Quality assessment of electroencephalography obtained from a "dry electrode" system.

This study examines the difference in application times for routine electroencephalography (EEG) utilizing traditional electrodes and a "dry electrode" headset. The primary outcome measure was the time to interpretable EEG (TIE). A secondary outcome measure of recording quality and interpretability was obtained from EEG sample review by two blinded clinical neurophysiologists. With EEG samples obtained from 10 subjects, the average TIE for the "dry electrode" system was 139s, and for the conventional recording 873s (p<0.001). The results support the hypothesis that such a "dry electrode" system can be applied with more than an 80% reduction in the TIE while still obtaining interpretable EEG.

Standardization of electrode placement for continuous patient monitoring: introduction of an assessment tool to compare proposed electrocardiogram lead configurations.

Numerous modified electrocardiogram (ECG) lead configurations are used for continuous patient monitoring, and they often produce waveforms substantially different from the standard 12-lead ECG. Serial comparisons between modified and standard lead configurations can lead to misdiagnosis and inappropriate therapy. To address this problem, an international Lead Summit was convened in June 2010 to discuss standardizing lead configurations for patient monitoring. The present article summarizes an invited presentation at the Lead Summit where the author proposed an assessment tool to compare the clinical utility of various lead configurations to provide a framework for ongoing and future discussions. The tool (0-20 points, with 20 being the perfect score) weighs 5 factors that are clinically important including equivalency to the standard ECG (6 points), patient comfort (4 points), noise immunity (4 points), noninterference with clinical interventions (3 points), and identifiable anatomical landmarks for accurate electrode placement (3 points). Because research is lacking for some of these factors (eg, patient comfort and noise immunity), studies conducted in patients in actual clinical environments are needed to reach consensus about ideal lead configurations for patient monitoring.

The effects of electrode impedance on data quality and statistical significance in ERP recordings.

To determine whether data quality is meaningfully reduced by high electrode impedance, EEG was recorded simultaneously from low- and high-impedance electrode sites during an oddball task. Low-frequency noise was found to be increased at high-impedance sites relative to low-impedance sites, especially when the recording environment was warm and humid. The increased noise at the high-impedance sites caused an increase in the number of trials needed to obtain statistical significance in analyses of P3 amplitude, but this could be partially mitigated by high-pass filtering and artifact rejection. High electrode impedance did not reduce statistical power for the N1 wave unless the recording environment was warm and humid. Thus, high electrode impedance may increase noise and decrease statistical power under some conditions, but these effects can be reduced by using a cool and dry recording environment and appropriate signal processing methods.

Brain tissue oxygen pressure monitoring in awake patients during functional neurosurgery: the assessment of normal values.

Local brain tissue oxygen (ptiO2) monitoring is frequently applied in patients at risk for cerebral ischemia. To identify ischemic thresholds, the normal range of local brain tissue oxygen pressure (ptiO2) values needs to be established. Ideally, such normal values are determined in healthy and awake subjects, so as to eliminate the possible influences of anesthetics on cerebral physiology or ptiO2. Thus far, however, such measurements have not been conducted, and to fill this void, we determined the ptiO2 values in normal white matter of awake patients undergoing functional stereotactic brain surgery. In 25 otherwise healthy patients, who underwent functional neurosurgery for treatment of a refractory movement disorder under local anesthesia, the ptiO2 of white matter was recorded continuously using a polarographic Clark type electrode monitoring system. Preoperative screening ruled out cognitive dysfunction or structural cerebral lesions. Reliable intraoperative ptiO2 values were obtained in 22 patients. After an adaptation period of 118+/-35 min (range, 47-171 min), we found an average normal ptiO2 of 22.6+/-7.2 mm Hg in the frontal white matter. In 11 patients, ptiO2 measurements were continued postoperatively for 24 h. During this period, a similar normal ptiO2 value of 23.1+/-6.6 mm Hg was found. No iatrogenic complications occurred. In conclusion, the normal ptiO2 of cerebral white matter is most likely lower than previously assumed. Further, the long adaptation time renders this widely applied monitoring instrument unreliable in detecting ischemia early after insertion and limits its usefulness for intraoperative monitoring.

