Use of EEG digital filtering and display for HPNS diagnosis

Divers show symptoms of a condition known as the high pressure nervous syndrome (HPNS) at depths greater than about 180 m. A symptom that possibly lends itself to an objective diagnosis is a change in the frequency content of the EEG signal with increased slow wave (theta, 4-8 Hz) and decreased fast wave (alpha, 8-13 Hz) activity. Therefore, a method was developed, first, to obtain the power in the EEG frequency bands by digital filtering and, second, to display band-indices and ratios during a dive, in a format suitable for rapid appraisal. The formulas for the cascaded 2nd order Butterworth bandpass filters are given together with the corresponding FORTRAN program. EEG signals from 3 subjects taped during the Duke University Atlantis IV dive to a depth of 650 m were analyzed and displayed by the above method as well as by a Fast Fourier Transfer signal analyzer. The results suggest the theta: alpha ratio, which is a one-parameter EEG evaluation in the frequency domain, is a valuable descriptor for the objective HPNS diagnosis.