Oxygen consumption of chronically stimulated skeletal muscle

The latissimus dorsi muscles of six dogs were made fatigue resistant by chronic electrical conditioning. Once the muscles were conditioned, oxygen consumption was measured during periods of exercise. The ratio of the tension developed to oxygen consumed during moderate stimulation (300 msec on) for the control and the electrically conditioned muscles was 16.3 +/- 3.5 and 36.5 +/- 6.7 kg-sec/ml oxygen, respectively. During intense stimulation (800 msec on) the ratio was 12.6 +/- 2.1 and 54.2 +/- 8.9 kg-sec/ml oxygen, respectively. Thus the conditioned muscle was able to develop and maintain tension with a considerably reduced oxygen expenditure. The increased efficiency of the conditioned muscle helps to explain its increased resistance to fatigue and the ability of pumping chambers constructed from electrically preconditioned skeletal muscle to perform sustained cardiac type work.     

Effects of collateral blood vessel ligation and electrical conditioning on blood flow in dog latissimus dorsi muscle

Utilization of skeletal muscle as a myocardial substitute requires it to undergo two major modifications: mobilization to the site of action and adaptation to continuous activity. We have examined the effects of collateral blood vessel ligation, which would accompany mobilization, on blood flow in control and electrically conditioned canine latissimus dorsi (LD) muscle. Blood flows were measured at rest and during a vigorous isometric fatigue test. In 22 control muscles, electrical stimulation during the fatigue test resulted in a sevenfold increase in muscle blood flow (0.26 +/- 0.18 ml/g/min at rest, 1.69 +/- 0.84 ml/g/min during stimulation). No difference was detected in flow to distal and proximal portions of the muscle. In three muscles where collateral vessels were ligated immediately before measurement of blood flow, flow in the proximal portion of the muscle was not significantly different from control, but in the distal portion, stimulation failed to elicit an increase in flow (0.12 +/- 0.13 ml/g/min at rest, 0.16 +/- 0.07 ml/g/min during stimulation). In animals allowed a 3-week recovery period following collateral vessel ligation, stimulation-induced increases in blood flow were detected but remained lower than control. Muscles which had been conditioned by continuous electrical stimulation for 6-7 weeks at 2 or 10 Hz generated less peak isometric tension than controls (peak tension = 4.5 +/- 1.7 kg control, 2.4 +/- 0.7 kg following 2 Hz conditioning, 1.6 +/- 0.4 kg following 10 Hz conditioning). However, these muscles demonstrated an increased resistance to fatigue.(ABSTRACT TRUNCATED AT 250 WORDS)     

Implantable extra-aortic balloon assist powered by transformed fatigue-resistant skeletal muscle

The hypothesis tested in this study was whether a skeletal muscle could be transformed to be fatigue resistant, to be used to power an implantable extra-aortic balloon assist device, and therefore to provide dynamically significant cardiac assistance. Eight dogs underwent implantation of an Itrel pacemaker to stimulate the thoracodorsal nerve over 8 to 18 weeks and transform the latissimus dorsi muscle. Biopsies of these muscles confirmed near complete (up to 98%) transformation into fatigue-resistance type I muscle fibers, identified by the adenosinetriphosphatase histochemical stains. Biochemical assays showed conversion of myosin isoforms to that of myocardial V3 phenotype, decreased activity of anaerobic glycolytic marker, and increased activity of aerobic enzyme marker, which indicated greater resemblance of such muscle to the myocardial fibers. In four dogs, the optimal stimulation parameters of such muscles in response to a burst stimulator, which synchronizes and summates the muscle contraction, were studied and compared with the contralateral, nontransformed muscle. Fatigue tests confirmed the marked fatigue resistance of the transformed muscle. In four dogs, a 100 ml balloon was placed beneath the transformed latissimus dorsi muscle and connected to the thoracic aorta with a Dacron graft. By means of the optimal burst-stimulating parameters identified above, the latissimus dorsi muscle was stimulated to contract during diastole, compressing the balloon to achieve diastolic augmentation while allowing the balloon to fill during systole. A 39% increase (p less than 0.001) in the "subendocardial viability index" (diastolic pressure-time index/tension-time index) was obtained as calculated from the left ventricular and ascending aortic pressure tracings. We conclude that the skeletal muscle can be transformed to resemble myocardium, which can generate sufficient force to provide hemodynamically significant and clinically relevant counterpulsation.     

