Chronic implantation of a skeletal muscle energy convertor for cardiac assist devices: a preliminary report

An efficient energy convertor capable of driving a variety of cardiac assist devices is being developed in goats. Muscle work in a linear configuration is converted to hydraulic energy and transmitted to an external test system that controls muscle loads during shortening contractions. This investigation focuses on the variation of muscle characteristics and optimal power output during muscle conditioning. The energy convertor was mounted on the rib cage, the latissimus dorsi insertion reattached to the device, and percutaneous hydraulic lines exited near the spine. Following device, stimulator, and intramuscular electrode implantation, a progressive conditioning protocol was initiated. Weekly biomechanical muscle characterization was performed in the conscious animal, with single twitch and tetanic contractions performed under isometric and isotonic conditions. The characterization data provide a measure of available power, as well as inputs, for a computer simulation that predicts optimal muscle power output and operating conditions. These ongoing implants provide insight into the available muscle power and suggest an implantable energy convertor is feasible. Development of an energy convertor is an important step toward tether free skeletal muscle powered cardiac assist. These studies will be expanded in number and duration to further investigate the effects of conditioning and identify improvements in device development.     

Mechanical advantage of skeletal muscle as a cardiac assist power source

The insertion of unconditioned latissimus dorsi muscle at the humerus was reattached in anesthetized goats to the first stage piston of a two stage mechanical-to-hydraulic energy convertor for a skeletal muscle powered ventricular assist device. To study a range of forces, pistons of different cross sectional areas were evaluated during thoracodorsal nerve stimulation. Each energy convertor piston was coupled hydraulically to an actuator on a Thoratec VAD (Thoratec Laboratories, Berkeley, CA) in a mock circulatory loop. Maximum force (70.1 +/- 10.8 N) was greatest for the largest piston, and stroke length (4.0 +/- 0.7 cm) was greatest for the smallest piston. However, maximum stroke work (1.2 +/- 0.5 J) and muscle powered VAD ejected stroke volume (45 +/- 17 ml) were greatest for the middle size piston. These results are consistent with a biomechanical model of whole muscle contraction that predicts that there is an optimum force that produces maximum cycle work. Thus, with a two stage energy convertor, by changing the ratio of the cross sectional areas of the energy convertor and muscle powered VAD actuator pistons, the effective mechanical advantage for the muscle can be optimized to produce more work output and muscle powered VAD flow. Skeletal muscle powered devices using such an energy convertor could provide completely implantable circulatory support free from batteries and other power conditioning hardware required with electromechanical systems.     

Clinical features of relative focal hyperfusion in patients with intracerebral hemorrhage detected by contrast-enhanced xenon CT

Optimization of the skeletal muscle contraction during cardiac assist is important to achieve maximal cardiac assist and yet avoid overstimulation that may injure skeletal muscle. Dynamic cardiomyoplasty suffers from lack of an objective, reproducible, and accurate technique to measure skeletal muscle shortening fraction after wrap and training of the muscle. A recruitment curve is considered the best way to select the proper stimulation level to achieve supramaximal contraction without overstimulating the muscle. A fluoroscopic technique of determining latissimus dorsi recruitment curve was evaluated in five goats undergoing dynamic cardiomyoplasty with an anterior cardio-subcutaneous wrap. Two pairs of stainless steel targets (0.5 and 1 cm of diameter) were implanted on each side of the muscle wrap. One pair of sonomicrometer crystals was also implanted. Displacement of the targets was measured under fluoroscopy at five different stimulation levels. Correlation coefficients between targets on the inside surface of the wrap and the sonomicrometer crystals, and targets on the outside surface of the wrap and the sonomicrometer crystals were 0.71 (P < 0.05) and 0.60 (P < 0.05), respectively. Targets on the inside surface of the wrap were more accurate than targets on the outside surface of the wrap for measurement of skeletal muscle shortening fraction and establishment of a recruitment curve. Adverse effects from the targets were not observed.     

