Key points• Delayed cord clamping improves circulatory stability in preterm infants at birth, but the underlying reason is not known.• In a new preterm lamb study we investigated whether delayed cord clamping until ventilation had been initiated improved pulmonary, cardiovascular and cerebral haemodynamic stability.• We demonstrated that ventilation prior to cord clamping markedly improves cardiovascular function by increasing pulmonary blood flow before the cord is clamped, thus further stabilising the cerebral haemodynamic transition.• These results show that delaying cord clamping until after ventilation onset leads to a smoother transition to newborn life, and probably underlies previously demonstrated benefits of delayed cord clamping.Abstract Delayed cord clamping improves circulatory stability in preterm infants at birth, but the underlying physiology is unclear. We investigated the effects of umbilical cord clamping, before and after ventilation onset, on cardiovascular function at birth. Prenatal surgery was performed on lambs (123 days) to implant catheters into the pulmonary and carotid arteries and probes to measure pulmonary (PBF), carotid (CaBF) and ductus arteriosus blood flows. Lambs were delivered at 126 ± 1 days and: (1) the umbilical cord was clamped at delivery and ventilation was delayed for about 2 min (Clamp 1st; n = 6), and (2) umbilical cord clamping was delayed for 3-4 min, until after ventilation was established (Vent 1st; n = 6). All lambs were subsequently ventilated for 30 min. In Clamp 1st lambs, cord clamping rapidly (within four heartbeats), but transiently, increased pulmonary and carotid arterial pressures (by ∼30%) and CaBF (from 30.2 ± 5.6 to 40.1 ± 4.6 ml min −1 kg −1 ), which then decreased again within 30-60 s. Following ventilation onset, these parameters rapidly increased again. In Clamp 1st lambs, cord clamping reduced heart rate (by ∼40%) and right ventricular output (RVO; from 114.6 ± 14.4 to 38.8 ± 9.7 ml min −1 kg −1 ), which were restored by ventilation. In Vent 1st lambs, cord clamping reduced RVO from 153.5 ± 3.8 to 119.2 ± 10.6 ml min −1 kg −1 , did not affect heart rates and resulted in stable blood flows and pressures during transition. Delaying cord clamping for 3-4 min until after ventilation is established improves cardiovascular function by increasing pulmonary blood flow before the cord is clamped. As a result, cardiac output remains stable, leading to a smoother cardiovascular transition throughout the early newborn period.

Background-Ventricular outputs cannot be used to assess systemic blood flow in preterm infants because they are confounded by shunts through the ductus arteriosus and atrial septum. However, flow measurements in the superior vena cava (SVC) can assess blood returning from the upper body and brain. Objectives-To describe a Doppler echocardiographic technique that measures blood flow in the SVC, to test its reproducibility, and to establish normal ranges. Design-SVC flow was assessed together with right ventricular output and atrial or ductal shunting. Normal range was established in 14 infants born after 36 weeks' gestation (2 measurements taken in the first 48 hours) and 25 uncomplicated infants born before 30 weeks (4 measurements taken in the first 48 hours). Intraobserver and interobserver variability were tested in 20 preterm infants. Results-In 14 infants born after 36 weeks, median SVC flow rose from 76 ml/kg/min on day 1 to 93 ml/kg/min on day 2; in 25 uncomplicated very preterm infants, it rose from 62 ml/kg/min at 5 hours to 86 ml/kg/min at 48 hours. The lowest SVC flow for the preterm babies rose from 30 ml/kg/min at 5 hours to 46 ml/kg/min by 48 hours. Median intraobserver and interobserver variability were 8.1% and 14%, respectively. In preterm babies with a closed duct, SVC flow was a mean of 37% of left ventricular output and the two measures correlated significantly. Keywords: preterm infants; superior vena cava; systemic blood flow; reproducibility A major problem in exploring the relation between early changes in the systemic blood flow and end organ injury, particularly to the brain in preterm newborn infants, is how to measure systemic blood flow. Blood pressure gives only a loose indication of systemic blood flow, 1 and the eVect of ductal shunts on left ventricular output and of atrial shunts on right ventricular output can cause either of these measures to overestimate the real systemic blood flow by up to 100%. Conclusions-This