Anatomic fitting studies of a total artificial heart in heart transplant recipients. Critical dimensions and prediction of fit

The ongoing quest for the first total artificial heart as destination therapy

Many patients with end-stage heart disease die because of the scarcity of donor hearts. A total artificial heart (TAH), an implantable machine that replaces the heart, has so far been successfully used in over 1,700 patients as a temporary life-saving technology for bridging to heart transplantation. However, after more than six decades of research on TAHs, a TAH that is suitable for destination therapy is not yet available. High complication rates, bulky devices, poor durability, poor biocompatibility and low patient quality of life are some of the major drawbacks of current TAH devices that must be addressed before TAHs can be used as a destination therapy. Quickly emerging innovations in battery technology, wireless energy transmission, biocompatible materials and soft robotics are providing a promising opportunity for TAH development and might help to solve the drawbacks of current TAHs. In this Review, we describe the milestones in the history of TAH research and reflect on lessons learned during TAH development. We summarize the differences in the working mechanisms of these devices, discuss the next generation of TAHs and highlight emerging technologies that will promote TAH development in the coming decade. Finally, we present current challenges and future perspectives for the field.

Anatomical human fitting of the BiVACOR total artificial heart

BACKGROUND

BiVACOR is a novel total artificial heart (TAH) utilizing a single centrifugal magnetically levitated rotor with the ability to modulate pulsatile flow. The device has been successfully tested in a bovine model. We undertook a multicenter anatomical and virtual fitting study of the BiVACOR in patients undergoing heart transplantation.

METHODS

10 patients were recruited across two heart transplant centers. A sterilized 1:1 titanium model of the device was inserted into the patient's chest post heart explant, prior to implantation of the donor heart. Measurements were recorded in situ. The device was then removed. Following this, retrospective 3D reconstructions were created from computed tomography chest scans to simulate a virtual fitting.

RESULTS

Mean age was 53 years (range 38-67). Mean BMI was 28 (range 20-37). Heart failure etiology was varied-with ischemic cardiomyopathy being the most common. Mean spine-to-sternum distance at the tenth thoracic vertebrae (T10) was 14 cm (range 11-18). Mean aorta to aortic Port distance was 0.2 cm (range 0-0.5). Mean pulmonary artery to pulmonary artery port distance was 4.2 cm (range 1-7). The device fitted suitably in all patients without gross distortion to the geometry between native vessel/chamber and port.

CONCLUSIONS

This study described the anatomical and virtual fitting of the BiVACOR TAH. The device fit well within the chest cavities of all 10 patients, who represented a variety of body morphologies and heart failure etiology.

A Preclinical Cadaver Fitting Study of Implantable Biventricular Assist Device - AnyHeart™

A multifunctional, Korean-made artificial heart (AnyHeart™) was developed, and prior to its clinical application, a cadaver-fitting study was performed. The study proposed to determine the optimal cannulation approach, implantation technique and route of the cannula to minimize the organ compression of AnyHeart™. The anatomical feasibility and a variety of surgical techniques were evaluated using ten preserved, human cadavers. Implanting AnyHeart™ with ease is possible using various approaches, including a median sternotomy, and a right or left lateral thoracotomy. The lateral thoracotomy approach is shown to be safe and reproducible, especially in patients who have already undergone an operation that used a median sternotomy. The results of this study will guide improvements in the designs of cannulae and AnyHeart™ for future clinical applications.

Anatomic fitting of total artificial hearts for in vivo evaluation

Successful anatomic fitting of a total artificial heart (TAH) is vital to achieve optimal pump hemodynamics after device implantation. Although many anatomic fitting studies have been completed in humans prior to clinical trials, few reports exist that detail the experience in animals for in vivo device evaluation. Optimal hemodynamics are crucial throughout the in vivo phase to direct design iterations and ultimately validate device performance prior to pivotal human trials. In vivo evaluation in a sheep model allows a realistically sized representation of a smaller patient, for which smaller third-generation TAHs have the potential to treat. Our study aimed to assess the anatomic fit of a single device rotary TAH in sheep prior to animal trials and to use the data to develop a three-dimensional, computer-aided design (CAD)-operated anatomic fitting tool for future TAH development. Following excision of the native ventricles above the atrio-ventricular groove, a prototype TAH was inserted within the chest cavity of six sheep (28-40 kg). Adjustable rods representing inlet and outlet conduits were oriented toward the center of each atrial chamber and the great vessels, with conduit lengths and angles recorded for future analysis. A three-dimensional, CAD-operated anatomic fitting tool was then developed, based on the results of this study, and used to determine the inflow and outflow conduit orientation of the TAH. The mean diameters of the sheep left atrium, right atrium, aorta, and pulmonary artery were 39, 33, 12, and 11 mm, respectively. The center-to-center distance and outer-edge-to-outer-edge distance between the atria, found to be 39 ± 9 mm and 72 ± 17 mm in this study, were identified as the most critical geometries for successful TAH connection. This geometric constraint restricts the maximum separation allowable between left and right inlet ports of a TAH to ensure successful alignment within the available atrial circumference.

