Evoked and event related potentials in chronic respiratory failure

OBJECTIVE: To assess the value of brainstem auditory evoked potentials and event related evoked potential (3rd positive component of evoked related potentials with latency of 300 millisecond, in evaluating cognitive dysfunction in patients with chronic respiratory failure.

METHODS: Thirty-two patients with chronic obstructive pulmonary disease and respiratory failure of mild to moderate severity, were assessed regarding their mental function, utilizing mini-mental state examination, arterial blood gases including PH, partial pressure of carbon dioxide, partial pressure of oxygen, and both brainstem auditory evoked potentials and event related evoked potential response. Twenty-five normal subjects, matched for age and sex, were also studied as a control group. The study was carried out during the year 1999 to 2000 in 3 hospitals; King Khalid University Hospital, King AbdulAziz University Hospital and Sahara Hospital, Riyadh, Kingdom of Saudi Arabia.

RESULTS: There were significant delay of event related evoked potential response in patients compared with controls (P<0.05). No significant difference was noted for brainstem auditory evoked potentials and mini-mental state examination scores were within normal limits in 78% of patients. When event related evoked potential were analyzed in comparison with blood gases and mini-mental state examination, there was a clear moderate correlation with severity of hypoxemia (r = -0.697). Correlation was also noted, but to a lesser degree with partial pressure of carbon dioxide (r = 0.52) and PH (r = 0.53). There was no correlation with mini-mental state examination.

CONCLUSION: The significant delay of event related evoked potential, which is considered the neuro-physiological correlate of cognition, points clearly to the presence of a certain degree of mental dysfunction in many of these patients, namely sub-clinical encephalopathy. These subtle changes commonly evade detection by conventional bed side test (mini-mental state examination), while detailed neuropsychological assessment is cumbersome and time consuming. So, event related evoked potential measurement may be an objective and practical test of subtle cognitive dysfunction in mild respiratory failure. Unfortunately, absolute event related evoked potential values may not be useful in individual patients, in view of its wide range. However, it is probably very helpful in the assessment of a group of subjects, such as trials of a new therapeutic modality. A follow-up study utilizing a larger group of patients, and formal neuropsychological mental assessment, will be expected to confirm and expand the present study’s conclusions.