A Novel Ex Vivo Heart Model for the Assessment of Cardiac Pacing Systems

Assessing Torque Transfer in Conduction System Pacing: Development and Evaluation of an Ex Vivo Model

BACKGROUND

Conduction system pacing (CSP) faces challenges in achieving reliable and safe deployments. Complex interactions between tissue and lead tip can result in endocardial entanglement, a drill effect that prevents penetration. No verified ex vivo model exists to quantitatively assess this relationship.

OBJECTIVES

The purpose of this study was to quantitatively characterize CSP lead tip to tissue responses for 4 commonly used leads.

METHODS

CSP leads (from Medtronic, Biotronik, Boston Scientific, and Abbott) were examined for helix rotation efficiency in ex vivo ovine right ventricular septa. A custom jig was utilized for rotation measurements. Fifteen turns were executed, documenting tissue-interface changes every 90° using high-resolution photography. Response curves (input rotation vs helix rotation) were evaluated using piecewise linear regression, with a focus on output vs input response slopes and torque breakpoint events.

RESULTS

We analyzed 3,840 quarter-turn CSP insertions with 4 different lead types. Helix rotations were consistently less than input: Abbott Tendril = 0.21:1, Medtronic 3830 = 0.21:1, Biotronik Solia = 0.47:1, and Boston Scientific Ingevity = 0.56:1. Torque breakpoint events were observed on average 7.22 times per insertion (95% CI: 6.08-8.35; P = NS) across all leads. In 57.8% of insertions (37 of 64), uncontrolled torque breakpoint events occurred, signaling unexpected excess helix rotations.

CONCLUSIONS

Using a robust ex vivo model, we revealed a muted helix rotation response compared with input turns on the lead, and frequent torque change events during insertion. This is critical for CSP implanters, emphasizing the potential for unexpected torque breakpoint events, and suggesting the need for novel lead designs or deployment methods to enhance CSP efficiency and safety.

Induced functional modulations of isolated large mammalian hearts

In this study we used Visible Heart® methodologies featuring cyclic temperature modulation of porcine hearts in order to establish characteristic temperature responses. This isolated and perfused model is a more predictable and modifiable analog for human heart preservation and isolates the response of the cardiac tissue. We comprehensively monitored isolated porcine hearts undergoing temperature change and demonstrated optimization of isolated cardiac function under mild hypothermia. We tracked metrics of cardiac function as continuous variables during temperature changes (~ 31 to 39 °C), eliciting a well-defined reduction in metabolic demand and in heart rate modulation. Optimization of function appeared to occur around 34.7 ± 0.9 °C (n = 13). Cardiac response was further investigated in the presence of active pacing in order to assess pacing capture and the heart's functional response without a means of regulating rate. Our results may have direct clinical implications for emerging heart preservation methods prior to transplantation, as well as benefits for investigators using isolated heart models for preclinical device testing. Clinically, this porcine model is a basis for finding new ways to extend the window of viability for transplantable organs, thereby restoring or improving graft function and potentially enhancing recipient outcomes.

The Visible Heart® project and methodologies: novel use for studying cardiac monophasic action potentials and evaluating their underlying mechanisms

INTRODUCTION

This review describes the utilization of Visible Heart® methodologies for electrophysiologic studies, specifically in the investigation of monophasic action potential (MAP) recordings, with the aim to facilitate new catheter/device design and development that may lead to earlier diagnosis, treatment, and ultimately a higher quality of life for patients with atrial fibrillation.

AREAS COVERED

We describe the historically proposed mechanisms behind which electrode is responsible for the MAP recording, new catheters for recording these signals, and how Visible Heart methodologies can be utilized to develop and test new technologies for electrophysiologic investigations.

