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C J Pomfrett

Publications and source records attributed to C J Pomfrett.

14 recordsLinked to original sources
PubMed2000

3D simulation of EIT for monitoring impedance variations within the human head.

A preliminary analysis is presented concerning the use of EIT for detecting impedance inhomogeneities within the human brain. The work to date is centred around the monitoring of two distinct impedance variations: those associated with the application of a carotid clamp during surgery and changes caused by the redistribution of blood flow during auditory stimuli. Using the commercially available Ansoft Maxwell package, a 3D finite element model of the human head has been developed to solve the forward problem. The model is hemispherical in shape and comprises regions of brain, cerebrospinal fluid, skull and skin and includes 16 scalp electrodes each of area 1 cm2. Results from simulations using the model suggest that an EIT system, incorporating diametric current excitation, would require a voltage measurement sensitivity of 100-120 dB in order to detect the impedance variations in the above cases.

Acoustic Stimulation↗
PubMed1999

Functional brain imaging during anesthesia in humans: effects of halothane on global and regional cerebral glucose metabolism.

BACKGROUND: Propofol and isoflurane anesthesia were studied previously with functional brain imaging in humans to begin identifying key brain areas involved with mediating anesthetic-induced unconsciousness. The authors describe an additional positron emission tomography study of halothane's in vivo cerebral metabolic effects. METHODS: Five male volunteers each underwent two positron emission tomography scans. One scan assessed awake-baseline metabolism, and the other scan assessed metabolism during halothane anesthesia titrated to the point of unresponsiveness (mean +/- SD, expired = 0.7+/-0.2%). Scans were obtained using a GE2048 scanner and the F-18 fluorodeoxyglucose technique. Regions of interest were analyzed for changes in both absolute and relative glucose metabolism. In addition, relative changes in metabolism were evaluated using statistical parametric mapping. RESULTS: Awake whole-brain metabolism averaged 6.3+/-1.2 mg x 100 g(-1) x min(-1) (mean +/- SD). Halothane reduced metabolism 40+/-9% to 3.7+/-0.6 mg x 100 g(-1) x min(-1) (P< or =0.005). Regional metabolism did not increase in any brain areas for any volunteer. The statistical parametric mapping analysis revealed significantly less relative metabolism in the basal forebrain, thalamus, limbic system, cerebellum, and occiput during halothane anesthesia. CONCLUSIONS: Halothane caused a global whole-brain metabolic reduction with significant shifts in regional metabolism. Comparisons with previous studies reveal similar absolute and relative metabolic effects for halothane and isoflurane. Propofol, however, was associated with larger absolute metabolic reductions, suppression of relative cortical metabolism more than either inhalational agent, and significantly less suppression of relative basal ganglia and midbrain metabolism.

Anesthesia, General↗
PubMed1998

Respiratory sinus arrhythmia and clinical signs of anaesthesia in children.

We have investigated changes in respiratory sinus arrhythmia (RSA) and compared these with clinical signs of anaesthesia in children. Children aged 3-10 yr were anaesthetized by gaseous induction with halothane and nitrous oxide. Multiple heart rate variability (HRV) spectra were obtained by power spectral analysis of continuous epochs of time from before introduction of halothane (baseline) until the pupils were central and fixed (stage 3). Measurement of RSA was performed by integration of the area under the spectral curve within the range of the respiratory frequency +/- 0.15 Hz. In all patients RSA decreased continuously during induction unless stimulation occurred with insertion of an airway. Values of RSA were compared at three times: baseline, loss of pharyngeal tone and stage 3. The decrease in RSA from baseline to loss of pharyngeal tone and from loss of pharyngeal tone to stage 3 was significant (P = 0.003 and P = 0.018, respectively). These results show that RSA can be related to the clinical signs of anaesthesia and has potential as a measure of depth of anaesthesia in children.

Anesthesia, Inhalation↗
PubMed1994

Respiratory sinus arrhythmia: comparison with EEG indices during isoflurane anaesthesia at 0.65 and 1.2 MAC.

