Monitoring cerebral perfusion and oxygenation: an elusive goal.

The impetus for cerebral hemodynamic monitoring in neurotrauma first arose from the original "talk and die" studies which described the group of head injured patients "who talk and then subsequently died". At necropsy, hypoxic or ischaemic brain damage was observed in a variable proportion of patients raising the possibility that systemic or cerebral hypoxia post trauma may have contributed to the poor neurological outcome. Improved understanding of the pathophysiology of neurotrauma influenced clinical practice in two ways: a) there was a plethora of monitoring modalities developed for evaluating cerebral hemodynamics and oxygenation and b) squeezing oxygenated blood through a swollen brain became the cornerstone of therapy in patients with head injury. Whilst there appears to be some agreement on the principles of management of neurotrauma, opinion still remains divided on what provides the best assessment of cerebral perfusion and oxygenation. Although initial monitoring was largely confined to global indices of brain oxygenation, refinement in technology has made the measurement of oxygen tensions further down in the oxygen cascade at the level of the tissue possible and applicable by the bedside. Metabolic monitoring of the brain is now possible with the use of a variety of biochemical indices and with the availability of microdialysis. The purpose of this review is to examine the various modes of monitoring cerebral oxygenation, critically review the literature concerning their use in day to day intensive care practice, outline their limitations and define possible indications for their use.