Impact of continuous-flow mechanical circulatory support on cerebrospinal fluid motility

BACKGROUND

Mechanical circulatory support (MCS), including ventricular assist devices (VADs), have emerged as promising therapeutic alternatives for end-stage congestive heart failure (CHF). The latest generation of these devices are continuous flow (CF) blood pumps. While there have been demonstrated benefits to patient outcomes due to CF-MCS, there continue to be significant clinical challenges. Research to-date has concentrated on mitigating thromboembolic risk (stroke), while the downstream impact of CF-MCS on the cerebrospinal fluid (CSF) flow has not been well investigated. Disturbances in the CSF pressure and flow patterns are known to be associated with neurologic impairment and diseased states. Thus, here we seek to develop an understanding of the pathophysiologic consequences of CF-MCS on CSF dynamics.

METHODS

We built and validated a computational framework using lumped parameter modeling of cardiovascular, cerebrovascular physics, CSF dynamics, and autoregulation. A sensitivity analysis was performed to confirm robustness of the modeling framework. Then, we characterized the impact of CF-MCS on the CSF and investigated cardiovascular conditions of healthy and end-stage heart failure.

RESULTS

Modeling results demonstrated appropriate hemodynamics and indicated that CSF pressure depends on blood flow pulsatility more than CSF flow. An acute equilibrium between CSF production and absorption was observed in the CF-MCS case, characterized by CSF pressure remaining elevated, and CSF flow rates remaining below healthy, but higher than CHF states.

CONCLUSION

This research has advanced our understanding of the impact of CF-MCS on CSF dynamics and cerebral hemodynamics.

Clinical implications of LDH isoenzymes in hemolysis and continuous-flow left ventricular assist device-induced thrombosis

Pump-induced thrombosis continues to be a major complication of continuous-flow left ventricular assist devices (CF-LVADs), which increases the risks of thromboembolic stroke, peripheral thromboembolism, reduced pump flow, pump failure, cardiogenic shock, and death. This is confounded by the fact that there is currently no direct measure for a proper diagnosis during pump support. Given the severity of this complication and its required treatment, the ability to accurately differentiate CF-LVAD pump thrombosis from other complications is vital. Hemolysis measured by elevated lactate dehydrogenase (LDH) enzyme levels, when there is clinical suspicion of pump-induced thrombosis, is currently accepted as an important metric used by clinicians for diagnosis; however, LDH is a relatively nonspecific finding. LDH exists as five isoenzymes in the body, each with a unique tissue distribution. CF-LVAD pump thrombosis has been associated with elevated serum LDH-1 and LDH-2, as well as decreased LDH-4 and LDH-5. Herein, we review the various isoenzymes of LDH and their utility in differentiating hemolysis seen in CF-LVAD pump thrombosis from other physiologic and pathologic conditions as reported in the literature.

Hybrid Continuous-Flow Total Artificial Heart

Clinical studies using total artificial hearts (TAHs) have demonstrated that pediatric and adult patients derive quality-of-life benefits from this form of therapy. Two clinically-approved TAHs and other pumps under development, however, have design challenges and limitations, including thromboembolic events, neurologic impairment, infection risk due to large size and percutaneous drivelines, and lack of ambulation, to name a few. To address these limitations, we are developing a hybrid-design, continuous-flow, implantable or extracorporeal, magnetically-levitated TAH for pediatric and adult patients with heart failure. This TAH has only two moving parts: an axial impeller for the pulmonary circulation and a centrifugal impeller for the systemic circulation. This device will utilize the latest generation of magnetic bearing technology. Initial geometries were established using pump design equations, and computational modeling provided insight into pump performance. The designs were the basis for prototype manufacturing and hydraulic testing. The study results demonstrate that the TAH is capable of delivering target blood flow rates of 1-6.5 L/min with pressure rises of 1-92 mm Hg for the pulmonary circulation and 24-150 mm Hg for the systemic circulation at 1500-10 000 rpm. This initial design of the TAH was successful and serves as the foundation to continue its development as a novel, more compact, nonthrombogenic, and effective therapeutic alternative for infants, children, adolescents, and adults with heart failure.

Pulmonary Valve Opening With Two Rotary Left Ventricular Assist Devices for Biventricular Support

Right ventricular failure is a common complication associated with rotary left ventricular assist device (LVAD) support. Currently, there is no clinically approved long-term rotary right ventricular assist device (RVAD). Instead, clinicians have implanted a second rotary LVAD as RVAD in biventricular support. To prevent pulmonary hypertension, the RVAD must be operated by either reducing pump speed or banding the outflow graft. These modes differ in hydraulic performance, which may affect the pulmonary valve opening (PVO) and subsequently cause fusion, valvular insufficiency, and thrombus formation. This study aimed to compare PVO with the RVAD operated at reduced speed or with a banded outflow graft. Baseline conditions of systemic normal, hypo, and hypertension with severe biventricular failure were simulated in a mock circulation loop. Biventricular support was provided with two rotary VentrAssist LVADs with cardiac output restored to 5 L/min in banded outflow and reduced speed conditions, and systemic and pulmonary vascular resistances (PVR) were manipulated to determine the range of conditions that allowed PVO without causing left ventricular suction. Finally, RVAD sine wave speed modulation (±550 rpm) strategies (co- and counter-pulsation) were implemented to observe the effect on PVO. For each condition, outflow banding had higher PVR (97 ± 20 dyne/s/cm⁵ higher) for when the pulmonary valve closed compared to reduced speed. In addition, counter-pulsation demonstrated greater PVO than co-pulsation and constant speed. For the purpose of reducing the risks of pulmonary valve insufficiency, fusion, and thrombotic event, this study recommends a RVAD with a steeper H-Q gradient by banding and further exploration of RVAD speed modulation.

