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

Electrophysiologic Monitoring in the Intensive Care Unit

Electroencephalography (EEG) and evoked potential studies are established monitoring tools in the neurological intensive care unit (ICU). These neurophysiologic techniques provide information on physiological state and response to therapy, and may aid diagnosis and prognosis. Serial studies or continuous monitoring may enable changes to be detected prior to irreversible deterioration in the patient's condition. Current computer technology allows simultaneous display and correlation of electrophysiologic parameters, cardiovascular state and intracranial pressure (ICP). Continuous EEG monitoring in the ICU has been shown to have a decisive or contributing impact on medical decision making in more than three-quarters of patients. In addition, continuous EEG monitoring has revealed previously unsuspected non-convulsive seizures in one-third of patients. SEPs and BAEPs can provide useful prognostic information in coma - however, these tests are etiologically nonspecific and must be carefully integrated into the clinical situation. Motor evoked potentials offer a potentially useful tool for evaluating motor system abnormalities in the ICU.

Evaluation and prognostication in coma

Electroencephalography (EEG) and evoked potential (EP) studies are neurophysiologic techniques which provide information on physiological state and response to therapy, and may aid diagnosis and prognosis. Serial studies or continuous monitoring may enable changes to be detected prior to irreversible deterioration in the patient's condition. Current computer technology allows simultaneous display and correlation of electrophysiologic parameters, cardiovascular state and ICP. Continuous EEG monitoring in the ICU has been shown to have a decisive or contributing impact on medical decision making in more than three-quarters of patients. In addition, continuous EEG monitoring has revealed previously unsuspected non-convulsive seizures in two-thirds of patients. Somatosensory and auditory EPs can provide useful prognostic information in coma patients, however, these tests are etiologically non-specific and must be carefully integrated into the clinical situation. Motor EPs offer a potentially useful tool for evaluating motor system abnormalities in the ICU. Thus, neurophysiologic tests are established monitoring tools in the neurological intensive care unit.

Somatosensory evoked potentials during hypoxia and hypocapnia in conscious humans

PURPOSE

The objective of the study was to evaluate the effects of moderate hypoxia and hypocapnia on the latency and amplitude of cortical somatosensory evoked potentials (SSEPs) in conscious human subjects.

METHODS

In ten volunteers the amplitude and latency of the cortical somatosensory evoked potentials were recorded during stimulation of the left posterior tibial nerve. Measurements of SSEPs and respiratory variables were made breathing ambient air, air containing a reduced oxygen percentage (17% O2, 14% O2 (n = 6) or 11% O2 (n = 10)), and again during voluntary hyperventilation breathing ambient air (PETCO2 = 20 mmHg, n = 10).

RESULTS

Hypoxia (11% O2) caused mild stimulation of ventilation (P < 0.05) but had no effects on the latency or amplitude of the SSEP. Lesser degrees of hypoxia had no effects. Hyperventilation caused a small (2-4%) decrease) in the latency of the SSEP and an increase in the amplitude of the SSEP (P < 0.05).

CONCLUSIONS

These findings in conscious subjects were consistent with previous observations in anaesthetized humans and anaesthetized dogs and show that the decrease in latency of the SSEP associated with hypocapnia is not due to changes in the depth of anaesthesia. These effects of hypocapnia may contribute to small variations in the latency of the SSEP when monitoring is performed during surgery, but are unlikely to be large enough to be of clinical concern.