Importance of adaptive stimulation of the latissimus dorsi muscle in cardiomyoplasty.

During cardiac synchronized latissimus dorsi (LD) stimulation, adaptive duration (AD) allows the pulse train length to be set as a percentage of the cardiac cycle. The LD contraction duration adjusts to variations in heart rate (HR). The effects of AD on LD work output and blood flow (BF) were investigated in nine dogs that underwent skeletal muscle ventricle (SMV) construction. The SMVs were stimulated according to the cardiomyoplasty protocol. BF and the pressure increase (delta P) generated during SMV contraction were monitored biweekly for 3 months. SMV contraction time increased significantly after training (P < 0.0001). The trained SMV could only partially contract at duration settings of 25% and 30%. Before training, the increase in mean pressure and BF during 35% AD increased proportionally with increasing HR. After training, BF and mean pressure decreased with increasing HR because of reduced peak pressure generation during partial SMV contractions. When duration was fixed at 200 msec, BF increased with HR to 80 bpm but decreased with additional increases in HR. At high rates and fixed duration, inadequate relaxation between contractions resulted in the inefficient muscle performance and reduced BF. AD did not impair BF and allowed the appropriate autoregulatory response to occur in the trained and untrained LD so that the supply-to-demand ratio did not become compromised over a large range of HR.

Conformational adaptation of muscle: implications in cardiomyoplasty and skeletal muscle ventricles.

In dynamic cardiomyoplasty and other forms of muscle-powered cardiac assist, the stretch that should be applied to the skeletal muscle to obtain optimal resting tension remains unclear. To test the hypothesis that skeletal muscle is capable of conformational adaptation over time, the effect of altered resting tension on the chronic performance of a skeletal muscle ventricle was studied. In 7 mongrel dogs, skeletal muscle ventricles constructed from the lastissimus dorsi muscle were stimulated to contract for 12 weeks against an implantable mock circulation. The preload pressure was altered, thereby varying the resting tension of the latissimus dorsi. One group (group I; n = 5) was maintained at a preload of 80 mm Hg, whereas a second group (group II; n = 2) was maintained at 20 mm Hg. Adaptation to preload was observed. After 12 weeks, the pressure increase generated by the skeletal muscle ventricle at a preload of 20 mm Hg was only 35 +/- 2 mm Hg for group I compared with 44 +/- 5 mm Hg for group II. At a preload of 80 mm Hg, the pressure increase was 61 +/- 4 mm Hg for group I and only 34 +/- 6 mm Hg for group II. Adaptation of the latissimus dorsi to a new resting tension has important implications in the use of skeletal muscle for cardiac assist. Stretching the latissimus dorsi to its in situ length during cardiomyoplasty is not required for future muscle performance to be optimal.

Blood flow to the latissimus dorsi muscle pouch during chronic counterpulsation stimulation.

The cyclic contraction of a skeletal muscle ventricle (SMV) stimulated in counterpulsation results in phasic perfusion of the muscle. Perfusion will occur primarily during cardiac systole when the muscle is relaxed. However, the resting preload of the SMV will be systolic arterial pressure, which will impede blood flow to the relaxed muscle. To determine the effect of chronic counterpulsation stimulation on the blood flow to an SMV and identify stimulation regimens that prevent the risk of chronic ischemia, SMVs were created in four mongrel dogs by implementing an implantable mock circulation device. The SMV was stimulated in counterpulsation for 4 weeks after a 2-week vascular delay period and 2 weeks of low-frequency muscle conditioning. During biweekly studies, the muscle was stimulated in four modes against preloads varying from 20 to 120 mm Hg. Resting blood flow decreased significantly at preloads greater than 60 mm Hg. Normalized blood flow increased between 10% and 30% during stimulation; greater increases corresponded to more demanding stimulation modes. The elevated blood flow during stimulation, however, decreased with increasing preload. Stroke work increased with increasing preload until preload exceeded 100 mm Hg. The decreased blood flow and increased stroke work occurring at higher preloads indicate that the supply/demand ratio becomes compromised with increasing preload. A hyperemic response occurred during the resting beats after a stimulated beat, increasing the volume blood flow by as much as 80%. This response occurred regardless of preload or stimulation rate. If the SMV relaxed before the onset of systole, a hyperemic response occurred within the stimulated beat.(ABSTRACT TRUNCATED AT 250 WORDS)