Minimally Invasive Left Ventricular Assist Device Explantation After Cardiac Recovery: Surgical Technical Considerations

A Case Report About Cardiac Arrest After Left Ventricular Assist Device Explantation

Hemodynamic unloading by left ventricular assist devices (LVADs) in patients with advanced heart failure can result in reverse remodeling and cardiac recovery allowing pump removal. Ventriculoplasty during explantation may cause fibrosis and ventricular scars which can provide a substrate for reentry, a common cause of ventricular arrythmias. In this case report, we describe the clinical course of a patient who suffered a cardiac arrest in the following few months after undergoing LVAD explantation because of cardiac recovery. This case highlights a potential risk related to complete LVAD explantation with ventriculoplasty and patch repair of the apex.

Advanced Heart Failure: Therapeutic Options and Challenges in the Evolving Field of Left Ventricular Assist Devices

Heart Failure is a chronic and progressively deteriorating syndrome that has reached epidemic proportions worldwide. Improved outcomes have been achieved with novel drugs and devices. However, the number of patients refractory to conventional medical therapy is growing. These advanced heart failure patients suffer from severe symptoms and frequent hospitalizations and have a dismal prognosis, with a significant socioeconomic burden in health care systems. Patients in this group may be eligible for advanced heart failure therapies, including heart transplantation and chronic mechanical circulatory support with left ventricular assist devices (LVADs). Heart transplantation remains the treatment of choice for eligible candidates, but the number of transplants worldwide has reached a plateau and is limited by the shortage of donor organs and prolonged wait times. Therefore, LVADs have emerged as an effective and durable form of therapy, and they are currently being used as a bridge to heart transplant, destination lifetime therapy, and cardiac recovery in selected patients. Although this field is evolving rapidly, LVADs are not free of complications, making appropriate patient selection and management by experienced centers imperative for successful therapy. Here, we review current LVAD technology, indications for durable MCS therapy, and strategies for timely referral to advanced heart failure centers before irreversible end-organ abnormalities.

HeartMate 3 Explantation Using Felt Plug for Ventriculotomy Occlusion

Left ventricular assist devices (LVAD) can be utilized for heart failure patients as a bridge to transplant, bridge to destination, or bridge to recovery. Given the lack of a universally accepted consensus for assessing myocardial recovery, techniques and strategies in LVAD explantation also vary. In addition, the incidence of LVAD explantation remains relatively low, and surgical techniques of explantation continue to be areas of interest. Our approach using a felt-plug Dacron technique is an effective way to preserve left ventricular geometry and cardiac function.

Novel Plug Device for HeartMate 3 Explantation: First Multicenter Experience

Patients undergoing explantation of left ventricular assist devices (LVADs) after improvement of myocardial function remain a minority. Nevertheless, considering the growing population of LVAD patients, increasing demand for new explantation strategies is expected. Herein, we present a retrospective review of seven patients undergoing HeartMate3 explantation with the use of a custom-made apical ring plug in four medical centers. The primary outcome was status at intensive care unit discharge. Secondary outcomes included perioperative complications and transfusions. Six out of seven patients were males. The median age at explantation and time on LVAD support was 35 years (range:13-73) and 10 months (range:9-24), respectively. No technical difficulties were experienced during plug implantation via a conventional sternotomy or through a left lateral thoracotomy, either with or without cardiopulmonary bypass. Perioperative transfusions ranged from 0 to 3 units/patient. No re-operations for bleeding, hemorrhagic, embolic, or plug-related infective events were observed. Heparin was started 6 hours after surgery as a bridge to oral anticoagulation (international normalized ratio: 2-2.5). All patients were discharged alive from intensive care unit. This novel plug device for HeartMate3 explantation was successfully and safely implanted in this first patient series. Notwithstanding, its use should still be considered off-label and larger studies are required to investigate its long-term results.

Biocompatibility of an apical ring plug for left ventricular assist device explantation: Results of a feasibility pre-clinical study

BACKGROUND

Patients receiving left ventricle assist devices (LVADs) as bridge to recovery remain a minority with 1%-5% of LVADs explanted after improvement of myocardial function. Nevertheless, considering the growing population of patients supported with LVADs, an increasing demand of new explantation strategies is expected in the near future. A novel plug for LVAD explantation has been developed and its biocompatibility profile needs to be proved. This study tested the biocompatibility of this novel plug in an in vivo ovine model.

