Friday August 21, 2009 (pediatric pearl day)
Uniqueness of Neonatal/ Infantile Myocardium compared to adult myocardium


Neonatal myocardium has a large supply of mitochondria, nuclei and endoplasmic reticulum to support cell growth and protein synthesis but these are non-contractile tissues which render the myocardium stiff and non-compliant. This may impair filling of the left ventricle and limit the ability to increase the cardiac output by increasing stroke volume (Frank Starling mechanism). Stroke volume is therefore relatively fixed and the only way of increasing cardiac output is by increasing heart rate. The cardiac index (defined as the cardiac output related to the body surface area to allow a comparison between different sizes of patients) is increased by 30-60 percent in neonates and infants to help meet the increased oxygen consumption.

The sympathetic nervous system is not well developed predisposing the neonatal heart to bradycardia. Anatomical closure of the foramen ovale occurs between 3 months and one year of age.

Neonatal myocardium is distinctly more sensitive to extracellular calcium levels than is mature myocardium. This has been ascribed to the poorly developed sarcoplasmic reticulum of neonatal myocardium and is dependent on serum calcium to maintain optimal contractility. Calcium is an important inotrope in newborns and its optimization is critical in low cardiac output syndrome.