Sequential changes of auditory brain stem responses in relation to intracranial and cerebral perfusion pressure and initiation of secondary brain stem damage.

The relationship of supratentorial intracranial pressure (ICP) and cerebral perfusion pressure (CCP) with serial changes in auditory evoked brain stem responses was investigated. Eighty-one patients without primary brain stem damage admitted to our emergency unit were studied. When ICP over 50 mm Hg persisted for 4 hours, the I-V interpeak latency was significantly prolonged. The threshold of this prolongation was 8 hours for the ICP over 45 mm Hg and 24 hours for that of over 40 mm Hg. The ICP of 35-40 mm Hg for 24 hours was the border zone. CCP did not show a significant relation with I-V interpeak latency changes. The loss of wave V was observed in a wide range of the ICP (30-147 mm Hg) and CPP (0-60 mm Hg). Wave III disappeared when the ICP exceeded 50 mm Hg. Wave I became undetectable with an ICP above 50 mm Hg or a CPP below 40 mm Hg. These results indicate that an increase of ICP over 40 mm Hg definitely initiates secondary brainstem dysfunction if it lasts for more than 24 hours and that the ICP should be reduced below this level, preferably below 35 mm Hg, to maintain brain function. The fact that both low CPP and high ICP were involved in the loss of wave I clearly shows that both ischaemia and displacement of the brain stem are the important pathophysiological factors for the disappearance of wave I.