Effects of fentanyl anesthesia on visual evoked potentials in humans

Evolution of flash visual evoked potentials to monitor visual pathway integrity during tumor resection: illustrative cases and literature review

Flash visual evoked potentials (fVEPs) provide a means to interrogate visual system functioning intraoperatively during tumor resection in which the optic pathway is at risk for injury. Due to technical limitations, fVEPs have remained underutilized in the armamentarium of intraoperative neurophysiological monitoring (IONM) techniques. Here we review the evolution of fVEPs as an IONM technique with emphasis on the enabling technological and intraoperative improvements. A combined approach with electroretinography (ERG) has enhanced feasibility of fVEP neuromonitoring as a practical application to increase safety and reduce error during tumor resection near the prechiasmal optic pathway. The major advance has been towards differentiating true cases of damage from false findings. We use two illustrative neurosurgical cases in which fVEPs were monitored with and without ERG to discuss limitations and demonstrate how ERG data can clarify false-positive findings in the operating room. Standardization measures have focused on uniformity of photostimulation parameters for fVEP recordings between neurosurgical groups.

Effects of Sevoflurane-Propofol-Balanced Anesthesia on Flash Visual Evoked Potential Monitoring in Spine Surgery: A Randomized Noninferiority Trial

BACKGROUND

Intraoperative flash visual evoked potential (FVEP) can be used to monitor visual function during spine surgery. However, it is limited due to the previous perception of its sensitivity to inhalation anesthesia. We conducted this trial to test the noninferiority of sevoflurane-propofol-balanced anesthesia (BA) versus popular propofol-based total intravenous anesthesia (TIVA) on the amplitude of FVEP during spine surgery.

METHODS

A total of 60 patients undergoing spine surgery were randomized to receive either sevoflurane-propofol-balanced anesthesia (BA group) or propofol-based total intravenous anesthesia (TIVA group) for anesthesia maintenance. We titrated the propofol plasma concentration to keep the bispectral index (BIS) values between 40 and 50. The primary outcome was the P100-N145 amplitudes of FVEP at 120 minutes after induction of anesthesia. The noninferiority margin (δ) was defined as 10% of the P100-N145 amplitude at 120 minutes after induction in the TIVA group. If the confidence interval (CI) for mean differences of P100-N145 amplitude at 120 minutes after induction between BA and TIVA groups lied above the lower limit of -δ with P < .025, we defined BA group was noninferior to TIVA group.

RESULTS

Fifty-nine patients were included in the final analysis. The amplitude of P100-N145 at 120 minutes after anesthesia induction in group BA was noninferior to group TIVA (3.8 [1.3] µV vs 3.2 [1.6] µV, -δ = -0.32, mean difference, 0.57, 95% CI, -0.18 to 1.33, P for noninferiority = .015).

CONCLUSIONS

The effect of 0.5 minimum alveolar concentration (MAC) of sevoflurane-propofol-balanced anesthesia on the P100-N145 amplitude of FVEP was noninferior to that of propofol-based TIVA under comparable BIS range.

Intraoperative monitoring of flash visual evoked potential under general anesthesia

In neurosurgical procedures that may cause visual impairment in the intraoperative period, the monitoring of flash visual evoked potential (VEP) is clinically used to evaluate visual function. Patients are unconscious during surgery under general anesthesia, making flash VEP monitoring useful as it can objectively evaluate visual function. The flash stimulus input to the retina is transmitted to the optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiation (geniculocalcarine tract), and visual cortical area, and the VEP waveform is recorded from the occipital region. Intraoperative flash VEP monitoring allows detection of dysfunction arising anywhere in the optic pathway, from the retina to the visual cortex. Particularly important steps to obtain reproducible intraoperative flash VEP waveforms under general anesthesia are total intravenous anesthesia with propofol, use of retinal flash stimulation devices using high-intensity light-emitting diodes, and a combination of electroretinography to confirm that the flash stimulus has reached the retina. Relatively major postoperative visual impairment can be detected by intraoperative decreases in the flash VEP amplitude.

A pilot study to determine whether visually evoked hemodynamic responses are preserved in children during inhalational anesthesia

BACKGROUND

Anesthetic depth is an important parameter to monitor during surgery, yet remains difficult to quantify, particularly in young children where developmental changes influence the electroencephalogram. A more fundamental physiological response to stimulation is the increase in cerebral blood flow secondary to increased metabolic demand, referred to as flow-metabolism coupling (FMC) and measurable using near-infrared spectroscopy (NIRS). Little is known about the effect of anesthesia on FMC; therefore, we studied visually evoked hemodynamic responses (VEHRs) using NIRS in children undergoing general anesthesia for minor surgical procedures.

