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W J Levy

Publications and source records attributed to W J Levy.

At least 19 recordsLinked to original sources
PubMed1999

The calibration and validation of a phase-modulated near-infrared cerebral oximeter.

OBJECTIVE: This study was undertaken to compare the cerebral oxygenation measured by an experimental phase-modulated near-infrared (NIR) spectroscopy system with capillary saturation estimated from jugular venous oxygen saturation. METHODS: Jugular venous catheters were placed in 30 patients undergoing carotid endarterectomy and 194 measurements of venous oxygen saturation were obtained intra operatively. Simultaneous measurement of optical path length at 754, 785, and 816 nm was performed using a phase-modulated near-infrared spectroscopy system. Optical calibration was performed using both an optical bench and a scattering mold. Hemoglobin saturation was calculated from NIR measurements using equations derived from diffusion theory. Capillary saturation was calculated from the arterial and venous saturations. RESULTS: Jugular venous saturations ranged from 41 to 92%. When calibrated using the optical bench, the NIR estimates of hemoglobin saturation deviated from estimated capillary values by an average of 2.6% bias and 4.3% deviation. No systematic bias was noted. NIR values derived from mold calibration were less accurate and precise (4.6% bias and 6.9% deviation.) Use of the initial venous sample as an in vivo calibration improved the accuracy of the mold calibration but did not alter the performance of the bench calibration. CONCLUSIONS: Under the conditions tested, an experimental phase-modulated near-infrared spectroscopy system calibrated using an optical bench agreed with capillary saturation estimated from jugular venous samples. Further work is necessary to demonstrate valid performance of the system under other conditions.

Aged↗
PubMed1999

Monitoring spinal cord motor and somatosensory evoked potentials in anesthetized primates.

Monitoring Motor Evoked Potential (MEP) to Transcranial Stimulation (TMS) monitoring (MEP) is a growing technique to assess motor function under anesthesia. The following primate study was conducted to analyze the non-myogenic spinal motor and sensory volleys and to examine their reproducibility under nitrous oxide-methohexidone anesthesia. The traveling periodic spinal descending MEP to TMS and ascending somatosensory (SEP) to posterior tibial nerve stimulation across the thoracic cord were recorded in 12 cynomolgus monkeys. Through a small T11-T12 laminotomy, an insulated stainless steel electrode was inserted into the epidural thoracic space. The potentials were analyzed under 50 vol% NO in O2 with methohexital (0.1-0.2 mg kg-1 min-1). A well-defined periodic TMS-MEPs and PTN-SEPs were recorded with high reproducibility and consistency in repeated trials under N2O-methohexital anesthesia. MEP tracing consisted of an initial peak (direct (D) wave), occurring at 2.43 (+/- 0.28) msec followed by subsequent five positive (indirect (I) waves). Spinal SEPs-MEPs were clearly defined, morphologically stable, and consistent over time under N2O-methohexitone anesthesia. The present primate study may set a model to monitor both modalities in anesthetized neurosurgical patients.

Anesthesia, General↗
PubMed1998

Relationships between cerebral blood flow velocities and arterial pressures during intra-aortic counterpulsation.

OBJECTIVE: To determine the effects of intra-aortic counterpulsation (IABP) on cerebral blood flow velocity. DESIGN: Prospective self-controlled study. SETTING: University hospital surgical intensive care unit. PARTICIPANTS: Nineteen cardiac surgical patients requiring perioperative IABP assistance. INTERVENTIONS: Simultaneous recording of transcranial Doppler middle cerebral artery blood flow velocity and arterial pressure in response to changes in the magnitude of augmentation and trigger ratio. MEASUREMENTS AND MAIN RESULTS: Instantaneous cerebral blood flow velocities correlated with arterial pressures during IABP (r = 0.650) at different magnitudes of augmentation and trigger ratios. The increase in arterial pressure during balloon inflation was associated with an increase in cerebral blood flow velocity, and the decrease in arterial pressure in response to balloon deflation was associated with a decrease in cerebral blood flow velocity that was dependent on the magnitude of augmentation. Different magnitudes of augmentation or trigger ratios had no effect on peak systolic cerebral blood flow velocity, mean cerebral blood flow velocity, mean arterial pressure, or the mean velocity-to-pressure ratio. Instantaneous cerebral blood flow velocity to arterial pressure ratios were lowest in response to balloon deflation at the time of pre-ejection. CONCLUSIONS: IABP modified the phasic profile of cerebral blood flow to reflect the arterial pressure waveform without affecting mean cerebral blood flow velocity. Peak systolic cerebral blood flow velocity was maintained in augmented beats despite the decreased systolic arterial pressure associated with afterload reduction. The acute decrease in cerebral blood flow velocity at pre-ejection was balanced by increased cerebral blood flow velocity during balloon inflation in diastole.