METHODS

Six adult Blackhead Persian female sheep received plug implantation on the cardiac apex via minimally invasive approach and were clinically observed up to 90 days. Echocardiography was performed to detect thrombus formation or further plug-related complications. After the observation period, euthanasia was performed and samples including the plug and the surrounding tissues were obtained to be analyzed with correlative light and electron microscopy. Organ necrosis, ischemia and peripheral embolism were investigated.

RESULTS

Three animals survived surgery and completed the follow-up time without experiencing clinical complications. Echocardiographic controls excluded the presence of an intracavitary thrombus in the left ventricle (LV). Autopsy confirmed no signs of local infection, LV thrombus or peripheral embolism. Light and electron microscopy revealed an intact epithelium covering a layer of connective tissue on the plug surface facing the heart lumen.

CONCLUSIONS

This novel apical plug for LVAD explantation allows for endothelial and connective tissue growth on its ventricular side within 90 days from surgery. Further studies are required to fully demonstrate the biocompatibility of this apical plug and investigate the optimal anticoagulation regimen to be applied after implantation.

Midterm Results of Minimally Invasive Left Thoracotomy Fully Magnetically Levitated Left Ventricular Assist Device Implantation

The aim of the study is to compare outcomes of the patients who underwent HeartMate3 (HM3) implantation with conventional sternotomy (CS) and minimally invasive thoracotomy technique (MILT). From June 2013 to April 2019, 50 patients who underwent isolated HM3 implantation were included. Patients were divided into two groups according to the operative technique; of 50 patients, 28 were implanted with CS and 22 with MILT. All patients were evaluated for mortality, need of inotrope, blood product transfusion, intensive care unit stay, time on mechanical ventilation, and postoperative right ventricular failure. Primary outcomes were early mortality and occurrence of adverse events. Patients characteristics were similar; preoperative central venous pressure (CVP) and the ratio of CVP to pulmonary capillary wedge pressure (CVP/PCWP) were significantly higher in the MILT group versus the CS group (p < 0.05). Intensive care unit stay was significantly shorter in the MILT group (p < 0.05). Incidence of bleeding requiring exploratory sternotomy and postoperative drainage on the first postoperative day were statistically higher in the CS group (p < 0.05). Right ventricular failure was higher in the CS group but was not statistically significant (p = 0.4). There was no significant difference in long-term adverse events. The Kaplan-Meier survival analysis showed no difference between the groups (p = 0.66). The HM3 assist system can be successfully implanted with the MILT technique, which has proved to be safe and reproducible and yields good clinical outcomes.

Expert Consensus Paper: Lateral Thoracotomy for Centrifugal Ventricular Assist Device Implant

BACKGROUND

The increasing prevalence of heart failure has led to the expanded use of left ventricle assist devices (VADs) for end-stage heart failure patients worldwide. Technological improvements witnessed the development of miniaturized VADs and their implantation through less traumatic non-full sternotomy approaches using a lateral thoracotomy (LT). Although adoption of the LT approach is steadily growing, a lack of consensus remains regarding patient selection, details of the surgical technique, and perioperative management. Furthermore, the current literature does not offer prospective randomized studies or evidence-based guidelines for LT-VAD implantation.

METHODS

A worldwide group of LT-VAD experts was convened to discuss these key topics openly. After a PubMed search and review with all authors, a consensus was reached and an expert consensus paper on LT-VAD implantation was developed.

RESULTS

This document aims to guide clinicians in the selection of patients suitable for LT approaches and preoperative optimization. Details of operative techniques are described, with an overview of hemisternotomy and bilateral thoracotomy approaches. A review of the best surgical practices for placement of the pump, inflow cannula, and outflow graft provides advice on the best surgical strategies to avoid device malpositioning while optimizing VAD function. Experts' opinions on cardiopulmonary bypass, postoperative management, and approaches for pump exchange and explant are presented. This review also emphasizes the critical need for multidisciplinary teams and specific training.

CONCLUSIONS

This expert consensus review provides a compact guide to LT for VAD implantation, from patient selection through intraoperative tips and postoperative management.

