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Biomedical subjects

W J Levy

Publications and source records attributed to W J Levy.

At least 37 records · Page 2Linked to original sources
PubMed1990

Multicenter study of general anesthesia. I. Design and patient demography.

A prospective randomized clinical trial of enflurane, fentanyl, halothane, and isoflurane is described. The 17,201 patients were stratified into two groups (preanesthetic medication and no preanesthetic medication) and were randomized to one of four study agents: enflurane, fentanyl, halothane, and isoflurane. Fifteen university-affiliated hospitals in the United States and Canada participated. All patients were first assessed preoperatively. Data were collected during anesthesia, in the immediate recovery period, and for up to 7 days after anesthesia/surgery. The mean age of the patients was 43 yr, the mean height 167 cm, and the mean weight 68 kg. Sixty-five percent of patients were female. In this study 90.7% of patients were classified as ASA Physical Status 1 or 2, and 34.7% of patients smoked. It is concluded that pooling of data across institutions was valid and does allow determination of the efficacy and relative safety of the four study agents.

Adult↗
PubMed1990

Multicenter study of general anesthesia. II. Results.

A prospective, stratified, randomized clinical trial of the safety and efficacy of four general anesthetic agents (enflurane, fentanyl, halothane, and isoflurane) was conducted in 17,201 patients (study population). Patients were studied before, during, and after anesthesia for up to 7 days. Nineteen patients died (0.11%), and in seven of these (0.04%) the anesthetic may have been a contributing factor. The rates of death, myocardial infarction, and stroke in the study population were so low (less than 0.15%) that no conclusions regarding the relative rates of these outcomes among the four anesthetic agents could be reached. The rates of 16 of 66 types of adverse outcomes in the study population were significantly different among the four study agents. Most of these outcomes were minor. However, severe ventricular arrhythmia (P less than 10(-6)) was more common with halothane, severe hypertension (P less than 10(-6)) and severe bronchospasm (P = 0.028) were more common with fentanyl, and severe tachycardia (P = 0.001) was more common with isoflurane. Recovery from anesthesia during the first 30 min was slowest in those patients who received halothane (P less than or equal to 0.001). In addition, patients who received fentanyl experienced less pain during the first hour in the recovery room (P less than 10(-6)). In conclusion, clinically important differences do exist for some outcomes among the four study agents.

Adult↗
PubMed1990

Electroencephalographic changes during brief cardiac arrest in humans.

Slowing and attenuation of the dominant frequency of the electroencephalogram (EEG) are changes commonly used to detect cerebral ischemia. To assess the validity of this method, the EEGs recorded during 93 episodes of circulatory arrest in ten normothermic, lightly anesthetized patients undergoing implantation of automatic internal cardioverting defibrillators (AICDs) were visually inspected for change. The number of events recorded for each patient varied from 5 to 18 and was a function of the duration and success of AICD testing in each patient. In 82 of 93 (88%) episodes, EEG changes were identified, and occurred an average of 10.2 s after the last normal heart beat. Of these 82, 67 (82%) illustrated slowing and attenuation. However, 15 (18%) of the hemodynamic events showed changes not previously described as indicative of cerebral ischemia: 6 (7%) showed a loss of delta-wave activity and 9 (11%) showed an increase in the amplitude of theta activity. Time to onset of these unusual changes (10.6 and 9.2 s, respectively) was not significantly different from that for EEG slowing and attenuation (10.2 s). Five of the ten subjects showed more than one pattern of EEG change. There was no significant difference in the time to onset of EEG change among individual patients, and neither were there differences in patterns of change associated with particular anesthetic agents. These results indicate that in normothermic, lightly anesthetized individuals, cerebral ischemia may cause changes in EEG pattern other than slowing and attenuation of dominant frequencies. These alternative patterns should be recognized as indicative of cerebral ischemia when intraoperative EEG monitoring is performed.

