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Background Despite increasing investigation in the area of cardiovascular instability in preterm infants,huge gaps in knowledge remain.Study of the hemodynamic characteristics in this population is inadequate.Methods A one-center,prospective,observational longitudinal cohort study at a third level Neonatal Intensive Care Unit enrolled 86 preterm infants.Of these,46 were low-birth-weight (LBW) newbos of a mean (SD) gestation-al age of 32.3 (1.1) weeks and a birth weight of 2,031 (1,684-2,320) g.Forty were very-low-birth-weight (VLBW) newbos with a gestational age of 28.4 (1.5) weeks and a birth weight of 1,255 (884-1,580) g.All infants underwent Doppler ultrasound examinations at 24,48,and 72 h after birth.Results SMII,DO2,MBP,LVCO,CI,and SVI in the VLBW infants were all significantly lower than the LBW infants,but SVRI was not different.Postnatal increases in MBP,SVRI were observed in VLBW and LBW groups that were not associated with changes in LVCO and DO2.The postnatal patte of SMII differed between the two groups.SMII increased with postnatal age in the LBW group and did not change significantly in the VLBW group.Conclusions SMII and DO2 were significantly lower in VLBW neonates during the first 72 h of life,and there was a direct relationship between inotropy,DO2,and birth weight over a range of maturities at birth.VLBW infants may be at higher risk for cardiac dysfunction when an additional challenge is encountered.