Preload sensitivity in cardiac assist devices

Novel Closed-Loop Control System of Dual Rotary Blood Pumps in Total Artificial Heart Based on the Circulatory Equilibrium Framework: A Proof-of-Concept In Vivo Study

OBJECTIVE

Total artificial heart (TAH) using dual rotary blood pumps (RBPs) is a potential treatment for end-stage heart failure. A well-noted challenge with RBPs is their low sensitivity to preload, which can lead to venous congestion and ventricular suction. To address this issue, we have developed an innovative closed-loop control system of dual RBPs in TAH. This system emulates the Frank-Starling law of the heart in controlling RBPs while monitoring stressed blood volume (V) based on the circulatory equilibrium framework. We validated the system in in-vivo experiments.

METHODS

In 9 anesthetized dogs, we prepared a TAH circuit using 2 centrifugal-type RBPs. We first investigated whether the flow and inlet atrial pressure in each RBP adhered to a logarithmic Frank-Starling curve. We then examined whether the RBP flows and atrial pressures were maintained stably during aortic occlusion (AO) and pulmonary cannula stenosis (PS), whether averaged flow of dual RBPs and bilateral atrial pressures were controlled to their predefined target values for a specific V, and whether this system could maintain the atrial pressures within predefined control ranges under significant changes in V.

RESULTS

This system effectively emulated the logarithmic Frank-Starling curve. It robustly stabilized the flow and atrial pressures during AO and PS without venous congestion or ventricular suction, accurately achieved target values in averaged flow and atrial pressures, and efficaciously maintained these pressures within the control ranges.

CONCLUSION

This system controls dual RBPs in TAH accurately and stably.

SIGNIFICANCE

This system may accelerate clinical application of TAH with dual RBPs.

Left ventricular unloading to facilitate ventricular remodelling in heart failure: A narrative review of mechanical circulatory support

Heart failure represents a dynamic clinical challenge with the continuous rise of a multi-morbid and ageing population. Yet, the evolving nature of mechanical circulatory support offers a variety of means to manage candidates who might benefit from such interventions. This narrative review focuses on the role of the main mechanical circulatory support devices, such as ventricular assist device, extracorporeal membrane oxygenation, Impella and TandemHeart, in the physiological process of ventricular unloading and remodelling in heart failure, highlighting their characteristics, mechanism and clinical outcomes. The outcome measures described include physiological changes (i.e., stroke volume or preload and afterload), intracardiac pressure (i.e., end-diastolic pressure) and extracardiac pressure (i.e., pulmonary capillary wedge pressure). Overall, all the above mechanical circulatory support strategies can facilitate the unloading of the ventricular failure through different mechanisms, which subsequently affects the ventricular remodelling process. These physiological changes start immediately after ventricular assist device implantation. The devices are indicated in different but overlapping populations and operate in distinctive ways; yet, they have evidenced performance to a favourable standard to improve cardiac function in heart failure, although this proved variable for different devices, and further high-quality trials are vital to assess their clinical outcomes further. Both Impella and TandemHeart are indicated mainly in cardiogenic shock and high-risk percutaneous coronary intervention patients; at the time the literature was evaluated, both devices were found to yield a significant improvement in haemodynamics but not in survival. Nevertheless, the choice of device strategy should be based on individual patient factors, including indication, to optimize clinical outcomes.

Mechanical Preload Reduction: Harnessing a Cornerstone of Heart Failure Management to Improve Clinical Outcomes

Decongestion is a cornerstone therapeutic goal for those presenting with decompensated heart failure. Current approaches to clinical decongestion include reducing cardiac preload, which is typically limited to diuretics and hemofiltration. Several new technologies designed to mechanically reduce cardiac preload are in development. In this review, we discuss the pathophysiology of decompensated heart failure; the central role of targeting cardiac preload; emerging mechanical preload reduction technologies; and potential application of these devices.

Cardiopulmonary Performance Among Heart Failure Patients Before and After Left Ventricular Assist Device Implantation

BACKGROUND

Patients with heart failure with reduced ejection fraction (HFrEF) have persistent impairments in functional capacity after continuous-flow left ventricular assist device (CF-LVAD) implantation.

OBJECTIVES

This study aims to characterize longitudinal changes in exercise hemodynamics and functional capacity among patients with HFrEF before and after CF-LVAD implantation.

METHODS

Ten patients underwent 3 invasive cardiopulmonary exercise tests on upright cycle ergometry with pulmonary artery catheterization: 1) Visit 1 before CF-LVAD implantation; 2) Visit 2 after device implantation with CF-LVAD pump speed held constant at baseline speed; and 3) Visit 3 with increases in pump speed during exercise (median: 1,050 rpm [IQR: 750-1,150 rpm] and 220 rpm [IQR: 120-220 rpm] for HeartMate 3 and HeartWare VAD, respectively). Hemodynamics and direct Fick cardiac output were monitored using pulmonary artery catheterization. Gas exchange metrics were determined using indirect calorimetry.

RESULTS

Maximal oxygen uptake (Visits 1, 2, and 3: 10.8 ± 2.5 mL/kg/min, 10.7 ± 2.2 mL/kg/min, and 11.5 ± 1.7 mL/kg/min; P = 0.92) did not improve after device implantation. Mean pulmonary arterial and pulmonary capillary wedge pressures increased significantly during submaximal and peak exercise on preimplantation testing (P < 0.01 for rest vs peak exercise) and remained elevated, with minimal change on Visits 2 and 3 regardless of whether pump speed was fixed or increased.

