Tidal volume and minute ventilation parameters derived from pacemaker impedance measurements can predict experimental heart failure development.

BACKGROUND: Specific respiratory patterns and periodic breathing have been associated with heart failure. Less is known regarding changes in tidal volume (TV) and minute ventilation (MV) as a result of early heart failure (HF) decompensation. METHODS AND RESULTS: Twelve adult Yucatan minipigs were implanted with a biventricular pacemaker and a left ventricular pressure sensor. HF was induced using high-rate pacing at 240 paces per minute for 2-4 weeks, followed by 2 weeks of recovery. Left ventricular pressure measurements and weekly echocardiograms verified the development of HF. The right and left ventricular intrathoracic impedance (RVITI and LVITI, respectively) signals were used to determine the respiratory parameters of rate, TV, and MV. Compared to baseline (BL), during HF, the TV dropped 68% for RVITI and 61% for LVITI (P < 0.0001 for both). Correspondingly, MV dropped 34% for RVITI and 27% for LVITI (P < 0.0001 for both). The daily medians of the respiratory rate (RR) and the longest breath interval (LBI) did not change significantly from BL to HF and recovery. However, circadian variation of the RR and the LBI became blunted during HF development. All derived respiratory parameters showed the reverse trend during the recovery period. CONCLUSION: TV and MV change independently from the RR in early HF decompensation. Tracking the changes of TV and MV with an implantable device may provide an additional method for early HF detection and assessment of the response to therapy.

Simulation of AV hysteresis pacing using an integrated dual chamber heart and pacer model.

Long term right ventricular apical pacing has been known to have adverse effects in cardiac function. The AV hysteresis (AVH) is a feature existing in many dual-chamber cardiac pacemakers that aims to minimize the right ventricular pacing, but its clinical efficacy remains inconclusive due to conflicting evidence from different studies. We have recently developed a novel integrated dual-chamber heart and pacer (IDHP) model, which can simulate various interactions between intrinsic heart activity and extrinsic cardiac pacing. In this study, we use the IDHP model to simulate various atrio-ventricular (AV) conduction pathologies, and to investigate the effects of an AVH algorithm on reducing right ventricular pacing. Our results show that the efficacy of AVH is dependent on the underlying cardiac conditions. While it can preserve intrinsic conduction during minor or moderate first degree AV block, its efficacy is reduced at higher degree AV block conditions. This pilot study further supports using the IDHP model to design and evaluate more advanced pacemaker algorithms for therapeutic interventions.

Heart rhythm and cardiac pacing: an integrated dual-chamber heart and pacer model.

Modern cardiac pacemaker can sense electrical activity in both atrium and ventricle, and deliver precisely timed stimulations to one or both chambers on demand. However, little is known about how the external cardiac pacing interacts with the heart's intrinsic activity. In this study, we present an integrated dual-chamber heart and pacer (IDHP) model to simulate atrial and ventricular rhythms in the presence of dual chamber cardiac pacing and sensing. The IDHP model is an extension and improvement of a previously developed open source model for simulating ventricular rhythms in atrial fibrillation and ventricular pacing. The new model takes into account more realistic properties of atrial and ventricular rhythm generators, as well as bi-directional conductions in atrium, ventricle, and the atrio-ventricular junction. Moreover, an industry-standard dual-chamber pacemaker timing control logic is incorporated in the model. We present examples to show that the new model can generate realistic cardiac rhythms in both physiologic and pathologic conditions, and simulate various interactions between intrinsic heart activity and extrinsic cardiac pacing. Among many applications, the IDHP model provides a new simulation platform where it is possible to bench test advanced pacemaker algorithms in the presence of different types of cardiac rhythms.

A simulation framework for dual chamber heart rhythm and cardiac pacing.