Progressive pressure expansion in skeletal muscle ventricle conditioning

Skeletal muscle ventricle (SMV) conditioning typically results in reduced muscle performance. This study investigated the effects of progressive SMV resting pressure expansion and dynamic muscle training on SMV pumping capability. SMVs were formed from latissimus dorsi muscle in five goats. Three experimental SMVs were conditioned against a compliant pneumatic implant system. SMV resting pressure was progressively increased as the SMV adapted to each increment. Resting pressure rose from 40 to 100-120 mmHg over an 8 week period of time. Two control SMVs were conditioned against a non expanded incompressible implant. Both experimental and control SMVs were electrically burst stimulated for at least 6 weeks after an initial 2 week vascular delay interval. Results demonstrate that 1) experimental SMVs increased in volume; 2) SMV passive and active (evoked isovolumetric pressure) pressure-volume curves adapted to the increasing or static resting volume; and 3) two of three experimental SMVs generated greater stroke volumes than control SMVs across a range of counterpulsation pressures and electrical stimulation parameters. Progressive pressure expansion using a compliant implant system improved final SMV pumping performance and merits further investigation.     

Skeletal muscle extra-aortic counterpulsation in dogs with dilated cardiomyopathy

Skeletal muscle extra-aortic counterpulsation was performed in seven dogs with dilated cardiomyopathy. A left latissimus dorsi dynamic descending thoracic aortomyoplasty was used as the autologous counterpulsator. Pulse train stimulation in diastole was used to initiate contraction and fibre type transformation. Two of the dogs died within 48 hours of surgery. The device was successfully activated in the five remaining dogs, but in one individual it failed within 48 hours of activation. Serial echocardiographic examinations of dogs in which the device functioned successfully (n = 4) showed trends towards the decrease in the left ventricular systolic internal dimension, left ventricular diastolic internal dimension, E-point to septal separation and left atrial diameter in systole seven to 14 days following the procedure, although these changes failed to persist in the long-term. The results suggest that skeletal muscle for cardiac assistances such as extra-aortic muscle counterpulsation, might be a therapeutic option for dogs with cardiac failure due to dilated cardiomyopathy.     

Instrumentation and techniques in equine fracture fixation

In recent years fracture fixation in the horse has changed significantly. New devices, mainly adapted from the human field, have been successfully introduced into large animal surgery. Examples of such implants include the DCS/DHS implant system, the self-tapping screw, the cannulated screw, and the pinless external fixator. However, new devices have also been developed exclusively for equine fracture management, including the interlocking intramedullary nail and the external skeletal fixation device. With these devices the surgeon has more options for repairing fractures in horses. Nevertheless, many problems are still unsolved. Indications exist that during the next few years new, exciting fracture fixation systems will be developed, providing further advancements in the quest for the ideal implant for horses. However, the development stages of these devices are such that mentioning the possibilities here is premature.     

Transient normalization of systolic and diastolic function after support with a left ventricular assist device in a patient with dilated cardiomyopathy

A 19-year-old man who had fulminant heart failure caused by an idiopathic dilated cardiomyopathy was supported with a left ventricular assist device for 183 days as a bridge to heart transplantation. At the time of intended transplantation it was noted that the patient's heart had returned to normal size, had a normal ejection fraction, and was able to maintain normal pressures and flows. In view of the apparent recovery of cardiac properties, the left ventricular assist device was explanted and the transplantation was not performed. However, the heart dilated, ejection fraction worsened, and the patient died of heart failure exacerbated acutely by a systemic viral illness. Although such recovery of systolic function is uncommon, as use of the left ventricular assist devices becomes more widespread other physicians might encounter similar findings and, in this regard, they might find our experience useful as they contemplate their treatment options.     

