Piotr Podziemski

A simple model of the right atrium of the human heart with the sinoatrial and atrioventricular nodes included

Existing atrial models with detailed anatomical structure and multi-variable cardiac transmembrane current models are too complex to allow to combine an investigation of long time dycal properties of the heart rhythm with the ability to effectively simulate cardiac electrical activity during arrhythmia. Other ways of modeling need to be investigated. Moreover, many state-of-the-art models of the right atrium do not include an atrioventricular node (AVN) and only rarely—the sinoatrial node (SAN). A model of the heart tissue within the right atrium including the SAN and AVN nodes was developed. Looking for a minimal model, currently we are testing our approach on chosen well-known arrhythmias, which were until now obtained only using much more complicated models, or were only observed in a clinical setting. Ultimately, the goal is to obtain a model able to generate sequences of RR intervals specific for the arrhythmias involving the AV junction as well as for other phenomena occurring within the atrium. The model should be fast enough to allow the study of heart rate variability and arrhythmias at a time scale of thousands of heart beats in real-time. In the model of the right atrium proposed here, different kinds of cardiac tissues are described by sets of different equations, with most of them belonging to the class of Liénard nonlinear dynamical systems. We have developed a series of models of the right atrium with differing anatomical simplifications, in the form of a 2D mapping of the atrium or of an idealized cylindrical geometry, including only those anatomical details required to reproduce a given physiological phenomenon. The simulations allowed to reconstruct the phase relations between the sinus rhythm and the location and properties of a parasystolic source together with the effect of this source on the resultant heart rhythm. We model the action potential conduction time alternans through the atrioventricular AVN junction observed in cardiac tissue in electrophysiological studies during the ventricular-triggered atrial tachycardia. A simulation of the atrio-ventricular nodal reentry tachycardia was performed together with an entrainment procedure in which the arrhythmia circuit was located by measuring the post-pacing interval (PPI) at simulated mapping catheters. The generation and interpretation of RR times series is the ultimate goal of our research. However, to reach that goal we need first to (1) somehow verify the validity of the model of the atrium with the nodes included and (2) include in the model the effect of the sympathetic and vagal ANS. The current paper serves as a partial solution of the 1). In particular we show, that measuring the PPI–TCL entrainment response in proximal (possibly-the slow-conducting pathway), the distal and at a mid-distance from CS could help in rapid distinction of AVNRT from other atrial tachycardias. Our simulations support the hypothesis that the alternans of the conduction time between the atria and the ventricles in the AV orthodromic reciprocating tachycardia can occur within a single pathway. In the atrial parasystole simulation, we found a mathematical condition which allows for a rough estimation of the location of the parasystolic source within the atrium, both for simplified (planar) and the cylindrical geometry of the atrium. The planar and the cylindrical geometry yielded practically the same results of simulations.

RS slope detection algorithm for extraction of heart rate from noisy, multimodal recordings.

Current gold-standard algorithms for heart beat detection do not work properly in the case of high noise levels and do not make use of multichannel data collected by modern patient monitors. The main idea behind the method presented in this paper is to detect the most prominent part of the QRS complex, i.e. the RS slope. We localize the RS slope based on the consistency of its characteristics, i.e. adequate, automatically determined amplitude and duration. It is a very simple and non-standard, yet very effective, solution. Minor data pre-processing and parameter adaptations make our algorithm fast and noise-resistant. As one of a few algorithms in the PhysioNet/Computing in Cardiology Challenge 2014, our algorithm uses more than two channels (i.e. ECG, BP, EEG, EOG and EMG). Simple fundamental working rules make the algorithm universal: it is able to work on all of these channels with no or only little changes. The final result of our algorithm in phase III of the Challenge was 86.38 (88.07 for a 200 record test set), which gave us fourth place. Our algorithm shows that current standards for heart beat detection could be improved significantly by taking a multichannel approach. This is an open-source algorithm available through the PhysioNet library.

Quantitative description of the 3D regional mechanics of the left atrium using cardiac magnetic resonance imaging.

The left atrium (LA) plays an important role in the maintenance of hemodynamic and electrical stability of the heart. One of the conditions altering the atrial mechanical function is atrial fibrillation (AF), leading to an increased thromboembolic risk due to impaired mechanical function. Preserving the regions of the LA that contribute the greatest to atrial mechanical function during curative strategies for AF is important. The purpose of this study is to introduce a novel method of regional assessment of mechanical function of the LA. We used cardiac MRI to reconstruct the 3D geometry of the LA in nine control and nine patients with paroxysmal atrial fibrillation (PAF). Regional mechanical function of the LA in pre-defined segments of the atrium was calculated using regional ejection fraction and wall velocity. We found significantly greater mechanical function in anterior, septal and lateral segments as opposed to roof and posterior segments, as well as a significant decrease of mechanical function in the PAF group. We suggest that in order to minimize the impact of the AF treatment on global atrial mechanical function, damage related to therapeutic intervention, such as catheter ablation, in those areas should be minimized.

