Midlatency median nerve evoked responses during recovery from propofol/sufentanil total intravenous anaesthesia

[Middle latency auditory evoked potentials in critical care children: preliminary study]

BACKGROUND

Middle latency auditory evoked potentials (MLAEP) reflect changes in electroencephalogram waves after an auditory signal and represent the earliest cortical response to acoustic stimulus. They are therefore used to measure variations in the level of consciousness. MLEAP have been used to measure the depth of anesthesia during surgical procedures, but experience in critical care patients is very limited.

OBJECTIVE

To analyze the utility of MLAEP for monitoring the level of sedation in critically ill children.

METHODS

Level of consciousness was monitored through MLAEP by placing special headphones and three sensors situated in the frontal and preauricular regions. Simultaneously, the level of sedation was measured using the COMFORT scale and the Bispectral Index (BIS) in distinct clinical situations.

RESULTS

We studied six critically ill children in whom MLAEP helped us to evaluate the level of consciousness: light sedation, natural sleep, deep sedation, sedation in a paralyzed child, and brain death. MLAEP showed a good correlation with the COMFORT scale and BIS in light and deep sedation and were effective in the early detection of brain death in one patient. In the paralyzed patient, MLAEP was able to detect undersedation. In one patient, a pacemaker interfered with the MLAEP signal.

CONCLUSIONS

MLAEP can be useful in evaluating the level of consciousness and sedation in critically ill children. Further studies with larger samples are required to analyze the limitations and reproducibility of this type of monitoring in children of different ages.

Representation of somatosensory evoked potentials using discrete wavelet transform

OBJECTIVE

Somatosensory evoked potentials (SEP) have been shown to be a useful tool in monitoring of the central nervous system (CNS) during anaesthesia. SEP analysis is usually performed by an experienced human operator. For automatic analysis, appropriate parameter extraction and signal representation methods are required. The aim of this work is to evaluate the discrete wavelet transform (DWT) as such a method for an SEP representation.

METHODS

Median nerve SEP were derived in 52 female patients, scheduled for elective surgery with SEP monitoring, under clinically proven conditions in the awake state. The discrete wavelet transform implemented as the multiresolution analysis was adopted for evaluating SEP. The suitability of the wavelet coefficients was investigated by calculating the error between the averaged response and the corresponding wavelet reconstructions.

RESULTS

SEP can be represented by a very small number of wavelet coefficients. Although the individual SEP waveform has an influence on the number and selection of wavelet coefficients, in all subjects more than 84% of the SEP waveform energy can be represented by a set 16 wavelet coefficients.

CONCLUSIONS

The discrete wavelet transformation provides an efficient tool for SEP representation and parameterisation. Depending on the specific problem the DWT, can be adjusted to the desired accuracy, which is important for the subsequent development of automatic SEP analysers.

Recovery of memory after general anaesthesia: clinical findings and somatosensory evoked responses

BACKGROUND

Mid-latency somatosensory evoked responses are used to monitor the integrity of the sensory pathways intra-operatively. They can quantify the effects of anaesthetics on the central nervous system. Mid-latency auditory evoked responses have been related to cognition during anaesthesia, but there are no detailed studies using median nerve somatosensory evoked responses (MnSSER).

METHODS

We studied 49 patients during recovery from general anaesthesia (isoflurane/nitrous oxide or propofol) to assess implicit and explicit memory function in relation to mid-latency MnSSER. The MnSSER recordings were made before anaesthesia, during steady-state anaesthesia, and at the end of the recovery period. The patients were interviewed 24 h later about their memory for the immediate wake up phase. Statistical analysis was by multivariate analysis of variance.

RESULTS

Out of 49 patients, 23 recalled the recovery period, 11 had implicit memory for an object shown to them during the recovery period, and 15 did not have any memory for the recovery period. At RECOVERY the patients with recall had significantly shorter MnSSER latencies N45 and P50 and inter-wave conduction times LatN35-LatP45 than patients without memory (P<0.05).

CONCLUSIONS

We conclude that MnSSER components warrant further investigation for studying the effects of anaesthetic drugs on cognitive function.