Differences in Neonatal and Adult Heart
1)Size at birth
- Both ventricles are of same size at birth,later left ventricle accommodates more pressure so hypertrophies than right ventricle.
- Neonatal heart is immature
- Myofibrils are arranged disorderly
- Small percentage of contractile proteins- 30% versus 60%
- Autonomic innervation is incomplete at birth
- Sympathetic innervation is decreased
- Catecholamine stores are decreased.
- Parasympathetic innervation is comparable with that of adult therefore vagal predominance is seen.
- Control of vascular tone and myocardial contractility depends on adrenal function and circulatory or exogenously administrated catacholamines.
- In the mature myocardium sarcoplasmic reticulum is the main source of calcium for excitation-contraction coupling but in neonatal hearts SR is only poorly developed.
- Also the neonatal myocardial cells are deficient in T tubules which cause coupling between cell membrane and sarcoplasmic reticulum.
- These two things lead to- no release and reuptake of calcium for contraction….. so transmembrane calcium transport has to occur for the development of tension in the muscles…. therefore newborns require greater levels of serum ionised calcium for optimum myocardial contractility.
- Neonatal immature heart has decreased compliance.
- RV and LV are more interrelated due to same size and decreased compliance so dysfunction of one leads to dysfunction of another and leads to biventricular failure.
- Reduced compliance also means that it is very sensitive to volume overload.
- Cardiac output depends on heart rate and sinus rhythm.
- Neonates ventricular function curve is shifted to right and downward.Over the physiological range of ventricular filling pressure stroke volume changes are small.This is a relatively fixed stroke volume type so it is more dependent on HR and sinus rhythm for cardiac output
6)Increase in pressure and volume both are poorly tolerated by neonatal heart.
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- by Dr Amarja