Continuous-Flow Cardiac Assistance: Effects on Aortic Valve Function in a Mock Loop

Management of aortic valve insufficiency in patients with continuous-flow left ventricular assist device: a republication of the review published in Japanese Journal of Artificial Organs

Since 2011, implantable ventricular assist devices have been a standard treatment for severe heart failure alongside heart transplantation in Japan. However, the limited availability of donors has led to a prolonged wait for transplants, now averaging 1719 days, intensifying the issue of aortic insufficiency in patients with continuous flow ventricular assist devices. These devices limit the opening of the aortic valve, leading to sustained closure and increased shear stress, which accelerates valve degradation. Risk factors for aortic insufficiency include having a smaller body surface area, being of advanced age, and the presence of mild aortic insufficiency prior to device implantation. In patients presenting with mild or moderate aortic regurgitation at the time of ventricular assist device implantation, interventions such as aortic valve repair or bioprosthetic valve replacement are performed with the aim of halting its progression. The choice of surgical procedure should be tailored to each patient's individual condition. The management of de novo aortic insufficiency in patients with continuous flow ventricular assist devices remains challenging, with no clear consensus on when to intervene. Interventions for significant aortic insufficiency typically consider the patient's symptoms and aortic insufficiency severity. De novo aortic insufficiency progression in continuous flow ventricular assist devices patients necessitates careful monitoring and intervention based on individual patient assessments and valve condition. This review was created based on a translation of the Japanese review written in the Japanese Journal of Artificial Organs in 2023 (Vol. 52, No. 1, pp. 77-80), with some modifications.

Mixing properties of coronary infusion catheters assessed by in vitro experiments and computational fluid dynamics

Aims

Continuous infusion thermodilution is an established technique for the assessment of absolute coronary blood flow and microvascular resistance due to its proven accuracy and reproducibility. However, for this technique to yield reliable measurements, direct and homogenous mixing of injected saline and blood is mandatory. This study aimed to assess and compare the mixing properties of two different microcatheters, namely the Rayflow® (with sideholes for infusion) and the Finecross® catheter (single end-hole for infusion), which are commonly used in the catheterization laboratory.

Methods and results

The study employed three different methods to evaluate the mixing properties of the catheters. Firstly, a qualitative assessment of mixing was performed using ink injections in an in vitro bench model of a coronary artery. Secondly, in analogy to the human catheterization laboratory, mixing properties over the length of the coronary artery were assessed semi-quantitatively by temperature measurements in the bench model. Lastly, a quantitative assessment was performed by 3D computational fluid dynamics, where the standard deviation and entropy ratio of the temperature over the cross-section in the coronary artery model were calculated for both catheters.

Conclusion

All three evaluation methods demonstrated that the Rayflow catheter's specific design leads to a more optimal, homogeneous mixture of blood and saline over both the cross-section and length of a coronary vessel, as compared with the standard end-hole catheter.

Mock circulatory loop applications for testing cardiovascular assist devices and in vitro studies

The mock circulatory loop (MCL) is an in vitro experimental system that can provide continuous pulsatile flows and simulate different physiological or pathological parameters of the human circulation system. It is of great significance for testing cardiovascular assist device (CAD), which is a type of clinical instrument used to treat cardiovascular disease and alleviate the dilemma of insufficient donor hearts. The MCL installed with different types of CADs can simulate specific conditions of clinical surgery for evaluating the effectiveness and reliability of those CADs under the repeated performance tests and reliability tests. Also, patient-specific cardiovascular models can be employed in the circulation of MCL for targeted pathological study associated with hemodynamics. Therefore, The MCL system has various combinations of different functional units according to its richful applications, which are comprehensively reviewed in the current work. Four types of CADs including prosthetic heart valve (PHV), ventricular assist device (VAD), total artificial heart (TAH) and intra-aortic balloon pump (IABP) applied in MCL experiments are documented and compared in detail. Moreover, MCLs with more complicated structures for achieving advanced functions are further introduced, such as MCL for the pediatric application, MCL with anatomical phantoms and MCL synchronizing multiple circulation systems. By reviewing the constructions and functions of available MCLs, the features of MCLs for different applications are summarized, and directions of developing the MCLs are suggested.

