Skeletal muscle ventricles in the pulmonary circulation: up to 16 weeks' experience.

Skeletal muscle ventricles (SMVs) were constructed from the right latissimus dorsi muscle of 8 mongrel dogs. After a 3-week vascular delay period, each SMV was electrically preconditioned with 2-Hz continuous stimulation of the thoracodorsal nerve for 6 weeks. A porcine-valved conduit was then anastomosed between the right ventricle and the SMV, with a second valved conduit connecting the SMV to the main pulmonary artery. The pulmonary artery was then ligated proximal to the conduit. The SMVs were stimulated to contract in 1:2 diastolic mode with a 33-Hz burst frequency. Effective right ventricular assist was achieved in all dogs. Cardiac output increased by 22.6% (1,799 +/- 97 versus 1,467 +/- 84 mL/min; p less than 0.001), systemic systolic arterial pressure by 9.3% (90.1 +/- 3.5 versus 82.4 +/- 3.9 mm Hg; p less than 0.005), and peak pulmonary artery pressure by 31.8% (27.8 +/- 2.0 versus 21.1 +/- 1.7 mm Hg; p less than 0.001) at the initiation of this study. In 6 dogs, effective right heart assist was sustained for periods of between 1 week and 12 weeks. Two dogs survived for longer than 3 months, though with evidence of deteriorating SMV function. These results demonstrate the feasibility of providing sustained right ventricular assist using this modified "Rastelli-SMV" configuration, which obviates the limitations imposed by low right atrial preload.     

Skeletal muscle ventricles as left atrial-aortic pumps: short-term studies.

In 5 dogs, skeletal muscle ventricles (SMVs) were constructed from the latissimus dorsi muscle and placed in the left hemithorax. After a 3-week vascular delay period, SMVs were electrically preconditioned with 2-Hz stimulation for 6 weeks. At a second operation, SMVs were connected between the left atrium and thoracic aorta by afferent and efferent aortic root homografts, and stimulated to contract in a 1:2 diastolic mode. At a mean left atrial pressure of 12.4 +/- 1.3 mm Hg and a burst stimulation frequency of 33 Hz, SMV stroke volume was initially 43% of that of the native left ventricle, achieving a flow equivalent to 21% of cardiac output (194 +/- 38 versus 902 +/- 85 mL/min). At 50-Hz stimulation, this figure rose to 27% (246 +/- 41 mL/min; p less than 0.05). Skeletal muscle ventricle power output (the product of stroke work and contraction rate) at 33 Hz was 0.016 +/- 0.003 W, increasing to 0.024 +/- 0.004 W at 50 Hz (p less than 0.05), corresponding to 14% and 22%, respectively, of left ventricular power output (0.11 +/- 0.012 W). After 4 hours of continuous pumping, four of the SMVs were still generating flows of more than 70% of starting values and more than 60% of initial power output. This study demonstrates that SMVs can function in the systemic circulation at physiologic left atrial preloads.     

Skeletal Muscle Ventricles: Frontiers in 1995

Abstract: Skeletal muscle ventricles (SMVs) constructed from electrically conditioned latissimus dorsi muscle (LDM) may become an alternative for assisting the failing heart. Left and right heart circulatory assist using SMVs has been performed successfully in both acute and chronic animal models. The configurations used to connect SMVs to the circulation have included a left atrium to aorta bypass, a left ventricle apex to aorta bypass, aortic counterpulsators, a cavopulmonary bypass, and a right ventricle to pulmonary artery bypass. One SMV used as an aortic counterpulsator functioned effectively in the circulation for more than 27 months. Recent application of the pericardium to the SMV as an inner layer and design changes in the connection of the SMV to the circulation have reduced the risk of thrombus formation and SMV rupture. Although several problems have yet to be solved, the goal of the SMV as a permanent circulatory assist device without the limitation of an external power source seems within reach.     