CONCLUSIONS

Among patients with HFrEF, cardiovascular hemodynamics and exercise capacity were similar after CF-LVAD implantation, regardless of whether patients exercised at fixed or adjusted pump speeds during exercise. Further research is needed to determine methods by which LVADs may alleviate the HFrEF syndrome after device implantation. (Effect of mechanIcal circulatoRy support ON exercise capacity aMong pAtieNts with heart failure [IRONMAN]; NCT03078972).

Balancing the ventricular outputs of pulsatile total artificial hearts

BACKGROUND

Maintaining balanced left and right cardiac outputs in a total artificial heart (TAH) is challenging due to the need for continuous adaptation to changing hemodynamic conditions. Proper balance in ventricular outputs of the left and right ventricles requires a preload-sensitive response and mechanisms to address the higher volumetric efficiency of the right ventricle.

METHODS

This review provides a comprehensive overview of various methods used to balance left and right ventricular outputs in pulsatile total artificial hearts, categorized based on their actuation mechanism.

RESULTS

Reported strategies include incorporating compliant materials and/or air cushions inside the ventricles, employing active control mechanisms to regulate ventricular filling state, and utilizing various shunts (such as hydraulic or intra-atrial shunts). Furthermore, reducing right ventricular stroke volume compared to the left often serves to balance the ventricular outputs. Individually controlled actuation of both ventricles in a pulsatile TAH seems to be the simplest and most effective way to achieve proper preload sensitivity and left-right output balance. Pneumatically actuated TAHs have the advantage to respond passively to preload changes.

CONCLUSION

Therefore, a pneumatic TAH that comprises two individually actuated ventricles appears to be a more desirable option-both in terms of simplicity and efficacy-to respond to changing hemodynamic conditions.

Vibrational characterization of cavitation in left ventricular assist device

BACKGROUND

The left ventricular assist device (LVAD) is a mechanical circulatory support device for patients with severe heart failure. Microbubbles caused by cavitation in the LVAD can potentially lead to physiological and pump-related complications. The aim of this study is to characterize the vibrational patterns in the LVAD during cavitation.

METHODS

The LVAD was integrated into an in vitro circuit and mounted with a high-frequency accelerometer. Accelerometry signals were acquired with different relative pump inlet pressures ranging from baseline (+20 mmHg) to -600 mmHg in order to induce cavitation. Microbubbles were monitored with dedicated sensors at the pump inlet and outlet to quantify the degree of cavitation. Acceleration signals were analyzed in the frequency domain to identify changes in the frequency patterns when cavitation occurred.

RESULTS

Significant cavitation occurred at the low inlet pressure (-600 mmHg) and was detected in the frequency range between 1800 and 9000 Hz. Minor degrees of cavitation at higher inlet pressures (-300 to -500 mmHg) were detected in the frequency range between 500-700, 1600-1700 Hz, and around 12 000 Hz. The signal power of the dominating frequency ranges was statistically significantly different from baseline signals.

CONCLUSION

Vibrational measurements in the LVAD can be used to detect cavitation. A significant degree of cavitation could be detected in a wide frequency range, while minor cavitation activity could only be detected in more narrow frequency ranges. Continuous vibrational LVAD monitoring can potentially be used to detect cavitation and minimize the damaging effect associated with cavitation.

An in vitro model to study suction events by a ventricular assist device: validation with clinical data

Introduction: Ventricular assist devices (LVADs) are a valuable therapy for end-stage heart failure patients. However, some adverse events still persist, such as suction that can trigger thrombus formation and cardiac rhythm disorders. The aim of this study is to validate a suction module (SM) as a test bench for LVAD suction detection and speed control algorithms. Methods: The SM consists of a latex tube, mimicking the ventricular apex, connected to a LVAD. The SM was implemented into a hybrid in vitro-in silico cardiovascular simulator. Suction was induced simulating hypovolemia in a profile of a dilated cardiomyopathy and of a restrictive cardiomyopathy for pump speeds ranging between 2,500 and 3,200 rpm. Clinical data collected in 38 LVAD patients were used for the validation. Clinical and simulated LVAD flow waveforms were visually compared. For a more quantitative validation, a binary classifier was used to classify simulated suction and non-suction beats. The obtained classification was then compared to that generated by the simulator to evaluate the specificity and sensitivity of the simulator. Finally, a statistical analysis was run on specific suction features (e.g., minimum impeller speed pulsatility, minimum slope of the estimated flow, and timing of the maximum slope of the estimated flow). Results: The simulator could reproduce most of the pump waveforms observed in vivo. The simulator showed a sensitivity and specificity and of 90.0% and 97.5%, respectively. Simulated suction features were in the interquartile range of clinical ones. Conclusions: The SM can be used to investigate suction in different pathophysiological conditions and to support the development of LVAD physiological controllers.

Left ventricular assist device in cardiac amyloidosis: friend or foe?

The prevalence of cardiac amyloidosis has progressively increased over the last years, being recognized as a significant cause of heart failure. In fact, the management of advanced heart failure is a cornerstone treatment of amyloid cardiomyopathy due to the frequent delay in its diagnosis. Left ventricular assist devices (LVADs) have been gaining importance in the scenario of end-stage heart failure, representing an alternative to heart transplant. However, only few studies have investigated the role of LVAD in restrictive cardiomyopathies such as cardiac amyloidosis, since there are several problems to consider. In fact, both anatomical factors and the restrictive physiology of this condition make LVAD implant a relevant challenge in this subset of patients. Furthermore, due to the systemic involvement of amyloidosis, several factors have to be considered after LVAD implant, such as an increased risk of bleeding and right ventricular failure. This review attempts to summarize the current evidence of LVAD in cardiac amyloidosis, especially focusing on the challenges that this cardiomyopathy imposes both to the implant and to its management thereafter.