Considerations of valvular heart disease in children with ventricular assist devices

Ventricular assist devices have become a valuable tool in the treatment of heart failure in children. The use of ventricular assist devices has decreased mortality in children with end-stage heart failure awaiting transplant. It is not uncommon for children with end-stage heart failure associated with cardiomyopathy or congenital heart disease to have significant systemic semilunar and atrioventricular valve regurgitation, which can impact the efficiency and efficacy of hemodynamic support provided by a ventricular assist device. Therefore, implanting clinicians should carefully assess for valve abnormalities that may need repair and impact device selection and cannulation strategy to effectively support this diverse population. The purpose of this review is to provide an overview of this important and relevant topic and to discuss strategies for managing these patients.

Factors influencing the functional status of aortic valve in ovine models supported by continuous-flow left ventricular assist device

Objectives

An acute animal experiment was performed to observe factors influencing the functional status of the aortic valve functional status after continuous‐flow left ventricular assist device (CF‐LVAD) implantation in an ovine model, and a physiologic predictive model was established.

Methods

A CF‐LVAD model was established in Small Tail Han sheep. The initial heart rate (HR) was set to 60 beats/min, and grouping was performed at an interval of 20 beats/min. In all groups, the pump speed was started from 2000 rpm and was gradually increased by 50–100 rpm. A multi‐channel physiological recorder recorded the HR, aortic pressure, central venous pressure, and left ventricular systolic pressure (LVSP). A double‐channel ultrasonic flowmeter was used to obtain real‐time artificial vascular blood flow (ABF). A color Doppler ultrasound device was applied to assess the aortic valve functional status. Multivariate dichotomous logistic regression was used to screen significant variables for predicting the functional status of the aortic valve.

Results

Observational studies showed that ABF and the risk of aortic valve closure (AVC) were positively correlated with pump speed at the same HR. Meanwhile, the mean arterial pressure (MAP) was unaltered or slightly increased with increased pump speed. When the pump speed was constant, an increase in HR was associated with a decrease in the size of the aortic valve opening. This phenomenon was accompanied by an initial transient increase in the ABF and MAP, which subsequently decreased. Statistical analysis showed that the AVC was associated with increased pump speed (OR = 1.02, 95% CI = 1.01–1.04, p = 0.001), decreased LVSP (OR = 0.95, 95% CI = 0.91–0.98, p = 0.003), and decreased pulse pressure (OR = 0.82, 95% CI = 0.68–0.96, p = 0.026). ABF or MAP was negatively associated with the risk of AVC (OR < 1). The prediction model of AVC after CF‐LVAD implantation exhibited good differentiation (AUC = 0.973, 95% CI = 0.978–0.995) and calibration performance (Hosmer–Lemeshow χ² = 9.834, p = 0.277 > 0.05).

Conclusions

The pump speed, LVSP, ABF, MAP, and pulse pressure are significant predictors of the risk of AVC. Predictive models built from these predictors yielded good performance in differentiating aortic valve opening and closure after CF‐LVAD implantation.

Mock circulatory test rigs for the in vitro testing of artificial cardiovascular organs

In vitro study plays an important role in the experimental study of cardiovascular dynamics. An essential hardware facility that mimics the blood flow changes and provides the required test conditions, a mock circulatory test rig (MCTR), is imperative for the execution of in vitro study. This paper examines the current MCTRs in use for the testing of artificial cardiovascular organs. Various aspects of the MCTRs are surveyed, including the necessity of in vitro study, the building of MCTRs, relevant standards, general system structure (e.g., the motion and driving, fluid, measurement subsystems), classification, motion driving mechanism of MCTRs, and the considerations for the modelling of the physiological impedance of MCTRs. Examples of the steady and pulsatile flow types of the MCTRs are introduced. Recent developments in MCTRs are inspected and possible future design improvements suggested. This study will help researchers in the design, construction, analysis, and selection of MCTRs for cardiovascular research.