Increased coronary artery blood flow with aortomyoplasty in chronic heart failure

BACKGROUND: We hypothesized that diastolic counter-pulsation using aortomyoplasty will increase coronary blood flow. METHODS: In dogs (n = 6, 20 to 25 kg), the left latissimus dorsi muscle was isolated, wrapped around the descending thoracic aorta, and conditioned by chronic electrical stimulation. Heart failure was induced by rapid ventricular pacing. In a terminal study, left ventricular and aortic pressures, and blood flow in the left anterior descending coronary artery and descending aorta were measured. The endocardial-viability ratio was calculated. RESULTS: Aortomyoplasty increased mean diastolic aortic pressure (70 +/- 5 to 75 +/- 5 mm Hg, p < 0.05) and reduced peak left ventricular pressure (86 +/- 4 to 84 +/- 4 mm Hg, p < 0.05), leading to a 16% increase in endocardial-viability ratio (1.29 +/- 0.05 to 1.49 +/- 0.05, p < 0.05). Coronary blood flow was increased by 15% (8.2 +/- 1.5 to 9.4 +/- 1.6 mL/min, p < 0.05). During muscle contraction, 2.7 +/- 0.5 mL was ejected from the wrapped aortic segment. CONCLUSIONS: These data demonstrate that aortomyoplasty provides successful diastolic counterpulsation after muscle conditioning and heart failure.     

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.     

Intrathoracic Skeletal Muscle Ventricles: A Feasibility Study

For skeletal muscle ventricles (SMVs) to be applied clinically, it is likely that they will have to be placed within the chest. Ease of subsequent connection to the circulation, and avoidance of significant lung compression, are factors that could influence SMV size and shape in a way that may prejudice their ability to pump effectively at physiological preloads. In five dogs, specially designed SMVs were constructed from the latissimus dorsi muscle, and placed in the apex of the left hemithorax. After a 3-week delay, the muscle was preconditioned electrically by 2-Hz continuous stimulation for 6 weeks. At a later thoracotomy, this positioning of SMVs permitted easy surgical access to the heart and great vessels. SMVs were then connected to a mock circulation device for functional evaluation. As right-sided pumps, at a preload of 10 mmHg, SMVs generated a stroke volume (SV) and stroke work (SW) exceeding that of the native right ventricle (SV = 8.9 +/- 0.8 vs 7.9 +/- 0.6 mL; SW = 0.44 +/- 0.03 vs 0.20 ergs x 10(6)). As left-sided pumps, also at a preload of 10 mmHg, SMV SV, and SW was roughly half that of the left ventricle (SV = 3.7 +/- 0.2 vs 7.9 +/- 0.6 mL; SW = 0.29 +/- 0.03 vs 0.57 +/- 0.05 ergs x 10(6)). SMVs may conveniently be positioned inside the chest, where they have the potential to function as left or right heart assist devices.     

Intrathoracic and Extrathoracic Skeletal Muscle Ventricles in Circulation: Left Ventricular Apex-to-Aorta Configuration

Skeletal muscle ventricles (SMVs) were constructed from the latissimus dorsi muscle in 12 dogs. In group I (n = 6), SMVs were placed intrathoracic, in the apex of the left hemlthorax. In group II (n = 6), SMVs were positioned extrathoracic between the chest wall and subcutaneous tissue. After a 3-week vascular delay period, SMVs were electrically precondltloned wlth 2-Hz continuous stimulation for 6 weeks. At a second procedure, a valved conduit was placed between the left ventrlcular (LV) apex and the SMV, and a second valved condult between the SMV and the thoracic aorta. The SMVs were stimulated to contract during dlestole at a 1:2 ratio wlth the heart. In group I, SMVs generated peak pressures of 91 ± 10 mmHg, pumped 47% of the systemic blood flow (0.73 ± 0.25 vs 1.54 ± 0.42 Umin; p < 0.05), and produced a 25% decrease In the LV systolic tension-time Index (TTI) (16.9 ± 2.7 vs 12.5 ± 3.3 mmHg sec; p < 0.05). In group II, SMV peak pressure was 93 ± 10 mmHg, SMVs pumped 51% of the systemlc blood flow (0.78 ± 0.10 vs 1.53 ± 0.42 L/min; p < 0.05), and the LV systolic TTI decreased 29% (14.0 ± 0.8 vs 9.9 ± 2.0 mmHg sec; p < 0.05). There was no significant difference between group I and II. These data Indicate that the SMV:LV apex-to-aorta configuration is the most effective method reported to date for skeletal muscle cardiac assist. Extrathoracic and Intrathoracic SMVs functioned equally well after connectlon to the circulation. (J Card Surg 1994;9:332–342)     