American Association of Neuromuscular & Electrodiagnostic Medicine evidenced-based review: use of surface electromyography in the diagnosis and study of neuromuscular disorders.

Surface electromyography (sEMG) measures myoelectrical signals recorded from sensors placed on the skin surface. The non-invasive nature of sEMG makes it a potentially useful technology for studying diseases of muscle and nerve. Reviews published by the American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM) and the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology (AAN), covering 1964-1994 and 1952-1998, respectively, concluded that sEMG adds no clinical utility over conventional needle EMG (nEMG) for the diagnosis of neuromuscular disease. The AANEM sEMG task force reevaluated the diagnostic utility and added value of this technology for the study of neuromuscular disease based on a contemporary review of relevant literature published between January 1994 and February 2006. The present review concludes that sEMG may be useful to detect the presence of neuromuscular disease (level C rating, class III data), but there are insufficient data to support its utility for distinguishing between neuropathic and myopathic conditions or for the diagnosis of specific neuromuscular diseases. sEMG may be useful for additional study of fatigue associated with post-poliomyelitis syndrome and electromechanical function in myotonic dystrophy (level C rating, class III data).

Spike sorting with hidden Markov models.

The ability to detect and sort overlapping spike waveforms in extracellular recordings is key to studies of neural coding at high spatial and temporal resolution. Most spike-sorting algorithms are based on initial spike detection (e.g. by a voltage threshold) and subsequent waveform classification. Much effort has been devoted to the clustering step, despite the fact that conservative spike detection is notoriously difficult in low signal-to-noise conditions and often entails many spike misses. Hidden Markov models (HMMs) can serve as generative models for continuous extracellular data records. These models naturally combine the spike detection and classification steps into a single computational procedure. They unify the advantages of independent component analysis (ICA) and overlap-search algorithms because they blindly perform source separation even in cases where several neurons are recorded on a single electrode. We apply HMMs to artificially generated data and to extracellular signals recorded with glass electrodes. We show that in comparison with state-of-art spike-sorting algorithms, HMM-based spike sorting exhibits a comparable number of false positive spike classifications but many fewer spike misses.

Comparison of two methods for quantitative assessment of unrestrained locomotion in the rat.

Changes in locomotor movements induced by central and peripheral nerve injury or obtained as a result of pharmacological treatment are increasingly being investigated in rats. Several methods have been used to assess changes in the main locomotor indices, most of which are based on video recordings, usually with low time resolution, or on X-ray cinematographic recordings. Other methods are based on qualitative visual locomotor scoring systems like the BBB scale. We have analyzed locomotor indices in freely moving rats using two methods that can give quantitative results and which may be readily automated. One is based on detecting the onsets of swing and stance phases with contact electrodes (CE), while the second is based on recording the bursts of electromyographic activity (EMG) from the flexor and extensor muscles of each limb during the swing and stance phases, respectively. Besides the investigation of spontaneous locomotion in intact rats, our study also included an examination of locomotion on a ladder using EMG recording and analysis of locomotor disturbances following spinal cord hemisection, for which combined application of the two methods appeared to be useful. Overall, the EMG method appears to be more versatile than the CE method, although the use of both methods in parallel is recommended.

Objective assessment of cervical spinal cord injury levels by transcranial magnetic motor-evoked potentials.