A Minimally Invasive Approach to Left Ventricular Assist Device Insertion Facilitates Subsequent Explant

A minimally invasive approach to left ventricular assist device (LVAD) insertion may benefit patients at the time of implant, but whether the approach to LVAD insertion influences the outcome of subsequent cardiovascular reoperations is unknown. Here we present the case of a 50-year-old male who underwent LVAD insertion through a minimally invasive approach and subsequently had left ventricular recovery. LVAD explant was performed without the use of any blood products or inotropic support. This case demonstrates that a minimally invasive approach to LVAD insertion may also facilitate subsequent device explant.

Effects of implanting a long-term left ventricle assist device on post-transplant outcomes

BACKGROUND

An increasing number of patients are receiving left ventricle assist devices as a bridge to heart transplantation. The aim of this study was to determine the difference between patients who received transplants from a left ventricle assist device and those who underwent heart transplantation without a prior left ventricle assist device implantation.

MATERIAL AND METHODS

The study included patients who underwent heart transplantation in our institute between January 2010 and November 2018. The following clinical variables were evaluated: donor characteristics, patient's pre-transplant demographical data, post-transplant data, and patient survival. Cardiac allograft vasculopathy progression was prospectively examined (after 1 month and 12 months after heart transplantation) by coronary optical coherence tomography. We were interested in the difference in 1- and 5-year survival between the left ventricle assist device and non-left ventricle assist device groups.

RESULTS

A total of 248 patients were identified; out of them, 48 patients received a left ventricle assist device before heart transplantation, whereas 200 had transplants with no prior left ventricle assist device implantation. There were no significant differences in any donor characteristics. The mean duration of cardiopulmonary bypass time in the non-left ventricle assist device group was 156 versus 175 min in the left ventricle assist device group (p = 0.009), blood loss was 650 versus 1045 mL (p < 0.001), the need to implant an extracorporeal membrane oxygenation was 10% versus 23% (p = 0.02). There was no difference in cardiac allograft vasculopathy progression between the groups 1 year after heart transplantation (p = 0.528). The 1- and 5-year survival, according to Kaplan-Meier, was 80% and 70% in the left ventricle assist device group, compared to 80% and 73%, respectively, in the non-left ventricle assist device group (Log-rank test: p = 0.945).

CONCLUSION

Our results indicate that patients undergoing heart transplantation from left ventricle assist devices suffer significantly more from intraoperative and post-operative complications; however, only insignificant cardiac allograft vasculopathy progression and survival differences between the two groups were observed.

Minimally invasive left ventricular assist device implantation: optimizing device design for this approach

Introduction: The global heart failure (HF) burden is expected to increase due to aging populations, increasing number of end-stage HF patients and adverse lifestyle changes. Mechanical circulatory support (MCS) devices such as left ventricular assist devices (LVADs) have become a promising treatment option for short-term and long-term circulatory support of end-stage HF patients.Areas covered: Recent developments in MCS technology have been focused on miniaturization leading to the development of minimally invasive surgical procedures for LVAD implantation. This helps overcome possible postoperative complications such as major incisions and poor outcomes due to infections, right heart failure, and bleeding. This article discusses clinical and technological developments in the field of minimally invasive procedures for LVAD implantation.Expert opinion: Most patients might benefit from minimally invasive LVAD implantation performed through a limited left lateral thoracotomy associated with an upper hemisternotomy or a right anterior thoracotomy. The thoracotomy approach can also be considered in case of pump exchange or pump explant. The success of these techniques is mainly based on the optimization of LVAD pump design, inflow cannula insertion, and outflow graft as well as driveline exit sites. The future direction of the LVAD field is likely to include less-invasive approaches and smartificial technologies.

First-in-man explantation of a HeartMate 3 left ventricular assist device via customized plug

Myocardial recovery in left ventricular assist device patients is a rare opportunity to explant the assist device. Despite myocardial recovery, these patients remain high-risk candidates. Short, effective procedures are the key to successful left ventricular assist device explantation. Conventional methods such as ventriculoplasty are extensive and challenging procedures. Thus, an explantation tool was developed in order to simplify HeartMate 3 (Abbott Cooperation, Abbott Park, Ill, USA) explantation. A customized metal plug was designed to fit into the HeartMate 3-sewing ring. We now report the successful first-in-man use of this novel plug for HeartMate 3 explantation.

