Somatosensory evoked potentials in intracranial hypertension: analysis of the effects of hypoxia

Abstract

The loss of somatosensory evoked potentials (SSEP's) was investigated in a feline model of intracranial hypertension. Threshold values of cerebral perfusion pressure (CPP) and cerebral blood flow (CBF) required for maintenance of SSEP's are defined using a mathematical model. The model describes loss of amplitude of SSEP's using the form of a dose-response curve. Amplitude of the SSEP's declined to 50% of control values at a CBF of 15 ml/100 gm/min and a CPP of 20 mm Hg in the normoxic animal; in the presence of mild hypoxia (8 to 9 kPa), a significant increase in these values to 18 ml/100 gm/min and 32 mm Hg, respectively, occurred. No reliable changes in latency or central conduction time were demonstrated. It is concluded that given adequate oxygenation, evoked electrical activity is lost at too low a level of CPP for this parameter to be useful in clinical monitoring. However, even mild hypoxia, when combined with intracranial hypertension, produces a major risk to neuronal integrity.

Electrophysiologic Monitoring in the Intensive Care Unit

“…Eidridge et al 64 evaluated the use of cortical SEPs to monitor cerebral perfusion pressure in patients with raised intracranial pressure. They found the SEP was not abolished until the cerebral perfusion pressure was as low as 25 mm Hg, which is far below the generally accepted level of 40-60 mm Hg.…”