An innovative, sensorless, pulsatile, continuous-flow total artificial heart: device design and initial in vitro study

BACKGROUND

We are developing a very small, innovative, continuous-flow total artificial heart (CFTAH) that passively self-balances left and right pump flows and atrial pressures without sensors. This report details the CFTAH design concept and our initial in vitro data.

METHODS

System performance of the CFTAH was evaluated using a mock circulatory loop to determine the range of systemic and pulmonary vascular resistance (SVR and PVR) levels over which the design goal of a maximum absolute atrial pressure difference of 10 mm Hg is achieved for a steady-state flow condition. Pump speed was then modulated at 2,600 +/- 900 rpm to induce flow and arterial pressure pulsation to evaluate the effects of speed pulsations on the system performance. An automatic control mode was also evaluated.

RESULTS

Using only passive self-regulation, pump flows were balanced and absolute atrial pressure differences were maintained at <10 mm Hg over a range of SVR (750 to 2,750 dyne.sec.cm(-5)) and PVR (135 to 600 dyne.sec.cm(-5)) values far exceeding normal levels. The magnitude of induced speed pulsatility affected relative left/right performance, allowing for an additional active control to improve balanced flow and pressure. The automatic control mode adjusted pump speed to achieve targeted pump flows based on sensorless calculations of SVR and CFTAH flow.

CONCLUSIONS

The initial in vitro testing of the CFTAH with a single, valveless, continuous-flow pump demonstrated its passive self-regulation of flows and atrial pressures and a new automatic control mode.

MagScrew TAH: an update

The MagScrew Total Artificial Heart (TAH) system is the result of a close collaboration among the Cleveland Clinic Foundation, Foster Miller Technologies, Wilson Greatbatch Ltd, and Whalen Biomedical Inc. The system components are the thoracic blood pumping unit with attached compliance chamber and refill port, implantable electronic control unit, implantable battery pack, transcutaneous energy transmission system, external battery pack, and a telemetry system for communication with the electronic control unit. System in vitro tests are underway for system characterization and durability demonstration, whereas in vivo tests were conducted to evaluate system performance and biocompatibility under physiologic conditions. The passively filling pump uses a left master alternate left and right ejection control mode and has a Starling law-like response to venous pressure. The in vitro tests documented excellent hydraulic pump performance with high device output of over 9 l/min at left atrial pressures below 12 mm Hg. Atrial balance was well maintained under all test conditions. The in vivo tests demonstrated good biocompatibility without use of anticoagulant therapy. Experimental durations have ranged between 0 and 92 days. Postexplant evaluation of tissue samples did not reveal any sign of thromboembolic events or tissue damage due to device operation.

In vitro controllability of the MagScrew total artificial heart system

The purpose of this study was to evaluate the in vitro responses to preload and afterload of our total artificial heart (TAH), the MagScrew TAH. The TAH consists of two blood pumps and a control logic, developed at the Cleveland Clinic, OH, and the MagScrew actuator and its electronic control system, developed by Foster-Miller Technologies, Inc., Albany, NY. Tests were performed on a mock circulatory loop, using water as a test fluid. Preload sensitivity of the Mag-Screw TAH demonstrated a Frank-Starling response to preload in automatic mode. A peak flow of 10 L/min was obtained, with a left atrial pressure of 13 mm Hg. The relationship between right atrial pressure and left atrial pressure was well balanced when tested with a left bronchial shunt flow of 5% and a range of pulmonary artery and aortic pressures. With respect to afterload response, the left pump showed a relatively low sensitivity, which allowed the pump to maintain perfusion over a wide range of aortic pressures. The right pump, on the other hand, was much more sensitive to pulmonary artery pressure, which provided a measure of protection against pulmonary congestion. The very effective physiologic response of the MagScrew TAH is believed to result from employment of a left master, alternating ejection control logic, high inherent sensitivity of the blood pumps to atrial pressure, a lower effective stroke volume for the right pump, and a scaling of right side motor ejection voltage to 80% of that used for the left side ejection.