2019 EACTS Expert Consensus on long-term mechanical circulatory support

Long-term mechanical circulatory support (LT-MCS) is an important treatment modality for patients with severe heart failure. Different devices are available, and many-sometimes contradictory-observations regarding patient selection, surgical techniques, perioperative management and follow-up have been published. With the growing expertise in this field, the European Association for Cardio-Thoracic Surgery (EACTS) recognized a need for a structured multidisciplinary consensus about the approach to patients with LT-MCS. However, the evidence published so far is insufficient to allow for generation of meaningful guidelines complying with EACTS requirements. Instead, the EACTS presents an expert opinion in the LT-MCS field. This expert opinion addresses patient evaluation and preoperative optimization as well as management of cardiac and non-cardiac comorbidities. Further, extensive operative implantation techniques are summarized and evaluated by leading experts, depending on both patient characteristics and device selection. The faculty recognized that postoperative management is multidisciplinary and includes aspects of intensive care unit stay, rehabilitation, ambulatory care, myocardial recovery and end-of-life care and mirrored this fact in this paper. Additionally, the opinions of experts on diagnosis and management of adverse events including bleeding, cerebrovascular accidents and device malfunction are presented. In this expert consensus, the evidence for the complete management from patient selection to end-of-life care is carefully reviewed with the aim of guiding clinicians in optimizing management of patients considered for or supported by an LT-MCS device.

A new tool for an explantation strategy of HeartMate 3™ left ventricular assist device

Although rare, recovery of cardiac function after mechanical support of the left ventricle is the most desirable therapeutic goal in the treatment of end-stage heart failure.Herein, we present a novel implantable device for explantation of the HeartMate 3™ left ventricular assist device (LVAD) in patients, following cardiac recovery, avoiding (re-) sternotomy.This article demonstrates the feasibility of applying this novel device, which fits into the sewing ring of the LVAD.This custom-made mechanical plug offers a promising alternative to standard device-explantation procedures.

Left ventricular assist device (LVAD) program in Chile: first successful experience in South America

Background

The need to have a variety of tools to deal with end-stage heart failure (ES-HF), along with the limited heart transplantation availability encouraged us to create a pilot Left ventricular assist device (LVAD) program in a public health care system hospital in Chile.

Methods

A retrospective analysis of the first nine patients of an ongoing LVAD program initiated on August 2013 was performed, completing an average of 30 months of follow-up. The most important events regarding to morbidity and mortality are described.

Results

Nine patients with ES-HF underwent LVAD implantation surgery; one of them died 23 days after surgery and another died after 11 months. One patient successfully underwent heart transplantation after 16 months of HeartWare ventricular assist device (HVAD) support; the other six patients remain in the program and have an average follow-up of 846 days at the time of this study (range, 23-1,481 days). The survival rate at 6, 12 and 18 months follow-up was 89%, 78% and 78% respectively.

Conclusions

This new pioneering LVAD program in Chile has been successful and now constitutes a vital adjunct to all who work in heart transplantation and ES-HF programs. It offers an effective therapeutic alternative when there is a severe donor shortage, in cases of atypical blood types, emergencies, exceptional cases with contraindication for heart transplantation or when there is important donor-receiver size mismatch.

Less invasive HeartMate 3 left ventricular assist device implantation

Background

The novel HeartMate 3 (Abbott, Chicago, IL, USA) left ventricular assist device (LVAD) was worldwide first implanted by Prof. Schmitto and his team in 2014 at the Hannover Medical School, Germany and received CE Mark approval in October, 2015 following completion of a clinical trial.

Methods

Although HeartMate 3 implantation in the clinical trial was restricted to conventional sternotomy, the small size of the pump allows for less-invasive implantation, generally associated with less trauma and reduced perioperative complication rates. Herein we describe our first experiences with a less-invasive implantation of the HeartMate 3 using an upper hemi-sternotomy combined with anterior lateral thoracotomy approach.

Results

Results demonstrate the feasibility of this novel, less invasive technique for HeartMate 3 LVAD implantation with diminished surgical trauma, less postoperative bleeding, maintenance of the chest stability, reduced need of blood product transfusion and earlier recovery.

Conclusions

The results of our study indicate that less-invasive implantation of the HeartMate 3 is technically feasible and offers several benefits for surgical outcome and may become the standard of care for LVAD implantation techniques.