Impact of Mechanical Circulatory Support on Exercise Capacity in Patients With Advanced Heart Failure

Approximately 6 million individuals have heart failure in the United States alone and 15 million in Europe. Left ventricular assist devices (LVAD) improve survival in these patients, but functional capacity may not fully improve. This article examines the hypothesis that patients supported by LVAD experience persistent reductions in functional capacity and explores mechanisms accounting for abnormalities in exercise tolerance.

Physiologic Data-Driven Iterative Learning Control for Left Ventricular Assist Devices

Continuous flow ventricular assist devices (cfVADs) constitute a viable and increasingly used therapy for end-stage heart failure patients. However, they are still operating at a fixed-speed mode that precludes physiological cfVAD response and it is often related to adverse events of cfVAD therapy. To ameliorate this, various physiological controllers have been proposed, however, the majority of these controllers do not account for the lack of pulsatility in the cfVAD operation, which is supposed to be beneficial for the physiological function of the cardiovascular system. In this study, we present a physiological data-driven iterative learning controller (PDD-ILC) that accurately tracks predefined pump flow trajectories, aiming to achieve physiological, pulsatile, and treatment-driven response of cfVADs. The controller has been extensively tested in an in-silico environment under various physiological conditions, and compared with a physiologic pump flow proportional-integral-derivative controller (PF-PIDC) developed in this study as well as the constant speed (CS) control that is the current state of the art in clinical practice. Additionally, two treatment objectives were investigated to achieve pulsatility maximization and left ventricular stroke work (LVSW) minimization by implementing copulsation and counterpulsation pump modes, respectively. Under all experimental conditions, the PDD-ILC as well as the PF-PIDC demonstrated highly accurate tracking of the reference pump flow trajectories, outperforming existing model-based iterative learning control approaches. Additionally, the developed controllers achieved the predefined treatment objectives and resulted in improved hemodynamics and preload sensitivities compared to the CS support.

A Sensorless Modular Multiobjective Control Algorithm for Left Ventricular Assist Devices: A Clinical Pilot Study

Background

Contemporary Left Ventricular Assist Devices (LVADs) mainly operate at a constant speed, only insufficiently adapting to changes in patient demand. Automatic physiological speed control promises tighter integration of the LVAD into patient physiology, increasing the level of support during activity and decreasing support when it is excessive.

Methods

A sensorless modular control algorithm was developed for a centrifugal LVAD (HVAD, Medtronic plc, MN, USA). It consists of a heart rate-, a pulsatility-, a suction reaction—and a supervisor module. These modules were embedded into a safe testing environment and investigated in a single-center, blinded, crossover, clinical pilot trial (clinicaltrials.gov, NCT04786236). Patients completed a protocol consisting of orthostatic changes, Valsalva maneuver and submaximal bicycle ergometry in constant speed and physiological control mode in randomized sequence. Endpoints for the study were reduction of suction burden, adequate pump speed and flowrate adaptations of the control algorithm for each protocol item and no necessity for intervention via the hardware safety systems.

Results

A total of six patients (median age 53.5, 100% male) completed 13 tests in the intermediate care unit or in an outpatient setting, without necessity for intervention during control mode operation. Physiological control reduced speed and flowrate during patient rest, in sitting by a median of −75 [Interquartile Range (IQR): −137, 65] rpm and in supine position by −130 [−150, 30] rpm, thereby reducing suction burden in scenarios prone to overpumping in most tests [0 [−10, 2] Suction events/minute] in orthostatic upwards transitions and by −2 [−6, 0] Suction events/min in Valsalva maneuver. During submaximal ergometry speed was increased by 86 [31, 193] rpm compared to constant speed for a median flow increase of 0.2 [0.1, 0.8] L/min. In 3 tests speed could not be increased above constant set speed due to recurring suction and in 3 tests speed could be increased by up to 500 rpm with a pump flowrate increase of up to 0.9 L/min.

Conclusion

In this pilot study, safety, short-term efficacy, and physiological responsiveness of a sensorless automated speed control system for a centrifugal LVAD was established. Long term studies are needed to show improved clinical outcomes.

Clinical Trial Registration

ClinicalTrials.gov, identifier: NCT04786236.

Control Strategy Design of a Microblood Pump Based on Heart-Rate Feedback

Based on the nonlinear relationship between heart rate and stroke volume, a flow model of left ventricular circulation was improved, and a variable-speed blood-pump control strategy based on heart-rate feedback was proposed. The control strategy was implemented on a system combining the rotary blood pump and blood circulation models of heart failure. The aortic flow of a healthy heart at different heart rates was the desired control goal. Changes in heart rate were monitored and pump speed was adjusted so that the output flow and aortic pressure of the system would match a normal heart in real time to achieve the best auxiliary state. After simulation with MATLAB, the cardiac output satisfied the ideal perfusion requirements at different heart rates, and aortic pressure demonstrated lifting and had good pulsatile performance when a variable-speed blood pump was used. The coupled model reflected the relationship between hemodynamic parameters at different heart rates with the use of the variable-speed blood pump, providing a theoretical basis for the blood-pump-assisted treatment of heart failure and the design of physiological control strategies.