Potential for left ventricular assistance by latissimus dorsi myograft--sequential effects of electrical preconditioning on skeletal muscle fiber type, blood flow and metabolic status

In order to evaluate the possibility of left ventricular assistance by latissimus dorsi (LD) myograft, we have studied contractile property and fatigue rates of skeletal muscle ventricle (SMV) constructed using canine LD muscles. Twenty three dogs were divided into 3 groups depending on the conditioning protocol of LD muscles; Group I (Control n = 12), Group II (Vascular delay n = 4) and Group III (Vascular delay and electrical preconditioning n = 7). SMVs in GIII dogs generated sufficient pressure and forward flow in a hydraulic test system with muscle stimulation at a burst-frequency of 50 Hz (SMV pressure 131 +/- 42 mmHg, Stroke volume 7.0 +/- 3.0 ml/beat). Although SMVs in GI and GII dogs could sustain flow for only 4.0 +/- 1.1 minutes and 32.4 +/- 14.0 minutes, respectively, SMVs in GIII were able to pump continuously for 107.5 +/- 15.0 minutes (p less than 0.01, vs GI and GII). Thermography surface temperature mapping revealed marked improvement of blood distribution of LD muscles in GII and GIII dogs. Flow rates of thoracodorsal artery during SMV stimulation were GI: 10.0 +/- 3.1 ml/minute/LD 100 g, GII: 15.0 +/- 3.7 ml/minutes/100 g and GIII: 20.7 +/- 2.5 ml/minutes/100 g (p less than 0.01 vs GI). The ratio of oxygen consumption to lactate output was GI: 0.33 +/- 0.10, GII: 0.36 +/- 0.09 and GIII: 1.56 +/- 0.97 (p less than 0.01 vs GI, p less than 0.05 vs GII). Histochemical examination of LD muscles using alkaline ATPase stain revealed muscle fiber type transformation of GIII muscles. These results suggest electrically preconditioned LD muscles have sufficient contractile property for partial left ventricular assistance, and highly fatigue-resistant properties resulted from muscle fiber transformation, improved muscle perfusion and metabolic changes.     

Acute descending aortomyoplasty induces coronary blood flow augmentation

BACKGROUND: Aortomyoplasty is a procedure aimed to improve cardiac output in patients suffering from heart failure. Stimulation of the latissimus dorsi muscle around the aorta produces hemodynamic effects similar to those of the intraaortic balloon pump. These may be maintained without the accompanying complications or the need for anticoagulation. The objective of this study was to test the acute effects of aortomyoplasty on coronary artery blood flow. METHODS: Eight mongrel dogs (18 to 30 kg) underwent acute descending aortomyoplasty. Several stimulation protocols were applied after wrapping of the latissimus dorsi muscle around the aorta in different surgical configurations. The left anterior descending coronary blood flow was measured using a transonic Doppler flow probe. Left ventricular and aortic pressures, proximal and distal to the aortomyoplasty site, were monitored continuously. RESULTS: Significant aortic diastolic pressure augmentation was expressed both as an increase in peak values, from 110 +/- 24 mm Hg to 120 +/- 24 mm Hg (p < 0.001) and as an increase in the diastolic integral, from 64 +/- 23 mm Hg x s to 84 +/- 37 mm Hg x s (p < 0.001). Concomitantly, peak left anterior descending coronary blood flow increased from 26 +/- 10 mL/min to 32 +/- 12 mL/min (p < 0.001). This was associated with an increase in the diastolic flow integral from 11 +/- 4 mL to 14 +/- 6 mL (p < 0.001). CONCLUSIONS: Descending aortomyoplasty induces significant augmentation of coronary blood flow. Optimal timing of muscle stimulation is important in achieving the best assist. This procedure may prove beneficial for end-stage ischemic patients.     