Minimally invasive surgery improves outcome of left ventricular assist device surgery in cardiogenic shock

Background

Left ventricular assist device (LVAD) (HVAD, Medtronic, Minneapolis, MN, USA) implantation is already a widely accepted treatment option for end-stage heart failure (HF) but also still considered as a rescue therapy for patients suffering from cardiogenic shock. Standard LVAD implantation techniques are often associated with high mortality rates and can result in severe complications, like bleeding or right heart failure (RHF). The aim of our study was to assess the outcome of Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) 1 patients (so called "crash and burn" patients) undergoing a LVAD implantation by standard or less invasive surgery.

Methods

We performed a retrospective evaluation of the 1-year outcome of 32 consecutive HF patients in cardiogenic shock, who underwent LVAD implantation in our institution. A total of 32 INTERMACS 1 patients were emergently operated. Fourteen patients (group A) were operated by using the "Hannover-VAD-technique", which is widely known to be less invasive (upper hemisternotomy and a left-sided anterolateral thoracotomy). In contrast, 18 patients (group B) were implanted with LVAD by using the standard technique (full sternotomy). The primary endpoint was survival after 1 year without device-related re-operations. Secondary endpoints included combined analyses of rates of RHF, respiratory failure and bleeding during the trial period.

Results

Baseline characteristics were similar in both groups. Survival after 1 year was higher in group A (69.7% vs. 50.0%). Technique-related adverse events (AEs) were also lower in the minimally invasive group, including a lower RHF (35.7% vs. 61.1%) and of further postoperative bleeding requiring surgery (14.3% vs. 33.3%).

Conclusions

LVAD surgery in INTERMACS 1 patients is associated with remarkably good outcome considering the already very high mortality of those patients, and compared to previously reported surgical outcomes. Our study indicates that minimally invasive LVAD implantation in cardiogenic shock decreases mortality and the incidence of postoperative AEs.

Left Ventricular Assist Devices - A State of the Art Review

Cardiovascular diseases are the leading cause of mortality rates throughout the world. Next to an insufficient number of healthy donors, this has led to increasing numbers of patients on heart transplant waiting lists with prolonged waiting times. Innovative technological advancements have led to the production of ventricular assist devices that play an increasingly important role in end stage heart failure therapy. This review is intended to provide an overview of current implantable left ventricular assist devices, different design concepts and implantation techniques. Challenges such as infections and thromboembolic events that may occur during LVAD implantations have also been discussed.

First experiences with HeartMate 3 follow-up and adverse events

BACKGROUND

The novel HeartMate 3 (HM3) left ventricular assist device (LVAD) received its CE mark in October 2015. It is a new compact LVAD featuring fully magnetically levitated pump, artificial pulse, large pump gaps, and a modular driveline. Here, we present outcomes and adverse events of a single-center cohort 6 months after HM3 implantation.

METHODS

We retrospectively studied a patient cohort of 27 patients who were supported with the HM3 at a single institution. We excluded patients with biventricular assist devices and other types of assist devices as well as LVAD exchange and re-operative procedures.

RESULTS

Twenty-seven patients were enrolled into the study. Within 1 year after HM3 implantation, 1 patient received a heart transplant and 3 patients died. Thirty-day survival was 88.9% and 6 months 85.2%. No pump thrombosis and no strokes were observed within 6 months. Right heart failure was diagnosed in 1 patient after HM3 implantation (3.7%). No technical complications of the pump were documented. No pump exchanges were necessary.

CONCLUSIONS

The novel LVAD HM3 has already shown good CE mark trial results. Within this first report after the CE mark trial, the 6-month survival after HM3 implantation was 85.2%. The HM3 showed excellent midterm results with 0% stroke and 0% pump thrombosis rates 6 months after implantation.