Nephrology Considerations in the Management of Durable and Temporary Mechanical Circulatory Support

Durable and temporary mechanical circulatory support (MCS) use is growing for a range of cardiovascular indications. Kidney dysfunction is common in people evaluated for or receiving durable or temporary MCS and portends worse outcomes. This kidney dysfunction can be due to preexisting kidney chronic kidney disease (CKD), acute kidney injury (AKI) related to acute cardiovascular disease necessitating MCS, AKI due to cardiac procedures, and acute and chronic MCS effects and complications. Durable MCS, with implantable continuous flow pumps, is used for long-term support in advanced heart failure refractory to guideline-directed medical and device therapy, either permanently or as a bridge to heart transplantation. Temporary MCS-encompassing in this review intra-aortic balloon pumps (IABP), axial flow pumps, centrifugal flow pumps, and venoarterial ECMO-is used for diverse situations: high-risk percutaneous coronary interventions (PCI), acute decompensated heart failure, cardiogenic shock, and resuscitation after cardiac arrest. The wide adoption of MCS makes it imperative to improve understanding of the effects of MCS on kidney health/function and of kidney health/function on MCS outcomes. The complex structure and functions of the kidney, and the complex health states of individuals receiving MCS, makes investigations in this area challenging, and current knowledge is limited. Fortunately, the increasing nephrology toolbox of noninvasive kidney health/function assessments may enable development and testing of individualized management strategies and therapeutics in the future. We review technology, epidemiology, pathophysiology, clinical considerations, and future directions in MCS and nephrology.

A Flow Sensor-Based Suction-Index Control Strategy for Rotary Left Ventricular Assist Devices

Rotary left ventricular assist devices (LVAD) have emerged as a long-term treatment option for patients with advanced heart failure. LVADs need to maintain sufficient physiological perfusion while avoiding left ventricular myocardial damage due to suction at the LVAD inlet. To achieve these objectives, a control algorithm that utilizes a calculated suction index from measured pump flow (SIMPF) is proposed. This algorithm maintained a reference, user-defined SIMPF value, and was evaluated using an in silico model of the human circulatory system coupled to an axial or mixed flow LVAD with 5–10% uniformly distributed measurement noise added to flow sensors. Efficacy of the SIMPF algorithm was compared to a constant pump speed control strategy currently used clinically, and control algorithms proposed in the literature including differential pump speed control, left ventricular end-diastolic pressure control, mean aortic pressure control, and differential pressure control during (1) rest and exercise states; (2) rapid, eight-fold augmentation of pulmonary vascular resistance for (1); and (3) rapid change in physiologic states between rest and exercise. Maintaining SIMPF simultaneously provided sufficient physiological perfusion and avoided ventricular suction. Performance of the SIMPF algorithm was superior to the compared control strategies for both types of LVAD, demonstrating pump independence of the SIMPF algorithm.

Mock circulatory loops used for testing cardiac assist devices: A review of computational and experimental models

Heart failure is a major health risk, and with limited availability of donor organs, there is an increasing need for developing cardiac assist devices (CADs). Mock circulatory loops (MCL) are an important in-vitro test platform for CAD's performance assessment and optimisation. The MCL is a lumped parameter model constructed out of hydraulic and mechanical components aiming to simulate the native cardiovascular system (CVS) as closely as possible. Further development merged MCLs and numerical circulatory models to improve flexibility and accuracy of the system; commonly known as hybrid MCLs. A total of 128 MCLs were identified in a literature research until 25 September 2020. It was found that the complexity of the MCLs rose over the years, recent MCLs are not only capable of mimicking the healthy and pathological conditions, but also implemented cerebral, renal and coronary circulations and autoregulatory responses. Moreover, the development of anatomical models made flow visualisation studies possible. Mechanical MCLs showed excellent controllability and repeatability, however, often the CVS was overly simplified or lacked autoregulatory responses. In numerical MCLs the CVS is represented with a higher order of lumped parameters compared to mechanical test rigs, however, complex physiological aspects are often simplified. In hybrid MCLs complex physiological aspects are implemented in the hydraulic part of the system, whilst the numerical model represents parts of the CVS that are too difficult to represent by mechanical components per se. This review aims to describe the advances, limitations and future directions of the three types of MCLs.

The role of renin-angiotensin system in patients with left ventricular assist devices

End-stage heart failure is a condition in which the up-regulation of the systemic and local renin-angiotensin-aldosterone system (RAAS) leads to end-organ damage and is largely irreversible despite optimal medication. Left ventricular assist devices (LVADs) can downregulate RAAS activation by unloading the left ventricle and increasing the cardiac output translating into a better end-organ perfusion improving survival. However, the absence of pulsatility brought about by continuous-flow devices may variably trigger RAAS activation depending on left ventricular (LV) intrinsic contractility, the design and speed of the pump device. Moreover, the concept of myocardial recovery is being tested in clinical trials and in this setting LVAD support combined with intense RAAS inhibition can promote recovery and ensure maintenance of LV function after explantation. Blood pressure control on LVAD recipients is key to avoiding complications as gastrointestinal bleeding, pump thrombosis and stroke. Furthermore, emerging data highlight the role of RAAS antagonists as prevention of arteriovenous malformations that lead to gastrointestinal bleeds. Future studies should focus on the role of angiotensin receptor inhibitors in preventing myocardial fibrosis in patients with LVADs and examine in greater details the target blood pressure for these patients.