Biomechanical characteristics of unconditioned and conditioned latissimus dorsi muscles used for cardiocirculatory assistance

An understanding of the biomechanical characteristics of striated skeletal muscles involved in cardiocirculatory assistance is a prerequisite to assess their efficacy and to evaluate their haemodynamic benefits. Six goats had their latissimus dorsi muscles evaluated by isometric strain gauge testing. Total tension, and both active and passive force development at different preloads were measured. The relationship between muscle impedance and starting length was also studied. Four additional muscles were submitted to isometric and isotonic strain gauge testing after 3 months of chronic electrical stimulation (Broussais Hospital protocol) with the contralateral muscle serving as a control. In isometric testing, both conditioned and unconditioned goat latissimus dorsi displayed a Frank-Starling length-tension curve, and a linear relationship between muscle impedance and starting length was found. Chronic stimulation preserved muscle mass and isometric force. Transformed muscles showed a mean 59% reduction of maximal shortening velocity; means (s.d.) residual shortening velocity at maximal work and power output was 0.17(0.07) m/s. The work and power output were both reduced 65% after stimulation, and the residual maximal power at optimal preload varied from approximately 7.7 and 9.6 W/kg. It is concluded that, following the Broussais protocol, the goat latissimus dorsi muscle retained mass and most of its isometric force-generating capacity, but lost significant work and power potential. The residual power output did not, however, preclude the possibility of a significant cardiocirculatory contribution, providing that the conditions for optimal energy transduction are adequately delineated.     

Homozygous deletion of the p16INK4A gene occurs more frequently in CD2+ than in CD2+ T-cell acute lymphoblastic leukemia

Right ventricular assist devices are an important part of the armamentarium of cardiac surgeons for the treatment of right-sided circulatory failure after cardiac transplantation or insertion of a left ventricular assist device. However, right ventricular assist device insertion can be technically challenging in the setting of pulmonary hypertension because of a number of concomitant anatomic and physiologic phenomena. We present a technique for the insertion of the right ventricular assist device outflow cannula that is easier and faster to insert, and safer to explant, especially if cardiopulmonary bypass is to be avoided.     

Effect of medication on biomechanical properties of rabbit bones: heparin induced osteoporosis

BACKGROUND: We hypothesize that the integrity of the latissimus dorsi muscle graft used to wrap the heart may affect the clinical outcome of patients undergoing dynamic cardiomyoplasty. METHODS: By correlating the pathologic findings with their clinical course in five patients who died 1 month to 6 years after dynamic cardiomyoplasty operation, we sought to discern findings that might shed light on the pathophysiology of cardiomyoplasty. RESULTS: Of the two patients who had a limited clinical response, one had an atrophic, edematous latissimus dorsi muscle with fatty infiltration resulting from cardiac cachexia, and the other had insufficient length of latissimus dorsi muscle to cover a large heart. The remaining patients responded well clinically without signs of pump failure and died at various intervals, mostly of arrhythmias. Autopsy findings included the following: (1) one patient with ischemic cardiomyopathy as the underlying disease had development of rich vascularity in the interface between the muscle wrap and the epicardium; whereas in four others with idiopathic cardiomyopathy, such evidence of collateralization was far less evident. (2) There was a variation in the skeletal muscle transformation achieved, with the fraction type I fatigue-resistant fiber in the muscle wrap ranging from 60% to 100%, in spite of the identical transformation protocol used. Such variation is believed to be genetically based. (3) In one patient, the skeletal muscle was paced to contract at 30 to 50 times/minute (2:1 ratio) for more than 5 years. Nevertheless, the pathologic specimen of the muscle wrap showed only minimal interstitial fibrosis. (4) Relatively thin muscle wrap around the heart found at autopsy could be atrophy but most likely was related to muscle transformation, which is known to reduce muscle mass and increase capillary density. (5) All skeletal muscle grafts showed geometric conformation to the shape of the epicardium and grossly looked as if they were an additional layer of the ventricular wall. Such conformation may facilitate the modulation of the ventricular remodelling process in the failing heart, as has been described both in clinical and experimental studies. CONCLUSIONS: Our findings are consistent with and support a number of mechanisms proposed for cardiomyoplasty. Thus preservation of latissimus dorsi muscle graft integrity may be important in the success of dynamic cardiomyoplasty.     

