Maturational Changes in Myocardial Contractile State of Newborn Lambs

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Abstract
Summary: There have been no investigations of chronologic changes in postnatal contractile state of both left and right ventricles of the newborn. Thus, we determined isovolumetric indices of contractility in 32 acutely instrumented newborn lambs (1 day to 12 wk; three exteriorized near term fetuses, and four adult sheep. These studies demonstrated a narked increase in inotropic properties of both ventricles which was especially pronounced during the first 3 days of extrauterine life (group I. left ventricle Vmax 3.2 ± 0.27 and VCEIO 3.2 ± 0.28 ML (muscle lengths/sec; right ventricle Vmax 2.1 ± 0.2 and VCEIO 2.2 ± 0.13 ML/sec) and was associated with elevated cardiac output (428 ± 52 cc/kg/min), largely resulting from an elevated stroke volume (2.1 ± 0.31cc/kg). During subsequent postnatal maturation, contractility indices of both ventricles gradually declined to values intermediate between newborn and adult [(group IV. 22 to 84 days) - left ventricle Vmax 1.78 ± 0.13 and VCEIO, 1.79 ± 0.15 ML/sec; right ventricle Vmax 1.48 ± 0.07 and VCEIO 1.31 ± 0.3 ML/sec]; associated with a corresponding decling in cardiac output (group IV, 136 ± 38 cc/kg/min), related in large measure in a decline in stroke volume (0.76 ± 0.13 cc/kg). Simulation of birth in exteriorized fetuses was associated with a sudden increase in contractile indices for both left (Vmax 2.1 ± .2 → 3.0 ± .3 and VCEIO 2.1 ± 0.2 → 3.1 ± ML/sec) and right (Vmax 2.0 ± 0.2 → 2.5 ± 0.2 and VCEIO 1.9 ± 0.1 → 2.5 ± 0.2 ML/sec) ventricles. We conclude that there is a postnatal elevation of ventricular contractile state which contributes to successful postnatal adaptation of the newborn cardiovescular system. Speculation: The increased metabolic demands associated with adaptation to extrauterine life are met by an increased cardiac output in the newborn. The necessary cardiac output may be accomplished in part through an elevation of myocardial contractile state, induced by postnatal increases in circulating hormones.

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