Adult↗
PubMed1990

The effect of etomidate on motor evoked potentials induced by transcranial magnetic stimulation in the monkey.

Etomidate (ET) is a known hypnotic agent in neuroanesthesia. This study was designed to examine the reliability of motor evoked potentials (MEPs) after transcranial magnetic stimulation in monkeys anesthetized intravenously with ET. The ET regimen was as follows: an initial dose (0.5 mg/kg) followed by 13 doses (0.2 mg/kg every 6-12 min; mean, 8.0 +/- 1.3 min). The total dose administered was 3.1 mg/kg. The magnetic coil was placed over the MEP scalp stimulation region. Evoked electromyographic responses were recorded from the contralateral abductor pollicis brevis (APB) and abductor hallucis (AH) muscles of the fore- and hindlimbs, respectively. Reproducible MEP responses were consistently recorded while the animal was under total ET anesthesia. The coil demography was altered and the MEP scalp topography was moderately reduced by ET injections. Significant threshold elevation was noted after a total dose of 1.7 mg/kg for APB responses and 0.5 mg/kg for AH responses (P less than 0.05). Marked prolongation of latency was observed after a dose of 0.5 mg/kg for APB MEPs and 2.5 mg/kg for AH MEPs (P less than 0.05). MEP amplitude responses showed marked variability. Repeated doses of ET produced a mean threshold rise of 14 to 28% for the APB and 19 to 29% for the AH. The mean latency delay was 5 to 11% for the APB and 0.5 to 8% for the AH, while the mean amplitude depression was 24 to 59% for the APB and 15 to 50% for the AH. Apparent seizure activity or abnormalities in behavior and feeding were not noted over a 1-year period.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Effects of incremental ketamine hydrochloride doses on motor evoked potentials (MEPs) following transcranial magnetic stimulation: a primate study.

the cumulative dose effect of ketamine hydrochloride (KH) on transcranial magnetic-induced motor evoked potentials (MEPs) was examined in monkeys. Electromyographic (EMG) responses were recorded from the contralateral abductor pollicis brevis (APB) and abductor hallucis (AH) muscles. MEP brain stimulation threshold, latency, and amplitude values were studied. After obtaining baseline recordings, increments of KH (5 mg/kg every 15-20 min) were given i.v. (50 mg/kg total dose). MEPs were repeatedly recorded following KH injections. No loss of potentials was encountered in any animal. However, KH induced significant MEP latency delay in doses >/=35-40 mg/kg and amplitude depression in doses >/=15-20 mg/kg (p < 0.01). Under various KH doses, the amplitude depression ranged from 13.6 to 45.5% for APB and 57.3 to 82% for AH compared to the control values. The MEP latency prolongation ranged from 3.5 to 18% for APB and 4.2 to 13.1% for AH. The stimulation threshold rise ranged from 6.7 to 14.7% for APB and 7 to 17.9% for AH. Statistical correlation was closest between cumulative KH doses and MEP latency prolongation. We conclude that, in the primate model, reliable MEP recording is feasible under deep KH anesthesia. However, awareness of drug-induced response alterations is essential during interpretation of intraoperative MEP changes. Further investigation is warranted regarding the specific dose effect in humans and safety of magnetic stimulation.

Journal Article↗
PubMed1990

The effect of nitrous oxide on transcranial magnetic-induced electromyographic responses in the monkey.

