Effects of biventricular shunt on pump characteristics in a maglev total artificial heart.

Severe left ventricular failure can progress to right ventricular failure, necessitating alternatives to heart transplantation, such as total artificial heart (TAH) treatment. Conventional TAHs encounter challenges associated with miniaturization and hemocompatibility owing to their reliance on mechanical valves and bearings. A magnetically levitated TAH (IB-Heart) was developed, utilizing a magnetic bearing. The IB-Heart features a distinctive biventricular shunt channel situated between the flow paths of the left and right centrifugal blood pumps, simplifying and miniaturizing its control system. However, the impact of these shunt channels remains underexplored. This study aimed to investigate the effects of shunt flow on pump characteristics and assess the IB-Heart's potential to regulate flow balance between systemic and pulmonary circulation. At a rotational speed of 2000 rpm and flow rate range of 0-10 L/min, shunt flow exhibited a minor impact, with a 1.4 mmHg (1.3%) effect on pump characteristics. Shunt flow variation of about 0.13 L/min correlated with a 10 mmHg pressure difference between the pumps' afterload and preload conditions. This variance was linked to changes in the inlet flow rates of the left and right pumps, signifying the ventricular shunt structure's capacity to mirror the function of an atrial shunt in alleviating pulmonary congestion. The IB-Heart's ventricular shunt structure enables passive regulation of left-right flow balance. The findings establish a fundamental technical groundwork for the development of IB-Hearts and TAHs with similar shunt structures. The innovative coupling of centrifugal pumps and the resultant effects on flow dynamics contribute to the advancement of TAH technology.

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.

Investigation of the inherent left-right flow balancing of rotary total artificial hearts by means of a resistance box.

With the incidence of end-stage heart failure steadily increasing, the need for a practical total artificial heart (TAH) has never been greater. Continuous flow TAHs (CFTAH) are being developed using rotary blood pumps (RBPs), leveraging their small size, mechanical simplicity, and excellent durability. To completely replace the heart with currently available RBPs, two are required; one for providing pulmonary flow and one for providing systemic flow. To prevent hazardous states, it is essential to maintain balance between the pulmonary and systemic circulation at a wide variety of physiologic states. In this study, we investigated factors determining a CFTAH's inherent ability to balance systemic and pulmonary flow passively, without active management of pump rotational speed. Four different RBPs (ReliantHeart HA5, Thoratec HMII, HeartWare HVAD, and Ventracor VentrAssist) were used in various combinations to construct CFTAHs. Each CFTAH's ability to autonomously maintain pressures and flows within defined ranges was evaluated in a hybrid mock loop as systemic and pulmonary vascular resistance (PVR) were changed. The resistance box, a method to quantify the range of vascular resistances that can be safely supported by a CFTAH, was used to compare different CFTAH configurations in an efficient and predictive way. To reduce the need for future in vitro tests and to aid in their analysis, a novel analytical evaluation to predict the resistance box of various CFTAH configurations was also performed. None of the investigated CFTAH configurations fully satisfied the predefined benchmarks for inherent flow balancing, with the VentrAssist (left) and HeartAssist 5 (right) offering the best combination. The extent to which each CFTAH was able to autonomously maintain balance was determined by the pressure sensitivity of each RPB: the sensitivity of outflow to changes in the pressure head. The analytical model showed that by matching left and right pressure sensitivity the inherent balancing performance can be improved. These findings may ultimately lead to a reduced need for manual speed changes or active control systems.

Pulsatile operation of a continuous-flow right ventricular assist device (RVAD) to improve vascular pulsatility.

