Functional properties of conditioned skeletal muscle: implications for muscle-powered cardiac assist

Microsystems for biomimetic stimulation of cardiac cells

The heart is a complex integrated system that leverages mechanoelectrical signals to synchronize cardiomyocyte contraction and push blood throughout the body. The correct magnitude, timing, and distribution of these signals is critical for proper functioning of the heart; aberrant signals can lead to acute incidents, long-term pathologies, and even death. Due to the heart's limited regenerative capacity and the wide variety of pathologies, heart disease is often studied in vitro. However, it is difficult to accurately replicate the cardiac environment outside of the body. Studying the biophysiology of the heart in vitro typically consists of studying single cells in a tightly controlled static environment or whole tissues in a complex dynamic environment. Micro-electromechanical systems (MEMS) allow us to bridge these two extremes by providing increasing complexity for cell culture without having to use a whole tissue. Here, we carefully describe the electromechanical environment of the heart and discuss MEMS specifically designed to replicate these stimulation modes. Strengths, limitations and future directions of various designs are discussed for a variety of applications.

Potential mechanisms for muscle-powered cardiac support

Biomechanical actuation of an implanted ventricular assist device (VAD) is an attractive means of providing long-term circulatory support. Studies show that energy from electrically stimulated skeletal muscle can, in principle, be used to provide tether-free cardiac assistance without the need for percutaneous drivelines or bulky energy transmission hardware. A mechanical prosthesis designed to harness the contractile power of in situ skeletal muscle has been developed in this laboratory that collects energy from the latissimus dorsi muscle and transmits it in the form of hydraulic power. In order to use this technique to pump blood however, a practical means to deliver this energy to the bloodstream must be devised. Presented here are six prospective mechanisms designed to accomplish this task, five of which also eliminate blood contacting surfaces that often lead to thromboembolic complications in chronic VAD patients.

In vivo demonstration of a self-sustaining, implantable, stimulated-muscle-powered piezoelectric generator prototype

An implantable, stimulated-muscle-powered piezoelectric active energy harvesting generator was previously designed to exploit the fact that the mechanical output power of muscle is substantially greater than the electrical power necessary to stimulate the muscle's motor nerve. We reduced to practice the concept by building a prototype generator and stimulator. We demonstrated its feasibility in vivo, using rabbit quadriceps to drive the generator. The generated power was sufficient for self-sustaining operation of the stimulator and additional harnessed power was dissipated through a load resistor. The prototype generator was developed and the power generating capabilities were tested with a mechanical muscle analog. In vivo generated power matched the mechanical muscle analog, verifying its usefulness as a test-bed for generator development. Generator output power was dependent on the muscle stimulation parameters. Simulations and in vivo testing demonstrated that for a fixed number of stimuli/minute, two stimuli applied at a high frequency generated greater power than single stimuli or tetanic contractions. Larger muscles and circuitry improvements are expected to increase available power. An implanted, self-replenishing power source has the potential to augment implanted battery or transcutaneously powered electronic medical devices.

Design Considerations for an Implantable, Muscle Powered Piezoelectric System for Generating Electrical Power

A totally implantable piezoelectric generator system able to harness power from electrically activated muscle would augment the power systems of implanted functional electrical stimulation devices by reducing the number of battery replacement surgeries or by allowing periods of untethered functionality. The generator design contains no moving parts and uses a portion of the generated power for system operation. A software model of the system was developed and simulations performed to predict the output power as the system parameters were varied within their constraints. Mechanical forces that mimic muscle forces were experimentally applied to a piezoelectric generator to verify the accuracy of the simulations and to explore losses due to mechanical coupling. Depending on the selection of system parameters, software simulations predict that this generator concept can generate up to 690 microW of power, which is greater than the power necessary to drive the generator, conservatively estimated to be 46 microW. These results suggest that this concept has the potential to be an implantable, self-replenishing power source and warrants further investigation.

Models of metabolic utilization predict limiting conditions for sustained power from conditioned skeletal muscle

Application and development of muscle powered cardiac assist devices is limited by the ability to predict the sustainable power output of in situ conditioned muscle under the expected loading conditions and geometrical constraints. Empirical definition of the sustained power limits and representative models of the bounding conditions where continuous power can be obtained are needed for device design and optimization. The latissimus dorsi muscles of four goats were chronically conditioned for 11 weeks with an implanted myostimulator. The ability to sustain power under isotonic conditions was evaluated across a range of contraction durations (100-600 ms) and rates (10-120 contractions/min). Muscles were characterized both biomechanically and myothermically to develop and evaluate three increasingly complex empirically-based models of metabolic utilization per contraction based on (1) the duty cycle, (2) a linear function of activation time, and (3) a multivariate-derived function of contraction duration, muscle load, and shortening distance. A clearly defined boundary for sustainable stimulation conditions was observed and was best represented by the linear metabolic model. These data provide both an empirical measure of chronically sustainable muscle power and predictive metabolic models that may be used to optimize the power harnessed for skeletal muscle actuated devices.

