Biomedical subjects

R H Arnott

Publications and source records attributed to R H Arnott.

5 recordsLinked to original sources

PubMed2004

Migrating from target-controlled infusion to closed-loop control in general anaesthesia.

The target-controlled infusion (TCI) technique has been successfully and commercially used in clinical general anaesthesia with the intravenous anaesthetic agent propofol. The technique is based on a population pharmacokinetic model and is an open-loop control system. Closed-loop control requires a reliable and consistent signal for feedback utilisation. With all anaesthetic agents the somatosensory evoked potentials (SEP) have been shown to give increased latency as anaesthetic depth is increased. Using infusion rate and SEP response data from rats anaesthetised with propofol a mathematical model was derived to describe the anaesthetic process. This model was used as a design reference to develop a proportional integral (PI) closed-loop control system using SEP as the feedback measure. A serials of 10 trials were conducted to investigate the difference between continuous bolus injection and infusion, all under closed-loop control. The trials showed that the use of SEPs in closed-loop control of anaesthesia is feasible.

Anesthesia, General↗

PubMed2003

Morphology of physiologically characterised ventral cochlear nucleus stellate cells.

Stellate cells within the ventral cochlear nucleus (VCN) are a diverse cell group that have been classified according to their size and morphology. Some of these stellate cell types constitute major projection neurones into the brainstem and directly into the inferior colliculus, while others are implicated in more local processing. It is still not clear whether a specific physiological profile is uniquely associated with each distinct type of stellate cell. To investigate such associations, we have analysed 23 units with a battery of physiological stimuli in vivo and then examined their shape and outputs following juxtacellular labelling with biocytin. Five physiologically identified groups of cells were filled. These formed two major response classes: onset cells and chopper cells. The two classes could be separated purely on morphological grounds. The onset cells had large somata, large symmetrical dendritic trees and profuse axonal branches that were restricted to the cochlear nucleus on one (On-L) or both sides (On-C) of the brainstem. The chopper cells had smaller, asymmetric, dendritic trees, which were either planar or marginal, had smaller somata and an output axon that left via the trapezoid body. We have confirmed profuse projections into the dorsal cochlear nucleus from all onset cells, and more focal projections from some members of all three groups of chopper cells.

Acoustic Stimulation↗

PubMed2001

The ability of inferior colliculus neurons to signal differences in interaural delay.

Sound localization in humans depends largely on interaural time delay (ITD). The ability to discriminate differences in ITD is highly accurate. ITD discrimination (Delta ITD) thresholds, under some circumstances, are as low as 10-20 micros. It has been assumed that thresholds this low could only be obtained if the outputs from many neurons were combined. Here we use Receiver Operating Characteristic analysis to compute neuronal Delta ITD thresholds from 53 cells in the inferior colliculus in guinea pigs. The Delta ITD thresholds of single neurons range from several hundreds of micros down to 20-30 micros. The lowest single-cell thresholds are comparable to human thresholds determined with similar stimuli. This finding suggests that the highly accurate sound localization of human observers is consistent with the resolution of single cells and need not reflect the combined activity of many neurons.

Animals↗

PubMed2000

Somatosensory evoked potentials for closed-loop control of anaesthetic depth using propofol in the urethane-anaesthetized rat.

Primary somatosensory cortical mass responses have been shown to exhibit dose-dependent changes in latency when general anaesthetics are administered. Here we describe a system in which the latency of evoked responses was measured automatically in real time in five animals. Latency changes were used to operate a closed-loop control of propofol delivery by intravenous infusion. The system attempted to induce and maintain a 1 ms increase in evoked response latency; this was reversed when infusion was discontinued. Allowing for the rapid and large biological fluctuations in the evoked response, this was achieved successfully. The system maintained a mean increase in latency of 1.27 (SD 0.42) ms. The mean statistical dispersion index of data obtained during the controlled period was 1.23 (0.3); in an ideal controllable system it approximates to 1. Such a system may provide a means for the automatic delivery of anaesthetics.

Anesthesia, Closed-Circuit↗

PubMed1999

The effect of etomidate on sensory transmission in the dorsal column pathway in the urethane-anaesthetized rat.

The effect of Etomidate, a general anaesthetic, on sensory afferent transmission was measured in the dorsal column pathway in urethane-anaesthetized rats. Extracellular recordings were made of peripherally evoked responses by single cells in the cuneate nucleus, ventroposterolateral nucleus of the thalamus and laminae IV-VI of the primary somatosensory cortex. Cortical mass responses were also recorded. In further experiments, cortical mass responses were evoked antidromically by stimulation in the pyramidal tract. The effect of incremental administration of Etomidate on evoked responses was recorded. These results are compared with the previously reported effects of urethane, a 'conventional' anaesthetic. Etomidate did not alter cuneate or ventroposterolateral thalamic cell responses but it caused a dose-dependent reduction in cortical cell responsiveness. It failed to alter antidromically evoked cortical mass responses. Etomidate differs from the majority of anaesthetics, which act in the thalamus, and appears to cause perturbation at the cortical level.

Anesthesia↗