EXPERT OPINION

When compared to traditional electrogram recordings, MAP waveforms provide clinical information vital to the understanding, diagnosis, and treatment of cardiac arrhythmias. New catheters and ablation technologies are routinely being assessed on reanimated large mammalian hearts (swine and human) in our laboratory. These abilities, combined with continued enhancements in imaging modalities and computational systems for electrical mapping, are being applied to the MAP catheter design process. Through this testing we are hopeful that the time from concept to product can be reduced, and that an array of MAP catheters can be placed in the hands of physicians, where they will improve patient outcomes.

Interaction of a Transapical Miniaturized Ventricular Assist Device With the Left Ventricle: Hemodynamic Evaluation and Visualization in an Isolated Heart Setup

New left ventricular assist devices (LVADs) offer both important advantages and potential hazards. VAD development requires better and expeditious ways to identify these advantages and hazards. We validated in an isolated working heart the hemodynamic performance of an intraventricular LVAD and investigated how its outflow cannula interacted with the aortic valve. Hearts from six pigs were explanted and connected to an isolated working heart setup. A miniaturized LVAD was implanted within the left ventricle (tMVAD, HeartWare Inc., Miami Lakes, FL, USA). In four experiments blood was used to investigate hemodynamics under various loading conditions. In two experiments crystalloid perfusate was used, allowing visualization of the outflow cannula within the aortic valve. In all hearts the transapical miniaturized ventricular assist device (tMVAD) implantation was successful. In the blood experiments hemodynamics similar to those observed clinically were achieved. Pump speeds ranged from 9 to 22 krpm with a maximum of 7.6 L/min against a pressure difference between ventricle and aorta of ∼50 mm Hg. With crystalloid perfusate, central positioning of the outflow cannula in the aortic root was observed during full and partial support. With decreasing aortic pressures the cannula tended to drift toward the aortic root wall. The tMVAD could unload the ventricle similarly to LVADs under conventional cannulation. Aortic pressure influenced central positioning of the outflow cannula in the aortic root. The isolated heart is a simple, accessible evaluation platform unaffected by complex reactions within a whole, living animal. This platform allowed detection and visualization of potential hazards.

Organomatics and organometrics: Novel platforms for long-term whole-organ culture

Organ culture systems are instrumental as experimental whole-organ models of physiology and disease, as well as preservation modalities facilitating organ replacement therapies such as transplantation. Nevertheless, a coordinated system of machine perfusion components and integrated regulatory control has yet to be fully developed to achieve long-term maintenance of organ function ex vivo. Here we outline current strategies for organ culture, or organomatics, and how these systems can be regulated by means of computational algorithms, or organometrics, to achieve the organ culture platforms anticipated in modern-day biomedicine.

An ex vivo platform to simulate cardiac physiology: a new dimension for therapy development and assessment

PURPOSE

Cardiac research and development of therapies and devices is being done with in silico models, using computer simulations, in vitro models, for example using pulse duplicators or in vivo models using animal models. These platforms, however, still show essential gaps in the study of comprehensive cardiac mechanics, hemodynamics, and device interaction. The PhysioHeart platform was developed to overcome these gaps by the ability to study cardiac hemodynamic functioning and device interaction ex vivo under in vivo conditions.

METHODS

Slaughterhouse pig hearts (420 ± 30 g) were used for their morphological and physiological similarities to human hearts. Hearts were arrested, isolated and transported similar to transplantation protocols. After preparation, the hearts were connected to a special circulatory system that has been engineered using physical and medical principles. Through coronary reperfusion and controlled cardiac loading, physiological cardiac performance was achieved while hemodynamic parameters were continuously monitored.

RESULTS

Normal cardiac hemodynamic performance was achieved both qualitatively, in terms of pulse waveforms, and quantitatively, in terms of average cardiac output (4 l/min) and pressures (110/75 mmHg). Cardiac performance was controlled and kept at normal levels for up to 4 hours, with only minor deterioration of hemodynamic performance.