Transcranial magnetic stimulation (TMS) to produce motor evoked potentials (MEPs) is a newly developed intraoperative modality to monitor functional integrity of the motor pathways. The present study was designed to examine the reliability of magnetic MEP recording under nitrous oxide (N2O) inhalation. Following ketamine injection (10 mg/kg i.m.) and endotracheal intubation, 16 monkeys were exposed to N2O:O2 mixture ratios 1:3, 1:1, and 3:1. Electromyographic (EMG) responses, evoked by pulsed magnetic fields applied extracranially to the scalp zone overlying motor cortex, were recorded from the contralateral fore- and hind limb flexor muscles. The scalp topography zone for contralateral muscle excitation was markedly reduced by 75 vol% N2O. Significant stimulation threshold elevation, latency prolongation, and amplitude depression were noted after inhalation of 75 vol% N2O (p <0.05) compared with </=50 vol% N2O inhalation. The mean range of brain stimulation rise was 1%, 7-8%, and 13-20%, and MEP latency delay was 3%, 3-6%, and 7-11% on exposure to 25%, 50%, and 75 vol% N2O, respectively. The mean amplitude response first increased by 20% and 7% with 25% and 50% N2O, respectively, and then reduced by 60% with 75 vol%. We conclude that, in a primate model, while magnetic MEPs could reliably be recorded under 25-75% N2O, the evoked motor responses were least affected by </=50 vol% inhalation. Knowledge of N2O-induced MEP alteration is essential when interpreting the response change while using this agent.

Journal Article↗
PubMed1989

Peripheral nerve repair across a gap studied by repeated observation in a new window implant chamber.

We have developed a new peripheral nerve chamber system which allows high resolution observation of the cellular events involved in nerve regeneration. The growth into the chamber is confined to a two-dimensional sheet resembling tissue culture. An intact blood supply forms within the chamber and by 100 days the nerve bridging the chamber has a nearly normal perineurium surrounding unmyelinated and myelinated axons. Degenerating axons are very rarely seen. The early growth in the chamber has a tendency to spread widely and form a two-dimensional sheet. This results in morphologies similar to those found in tissue culture and facilitates observation of individual elements. The initially wide tissue growth gradually re-models to form a bridge with a constant width similar to the width of the peroneal nerve. Occasionally a 'side arm' containing myelinated axons was retained even though the majority of axons appeared to loop back and rejoin the main bridge. Prefilling the chamber with Matrigel did not produce a significant enhancement of growth rate over that found following prefilling with sterile saline but did result in a more normally organized structure in the long term. Proximal ingrowth occurred at a similar rate in the absence of the distal stump. The structure of the proximal stump in the absence of the distal stump was similar to the structure when both stumps were present, including the presence of myelinated axons near the proximal port by 20 days. However, at subsequent stages the absence of a distal stump led to withdrawal of the proximal growth.

Animals↗
PubMed1988

Motor-evoked potentials of tongue musculature in dogs.

Transcranial brain stimulation by either electrical or noninvasive magnetic stimulation has produced motor responses of limb musculature. However, recordings from muscles innervated by cranial nerves have not previously been reported. A right-sided craniotomy was performed in 18 dogs. An electric stimulus was delivered to the motor cortex via surface electrodes and evoked EMG responses were recorded from the tongue musculature to determine conduction time. Magnetic stimulation of the motor cortex yielded EMG latencies similar to those obtained by electrical direct cortical stimulation, thus demonstrating the potential of magnetic stimulation in intraoperative and outpatient clinical evaluation of hypoglossal nerve disorders. For the first time, it appears possible to noninvasively stimulate the cranial nerves proximal to the point of injury and to evaluate more central conduction pathways.

Action Potentials↗
PubMed1988

Motor-evoked potentials of facial musculature in dogs.

Motor responses of limb musculature have been elicited by transcranial brain stimulation using electrical and magnetic stimulation. Recording from muscles innervated by cranial nerves has not previously been reported. A unilateral hemispheric craniotomy was performed in 24 dogs. An electric stimulus was delivered directly to the exposed motor cortex via surface electrodes. Evoked electromyographic responses to motor cortex stimulation were recorded from the contralateral orbicularis oculi and orbicularis oris muscles. Preliminary investigation into the use of magnetic stimulation of the motor cortex is compared to direct electrical cortical stimulation. This study demonstrates the potential for magnetic stimulation in the intraoperative and outpatient clinical evaluation of facial nerve disorders. It appears possible, for the first time using magnetic stimulation, to noninvasively stimulate these nerves proximal to the point of injury and to evaluate more central conduction pathways.

Animals↗
PubMed1987

Electroencephalographic evidence of cerebral ischemia during acute extracorporeal hypoperfusion.

Although the incidence of neurologic complications after cardiopulmonary bypass appears to be decreasing, such complications continue to occur regardless of the perfusion techniques employed. In order to assess the ischemic potential of brief episodes of hypoperfusion, eight-channel EEG power spectra were recorded from 15 patients undergoing high-pressure, high-flow extracorporeal perfusion. Reductions in flow were performed 49 times to facilitate surgical manipulations, and on ten occasions (20%) resulted in EEG evidence of cerebral ischemia. Such ischemia was observed in 6 of 18 (33%) episodes during normothermia, but 0 of 18 episodes during stable hypothermia. In 4 of 13 (31%) cases of hypoperfusion during cooling at the start of cardiopulmonary bypass, EEG evidence of cerebral ischemia was also observed. These results indicate that brief episodes of hypoperfusion during normothermia or incomplete hypothermia often produce electroencephalographic evidence of cerebral ischemia.

Body Temperature↗
PubMed1987

Effect of epoch length on power spectrum analysis of the EEG.

To study the effect of epoch length on the variability of power spectrum analysis of the EEG, 22 64-s segments of EEG were analyzed using epoch lengths of 2, 4, 8, 16, and 32 s. Nine of these segments exemplified EEG changes during transient anesthetic states or surgical conditions. Epoch-to-epoch variability was computed within frequency bins for all segments, and ANOVA with hierarchal classification was used to determine the length of the EEG segment necessary to identify a statistically significant change in those EEG segments recorded during changing conditions. In 16 segments, the epoch-to-epoch variability with power spectra were computed using 2-s epochs was significantly less than the variability when power spectra were computed using longer epoch lengths. In five segments, no significant difference existed between the variance at 2-s epochs and longer (4-s) epochs. In one case, an EEG containing a burst-suppression pattern, the variability was significantly increased when 2-s epochs were used. Analysis using 2-s epochs also identified changes more rapidly than analysis using any longer epoch length in eight of nine segments, and the differences were clinically significant as well (over 30 s faster when 2-s epochs were used instead of 16-s epochs). These findings suggest the preferability of short epoch lengths when power spectrum analysis is used for intraoperative EEG monitoring.

Analysis of Variance↗
PubMed1987

Motor evoked potentials in the dog: effects of global ischemia on spinal cord and peripheral nerve signals.

Motor evoked potentials (MEPs) in cats, rats, and humans have been reported. They appear promising as a test of central nervous system function, and they are sensitive not only to mechanical injury but also to ischemia. In mechanical trauma, the peripheral nerve response is much more sensitive to damage than the cord response, with a lower threshold and an earlier disappearance. We are reporting that the MEP can also be produced in the dog and that, under conditions of cardiac arrest induced by fibrillation, the peripheral nerve response disappears first at about 30 seconds and then the spinal cord response disappears at about 10 to 13 minutes. The late disappearance of the spinal cord response raises serious questions about its role as an adequate injury monitor. The most useful warning feature of the spinal cord response is an increase in amplitude during the critical first 2 minutes of arrest. Latency changes in the cord and peripheral nerve response did not seem as useful as amplitude changes in terms of providing adequate detection of injury. We also evaluated the peripheral nerve signals to determine whether they are partially volume-conducted weak muscle responses, and evidence substantiates their nonmuscle origin.

Animals↗
PubMed1987

Proposal for bipolar coagulation of the sella turcica wall during pituitary tumor operation.