Despite the widespread acceptance of rotary blood pump (RBP) in clinical use over the past decades, the diminished flow pulsatility generated by a fixed speed RBP has been regarded as a potential factor that may lead to adverse events such as vasculature stiffening and hemorrhagic strokes. In this study, we investigate the feasibility of generating physiological pulse pressure in the pulmonary circulation by modulating the speed of a right ventricular assist device (RVAD) in a mock circulation loop. A rectangular pulse profile with predetermined pulse width has been implemented as the pump speed pattern with two different phase shifts (0% and 50%) with respect to the ventricular contraction. In addition, the performance of the speed modulation strategy has been assessed under different cardiovascular states, including variation in ventricular contractility and pulmonary arterial compliance. Our results indicated that the proposed pulse profile with optimised parameters (Apulse = 10000 rpm and ωmin = 3000 rpm) was able to generate pulmonary arterial pulse pressure within the physiological range (9-15 mmHg) while avoiding undesirable pump backflow under both co- and counter-pulsation modes. As compared to co-pulsation, stroke work was reduced by over 44% under counter-pulsation, suggesting that mechanical workload of the right ventricle can be efficiently mitigated through counter-pulsing the pump speed. Furthermore, our results showed that improved ventricular contractility could potentially lead to higher risk of ventricular suction and pump backflow, while stiffening of the pulmonary artery resulted in increased pulse pressure. In conclusion, the proposed speed modulation strategy produces pulsatile hemodynamics, which is more physiologic than continuous blood flow. The findings also provide valuable insight into the interaction between RVAD speed modulation and the pulmonary circulation under various cardiovascular states.

Preliminary design of the internal geometry in a minimally invasive left ventricular assist device under pulsatile-flow conditions.

PURPOSE: A minimally invasive, partial-assist, intra-atrial blood pump has been proposed, which would unload the left ventricle with a flow path from the left atrium to the arterial system. Flow modulation is a common strategy for ensuring washout in the pump, but it can increase power consumption because it is typically achieved through motor-speed variation. However, if a pump's performance curve had the proper gradient, flow modulation could be realized passively. To achieve this goal, we propose a pump performance operating curve as an alternative to the more standard operating point. METHODS AND RESULTS: Mean-line theory was employed to generate an initial set of geometries that were then tested on a hydraulic test rig at ~20,000 r/min. Experimental results show that the intra-atrial blood pump performed below the operating region; however, it was determined that smaller hub diameter and longer chord length bring the performance of the intra-atrial blood pump device closer to the operating curve. CONCLUSION: We found that it is possible to shape the pump performance curve for specifically targeted gradients over the operating region through geometric variations inside the pump.

Application of Adaptive Starling-Like Controller to Total Artificial Heart Using Dual Rotary Blood Pumps.

The successful clinical applicability of rotary left ventricular assist devices (LVADs) has led to research interest in devising a total artificial heart (TAH) using two rotary blood pumps (RBPs). The major challenge when using two separately controlled LVADs for TAH support is the difficulty in maintaining the balance between pulmonary and systemic blood flows. In this study, a starling-like controller (SLC) hybridized with an adaptive mechanism was developed for a dual rotary LVAD TAH. The incorporation of the adaptive mechanism was intended not only to minimize the risk of pulmonary congestion and atrial suction but also to match cardiac demand. A comparative assessment was performed between the proposed adaptive starling-like controller (A-SLC) and a conventional SLC as well as a constant speed controller. The performance of all controllers was evaluated by subjecting them to three simulated scenarios [rest, exercise, head up tilt (HUT)] using a mock circulation loop. The overall results showed that A-SLC was superior in matching pump flow to cardiac demand without causing hemodynamic instabilities. In contrast, improper flow regulation by the SLC resulted in pulmonary congestion during exercise. From resting supine to HUT, overpumping of the RBPs at fixed speed (FS) caused atrial suction, whereas implementation of SLC resulted in insufficient flow. The comparative study signified the potential of the proposed A-SLC for future TAH implementation particularly among outpatients, who are susceptible to variety of clinical scenarios.

Rapid Speed Modulation of a Rotary Total Artificial Heart Impeller.

Unlike the earlier reciprocating volume displacement-type pumps, rotary blood pumps (RBPs) typically operate at a constant rotational speed and produce continuous outflow. When RBP technology is used in constructing a total artificial heart (TAH), the pressure waveform that the TAH produces is flat, without the rise and fall associated with a normal arterial pulse. Several studies have suggested that pulseless circulation may impair microcirculatory perfusion and the autoregulatory response and may contribute to adverse events such as gastrointestinal bleeding, arteriovenous malformations, and pump thrombosis. It may therefore be beneficial to attempt to reproduce pulsatile output, similar to that generated by the native heart, by rapidly modulating the speed of an RBP impeller. The choice of an appropriate speed profile and control strategy to generate physiologic waveforms while minimizing power consumption and blood trauma becomes a challenge. In this study, pump operation modes with six different speed profiles using the BiVACOR TAH were evaluated in vitro. These modes were compared with respect to: hemodynamic pulsatility, which was quantified as surplus hemodynamic energy (SHE); maximum rate of change of pressure (dP/dt); pulse power index; and motor power consumption as a function of pulse pressure. The results showed that the evaluated variables underwent different trends in response to changes in the speed profile shape. The findings indicated a possible trade-off between SHE levels and flow rate pulsatility related to the relative systolic duration in the speed profile. Furthermore, none of the evaluated measures was sufficient to fully characterize hemodynamic pulsatility.

In Vitro Evaluation of the Dual-Diffuser Design for a Reversible Rotary Intra-Aortic Ventricular Assist Device.

The intra-aortic ventricular assist device (IntraVAD) is a miniature intra-aortic axial-flow ventricular assist device (VAD) that works in series with the left ventricle (LV) to assist the compromised heart. Previous in vitro results have shown that the IntraVAD can successfully increase coronary perfusion and offload ventricular volume by operating in reverse-rotation control (RRc) mode. The RRc mode includes forward rotation in systole and reverse rotation (RR) in diastole. It is necessary to derive a new diffuser design that can be used for the bi-directional rotation of the IntraVAD. In this work, a dual-diffuser set (DDS) was proposed to replace the conventional inducer and diffuser upstream and downstream of the pump. The DDS comprised two diffusers, located on both sides of the impeller, omitting the conventional inducer and diffuser. Different configurations of the DDS were designed and manufactured with various combinations of curved and straight blades. All configurations were initially tested in continuous flow, then in a pulsatile mock circulatory loop. A weighted normalized scalar (WNS) was proposed to comprehensively evaluate the hemodynamic effect of the DDS with different configurations. The results show that the maximum of WNS occurred when the upstream diffuser had equal numbers of curved and straight blades and the downstream diffuser had only curved blades. This indicates such a dual-diffuser design for the IntraVAD can give an optimal cardiac assistance potentially improving ventricular contractility, thereby restoring heart function.

In Vivo Evaluation of Active and Passive Physiological Control Systems for Rotary Left and Right Ventricular Assist Devices.

Preventing ventricular suction and venous congestion through balancing flow rates and circulatory volumes with dual rotary ventricular assist devices (VADs) configured for biventricular support is clinically challenging due to their low preload and high afterload sensitivities relative to the natural heart. This study presents the in vivo evaluation of several physiological control systems, which aim to prevent ventricular suction and venous congestion. The control systems included a sensor-based, master/slave (MS) controller that altered left and right VAD speed based on pressure and flow; a sensor-less compliant inflow cannula (IC), which altered inlet resistance and, therefore, pump flow based on preload; a sensor-less compliant outflow cannula (OC) on the right VAD, which altered outlet resistance and thus pump flow based on afterload; and a combined controller, which incorporated the MS controller, compliant IC, and compliant OC. Each control system was evaluated in vivo under step increases in systemic (SVR ∼1400-2400 dyne/s/cm(5) ) and pulmonary (PVR ∼200-1000 dyne/s/cm(5) ) vascular resistances in four sheep supported by dual rotary VADs in a biventricular assist configuration. Constant speed support was also evaluated for comparison and resulted in suction events during all resistance increases and pulmonary congestion during SVR increases. The MS controller reduced suction events and prevented congestion through an initial sharp reduction in pump flow followed by a gradual return to baseline (5.0 L/min). The compliant IC prevented suction events; however, reduced pump flows and pulmonary congestion were noted during the SVR increase. The compliant OC maintained pump flow close to baseline (5.0 L/min) and prevented suction and congestion during PVR increases. The combined controller responded similarly to the MS controller to prevent suction and congestion events in all cases while providing a backup system in the event of single controller failure.

In Vitro Comparison of Active and Passive Physiological Control Systems for Biventricular Assist Devices.

The low preload and high afterload sensitivities of rotary ventricular assist devices (VADs) may cause ventricular suction events or venous congestion. This is particularly problematic with rotary biventricular support (BiVAD), where the Starling response is diminished in both ventricles. Therefore, VADs may benefit from physiological control systems to prevent adverse events. This study compares active, passive and combined physiological controllers for rotary BiVAD support with constant speed mode. Systemic (SVR) and pulmonary (PVR) vascular resistance changes and exercise were simulated in a mock circulation loop to evaluate the capacity of each controller to prevent suction and congestion and increase exercise capacity. All controllers prevented suction and congestion at high levels of PVR (900 dynes s cm(-5)) and SVR (3000 dynes s cm(-5)), however these events occurred in constant speed mode. The controllers increased preload sensitivity (0.198-0.34 L min(-1) mmHg(-1)) and reduced afterload sensitivity (0.0001-0.008 L min(-1) mmHg(-1)) of the VADs when compared to constant speed mode (0.091 and 0.072 L min(-1) mmHg(-1) respectively). The active controller increased pump speeds (400-800 rpm) and pump flow by 2.8 L min(-1) during exercise, thus increasing exercise capacity. By reducing suction and congestion and by increasing exercise capacity, the control systems presented in this study may help increase quality of life of VAD patients.

Hemodynamic effects of synchronizing an intra-aortic VAD in reverse-rotation control with left ventricle: a mock loop study.

The IntraVAD is an intra-aortic left ventricular assist device (LVAD) to be located in the ascending aorta. In order to enhance unloading and promote coronary flow for the left ventricle (LV), an operating mechanism, reverse-rotation control (RRc) mode, has been developed for the IntraVAD and tested in vitro in a mock circulation loop (MCL). The RRc mode consists of forward rotation (FR) and reverse rotation (RR). The synchronization between the IntraVAD and the LV was studied to offload the ventricle more effectively and to improve myocardial perfusion. The percentage time length of the FR period in the cardiac cycle (Tlf) and time offset between the central-lines of the FR period and the LV systole (Toc) are two parameters of the RRc mode that were varied to adjust the synchronization between the IntraVAD and the LV. The ejection fraction (EF), coronary perfusion pressure (CPP), and arterial pulsatility index (API) were measured at different Tlf and Toc values. These hemodynamic results closely correlated to the LV unloading, coronary perfusion, and peripheral arterial pulsatility. The EF, CPP and API were fed into a weighted normalized scalar (WNS) which was implemented to comprehensively evaluate the hemodynamic influence. The WNS result shows that the overall hemodynamic response is more sensitive to the changes in Toc value than Tlf value. The result shows a significant reduction in LV afterload by starting the FR before LV contraction, then switching to RR at the onset of ventricular dilation. The optimal phase shift of -π/5 was found to precede LV contraction, indicating that changes in LV afterload are more sensitive to the phase shift at the start of the ventricular systole than at the end. Thus, a phase advance between intra-aortic pumps and the LV is critical to unload the ventricle and promote myocardial recovery.

Total artificial hearts: past, present, and future.

A practical artificial heart has been sought for >50 years. An increasing number of people succumb to heart disease each year, but the number of hearts available for transplantation remains small. Early total artificial hearts mimicked the pumping action of the native heart. These positive-displacement pumps could provide adequate haemodynamic support and maintain the human circulation for short periods, but large size and limited durability adversely affected recipients' quality of life. Subsequent research into left ventricular assist devices led to the use of continuous-flow blood pumps with rotating impellers. Researchers have attempted to integrate this technology into modern total artificial hearts with moderate clinical success. The importance of pulsatile circulation remains unclear. Future research is, therefore, needed into positive-displacement and rotary total artificial hearts.

A compliant, banded outflow cannula for decreased afterload sensitivity of rotary right ventricular assist devices.