Mechanical behavior of bovine pericardium treated with hyaluronic acid derivative for bioprosthetic aortic valves

We studied the mechanical behavior of bovine pericardium (BP) after anticalcification treatment using hyaluronic acid (HA) derivative. To simulate the physiological environment and stimulate the calcification process, the BP samples were immersed into simulated body fluid solution. We conducted scanning electron microscopy with energy dispersive X-ray spectrometry, and uniaxial mechanical tests of HA-treated and non-treated samples. Although our microstructural analyses indicated that the HA treatment actually prevents the formation of calcium phosphate deposits, the mechanical tests show significant increase of stiffness of the HA-treated samples. Using data from our mechanical tests as input parameters, we performed finite element (FE) computer simulations to estimate how this increase in the BP stiffness affects the stress distribution in the bioprosthetic leaflet. Although the maximum stress observed during the closing phase of the membrane in vivo is below the experimental yield stress in all cases we analyzed, our FE results indicate that increase of BP stiffness due to HA anticalcification treatment results in higher risk of disruption and failure of the leaflets in bioprosthetic heart valves. Since our FE results indicate that the commissure and the fixed edge are the regions that withstand the highest mechanical stresses during the closing phase, new designs of the valve might be efficient to enhance the endurance of the prosthesis. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 2273-2280, 2019.

Blood damage in Left Ventricular Assist Devices: Pump thrombosis or system thrombosis?

Introduction:

Despite significant technical advancements in the design and manufacture of Left Ventricular Assist Devices, post-implant thrombotic and thromboembolic complications continue to affect long-term outcomes. Previous efforts, aimed at optimizing pump design as a means of reducing supraphysiologic shear stresses generated within the pump and associated prothrombotic shear-mediated platelet injury, have only partially altered the device hemocompatibility.

Methods:

We examined hemodynamic mechanisms that synergize with hypershear within the pump to contribute to the thrombogenic potential of the overall Left Ventricular Assist Device system.

Results:

Numerical simulations of blood flow in differing regions of the Left Ventricular Assist Device system, that is the diseased native left ventricle, the pump inflow cannula, the impeller, the outflow graft and the anastomosed downstream aorta, reveal that prothrombotic hemodynamic conditions might occur at these specific sites. Furthermore, we show that beyond hypershear, additional hemodynamic abnormalities exist within the pump, which may elicit platelet activation, such as recirculation zones and stagnant platelet trajectories. We also provide evidences that particular Left Ventricular Assist Device implantation configurations and specific post-implant patient management strategies, such as those allowing aortic valve opening, are more hemodynamically favorable and reduce the thrombotic risk.

Conclusion:

We extend the perspective of pump thrombosis secondary to the supraphysiologic shear stress environment of the pump to one of Left Ventricular Assist Device system thrombosis, raising the importance of comprehensive characterization of the different prothrombotic risk factors of the total system as the target to achieve enhanced hemocompatibility and improved clinical outcomes.

Current status of mechanical circulatory support for treatment of advanced end-stage heart failure: successes, shortcomings and needs

INTRODUCTION

Heart failure (HF) remains a major global burden in terms of morbidity and mortality. Despite advances in pharmacological and resynchronization device therapy, many patients worsen to end-stage HF. Although the gold-standard treatment for such patients is heart transplantation, there will always be a shortage of donor hearts. Areas covered: A left ventricular assist device (LVAD) is a valuable option for these patients as a bridge measure (to recovery, to candidacy for transplant, or to transplant itself) or as destination therapy. This review describes the current indications for and complications of the most commonly implanted LVADs. In addition, we review the potential and promising new LVADs, including the HeartMate 3, MVAD, and other LVADs. Studies investigating each were identified through a combination of online database and direct extraction of studies cited in previously identified articles. Expert commentary: The goal of LVADs has been to fill the gap between patients with end-stage HF who would likely not benefit from heart transplantation and those who could benefit from a donor heart. As of now, the use of LVADs has been limited to patients with end-stage HF, but next-generation LVAD therapy may improve both survival and quality of life in less sick patients.

Continuous Monitoring of Aortic Valve Opening in Rotary Blood Pump Patients

GOAL

Rotary blood pumps (RBPs) typically support the left ventricle by pumping blood from the ventricle to the aorta, partially bypassing the aortic valve (AV). Monitoring the AV opening during RBP support would provide important information about cardiac-pump interaction. However, currently this information is not continuously available. In this study, an algorithm to determine AV opening using available pump signals was evaluated in humans.