Convoluted morphology, limited visualization, and the lack of appropriate instruments limit the surgical approach to the walls of the sella turcica. They are, however, a site of local invasion and retained tumor fragments in cases of pituitary lesions. Effective bipolar coagulation within the sella turcica could reduce recurrence from local invasion, but it is made difficult because rigid bipolar forceps cannot fit effectively or safely against the complex contours of the walls there. Effective coagulation and thorough tumor removal are further frustrated by the lack of direct visualization of either the sella walls or the part of the floor adjacent to the surgical opening. It would be desirable, however, to coagulate the dura mater and retained tumor fragments. As an aid in this problem, we are reporting the use of a flexible bipolar coagulation system in two forms and of a mirror system for visualization in the sella without requiring the removal of one hand from surgical maneuvers. A pair of parallel flexible wire extensions to the standard bipolar forceps can be shaped by the surgeon into a contour that will coagulate the specific local terrain. A web of wire woven into one surface of a cotton patty, which is a bipolar coagulation system, supplements the wire loops. This allows the walls of the sella turcica to be coagulated more effectively where the contours are difficult to gauge and not well visualized. For visualization of the sella turcica walls and floor, we have used a small front surface mirror 1.0 mm thick.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1987

Transcranial stimulation of the motor cortex to produce motor-evoked potentials.

Monitoring of the nervous system using evoked potentials is a developing tool. A new evoked potential, the motor-evoked potential (MEP), based on a traditional test, uses brain stimulation to monitor the motor system. The MEP complements the existing modalities, which are wholly sensory. The MEP can be prompted by direct stimulation of the motor cortex, but, in a more general way, by transcranial stimulation. Electric or magnetic means can be used. One electric system involves placement of an electrode on the scalp over the motor cortex, paired with a cathodal plate on the roof of the hard palate. Recording electrodes are placed over the spinal cord, peripheral nerves, and muscles. Signals are recorded with a standard, evoked-potential, signal-averaging computer. Animal studies indicate that the electric stimulus activates primarily the pyramidal system to produce a descending evoked potential in the ventral and dorsolateral spinal cord. It is more sensitive than the sensory-evoked potential to spinal cord injury produced by the weight-drop method in cats. The peripheral nerve responses, much more sensitive to injury than the cord responses, can be altered by metabolic abnormalities. To date, the MEP has been an accurate indicator of ambulation in chronic spinal cord injury in animals. Parallel clinical development in the operating room has shown that the test is valuable; and it monitors brainstem or cortex manipulation, as well as cord manipulation. Safety studies are encouraging. The MEP is developmentally and technically demanding, but it has produced high quality signals.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Motor evoked potential as a predictor of recovery in chronic spinal cord injury.

In injury to the spinal cord, motor function is the most critical system affected. The introduction of motor evoked potential (MEP) monitoring techniques over the past 5 years has opened new possibilities for the assessment of motor function. We examined the MEP change in 30 cats that received weight drop injuries to the thoracic spinal cord and correlated it with function. The MEPs were recorded above and below the injury and in the sciatic nerves. In these animals, we found that the peripheral nerve response to the MEP was the most sensitive to injury, disappearing immediately upon weight drop. The MEP at the spinal cord level was somewhat more sensitive to injury than the SSEP at the cord level, and the SSEP at the cortex was the least sensitive to injury. The MEP spinal cord signal below the lesion showed both a latency increase and a substantial amplitude decrease after impact. The latency increase, however, was of relatively small absolute magnitude making the amplitude change easier to monitor. The animals were followed chronically, and 17 of the 30 regained ambulation. In all animals (followed up to 2 months), the peripheral nerve signals returned at or immediately before the time of ambulation. In none of the animals who failed to regain a peripheral nerve response was ambulation attained. This supports a strong correlation of the peripheral nerve response with walking. However, the peripheral nerve response was only a short term predictor. Examination of the spinal cord signal showed that the MEP spinal cord signal below the lesion as a percentage above the lesion was a significant correlate of current ambulation recovery, with a correlation coefficient of 0.55. This suggests that evaluation of the SSEP and MEP spinal cord signals may be able to predict longer term recovery in animals and perhaps humans.

Animals↗