Controlling the flow balance: In vitro characterization of a pulsatile total artificial heart in preload and afterload sensitivity

The objective of this study is to identify the preload and afterload sensitivity of the ReinHeart TAH 2.0. For adequate left-right flow balance, the concept of a reduced right stroke volume (by about 10%) and active adaption of the right diastole duration are evaluated concerning the controllability of the flow balance. This study used an active mock circulation loop to test a wide range of preload and afterload conditions. Preload sensitivity was tested at atrial pressures (APs) between 4 and 20 mm Hg. Left afterload was varied in a range of 60-140 mm Hg mean aortic pressure (MAP), right afterload was simulated between 15 and 40 mm Hg. Four scenarios were developed to verify that the flow difference fully covers the defined target range of 0-1.5 L/min. Although a positive correlation between inlet pressure and flow is identified for the right pump chamber, the left pump chamber already fills completely at an inlet pressure of 8-10 mm Hg. With increasing afterload, both the left and right flow decrease. A positive flow balance (left flow exceeds right flow) is achieved over the full range of tested afterloads. At high APs, the flow difference is limited to a maximum of 0.7 L/min. The controllability of flow balance was successfully evaluated in four scenarios, revealing that a positive flow difference can be achieved over the full range of MAPs. Under physiological test conditions, the linear relationship between flow and heart rate was confirmed, ensuring good controllability of the TAH.

Impairments in Blood Pressure Regulation and Cardiac Baroreceptor Sensitivity Among Patients With Heart Failure Supported With Continuous-Flow Left Ventricular Assist Devices

BACKGROUND

Continuous-flow (CF) left ventricular assist devices (LVADs) improve outcomes for patients with advanced heart failure (HF). However, the lack of a physiological pulse predisposes to side-effects including uncontrolled blood pressure (BP), and there are little data regarding the impact of CF-LVADs on BP regulation.

METHODS

Twelve patients (10 males, 60±11 years) with advanced heart failure completed hemodynamic assessment 2.7±4.1 months before, and 4.3±1.3 months following CF-LVAD implantation. Heart rate and systolic BP via arterial catheterization were monitored during Valsalva maneuver, spontaneous breathing, and a 0.05 Hz repetitive squat-stand maneuver to characterize cardiac baroreceptor sensitivity. Plasma norepinephrine levels were assessed during head-up tilt at supine, 30^o and 60^o. Heart rate and BP were monitored during cardiopulmonary exercise testing.

RESULTS

Cardiac baroreceptor sensitivity, determined by Valsalva as well as Fourier transformation and transfer function gain of Heart rate and systolic BP during spontaneous breathing and squat-stand maneuver, was impaired before and following LVAD implantation. Norepinephrine levels were markedly elevated pre-LVAD and improved-but remained elevated post-LVAD (supine norepinephrine pre-LVAD versus post-LVAD: 654±437 versus 323±164 pg/mL). BP increased during cardiopulmonary exercise testing post-LVAD, but the magnitude of change was modest and comparable to the changes observed during the pre-LVAD cardiopulmonary exercise testing.

CONCLUSIONS

Among patients with advanced heart failure with reduced ejection fraction, CF-LVAD implantation is associated with modest improvements in autonomic tone, but persistent reductions in cardiac baroreceptor sensitivity. Exercise-induced increases in BP are blunted. These findings shed new light on mechanisms for adverse events such as stroke, and persistent reductions in functional capacity, among patients supported by CF-LVADs. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT03078972.

Biomechanical effects of the novel series LVAD on the aortic valve

BACKGROUND AND OBJECTIVE

The series type of LVAD (i.e., BJUT-II VAD) is a novel left ventricular assist device, whose effects on the aortic valve remain unclear.

METHODS

The biomechanical effects of BJUT-II VAD on the aortic valve were investigated by using a fluid-structure interaction method. The geometric model of BJUT-II VAD was virtually implanted into the ascending aorta to generate the realistic flow pattern for the aortic valve (i.e., support). In addition, the biomechanical states of the aortic valve without BJUT-II VAD support was computed as control (i.e., control case).

RESULTS

Results demonstrated that the biomechanical effects of BJUT-II VAD were quite different from that resulting from traditional "bypass LVAD." Compared with those in the control case, BJUT-II VAD support could significantly reduce the stress load of the leaflet (maximum stress, 0.5 MPa in the control case vs. 0.12 MPa in the support case). Similarly, the rapid valve opening time (100 ms in the control case vs. 175 ms in the support case) and rapid valve closing time (50 ms in the control case vs. 150 ms in the support case) in the support case were obviously longer than those in the control case. Moreover, BJUT-II VAD support reduced retrograde blood flow during the diastolic phase and significantly changed the distribution of WSS of the leaflets.

CONCLUSIONS

In summary, while unloading the left ventricle, BJUT-II VAD could provide beneficial biomechanical states for the aortic leaflets, thereby reducing the risk of aortic valve disease.

Validity of Hemodynamic Monitoring Using Inert Gas Rebreathing Method in Patients With Chronic Heart Failure and Those Implanted With a Left Ventricular Assist Device

OBJECTIVE

The present study assessed agreement between resting cardiac output estimated by inert gas rebreathing (IGR) and thermodilution methods in patients with heart failure and those implanted with a left ventricular assist device (LVAD).

METHODS AND RESULTS

Hemodynamic measurements were obtained in 42 patients, 22 with chronic heart failure and 20 with implanted continuous flow LVAD (34 males, aged 50 ± 11 years). Measurements were performed at rest using thermodilution and IGR methods. Cardiac output derived by thermodilution and IGR were not significantly different in LVAD (4.4 ± 0.9 L/min vs 4.7 ± 0.8 L/min, P = .27) or patients with heart failure (4.4 ± 1.4 L/min vs 4.5 ± 1.3 L/min, P = .75). There was a strong relationship between thermodilution and IGR cardiac index (r = 0.81, P = .001) and stroke volume index (r = 0.75, P = .001). Bland-Altman analysis showed acceptable limits of agreement for cardiac index derived by thermodilution and IGR, namely, the mean difference (lower and upper limits of agreement) for patients with heart failure -0.002 L/min/m² (-0.65 to 0.66 L/min/m²), and -0.14 L/min/m² (-0.78 to 0.49 L/min/m²) for patients with LVAD.

