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.
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This work is fully funded by Biotronik GmbH. J. L. and D. M. are both employees of Micro Systems Engineering Inc., a wholly owned subsidiary of Biotronik. Interested readers can contact the authors for using the software program of the present model for research purpose.
Author information Authors and AffiliationsMicro Systems Engineering, Inc., 6024 SW Jean Road, Lake Oswego, OR, 97035, USA
Jie Lian & Dirk Müssig
Correspondence to Jie Lian.
Appendices Appendix A: Basics of Pacemaker TimingModern pacemakers have many therapeutic and diagnostic features, which are supported by hundreds of sophisticated pacer-specific timers and windows. While it is beyond the scope of this paper to discuss the detailed pacemaker timing logic, the basic concept of dual-chamber pacemaker sensing and pacing is summarized below.
Figure A1 illustrates some common pacemaker windows used for device sensing. After each AP or VP, a blanking window is applied to the same channel and another blanking window is applied to the other channel to prevent device from sensing the pacing artifact. Any electrical event occurring in these blanking windows is not seen by the pacemaker. Each AP (or VP) also starts an atrial (or ventricular) refractory window, when an atrial (or ventricle) event can be seen, but usually is ignored by the pacemaker. A VP triggers both a far-field window and a post-ventricular atrial refractory period in the atrial channel. The former prevents far-field sensing of ventricular depolarization, and the latter prevents tracking of retrograde AS event that may lead to pacemaker-mediated tachycardia (see Fig. 11). On the other hand, an AP triggers a safety window in the ventricle channel. If an intrinsic ventricular activity is detected during this window (could not be differentiated from cross-talk), a VP will be triggered at the end of the safety window to prevent ventricular asystole. In addition, an upper tracking interval is applied after each VP, which sets the upper limit to the ventricular pacing rate in DDD mode (see Fig. 9). Similar windows are also applied after each AS or VS event (not shown).
Figure A1Schematic illustration of the pacemaker windows used for sensing
Figure A2 illustrates the basic timing concept of pacemaker in (a) DDD mode and (b) DDI mode. DDD mode is an atrial-tracking mode, i.e., pacer’s basic interval and AV delay start with each AP or AS. Timeout of the basic interval triggers an AP, and timeout of the AV delay triggers a VP. An AS event detected during the basic interval inhibits the AP, and a VS event detected during the AV delay inhibits the VP. Additionally, a pacer-classified ventricular extra-systole reschedules the next AP at the calculated VA delay. Contrarily, DDI mode is a non-atrial tracking mode. The pacer’s basic interval and VA delay starts with each VP or VS. Timeout of the basic interval triggers a VP, and timeout of the VA delay triggers an AP. A VS event detected during the basic interval inhibits the VP, and an AS event detected during the VA delay inhibits the AP.
Figure A2Schematic illustration of the pacemaker timing in (a) DDD mode and (b) DDI mode. BI: basic interval; AVD: AV delay; VAD: VA delay; VES: ventricular extra-systole
Appendix B: Model ParametersThe simulation loads model parameters from an external configuration file. The model parameters are grouped into nine parts, corresponding to the simulation environment and eight model components. Most parameters inherited from the AF–VP model have been described elsewhere,12,16 whereas those extended heart model parameters and pacer specific model parameters can be understood from the text or comments. An exemplary set of model parameters is listed in Table B1. The parameters that are changed to produce Figs. 6–11 are, respectively, listed in Table B2.
Table B1 List of an exemplary set of model parameters Table B2 List of model parameter changes from Table B1 to produce Figs. 6–11 Appendix C: Model Event HandlingThis section describes the IDHP event handling services shown in Figs. 4 and 5.
The ArgOutput service is called upon an atrial rhythm generator output (Fig. C1a). The model gets the atrial impulse strength (ΔV) and predicts the arrival time of the next impulse. No atrial depolarization is possible if the atrium is still refractory; otherwise, an antegrade atrial activation wave is generated. The VrgOutput service for ventricle rhythm generator output (Fig. C1b) is similarly implemented, except that the ventricular impulse is assumed strong enough that it can bring the AVJ membrane potential (V m) to the depolarization threshold (V T).