Biventricular support with dual rotary ventricular assist devices (VADs) has been implemented clinically with restriction of the right VAD (RVAD) outflow cannula to artificially increase afterload and, therefore, operate within recommended design speed ranges. However, the low preload and high afterload sensitivity of these devices increase the susceptibility of suction events. Active control systems are prone to sensor drift or inaccurate inferred (sensor-less) data, therefore an alternative solution may be of benefit. This study presents the in vitro evaluation of a compliant outflow cannula designed to passively decrease the afterload sensitivity of rotary RVADs and minimize left-sided suction events. A one-way fluid-structure interaction model was initially used to produce a design with suitable flow dynamics and radial deformation. The resultant geometry was cast with different initial cross-sectional restrictions and concentrations of a softening diluent before evaluation in a mock circulation loop. Pulmonary vascular resistance (PVR) was increased from 50 dyne s/cm(5) until left-sided suction events occurred with each compliant cannula and a rigid, 4.5 mm diameter outflow cannula for comparison. Early suction events (PVR ∼ 300 dyne s/cm(5) ) were observed with the rigid outflow cannula. Addition of the compliant section with an initial 3 mm diameter restriction and 10% diluent expanded the outflow restriction as PVR increased, thus increasing RVAD flow rate and preventing left-sided suction events at PVR levels beyond 1000 dyne s/cm(5) . Therefore, the compliant, restricted outflow cannula provided a passive control system to assist in the prevention of suction events with rotary biventricular support while maintaining pump speeds within normal ranges of operation.

Hemodynamic response to exercise and head-up tilt of patients implanted with a rotary blood pump: a computational modeling study.

The present study investigates the response of implantable rotary blood pump (IRBP)-assisted patients to exercise and head-up tilt (HUT), as well as the effect of alterations in the model parameter values on this response, using validated numerical models. Furthermore, we comparatively evaluate the performance of a number of previously proposed physiologically responsive controllers, including constant speed, constant flow pulsatility index (PI), constant average pressure difference between the aorta and the left atrium, constant average differential pump pressure, constant ratio between mean pump flow and pump flow pulsatility (ratioP I or linear Starling-like control), as well as constant left atrial pressure ( P l a ¯ ) control, with regard to their ability to increase cardiac output during exercise while maintaining circulatory stability upon HUT. Although native cardiac output increases automatically during exercise, increasing pump speed was able to further improve total cardiac output and reduce elevated filling pressures. At the same time, reduced venous return associated with upright posture was not shown to induce left ventricular (LV) suction. Although P l a ¯ control outperformed other control modes in its ability to increase cardiac output during exercise, it caused a fall in the mean arterial pressure upon HUT, which may cause postural hypotension or patient discomfort. To the contrary, maintaining constant average pressure difference between the aorta and the left atrium demonstrated superior performance in both exercise and HUT scenarios. Due to their strong dependence on the pump operating point, PI and ratioPI control performed poorly during exercise and HUT. Our simulation results also highlighted the importance of the baroreflex mechanism in determining the response of the IRBP-assisted patients to exercise and postural changes, where desensitized reflex response attenuated the percentage increase in cardiac output during exercise and substantially reduced the arterial pressure upon HUT.

In vitro study of an intra-aortic VAD: Effect of reverse-rotating mode on ventricular recovery.