Far-field effect in unipolar electrograms revisited: High-density mapping of atrial fibrillation in humans.

Unipolar electrogram can detect local as well as remote electrical activity of the heart. Information on how the amplitude and morphology of the recorded signal changes with the distance from the source tissue undergoing depolarization can help to better understand unipolar electrograms fractionation and provide insights into the passive conduction properties of the atrial tissue. Ten second unipolar atrial fibrillation (AF) electrograms were recorded using high-density electrode array from the posterior left atrium (LA) and right atrium (RA) of 19 (8 persistent - PERS & 11 paroxysmal - PAF) AF patients undergoing cardiac surgery. Conduction along lines of conduction block was detected in the recorded activation patterns by a proposed automated algorithm. Changes of the amplitude of the unipolar electrogram with increasing distance from the conduction blocks were assessed and compared to predictions of a theoretical model. For each recording, the median far-field decay space constant (FF0.5) was calculated. Overall, we found a significant difference between FF0.5 for patients with paroxysmal and persistent AF. Estimation of maximum FF0.5 from both RA and LA resulted in a mean FF0.5 of 1.5±0.2 mm for PERS patients and 2.1±0.6 mm for PAF patients (p=0.03). Moreover, detected conduction blocks demonstrated high spatial organization and appeared in distinctive areas of the mapped area in all patients, regardless of the type of AF, while the total number of detected block lines was higher in PERS patients.

Effect of the restitution properties of cardiac tissue on the repeatability of entrainment mapping response.

Background—The difference between the postpacing interval (PPI) and the tachycardia cycle length (TCL; PPI−TCL) is a useful tool in mapping macro-reentrant tachycardias. However, entrainment pacing causes some perturbation of the conduction velocity within the tachycardia circuit, which may affect the repeatability and consequently the accuracy of the measurement of PPI−TCL. The aim of this study was to assess PPI−TCL repeatability both in vivo and in silico. Methods and Results—In the experimental part, entrainment pacing was performed twice at each of the 124 tested sites for 30 patients undergoing radiofrequency ablation of atrial and ventricular re-entrant arrhythmias. A similar protocol was used in a simplified computer model of the cardiac tachycardia circuit in a 2-dimensional tissue strip using a Fenton–Karma model of cardiac tissue. In vivo, in the case of fast tachycardias (<350 ms), PPI−TCL variability observed was doubled compared with slow tachycardias (>350 ms; 95% Limits of Agreement ranged from −21.4 to 21.6 ms for TCL<350 ms and from −10.8 to 11.5 ms for TCL>350 ms). Simulations show that this increase of variability may be because of the oscillations of the conduction velocity inside the tachycardia circuits. The effect of the restitution properties of cardiac tissue on the outcome of entrainment pacing is discussed. Conclusions—PPI−TCL is characterized by a high repeatability with the differences between the results for individual stimulations of ⩽20 ms. The variability of this parameter is significantly lower in the case of slow tachycardias.

Stationary atrial fibrillation properties in the goat do not entail stable or recurrent conduction patterns

Introduction: Electro-anatomical mapping of the atria is used to identify the substrate of atrial fibrillation (AF). Targeting this substrate by ablation in addition to pulmonary vein ablation did not consistently improve outcome in clinical trials. Generally, the assessment of the substrate is based on short recordings (≤10 s, often even shorter). Thus, targeting the AF substrate assumes spatiotemporal stationarity but little is known about the variability of electrophysiological properties of AF over time. Methods: Atrial fibrillation (AF) was maintained for 3–4 weeks after pericardial electrode implantation in 12 goats. Within a single AF episode 10 consecutive minutes were mapped on the left atrial free wall using a 249-electrode array (2.25 mm inter-electrode spacing). AF cycle length, fractionation index (FI), lateral dissociation, conduction velocity, breakthroughs, and preferentiality of conduction (Pref) were assessed per electrode and AF property maps were constructed. The Pearson correlation coefficient (PCC) between the 10 AF-property maps was calculated to quantify the degree spatiotemporal stationarity of AF properties. Furthermore, the number of waves and presence of re-entrant circuits were analyzed in the first 60-s file. Comparing conduction patterns over time identified recurrent patterns of AF with the use of recurrence plots. Results: The averages of AF property maps were highly stable throughout the ten 60-s-recordings. Spatiotemporal stationarity was high for all 6 property maps, PCC ranged from 0.66 ± 0.11 for Pref to 0.98 ± 0.01 for FI. High stationarity was lost when AF was interrupted for about 1 h. However, the time delay between the recorded files within one episode did not affect PCC. Yet, multiple waves (7.7 ± 2.3) were present simultaneously within the recording area and during 9.2 ± 11% of the analyzed period a re-entrant circuit was observed. Recurrent patterns occurred rarely and were observed in only 3 out of 12 goats. Conclusions: During non-self-terminating AF in the goat, AF properties were stationary. Since this could not be attributed to stable recurrent conduction patterns during AF, it is suggested that AF properties are determined by anatomical and structural properties of the atria even when the conduction patterns are very variable.