CONCLUSIONS

IGR is a valid method for estimating cardiac output and should be used in clinical practice to complement the evaluation and management of chronic heart failure and patients with an LVAD.

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.

Physiology of the Assisted Circulation in Cardiogenic Shock: A State-of-the-Art Perspective

Mechanical circulatory support (MCS) has made rapid progress over the last 3 decades. This was driven by the need to develop acute and chronic circulatory support as well as by the limited organ availability for heart transplantation. The growth of MCS was also driven by the use of extracorporeal membrane oxygenation (ECMO) after the worldwide H1N1 influenza outbreak of 2009. The majority of mechanical pumps (ECMO and left ventricular assist devices) are currently based on continuous flow pump design. It is interesting to note that in the current era, we have reverted from the mammalian pulsatile heart back to the continuous flow pumps seen in our simple multicellular ancestors. This review will highlight key physiological concepts of the assisted circulation from its effects on cardiac dynamic to principles of cardiopulmonary fitness. We will also examine the physiological principles of the ECMO-assisted circulation, anticoagulation, and the haemocompatibility challenges that arise when the blood is exposed to a foreign mechanical circuit. Finally, we conclude with a perspective on smart design for future development of devices used for MCS.

Medical Management of Left Ventricular Assist Device Patients: A Practical Guide for the Nonexpert Clinician

Left ventricular assist devices (LVADs) provide short- or long-term circulatory support to improve survival and reduce morbidity in selected patients with advanced heart failure. LVADs are being used increasingly and now have expanded indications. Health care providers across specialties will therefore not only encounter LVAD patients but play an integral role in their care. To accomplish that, they need to understand the elements of LVAD function, physiology and clinical use. This article provides a concise overview of the medical management of LVAD patients for nonexpert clinicians. Our presentation includes the basics of LVAD physiology, design, and operation, patient selection and assessment, medical management, adverse event identification and management, multidisciplinary care, and management of special circumstances, such as noncardiac surgery, cardiac arrest, and end-of-life care. The clinical examination of LVAD patients is unique in terms of blood pressure and heart rate assessment, LVAD "hum" auscultation, driveline and insertion site inspection, and device parameter recording. Important potential device-related adverse events include stroke, gastrointestinal bleeding, hematologic disorders, device infection, LVAD dysfunction, arrhythmias, and heart failure. Special considerations include the approach to the unconscious or pulseless patient, noncardiac surgery, and palliative care. An understanding of the principles presented in this paper will enable the nonexpert clinician to be effective in collaborating with an LVAD center in the assessment, medical management, and follow-up of LVAD patients. Future opportunities and challenges include the improvement of device designs, greater application of minimally invasive surgical implantation techniques, and management of health economics in cost-constrained systems like those of Canada and many other jurisdictions.

Preload Sensitivity with TORVAD Counterpulse Support Prevents Suction and Overpumping

PURPOSE

This study compares preload sensitivity of continuous flow (CF) VAD support to counterpulsation using the Windmill toroidal VAD (TORVAD). The TORVAD is a two-piston rotary pump that ejects 30 mL in early diastole, which increases cardiac output while preserving aortic valve flow.

METHODS

Preload sensitivity was compared for CF vs. TORVAD counterpulse support using two lumped parameter models of the cardiovascular system: (1) an open-loop model of the systemic circulation was used to obtain ventricular function curves by isolating the systemic circulation and prescribing preload and afterload boundary conditions, and (2) a closed-loop model was used to test the physiological response to changes in pulmonary vascular resistance, systemic vascular resistance, heart rate, inotropic state, and blood volume. In the open-loop model, ventricular function curves (cardiac output vs left ventricular preload) are used to assess preload sensitivity. In the closed-loop model, left ventricular end systolic volume is used to assess the risk of left ventricular suction.

RESULTS

At low preloads of 5 mmHg, CF support overpumps the circulation compared to TORVAD counterpulse support (cardiac output of 3.3 L/min for the healthy heart, 4.7 with CF support, and 3.5 with TORVAD counterpulse support) and has much less sensitivity than counterpulse support (0.342 L/min/mmHg for the healthy heart, 0.092 with CF support, and 0.306 with TORVAD counterpulse support). In the closed-loop model, when PVR is increased beyond 0.035 mmHg s/mL, CF support overpumps the circulation and causes ventricular suction events, but TORVAD counterpulse support maintains sufficient ventricular volume and does not cause suction.

CONCLUSIONS

Counterpulse support with the TORVAD preserves aortic valve flow and provides physiological sensitivity across all preload conditions. This should prevent overpumping and minimize the risk of suction.

