A study of cerebral blood flow autoregulation in the extremely premature, very low birth weight baby

Introduction Preterm very low birth weight babies are at high risk of brain injury. Despite improvement in mortality rates the morbidity from brain injury in these babies remains high. Whilst some of the brain injury is bound up in the causation of their prematurity and is therefore antenatal in origin there remains a percentage that suffer from postnatally acquired brain injury. These babies are at high risk of intra cranial haemorrhage which usually occurs within the first 72 hrs of birth. This haemorrhage can in severe cases lead to brain injury. The cause of the haemorrhage is probably multi factorial and animal models have been constructed in attempt to explain this. The animal models would suggest that the most likely sequence of events is a period of low blood pressure (hypotension) followed by a period of hyperperfusion causing the haemorrhage to occur. However low systemic blood pressure does not necessarily equate to low cerebral blood flow (CBF). This is because humans have the ability to autoregulate their CBF. This in essence means that despite variable systemic pressure the cerebral blood vessels react to keep the CBF constant. If this process fails the blood vessels are unable to react (vasoparalysis) and CBF must directly relate to systemic blood pressure (otherwise known as mean arterial pressure (MAP).) This is illustrated in figure 1. below: