Quantifying cerebral autoregulation in health and disease.
OBJECTIVE
To review methods of quantifying human cerebral autoregulation in health and disease.


METHODS
Articles and published abstracts on methods to quantify cerebral autoregulation in health and disease.


RESULTS
Cerebral autoregulation is defined as the relationship between cerebral blood flow and cerebral perfusion pressure. Complex neurohumoral processes are involved in myogenic and metabolic mechanisms to maintain cerebral blood flow at a constant level in the presence of fluctuating systemic and cerebral perfusion pressures. Despite advances in physiological measurement, there is no standard measurement of cerebral blood flow and quantifying cerebral autoregulation remains problematic. Clinical monitors such as transcranial Doppler and jugular bulb oximetry have high levels of error with poor specificity and sensitivity. Cerebral autoregulation is impaired in traumatic brain injury and subarachnoid haemorrhage, so that cerebral blood flow becomes pressure-passive. Hypotension is associated with significant secondary neuronal damage following traumatic brain injury. Hypertensive emergencies represent failure of the upper autoregulatory threshold, often with devastating neurological consequences. The monitoring and treatment of autoregulatory failure remains limited and is essentially directed at maintaining an appropriate systemic blood pressure. Consequently, the use of strategies to manipulate cerebral perfusion requires care and circumspection.


CONCLUSIONS
Cerebral autoregulation is impaired with brain injury with cerebral blood flow often becoming pressure-passive. The monitoring and treatment of autoregulatory failure is limited and usually directed at maintaining systemic blood pressure with the effectiveness of this strategy often being unknown.
Department of Intensive Care Medicine, The St George Hospital, Sydney, NSW, Australia. j.myburgh@unsw.edu.au
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Myburgh