Hydraulic pouches of canine latissimus dorsi. Potential for left ventricular assistance

We have studied the fatigue rates of hydraulic pouches constructed in the form of a multilayered conical spiral using the latissimus dorsi muscle of 17 beagles. The roles that electrical muscle conditioning and early interruption of collateral blood supply have in the prevention of pouch fatigue were evaluated. The length of time that a pouch could generate flow in a hydraulic test system was measured; afterload was set at 80 mm Hg and preload 24 mm Hg. Pouches (N = 3) fashioned from muscles subject to neither electrical conditioning nor a vascular delay generated an initial flow of 990 +/- 346 ml/min, but could sustain flow for only 2.3, 3.8 and 3.6 minutes. Pouches (N = 5) constructed with electrically unconditioned muscles after a vascular delay (median 3 weeks) demonstrated a variable improvement in fatigue rates (initial flow 826 +/- 265 ml/min; time to no forward flow, 2.5, 7.5, 7.5, 10, and 200 minutes). Four of six pouches that received the benefit of long-term electrical muscle conditioning and a vascular delay (N = 6) were able to generate flow for a 4 hour period, at which time the experiment was terminated (initial flow 478 +/- 204 ml/min; final flow 195 +/- 157 ml/min). After the 4 hour fatigue test was completed, one electrically conditioned pouch was placed in series with the heart and served as a counterpulsator. The initial volume of blood pumped by the muscle pouch was 262 ml/min or 13.8% of cardiac output. After the pouch had contracted at a rate of approximately 45 beats/min for 1 hour, the volume of blood pumped was 178 ml/min, or 11% of cardiac output. In three other animals a pouch was fashioned and then left in situ for a 1 to 3 week period before hydraulic testing. These pouches generated significant initial flows (390 +/- 60 ml/min), which demonstrates the feasibility of further study of permanent pouches. These results suggest that permanent electrical muscle conditioning and perhaps a vascular collateral delay might permit an auxiliary skeletal muscle-powered ventricle to assume a portion of left ventricular function.     

Activity–rest stimulation of latissimus dorsi for cardiomyoplasty: 1-year results in sheep

Background. In dynamic cardiomyoplasty electrostimulation achieves full transformation of the latissimus dorsi (LD); therefore, its slowness limits the systolic support. Daily activity–rest could maintain partial transformation of the LD.

Methods. Sheep LD were burst-stimulated either 10 or 24 hours/day. Before and 2, 4, 6, and 12 months after stimulation, LD power output, fatigue resistance, and tetanic fusion frequency were assessed. Latissimus dorsi were biopsied at 6 months, and sheep sacrificed at 12 months.

Results. After 1 year of 10 hours/day stimulation LD was substantially conserved and contained large amounts of fast type myosin. From 2 months to 1 year of stimulation the power per muscle of the daily rested LD was greater than that of the left ventricle, being three to four times higher than in the 24-hour/day stimulation.

Conclusions. If extended to humans, these results could be the rationale for the need of a cardiomyostimulator, whose discontinuous activity could offer to patients the long-standing advantage of a faster and powerful muscle contraction.     