Respiratory sinus arrhythmia (RSA) is a cyclical variation in heart rate during breathing, where the heart rate increases during inspiration and decreases during expiration. RSA and the electroencephalogram (EEG) were monitored in 10 patients undergoing elective surgery with isoflurane and nitrous oxide in oxygen anaesthesia after induction with propofol. All patients were subject to controlled ventilation and recovery from competitive neuromuscular block was facilitated by neostigmine and glycopyrronium (seven patients) or atropine (three patients). Median and spectral edge (95%) frequencies of the raw EEG were derived off-line. RSA and EEG indices were obtained during preinduction (baseline), induction, incision, 0.65 and 1.2 MAC of isoflurane maintenance during surgery and recovery. Significant decreases in the level of RSA, median and spectral edge frequencies were observed during induction and significant increases in all indices were observed at recovery in all patients. Significant decreases in the median and spectral edge EEG frequencies occurred in patients treated with atropine both to counteract bradycardia after propofol induction and at antagonism of neuromuscular block (n = 3), compared with patients treated with glycopyrronium (n = 7). In contrast, the level of RSA did not decrease significantly with atropine. It is concluded that measurements of RSA could form the basis of a useful index of anaesthetic depth during isoflurane anaesthesia, even during the use of pharmacologically appropriate doses of atropine. However, any effects of atropine on the raw EEG and on indices derived from the EEG, should be characterized further so that these effects are not confused with changes in anaesthetic depth.

Adult↗
PubMed1993

Respiratory sinus arrhythmia: an index of light anaesthesia.

This study was designed to test if changes in the degree of respiratory sinus arrhythmia (RSA) can be used as an index of light anaesthesia. An on-line, real-time data logging system was used to record simultaneously the EEG and ECG waveforms from 10 patients undergoing routine surgery using i.v. propofol (Diprivan) anaesthesia. The degree of RSA was determined in real-time. The median frequency of the EEG was derived off-line and correlated with the degree of RSA. Time series analysis was performed on the derived indices off-line. Significant changes in the degree of RSA occurred in response to changes in propofol infusion in all patients; these changes corresponded also to changes in the median frequency of the EEG. Heart rate and ventilatory frequency are measured routinely during surgery and it is suggested that on-line monitoring of RSA, derived from these standard signals, provides a more convenient and objective index of lightening anaesthesia than either EEG analysis or classical estimates of anaesthetic depth based on arterial pressure or heart rate.

Adult↗
PubMed1993

Respiratory sinus arrhythmia: a new, objective sedation score.

We tested if microcomputer-based measurements of heart rate variability and respiratory sinus arrhythmia (RSA) could be used as the basis of an objective sedation score. Measurements were obtained in eight ICU patients before, during and after physiotherapy. Patients were sedated with propofol and alfentanil and paralysed with atracurium. Mean ECG R-R interval showed little variation, changing from 646.15 (SD 203.15) ms to 596.08 (181.75) ms and 633.98 (184.53) ms before, during and after physiotherapy, respectively (not significant). However, the degree of respiratory sinus arrhythmia, determined using circular statistical analysis, increased significantly, from 0.14 (0.11) to 0.24 (0.15), during physiotherapy and returned to control after physiotherapy (P < 0.05). Changes in respiratory sinus arrhythmia may provide an objective measurement of sedation in ICU patients and could form the basis of a simple sedation scoring system.

Adult↗
PubMed1992

Effect of physiotherapy on the auditory evoked response of paralysed, sedated patients in the intensive care unit.