Is Skeletal Muscle Ventricle Chronic Stability Dependent on Wall Stress? Design Implications

Chronic skeletal muscle ventricle (SMV) stability is essential for clinical implementation. SMVs in animal models have chronically expanded or collapsed when exposed to physiologic pressures. SMV wall stress is a more appropriate indicator than pressure or geometry to compare SMVs between studies. SMV wall tensions during conditioning were determined for SMVs that collapsed, expanded, or were isovolumetric in a previous study. Wall stresses in SMVs that expanded (2.76 +/- 0.803 N/cm(2)) were significantly greater than isovolumetric SMVs (0.89 +/- 0.450) and SMVs that collapsed (0.88 +/- 0.451). These data support the existence of minimum and maximum wall stresses for SMV volume stability and provide empiric estimates for SMV design. Scaling SMV designs from animal models with smaller volumes and similar pressures may result in greater wall stresses in clinical designs. Therefore, the use of volume limiting implants or an isovolumetric conditioning phase to increase the wall stress expansion threshold may be required.

Improved mechanism for capturing muscle power for circulatory support

Although it is now understood that trained skeletal muscle can generate enough steady-state power to provide significant circulatory support, there are currently no means by which to tap this endogenous energy source to aid the failing heart. To that end, an implantable muscle energy converter (MEC) has been constructed and its function has been improved to optimize durability, anatomic fit, and mechanical efficiency. Bench tests show that MEC transmission losses average less than 10% of total work input and that about 85% of this muscle power is successfully transferred to the working fluid of the pump. Results from canine implant trials confirm excellent biocompatibility and demonstrate that contractile work of the latissimus dorsi muscle-measured to 290 mJ/stroke in one dog-can be transmitted within the body at levels consistent with cardiac assist requirements. These findings suggest that muscle-powered cardiac assist devices are feasible and that efforts to further develop this technology are warranted.

Expression of inducible nitric oxide synthase mRNA and nitric oxide production during the development of liver abscess in hamster inoculated with Entamoeba histolytica

The present study analyzed iNOS and eNOS mRNA expression and NO production during development of hepatic abscess caused by Entamoeba histolytica trophozoites. One 374-bp sequence, which displayed 88% identity to mammalian iNOS protein, was isolated from LPS-stimulated peritoneal hamster macrophages. A separate 365-bp cDNA sequence showed 99% identity with eNOS protein. iNOS mRNA was detected in hamsters during formation of amoebic liver abscesses, but not in control hamsters. eNOS mRNA expression was not modified. Serum nitrite concentration in hamsters infected with E. histolytica was 33 +/- 6 microM, in control hamsters was 20 +/- 3 microM. The study shows that iNOS mRNA expression and NO production are induced by E. histolytica trophozoites during amoebic liver abscess formation. However, in spite of iNOS mRNA expression and NO production, E. histolytica trophozoites induced liver abscess formation in hamster.

Linear performance characteristics of latissimus dorsi muscle: potential for cardiac assistance

Knowledge of the quantitative performance capabilities of skeletal muscle in a linear geometry is necessary to predict the performance and to optimize the design of linearly configured, skeletal muscle powered cardiac assist devices (MCADs). This study determined the performance characteristics of goat latissimus dorsi muscle (LDM) using a linear, ex vivo experimental apparatus. In five goats, the LDM (130.6 +/- 18.8 g) was dissected free of its distal attachments, connected to a series of weights (500 to 2250 g), and maximal tetanic contractions were elicited via thoracodorsal nerve stimulation. Second order polynomial equations were derived to represent each of the following variables versus load: muscle prestretch (range 1.5 to 5.4 cm), contraction duration (220 to 360 milliseconds), contraction shortening distance (13.5 to 10.9 cm), contraction velocity (60 to 31 cm/s), generated stroke power (3 to 7 W), and stroke work (0.7 to 2.4 J). Analysis of the potential stroke volumes obtained with a linearly configured, cylindrically shaped MCAD directly coupled to the circulation indicate that a feasible MCAD operating region exists based on the LDM performance data across a range of device geometries and mean ejection pressures.

Skeletal muscle ventricle pressure-volume properties conform to dynamic and static conditioning

BACKGROUND

Chronic changes in skeletal muscle ventricle (SMV) size and strength can directly affect performance and stability. These changes may depend on the conditioning protocol or implant system. Therefore the effects of conditioning protocols on SMV geometry and contractility must be identified for optimal SMV design and application.