Cardiac recovery has been observed in end-stage heart failure patients with mechanical circulatory support. An intra-aortic ventricular assist device (IntraVAD) is a novel rotary blood pump designed to operate in the ascending aorta behind the aortic valve, working in series with the compromised left ventricle (LV). Such a device requires optimal motion control in order to enhance the myocardial perfusion and thus promote cardiac recovery. Therefore, a reverse-rotating control (RRc) mode has been proposed to increase the mean arterial pressure (MAP) in diastole where the most coronary flow occurs. The RRc mode consists of two motions - forward rotating speed (FS) and reversely rotating speed (RS). The capability of cardiac recovery of three control modes, including `continuous', `on/off ' and `RRc' modes, was evaluated in vitro. Stroke work (SW), ventricular volume, coronary perfusion pressure (CPP), and arterial pulsatility index (API) were used to evaluate LV unloading, myocardial perfusion and arterial pulsatility. The results show that, all three modes increased the LV stroke work (0.98W vs 1.00W vs 1.01W for continuous, on/off and RRc, respectively; baseline 0.9W) and decreased both end-diastolic volume (EDV) and end-systolic volume (ESV). The "RRc" mode improved CPP significantly (78.4 mm Hg compared to 66.4 mmHg and 70.9 mm Hg for continuous and on/off modes; baseline 71 mm Hg). The arterial pulsatility was higher in `RRc' mode (0.84 compared to 0.43 and 0.59; baseline 0.48). In summary, the IntraVAD operating in the RRc mode can successfully unload the LV, enhance the myocardial perfusion, and restore the arterial pulsatility; therefore, it could be a promising therapeutic option to bridge heart failure patients to recovery.

A hybrid mock circulation loop for a total artificial heart.

Rotary blood pumps are emerging as a viable technology for total artificial hearts, and the development of physiological control algorithms is accelerated with new evaluation environments. In this article, we present a novel hybrid mock circulation loop (HMCL) designed specifically for evaluation of rotary total artificial hearts (rTAH). The rTAH is operated in the physical domain while all vasculature elements are embedded in the numerical domain, thus combining the strengths of both approaches: fast and easy exchange of the vasculature model together with improved controllability of the pump. Parameters, such as vascular resistance, compliance, and blood volume, can be varied dynamically in silico during operation. A hydraulic-numeric interface creates a real-time feedback loop between the physical and numerical domains. The HMCL uses computer-controlled resistance valves as actuators, thereby reducing the size and number of hydraulic elements. Experimental results demonstrate a stable interaction over a wide operational range and a high degree of flexibility. Therefore, we demonstrate that the newly created design environment can play an integral part in the hydraulic design, control development, and durability testing of rTAHs.

Optimal management of the critically ill: anaesthesia, monitoring, data capture, and point-of-care technological practices in ovine models of critical care.

Animal models of critical illness are vital in biomedical research. They provide possibilities for the investigation of pathophysiological processes that may not otherwise be possible in humans. In order to be clinically applicable, the model should simulate the critical care situation realistically, including anaesthesia, monitoring, sampling, utilising appropriate personnel skill mix, and therapeutic interventions. There are limited data documenting the constitution of ideal technologically advanced large animal critical care practices and all the processes of the animal model. In this paper, we describe the procedure of animal preparation, anaesthesia induction and maintenance, physiologic monitoring, data capture, point-of-care technology, and animal aftercare that has been successfully used to study several novel ovine models of critical illness. The relevant investigations are on respiratory failure due to smoke inhalation, transfusion related acute lung injury, endotoxin-induced proteogenomic alterations, haemorrhagic shock, septic shock, brain death, cerebral microcirculation, and artificial heart studies. We have demonstrated the functionality of monitoring practices during anaesthesia required to provide a platform for undertaking systematic investigations in complex ovine models of critical illness.

Physiological control of dual rotary pumps as a biventricular assist device using a master/slave approach.

Dual rotary left ventricular assist devices (LVADs) can provide biventricular mechanical support during heart failure. Coordination of left and right pump speeds is critical not only to avoid ventricular suction and to match cardiac output with demand, but also to ensure balanced systemic and pulmonary circulatory volumes. Physiological control systems for dual LVADs must meet these objectives across a variety of clinical scenarios by automatically adjusting left and right pump speeds to avoid catastrophic physiological consequences. In this study we evaluate a novel master/slave physiological control system for dual LVADs. The master controller is a Starling-like controller, which sets flow rate as a function of end-diastolic ventricular pressure (EDP). The slave controller then maintains a linear relationship between right and left EDPs. Both left/right and right/left master/slave combinations were evaluated by subjecting them to four clinical scenarios (rest, postural change, Valsalva maneuver, and exercise) simulated in a mock circulation loop. The controller's performance was compared to constant-rotational-speed control and two other dual LVAD control systems: dual constant inlet pressure and dual Frank-Starling control. The results showed that the master/slave physiological control system produced fewer suction events than constant-speed control (6 vs. 62 over a 7-min period). Left/right master/slave control had lower risk of pulmonary congestion than the other control systems, as indicated by lower maximum EDPs (15.1 vs. 25.2-28.4 mm Hg). During exercise, master/slave control increased total flow from 5.2 to 10.1 L/min, primarily due to an increase of left and right pump speed. Use of the left pump as the master resulted in fewer suction events and lower EDPs than when the right pump was master. Based on these results, master/slave control using the left pump as the master automatically adjusts pump speed to avoid suction and increases pump flow during exercise without causing pulmonary venous congestion.