Effects of Muscle Length, Frequency of Stimulation, and Fatigue on the Isometric Tension in Canine Latissimus Dorsi

Abstract: An experimental protocol was designed to study the mechanical response of the canine latissimus dorsi muscle stimulated to contract isometrically. Active and passive tensions were measured with the muscle's initial length varying within 10% of its physiologic length in situ. The force-frequency relationship was obtained at frequencies of stimulation of 1, 10, 15, 30, and 60 Hz. Muscle fatigability was assessed during 3 min of successive contractions. Tests were performed in pedicled muscles of anesthetized mongrel dogs (n = 10). Force-length characteristics were found to affect evoked tension markedly. Maximal active tension was generated near the muscle length in situ. Specific isometric tension measured with unit pulse stimulation was 2 ± 0.4 N/cm², and the time to peak twitch was 92.8 ± 2.67 ms. With 60 Hz of stimulation frequency, the tension was 9.31 ± 0.32 N/cm², and the time to peak tension was 216.05 ± 16.28 ms. After the fatigue test, the tension generated decreased to 62.5% of its initial value, and this decline was paralleled by the rate of tension development and tension relaxation.     

Diabetes-induced alterations in latissimus dorsi muscle properties impair effectiveness of dynamic cardiomyoplasty in rats

Short-term diabetes was induced in male Wistar rats with streptozotocin injection. The effects of diabetes on latissimus dorsi (LD) muscle contractile and biochemical properties and acute cardiomyoplasty (CDM) were assessed and compared with data from 16 control rats. Isometric force, contractile properties, and fatigue were measured in electrically stimulated muscles (0.3 ms, 1-256 Hz), and Na+K+ and Ca2+ATPase activities were quantified in muscle membrane preparations. Systolic arterial pressure and aortic blood flow were recorded at rest and during LD muscle stimulation. Compared with control muscle, diabetic muscle showed smaller maximum specific tetanic tension and lower rates of rise and fall in force. Diabetic LD muscle also showed lower muscle enzyme activities. Twitch tension and fatigue did not differ between groups. Smaller increases in aortic flow and systolic pressure after CDM were found in diabetic rats compared to controls. The marked decrease in CDM effectiveness in diabetic rats likely reflected the alterations in muscle properties associated with diabetes.     

Skeletal muscle ventricle used for right ventricle assistance

There are a number of advantages in using an electrically stimulated autogenous skeletal muscle to construct an auxiliary ventricle to assist a heart. The purpose of this study was to determine the feasibility of biological right ventricular assistance using long-term electrically stimulated skeletal muscle grafts. In fourteen dogs, the latissimus dorsi muscles and the right thoracodorsal nerves were exposed and unipolar pulse generator was implanted. The initial rate of 70 cycle/min. was increased to a rate of 100 cycle/min. Six or 12 months later, the latissimus dorsi was wrapped around a latex pouch equipped with inflow and outflow valved conduit (skeletal muscle ventricles; SMVs). The SMVs were connected to main pulmonary artery and right atrium. These SMVs were stimulated 20 Hz for 200 msec at a fixed rate of 90 cycle/min, the hemodynamic changes with or without skeletal muscle ventricular assistance (SMVA) were measured. In as animals the circulation failed after total right ventricular bypass without SMVA. But the SMVA increased aortic blood pressure, aortic blood flow, left atrial pressure and peak pulmonary pressure significantly. There was a linear correlation between central venous pressure and skeletal muscle ventricular assist flow. Histologic studies showed the conditioned muscles had a greater percentage of slow-twitch, fatigue resistant fibers on ATPase stain. These results suggested the long-term electrical conditioning skeletal muscle could be possible to use SMVs in humans to provide support in children with some types of congenital heart disease.     

Experimental study on potential for cardiac assist by latissimus dorsi myograft--an importance of muscle ischemia