Towards application of complexity measures of atrial electrograms to predict outcome of the ablation procedure

The aim of this study was to assess the reliability of the complexity analysis of a single electrogram as a predictor of spontaneous termination of Atrial Fibrillation (AF) during ablation procedure. Left and right atrial endocardial bipolar electrograms from two locations (High Right Atrium - HRA and Coronary Sinus - CS) were recorded before ablation treatment of AF (at baseline) in 36 patients. Information about the ablation outcome (cardioversion - CV or spontaneous termination - TEKM) was collected. For each electrogram, algorithmic complexity (AC) and Shannon entropy was calculated. Baseline electrograms from electrodes located in HRA had significantly lower algorithmic complexity than from electrodes located in Cs. Only the Shannon Entropy of electrogram measured at CS showed significant diference between CV and TERM (p=0.03), while in case of algorithmic complexity only a trend towards significance was found (p=0. 08). Electrogram complexity parameters used in clinical practice did not distinguish the ablation outcome groups, with Shannon entropy showing the most significant diference.

Far-field effect in unipolar electrograms recorded from epicardial and endocardial surface: Quantification of epi-endo dissociation during atrial Fibrillation in Humans

In this study we explore whether endo-epicardial dissociation during atrial fibrillation (AF) can explain origin of some of the far-field components in unipolar electrograms. To assess the number of far-field deflections in unipolar electrograms that have a source on the contralateral side of the atrial wall we used simultaneous endo-epicardial high-resolution contact mapping. 30s endo-epicardial electrograms were recorded using two 64 electrode arrays directly opposing each other, placed on the right atrial wall in 5 patients with persistent AF. For all far-field deflections that could not be explained by local activation within the same plane, we searched for a passing wavefront on the other side of the atrial wall. 74±3% of detected far-field deflections could be explained by activation on the same side of the atrial wall. Within the remaining deflections, 42±5% had a source in the activity taking place directly on the other side of the atrial wall. 15±3% of all detected far-fields were of unknown origin. High proportion of the far-field deflections detected using contact mapping during AF results from endo-epicardial dissociation may have an impact on proper annotation of local activity, and therefore on identification of conduction patterns. Calculating the number of far-field deflections in unipolar electrograms due to endo-epi dissociation may help to quantitatively describe transmural dissociation.

Electrogram coupling as a measure of local conduction during atrial fibrillation

Epicardial wavefront conduction patterns during atrial fibrillation (AF) can be recorded using high-density contact mapping. Quantification of electrogram morphology similarity at adjacent recording sites during AF may characterize substrate complexity without the need for electrogram annotation. Electrogram coupling was quantified as the decay in reconstruction quality of a central electrogram by its neighbors at increasing distance. Coupling was computed in patients in paroxysmal AF (PAF, n=12) and persistent AF (persAF, n=9) with a 16 ×16 grid of electrodes (1.5mm electrode distance) with nonnegative least squares using only complete, symmetric topologies. Half-decay distance c0.5 was compared to conventional conduction-related contact mapping parameters. Electrogram coupling was weaker in persAF than in PAF (c0.5 (median±MAD): 2.4±0.5mm vs. 3.2±1.2mm, p<;0.02). High correlation was found between mean c0.5 and CV (ρ=0.80, p<;0.001). Other parameters showed only moderate or no correlation. Differences in AF conduction velocity between patients can be accurately described using a surrogate parameter based on the degree of electrogram coupling. This technique can for instance be applied to high-density contact recordings to quickly assess the Class I effect of antiarrhythmic drugs, both in atrial and ventricular recordings.