Reinventing the displacement left ventricular assist device in the continuous-flow era: TORVAD, the first toroidal-flow left ventricular assist device.

Abstract

Over the past two decades, left ventricular assist device (LVAD) support has emerged as standard therapy for adult patients with advanced, life-threatening heart failure. First-generation pulsatile devices have been replaced by rotary blood pumps that continuously unload the failing left ventricle. So-called continuous-flow LVADs (CF-LVADs) are smaller, more reliable, durable, and energy efficient, and less traumatic to implant than first-generation devices. Survival of 80% at one year and 70% at two years has been widely achieved (1). However, CF-LVADs have introduced a new, non-pulsatile physiology in which blood encounters supraphysiologic shear stress, and as a result, unforeseen consequences have emerged. CF-LVADs operate with an impeller that spins at thousands of revolutions per minute (RPM) to generate forward flow. Within current-generation devices, blood passes through narrow flow gaps (50 to 500 µm) at high velocity. Consequently, blood encounters shear stress that exceeds physiologic values by more than two orders of magnitude. Indeed, current-generation LVADs generate peak shear stress of up to 1,500 Pa (2). For reference, physiologic levels of intravascular shear stress in mammals are approximately two to eight Pa (2). Irrefutable evidence demonstrates that supraphysiologic shear stress from CF-LVADs causes blood trauma that contributes to adverse events. Pathologic degradation of von Willebrand factor (VWF) leads to an acquired bleeding diathesis, gastrointestinal angiodysplasia, and gastrointestinal bleeding in 20% to 40% of LVAD patients (3,4). Platelet activation and (at least subclinical) hemolysis cause a procoagulant state that contributes to LVAD thrombosis and thromboembolism (5). As a result, “LVAD hemocompatibility”, a term that characterizes the clinical impact of biophysical interactions and blood trauma at the blood-device interface, is gaining attention as a major area for improvement of future-generation LVADs. Lack of pulsatile blood flow has also raised concerns. Potential benefits of pulsatility include prevention of aortic valve thrombosis, leaflet fusion, de novo aortic insufficiency, favorable myocardial remodeling, avoidance of arterial stiffening, better end-organ function, reduced bleeding, and reduced stroke (6). Consequently, pulsation algorithms are being developed to generate pulsatility with CF-LVADs. Also problematic, CF-LVADs operate at fixed RPM, independent of patient physiology. These devices do not sense or adjust device parameters in response to dynamic changes in cardiac rate, rhythm, preload, afterload, intracardiac hemodynamics, or systemic metabolic demands. “Physiologic control”, a term used to describe automatic responsiveness to changes in patient physiology, is another focus for improvement of future-generation LVADs. Devices with physiologic control may sense and adjust to pathologic conditions such as exacerbation of heart failure, hypotension, hypovolemia, and malignant arrhythmia. Optimization of ventricular (un)loading may increase favorable myocardial remodeling and facilitate LVAD weaning and explantation in the setting of myocardial recovery. Finally, full sternotomy and cardiopulmonary bypass (CPB) deter referral of less sick patients for LVAD therapy. Minimally-invasive implantation strategies without CPB that include mini-thoracotomy, subxiphoid access, device placement in an infraclavicular pacemaker-like pocket, and percutaneous implantation may increase referral of earlier-stage heart failure patients and increase the public health impact of LVAD support. In summary, next-generation LVADs are needed to reduce adverse events and improve outcomes. Toroidal-flow is a novel LVAD flow strategy designed to improve hemocompatibility while providing pulsatility with physiologic device control. As such, the first-of-its-kind, toroidal-flow TORVAD is a disruptive technology with the potential to alter mechanical circulatory support therapy. TORVAD Windmill Cardiovascular Systems, Inc. (Austin, TX, USA) is developing the TORVAD (2,7-10), a positive-displacement, rotary LVAD (Figure 1). The toroidal-flow mechanism generates low shear stress (7), minimal blood trauma (2), pulsatile blood flow (9), and operates with physiologic control (8,10). Open in a separate window Figure 1 The Toroidal-Flow TORVAD, Device and Mechanism of Flow. (A) The toroidal-flow TORVAD consists of an inflow cannula with sewing ring, torus pumping chamber, and outflow graft. An epicardial ECG lead senses the patient’s native heart rhythm and triggers TORVAD support with asynchronous or synchronous pulsatile, counter-pulsatile, or co-pulsatile pumping modes. (B) To simultaneously fill and eject, the TORVAD spins two magnetic pistons (Pa and Pb) in sequence within the doughnut-shaped torus chamber. During support, each piston remains stationary while the other piston spins. While the first piston (Pa) is temporarily fixed as a virtual valve between the torus inflow and outflow, the second piston (Pb) rotates within the torus to eject 30 mL of blood. After one cycle, the pistons switch positions, and the first piston (Pa) spins while the second piston (Pb) remains stationary. The result is unidirectional, pulsatile blood flow with low shear stress and a high level of physiologic control. ECG, electrocardiogram.