A Muscle-Powered Counterpulsation Device for Tether-Free Cardiac Support: Form and Function1

A fundamental breakthrough in long-term circulatory support technology is needed to drive the field forward. The reasons for this are twofold. First, despite decades of research extracorporeal drive systems for long-term ventricular assist devices (VADs) continue to be extremely problematic in terms of infection risk and patient quality-of-life. Second, bleeding and thrombosis associated with exposure to blood-contacting surfaces are life-threatening complications that have yet to be resolved in the setting of long-term circulatory support. Together, these problems have proven difficult, if not impossible, to overcome using traditional blood pump technologies, and there is no indication that the latest generation of rotary devices will prove to be any more successful in the long run. It is therefore important that researchers continue to explore new and innovative ways to assist the failing heart for patients who require long-term circulatory support.Given the persistent difficulties encountered with existing blood pump technologies used in the chronic setting, the promise of safe, reliable long-term circulatory support is likely to remain unfulfilled unless and until there is a fundamental change in the way we approach this problem. Research performed by this laboratory suggests that the best way to deal with the intractable problems created by drivelines and blood-contacting surfaces is to avoid them altogether by harnessing the body's own endogenous energy stores (i.e., skeletal muscle) and applying this power to the external surface of the heart or ascending aorta.This is an ambitious undertaking to be sure, but our group starts with a critical leg up as we have already solved several of the most fundamental challenges in harnessing muscle power through prior studies at both Allegheny General Hospital (Pittsburgh, PA) and Carnegie Mellon University. In fact, research to develop an implantable muscle energy converter (MEC) for transforming the contractile energy of the latissimus dorsi muscle (LDM) into hydraulic power has been underway for well over a decade now under the auspices of the Whitaker Foundation and the National Institutes of Health [1]. Work to date has yielded a unique implantable device (Fig. 1) with excellent anatomic fit, mechanical durability, and energy transfer efficiency with the capacity to transmit 1.37 J of contractile work per stroke cycle [2].Here, I describe one possible mechanism by which the MEC may be used to support the failing heart without recourse to external power sources or blood-contacting surfaces. Specifically, this technical brief details the design of a working muscle-powered VAD (mVAD) in its simplest incarnation: extravascular balloon counterpulsation.Arguably the most conservative approach to MEC-powered circulatory support would be to use this pump to drive a counterpulsation balloon similar to the C-pulse extra aortic counterpulsation (EAC) device developed by Sunshine Heart, Inc. (Eden Prairie, MN) [3]. This device is based on established intra-aortic balloon pump (IABP) technology and, like all conventional IABPs, assists the failing heart by both reducing the workload of the left ventricle (LV) and increasing coronary blood flow. Unlike conventional IABPs, however, the C-pulse does not contact the bloodstream and so requires no anticoagulants, which makes it attractive as a means for long-term circulatory support. The proposed arrangement, shown in Fig. 2, would involve inflating a balloon wrapped around the ascending aorta so that a 20 mL bolus of blood is displaced from the vessel between cardiac beats.While the C-pulse counterpulsation system continues to show great promise in clinical trials with regard to the relief of symptoms in class III and ambulatory class IV heart failure patients, infection remains the most common adverse device-related event. The persistence and severity of this problem have recently prompted Sunshine Heart to begin development of a fully implantable system that uses transcutaneous energy transmission (TET) coils to power the pump across (rather than through) the skin. However, while transcutaneous electrical actuation does eliminate the percutaneous drive-line, which is of significant benefit, there is also a significant cost associated with this approach. Apart from being expensive in the financial sense, TET systems are notoriously delicate, hard to maintain, and require the patient to carry an external power source/controller connected to a transmission coil positioned (precisely) over the implanted receiver coil. This raises serious quality-of-life issues as patients must pay close attention to coil placement owing to the fact that the range of positions where power can be transmitted efficiently is not wide. The muscle-powered version, on the other hand, would require only the MEC, a muscle stimulator, and the C-pulse device—all implanted with no external components. This would not only be less expensive and maintenance free but would also offer patients a much better quality-of-life.To achieve the goal of implementing a completely self-contained mVAD system based on established MEC technology, device development will proceed in three stages. The first step in this process will be to build prototype mVAD counterpulsation devices and test them on the bench to document functional parameters and assess overall mechanical reliability. The second task will be to implant prototype mVAD systems (acute experiments) in animals with drug-induced heart failure to refine implantation methods, evaluate anatomic fit, document acute biological reactions at tissue/device interface surfaces, and assess short-term hemodynamic effects. The final step will be to perform short-term (28-day) implant studies in animals with ischemic heart failure to establish in vivo system functionality, document chronic biological reactions at tissue/device interface surfaces, quantify prolonged hemodynamic effects, and determine the overall efficacy of the mVAD system.The feasibility of any MEC-based mVAD scheme rests upon two fundamental postulates: (1) that the LDM is able to generate steady-state power at levels sufficient to support the failing heart and (2) that the MEC device can efficiently capture and transmit that power in a form suitable for mVAD actuation. In aggregate, the evidence supporting these enabling conditions is both clear and unequivocal. In the 40 years since the discovery that impulse activity drives skeletal muscle fiber-type expression, scientists have amassed a large body of evidence regarding the steady-state work capacity of electrically conditioned skeletal muscle. Taken together, these studies show that skeletal muscle conditioned to express 100% type 1 (slow) fibers can produce sustainable work rates up to 4.0 W/kg and that the human LDM in particular, when fully conditioned, can match the work output of a healthy left ventricle. In addition, our own studies have shown that the MEC device can transfer muscular stroke work at levels up to 1.37 J with an efficiency that exceeds 90% [2]. So the real question facing mVAD researchers today is not whether fully conditioned LDM is energetically “up to the task,” but, rather, how best to engineer a means to capture this contractile energy and deliver it to the bloodstream.What makes the development of mVADs so compelling is the simple fact that, despite five-plus decades of artificial heart research, severe complications associated with percutaneous drivelines and blood-contacting surfaces continue to plague VAD recipients to the present day. The use of electrically stimulated skeletal muscle as an endogenous power source has the potential to achieve the same objectives as conventional VADs while providing several key benefits that address these longstanding problems. One major advantage is the elimination of all external hardware elements, including batteries, charging stations, and controllers. Another is the minimization and simplification of implanted components. In the case described here, coupling the MEC to an extra-aortic counterpulsator not only eliminates the need for internal electric actuators, coils, motors, and cannula but also avoids thrombotic and bleeding complications associated with all blood-contacting devices.Overall, these improvements to system safety and reliability have the potential to drastically reduce the cost of long-term cardiac support, increasing its viability from a societal perspective while improving patient quality-of-life.This work was supported by the National Institutes of Health (NIH 1 R01 EB019468-01A1).