Figure C1The event handling flowcharts for (a) atrial rhythm generator output and (b) ventricle rhythm generator output
Atrial fusion (AtrFusion) terminates both antegrade and retrograde atrial activation waves (Fig. C2a), and ventricle fusion (VtrFusion) annihilates ventricular activation waves in both directions (Fig. C2b).
Figure C2The event handling flowcharts for (a) atrial fusion and (b) ventricular fusion
As shown in Fig. C3a, the antegrade atrial conduction stops when it hits the AVJ (AntHitAvj). If AVJ is in phase 4, then V m has a step increase of ΔV, and antegrade AVJ activation is flagged if V m ≥ V T. If AVJ is still refractory, then its refractory period is extended (electrotonic modulation). If there is a retrograde wave in the AVJ and the atrial impulse has supra-threshold strength, then the retrograde AV conduction is disabled. The counterpart service for retrograde invasion of AVJ (RetHitAvj) is similarly implemented (Fig. C3b).
Figure C3The event handling flowcharts for (a) antegrade invasion of AVJ and (b) retrograde invasion of AVJ
The AnteEscAvj service is called upon completion of the antegrade AV conduction (Fig. C4a). It also enables the AVJ for recovery, and starts an antegrade ventricle activation wave if the ventricle is not refractory. Similarly, the RetrEscAvj service is implemented (Fig. C4b).
Figure C4The event handling flowcharts for (a) antegrade escaping from AVJ and (b) retrograde escaping from AVJ
The ActivateAvj service (Fig. C5a) is called when V m ≥ V T. It calculates the AV conduction time based on elapsed AVJ recovery time. If the AVJ is antegrade (or retrograde) excited, then an antegrade (or retrograde) AVJ activation wave is generated if there is no activation wave in the opposite direction; otherwise, the opposite activation wave is stopped (AVJ fusion). Start of the AVJ phase 4 (StartAvjPh4) resets V m to the resting potential (V R) and enables AVJ to recover (Fig. C5b). The StartAvjRef service (Fig. C5c) flags the AVJ refractory phase, calculates the refractory period based on the AVJ recovery time, and then starts the AVJ refractory timer.
Figure C5The event handling flowcharts for (a) activation of AVJ, (b) start AVJ phase 4, and (c) start AVJ refractory period
The AtrPace service first checks if the AP amplitude is above the atrial threshold and if the atrium is non-refractory (Fig. C6a). Only if both conditions are met, an antegrade atrial activation wave is generated with supra-threshold strength, and the atrial rhythm generator is reset. Any AP (capture or non-capture) will affect pacer’s operation, such as interval measurement, timer update, and marker annotation. Similar implementation of the VtrPace service is shown in Fig. C6b.
Figure C6The event handling flowcharts for (a) atrial pacing and (b) ventricular pacing
The AtrSense service is called upon detection of any electrical activity by the atrial lead (Fig. C7a). If the sense is caused by retrograde atrial activation, then the atrial rhythm generator is reset. For each atrial sense (excluding blanked ones), pacer will classify its event type, update various counters, timers, intervals, and record the event markers. Similar implementation of the VtrSense service is shown in Fig. C7b.
Figure C7The event handling flowcharts for (a) atrial sensing and (b) ventricular sensing
About this article Cite this articleLian, J., Müssig, D. Heart Rhythm and Cardiac Pacing: An Integrated Dual-Chamber Heart and Pacer Model. Ann Biomed Eng 37, 64–81 (2009). https://doi.org/10.1007/s10439-008-9585-x
Received: 08 May 2008
Accepted: 10 October 2008
Published: 18 October 2008
Issue Date: January 2009
DOI: https://doi.org/10.1007/s10439-008-9585-x
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