We have studied contractile property and fatigue rates of skeletal muscle ventricle (SMV) constructed using the latissimus dorsi muscles of 11 dogs. The role of early interruption of collateral blood supply in the prevention of muscle ischemia and SMV fatigue was evaluated. Systolic function of SMV was measured in a hydraulic test system; afterload was set at 70 mmHg and preload 15 or 25 mmHg. Control SMV (GI: N = 7), which was fashioned immediately after interruption of collateral blood supply, generated an initial SMV pressure of 222 +/- 50 mmHg and stroke volume of 15 +/- 7 ml/beat with muscle stimulation at a burst-frequency of 50 Hz, but could sustain flow for only 3.5 +/- 0.8 minutes. SMV subjected to a vascular delay (Group II: N = 4) demonstrated improvement of fatigue rates; duration of flow 32.4 +/- 14.0 and sufficient contractile property (initial SMV pressure 182 +/- 17 mmHg, stroke volume 1- +/- 2 ml/beat). Thermography surface temperature mapping revealed remarkable improvement of blood distribution in GII muscles. Flow rates of thoracodorsal artery were significantly greater in GII muscles compared to those in GI muscles (15.0 +/- 3.7 ml/min/LD 100 g, 10.1 +/- 3.1 ml/min/LD 100 g, p less than 0.05, respectively). Despite significant improvement of functional durability in GII muscles, the ratio of oxygen consumption to lactate output was not different between 2 groups. These results suggest that early interruption of collateral blood supply can minimize muscle ischemia, resulting in diminishing fatigue of latissimus dorsi muscles without changes in skeletal muscle metabolism.     

Performance of dynamic and isovolumetric trained skeletal muscle ventricles

BACKGROUND: Dynamic training and maintaining muscle tension are important factors during skeletal muscle ventricle (SMV) conditioning that may improve SMV performance. This study sought to determine the effects of dynamic muscle training and progressive SMV resting pressure expansion on SMV pumping capability. MATERIALS AND METHODS: SMVs were constructed in 14 goats using the left latissimus dorsi muscle. 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 mmHg (n = 5, HiP group) or maintained at 40 mmHg (n = 4, LowP group) during conditioning. After 8 to 10 weeks of electrical stimulation conditioning, SMVs were connected to a counterpulsation mock circulation system and SMV pumping performance evaluated across a range of pressures and stimulation parameters. RESULTS: SMV pumping performance was similar in each group. Stroke works generally increased with pressure and reached a plateau in all groups above 80 mmHg (120 msec contraction approximately 80 mJ/stroke; 480 msec contraction approximately 180 mJ/stroke). Stroke volumes decreased with pressure except at high stimulation levels where loading effects were observed. Chronic changes in SMV volume significantly effected pumping performance. CONCLUSIONS: These data suggest that acute pumping performance is not different between 8 to 10 weeks of dynamic or isovolumetric training if SMV volume is not constrained. A potentially improved SMV conditioning protocol is proposed that determines, positions, and maintains SMV volume near the initial volume for maximal isovolumetric pressure during conditioning.     

Impaired neuromuscular transmission after recovery of the train-of-four ratio

BACKGROUND: Residual neuromuscular blockade may increase the risk of development of post-operative pulmonary complications, but is difficult to detect clinically. It was speculated that patients may have impaired neuromuscular transmission after surgery of long duration, despite the recovery of the train-of-four (TOF) ratio. METHODS: The muscle force (mechanomyography), motor compound muscle action potential amplitude and fatigue of the adductor pollicis (AP) muscle were assessed after recovery of the TOF ratio to 0.9. Thirteen patients receiving repetitive administration of neuromuscular blocking agents (NMBAs) during surgery (median, 5.3 h; interquartile range, 3.4-6 h) were studied post-operatively in the intensive care unit. At the time of the measurements, patients were scheduled for extubation and the AP TOF ratio amounted to a mean (standard deviation, SD) of 0.94 (0.05). Six healthy volunteers of similar age, weight and gender were studied for comparison. Force-frequency curves were generated by stimulation (10-80 Hz) of the ulnar nerve, and the AP electromyogram (EMG) amplitude was measured, in parallel, before and after evoked muscle fatigue. RESULTS: The maximum AP force at a stimulation frequency of 20-80 Hz was significantly lower in patients than in controls [40 N (16 N) vs. 65 N (18 N) at 80 Hz]. In patients, but not in controls, the EMG amplitude decreased with increasing nerve stimulation frequency, and a tetanic fade of both force and EMG, amounting to 0.41 (0.33) (EMG) and 0.61 (0.35) (mechanomyography) at 80 Hz, was observed. Force after fatiguing contractions did not differ between the groups. CONCLUSION: After repetitive administration of NMBAs during surgery, even with recovery of the TOF ratio to 0.9 or more, muscle weakness from impaired neuromuscular transmission can occur. The clinician should consider that post-operative recovery of the TOF ratio to 0.9 does not exclude an impairment of neuromuscular transmission.     

