Are We Fortune Tellers or Healers?*
A number of devices have been developed that seek to detect a change in a parameter that reflects a deterioration or an improvement in neurologic function. Unfortunately, their success has been hampered by a number of issues. Invasive probes are available but all suffer from one or more limitations. The parameter measured may be an uncertain surrogate for brain function (microdialysis, brain tissue oxygen tension [pBO2]) or only detect extreme dysfunction (intracranial pressure [ICP]). The field of view of many is extremely small, and the variable measured differs depending on placement relative to region of injury (pBO2, cerebral blood flow [CBF] and microdialysis), which has proven difficult to identify (2). Other noninvasive devices suffer from technical limitations that have yet to be resolved (near infrared spectroscopy [NIRS]) or the signal derived requires extensive processing and interpretation (electroencephalogram).
Recently, a different approach has sought to make use of spontaneous fluctuation in monitored variables and how they correlate with each other (3). Assessment of dynamic autoregulation correlates some index of brain function (ICP, transcranial Doppler [TCD] velocity, pBO2, NIRS) with blood pressure (BP) or perfusion pressure. The “autoregulatory index” is thought to reflect the brain’s ability to maintain a constant blood flow over a range of cerebral perfusion pressure (CPP). Cerebrovascular tone is modified by a complex cerebrovascular system that is mediated by chemical, musculature, and neuronal processes. All else being equal, cerebrovascular resistance is adjusted to keep CBF constant as CPP changes. It is critical to understand, however, that the goal of the system is to maintain adequate delivery of substrate (glucose and oxygen) to meet metabolic demands (4). Thus in addition to the changes in CPP, it must integrate metabolic rate, hemoglobin, oxygen saturation, and pH. Impaired dynamic autoregulation has been detected in traumatic brain injury (TBI), subarachnoid hemorrhage (SAH), ischemic stroke, and intracerebral hemorrhage and has been associated with poor outcome.
The definitive test of the utility of a monitoring device is establishing that it can detect a deterioration, lead to a change in treatment, and lead to improved outcome. Few, if any, devices have met this lofty standard. Recently, the utility of the most widely used device for neurologic monitoring, the ICP monitor, was questioned when a controlled trial demonstrated that equivalent outcomes could be achieved in severe TBI patients with and without the use of ICP monitors (5). Although this trial does not indicate that ICP monitoring is not useful, it does illustrate the difficulty in demonstrating that a monitoring device leads to improved outcome (6) and illustrates that demanding such proof before employing a device or system would leave us with few options.
How then should we decide if a monitoring system should be used? Two strategies have been employed: demonstrating that the monitoring system can detect a change in response to an intervention or demonstrating that it can predict outcome. The latter is the one most frequently used.