By investigating new methods of assessing a babies circulatory status we intend to help create a new definition of circulatory failure which will in turn help doctors create further studies to identify infants with circulatory failure and find the best ways of treating this condition in babies. So far research in this area has been confined to babies born at less than 33 weeks gestation. We intend to extend the knowledge in this area to infants older that 33 weeks gestation.