Assessment of Systemic Blood Flow in the Newborn Infant

Abstract

Preterm infants are vulnerable to brain injury which is thought to be caused partly by abnormalities in cerebral perfusion. However the accurate assessment of cerebral and general systemic perfusion remains a challenge in the newborn infant. Commonly used clinical parameters such as blood pressure and blood lactate concentrations are imperfect predictors of blood flow. Cardiac output measurements used in older children do not reflect true systemic perfusion in the neonate due to shunting of blood through persisting fetal pathways. Echocardiographic measurements of descending aortic (DAo) and superior vena caval (SVC) blood flow may provide more reliable assessment of neonatal systemic perfusion. This thesis evaluates these techniques in the first days of postnatal life. Measures of flow volume in the SVC and DAo were found to be feasible in the vast majority of infants, and were performed without significantly affecting cardiorespiratory status. Assessment of SVC flow volume showed similar repeatability to other measures of blood flow in neonates when assessed by a single observer, as did assessment of velocity of flow in the DAo. We then used these techniques to further assess the transitional circulation, and found no evidence of a positive association between arterial blood pressure and volume of systemic perfusion. Contrary to previous assumptions that ductal shunting compromises systemic perfusion, we found that left ventricular output tended to increase with increasing shunt through the ductus arteriosus, thereby maintaining upper, though not necessarily lower, body perfusion. There was an association between very low levels of flow in individuals and some adverse outcomes that had a strong circulatory component to their pathophysiology (periventricular haemorrhage and necrotising enterocolitis). However low blood flow in the SVC or DAo did not predict poor outcome within the entire cohort. Assessments of SVC and DAo flow in the neonate are feasible, relatively repeatable and have already enhanced our understanding of the pathophysiology of the transitional circulation. These and other techniques to monitor systemic blood flow in the neonate may aid identification of circulatory failure, act as short-term endpoints in clinical trials of interventions supporting the circulation, and eventually improve neurodevelopmental outcome in preterm infants.