The Effect of Low Birth Weight on the Microcirculation in the 1st Year of Life

Abstract Introduction: Capillary rarefaction, defined as a reduction in capillary density, is an established hallmark of essential hypertension. Low birth weight (LBW) infants, known to have an increased risk of developing hypertension as adults, were unexpectedly found to have a significantly higher capillary density at birth when compared to normal birth weight (NBW) infants. We therefore hypothesised that there is a microcirculatory window in the 1st year of life of LBW infants, during which a process of extensive capillary loss, known as “hyperpruning”, occurs. Methods: The George’s Capillary Rarefaction Offspring Study (G-CROS) is a longitudinal, multi-centre study of which 284 infants were NBW, born at term, and 77 were LBW. Intravital microscopy was used to measure the functional (also known as basal) capillary density (BCD), and the structural (also known as maximal) capillary density (MCD) at birth, 3 months, 6 months and 12 months. Results: LBW infants had a significantly higher capillary density at birth when compared to NBW infants (p < 0.0001). NBW infants showed a gradual reduction in capillary density between birth and 12 months, with their greatest reduction occurring between birth and 3 months (BCD mean difference = -27.62 cap/field, p <0.0001 and MCD mean difference = -31.49 cap/field, p <0.0001). Similarly, the most significant reduction in BCD and MCD, in the LBW cohort, was between birth and 3 months (BCD mean difference = -47.01 cap/field, p < 0.0001 and MCD mean difference = -48.01 cap/field, p < 0.0001). However, within this same 3-month interval, LBW infants demonstrated a significantly higher percentage reduction in BCD (mean difference = 7.81%, p = 0.0194) and MCD (mean difference = 8.29%, p = 0.0361) when compared to NBW infants. Conclusions: Once again, LBW infants were shown to have a higher capillary density than NBW infants at birth. However, this study has shown for the first time that there appears to be a microcirculatory window in the first 3 months of life during which LBW infants undergo capillary “hyperpruning”. Further follow-up studies are required to investigate the role of early capillary rarefaction in the pathogenesis of hypertension, as well as the mechanisms orchestrating these early microcirculatory changes.