A previously developed open source computer model allows realistic simulation of ventricular intervals in atrial fibrillation, while taking into account of ventricular pacing. In this paper, we further improve this model and present a new simulation framework based on an integrated dual-chamber heart and pacer (IDHP) model. The IDHP model incorporates more realistic atrial and ventricular rhythm generators and an industry-standard dual-chamber pacemaker timing control logic. Moreover, it simulates various interactions between intrinsic heart activity and extrinsic cardiac pacing. The IDHP model provides a new simulation platform where it is possible to bench test advanced pacemaker algorithms in the presence of different types of cardiac rhythms.

Tiletamine-zolazepam and xylazine is a potent cardiodepressive combination: a case report.

Intramuscular injection of tiletamine-zolazepam and xylazine is commonly used as a preanesthetic for veterinary surgical procedures and for short-term restraint. However, this combination can have marked cardiodepressive and hypothermic effects that persist for hours to days. Here we present a case report of these effects in a swine heart failure model.

Ventricular rate smoothing for atrial fibrillation: a quantitative comparison study.

AIMS: To quantitatively compare the ventricular rate-smoothing (VRS) effects of different ventricular pacing (VP) protocols for atrial fibrillation (AF). METHODS AND RESULTS: Using a recently developed open-source model that can simulate the ventricular response in AF and VP, the performance of fixed-rate pacing and four previously published VRS algorithms were assessed by the mean RR (mRR), the root mean square of successive RR differences (RMSSD), the percentage of ventricular senses (VS%), and the percentage of short RR intervals (sRR%). All pacing protocols cause rate-dependent reduction of RMSSD, VS%, and sRR% with or without shortening of mRR compared to spontaneous AF. Fixed-rate pacing was more sensitive to the intrinsic rate than the VRS algorithms. The performance was generally comparable between different VRS algorithms, although higher mRR and VS% can be achieved at the expense of larger RMSSD and sRR%. CONCLUSION: The effect of VP on ventricular rhythm in AF depends on both intrinsic rate and the aggressiveness of the pacing protocol. Adequate rate control is necessary for effective operation of the VRS algorithm. Choosing VRS algorithm should balance between the beneficial effects of rate regularization and the negative effects of increasing heart rate and percentage of VP.

On the role of ventricular conduction time in rate stabilization for atrial fibrillation.

AIM: Ventricular pacing (VP) could stabilize the ventricular rhythm in atrial fibrillation (AF). This study investigates the role of ventricular conduction time (VCT) in rate stabilization for AF. METHODS AND RESULTS: A novel computer model was used to generate various patterns of RR intervals in AF. For each model configuration, the rate stabilization effect of VP was compared with respect to different VCTs. In all tested cases, the ventricular rate in AF could be stabilized at pacing intervals longer than the shortest spontaneous RR intervals. For each model configuration, slightly longer pacing interval (difference <100 ms) was needed to achieve 95% VP when the antegrade/retrograde VCT was increased from 10/10 to 110/110 ms, whereas the VCT had less effect at lower pacing rate. Although longer VCT was associated with increased percentage of ventricular fusion, its role was diminished at higher pacing rate when more retrograde waves could conduct to the atrium. CONCLUSION: Ventricular conduction time has limited effects on rate stabilization, which could be explained by multi-level interactions between antegrade waves induced by AF and retrograde waves induced by VP.

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

BACKGROUND: Realistic 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. METHODS: We 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. RESULTS: Examples 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. CONCLUSION: The 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.

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.

Validation of a novel atrial fibrillation model through simulated atrial pacing protocols.

We have previously reported a novel model to elucidate the effects of ventricular pacing (VP) on RR intervals during atrial fibrillation (AF). This model treats the AV junction (AVJ) as a lumped structure with defined conductivity, refractoriness and automaticity. We have shown that this model could account for various patterns of RR intervals that are consistent with experimental observations. In this study, we further validate this model by comparing its behavior with that of a real AVJ obtained in isolated rat heart preparation, through application of programmed atrial pacing protocols. We demonstrate that the AV conduction time and ventricular response of the present model are consistent with experimental findings, thus providing additional evidence to support the validity of our model.

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.