CONCLUSIONS

With the PhysioHeart platform we were able to reproduce normal physiological cardiac conditions ex vivo. The platform enables us to study, under different but controlled physiological conditions, form, function, and device interaction through monitoring of performance parameters and intra-cardiac visualization. Although the platform has been used for pig hearts, application of the underlying physical and engineering principles to physiologically comparable hearts from different origin is rather straightforward.

Cardiac device testing enhanced by simultaneous imaging modalities: the Visible Heart, fluoroscopy and echocardiography

The evaluation of implantable cardiac devices requires innovative and critical testing in all phases of the design process. The recent development of an in vitro isolated heart model, the Visible Heart, allows for novel examination of device-tissue interactions to evaluate device placement and performance within a functioning heart. This model provides product designers with a rapid method to critically assess prototypes, thus expediting design decisions. The future of cardiac devices will include beating heart procedures (e.g., transcatheter-delivered valves), and such implantations will probably utilize simultaneous multiple imaging modalities. These imaging modes allow for verification of proper device positioning, refinement of device/deployment procedures, and eventual evaluation of resultant cardiac function within experimental and clinical settings.

Advances in swine biomedical model genomics

This review is a short update on the diversity of swine biomedical models and the importance of genomics in their continued development. The swine has been used as a major mammalian model for human studies because of the similarity in size and physiology, and in organ development and disease progression. The pig model allows for deliberately timed studies, imaging of internal vessels and organs using standard human technologies, and collection of repeated peripheral samples and, at kill, detailed mucosal tissues. The ability to use pigs from the same litter, or cloned or transgenic pigs, facilitates comparative analyses and genetic mapping. The availability of numerous well defined cell lines, representing a broad range of tissues, further facilitates testing of gene expression, drug susceptibility, etc. Thus the pig is an excellent biomedical model for humans. For genomic applications it is an asset that the pig genome has high sequence and chromosome structure homology with humans. With the swine genome sequence now well advanced there are improving genetic and proteomic tools for these comparative analyses. The review will discuss some of the genomic approaches used to probe these models. The review will highlight genomic studies of melanoma and of infectious disease resistance, discussing issues to consider in designing such studies. It will end with a short discussion of the potential for genomic approaches to develop new alternatives for control of the most economically important disease of pigs, porcine reproductive and respiratory syndrome (PRRS), and the potential for applying knowledge gained with this virus for human viral infectious disease studies.

In vivo versus in vitro comparison of swine cardiac performance: Induction of cardiodepression with halothane

An in situ versus in vitro comparison of relative dose-dependent effects of halothane on cardiac performance was investigated, including ventricular systolic/diastolic function. Such comparative studies may be of interest to individuals working on heart failure models, cardiac device testing, or xenotransplantation. Normal swine (n=9) received halothane at levels of 0.25, 0.5 and 1 MAC (minimum alveolar concentration) for 30 min each. Parameters assessed included: 1) heart rate; 2) arterial blood pressure; 3) pulmonary artery, central venous, left and right ventricular pressures; 4) cardiac output; 5) end-expiratory CO(2) and halothane levels; 6) cardiac temperature; and 7) arterial blood gases. Hearts were removed using standard cardioplegic procedures and reperfused in four-chamber working mode (n=8); again the effects of increasing halothane concentrations on cardiac performance were analyzed. When comparing biventricular depressive effects (negative inotropic, negative lusitropic) of halothane in vivo and in vitro, there were distinct quantitative differences. The negative lusitropic effects were less pronounced in vitro; this was especially true for the right ventricle. Yet, in vitro, halothane at all doses induced more pronounced decreases in left heart output compared to the right. The large mammalian isolated four-chamber working heart model allows for novel assessment of pharmacodynamics and/or evaluation of cardiac devices under a range of hemodynamic performances. Halothane, a cardiodepressive agent, induced direct myocardial depressive effects in vitro similar to those recorded in vivo; hence additional systemic effects are considered to play a minor role in ultimate performances, e.g., compensatory responses due to autonomic controls.