In vivo studies of the MagScrew total artificial heart in calves

The purpose of this study was to evaluate the in vivo pump performance of our total artificial heart (TAH), the "MagScrew TAH." The TAH consists of a blood pump and control logic developed at the Cleveland Clinic and the MagScrew actuator and electronic control system developed by Foster-Miller Technologies, Inc. (Albany, NY). MagScrew TAH implantation was performed in two calves. Study durations were 50 and 5 days. The causes of termination were prosthetic valve endocarditis in one case and cable failure in the other. Mean left pump flow ranged from 8.0 to 9.7 L/min, with left atrial pressure of 3.0 to 16.0 mm Hg. Preload sensitivity of the MagScrew TAH demonstrated a Frank-Starling response to preload in automatic mode. The relationship between right and left atrial pressure was well balanced. Mean arterial pressure and mean pulmonary artery pressure were maintained within physiologic ranges over study duration. There were no signs of bleeding, hemolysis, or organ failure. The MagScrew TAH showed physiologic pump performance, and hemodynamics were well maintained without any organ failure. Further development testing will bring the MagScrew TAH to the point of preclinical readiness testing.

Noninvasive assessment method to determine the anatomic compatibility of an implantable artificial heart system

To assess the anatomic compatibility of an artificial heart (AH), we attempted to develop a computer environment that would facilitate a reliable simulation of an AH implanted in the human thorax. A three-dimensional thoracic computer model with a ventricle-resected heart was constructed, by using manually extracted contour points of the aorta, pulmonary artery, atria, atrioventricular valves, diaphragm, and thoracic wall from a set of consecutive CT images. Such a model enabled simulation of an AH implantation by orienting the AH model in it. Error evaluation on CT imaging and contour extraction with a Plexiglas cylindrical phantom showed that the diameter of the extracted phantom contour was approximately 2 mm smaller than its original with a standard deviation of <0.5 mm. Errors in contour and surface reconstruction could be reduced to far less than 1 mm under constrained conditions. A study on the influence of breathing revealed that variations in some thoracic dimensions between inspiration and expiration could reach 10 mm. In summary, computer simulation of AH implantation is a worthwhile approach with acceptable accuracy, although further considerations of extreme thoracic situations will be required.

Auxiliary total artificial heart: A compact electromechanical artificial heart working simultaneously with the natural heart

Leading international institutions are designing and developing various types of ventricular assist devices (VAD) and total artificial hearts (TAH). Some of the commercially available pulsatile VADs are not readily implantable into the thoracic cavity of smaller size patients because of size limitation. The majority of the TAH dimensions requires the removal of the patients' native heart. A miniaturized artificial heart, the auxiliary total artificial heart (ATAH), is being developed in these authors' laboratories. This device is an electromechanically driven ATAH using a brushless direct current (DC) motor fixed in a center metallic piece. This pusher plate-type ATAH control is based on Frank-Starling's law. The beating frequency is regulated through the change of the left preload, assisting the native heart in obtaining adequate blood flow. With the miniaturization of this pump, the average sized patient can have the surgical implantation procedure in the right thoracic cavity without removing the native heart. The left and right stroke volumes are 35 and 32 ml, respectively. In vitro tests were conducted, and the performance curves demonstrate that the ATAH produces 5 L/min of cardiac output at 180 bpm (10 mmHg of left inlet mean pressure and 100 mm Hg of left outlet mean pressure). Taking into account that this ATAH is working along with the native heart, this output is more than satisfactory for such a device.

Three-dimensional thoracic modeling for an anatomical compatibility study of the implantable total artificial heart

Anatomical compatibility with the thoracic cavity is important in developing a completely implantable total artificial heart (TAH). The purpose of this study was to examine the optimal morphology of our TAH and to develop a computer implantation simulation environment for preoperational discussion. As the first stage, we constructed a prototype simulation system by creating a 3-D surface model of a thoracic cavity and one of a TAH. In this system, the thoracic surface model, composed of the aorta, pulmonary artery, ventricle resected heart, diaphragm, and thoracic wall, was created based on the organ contours extracted from electrocardiogram (ECG)-synchronized ultrafast computer tomographic images. The accuracy of the thoracic model was discussed using a phantom. The fitting study can be performed on the computer with the model of the thorax and that of the TAH. In the future, construction of a database for the thoracic cavities of heart failure patients is planned to improve the morphology of the TAH.