Skeletal Muscle Ventricles with Efferent Valved Homograft

A bstract Skeletal muscle ventricles (SMVs) were constructed from the latissimus dorsi muscle in seven beagles. Following 3 weeks of vascular delay and 6 weeks of electrical conditioning, the SMVs were connected in series with the thoracic aorta using a valved aortic homograft for the efferent limb. The SMVs were stimulated to contract synchronously during diastole. Effective aortic diastolic counterpulsation was achieved in all dogs, with an average 24.2%± 15.3% improvement in diastolic pressure. In two animals surviving beyond 3 months, increase in SMV function was noted over time. Appropriate aortic homograft valve function was documented by echocardiogram. Acute reversible heart failure was induced with propranolol in one dog alive after 126 days. A 61.3% reduction in cardiac output and a 37.6% reduction in mean arterial blood pressure were achieved. During profound low cardiac output, SMV stimulation with 33 Hz and 50 Hz improved cardiac output by 16.9% and 17.8%, improved the tension time index by 14.9% and 16.1%, and improved the endocardial viability ratio by 34.1% and 34.1%, respectively. These results again demonstrate the long-term effectiveness of SMVs as aortic counterpulsators. A valve in the efferent limb of the SMV system functions appropriately over time and may improve the efficiency of the system.     

The hemodynamic function of intrathoracic skeletal muscle ventricles after recovery from surgery in pigs

The shortage of donor organs for heart transplantation highlights the need for new approaches to end-stage heart failure. A promising experimental technique is the use of pumping chambers formed from the latissimus dorsi muscle. We formed such skeletal muscle ventricles (SMVs) and connected them to the descending thoracic aorta in a single surgical procedure in pigs. Activation of conditioned SMVs from the end of systole for 80% of diastole increased mean aortic diastolic blood pressure by 11.2 +/- 1.6% in 1 animal and by 15.8 +/- 0.3% in another. The left-ventricular stroke work in the postassisted beat was decreased by 8.7 +/- 5.8% and 10.1 +/- 2.2% and the overall stroke work by 7.4 +/- 1.2% and 9.4 +/- 0.8%. The key to forming and connecting the SMV in a single procedure was the use of a composite homograft lining. In future clinical practice, this component could be replaced by a synthetic composite or by a tissue lining produced in vitro.     

Pericardium-Lined Skeletal Muscle Ventricles: Up to Two Years'' In-Circulation Experience

Background. Skeletal muscle ventricles (SMVs) are autologous pumping chambers constructed from skeletal muscle. Skeletal muscle ventricular rupture and thromboembolism have complicated chronic models of this method of skeletal muscle cardiac assist.

Methods. The SMVs were constructed from the latissimus dorsi muscle in 10 dogs. The inner surface of each SMV was lined with autologous pericardium harvested at the time of SMV construction. After a 3-week period of vascular delay and 6 weeks of electrical conditioning to convert the muscle to a fatigue-resistant state, SMVs were connected to the descending thoracic aorta and stimulated to contract during cardiac diastole.

Results. Initial hemodynamics revealed that SMV contraction at 33 Hz increased diastolic pressure 24.7% (60.8 ± 7.3 mm Hg versus 80.3 ± 8.8 mm Hg). Skeletal muscle ventricle relaxation decreased presystolic pressure 14.4% (59.9 ± 7.7 mm Hg versus 51.3 ± 7.5 mm Hg) and decreased peak systolic pressure 4.1% (90.2 ± 7.3 mm Hg versus 86.5 ± 5.8 mm Hg). Hemodynamics were assessed at 1 to 2 weeks, then at 1, 2, 3, and 6 months, and at 6-month intervals thereafter. Hemodynamic performance remained stable for the duration of this study. After 2 years of pumping continuously in circulation, SMV contraction resulted in a 34.8% augmentation of diastolic pressure (63.6 ± 6.6 mm Hg versus 85.3 ± 6.4 mm Hg), a 17.2% decrease in presystolic pressure (54.7 ± 3.73 mm Hg versus 45.3 ± 4.1 mm Hg), and a 4.2% decrease in peak systolic pressure (95.3 ± 10.4 mm Hg versus 91.3 ± 12.3 mm Hg). Three dogs survived to 2 years with the SMVs in circulation. No animal showed evidence of thromboembolism during serial echocardiography or at autopsy and no SMVs ruptured.

Conclusions. These data demonstrate that SMVs can provide effective hemodynamic assist over an extended period without specific complications related to the SMVs.     

Skeletal muscle ventricles for total heart replacement.

