possibility of instantaneous or shortly delayed atrial pacing triggered by a sensed or paced ventricular event. The pathological phenomena modelled in the experiments simulate different possibilities of tachycardia initiation. These disorders include: (1) single atrial premature beats (APBs), (2) salvos of APBs, (3) closely coupled pairs of APBs, (4) ventricular premature beats initiating an antidromic reentry tachycardia, and (5) ventricular ectopic beats initiating an AV nodal reentry tachycardia. The computational results prove that many possible mechanisms of initiation of junctional reentry tachycardia are beyond the prophylactic capabilities of current sophisticated DDD pacemakers (A). The results also show that the suggested pacing mode (B) improves anti-tachycardia prophylaxis even when responding to complex pathological episodes of the natural cardiac activity. Future development of the suggested mode (B) is discussed.

Complexity of AV nodal function: complex nodal structure or complex behavior of nodal elements?

This paper discusses the hypothesis that the complex behavior of the atrioventricular (AV) node, with particular reference to Wenckebach periods, may be explained by the complexity of nodal structure and that complicated function of individual AV nodal fibers need not necessarily be considered. To prove this hypothesis, a computer model of cardiac excitation has been employed to simulate AV nodal function for two different anisotropic AV nodal model images. The experimental computer results show that the pattern of Wenckebach periods, and decremental and concealed AV nodal conduction can be explained without considering decremental conduction properties of individual AV nodal cells.

A one-dimensional model of atrioventricular nodal conduction

A computer model of the cardiac conduction process and an integral equation mathematical model have been employed to study the functions of the atrioventricular (AV) node. Special attention has been paid to the dependence of conduction delay on cycle length. The models have been used to evaluate the question as to whether simple cycle length dependences of AV nodal conduction could cause the oscillations of cycle length which are sometimes observed in AV reentry tachycardia. The models consider the AV node as a linear structure in which the depolarisation wavefront radiates in one dimension only. A model representing the node by two parallel linear structures has also been examined. Some results obtained do not conform with those of natural circumstance and clinical experiments, suggesting that the above hypotheses of AV nodal function are too simple and restricted to explain the natural processes.