METHODS

Pump speed changes were performed in 15 RBP patients to elicit opening of the AV. Simultaneously to pump data recordings, the AV was continuously monitored using echocardiography. The algorithm, which classifies the AV state utilizing three features (skewness, kurtosis, and crest factor) calculated from the pump flow waveform, was compared to echocardiography by using cross-validation analysis. Additionally, numerical simulation was used to evaluate effects of different pump characteristics and cannula length, as well as mitral valve insufficiency on the AV opening detection method.

RESULTS

More than 7000 heart beats were analyzed. The correct classification rate using the developed algorithm was 91.1% (sensitivity 91.0%, specificity 91.2%). Numerical simulations showed that the flow waveform shape used for AV opening detection is preserved under the different conditions studied.

CONCLUSION

This study demonstrates that the AV opening can be reliably detected in RBP patients using available pump data.

SIGNIFICANCE

Once implemented in RBP controllers, this method will provide a novel tool to improve the management of RBP patients, particularly for adjustments of the pump speed and flow and for the evaluation of the assisted cardiac function.

A novel passive left heart platform for device testing and research

Integration of biological samples into in vitro mock loops is fundamental to simulate real device's operating conditions. We developed an in vitro platform capable of simulating the pumping function of the heart through the external pressurization of the ventricle. The system consists of a fluid-filled chamber, in which the ventricles are housed and sealed to exclude the atria from external loads. The chamber is connected to a pump that drives the motion of the ventricular walls. The aorta is connected to a systemic impedance simulator, and the left atrium to an adjustable preload. The platform reproduced physiologic hemodynamics, i.e. aortic pressures of 120/80 mmHg with 5 L/min of cardiac output, and allowed for intracardiac endoscopy. A pilot study with a left ventricular assist device (LVAD) was also performed. The LVAD was connected to the heart to investigate aortic valve functioning at different levels of support. Results were consistent with the literature, and high speed video recordings of the aortic valve allowed for the visualization of the transition between a fully opening valve and a permanently closed configuration. In conclusion, the system showed to be an effective tool for the hemodynamic assessment of devices, the simulation of surgical or transcatheter procedures and for visualization studies.

Can Intraoperative Transesophageal Echocardiography Predict Postoperative Aortic Insufficiency in Patients Receiving Implantable Left Ventricular Assist Devices?

OBJECTIVE

Aortic insufficiency (AI) develops in 25% of patients after left ventricular assist device (LVAD) insertion. The objective of this study was to evaluate the occurrence of new-onset AI upon initiation of cardiopulmonary bypass (CPB) required for LVAD insertion and the potential ability of this new-onset AI to predict development of post-LVAD insertion AI.

DESIGN

Forty-one patients undergoing LVAD insertion were studied. Intraoperative transesophageal echocardiography (TEE) evaluation was performed at baseline (post-induction, pre-sternotomy), 5 minutes after CPB initiation, and post-chest closure. Patients were followed up postoperatively for development of AI.

SETTING

Single university hospital.

PARTICIPANTS

Patients undergoing elective LVAD insertion.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

At baseline, 35 patients exhibited none-trace AI, 4 exhibited mild AI, 2 exhibited moderate AI, and none exhibited severe AI. After initiation of CPB, 34 patients exhibited no change in degree of AI yet 7 exhibited an increase in AI severity. However, all 7 patients exhibited no change in degree of AI at chest closure and one exhibited a decrease in AI severity. Four patients developed at least moderate AI during the postoperative period (range 3-8 months). However, only one of these patients exhibited an increase in AI severity after initiation of CPB for LVAD insertion. No significant changes in aortic root measurements were observed during the entire intraoperative period (within patients nor between patients with/without development of at least moderate postoperative AI).

CONCLUSIONS

One in 5 patients undergoing LVAD insertion will demonstrate an increase in AI severity at CPB initiation without changes in aortic root measurements. None of the information obtained from intraoperative TEE seemed to predict development of at least moderate postoperative AI.

Aortic Valve Function Under Support of a Left Ventricular Assist Device: Continuous vs. Dynamic Speed Support

Continuous flow left ventricular devices (CF-LVADs) support the failing heart at a constant speed and alters the loads on the aortic valve. This may cause insufficiency in the aortic valve under long-term CF-LVAD support. The aim of this study is to assess the aortic valve function under varying speed CF-LVAD support. A Medtronic freestyle valve and a Micromed DeBakey CF-LVAD were tested in a mock circulatory system. First, the CF-LVAD was operated at constant speeds between 7500 and 11,500 rpm with 1000 rpm intervals. The mean pump outputs obtained from these tests were applied in varying speed CF-LVAD support mode using a reference model for the pump flow. The peak of the instantaneous pump flow was applied at peak systole and mid-diastole, respectively. Ejection durations and in the aortic valve were the longest when the peak pump flow was applied at mid-diastole among the CF-LVAD operating modes. Furthermore, mean aortic valve area over a cardiac cycle was highest when the peak pump flow was applied at mid-diastole. The results show that changing phase of the reference flow rate signal may reduce the effects of the CF-LVADs on altered aortic valve closing behavior, without compromising the overall pump support level.

Management of aortic insufficiency in the continuous flow left ventricular assist device population

With the current generation of continuous-flow (CF) left ventricular assist devices (LVADs), patients are able to be supported for longer periods of time. As a result, there has been increasing focus on long-term complications from prolonged mechanical circulatory support, such as acquired aortic insufficiency (AI). In the presence of an LVAD, AI leads to a blind circulatory loop, with a portion of LVAD output regurgitating through the aortic valve (AV) into the left ventricle and back again through the device, limiting effective forward flow and ultimately leading to organ malperfusion and increased left ventricular diastolic pressures. The AV also experiences abnormal biomechanics as a result of limited valve opening in the presence of a CF LVAD. Increased shear stress, elevated transvalvular pressure gradients, and decreased valve open time all contribute to acquired AI. The prognosis of moderate to severe AI in LVAD patients is generally poor and leads to a higher rate of AV replacement and potentially reduced survival. However, there are no evidence-based guidelines for management of this challenging population. In severe AI, experts generally advocate AV replacement or repair, while lesser degrees of AI can be managed medically and/or with adjustments in pump parameters.

Assessment of aortic valve opening during rotary blood pump support using pump signals

During left ventricular support by rotary blood pumps (RBPs), the biomechanics of the aortic valve (AV) are altered, potentially leading to adverse events like commissural fusion, valve insufficiency, or thrombus formation. To avoid these events, assessment of AV opening and consequent adaptation of pump speed seem important. Additionally, this information provides insight into the heart-pump interaction. The aim of this study was to develop a method to assess AV opening from the pump flow signal. Data from a numerical model of the cardiovascular system and animal experiments with an RBP were employed to detect the AV opening from the flow waveform under different hemodynamic conditions. Three features calculated from the pump flow waveform were used to classify the state of the AV: skewness, kurtosis, and crest factor. Three different classification algorithms were applied to determine the state of the AV based on these features. In the model data, the best classifier resulted in a percentage of correctly identified beats with a closed AV (specificity) of 99.9%. The percentage of correctly identified beats with an open AV (sensitivity) was 99.5%. In the animal experiments, specificity was 86.8% and sensitivity reached 96.5%. In conclusion, a method to detect AV opening independently from preload, afterload, heart rate, contractility, and degree of support was developed. This algorithm makes the evaluation of the state of the AV possible from pump data only, allowing pump speed adjustment for a frequent opening of the AV and providing information about the interaction of the native heart with the RBP.

In vitro evaluation of aortic insufficiency with a rotary left ventricular assist device

Aortic insufficiency (AI) is usually repaired prior to rotary blood pump (RBP) implantation but can develop during support due, in part, to the sustained RBP-induced high pressure gradient across the aortic valve. Repair of the aortic valve before or during RBP support predisposes these critically ill patients to even higher risks. This study used an in vitro mock circulation loop to identify the severity of AI and/or left heart failure (LHF) that might benefit from valve repair while investigating RBP operating strategies to reduce the hemodynamic influence of AI. Reproduction of AI with RBP-supported LHF reduced device efficiency, particularly in the more severe cases of AI and LHF. The requirement for repair or closure of the aortic valve was demonstrated in all conditions other than those with only mild AI. When a sinusoidal RBP speed pulse was induced, small changes in systemic flow rate and regurgitant volume were observed with all degrees of AI. Variation of the pulse phase delay only resulted in minor changes to systemic flow rate, with a maximum difference of 0.17 L/min. Although the clinical implications of these small changes may be insignificant, changes in systemic flow rate and transvalvular pressure were shown when the sinusoidal RBP speed pulse was applied with no AI. In these cases, transvalvular pressure was reduced by up to 8% through sinusoidal copulsation of the RBP, which may prevent or delay the onset of AI. This in vitro study suggests that surgical intervention is required with moderate or worse AI and that RBP operating strategies should be further explored to delay the onset and reduce the harmful effects of AI.

Effects of the HeartMate II left ventricular assist device as observed by serial echocardiography

BACKGROUND

The HeartMate II is the most frequently used left ventricular assist device (LVAD) in patients with end-stage heart failure. There is a paucity of data regarding its longitudinal cardiac effects, particularly that on diastole.

METHODS

This retrospective study was an evaluation of echocardiograms preoperatively and at 3, 6, and 12 months postoperatively in patients with a HeartMate II. Measurements included left ventricle (LV) dimensions, ejection fraction (EF), right ventricle (RV) size and function, parameters of diastolic function, and an analysis of mitral regurgitation (MR), tricuspid regurgitation (TR), aortic insufficiency (AI), and aortic valve thickening.

RESULTS

Forty-seven patients were evaluated. LV dimensions decreased but EF, RV size, and RV function were unchanged. Right ventricular systolic pressure (RVSP) and diastolic parameters including mitral inflow E/A, deceleration time (DT), pulmonary vein inflow, left atrial size, and overall diastolic grade improved. LV relaxation measured by tissue Doppler (e') was unchanged and the E/e' ratio was also unchanged. Regarding valve function, MR decreased, TR was unchanged, and the aortic valve became increasingly thickened with increased AI severity.

CONCLUSIONS

The HeartMate II unloads the LV as shown by decreased LV size, decreased MR, reduced RVSP, and improved patterns of mitral inflow. However, neither systolic function nor diastolic relaxation improves in this cohort. RV size and function also remain unchanged. The aortic valve shows increased thickening and AI likely from valve leaflet fusion. These results provide detailed functional and hemodynamic information regarding the longitudinal effects of the HeartMate II.

End-of-life discontinuation of destination therapy with cardiac and ventilatory support medical devices: physician-assisted death or allowing the patient to die?

BACKGROUND

Bioethics and law distinguish between the practices of "physician-assisted death" and "allowing the patient to die."

DISCUSSION

Advances in biotechnology have allowed medical devices to be used as destination therapy that are designed for the permanent support of cardiac function and/or respiration after irreversible loss of these spontaneous vital functions. For permanent support of cardiac function, single ventricle or biventricular mechanical assist devices and total artificial hearts are implanted in the body. Mechanical ventilators extrinsic to the body are used for permanent support of respiration. Clinical studies have shown that destination therapy with ventricular assist devices improves patient survival compared to medical management, but at the cost of a substantial alteration in end-of-life trajectories. The moral and legal assessment of the appropriateness and permissibility of complying with a patient's request to electively discontinue destination therapy in a life-terminating act in non-futile situations has generated controversy. Some argue that complying with this request is ethically justified because patients have the right to request withdrawal of unwanted treatment and be allowed to die of preexisting disease. Other commentators reject the argument that acceding to an elective request for death by discontinuing destination therapy is 'allowing a patient to die' because of serious flaws in interpreting the intention, causation, and moral responsibility of the ensuing death.

SUMMARY

Destination therapy with cardiac and/or ventilatory medical devices replaces native physiological functions and successfully treats a preexisting disease. We posit that discontinuing cardiac and/or ventilatory support at the request of a patient or surrogate can be viewed as allowing the patient to die if--and only if--concurrent lethal pathophysiological conditions are present that are unrelated to those functions already supported by medical devices in destination therapy. In all other cases, compliance with a patient's request constitutes physician-assisted death because of the pathophysiology induced by the turning off of these medical devices, as well as the intention, causation, and moral responsibility of the ensuing death. The distinction between allowing the patient to die and physician-assisted death is pivotal to the moral and legal status of elective requests for death by discontinuing destination cardiac and/or ventilatory medical devices in patients who are not imminently dying. This distinction also represents essential information that must be disclosed to patients and surrogates in advance of consent to this type of therapy.