METHODS

Skeletal muscle ventricles were constructed in 14 goats using the left latissimus dorsi muscle. The SMVs were conditioned with a 40 mL constant-volume isovolumetric implant (n = 5, IsoVol group) or a compliant pneumatic system that allowed dynamic shortening and direct exposure to resting pressures. Dynamic SMV resting pressure was either progressively increased from 40 to 100 to 120 mm Hg (n = 5, high pressure [HiP] group) or maintained at 40 mm Hg (n = 4, low pressure [LowP] group) during conditioning. The SMV pressure and volume characteristics were monitored daily.

RESULTS

All HiP SMVs expanded in volume during conditioning after exposure to physiologic pressures. Three of 4 LowP SMVs decreased in volume during conditioning. Skeletal muscle ventricle passive and active (isovolumetric evoked pressure) pressure-volume curves shifted toward the increasing, stable, and decreasing volumes in HiP, IsoVol, and LowP SMVs respectively.

CONCLUSIONS

Frequent monitoring of SMV characteristics during conditioning enabled progressive pressure training and is a valuable tool to evaluate SMV conformation. Chronic SMV adaptation is dependent on the conditioning protocol or implant system utilized. Demonstration of SMV expansion at physiologic pressures suggests that clinical sized SMVs may be chronically unstable unless a supporting implant system is utilized or SMV compliance is reduced. Therefore the mechanisms effecting chronic expansion should be further defined to optimally design SMVs for clinical implementation.

An analysis of responses to levosimendan in the pulmonary vascular bed of the cat

Calcium-sensitizing drugs, such as levosimendan, are a novel class of drug therapy for heart failure. We investigated the hypothesis that levosimendan is a pulmonary vasodepressor mediated through inhibition of phosphodiesterase, adenosine triphosphate (ATP)-dependent potassium channels, or both. We investigated responses to the calcium sensitizer levosimendan in the pulmonary vascular bed of the cat under conditions of controlled pulmonary blood flow and constant left atrial pressure when lobar arterial pressure was increased to a high steady level with the thromboxane A(2) analog U-46619. Under increased-tone conditions, levosimendan caused dose-related decreases in lobar arterial pressure without altering systemic arterial and left atrial pressure. Responses to levosimendan were significantly attenuated, although not completely, after the administration of U-37883A, a vascular selective nonsulfonylurea ATP-sensitive K(+)-channel-blocking drug. Responses to levosimendan were not significantly different after the administration of the nitric oxide synthase inhibitor L-N(5)-(1-iminoethyl)-ornithine or the cyclooxygenase inhibitor sodium meclofenamate or when lung ventilation was interrupted. These data show that levosimendan has significant vasodilator activity in the pulmonary vascular bed of the cat. They also suggest that pulmonary vasodilator responses to levosimendan are partially dependent on activation of ATP-sensitive K(+) channels and independent of the synthesis of nitric oxide, activation of cyclooxygenase enzyme, or changes in bronchomotor tone in the pulmonary vascular bed of the cat.

IMPLICATIONS

Calcium-sensitizing drugs, such as levosimendan, are a novel class of drug therapy for heart-failure treatment. The lung circulation affects both right- and left-sided heart failure. Levosimendan decreased lobar arterial pressure via a partial K(+)(ATP) (potassium channel sensitive to intracellular adenosine triphosphate levels)-dependent mechanism. These data suggest that, in addition to calcium-sensitizing activity, levosimendan decreases pulmonary resistance, which may also aid in the treatment of heart failure.

Method for anchoring biomechanical implants to muscle tendon and chest wall

Reliable tissue fixation is of fundamental importance to the successful development of muscle powered motor prostheses. This report describes a series of canine implant trials used to develop stable tissue-device interface mechanisms. Muscle pumps were fitted with prototype tendon and chest wall anchoring schemes and secured to the ribs and humeral insertion of latissimus dorsi (LD) muscles. LD stimulation was initiated 1 week postimplantation and continued throughout the implant period to stress these fixation sites. Design modification and implant testing were continued until both muscle and chest wall attachment points were found to be stable. Chest wall fixation was best achieved using perforated metallic plates wired to the ribs, as opposed to bone screws or wire mesh, which were subject to degradation. Direct attachment of the native tendon by means of spiked clamping plates proved ineffective. Stable muscle attachment was ultimately achieved by replacing the humeral tendon with an artificial substitute formed from fine polyester fibers gathered into 6-8 bundles and sewn into the LD insertion. Braided into a single cord, these fibers were fixed to the device by means of spiked clamping plates. Based on these findings, we conclude that perforated anchor plates and multifibrous artificial tendons can function as effective tissue-device interface mechanisms.

