Brain Vascular Reactivity to Hypothermic Circulatory Arrest With Antegrade Cerebral Perfusion During Aortic Arch Surgery

Overview

Many neonates and infants who undergo complex cardiac surgery are affected by neurological developmental delays. Whilst catastrophic events are immediately identifiable from clinical examination or by macro changes on MRI or CT scans, smaller changes are often not immediately visible or detected. This is an observational pilot study examining brain vascular reaction to hypothermic circulatory arrest with antegrade cerebral perfusion and neuro-protection techniques during aortic arch surgery in neonates and infants. A combination of duplex ultrasound and transcranial doppler will be used to record in-depth information on the cerebrovascular changes that occur during the entire length of the surgical procedure and during the early postoperative period. The proposed techniques and equipment are non-invasive and are in use clinically to evaluate brain perfusion in a similar age group. During aortic arch surgery, the patient's body and brain temperature is reduced to values between 18 and 24 degrees centigrade in order to decrease metabolic demand that provides a form of metabolic protection. However, there is no consensus within the clinical community regarding the optimal temperature at which to perform surgery. Moreover, in order to improve cerebral perfusion, the brain is perfused via the right internal carotid artery with cold blood. At Alder Hey Children Hospital, this surgery is undertaken by the three surgeons but, due to clinical preference, differs in relation to the temperature at which surgery is undertaken. This provides the opportunity to observe the impact of different temperatures on cerebral vascular reactivity in neonates and young infants The arguments for future comparisons and a larger randomised study will be made based on the information gained from this observational study.

Children who have undergone cardiac surgery are at high risk of neurological disability. Adequate cerebral perfusion during the surgery is paramount to minimise the risk of brain injury which is a well-documented complication. Whilst this is easily identifiable through brain imaging post-operatively, less is known about intracerebral vascular reactivity that arise due to inadequate cerebral perfusion during and immediately following surgery which may result in long-term neurological deficits. Some of these changes have been observed solely through long term evaluation of childhood behaviours, as is seen in the Boston Circulatory Arrest Trial. Aortic arch surgical repair surgery is performed in acynotic patients with conditions like Hypoplastic Aortic Arch or Interrupted Aortic Arch, or in cyanotic patients for diagnosis such as Hypoplastic Left Heart Syndrome, where a single ventricle repair is undertaken by means of a Norwood type operation. Aortic arch surgery is performed with the aid of cardiopulmonary bypass and requires stopping the body's circulation for the time employed to repair the aortic arch. In order to provide protection to the body and to the brain, the patient's temperature is reduced to 18-25C to minimise metabolic requirements. This is particularly important for the brain so circulation can be stopped while operating on the aortic arch in a bloodless field. In recent years, techniques aimed at preserving brain perfusion during circulatory arrest have been developed. Antegrade cerebral perfusion of cold blood (18-25C) via the right internal carotid artery has been successfully employed and it is routinely used at Alder Hey Children's Hospital. Nevertheless, target brain temperature, the rate of cooling, perfusion pressure and PCO2 in arterial blood significantly influence cerebral vascular reactivity and perfusion. The optimal temperature to ensure neuroprotection is currently unknown and the temperature for cooling is based on surgeon preference rather than solid evidence. Currently, clinical practice in the UK is to employ near infrared spectroscopy (NIRS) as an index of cerebral perfusion. Whilst this is a simple and non-invasive tool, it only measures oxygen levels in the frontal lobes, and is purely indicative of oxygenation, not absorption of oxygen by the cerebral tissue. An injured brain will continue to show normal levels of saturation on NIRS despite hypoxia (lack of oxygen) induced injury. Novel imaging techniques are now available that can provide more in-depth information on real time cerebral perfusion and quantify the magnitude of changes during and following surgery in children. One research study has employed Transcranial Doppler to monitor cerebral velocity during aortic arch surgery whilst on cardiopulmonary bypass with the aim of maintaining cerebral velocity during surgery at a level similar to before cooling. More recently, duplex ultrasound to image the cerebral vessels through the fontanelle has been employed to obtain perfusion data from both hemispheres of the brain during aortic arch surgery. Taken together, these measurement techniques provide unique novel insight into the cerebrovascular changes which occur during the surgical and post-operative period.