In-vitro investigation of the hemodynamic responses of the cerebral, coronary and renal circulations with a rotary blood pump installed in the descending aorta

This study investigates the hemodynamic responses of the cardiovascular system when a rotary blood pump is operating in the descending aorta, with a focus on the cerebral, coronary and renal autoregulation, using our in-house cardiovascular emulator. Several improvements have been made from our previous studies. A novel coronary system was developed to replicate the native coronary perfusion. Three pinch valves actuated by stepper motors were used to simulate the regional autoregulation systems of the native cerebral, coronary and renal circulations. A rotary pump was installed in the descending aorta, in series with the heart, and the hemodynamic responses of the cardiovascular system were investigated with a focus on cerebral, coronary and renal circulation over a wide range of pump rotor speeds. Experiments were performed twice, once with the autoregulation systems active and once with the autoregulation systems inactive, to reflect that there will be some impairment of autoregulatory systems in a patient with heart failure. It was shown that by increasing the rotor speed to 3000 rpm, the cardiac output was improved from 2.9 to 4.1 L/min as a result of an afterload reduction induced by the pressure drop upstream of the pump. The magnitudes of changes in perfusion in the cerebral, coronary and renal circulations were recorded with regional autoregulation systems active and inactive.

HeartWare left ventricular assist device for the treatment of advanced heart failure

The importance of mechanical circulatory support in the therapy of advanced heart failure is steadily growing. The rapid developments in the field of mechanical support are characterized by continuous miniaturization and enhanced performance of the assist devices, providing increased pump durability and prolonged patient survival. The HeartWare left ventricular assist device system (HeartWare Inc., Framingham, MA, USA) is a mechanical ventricular assist device with over 8000 implantations worldwide. Compared with other available assist devices it is smaller in size and used in a broad range of patients. The possibility of minimally invasive procedures is one of the major benefits of the device – allowing implants and explants, as well as exchanges of the device with reduced surgical impact. We present here a review of the existing literature on the treatment of advanced heart failure using the HeartWare left ventricular assist device system.

Heart-Failing Jehovah's Witness Patient Successfully Treated by Minimally Invasive LVAD Implantation without Any Blood Transfusions

Left ventricular assist device (LVAD) implantation without the use of any blood products is considered as a challenge. We present a 48-year-old patient (175 cm, 75 kg, body mass index 24.5 kg/m²) who was admitted to our clinic due to end-stage heart failure. The patient is a Jehovah's witness. He declined full blood transfusion as well as use of coagulation factors. Therefore, we interdisciplinary decided to implant an LVAD via minimally invasive technique. The patient experienced an uneventful intrahospital stay and was successfully discharged home on his 17th postoperative day, proving that LVAD implantation without blood transfusions is possible by using a minimally invasive technique. This benefits not only Jehovah's witness patients, but also all patients in need of an LVAD.

Minimally-invasive LVAD Implantation: State of the Art

Nowadays, the worldwide number of left ventricular assist devices (LVADs) being implanted per year is higher than the number of cardiac transplantations. The rapid developments in the field of mechanical support are characterized by continuous miniaturization and enhanced performance of the pumps, providing increased device durability and a prolonged survival of the patients. The miniaturization process enabled minimally-invasive implantation methods, which are associated with generally benefitting the overall outcome of patients. Therefore, these new implantation strategies are considered the novel state of the art in LVAD surgery.

In this paper we provide a comprehensive review on the existing literature on minimally-invasive techniques with an emphasis on the different implantation approaches and their individual surgical challenges.

Minimally invasive ventricular assist device surgery

The use of mechanical circulatory support to treat patients with congestive heart failure has grown enormously, recently surpassing the number of annual heart transplants worldwide. The current generation of left ventricular assist devices (LVADs), as compared with older devices, is characterized by improved technologies and reduced size. The result is that minimally invasive surgery is now possible for the implantation, explantation, and exchange of LVADs. Minimally invasive procedures improve surgical outcome; for example, they lower the rates of operative complications (such as bleeding or wound infection). The miniaturization of LVADs will continue, so that minimally invasive techniques will be used for most implantations in the future. In this article, we summarize and describe minimally invasive state-of-the-art implantation techniques, with a focus on the most common LVAD systems in adults.

Kidney dysfunction and left ventricular assist device support: a comprehensive perioperative review

Left ventricular assist devices (LVADs) are used increasingly as a bridge to transplantation or as destination therapy in end-stage heart failure patients who do not respond to optimal medical therapy. Many of these patients have end-organ dysfunction, including advanced kidney dysfunction, before and after LVAD implantation. Kidney dysfunction is a marker of adverse outcomes, such as increased morbidity and mortality. This review discusses kidney dysfunction and associated management strategies during the dynamic perioperative time period of LVAD implantation. Furthermore, we suggest potential future research directions to better understand the complex relationship between renal pathophysiology and mechanical circulatory support.