In vitro evaluation of an adaptive Starling-like controller for dual rotary ventricular assist devices

Rotary ventricular assist devices (VADs) operated clinically under constant speed control (CSC) cannot respond adequately to changes in patient cardiac demand, resulting in sub-optimal VAD flow regulation. Starling-like control (SLC) of VADs mimics the healthy ventricular flow regulation and automatically adjusts VAD speed to meet varying patient cardiac demand. The use of a fixed control line (CL - the relationship between ventricular preload and VAD flow) limits the flow regulating capability of the controller, especially in the case of exercise. Adaptive SLC (ASLC) overcomes this limitation by allowing the controller to adapt the CL to meet a diverse range of circulatory conditions. This study evaluated ASLC, SLC and CSC in a biventricular supported mock circulation loop under the simulated conditions of exercise, sleep, fluid loading and systemic hypertension. Each controller was evaluated on its ability to remain within predefined limits of VAD flow, preload, and afterload. The ASLC produced superior cardiac output (CO) during exercise (10.1 L/min) compared to SLC (7.3 L/min) and CSC (6.3 L/min). The ASLC produced favourable haemodynamics during sleep, fluid loading and systemic hypertension and could remain within a predefined haemodynamic range in three out of four simulations, suggesting improved haemodynamic performance over SLC and CSC.

Numerical modeling of continuous-flow left ventricular assist device performance

Responses of five rotary blood pumps, namely HeartAssist 5, HeartMate II, HeartWare, Sputnik 1, and Sputnik 2, were extensively assessed in six test cases using a mathematical model of the cardiovascular system. Data for the rotary pumps were derived from pressure-flow curves reported in the literature. The test cases were chosen to attempt to cover most common clinical conditions, such as partial or full support or transitions between different levels of ventricular support. The investigated parameters are collected in a table and presented in figures, such as pressure-volume loops, H-Q curves, pump flow, and aortic pressure waveforms. HeartAssist, Sputnik 1, and Sputnik 2 pumps provide comparable level of aortic pressure, pump flow pulsatility PI(Q_P), and aortic pressure pulsatility PI(AoP) due to the similarity of pressure-flow characteristic curves of these pumps. HeartMate II provides a minimal backflow among other investigated rotary blood pumps due to the maximum pressure head at zero flow. HeartWare provides minimal pulsation of flow, which is confirmed by a flow range from -2 to 7 L/min in case 1. At the same time, the greatest degree of unloading was demonstrated by the HeartWare due to the flatness of the pressure-flow curve shape. The conclusions were made based on the obtained results, including the influence of pressure-flow curve shape on the pump performance and occurrences of adverse events, such as backflow or suction. For example, the increase of the pressure head at zero flow decreases the likelihood of backflow through the pump, and with it, increasing the flow under minimal pressure head increases the likelihood of suction.

How can LVAD support influence ventricular energetics parameters in advanced heart failure patients? A retrospective study

BACKGROUND AND OBJECTIVE

Here we present a retrospective analysis of six heart failure patients previously discussed at a multidisciplinary team meeting. Only three out of six patients underwent LVAD insertion as the most appropriate management option.

METHODS

We sought to reproduce the baseline conditions of these patients on hospital admission using our cardiovascular software simulator (CARDIOSIM^©). Subsequently, we simulated the effects of LVAD support and drug administration on left and right ventricular energetics parameters. LVAD assistance was delivered by CARDIOSIM^© based on the module reproducing the behaviour of the Berlin Heart INCOR pump.

RESULTS

The results of our simulations were in agreement with the multidisciplinary team meeting outcome. The analysis of ventricular energetics parameters based on external work and pressure volume area confirmed LVAD support as a beneficial therapeutic option for the three patients considered eligible for this type of treatment. The effects induced by LVAD support and drugs administration showed specific patterns between the two groups of patients.

CONCLUSION

A quantitative approach with the ability to predict outcome during patient's assessment may well be an aid and not a substitute for clinical decision-making.

Computational analysis of the hemodynamic characteristics under interaction influence of β-blocker and LVAD

Background

Hemodynamic characteristics of the interaction influence among support level and model of LVAD, and coupling β-blocker has not been reported.

Methods

In this study, the effect of support level and model of LVAD on cardiovascular hemodynamic characteristics is investigated. In addition, the effect of β-blocker on unloading with LVAD is analyzed to elucidate the mechanism of LVAD coupling β-blocker. A multi-scale model from cell level to system level is proposed. Moreover, LVAD coupling β-blocker has been researching to explain the hemodynamics of cardiovascular system.

Results

Myocardial force was decreased along with the increase of support level of LVAD, and co-pulse mode was the lowest among the three support modes. Additionally, the β-blocker combined with LVAD significantly reduced the left ventricular volume compared with LVAD support without β-blocker. However, the left ventricular pressure under both cases has no significant difference. External work of right ventricular was increased along with the growth of support level of only LVAD. The LVAD under co-pulse mode achieved the lowest right-ventricular EW among the three support modes.

Conclusions

Co-pulse mode with β-blocker could be an optimal strategy for promoting cardiac structure and function recovery.

Implantable Ventricular Assist Device Use and Outcomes in People With End-Stage Renal Disease

Background

People with end‐stage renal disease (ESRD) are at risk for advanced heart failure, but little is known about use and outcomes of durable mechanical circulatory support in this setting. We examined use and outcomes of implantable ventricular assist devices (VADs) in a national ESRD cohort.

Methods and Results

We performed a retrospective cohort study of Medicare beneficiaries with ESRD who underwent implantable VAD placement from 2006 to 2014. We examined in‐hospital and 1‐year mortality, all‐cause and cause‐specific hospitalizations, and heart/kidney transplantation outcomes. We investigated as predictors demographic factors, time‐period of VAD implantation, primary or post‐cardiotomy implantation, and duration of ESRD before VAD implantation. We identified 96 people with ESRD who underwent implantable VAD placement. At time of VAD implantation, 74 (77.1%) were receiving hemodialysis, 10 (10.4%) were receiving peritoneal dialysis and 12 (12.5%) had renal transplant. Time from incident ESRD to VAD implantation was median 4.0 (interquartile range 1.1, 8.2) years. Mortality during the implantation hospitalization was 40.6%. Within 1 year of implantation 61.5% of people had died. On multivariable analysis, males had half the mortality risk of females. Lower mortality risk was also seen with VAD implantation in a primary setting, and with more recent year of implantation, but these results did not reach statistical significance.