Adult↗
PubMed1997

The influence of demographic factors on phase-modulated spectroscopy in adults.

The increased complexity of phase-modulated spectroscopy (PMS) compared with incoherent light techniques of near-infrared spectroscopy is justified if measurement of path length is necessary. In order to assess the variability of optical transmission in the head of adults, 96 subjects of varying age, gender and skin pigmentation were studied using an experimental three-wavelength time-shared PMS device. Optical path length was measured at each wavelength, and saturation and haemoglobin-free path length were calculated. Path length varied linearly with the separation of the optical probes, but gender, age or skin pigmentation were not associated with differences in path length. Haemoglobin saturation averaged 68% and varied with age in a non-uniform fashion. Haemoglobin-free path length differed between genders, being 8% longer in women than in men. Measurements could not be performed at 5 cm due to optical attenuation in 36% of subjects. These subjects were more often young, but they were not otherwise distinguished by gender or pigmentation differences. Subjects in whom measurements could be obtained at 5 cm had longer haemoglobin-free path lengths than did subjects in whom optical attenuation prevented these measurements. These data confirm the occurrence of significant differences in optical transmission in adults and support the use of PMS techniques that measure path length.

Adult↗
PubMed1997

Partial regeneration of the sciatic nerve in rats enhances motor excitability to magnetic stimulation.

Motor-evoked potentials elicited from transcranial magnetic stimulation were recorded from both cranial tibial muscles of 10 Lewis rats. One sciatic nerve was then transected and surgically reanastomosed and allowed to regenerate for 6 weeks. Walking function was tested weekly using a footprint test giving a functional sciatic nerve index. At the end of the study motor-evoked potentials were again performed, as were nerve conduction velocities and supramaximal stimulation of operated and unoperated sciatic nerves. The nerve was evaluated for myelinated axon numbers. Functional sciatic index, numbers of myelinated axons, nerve conduction velocities, and supramaximal stimulation demonstrated the regenerated nerve to be functional, but to a lesser extent than the unoperated nerve. In all cases the amplitude of the compound muscle action potential evoked from transcranial magnetic stimulation of the motor cortex was greater in the operated motor pathway. This finding demonstrates hyperexcitability of the motor pathway to magnetic stimulation during peripheral nerve regeneration.

Animals↗
PubMed1995

Near-infrared measurement of cerebral oxygenation. Correlation with electroencephalographic ischemia during ventricular fibrillation.

BACKGROUND: The application of phase-modulated near-infrared techniques for measurement of the oxygen saturation of cerebral tissue requires both validation by conventional measures of cerebral oxygenation and determination of normal and abnormal values. This study was undertaken to validate phase-modulated near-infrared measurements of cerebral oxygen saturation by comparing them with electroencephalographic evidence of cerebral ischemia during implantation of cardioverting defibrillators. This comparison also yields an estimate of the ischemic threshold as measured with near-infrared techniques. METHODS: Electroencephalograms and near-infrared measurements were performed during 85 episodes of ventricular fibrillation in ten patients. Light at 754, 785, and 816 nm was modulated at 200 MHz, transmitted through the skull, and the path lengths of the reflected light were determined by measuring the phase shifts. The electroencephalogram was inspected for changes during the hypotension associated with the arrhythmia and the oxygen saturation was calculated from the near-infrared path lengths. Changes in oxygen saturation were then compared with electroencephalographic evidence of cerebral ischemia. RESULTS: The mean saturation before fibrillation was 56.5% +/- 1.2 (SEM). In 40 (47%) of the events, electroencephalographic evidence of ischemia was observed. Such changes were related to the minimum saturation observed during ventricular fibrillation (44% +/- 2.5 vs. 56% +/- 1.9 in the absence of electroencephalographic changes; P < 0.001). The ischemic threshold was estimated to be 47% saturation. The sensitivity of this technique was estimated to be 0.6, the specificity 0.84, and the predictive accuracy 0.73. CONCLUSIONS: Near-infrared measurements reflect changes in cerebral oxygenation as indicated by electroencephalographic evidence of cerebral ischemia.

Aged↗
PubMed1993

Cardiovascular stability with rapid intravenous infusion of ondansetron.

The acute cardiovascular effects of rapid iv administration of the antiemetic ondansetron, a selective serotonin (5-HT3) receptor antagonist were determined in a randomized, blinded, placebo-controlled study. Measurements of heart rate, blood pressure, oxygen saturation and respiratory rate were made preoperatively over a five-minute period which followed a two-minute infusion of the medication. Intraoperative and postoperative data were not collected. None of the variables recorded changed significantly during the infusion or in the observation period which followed. Within the limitations of this study, we detected no cardiovascular change in the five minutes between the end of the drug infusion and the induction of anaesthesia.