In vitro and in vivo characterization of three different modes of pump operation when using a left ventricular assist device as a right ventricular assist device.

Dual rotary left ventricular assist devices (LVADs) have been used clinically to support patients with biventricular failure. However, due to the lower vascular resistance in the pulmonary circulation compared with its systemic counterpart, excessively high pulmonary flow rates are expected if the right ventricular assist device (RVAD) is operated at its design LVAD speed. Three possible approaches are available to match the LVAD to the pulmonary circulation: operating the RVAD at a lower speed than the LVAD (mode 1), operating both pumps at their design speeds (mode 2) while relying on the cardiovascular system to adapt, and operating both pumps at their design speeds while restricting the diameter of the RVAD outflow graft (mode 3). In this study, each mode was characterized using in vitro and in vivo models of biventricular heart failure supported with two VentrAssist LVADs. The effect of each mode on arterial and atrial pressures and flow rates for low, medium, and high vascular resistances and three different contractility levels were evaluated. The amount of speed/diameter adjustment required to accommodate elevated pulmonary vascular resistance (PVR) during support with mode 3 was then investigated. Mode 1 required relatively low systemic vascular resistance to achieve arterial pressures less than 100 mm Hg in vitro, resulting in flow rates greater than 6 L/min. Mode 2 resulted in left atrial pressures above 25 mm Hg, unless left heart contractility was near-normal. In vitro, mode 3 resulted in expected arterial pressures and flow rates with an RVAD outflow diameter of 6.5 mm. In contrast, all modes were achievable in vivo, primarily due to higher RVAD outflow graft resistance (more than 500 dyn·s/cm(5)), caused by longer cannula. Flow rates could be maintained during instances of elevated PVR by increasing the RVAD speed or expanding the outflow graft diameter using an externally applied variable graft occlusion device. In conclusion, suitable hemodynamics could be produced by either restricting or not restricting the right outflow graft diameter; however, the latter required an operation of the RVAD at lower than design speed. Adjustments in outflow restriction and/or RVAD speed are recommended to accommodate varying PVR.

Modeling of a rotary blood pump.

The accurate representation of rotary blood pumps in a numerical environment is important for meaningful investigation of pump-cardiovascular system interactions. Although numerous models for ventricular assist devices (VADs) have been developed, modeling methods for rotary total artificial hearts (rTAHs) are still required. Therefore, an rTAH prototype was characterized in a steady flow, hydraulic test bench over a wide operational range for pump and hydraulic parameters. In order to develop a generic modeling method, a data-driven modeling approach was chosen. k-Nearest-neighbors, artificial neural networks, and support vector machines (SVMs) were the machine learning approaches evaluated. The best performing parameters for each algorithm were determined via optimization. The resulting multiple-input-multiple-output models were subsequently assessed under identical conditions, and a SVM with a radial basis function kernel was identified as the best performing. The achieved root mean squared errors were 0.03 L/min, 0.06 L/min, and 0.18 W for left and right flow and motor power consumption, respectively. In comparison with existing models for VADs, the flow errors are more than 70% lower. Further advantages of the SVM model are the robustness to measurement noise and the capability to operate outside of the trained parameter range. This proposed modeling method will accelerate further device refinements by providing a more appropriate numerical environment in which to evaluate the pump-cardiovascular system interaction.