Conclusions

Medicare beneficiaries with ESRD are undergoing durable VAD implantation, often several years after incident ESRD, although in low numbers. Mortality is high among these patients, highlighting the need for investigations to improve treatment selection and management.

Living Without a Pulse: The Vascular Implications of Continuous-Flow Left Ventricular Assist Devices

Pulsatility seems to have a teleological role because evolutionary hierarchy favors higher ordered animals with more complex, multichamber circulatory systems that generate higher pulse pressure compared with lower ordered animals. Yet despite years of such natural selection, the modern generation of continuous-flow left ventricular assist devices (CF-LVADs) that have been increasingly used for the last decade have created a unique physiology characterized by a nonpulsatile, nonlaminar blood flow profile with the absence of the usual large elastic artery Windkessel effect during diastole. Although outcomes and durability have improved with CF-LVADs, patients supported with CF-LVADs have a high rate of complications that were not as frequently observed with older pulsatile devices, including gastrointestinal bleeding from arteriovenous malformations, pump thrombosis, and stroke. Given the apparent fundamental biological role of the pulse, the purpose of this review is to describe the normal physiology of ventricular-arterial coupling from pulsatile flow, the effects of heart failure on this physiology and the vasculature, and to examine the effects of nonpulsatile blood flow on the vascular system and potential role in complications seen with CF-LVAD therapy. Understanding these concomitant vascular changes with CF-LVADs may be a key step in improving patient outcomes as modulation of pulsatility and flow characteristics may serve as a novel, yet simple, therapy for reducing complications.

Mechanical circulatory support devices: methods to optimize hemodynamics during use

INTRODUCTION

Mechanical circulatory support (MCS) is an increasingly utilized mode of therapy in the management of advanced heart failure, both as bridge to heart transplantation and destination therapy. As MCS becomes more prevalent, it is ever more important to understand the complex hemodynamics of these devices, as well as the strategies for hemodynamic optimization. Areas covered: This review provides an overview of hemodynamics in the normal human heart and the failing heart. We discuss the various short-term mechanical circulatory support devices and their hemodynamic consequences. We will then discuss the differences between left ventricular assist devices, and the impact of these differences on hemodynamics. We will describe the strategies for hemodynamic optimization using echocardiographic and invasive ramp studies. Finally, we will discuss the impact of speed changes with exercise and discuss future directions for advancements in MCS therapies. Expert commentary: We advocate for a deeper understanding of the hemodynamics underpinning MCS devices. We also recommend the more widespread use of ramp studies for speed optimization, which have been well validated across a number of different left ventricular assist device types.

Sensor-Based Physiologic Control Strategy for Biventricular Support with Rotary Blood Pumps

Rotary biventricular assist devices (BiVAD) are becoming a clinically accepted treatment option for end-stage biventricular failure. To improve BiVAD efficacy and safety, we propose a control algorithm to achieve the clinical objectives of maintaining left-right-sided balance, restoring physiologic flows, and preventing ventricular suction. The control algorithm consists of two proportional-integral (PI) controllers for left and right ventricular assist devices (LVAD and RVAD) to maintain differential pump pressure across LVAD (ΔPL) and RVAD (ΔPR) to provide left-right balance and physiologic flow. To prevent ventricular suction, LVAD and RVAD pump speed differentials (ΔRPML, ΔRPMR) were maintained above user-defined thresholds. Efficacy and robustness of the proposed algorithm were tested in silico for axial and centrifugal flow BiVAD using 1) normal and excessive ΔPL and/or ΔPR setpoints, 2) rapid threefold increase in pulmonary vascular or vena caval resistances, 3) transient responses from exercise to rest, and 4) ventricular fibrillation. The study successfully demonstrated that the proposed BiVAD algorithm achieved the clinical objectives but required pressure sensors to continuously measure ΔPL and ΔPR. The proposed control algorithm is device independent, should not require any modifications to the pump or inflow/outflow cannulae/grafts, and may be directly applied to current rotary blood pumps for biventricular support.

Optimisation of the Sputnik-VAD Design

Purpose

Miniaturisation of VADs can offer important benefits, including less invasive implantation techniques and more versatility in patient selection. The aim of this work was to reduce the weight, size, and energy consumption of the Sputnik VAD.

Methods

The second generation of the Sputnik VAD was developed with a set of changes in construction. The head pressure-flow rate (H-Q) and power consumption-flow rate curves for the Sputnik VADs were measured at different rotational speeds. Computational fluid dynamics (CFD) were used for operating condition simulation and the LVADs were compared under the simulated physiological conditions.

Results

The slope of the H-Q curves for the Sputnik 1 VAD remains almost invariable over the entire range of the measured flow rate, in contrast to the curves for the Sputnik 2 VAD, which become flat in the high flow-rate region. Despite the design modification, the operating rotor speed remained invariable. The preload sensitivity of the Sputnik VAD is higher than that of the other rotary blood pumps and amounts to 0.111 ± 0.0092 L min−1 mmHg−1. The power consumption for the Sputnik 2 VAD is lower over the entire speed range, except for at 5,000 rpm.

Conclusions

The pump weight was reduced from 246 to 205 g, the pump length was decreased from 82 to 66 mm, and the pump diameter was decreased from 32 to 29 mm. The total energy consumption of the pump was reduced by 15%.