A comparison of the active stiffness of fetal and adult cardiac muscle.

This report described the in vitro analysis of the series elasticity of ventricular myocardium isolated from five fetal lambs and six adult sheep. Active compliance measurements were obtained by quick releases utilizing a closed loop servosystem and electromagnetic positioning device to control cardiac muscle length or tension. Right ventricular moderator bands were obtained from five fetuses (average 139 days of gestation, term=147 days), and compared with two adult moderator bands and four adult right ventricular trabeculae carneae. After correction for equipment coupling the experimental load extension data were fit to an exponential expression by a least squares technique: strain=1/b ln (1 + stress/a). When fetal and adult data were compared, no age-related differences in the constants were observed (a=0.185 +/- 0.062 SE (fetal) and 0.159 +/- 0.03 g/mm2 (adult); b=40.4 +/- 5.2 (fetal) and 43.1 +/- 5.2 (adult); and ab=7.39 +/-2.34 (fetal) and 5.27 +/- .99 (adult). However, significant variation existed in the histologically determined amount of contractile tissue present in muscle samples from both fetus (57-85%, average=74.0 +/- 4.9) and adult (64-94%, average=82.3 +/- 5.3). Wihtout regard to age, a signficant correlation was found between the b stiffness constant and the amount of contractile mass present in cardiac muscle. Thus, it is concluded that there is no age-related difference in the stiffness of fetal and adult ventricular muscle. However, there is a significant relationship between active compliance and true muscle mass.