Skeletal muscle ventricles (SMV) were constructed from canine left latissimus dorsi muscle. The animals were divided into three groups: group A (n = 5), SMVs rested 4 weeks without electrical conditioning; group B (n = 6), SMVs rested 4 weeks and then electrically conditioned for 6 weeks; group C (n = 5), SMVs rested 18 weeks without electrical conditioning. At the end of each protocol, the SMVs were acutely tested by connecting them to a mock-circulation device. The SMVs in group C developed stroke work at physiologic preloads superior to any previously reported, as high as 194% of left ventricular stroke work at afterloads of 80 mmHg. The SMVs in group B developed work outputs equivalent to 53% of the left ventricle, which is still more than four times that of the right ventricle. The results show that it is possible to harvest sufficient work from skeletal muscle ventricles to fully replace cardiac function at physiologic preloads.     

Skeletal muscle ventricles with a single-limb conduit: the importance of hemodynamic design

BACKGROUND: Skeletal muscle ventricles (SMVs) connected to the descending thoracic aorta have the potential for providing long-term diastolic augmentation. A successful existing design employs a bifurcated conduit, but aortic constriction between the limbs of the conduit is required to ensure obligatory flow-through. Here we evaluate an alternative approach in which connection to the aorta is made by a single-limb conduit. METHODS: In two groups of dogs SMVs were constructed from the left latissimus dorsi muscle and connected to the circulation via a single-limb conduit of length 110-120 mm (Group 1, n = 5) or 70 mm (Group 2, n = 5). The animals were followed over 10 weeks. RESULTS: Although all animals showed significant augmentation of diastolic aortic pressure at the outset, substantial thrombus developed in the SMVs of both groups. The results were analyzed by reference to design criteria for a single-limb conduit SMV, developed from empirical, in-vitro flow studies and formulated mathematically. CONCLUSION: The SMVs constructed in this experiment appeared to meet the criteria for adequate mixing of blood within the ventricle. They did not, however, achieve adequate exchange of blood with the circulation. Thrombosis was therefore attributable to excessive residence time of blood in the SMV and conduit. Both the experimental study and the mathematical analysis point to the need for SMVs of this configuration to be constructed closer to the aorta. Preliminary results are reported for such an experiment in the pig, in which the SMV was thrombus-free when terminated electively after 1 week.     

Bilateral latissimus dorsi cardiomyoplasty.

This study was undertaken to test the hypothesis that a bilateral latissimus dorsi cardiomyoplasty provides greater hemodynamic augmentation than a unilateral procedure. Two types of bilateral procedure and a left posterior cardiomyoplasty were tested in each of 8 mongrel dogs. R-wave synchronous muscle pacing was achieved with a programmable burst stimulator. Hemodynamic variables of stimulated beats were compared with those of a nonstimulated baseline using paired t tests. The effects of a double anterior muscle wrap were equal to a right anterior/left posterior configuration. Therefore, the data on the two types of bilateral procedure were combined and compared with the left wrap. Stimulation of the bilateral cardiomyoplasty resulted in significant increases in right ventricular pressure (44 +/- 3.1 versus 26 +/- 1.8), first derivative of right ventricular pressure (595 +/- 117 versus 196 +/- 14), pulmonary artery pressure (34 +/- 1.9 versus 23 +/- 1.6), left ventricular pressure (90 +/- 5.9 versus 69 +/- 5.3), first derivative of left ventricular pressure (1454 +/- 141 versus 1072 +/- 107), aortic pressure (80 +/- 5.4 versus 67 +/- 4.9), and peak aortic flow (9.4 +/- 1.1 versus 7.7 +/- 0.8) (p less than 0.05). Significant increases in all of these variables also occurred with stimulation of the left cardiomyoplasty, but the increases in right ventricular pressure, first derivative of right ventricular pressure, pulmonary artery pressure, and aortic pressure were larger for the bilateral than the left cardiomyoplasty. The bilateral and the left procedure can each augment systolic ventricular function. The bilateral procedure appears to have greater effects, especially on right ventricular function.     

Dynamic cardiomyoplasty acutely impairs left ventricular diastolic function.

