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J Thorac Cardiovasc Surg 1994;107:868-878
© 1994 Mosby, Inc.

CARDIOPULMONARY BYPASS, MYOCARDIAL MANAGEMENT, AND SUPPORT TECHNIQUES

The relation between latissimus dorsi skeletal muscle structure and contractile function after cardiomyoplasty

Charleston, S.C.

Supported in part by National Institutes of Health grant HL45024 (F.G.S.) and Medtronic, Inc., Minneapolis, Minn.

Received for publication June 9, 1993. Accepted for publication Aug. 30, 1993. Address for reprints: John M. Kratz, MD, Division of Cardiothoracic Surgery, Medical University of South Carolina, 1717 Ashley Ave., Charleston, SC 29425.

Abstract

Past reports suggest that structural changes within the latissimus dorsi muscle occur with chronic electrical stimulation during cardiomyoplasty. However, the specific changes in the structure of the latissimus dorsi muscle and the relation to muscle contractile function with cardiomyoplasty are unknown. Accordingly, this study examined regional changes in latissimus dorsi muscle structure and function after cardiomyoplasty. The left latissimus dorsi muscle was mobilized and wrapped around the heart in pigs with the use of standardized techniques and the latissimus dorsi muscle chronically paced at ambient heart rates (90 beats/min; 20 Hz, 5 V amplitude, n = 6). After 6 weeks, the paced latissimus dorsi muscle and the contralateral control muscle were removed and divided into proximal (0 to 3 cm), middle (3 to 6 cm), and distal (6 to 12 cm) regions. By computer-assisted morphometry, muscle cell myofibril volume, cross-sectional area, and collagen percent area were determined. In the paced latissimus dorsi muscle, myofibril volumes increased by more than 50% in the proximal and middle regions compared with those in the contralateral control muscle. However, myofibril volumes were significantly lower in the distal region of the paced latissimus dorsi muscle compared with those in control muscles (33% ± 5% versus 20% ± 3%, p < 0.05). In the paced latissimus dorsi muscle, cross-sectional area was significantly reduced from that of control muscles in all regions. A further reduction in cross-sectional area was noted in the distal region of the paced latissimus dorsi muscle compared with that in both the contralateral control muscle and the proximal and middle regions of the paced latissimus dorsi muscle. Collagen content significantly increased in the paced latissimus dorsi muscle compared with that in control muscle with a more fibrotic pattern observed in the distal region. Latissimus dorsi muscle strips (less than 2 mm² cross-sectional area) were harvested, and peak and velocity of tension development were examined after field electrical stimulation at 0.2 to 1.2 Hz. At 0.2 Hz, the velocity of tension development was unchanged in the paced latissimus dorsi muscle compared with that in control muscle. However, peak tension development degraded by only 28% in the paced latissimus dorsi muscles but fell by 51% in control muscles with increased stimulation frequencies. In summary, the contractile function of the chronically stimulated latissimus dorsi muscle was associated with fatigue resistance and increased contractile protein content. However, more distal regions of the paced latissimus dorsi muscle demonstrated atrophy and fibrosis. These region-specific changes within the chronically stimulated latissimus dorsi muscle may significantly influence the long-term performance of this skeletal muscle after cardiomyoplasty. (J THORAC CARDIOVASC SURG 1994;107:868-78)

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