Mechanical Circulatory Support for Advanced Heart Failure: Are We about to Witness a New "Gold Standard"?

The impact of left ventricular assist devices (LVADs) for the treatment of advanced heart failure has played a significant role as a bridge to transplant and more recently as a long-term solution for non-eligible candidates. Continuous flow left ventricular assist devices (CF-LVADs), based on axial and centrifugal design, are currently the most popular devices in view of their smaller size, increased reliability and higher durability compared to pulsatile flow left ventricular assist devices (PF-LVADs). The trend towards their use is increasing. Therefore, it has become mandatory to understand the physics and the mathematics behind their mode of operation for appropriate device selection and simulation set up. For this purpose, this review covers some of these aspects. Although very successful and technologically advanced, they have been associated with complications such as pump thrombosis, haemolysis, aortic regurgitation, gastro-intestinal bleeding and arterio-venous malformations. There is perception that the reduced arterial pulsatility may be responsible for these complications. A flow modulation control approach is currently being investigated in order to generate pulsatility in rotary blood pumps. Thrombus formation remains the most feared complication that can affect clinical outcome. The development of a preoperative strategy aimed at the reduction of complications and patient-device suitability may be appropriate. Patient-specific modelling based on 3D reconstruction from CT-scan combined with computational fluid dynamic studies is an attractive solution in order to identify potential areas of stagnation or challenging anatomy that could be addressed to achieve the desired outcome. The HeartMate II (axial) and the HeartWare HVAD (centrifugal) rotary blood pumps have been now used worldwide with proven outcome. The HeartMate III (centrifugal) is now emerging as the new promising device with encouraging preliminary results. There are now enough pumps on the market: it is time to focus on the complications in order to achieve the full potential and selling-point of this type of technology for the treatment of the increasing heart failure patient population.

Rotary Blood Pumps as Long-Term Mechanical Circulatory Support: A Review of a 15-Year Berlin Experience

This article reports our 15-year single-center experience with rotary blood pumps (RBPs) as long-term mechanical circulatory support (MCS) with emphasis on outcomes. For more than 15-year period, we have used various RBPs as bridge to transplantation or to myocardial recovery. Our group performed the first human implantation worldwide of RBCs, the MicroMed DeBakey ventricular assist device in November 1998 in a patient with end-stage heart failure who was supported for 47 days until his death. Based on this initial experience, we recognized the feasibility of providing long-term support and since then it has been our primary armamentarium in treating patients with heart failure. Between 1987 and September 2013, we have implanted 2208 ventricular assist devices ranging from pulsatile to continuous-flow systems, as short-term, long-term, or permanent support in patients with end-stage heart failure. In total, 1009 RBPs were implanted on 908 patients, and their outcomes are reported here. We have shared some milestones in MCS including the first implantation of Jarvik 2000 on the oldest patient (81-year old) in 2008 and the first worldwide implantation of a biventricular HeartWare. Over time, implantation techniques, anticoagulation, and postoperative care have been modified and individualized. A relevant aspect of our experience has been the incidence of pump thrombosis. This is particularly frustrating because the problem has occurred in the setting of full anticoagulation and antiplatelet therapy, guided by strict anticoagulation monitoring. It has become clear to us that the devices are still not perfect. Technical pump failures such as cable breaks also occur, prompting urgent pump exchange, and infection. A 15-year cumulative mortality rate is 46.9%. This report emphasizes that MCS with RBPs has evolved into a routine treatment in heart failure and is a highly feasible option for permanent therapy particularly for elderly patients.

Beyond the VAD: Human Factors Engineering for Mechanically Assisted Circulation in the 21st Century

Thousands of ventricular assist devices (VADs) currently provide circulatory support to patients worldwide, and dozens of heart pump designs for adults and pediatric patients are under various stages of development in preparation for translation to clinical use. The successful bench-to-bedside development of a VAD involves a structured evaluation of possible system states, including human interaction with the device and auxiliary component usage in the hospital or home environment. In this study, we review the literature and present the current landscape of preclinical design and assessment, decision support tools and procedures, and patient-centered therapy. Gaps of knowledge are identified. The study findings support the need for more attention to user-centered design approaches for medical devices, such as mechanical circulatory assist systems, that specifically involve detailed qualitative and quantitative assessments of human-device interaction to mitigate risk and failure.

Rotary blood pumps as definitive treatment for severe heart failure

Rotary blood pumps are increasingly recognized as mainstream therapy for severely symptomatic heart failure. Carefully targeted refinements in patient selection and postoperative care have substantially reduced the adverse event burden. These improvements translate into better survival and quality of life in comparison with medical management. Medium-term outcomes now compete favorably with cardiac transplantation, although evidence-based outcome data indicate that transplant and ‘lifetime’ left ventricular-assist device (LVAD) candidates are fundamentally different. Significant challenges remain in relation to neurological injury and right heart failure, which may continue to limit exercise capacity. In the meantime, both physician awareness and patient access to LVAD technology remain limited. The debate is rarely between cardiac transplant or lifetime LVAD. It should focus on the choice between pump versus palliative care for the thousands of patients of all age groups who are judged ineligible for transplantation. Comprehensive healthcare systems must consider contemporary evidence and provide the most symptomatic of heart failure patients with effective care. Cardiac resynchronization therapy is no longer the ceiling for this.