Volker Lang

Computer modeling of ventricular rhythm during atrial fibrillation and ventricular pacing

We propose a unified atrial fibrillation (AF)-ventricular pacing (VP) (AF-VP) model to demonstrate the effects of VP on the ventricular rhythm during atrial fibrillation AF. In this model, the AV junction (AVJ) is treated as a lumped structure characterized by refractoriness and automaticity. Bombarded by random AF impulses, the AVJ can also be invaded by the VP-induced retrograde wave. The model includes bidirectional conduction delays in the AVJ and ventricle. Both refractory period and conduction delay of the AVJ are dependent upon its recovery time. The electrotonic modulation by blocked impulses is also considered in the model. Our simulations show that, with proper parameter settings, the present model can account for most principal statistical properties of the RR intervals during AF. We further demonstrate that the AV conduction property and the ventricular rate in AF depend on both AF rate and the degree of electrotonic modulation in the AVJ. Finally, we show that multilevel interactions between AF and VP can generate various patterns of ventricular rhythm that are consistent with previous experimental observations

Fast communication: A simple method to quantify the morphological similarity between signals

We propose a simple index, termed adaptive signed correlation index (ASCI), to quantify the morphological similarity between signals. The ASCI between two signals is calculated by trichotomizing each signal based on predefined three signal subspaces, then calculating the signed correlation of the trichotomized vectors. Examples are shown to compare ASCI with conventional correlation coefficients with respect to the effects of signal perturbation and additive noise. The ASCI provides a robust and efficient measure of morphological similarity and has particular applications in embedded systems involving biological signal analysis.

Open source model for generating RR intervals in atrial fibrillation and beyond

BackgroundRealistic modeling of cardiac inter-beat (RR) intervals is highly desirable for basic research in cardiac electrophysiology, clinical management of heart diseases, and developing signal processing tools for ECG analysis.MethodsWe present an open source computer model that is capable to generate realistic time series of RR intervals in both physiologic and pathologic conditions. Detailed model structure and the software implementation are described.ResultsExamples are provided on how to use this model to generate RR intervals in atrial fibrillation with ventricular pacing, normal sinus rhythm with heart rate variability, and typical atrial flutter with atrioventricular block. The extensibility of the model is also discussed.ConclusionThe present computer model provides a unified platform wherein various types of ventricular rhythm can be simulated. The availability of this open source model promises to support and stimulate future studies.

Increased monophasic action potential dispersion in endothelin-1-induced ventricular arrhythmias.

The aim of this study was to investigate the changes in monophasic action potentials (MAP) from different sites in the heart and to determine MAP dispersion during endothelin-1 (ET-1) infusion. Standard ECG, left ventricular anterior, right ventricular lateral, right ventricular septal, and right ventricular apical MAPs and intra-arterial blood pressure were monitored in seven anesthetized open-chest mongrel dogs. After radiofrequency atrioventricular node ablation, ventricular pacing (70/min) was performed and intracoronary ET-1 (60 pmol/min) was administered into the left anterior descending coronary artery. Both MAPd90 and MAPd90 dispersion increased significant during ET-1 infusion. The onset of spontaneous monomorphic and polymorphic sustained ventricular tachycardias (sVT) was observed in five dogs (around 40 min), and nonsustained VTs (nsVT) developed in another two dogs. The increases in MAP and MAP dispersion lasted until the appearance of polymorphic nsVTs and sVTs, but at the time of these VTs this difference decreased. At the termination of the experiments, ventricular fibrillation occurred in six cases. In four cases third-phase early afterdepolarizations were recorded. Our results suggest that increased MAP dispersion and development of EAD contribute to the arrhythmogenic action of ET-1, and these phenomena might explain the pathogenesis of a wide variety of ventricular arrhythmias with different morphology observed in this study.

Optimizing the Geometry of Implantable Leads for Recording the Monophasic Action Potential with Fractally Coated Electrodes

This study investigates the influence of various lead geometry on intracardial signals like the monophasic action potential (MAP) to optimize the geometry of implantable MAP leads. The experimental results were compared with a field theoretical approach to the origin of MAP from the transmembrane potential (TAP). During the experiments several lead geometries (tip surface: 1.3 to 12 mm2; tip‐ring distance: 0.8 mm to 25 cm; ring surface: 1.8mm2 to 40 mm2) were investigated in endo‐ and epicardial positions in 12 dogs (17±9 kg). The electrodes were fixed passively (tines) or actively (screws). MAP was recorded during several interventions and correlated with MAP measured using an Ag‐AgCl MAP catheter. The experimental results showed that small tips provided high MAP amplitudes with less pressure. No difference was observed using active and passive fixations. A tip‐ring distance smaller than 5 mm with a ring surface smaller than the tip (<5 mm2) avoided artifacts in the repolarization course. For the theoretical approach the quasistatic, anisotropic bidomain model was calculated in smalt unity volumes Vi where the TAP φm was constant and represented by the current density J. Two solutions for electrode positions at and outside the heart were achieved. By superposition of each solution φei the summed potential at the electrode position was calculated. The theoretical findings show in good correlation with the experimental results that a larger distance than 10 mm leads to distortions in repolarization course by signals proportional to φout.