The Characterization of Contractile and Myoelectric Activities in Paralyzed Tibialis Anterior Post Electrically Elicited Muscle Fatigue

This study aimed to understand the myoelectric and mechanical characteristics of muscle recovering from electrically elicited fatigue. A modified Burke fatigue protocol was delivered to activate the tibialis anterior of 13 spinal cord injured subjects for 4 min. Before and after the fatigue protocol, a series of pulse trains was delivered to induce three twitches and a fused contraction at 0, 1, 3, and 5 min and then followed every 5 min for 60 min. The recovery processes of the ankle dorsiflexion torque and the evoked electromyography (EMG) parameters were analyzed and characterized by a first‐order exponential equation. The recovery process was found to be faster in regard to tetanic muscle contraction. Factors relating to low‐frequency fatigue, postfatigue potentiation, and the quickly normalized relaxation rate were taken into account for the discussion of this result. During the recovery process, the disassociation was found not only between twitch and tetanic contractions but also between mechanical and myoelectric activities. After the complete normalization of EMG parameters from about 15 min post fatigue, the tetanic force recovered incompletely to an asymptotic level.     

Voltage-dependent performance of skeletal muscle pouches: implications for cardiomyoplasty.

Cardiomyoplasty, a new therapy for heart failure, uses autologous skeletal muscle to mechanically assist the heart. The success of dynamic cardiomyoplasty is critically dependent on the contraction strength of the assisting skeletal muscle. Unlike cardiac muscle, skeletal muscle contracts in a graded response to electrical stimulation. However, in current cardiomyoplasty practice, no systematic technique exists to set the stimulating voltage effecting skeletal muscle contraction. The stimulating voltage is simply set to some multiple of the "threshold" voltage. Furthermore, researchers do not consider the role of stimulating voltage when they determine the amount of assistance afforded during cardiomyoplasty. To more accurately assess the value of this heuristic voltage-setting technique, we investigated the role of stimulating voltage on the strength of contraction of the latissimus dorsi muscle. Six New Zealand white rabbits had isovolumic hydraulic pouches constructed from the latissimus dorsi muscle. The muscles were wrapped around a compliant balloon in which isovolumic pressure development was measured during tetany-inducing burst (pulse-train) stimulation. The tetanic plateau of the pouch pressure record was used to measure the effects of stimulating voltage on skeletal muscle contraction. Results indicated that (1) increasing stimulating voltage from two to four times the "threshold" voltage increased normalized pouch pressure from 0.38 +/- 0.21 to 0.78 +/- 0.12 (mean +/- SD) (p < 0.05); (2) the threshold-normalized voltage necessary to cause maximal muscle contraction varied widely (5.7 +/- 2.0, mean +/- SD; range, 3.1 to 9.3); and (3) the current achieving maximal pressure development varied from 5.6 to 31.4 mA (19.9 +/- 10.4 mA).(ABSTRACT TRUNCATED AT 250 WORDS)     

Circulatory benefits of diastolic counterpulsation in an ischemic heart failure model after aortomyoplasty