Auditory evoked response (AER) was recorded before, during and after physiotherapy in 11 paralysed (atracurium 0.56 (SD) 0.13 mg kg-1 h-1), sedated (propofol 2.2 (1.0) mg kg-1 h-1; fentanyl 4.4 (2.3) micrograms kg-1 h-1) and critically ill patients undergoing ventilation in the intensive care unit (ICU). The latency of the negative wave, NB, was reduced by physiotherapy (mean 44.8 (SD) 7.9 ms before, 41.0 (6.8) ms during (P less than 0.01, non-parametric Friedman test) and 45.6 (6.3) ms after physiotherapy); NB amplitude showed no consistent change (-0.81 (1.4) microV, -0.81 (1.5) microV and -0.71 (1.3) microV, respectively). NB latency responded to patient arousal at constant levels of sedation and this requires further evaluation as a means of monitoring sedation in paralysed patients in the ICU.

Adult↗
PubMed1991

Interocular mismatch in spatial frequency and directionality characteristics of striate cortical neurones.

Spatial-frequency dependence of directional tuning and directional bias was compared, for both eyes, in four previously established discrete classes of binocular feline striate cortical neurones. Two classes (respectively direction-selective or bidirectional at optimal spatial frequency) were directionality invariant at all spatial frequencies. In the remaining two classes, both direction-biased at optimal spatial frequency, directional bias either altered or reversed with change in spatial frequency. In all four classes, the directional tuning of a majority of neurones sharpened at high spatial frequency through either eye, although the bandpass characteristics were sometimes dissimilar for the two eyes. All neurones were of the same type through either eye. Amongst the two classes of direction-biased neurones, the strength of bias was commonly different through the two eyes. Where reversal of bias occurred, that reversal took place at different spatial frequencies for each eye. Thus, the direction and orientation preferences of cortical neurones are fixed at optimal spatial frequency, but their envelope of tuning to a gamut of spatial frequencies is not. These differences are potentially related to binocular coding of visual perspective, including dynamic object rotation in visual space.

Animals↗
PubMed1990

Directionality of cat striate cortical neurones: contribution of suppression.

Direction-selective or direction-biased striate cortical neurones were assessed for absence or incidence of suppression of firing, maximal at 90 degrees or 180 degrees ("null" suppression) to the optimal direction, in 327 neurones recorded from the striate cortex of cats anaesthetized with N2O/O2/halothane. Stimuli were light or dark bars moving over uniform or stationary textured backgrounds; or square-wave gratings of optimal spatial frequency and velocity. Five identified directionality groups were correlated with neuronal class and a range of other receptive field properties. Suppression maximal at 90 degrees to optimum was common amongst direction-biased neurones, rare amongst direction-selective neurones. In the latter group, null suppression (maximal at 180 degrees to optimum) was more prevalent than at 90 degrees. Standard complex cells constituted the majority of complex neurones. They were more commonly direction-biased and less commonly showed suppression than special complex cells. The latter comprised the majority of direction-selective neurones with 180 degrees suppression. Endstopping was seen more frequently in special complex cells, but for each functional class was similarly distributed between the different directionality groups. Based on the mean and mode of partially overlapping distributions, for all neuronal classes direction-selective neurones were more broadly tuned than direction-biased neurones. Special complex neurones were appreciably more broadly tuned than standard complex neurones; those with suppression at 180 degrees were the most broadly tuned neurones in the cortex. Direction-biased neurones with suppression at 90 degrees to optimum were more sharply tuned than those lacking such suppression. Direction-selective neurones had larger receptive fields than direction-biased neurones.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Influence of spatial frequency on tuning and bias for orientation and direction in the cat's striate cortex.

