Myocardial Function in Infants During Postnatal Transition and With Further Maturation: A longitudinal echocardiographic study in preterm and term infants
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Assessment of the neonatal circulation has traditionally been based on indirect clinical signs such as blood pressure, skin perfusion, body temperature, capillary refill time and biochemical markers. However, applying these signs and markers is inadequate for evaluation of the cardiovascular status in infants and may lead to erroneous conclusions. The use of echocardiography to assess circulation and ventricular function in the absence of structural cardiac malformations is increasingly applied in intensive care settings, both in adults and in children. In the neonatal population, and especially in preterm infants, few studies have investigated echocardiographic methods for the assessment of myocardial function. This thesis includes three papers focusing on different methods for assessing the myocardial long axis function in moderate preterm and term born infants. The longitudinal study design was chosen to incorporate the dynamic nature of fetalneonatal cardiovascular transition and maturation after birth. Paper I serially assessed left and right pulsed wave tissue Doppler velocities of the atrioventricular (AV) annulus in moderately preterm infants. Paper II investigated long-axis systolic AV annulus excursions by conventional and colour tissue Doppler imaging (cTDI) in the same group of preterm infants. In paper III, corresponding indices of myocardial function were compared in healthy term and preterm infants. Our studies showed that serial assessment of ventricular function by conventional and tissue Doppler imaging echocardiography was feasible in both preterm and term infants. Intra- and inter-observer variabilities were acceptable. Only small variations in ventricular functional indices were found between the first three days in the preterm infants, suggesting that a fairly stable haemodynamic status with respect to ventricular function was already established within the first day of life. All indices of ventricular function increased with postnatal age in term and preterm infants, probably due to change both in heart size and postnatal maturation. The indices were higher in the term compared to the preterm group both in the late transitional phase and after postnatal maturation. However, at term equivalent age in the preterm group, ventricular indices were higher compared to the term group at the same gestational age (GA), probably due to enhanced postnatal maturation in the preterm group. The term infants after postnatal maturation had the highest indices of ventricular function, the largest hearts and the most advanced gestational and postnatal age. All these factors probably contributed to the advanced ventricular function. The ventricular functional measures are related to heart size. Normalizing the myocardial indices by left ventricular (LV) length removed, and to some extent reversed, the differences between term and preterm infants. This suggests that myocardial function relative to the size of the heart was not inferior in the preterm infants compared to the term infants, and that normalization of myocardial indices by heart size might be appropriate in infants. The indices of myocardial function applied in our studies were feasible and reproducible. The indices might serve as useful tools in assessing cardiac function in the specified groups of infants in a clinical setting. Further research is needed to evaluate the applicability and usefulness of these methods in various medical conditions and in lower GA groups.