Adult↗
PubMed1993

The use of a cap-shaped coil for transcranial magnetic stimulation of the motor cortex.

A cap-shaped coil is introduced as a superior design for inducing transcranial magnetic motor evoked potentials for spinal cord monitoring. Evaluation of the magnetic characteristics of the cap coil showed higher induced electrical fields at and below the depth of the cortical surface, compared to a 9-cm, butterfly-shaped coil. Twenty normal adults were stimulated with the cap coil and a 9-cm round coil in three positions. Compound muscle action potentials were recorded from the left and right abductor digiti minimi and anterior tibialis muscles. The cap coil induced potentials with higher intensities and lower variability between consecutive stimuli. The cap coil was also more able to simultaneously induce motor evoked potentials from the four muscles studied. This coil design should provide superior means of inducing transcranial magnetic motor evoked potentials in multiple muscles.

Arm↗
PubMed1992

Comparison of desflurane with propofol in outpatients undergoing peripheral orthopedic surgery.

This study was undertaken to compare desflurane with propofol anesthesia in outpatients undergoing peripheral orthopedic surgery. Data were combined from two institutions participating in a multicenter study. Ninety-one patients, ASA physical status I or II, were each randomly assigned to one of four groups. After administration of fentanyl (2 micrograms/kg) and d-tubocurarine (3 mg), intravenous propofol was administered to induce anesthesia in groups I and II and desflurane in groups III and IV. Maintenance was provided by desflurane/N2O in groups I and III, propofol/N2O in group II, and desflurane/O2 in group IV. Emergence and recovery variables, psychometric test results, and side effects were recorded by observers unaware of the experimental treatment. Patients in group II experienced less nausea than other groups (P = 0.002) despite this group having required more intraoperative fentanyl supplementation than groups III and IV (P = 0.01). Time to emergence, discharge, and psychometric test results were similar in all groups. Desflurane appears to be comparable with propofol as an outpatient anesthetic, facilitating rapid recovery and discharge home.

Adult↗
PubMed1991

The effect of an anesthetic induction dose of midazolam on motor potentials evoked by transcranial magnetic stimulation in the monkey.

The effect of a hypnotic dose (0.5 mg/kg) of midazolam (MDZ) on motor evoked potentials (MEPs) was examined in 12 monkeys. MEPs were elicited by transcranial magnetic stimulation (TMS) and the resultant potentials recorded from abductor pollicis brevis (APB) and anterior tibialis (AT) muscles contralateral to the stimulation site. After administration of MDZ, sequential MEP recordings were obtained at postinduction, hypnosis, awakening, emergence, and recovery periods. The results were compared with control values using one-way analysis of variance and Tukey's post-hoc test. Under hypnosis, MEP reproducibility was problematic as the potentials were occasionally ill identified and questionable. MDZ resulted in marked MEP scalp field reduction, coil demography alteration, stimulation threshold elevation, and amplitude suppression (p <0.01). Latency response was unaltered. During hypnosis, awakening, and recovery periods, the mean APB and AT thresholds were elevated by 39, 23, and 0% and by 60, 34, and 4% respectively; while APB and AT amplitudes were depressed by 95, 86, and 53% and by 99, 91, and 60%, respectively. We conclude that an induction dose of MDZ can produce profound and prolonged attenuation of TMS MEPs. The drug inhibitory effect on MEPs may persist after recovery. Anesthetic doses of MDZ should cautiously be used in the settings of MEP monitoring.

Journal Article↗
PubMed1991

The effect of neuroleptanalgesia (droperiodol-fentanyl) on motor potentials evoked by transcranial magnetic stimulation in the monkey.