Long-term survival after treatment of malignant colonic polyps

For the application of the latissimus dorsi muscle (LDM) to circulatory assist, the muscle is stimulated with co- or counterpulsation during the cardiac cycle. The purpose of this study was to evaluate the blood supply to the LDM and its muscular performance during each respective stimulation. The origin of the LDM was connected to a tension gauge, a potentiometer, and 1 kg of weight in series. The LDM was stimulated at a ratio of 1:1 of heart to muscle contraction for 10 min. Copulsatile stimulation made thoracodorsal arterial flow (TDF) predominant during cardiac diastole. In counterpulsatile stimulation, TDF occurred predominantly during cardiac systole. Between the 2 patterns of stimulation, no significant differences were observed in the mean TDF rate during 1 cardiac cycle. The maximal force, maximal contraction length, and power of the LDM also did not differ significantly. These results suggest that despite the difference of the TDF profile, LDM performance may be comparable between co- and counterpulsatile stimulation for the application of the LDM to circulatory assist.     

Factors influencing pressurization of the femoral canal during cemented total hip arthroplasty

Successful cement pressurization with total hip arthroplasty depends on the capacity of the cement gun and its ability to pressurize the canal and the integrity of the intramedullary plug and the proximal seal used to contain the cement bolus during pressurization. In the laboratory, the authors measured the volume of cement delivered by two cement guns (from Zimmer, Warsaw, IN, and Howmedica, Rutherford, NJ) in comparison with typical values for the volume of the medullary canal following standard surgical preparation. The two cement guns studied delivered 93 and 138 mL cement, respectively. In comparison, the volume of the intramedullary canal ranged from 35 to 70 mL using a standard femoral prosthesis (Precision Hip System, Howmedica). Peak pressures developed during cement injection using the cement guns were 73.6 +/- 27.1 psi for the Zimmer system and 47.3 +/- 16.9 psi for the Howmedica system. Both devices were able to sustain a minimum pressure of at least 6.5 psi through cementing when used in conjunction with a flexible pressurizing seal. The mechanical performance of five designs of intramedullary plugs was assessed by monitoring plug displacement during cement pressurization in reamed cortical specimens. The performance of each device was judged by its ability to withstand cement pressures of 50 psi without displacement within the medullary canal. On the basis of this test, the probability that these plugs would exceed this criterion when used with the femur was estimated to range from 24 to 94%. Few of the commercially available plugs were able to withstand cement pressures routinely generated using standard cement delivery systems.     

Configuration of linear dynamic cardiomyoplasty for hypoplastic right ventricle

BACKGROUND: The purpose of this study is to determine feasibility of linear dynamic cardiomyoplasty with a briefly preconditioned latissimus dorsi in the experimental model simulating patch enlargement of the hypoplastic right ventricle. METHODS: In 8 mongrel dogs, a diminished right ventricular chamber was reconstructed with an extended pericardial patch. A left latissimus dorsi, preconditioned for 2 weeks (2 Hz) after a previous 2-week vascular delay period, was placed on the patch, with the muscle fiber oriented in parallel to the right ventricular long axis. RESULTS: Graft pacing with trained-pulses of 25 Hz at a 1:1 ratio showed significant augmentation of pulmonary flow and pressure (158% +/- 21%, 156% +/- 14%, respectively), contributing to restoring right ventricular function comparable with preoperative control, which was also confirmed by the right ventricular function curve and pressure-volume relationship analyzes. Continuous pacing was performed in 4 animals for 7 hours without evidence of muscle fatigue, implying feasibility of "working conditioning" after minimum preconditioning for this type of right heart assist. CONCLUSIONS: Linear latissimus dorsi myoplasty can restore normal right ventricular performance at a physiologic preload, and may provide a surgical option for the hypoplastic right ventricle.     