OBJECTIVE: Aortomyoplasty is an experimental surgical procedure in which the latissimus dorsi muscle is wrapped around the thoracic aorta and stimulated to contract during diastole, providing diastolic counterpulsation. We hypothesized that aortomyoplasty could improve cardiac function in a chronic ischemic heart failure model, similar to the improvement seen with the intra-aortic balloon pump. METHODS: Six dogs (25-30 kg) successfully underwent aortomyoplasty followed by serial coronary microembolization. Ejection fraction decreased from 63.5% to 36.5%. Two weeks after the final microembolization, the muscle was conditioned for 4 months to achieve fatigue resistance. One year after aortomyoplasty, hemodynamic studies during 1 hour of aortomyoplasty and 1 hour of intra-aortic balloon counterpulsation determined mean diastolic aortic pressure, peak left ventricular pressure, and endocardial viability ratio for assisted and unassisted beats. Cardiac output, stroke volume, and parameters of cardiac function were also measured. RESULTS: Endocardial viability ratio increased by 23.8% +/- 7.9% (P =.001) with aortomyoplasty counterpulsation and by 22.7% +/- 12.9% (P =.021) with the intra-aortic balloon pump. Both aortomyoplasty and the intra-aortic balloon pump significantly increased mean diastolic aortic pressure and reduced peak left ventricular pressure. Improvements in cardiac function with aortomyoplasty and the intra-aortic balloon pump were similar. Cardiac output increased from 2.61 +/- 0.88 to 3.07 +/- 1.06 L/min (P =.006), and index of afterload decreased from 5.4 +/- 1.4 to 4.8 +/- 1.4 mm Hg/mL (P =.02) during 1 hour of aortomyoplasty counterpulsation. CONCLUSION: One year after the procedure, aortomyoplasty counterpulsation provided diastolic augmentation and improved cardiac performance similar to the improvement provided by the intra-aortic balloon pump in a chronic ischemic heart failure model. Aortomyoplasty has the potential to benefit patients with ischemic heart disease refractory to current therapies.     

Latissimus dorsi dynamic cardiomyoplasty of the right ventricle. Potential for use as a partial myocardial substitute

Full-thickness right ventricular latissimus dorsi dynamic cardiomyoplasty with the Medtronic Cardiomyostimulator (Medtronic, Inc., Minneapolis, Minn.) was performed in a chronic canine model. In one group (n = 2) the latissimus dorsi was electrically preconditioned before cardiomyoplasty. In a second group (n = 3) cardiomyoplasty was performed and the muscle was progressively stimulated, with conditioning accomplished while the latissimus dorsi was functioning on the ventricle. The contribution of the stimulated latissimus dorsi to global ventricular function was assessed, and the effects of varying muscle stimulation parameters on latissimus dorsi function and hemodynamics were examined. Right ventricular systolic pressure increased 8%, from 23.2 +/- 0.95 to 25.1 +/- 1.5 mm Hg. The rate of pressure rise increased 37% from 226 +/- 13 to 309 +/- 12 mm Hg/sec. Right ventricular ejection fraction was measured in two dogs and increased 29% with latissimus dorsi stimulation, from 51.5% +/- 13.5% to 66.5% +/- 14.5%. Although the sample size was small, there was no difference observed between the preconditioned and nonpreconditioned groups. Right ventricular systolic pressure, rate of pressure rise, and percent latissimus dorsi fiber shortening increased as voltage and burst frequency of the muscle stimulus increased, whereas increasing the burst duration had little effect in two dogs so studied. Latissimus dorsi dynamic cardiomyoplasty can function as a partial myocardial replacement in a chronic canine model, apparently without preconditioning of the muscle. The degree of cardiac assist obtained with cardiomyoplasty appears to be influenced by the voltage and frequency of the stimulus applied to the muscle. Although it is unclear whether these results can be extrapolated to the left ventricle, this technique may find application in the treatment of ventricular aneurysm or ventricular tumor.     