Sustained skeletal muscle power for cardiac assist devices: implications of metabolic constraints

A device to harness power from skeletal muscle contracting in a linear configuration is under development. This application requires a sustained level of power that is dependent upon muscle mechanics and metabolic properties. A biomechanical muscle model and a metabolic model constructed from experimental data were used to predict maximum power available in a sustainable region of loading and stimulation conditions. Latissimus dorsi (LD) of four goats were evaluated in vivo after a 10 week in situ conditioning protocol with an implanted Telectronics myostimulator. The LD insertion was reconnected to a hydraulic loading system, allowing isometric and isotonic contractions for biomechanical characterization. Metabolic utilization was measured by a thermister based myothermic technique. Brief fatigue tests of working isotonic contractions revealed stimulation conditions associated with sustained power. The results show metabolic utilization was dependent on contraction duration, rate, force, and stroke. The region of sustainable contractions was found for a range of durations of 0.1 to 0.6 sec and rates of 10 to 120 bpm. The boundary for the sustainable power region was well approximated by a constant value of metabolic utilization. A constant duty cycle (contraction to cycle duration ratio) also approximated the sustained power but differed by as much as 30% during the shorter contraction durations. The results demonstrate that a mechanical muscle model can predict maximum sustained power when the operating conditions are constrained to a sustainable range determined by a metabolic model. Furthermore, metabolic constraints influence the optimum conditions for sustained power needed in the design of skeletal muscle powered assist devices.

Method for measuring long-term function of muscle-powered implants via radiotelemetry

Long-term remote monitoring of muscle-powered implants has been made possible with development of an adjustable workload that can be remotely monitored to assess device function. This technique obviates the need for percutaneous access lines and allows test animals to remain untethered, eliminating deleterious effects caused by infection, sedation, or animal stress. Hardware components include a latex bladder fixed within a hermetically sealed canister, multichannel implantable telemetry unit, and subcutaneous access port (for pressure charge adjustment). To validate this method, in vitro tests were performed by using a third-generation muscle energy converter designed to function as an implantable hydraulic pump. Two channels of telemetered pressure data were collected and used to calculate six indexes of device function. Calculated parameters were then compared with measured values to determine accuracy. Correlation between measured and calculated parameters was high in all instances, with most estimates yielding errors of <3%. These results demonstrate the utility of this approach and support its use as a means to monitor muscle-powered devices during long-term animal trials.

A muscle-powered energy delivery system and means for chronic in vivo testing

Electrically stimulated skeletal muscle represents a potentially unlimited source of energy for the actuation of motor prostheses. Devices to harvest and deliver contractile power have proven mechanically feasible, but long-term efficacy has not been demonstrated. This report describes recent refinements in muscle energy converter (MEC) design and details the development of an implantable afterload chamber (IAC) designed to facilitate implant testing. The IAC comprises a fluid-filled bladder housed within a titanium cylinder that connects directly to the MEC. A vascular access port allows percutaneous measurement and adjustment of air pressure within the housing and provides a means both to monitor MEC function and to control hydraulic loading conditions. Data from in vitro tests show that IAC pressure mirrors changes in MEC-piston displacement over a wide range of actuation speeds and stroke lengths. Stroke lengths and actuation forces calculated from IAC pressure readings were typically found to be within 5% of measured values. This testing scheme may yield important information in regard to the ability to harness energy from in situ muscle over prolonged periods.

Intermittent stimulation enhances function of conditioned muscle

Skeletal muscle is highly adaptable in that its metabolic and contractile characteristics are largely regulated by its pattern of use. It is known that muscle phenotype can be manipulated via chronic electrical stimulation to enhance fatigue resistance at the expense of contractile power. Type 2A fibers are fatigue resistant, powerful, and considered most desirable for cardiac assist purposes. We have found that 12-wk of intermittent-burst stimulation produces a high percentage of 2A fibers and increases fatigue resistance and power in rabbit latissimus dorsi muscle. Fixed-load endurance tests were used to quantify fatigue resistance among normal and trained muscle groups. Control muscles were found to fatigue completely within 10-20 min. Muscles stimulated continuously for 6 wk retained 35% (71.5 +/- 19.5 g. cm) of their initial stroke work at 40 min. Muscles stimulated 12 h/day for 12 wk had the highest initial stroke work (449.7 +/- 92.4 g. cm) and the highest remaining stroke work (234.7 +/- 50.1 g. cm) at 40 min. Results suggest that employing regular resting periods during conditioning preserves strength in fatigue-resistant muscle.