BACKGROUND: The neurologic examination serves as the optimal method to record the level of spinal cord injury (SCI). However, this test is subject to interexaminer variability. To address this shortcoming, we describe a technique that uses transcranial magnetic motor-evoked potentials (tcMMEPs) and dermatomal somatosensory-evoked potentials (d-SSEPs) to more accurately measure the precise level of SCI. METHODS: Two groups of subjects were studied: (1) complete cervical SCI (n = 10) and (2) neurologically intact volunteers (n = 10). Two additional patients were evaluated: one with a cervical central spinal cord syndrome and another with a head injury with a suspected cervical SCI. Each subject underwent upper extremity tcMMEPs and d-SSEPs. RESULTS: Transcranial magnetic motor-evoked potentials were elicited from all upper limb myotomes (C4-T1, bilaterally) in neurologically intact volunteers (20 sides). The level of injury was determined using tcMMEPs by observing the lowest level of measurable response. The level of injury obtained using tcMMEPs was the same as that determined by neurologic examination in 13 (65%) of the 20 sides. In 7 sides, tcMMEP responses were obtained 1 level lower than that assessed by physical examination. Dermatomal somatosensory-evoked potentials were obtained from all dermatomes of volunteers tested in the laboratory compared with only 5 of the 9 patients with SCI who underwent d-SSEP testing. CONCLUSION: Testing using tcMMEPs provides an objective supplement to the neurologic examination after acute cervical SCI. Dermatomal somatosensory-evoked potentials were of limited value in determining the level of cervical SCI.

Nyquist interpolation improves neuron yield in multiunit recordings.

Multiunit electrodes, in particular tetrodes and polytrodes, are able to isolate action potentials from many neurons simultaneously. However, inaccuracies in the post-acquisition reconstruction of recorded spike waveforms can affect the reliability of spike detection and sorting. Here we show that bandlimited interpolation with sample-and-hold delay correction reduces waveform variability, leading to improved reliability of threshold-based event detection and improved spike sorting accuracy. Interpolation of continuously acquired data is, however, computationally expensive. A cost-benefit analysis was made of varying sampling rates from 12.5 kHz (no interpolation) to 100 kHz (eight times oversampling, with respect to the Nyquist frequency), taking into consideration the final application of the data. For most purposes, including spike sorting, sample rates below 25 kHz with bandlimited interpolation to 50 kHz were ideal, with negligible gains above this rate. A practical benefit, especially for large electrode arrays, is that the bandwidth and storage requirements can be greatly reduced by using data acquisition rates at or slightly above the Nyquist frequency.

Assessment of single motor unit conduction velocity during sustained contractions of the tibialis anterior muscle with advanced spike triggered averaging.

This paper describes an improved spike triggered averaging technique for the assessment of control properties and conduction velocity (CV) of single motor units (MUs) of the tibialis anterior muscle during voluntary muscle contractions. The method is based on the detection of multi-channel surface EMG signals (with linear electrode arrays) and intramuscularly recorded single MU action potentials (MUAPs). Intramuscular electrodes were inserted in the muscle taking into account the MU structural properties (innervation zone, tendon locations, length of the fibers), assessed by the linear array surface EMG detection technique. An algorithm for intramuscular EMG signal decomposition is used to identify single MUAP trains. The MUAPs detected by the intramuscular EMG decomposition algorithm were used to trigger and average the multi-channel EMG signals. CV of single averaged surface MUAPs was estimated by the use of advanced signal processing methods based on multi-channel recordings which allow to consistently reduce the variance of CV estimates compared with traditional two channel delay estimators. The number of averaged potentials can thus be limited, resulting in high temporal resolution CV estimates. The developed technique was tested on recordings from the tibialis anterior muscle in 11 volunteers during fatigue. It was shown that the method allows the assessment of single MU CV changes (fatigue) as small as 0.1 m/s with less than 2 s data epochs. The method allows reliable assessment of firing rate and conduction properties of single MUs with applications for the investigation of central and peripheral fatigue mechanisms.

Monte Carlo simulation studies of EEG and MEG localization accuracy.

Both electroencephalography (EEG) and magnetoencephalography (MEG) are currently used to localize brain activity. The accuracy of source localization depends on numerous factors, including the specific inverse approach and source model, fundamental differences in EEG and MEG data, and the accuracy of the volume conductor model of the head (i.e., the forward model). Using Monte Carlo simulations, this study removes the effect of forward model errors and theoretically compares the use of EEG alone, MEG alone, and combined EEG/MEG data sets for source localization. Here, we use a linear estimation inverse approach with a distributed source model and a realistic forward head model. We evaluated its accuracy using the crosstalk and point spread metrics. The crosstalk metric for a specified location on the cortex describes the amount of activity incorrectly localized onto that location from other locations. The point spread metric provides the complementary measure: for that same location, the point spread describes the mis-localization of activity from that specified location to other locations in the brain. We also propose and examine the utility of a "noise sensitivity normalized" inverse operator. Given our particular forward and inverse models, our results show that 1) surprisingly, EEG localization is more accurate than MEG localization for the same number of sensors averaged over many source locations and orientations; 2) as expected, combining EEG with MEG produces the best accuracy for the same total number of sensors; 3) the noise sensitivity normalized inverse operator improves the spatial resolution relative to the standard linear estimation operator; and 4) use of an a priori fMRI constraint universally reduces both crosstalk and point spread.