Contractile response of different segments of the latissimus dorsi muscle to chronic stimulation

Objective: This study was planned to investigate if there is any difference in terms of the muscle force between the distal and proximal segments of the latissimus dorsi muscle.Subjects and Methods: An implantable mock circulation system was placed around the latissimus dorsi muscle. The wrapping procedure around the implantable mock circulation was performed by using two different latissimus dorsi muscle segments. In group 1, the very proximal and in group 2, very distal part of the latissimus dorsi were wrapped. The main differences is the blood supply to the distal part of the latissimus dorsi that was interrupted during dissection. During the stimulation period which lasted 120 minutes, the pressure developed in this system and adenosine triphosphate (ATP) levels were measured.Results: The stimulation at 20 Hz did not result in any change in pressure and metabolic data. When it was switched to 43 and 85 Hz, ATP levels decreased with a resultant drop in pressure in group 2. However ATP levels were 15.9±2.2 μmol/gr and 14.8±2.5 μmol/gr in group 1, 12.0±1.4 μmol/gr and 6.1±1.2 μmol/gr in group 2 at 43 and 85 Hz respectively (p<0.05) at the end of the 90 minutes. The pressures at the same time interval were 89±11 and 102±7 mmHg in group 1, 61±7 and 65±8 mmHg in group 2 (p<0.05).Conclusion: In this study, we demonstrated that changes in the distal segment of the latissimus dorsi muscle affects its performance in terms of metabolic and pressure changes during high frequency electrical stimulation at 43 and 85 Hz.     

Cardiomyoplasty: Studies on Goat Latissimus Dorsi Blood Flow and Muscle Damage Following Surgical Dissection and Chronic Electrical Stimulation

A bstract Background : Dynamic cardiomyoplasty has shown promise as a surgical treatment for congestive heart failure, however, skeletal muscle damage has been reported in the latissimus dorsi muscle flap. Possible etiologies for the muscle damage include surgical dissection of the latissimus dorsi muscle with interruption of collateral blood supply, as well as chronic electrical stimulation of the muscle. Methods : To investigate these possible etiologies, we conducted a series of experiments using the goat model, evaluating blood flow and muscle morphology following surgical dissection and chronic stimulation of the latissimus dorsi muscle. Four different conditions were evaluated: (1) latissimus dorsi muscle that was neither dissected nor chronically stimulated; (2) latissimus dorsi muscle that was stimulated, but not dissected; (3) latissimus dorsi muscle that was surgically dissected, but not chronically stimulated; and (4) latissimus dorsi muscle that was both surgically dissected and chronically stimulated. Conclusion : We concluded that skeletal muscle damage resulted primarily from the surgical dissection, whereby the collateral blood supply to the latissimus dorsi muscle was interrupted and not primarily as a result of chronic electrical stimulation.     

Skeletal muscle ventricle aortic counterpulsation: function during chronic heart failure

BACKGROUND: Skeletal muscle ventricles (SMVs) are pumping chambers formed from latissimus dorsi muscle. The SMV aortic counterpulsator model has been proven to be stable in the long term and provide effective diastolic pressure augmentation in normal dogs. This study seeks to prove that the aortic counterpulsator model can function effectively in chronic heart failure. METHODS: In 6 dogs, pericardium-lined SMVs were created from latissimus dorsi muscle and electrically conditioned for fatigue resistance. Each SMV was attached to the descending thoracic aorta with a two-limb bifurcated graft and the aorta ligated between the limbs. The SMV was stimulated to contract during cardiac diastole at 1:2 to 1:3 ratio. Rapid ventricular pacing was initiated at 220 to 230 beats/min for 7 weeks to induce chronic heart failure. RESULTS: SMV contraction resulted in augmentation of the diastolic pressure time-index by 12.1% (32.8+/-15.4 versus 36.1+/-14.7 mm Hg-s, p < 0.05) at baseline, then by 33.6% (12.9+/-4.4 versus 16.8+/-4.3 mm Hg-s, p < 0.05) after 7 weeks of rapid ventricular pacing. After 7 weeks of rapid ventricular pacing, SMV counterpulsation provided significant afterload reduction with increases in peak left ventricular ejection velocity and stroke volume of 22.7% (142+/-55 versus 168+/-45 mL/s, p < 0.05) and 6.2% (13.0+/-5.1 versus 13.7+/-5.2 mL, p < 0.05), respectively. Coronary blood flow was measured in 3 animals at the 7-week measurement; augmentation averaged 47.6% (0.357+/-0.29 versus 0.432+/-0.26 mL/beat, p < 0.05). CONCLUSIONS: The SMV aortic counterpulsator provides improved cardiac assistance relative to the failing heart.     