In patients with congestive heart failure, medical treatment has a high rate of mortality and morbidity, and transplantation is limited by the availability of donor hearts. Dynamic cardiomyoplasty is being investigated as surgical therapy to improve left ventricular function in these patients. To evaluate the early postoperative effects of this procedure on left ventricular diastolic function, we studied seven dogs through the use of sonomicrometry and micromanometry in a canine model of dynamic cardiomyoplasty. Left ventricular diastolic parameters were determined before wrapping the latissimus dorsi muscle (baseline), after latissimus dorsi muscle wrap but without stimulation, and with synchronous left ventricular contraction-latissimus dorsi muscle stimulation. End-diastolic pressure was increased in both conditions after latissimus dorsi muscle wrap (without stimulation, 5 +/- 1; with stimulation, 6 +/- 2 mm Hg; p < 0.05) compared with baseline (3 +/- 2 mm Hg). The peak rate of diastolic pressure decay was greater at baseline (1560 +/- 370 mm Hg/sec) than after latissimus dorsi muscle wrap, both without (1260 +/- 330 mm Hg/sec, p < 0.01) and with (1120 +/- 420 mm Hg/sec, p < 0.01) stimulation. The constant of pressure decay was prolonged both without (53 +/- 10 seconds, p < 0.05) and with (62 +/- 11 seconds, p < 0.01) latissimus dorsi muscle stimulation compared with the baseline (38 +/- 5 seconds). Compared with baseline (0.2 +/- 0.2 cm-2), the constant of passive chamber stiffness increased after the latissimus dorsi muscle was wrapped around the heart (1.6 +/- 0.7 cm-2, p < 0.05) and with stimulation (2.1 +/- 1.0 cm-2, p < 0.01). The maximal diastolic filling rate (baseline, 18.1 +/- 6.7; without stimulation, 16.6 +/- 8.9; with stimulation, 16.6 +/- 4.1 cm2/sec, not significant) and end-diastolic short-axis area (baseline, 7.3 +/- 2.3; without stimulation, 7.4 +/- 2.1; with stimulation, 7.5 +/- 2.3 cm2, not significant) were similar among the three conditions. The latissimus dorsi muscle wrap prolonged relaxation and increased left ventricular passive stiffness. Synchronous latissimus dorsi muscle stimulation with left ventricular contraction did not improve diastolic function in this model. The results suggest that in the early postoperative period, dynamic cardiomyoplasty impairs diastolic function.     

Assessment of Skeletal Muscle Ventricle Function Using Tissue Velocity Imaging

Abstract Background and Aims: Skeletal muscle ventricles (SMVs) are a potential power source for circulatory assistance. Noninvasive assessment of SMVs is desirable in long-term studies of SMV function. This study evaluated whether tissue velocity imaging (TVI) indices of function correlate with invasive measurements of output and pressure generation and examined the potential of TVI to provide information about SMV geometry and wall contraction characteristics. Methods: SMVs were constructed in six sheep. After electrical conditioning, SMVs were connected to a mock circulation and stimulated with supramaximal 30-Hz and 50-Hz bursts to contract 35 times/min. The SMVs were tested over a range of preloads, and afterload was adjusted to simulate systemic (80 mmHg) and right ventricular (30 mmHg) loading conditions. Stroke volume and pressure were measured invasively, and stroke work was calculated. TVI was used to measure velocities in two opposing SMV walls, providing a simple wall motion score (WMS). This was evaluated against stroke volume, stroke work, and pressure development. Results: 50-Hz stimulation frequency and high preload optimized SMV performance. Optimal SMV performance indices (mean at 50 Hz) were as follows: (a) right ventricular loading conditions (preload 30 mmHg), stroke volume 17.6 mL (SEM 3.2), peak pressure over afterload 44.2 mmHg (10.9), stroke work 0.05 J (0.02); (b) systemic loading conditions (preload 60 mmHg), stroke volume 10.1 mL (3.2), peak pressure over afterload 58 mmHg (14.6), stroke work 0.08 J (0.03). With low preloads, geometric anomalies were noted in the SMVs using TVI. Collapse of the SMVs and dyskinesis were observed, which normalized with higher preloads. Persistent dyskinesis was noted in one SMV and was associated with poor performance. Correlations (at optimal loading and stimulation settings) were as follows: systemic loading conditions, stroke volume versus WMS, 0.92 (p = 0.026); peak pressure versus WMS 0.89 (p = 0.045); stroke work versus WMS, r = 0.91 (p = 0.046). Right ventricular loading conditions were as follows: stroke volume versus WMS, 0.63 (p = 0.25); peak pressure versus WMS, 0.66 (p = 0.22); stroke work versus WMS, 0.45 (p = 0.39). Conclusion: Under systemic loading conditions, TVI indices of SMV wall motion mirror invasive indices of performance, suggesting that TVI may be a useful tool for long-term noninvasive monitoring of SMV function.     

