N.A.M. de Beer

Information processing during cardiac surgery: an event related potential study

The aim of this study was to investigate whether information processing persists during general anesthesia, and if so, to determine the relationship between the degree of cognitive processing measured during anesthesia and the presence or absence of intraoperative memories measured after anesthesia. Subjects were 12 patients, undergoing cardiac surgery with propofol/alfentanil anesthesia. During several periods of the operation, event related potentials (ERPs) to frequent and infrequent tones of different pitch were analyzed. After the operation, a word recognition task with ERP recording was administered to determine whether intraoperatively presented words would elicit a (covert) recognition reaction in the brain. ERP wave forms could be obtained during the intraoperative recording periods but differed substantially from those in the awake state. The presence of ERP components up to 500 msec after stimulus presentation suggests that auditory information processing continued during anesthesia up to a certain level of cognition. Intraoperative ERPs to frequent and infrequent tones were not different from each other implying that differences in pitch could not be detected. The postoperative results demonstrated evidence for intraoperative memories in 3 patients. For 2 of these 3 patients, low propofol levels as well as reliable ERPs with large amplitudes were found close to the moment of information presentation. The results emphasize the importance of combining intra- and postoperative measurements and suggest that late ERP components might be used as indicators of an increased risk of auditory perception.

Customized spectral band analysis compared with conventional Fourier analysis of heart rate variability in neonates

A customized filtering technique is introduced and compared with fast Fourier transformation (FFT) for analyzing heart rate variability (HRV) in neonates from short-term recordings. FFT is classically the most commonly used spectral technique to investigate cardiovascular fluctuations. FFT requires stability of the physiological signal within a 300 s time window that is usually analyzed in adults. Preterm infants, however, show characteristics of rapidly fluctuating heart rate and blood pressure due to an immature autonomic regulation, resulting in non-stationarity of these signals. Therefore neonatal studies use (half-overlapping or moving) windows of 64 s length within a recording time of 2-5 min. The proposed filtering technique performs a filtering operation in the frequency range of interest before calculating the spectrum, which allows it to perform an analysis of shorter periods of only 42 s. The frequency bands of interest are 0.04-0.15 Hz (low frequency, LF) and 0.4-1.5 Hz (high frequency, HF). Although conventional FFT analysis as well as the proposed alternative technique result in errors in the estimation of LF power, due to spectral leakage from the very low frequencies, FFT analysis is more sensitive to this effect. The response times show comparable behavior for both the techniques. Applying both the methods to heart rate data obtained from a neonate before and after atropine administration (inducing a wide range of HRV), shows a very significant correlation between the two methods in estimating LF and HF power. We conclude that a customized filtering technique might be beneficial for analyzing HRV in neonates because it reduces the necessary time window for signal stability.

Multivariate analysis of the EEG and auditory evoked potentials for monitoring depth of anesthesia

A principal factor analysis was performed on EEG and AEP parameters, simultaneously recorded during anesthesia. The results showed that parameters from the EEG spectrum and the latencies of peaks In the AEP change with the level of anesthesia, while the amplitudes of the AEP reflect a balance between the sedative effect of anesthesia and the arousing effect of surgical stimulation. It is concluded that the EEG and AEP monitor different aspects of anesthesia.<<ETX>>