Hemodynamic response to in situ latissimus dorsi muscle stimulation: implications in dynamic cardiomyoplasty

Dynamic cardiomyoplasty (DCM) involves the electrical stimulation of a pedicled latissimus dorsi muscle flap wrapped around the falling ventricle as a means of cardiac assist. To further elucidate a potential neurohumoral mechanism for improvement of cardiac output after myoplasty, we evaluated the hemodynamic effects of in situ stimulation of the latissimus dorsi muscle (in the absence of cardiomyoplasty). In seven mongrel dogs, a nerve cuff electrode (Medtronic 6901) was placed around the left thoracodorsal nerve (TDN). This was attached to a pulse generator (Medtronic, Itrel 7420), delivering a 4.0 volt, 0.19 second on, 0.81 second off, 33 Hz, 210 microsecond pulse width, cyclic bursts similar to that used in DCM. Stroke volume index (SVI) and other hemodynamic parameters as well as plasma norepinephrine (NE) levels were measured at five stages: baseline, stimulator on at 0, 2, and 5 minutes, and stimulator off at 30 minutes after. The animals were then subjected to 4 weeks of rapid pacing at 240 beats/min (Medtronic 8329) to induce heart failure, and as the rapid pacing was discontinued, measurements were repeated as above. After rapid pacing, cardiac function was significantly depressed, and NE was elevated (133 +/- 69 versus 500 +/- 353 pg/mL, p < 0.05). In the normal hearts, TDN stimulation increased SVI, heart rate, systemic pressure, and NE levels. In heart failure, however, no significant changes in cardiac function and NE levels were noted. In conclusion, our data indicate that in the normal hearts, afferent impulses from TDN stimulation alone may augment cardiac function by means of a neurohumoral effect that is not seen in severe heart failure. The implications of these findings in DCM are discussed.     

Positive panel reactive antibody titers in patients bridged to transplantation with a mechanical assist device: risk factors and treatment

Patients who are bridged-to-transplantation with mechanical support have a high incidence of pretransplant sensitization defined by panel reactive antibody (PRA) titers greater than 10. Risk factors for positive PRA in patients with assist devices were investigated. From 1993 to 1997, 17 patients underwent implant surgery with CardioWest C-70 total artificial hearts (TAHs; CardioWest Technologies, Inc., Tucson, AZ), and 13 with Novacor left ventricular assist systems (LVASs; Baxter Healthcare, Novacor Division, Oakland, CA) for bridge-to-transplantation at this institution. Two patients died during implantation of the assist devices. Of the remaining 28 patients, four (14%) were women (3 with TAHs and 1 with an LVAS). All four women (100%) had a positive PRA, whereas only two of the 24 men (8%) had positive PRA (p < 0.0001). The transfusion histories of these patients were reviewed. Using chi-squared analysis (alpha = 0.05), the PRA levels were independent of transfusion of packed red blood cells and fresh frozen plasma. There was an association, however, between platelet transfusions and PRA levels. The times on device awaiting cardiac transplantation were also compared between the PRA positive and PRA negative groups. The average time to transplantation for PRA positive patients was 116 days, whereas the average waiting time for the PRA negative patients was 55 days (p = 0.05). Based on these data, a female patient with consistently positive PRA (93%) after TAH implantation underwent a transplant on post implant day 25 despite a positive lymphocytotoxic crossmatch with the donor. She was treated with plasmapheresis during cardiopulmonary bypass at the time of transplantation, and with four further treatments post transplant. As of this writing, she is alive and well on our standard triple immunotherapy. Therefore, women who are bridged-to-transplantation with assist devices are at risk for positive PRA. It is recommended that patients who are bridged-to-transplantation with assist devices and have high PRA levels be treated with perioperative plasmapheresis. With this aggressive approach, it may no longer be necessary to keep patients on mechanical support for prolonged periods, but possible to perform transplants as soon as suitable donors become available.     

Analytic processing elicits right ear superiority in monaurally presented speech

Experiments were designed to assess the performance of an intracorporeal (abdominal) left ventricular assist device (ALVAD) in the presence of induced tachycardias, multiple premature ventricular contractions (PVC's), and ventricular fibrillation in calves. Performance criteria were the degree of left ventricular unloading and the per cent cardiac output assumed by the ALVAD. During synchronous pumping, left ventricular unloading was complete and the entire cardiac output was captured by the device. During induced tachycardias up to rates of 120 beats per minute, these degrees of performance were maintained. At rates in excess of 120 beats per minute, performance declined due to decreased biologic stroke volumes and prosthetic filling times. In the presence of induced PVC's, performance during synchronous pumping decreased because of erratic R-wave sensing. Left ventricular unloading was complete but irregular, and the total cardiac output was captured. When asynchronous pumping was utilized, mean left ventricular systolic pressures increased, but total cardiac output was still captured. During induced ventricular fibrillation, ALVAD actuation maintained cardiac outputs equal to control values for periods up to 5 1/2 hours. These experiments indicate that, during normal sinus rhythm, synchronous pumping is optimal; asynchronous pumping is optimal during complex dysrhythmias; and either can be utilized to support the circulation with varying degrees of left ventricular unloading.     

Spectroscopic study of Y210C lambda-repressor: implications for cooperative interaction

The development of a percutaneous artificial internal organ system requires a reliable biocompatible connection between the external environment and the inside of the human body. Such is necessary for the success of a permanent left ventricular assist device. However, the search for a satisfactory interface at the epidermal level has proven to be difficult. Carbon has been proposed for this application, but its texture does not typically promote ingrowth from surrounding tissue. We have therefore employed a new processing method to produce a fine trabecularized carbon implant. The method for preparing the implant involves infiltrating low temperature pyrolytic carbon into the surface of a carbon core which is wrapped with carbon fabric. This results in a tightly woven porous structure of carbon (carbon fiber diameter: 35-50 microm, maximal pore size >200 microm) with gradually increasing porosity from 15-75%. We implanted test samples percutaneously in a calf for in vivo histological evaluation. Thirty days after implantation epidermal downgrowth was minimal. Microscopic analysis revealed that a thin fibrous capsule surrounded the implant, and mature connective tissue with accompanying blood vessels filled the pores of the fine trabecularized carbon layer. From these results we suggest that fine trabecularized carbon is ideally suited for a percutaneous device system in a permanent left ventricular assist device.     

Twenty-four hour ambulatory blood pressure: a sample from a normal British population

Breast reconstruction with the latissimus dorsi muscle flap combined with a saline-filled implant was performed. Using an endoscope, additional operative scarring due to muscle harvesting was avoided. The operative techniques were easier than usual endoscopic surgery because most of the procedures were performed under direct vision. The procedures are presented in this article.     

Progress in the development of a transcutaneously powered axial flow blood pump ventricular assist system

Development of the Jarvik 2000 intraventricular assist system for long-term support is ongoing. The system integrates the Jarvik 2000 axial flow blood pump with a microprocessor based automatic motor controller to provide response to physiologic demands. Nine devices have been evaluated in vivo (six completed, three ongoing) with durations in excess of 26 weeks. Instrumented experiments include implanted transit-time ultrasonic flow probes and dual micromanometer LV/AoP catheters. Treadmill exercise and heart pacing studies are performed to evaluate control system response to increased heart rates. Pharmacologically induced cardiac dysfunction studies are performed in awake and anesthetized calves to demonstrate control response to simulated heart failure conditions. No deleterious effects or events were encountered during any physiologic studies. No hematologic, renal, hepatic, or pulmonary complications have been encountered in any study. Plasma free hemoglobin levels of 7.0 +/- 5.1 mg/dl demonstrate no device related hemolysis throughout the duration of all studies. Pathologic analysis at explant showed no evidence of thromboembolic events. All pump surfaces were free of thrombus except for a minimal ring of fibrin, (approximately 1 mm) on the inflow bearing. Future developments for permanent implantation will include implanted physiologic control systems, implanted batteries, and transcutaneous energy and data transmission systems.     

精密油液温度控制在高温液压源中的应用

本文设计了一种以智能仪表为核心的控制器件，用于为外部提供温度可控的液压源，并对液压系统的性能参数进行测量显示。结果表明，该高温液压油源具有设计合理、运行稳定、可靠性高、压力波动小、噪音小等特点，较好的发挥了智能仪表及变频器在高温液压油源中的作用。