METHOD

We recruited 23 children (aged 2-5 years), undergoing surgery requiring general anesthesia. VEHRs were measured for 30 min using NIRS, including 5 min of baseline recording after anesthetic induction. Parameters recorded using NIRS included the concentrations of oxygenated (oxy), deoxygenated (deoxy), and total hemoglobin (Hb), which were separated into epochs for evoked response analysis after filtration of motion artifact and baseline drift. Goodness-of-fit statistics and classification rules were used to determine the existence of evoked responses, and a modified Gaussian equation was used to model each evoked response.

RESULTS

Near-infrared spectroscopy data were recorded in 20/23 children, of whom nine showed a VEHR. No responses were seen in the baseline control data. When examining the relationship between VEHR and anesthetic agents, we noted that for 8/10 patients in whom preoperative or intraoperative fentanyl were administered and VEHRs were absent.

CONCLUSION

We have shown that VEHRs can be detected using NIRS in some anesthetized children. Administration of fentanyl was associated with an absence of VEHRs. The mechanism underlying this association is unclear.

Intraoperative flash VEPs are reproducible in the presence of low amplitude EEG

Flash visual evoked potentials (FVEPs) are often irreproducible during surgery. We assessed the relationship between intraoperative FVEP reproducibility and EEG amplitude. Left then right eyes were stimulated by goggle light emitting diodes, and FVEPs were recorded from Oz–Fz′ (International 10-20 system) in 12 patients. Low cut filters were ≤5 Hz in all patients; two patients also had recordings using 10 and 30 Hz. The reproducibility of FVEP and the amplitude of the concomitant EEG from C4′–Fz were measured. Nine patients had low amplitude EEG (<30 μV); reproducible FVEPs were obtained from all eyes with normal pre-operative vision. The other three patients had high amplitude EEG (>50 μV); FVEPs were absent from three of four eyes with normal pre-operative vision (the other normal eye had a present but irreproducible FVEP). Raising the low cut filter to 10 and 30 Hz (in two patients) progressively reduced EEG and FVEP amplitude, reduced amplifier blocking time and improved FVEP reproducibility. FVEPs were more reproducible in the presence of low amplitude EEG than high amplitude EEG. This is the first report describing the effect of EEG amplitude on FVEP reproducibility during surgery

Ischemic orbital compartment syndrome after posterior spinal surgery

STUDY DESIGN

Case report.

OBJECTIVE

To describe a patient with ischemic orbital compartment syndrome after undergoing spinal surgery in a prone position.

SUMMARY OF BACKGROUND DATA

Ocular complications after nonocular surgery are devastating. The most common etiology is ischemic optic neuropathy. Orbital compartment syndrome with ischemic change is quite rare and urgent management is necessary.

METHODS

Early diagnosis and surgical intervention was performed for ischemic compartment syndrome after posterior spinal surgery was done.

RESULTS

Despite immediate management, the patient remained blind and ptotic 3 months after the injury.

CONCLUSION

Understanding ocular complications after spinal surgery is vital. Although rare, but the most troublesome complication is ischemic orbital compartment syndrome.

Motor signs of wakefulness during general anaesthesia with propofol, isoflurane and flunitrazepam/fentanyl and midlatency auditory evoked potentials

Auditory evoked potentials have been used as an indicator of awareness. In the present study we combined epidural analgesia with three techniques of general anaesthesia. Motor signs of intra-operative wakefulness were documented and assessed along with cardiovascular changes and with midlatency auditory evoked potentials. Thirty patients undergoing elective laparotomy were studied as follows: first continuous epidural analgesia was used in all patients to block painful sensation to the level of T5. Intravenous general anaesthesia was induced with propofol (2.5 mg.kg-1 b.w., group 1, n = 10), thiopentone (5 mg.kg-1 b.w., group 2, n = 10) or etomidate (0.2 mg.kg-1 b.w., group 3, n = 10) and maintained with a propofol (3-5 mg.kg-1, group 1), isoflurane (0.4-0.8 Vol%, group 2), flunitrazepam and fentanyl (0.005 mg.kg-1 b.w.) bolus injection every 20 to 30 s (group 3). Heart rate and arterial pressure were recorded continuously. Purposeful movements of the limbs, eye-opening or other movements as well as coughing were documented as motor signs of intra-operative wakefulness. Auditory evoked potentials were recorded in the awake state, after induction and during maintenance of general anaesthesia. Motor signs of intra-operative wakefulness occurred statistically significantly more often in the patients of the flunitrazepam/fentanyl group than in those of the propofol and isoflurane group. There was no correlation between wakefulness and cardiocirculatory parameters. In the awake patients midlatency auditory evoked potentials had high peak to peak amplitudes and a periodic waveform.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral and electrophysiological aspects of nitrous oxide dependence