MegaDyne Mega 2000 return electrode.

The Mega 2000 return electrode, manufactured by MegaDyne Medical Products, is a reusable, capacitively coupled return electrode used in monopolar electrosurgery. It consists of a large--almost two feet by three feet--sheet of flexible conductive fabric that is enclosed in a nonlatex, urethane insulating material. Unlike conductive return electrodes, which are applied adhesively to the patient, the Mega 2000 does not come into direct contact with the patient. Rather, it is placed on an OR table and covered by a protective sheath and, at most, a cover sheet and draw sheet. The patient lies on top of these sheets. The electrode forms a large capacitor with the patient, capacitively coupling the patient into the electrosurgical circuit. Because it is reusable--only the protective sheath is replaced--the Mega 2000 is touted as being a significantly less expensive alternative to traditional conductive return electrodes. And because it is not affixed to the patient, it offers potential advantages for patients having frail skin or extensive skin and tissue damage that would make the use of adhesive return electrodes difficult or impossible. While some healthcare facilities have considered using this return electrode because of its potential clinical and economic benefits, others have questioned its safety because it differs in design from a traditional return electrode. Therefore, we have examined its safety, efficacy, and cost-effectiveness in this Evaluation. We also offer guidance for facilities trying to decide whether to buy the Mega 2000.

Automated external defibrillators.

This study is an update to our August-September 1995 Evaluation of automated external defibrillators (AEDs) in which we evaluated nine AEDs from three manufacturers. In the current issue, we evaluate three additional semiautomated AEDs from three manufacturers. (We also present update information, including new rankings, for the previously evaluated units.) We tested the new units against the same criteria and rated and ranked them using the same scheme as in our original Evaluation. Specifically, we judged the suitability of these units for two primary clinical applications: prehospital use and in-hospital use. In addition, in this Update, we rated and ranked all of the units evaluated to date for a third clinical application: public access defibrillation (PAD) use. In this Update, we briefly describe the purpose and use of AEDs in the Introduction. For more detailed information about this technology, the environments in which AEDs are used, and the factors to consider when selecting this type of device, we encourage readers to refer to our original AED Evaluation (Health Devices 24[8-9], August-September 1995). Also, because our criteria and test methods have not changed since the original study, we have not repeated them in this issue. Again, readers should refer to the previous Evaluation for this information. Furthermore, as described in the inset on page 272, the format of this Update differs from that of traditional Health Devices Evaluations. Most notably, all of the characteristics, test results, and ratings for each unit are presented in an individual Product Profile. Following the profiles for the three units, we present a comprehensive Conclusions section, detailing how the newly evaluated units compare with those evaluated in our original study.

Guidance section: ensuring monopolar electrosurgical safety during laparoscopy.

In this study, we have discussed how direct leakage currents flowing through defects in active electrode insulation or capacitive leakage currents originating from the shaft of the active electrode can cause serious patient injuries during laparoscopic monopolar electrosurgical procedures. We have also discussed a number of other surgical scenarios, varying in likelihood, severity, and cause, in which electrosurgical injuries can occur during laparoscopy. This section provides a discussion of protective measures that hospitals can take to reduce the unique risks of serious patient injuries posed by monopolar electrosurgery used laparoscopically. In the Protective Measures for Laparoscopic Monopolar Electrosurgery table, we list our recommendations for reducing the risk of electrosurgical injuries in four common scenarios, as described in the Clinical and Technical Overview.