in a "reverberating flow" pattern, with forward flow during systole, and backward flow during diastole, signifying the onset of intracranial circulatory arrest. Based on these considerations, many investigators have proposed using TCD as a non-invasive monitor of ICP, and preliminary results are promising.
Autoregulation Testing Patients who suffer traumatic brain injury or SAH frequently develop impaired cerebral autoregulation, increasing the risk of brain injury with sudden changes in systemic blood pressure. Elevation of blood pressure may increase the risk of vasogenic edema, whereas decrease in blood pressure may result in cerebral ischemia. Elucidation and quantification of the state of autoregulation would facilitate clinical management of these patients. Furthermore, it has been shown that delayed ischemic deficits are more likely to develop in patients with the combination of vasospasm and impaired cerebral autoregulation, as determined by TCD.
A number of different methods have been investigated. These include:
Spontaneous relationship between blood pressure and flow velocity changes. With intact cerebral autoregulation there is a negative correlation between changes in blood pressure and change in flow velocity, and a positive correlation when autoregulation is impaired. By monitoring both blood pressure and flow velocity simultaneously over multiple time epochs, the state of cerebral autoregulation can be qualitatively determined.
Transient hyperemic response. When autoregulation is intact, compression of the extracranial internal carotid artery for 7-10 seconds will result in a transient hyperemic response in the ipsilateral middle cerebral artery.
Dynamic autoregulation. When autoregulation is intact, a transient decrease in blood pressure effected by sudden deflation of inflated thigh cuffs will cause a very brief decrease in middle cerebral artery flow velocity, rapidly returning to baseline value. Static autoregulation. Below the upper limit of autoregulation, elevation of systemic blood pressure using a vasopressor
(phenylephrine) will not affect cerebral artery flow velocity when autoregulation is intact.
Computed tomography of the brain is the most important diagnostic imaging modality in the care of the critically ill neurological patient. Patients with traumatic head injury may require daily CT during the initial course, and more often if the neurological status is fluctuating. It is important to obtain CT whenever there is sudden deterioration in the neurological status or sudden increase in ICP to rule out surgically correctable causes. Magnetic resonance (MR) imaging is more helpful in delineating infarcts and ischemic lesions, particularly with diffusion-weighted and perfusion-weighted imaging. However, it is time consuming and generally not as available as CT. Angiography is essential in the establishment of vasospasm and institution of interventional treatment.
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