We examined the effect of 70% nitrous oxide (N2O) on locomotion and visual-evoked potentials (VEP) in rats. The animals exposed to N2O showed an initial decrease of locomotion, followed by development of tolerance and unaltered motor activity during N2O withdrawal. Similarly, an initial decrease of VEP amplitudes was followed by tolerance to N2O. In addition, some amplitudes (N2-P3, P3-N3, and N3-P4) exceeded the control values, indicating an increase of neuronal excitability of the visual system during a long lasting exposure to N2O. The increase of VEP amplitudes was further potentiated by cessation of this gas. The VEP latencies after initial increase, returned to normal and remained unaltered during N2O withdrawal suggesting that the speed of neurotransmission is not essentially changed during chronic exposure to N2O. However, a significant increase of neuronal excitability during chronic N2O exposure, which further increased by cessation of N2O, could be of clinical importance. Therefore, monitoring of VEP, particularly the amplitude values, may significantly improve a detection of altered neuronal excitability during anaesthesia and drug withdrawal.

Automated evoked potential monitoring

We have developed a computer system for use in the operating room and intensive care unit for the automated acquisition and classification of flash evoked potentials. The project goals were to: (1) acquire evoked potentials without user intervention; (2) recognize, label, and trend important parameters; and (3) alarm if parameters deviate too far from normal. The system consists of a Nicolet Pathfinder, programmed for periodic flash visual evoked potential acquisition and a personal computer for analysis. Analysis software utilizes the C+ +language. The system has been tested on sixteen cases, determinations of specific waveforms were validated ex post facto by two experts. Three parameters in each of three channels were correctly recognized in more than 87% of all waveforms acquired. No signals at all were obtained in only 0.8% of all samples.

[Effect of anesthetics on sensory evoked potentials]

The authors have reviewed the main effects of anaesthetic agents on sensory evoked potentials (EPs) in the auditory, somatosensory and visual modalities. Knowledge of EP changes induced by anesthetic drugs is important to avoid false alarms when monitoring neural structures at risk during surgery. Intraoperative EP monitoring is all the more efficient as the following points are taken into account: 1) whatever the sensory modality considered, EPs are more attenuated by volatile halogenated agents than by intravenous drugs; 2) the cortical components of EPs are more sensitive to anesthetic drugs than the brainstem components; 3) in each modality, the first component of the "primary cortical complex" is less attenuated by anaesthetic agents than the following cortical waves; 4) continuous administration of anaesthetic agents rather than acute administration (bolus) is preferred during EP monitoring. EPs also represent an objective means to assess the depth of surgical anaesthesia, since they may provide a reliable index of cortical depression. Amplitude changes of middle-latency auditory responses (Pa/Nb) seem to be a good marker for estimating depth of anaesthesia with the aid of EPs.

Visual evoked potentials during thiopentone-fentanyl-nitrous oxide anaesthesia in humans

Visual evoked potentials (VEP) during thiopentone-fentanyl-nitrous oxide anaesthesia were studied in 15 healthy patients undergoing non-neurosurgical procedures. The VEP was recorded before and at 1 and 2 min after induction of anaesthesia with 5-6 mg.kg-1 of thiopentone. After recording the 1 and 2 min VEPs, anaesthesia was maintained with a fentanyl-nitrous oxide-oxygen combination, and further recordings were made at 5, 10, 15 and 20 min after induction. The 1 and 2 min VEPs showed a marked decrease in the amplitudes. Latencies were increased. The amplitudes gradually returned to the control level at 15 min, while the latencies remained increased throughout the study period. In conclusion, thiopentone should be avoided during the critical period of VEP recording. Once it is given, at least 15 min should elapse before an appropriate interpretation of the VEP can be made. Thiopentone-fentanyl-nitrous oxide anaesthesia slightly increases the latencies of the VEP. These effects should be considered in the interpretation of the VEP when thiopentone-fentanyl-nitrous oxide anaesthesia is used.