Effect of Adaptive Pulse Train Duration on Latissimus Dorsi Blood Flow

Abstract: Blood flow, intramuscular pressure, and stroke work of the trained latissimus dorsi muscle (LD) were measured during electrical stimulation at contraction rates between 20 and 160/min using pulse trains of 2 to 6 pulses in length. Epimysial electrodes and intramuscular pressure sensors were implanted in the LD of 5 dogs. The muscle remained in situ. After 12 weeks of a progressive training protocol, LD blood flow (BF) was measured using an ultrasonic flow probe and work (SW) was determined from the measured force and shortening. For pulse trains of 2 or 3 pulses, BF increased with rate, and SW was maintained at all rates. For 4 pulses, BF and SW decreased when the contraction rate exceeded 120/min. SW decreased above 100/min and 80/min for 5 and 6 pulses, respectively. An upper rate limit dependent upon the pulse train duration exists above which BF and SW decline. Exceeding these upper rate limits should be avoided in cardiomyoplasty. Excessive stimulation rates could be detrimental to the muscle by creating a metabolic insufficiency or ischemia. The cardiac assistance benefit is compromised as SW declines during high contraction rates of long pulse train duration.     

Functional assessment of skeletal muscle ventricles after pumping for up to four years in circulation

BACKGROUND: The successful treatment of cardiac failure by heart transplantation is severely limited by the shortage of donor organs, and alternative surgical approaches are needed. An experimental approach that holds considerable promise is the skeletal muscle ventricle (SMV), an auxiliary blood pump formed from a pedicled graft of latissimus dorsi muscle and connected to the circulation in a cardiac assist configuration. Adaptive transformation, or conditioning, by electrical stimulation enables the skeletal muscle to perform a significant proportion of cardiac work indefinitely without fatigue. METHODS: In 10 dogs, SMVs were constructed from the latissimus dorsi muscle, lined internally with pericardium, and conditioned by electrical stimulation to induce fatigue resistant properties. The SMVs were connected to the descending thoracic aorta via two 12-mm Gore-Tex conduits and the aorta was ligated between the two grafts. The SMV was stimulated to contract during the diastolic phase of alternate cardiac cycles. The animals were monitored at regular intervals. RESULTS: At initial hemodynamic assessment, SMV contraction augmented mean diastolic blood pressure by 24.6% (from 61 +/- 7 to 76 +/- 9 mm Hg). Presystolic pressure was reduced by 15% (from 60 +/- 8 to 51 +/- 7 mm Hg) after an assisted beat. Four animals died early, 1 from a presumed arrhythmia, and 3 during propranolol-induced hypotension. The other 6 animals survived for 273, 596, 672, 779, 969, 1,081, and 1,510 days. Diastolic augmentation was 27.4% at 1 year (93 +/- 9 vs 73 +/- 6 mm Hg; n = 5), 34.7% at 2 years (85 +/- 6 vs 63 +/- 7 mm Hg; n = 3), 21.2% (89 +/- 10 vs 73 +/- 8 mm Hg; n = 2) at 3 years, and 34.5% (78 vs 58 mm Hg; n = 1) after 4 years in circulation. After 4 years, the isolated SMV was able to maintain a pressure of over 80 mm Hg while ejecting fluid at 20 mL/s. No animal showed evidence of SMV rupture or thromboembolism. CONCLUSIONS: The SMVs in this study provided effective and stable hemodynamic assistance over an extended period of time. There was no evidence that the working pattern imposed on the muscular wall of the SMV compromised its viability. Areas of fibrofatty degeneration were suggestive of early damage that future protocols should seek to minimize.