First results of an implantable sensor for blood flow measurement

Information about the hemodynamic state of the human organism is very important in the electrotherapy of the heart. In the therapy of tachycardias with implantable defibrillators ventricular and supraventricular tachycardias have to be discriminated from each other to avoid unnecessary electrical shocks. In this study two new implantable sensors for blood flow measurements are evaluated. The first sensor determines the change of conductivity of blood depending on the flow rate. This effect is based on shear forces which lead to an orientation of cellular blood components causing anisotropic electrical properties of the blood. For measuring the relative conductivity of blood a bipolar pacemaker lead is used. The second sensor principle is based on the measurement of the resistance of a heated resistor. This depends on the blood flow due to thermal transport of the blood. Both hemodynamic sensors were implanted in dogs in the coronary sinus, the vena cava superior and the right outflow tract. The sensor signal-the impedance of the blood or the powered resistor respectively-was measured during electrical stimulation of the heart and during atrial and ventricular fibrillation. The course of the signal of the blood conductivity sensor reflects the course of the mean arterial blood pressure (MAPB) during the heart cycle. The absolute values of the sensed signals depend on the position of the sensor. In conclusion, both sensor principles offer information about blood flow and cardiac output, which can be used for a better control of ICD therapy or new pacing algorithms.

Mechanisms of Inappropriate Defibrillator Therapy in a Modern Cohort of Remotely Monitored Patients

Defibrillator (ICD) technology and monitoring are evolving rapidly. We investigated the mechanisms of inappropriate ICD therapies in a modern cohort of patients followed at our institution via remote monitoring.

Computer modelling of ventricular rhythm during atrial fibrillation and ventricular pacing

In this paper, we describe a computer model of the ventricular response to atrial fibrillation (AF) in the presence of ventricular pacing (VP). In this model, the AV junction (AVJ) is treated as a lumped structure with defined electrical properties including refractory period and automaticity. The AVJ is randomly bombarded by AF impulses, and can be invaded by the retrograde activation wave induced by the VP as well. The model further incorporates bidirectional conduction delays in both AVJ and ventricle, and the recovery-dependency of the AVJ properties is also considered. Simulations show that dependent upon model parameter settings, multilevel AF-VP interactions may occur to generate various ventricular rhythms that are consistent with experimental observations. This model may be used to study the mechanism of AF-VP interaction and to develop VP-based ventricular rate smoothing algorithms for ventricular rhythm control during AF.

SIMULTANEOUS RECORDINGS OF THE MONOPHASIC ACTION POTENTIAL WITH SILVER CHLORIDE- AND IR-COATED ELECTRODES

Ag AgCI and Ir‐coated electrodes allow the recording of the monophasic action potential (MAP) due to their electrical properties like non‐polarisability. This study investigates the correlation of MAP recorded with both types of electrodes. In 20 mongrel dogs (18 ± 6 kg) an Ag/AgCI and an Ir‐coated catheter (Ir) were placed endo‐cardially in the apex of the right ventricle. The effects of isoproterenol and verapamil were investigated during spontaneous rhythm and stimulation simultaneously recorded with both types of electrodes in 10 dogs without AV‐node ablation. The correlation at different heart rates were investigated in 10 other dogs with complete AV‐block. The morphology and amplitudes of MAP were comparable (AgCl: 15±7 mV; Ir: 13±8 mV). Following an i.v. bolus of 2μg/kg isoproterenol the spontaneous rate increased (175±18 to 245±25 bpm). During stimulation with 250 ms cycle length the duration shortened (MAPd90: AgCl: 160 ± 11 to 130 ± 12 ms; Ir: 154 ± 18 to 128±15 ms). The alterations reversed after 20 mm. An i.v. bolus of 0.2 mg/kg verapamil decreased the spontaneous rate (167±11 to 104 ± 23 bpm) and lengthened the MAPd90 (AgCl: 182 ± 14 to 220±13 ms; Ir: 174 ± 16 to 216, 21 ms) at 300 ms stimulation. The correlation between the MAPd90 of both lead types was r=0.98 during all measurements. Under the effect of beta‐agonist and Ca2+ ‐antagonist medication MAP showed a strong correlation recorded with both types of electrodes. Thus, both leads allow the recording of MAP but only the Ir‐electrodes with their long‐term stability are implantable and allows us to control the effects of drugs with implantable devices.

Adaptive Ventricular Rate Smoothing During Atrial Fibrillation: A Pilot Comparison Study

An adaptive ventricular rate smoothing (VRS) algorithm is developed to regularize the ventricular rate during atrial fibrillation (AF) by means of ventricular pacing (VP). Using a quantitative AF-VP model, we conduct pilot study to compare its performance with three other VRS algorithms. Simulations show that all VRS algorithms are effective to stabilize the heart rate during AF when intrinsic ventricular rate is not higher than the maximum pacing rate. The effect of VRS is diminished as the intrinsic ventricular rate increases, whereas slower intrinsic ventricular rate renders more aggressive VP. Compared to other methods, the Adaptive-VRS algorithm tends to stabilize the ventricular rate during AF with less VP, while intrinsic ventricular responses with physiological rate and rhythm are preferably preserved