Directionality, orientation and spatial frequency tuning were determined for 108 neurones recorded extracellularly from the striate cortex of anaesthetized cats. Significant sharpening of orientation selectivity with increasing spatial frequency was seen in all simple neurones and the overwhelming majority of complex neurones. Orientation selectivity sharpened in 90 and broadened in only 10 of 100 fully characterized neurones. At least four distinct classes of neurone could be characterized on the basis of their directionality at optimal spatial frequency, and the presence or absence of changes in directionality over a range of spatial frequencies: in two classes, directionality was spatial-frequency dependent; in the remaining two it was invariant. With two exceptions Type A neurones (23 cells) were direction-selective; they were narrowly tuned for orientation and spatial frequency, and their directionality was invariant with spatial-frequency. The majority of neurones (52 cells) were Type B, most of which were direction-biased; their bias for direction varied systematically with spatial frequency. Type C were direction-biased and spatial-frequency selective (5 cells), but showed a clear reversal of bias with change in spatial frequency. Type D, a subset of direction-biased cells, were bidirectional and spatial-frequency invariant (8 cells), with comparable response strengths to motion in two opposing directions at all spatial frequencies. These response types crossed traditional boundaries between categories of simple and complex neurones, assigned on the basis of spatial summation, presence or absence of end-inhibition, and receptive field size.

Animals↗
PubMed1989

Directional and orientational tuning of feline striate cortical neurones: correlation with neuronal class.

In a subset of 327 simple and complex cells from the striate cortex of cats anaesthetized with N2O/O2/halothane, a range of receptive field properties were compared. These included directional and orientational selectivity, tuning and symmetry; endstopping; receptive field dimensions; length summation; texture sensitivity; ocular dominance; and resting discharge levels. These properties were related to neuronal class (simple or complex) and to the special, intermediate and standard subdivisions of the complex cell category. Special complex cells showed a high incidence of direction selectivity, were less sharply tuned for orientation, more commonly endstopped, more strongly binocular, tended to have higher resting discharge levels and exhibited greater sensitivity to motion of randomly textured patterns than the other classes of neurones. The remaining classes of complex cells, together with simple cells, were more commonly direction-biased or bidirectional, and more selective for orientation than special complex cells. Standard complex cells were marginally more symmetrically tuned for orientation than the other groups. Simple cells represented the most sharply orientation tuned neurones in the cortex; unlike complex cells of all groups they were insensitive to texture motion, generally had lower levels of maintained discharge, and showed least integration of inputs between the two eyes. Assessed by appropriate measures (minimum response fields in special complex cells; length summation in standard complex cells), standard complex cells had significantly larger receptive fields than special complex cells.

Animals↗
PubMed1989

Neural motion after-effects in the cat's striate cortex: orientation selectivity.

Single striate cortical neurones were recorded from adult cats, lightly anaesthetized with N2O/O2/halothane. The receptive fields for the dominant eye were subjected to direction-specific adaptation by a square-wave grating of optimal spatial frequency and velocity, drifting continuously in each neurone's preferred direction. Recovery of the neural motion after-effect induced by prior adaptation was assessed with the same grating pattern which now moved alternately in the preferred and opposite directions. In controls the same tests for recovery followed a period of exposure to a uniform field of identical luminance to the adapting grating. Three sets of measurements were made to establish whether the adaptation was orientation- as well as direction-specific. In the first, test grating orientation was maintained constant and optimal for each neurone whilst adapting orientation was systematically varied. In the second, test orientation was varied whilst maintaining adapting orientation constant. In the third set, adapting and test orientations were initially fixed at each neurone's optimum; they were next set, non-optimally to one side of the optimum. Results from the latter configuration were compared with similar tests in which the test grating remained at that non-optimal orientation whilst the orientation of the adapting grating was now altered to a new point on the other flank of each neurone's orientation tuning curve that was matched for strength of adaptation. Thus the degree of adaptation was identical in each case, but zero orientation difference between adapting and test gratings in one case was contrasted with a substantial orientation difference in the other. The results from all three sets of data were unequivocal: in simple neurones, and in standard and intermediate classes of complex neurones, but not in special complex neurones, the sequential effects of adapting gratings on the responses and sensitivity to subsequently presented test gratings were maximal when their orientations were matched and optimal for each neurone, less marked when orientations were matched but non-optimal. In conclusion, adaptation induced by pattern motion was orientation- as well as direction-specific only in standard (length summating) and intermediate complex neurones, and in simple cells; in special complex neurones it was not.

Adaptation, Ocular↗