Skeletal muscle extraaortic counterpulsation. A true arterial counterpulsation

Reduction of left ventricular work load during systole, a critical component of arterial counterpulsation, has not previously been documented for skeletal muscle-powered extraaortic counterpulsation. To assess its capacity for afterload reduction, a skeletal muscle extraaortic counterpulsator was connected to the thoracic aorta and counterpulsated. Canine hearts (n = 7) were instrumented with left ventricular Millar catheters (Millar Instruments, Inc., Houston, Tex.) for pressure measurements and with piezoelectric ultrasonic crystals for measurement of the left ventricular minor axis dimension and wall thickness. During systole, skeletal muscle extraaortic counterpulsation resulted in a significant change in all three determinants of left ventricular circumferential wall stress compared with control conditions (no counterpulsation). Pressure decreased (peak systole, 100 +/- 5 versus 75 +/- 6 mm Hg; p less than 0.05 by paired t test), minor axis dimension decreased (end systole, 46.4 +/- 1.1 versus 45.8 +/- 1.1 mm; p less than 0.05 by paired t test), and wall thickness increased (end systole, 10.4 +/- 0.7 versus 10.6 +/- 0.7 mm; p less than 0.05 by paired t test). Left ventricular wall stress/dimension work loops showed a shift downward and to the left, a shift consistent with afterload reduction. The mean systolic left ventricular wall stress was significantly reduced, from 67.3 +/- 10.6 to 47.7 +/- 8.1 10(3) dyne/cm2 (p less than 0.05 by paired t test). Skeletal muscle extraaortic counterpulsation increased the diastolic aortic pressure from 72 +/- 6 to 105 +/- 8 mm Hg (p less than 0.05 by paired t test). Our data, which documented the counterpulsator's direct effects on left ventricular functional mechanics, showed that skeletal muscle extraaortic counterpulsation is capable of both diastolic augmentation of arterial pressure and systolic unloading of the left ventricle. Skeletal muscle extraaortic counterpulsation has potential application for ventricular unloading in the treatment of chronic end-stage heart failure.     

The Muscle-Powered Dual-Chamber Counterpulsator: Rheologically Superior Implantable Cardiac Assist Device

To enable long-term studies of a totally implantable cardiac assist device powered by transformed fatigue-resistant skeletal muscle, we developed a dual-chamber extraaortic counterpulsation system that uses hydraulic fluid for power transfer. Six dogs had our dual-chamber extraaortic counterpulsator implanted, 2 of which had undergone prior transformation of their latissimus dorsi muscle. The blood pump, with a Dacron graft at each end, was anastomosed end-to-side and parallel to the thoracic aorta, allowing continuous blood flow to minimize thrombus formation caused by stasis and turbulence. The blood pump was powered by a hydraulic bulb placed beneath the latissimus dorsi muscle. The latissimus dorsi muscle was stimulated to contract during diastole using a synchronized burst electrical stimulator. The ratio of diastolic pressure time product over systolic time tension index, which reflects the myocardial oxygen supply and demand ratio, was calculated from ascending aortic pressure tracings. A consistent increase in this ratio of 44% in 4 dogs with nontransformed latissimus dorsi muscle and of 70% in 2 dogs with transformed latissimus dorsi muscle was obtained when the device was activated. Preliminary chronic implantation studies using a Medtronic cardiomyo-stimulator (Model SP1005) as the burst stimulator for our dual-chamber extraaortic counterpulsator produced an average augmentation in aortic diastolic pressure of 34 mm Hg for up to six days.Our results indicate that, with further refinement of this device, a long-term totally implantable cardiac assist device powered by endogenous skeletal muscle will be feasible.     

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.