Motor evoked potentials (MEPs) after transcranial magnetic stimulation (TMS) have been examined in 12 monkeys under neuroleptanalgesia (NLA). Compound muscle action potentials were recorded from abductor policis brevis (APB) and gastroncnemius (GN) muscles contralateral to the stimulation site. After obtaining baseline tracings during emergence from methohexitone, 10 mg/kg i.m., NLA was induced using droperiodol, 0.3 mg/kg i.v. followed by fentanyl, 0.006 mg/kg i.v. Sequential MEP recordings were obtained 10 min after i.v. droperiodol, 2, 8, and 16 min after i.v. fentanyl, and during recovery. Replicable TMS MEPs were consistently recorded under NLA. However, droperidol and fentanyl caused significant stimulation threshold elevation, amplitude depression, and latency delay compared to control values (p <0.01). Ten minutes after droperiodol administration, the APB-GN threshold, amplitude, and latency values (mean +/- SD) were 0.81 +/- 0.2-0.84 +/- 0.1 T (baseline 0.57 +/- 0.1-0.59 +/- 0.1 T), 3.4 +/- 2.1-4.0 +/- 2.5 mV (baseline 8.0 +/- 3.7-9.0 +/- 3.7 mV), and 15.8 +/- 1.3-21.1 +/- 1.2 ms (baseline 14.9 +/- 1.2-20.1 +/- 1.3 ms), respectively. Addition of fentanyl resulted in further response deterioration. Two minutes after fentanyl injection, the APB-GN threshold, amplitude, and latency values were 0.88 +/- 0.18-0.95 %% 0.15 T, 2.1 +/- 1.7-2.0 +/- 2.1 mV, and 16.0 +/- 1.4-21.9 +/- 1.3 ms, respectively. Subsequent MEPs revealed gradual response improvement but, in contrast to baseline, remained markedly altered (p <0.05). During the recovery period (53 +/- 6 min), the APB-GN threshold, amplitude, and latency measurements were 0.66 +/- 0.1-0.77 +/- 0.2 T, 4.4 +/- 3.1-4.2 +/- 2.9 mV, 15.5 %% 1.4-20.9 +/- 1.7 ms, respectively. We conclude that, in a primate model, NLA maintains measurable TMS MEPs. Nevertheless, droperiodol and fentanyl produce significant and prolonged response alterations. Knowledge of these changes, while administering NLA drugs intraoperatively, is essential to interpretation of MEP data.

Journal Article↗
PubMed1991

Cerebellar evoked potentials and motor evoked potentials in the spinal cord of rats.

In rats, the cerebellar evoked potentials (CEPs) were monitored along the spinal cord following the electrical stimulation of cerebellar cortex. The CEPs monitored at thoracic and lumbar cord consisted of from 1 to 3 waves with conduction velocities of 16-20.28 m/sec. The amplitudes of these waves declined dramatically as the frequency of the stimulation increased above 50 Hz, indicating that the CEPs were either evoked or conducted polysynaptically. The pathway conducting CEPs were studied using acute spinal cord lesioning and intracord recording methods. The pathways conducting CEP and MEP were also compared in the same preparation in controlled spinal-cord-lesioned animals. The CEPs were mainly conducted along the ventral and lateral funiculi where the reticulospinal tracts and the vestibulospinal tracts are located. Simultaneous disappearance of both MEP and CEP after controlled spinal cord section seems to suggest the 2 different evoked potentials are conducted along the same pathways in the spinal cord of rats. Species difference in conduction pathways of MEP and CEP were compared and discussed.

Animals↗
PubMed1991

Mapping of motor cortex gyral sites non-invasively by transcranial magnetic stimulation in normal subjects and patients.

Transcranial stimulation offers the opportunity to replace a standard clinical method of invasive brain cortical mapping. A figure '8' magnetic coil with a 1-3-cm area of stimulation is combined with a new method of creating magnetic resonance imaging (MRI) scans which represent the cortical surface (mapping cuts) and thus establishing common landmarks between the MRI and the scalp for cortical mapping. This assigns gyral sights to scalp sites of transcranial stimulation. We report the development and evaluation of these techniques in 6 normals and 10 patients. Cortical maps were shown to enlarge during voluntary contraction. They were variable between individuals, as reported for direct cortical mapping. Maps showed a degree of short and longer term consistency within an individual. The maps were influenced by lesions. They also provided clinical information of the relation of functioning cortex to brain tumors that could influence clinical management including surgical decisions. A consideration of safety of the method in comparison to direct brain stimulation indicates that it falls within existing parameters for electrical stimulation of the brain, and could reduce the risks of the present standard by lowering charge density and the need for invasive methods. Broader application could lead to development of functional imaging methods, which would be a useful preoperative planning tool and investigative method.

Action Potentials↗
PubMed1990

Focal magnetic coil stimulation reveals motor cortical system reorganized in humans after traumatic quadriplegia.

A figure of '8' magnetic coil (MC) was used to stimulate focally the motor cortex of two adult, traumatic quadriplegics and three normal adults. The two patients were injured approximately 2 years previously and had intense physiotherapy, including biofeedback training of biceps and deltoid muscles, respectively, which were the most caudal muscles spared. The focal MC elicited compound motor action potentials (CMAPs) from these muscles from a much wider area of scalp than in the normal subjects. Latency of biceps and deltoid CMAPs were inversely related to CMAP amplitude. A reorganization of the motor cortical projection system is inferred, in which areas normally eliciting digit movements instead activate muscles in quadriplegics just above the spinal level. The reorganization applies also to the central sense of movement normally elicited by focal frontal cortex stimulation. Possible mechanisms of the reorganization and an implication for rehabilitation are discussed.

Action Potentials↗