William A. MacKay

Clinical trial of a new lightwand device (Trachlight) to intubate the trachea.

Background : Transillumination of the soft tissue of the neck using a lighted stylet (lightwand) is an effective and safe intubating technique. A newly designed lightwand (Trachlight) incorporates modifications to improve the brightness of the light source as well as flexibility. The goal of this study was to determine the effectiveness and safety of this device in intubating the trachea of elective surgical patients. Methods : Healthy surgical patients were studied. Patients with known or potential problems with intubation were excluded. During general anesthesia, the tracheas were intubated randomly using either the Trachlight or the laryngoscope. Failure to intubate was defined as lack of successful intubation after three attempts. The duration of each attempt was recorded as the time from insertion of the device into the oropharynx to the time of its removal. The total time to intubation (TTI), an overall measure of the ease of intubation, was defined as the sum of the durations of all (as many as three) intubation attempts. Complications, such as mucosal bleeding, lacerations, dental injury, and sore throat, were recorded. Results : Nine hundred fifty patients (479 in the Trachlight group and 471 in the laryngoscope group) were studied. There was a 1% failure rate with the Trachlight, and 92% of intubations were successful on the first attempt, compared with a 3% failure rate and an 89% success rate on the first attempt with the laryngoscope (P not significant). All failures were followed by successful intubation using the alternate device. The TTI was significantly less with the Trachlight compared with the laryngoscope (15.7 ± 10.8 vs. 19.6 ± 23.7 s). For laryngoscopic intubation, the TTI was longer for patients with limited mandibular protrusion and mentohyoid distance, with a larger circumference of the neck, and with a high classification according to Mallampatti et al. However, there was no relation between the TTI and any of the airway parameters for Trachlight. There were significantly fewer traumatic events in the Trachlight group than in the laryngoscope group (10 vs. 37). More patients complained of sore throat in the laryngoscope group than in the Trachlight group (25.3% vs. 17.1%). Conclusions : In contrast to laryngoscopy, the ease of intubation using the Trachlight does not appear to be influenced by anatomic variations of the upper airway. Intubation occasionally failed with the Trachlight but in all cases was resolved with direct laryngoscopy. The failures of direct laryngoscopy were resolved with Trachlight. Thus the combined technique was 100% successful in intubating the tracheas of all patients.

Unit activity in the cerebellar nuclei related to arm reaching movements

Single units in the fastigial, interpositus and dentate nuclei of two stump-tail macaque monkeys were studied in relation to a right arm, visually guided reaching task. Of 638 recorded cells, 149 showed activity changes correlated to the task, including 24 in the contralateral fastigial and interpositus. Reach-related discharge patterns fell into two broad categories, tonic and phasic. Tonic responses were maintained throughout the reach with no observable relation to kinematic parameters. Most of the task-related activity occurred during the upward lift of the arm toward the target button, with a drop-off as the arm was lowered toward the rest plate. Phasic response cells fired bursts (or suppressed discharge) at specific points in the arm trajectory, most commonly during the lift phase. Many had a sharp drop in discharge when the shoulder flexion torque was transiently reversed to decelerate the arm. For either type, restricted directional specificity was rarely seen in any nucleus, and correlations with recorded EMGs were weak. Visual responses to target button illumination were observed in both the fastigial and dentate nuclei, but did not necessarily correspond with the button giving the best movement-related response. Task-related activity changes started earliest in the fastigial nuclei and latest in the interpositus nuclei. The data suggested that cerebellar output facilitates motor centers in a rather general manner, but at precisely determined times.

Neuronal correlates in posterior parietal lobe of the expectation of events

During studies of response properties of single units in the posterior parietal cortex of 6 awake monkeys, 168 neurons were encountered (7.1% of examined units) which showed anticipatory types of activity. These neurons were found on either side of the intraparietal sulcus. In area 5, this expectation activity was expressed as a change in discharge rate whenever a specific body part (e.g. hand or shoulder) was approached by the investigator as though contact would be made. Invariably the neurons also responded to cutaneous and/or proprioceptive stimulation of the target body area. In area 7a the same type of response was also found but not always with a corresponding somatosensory receptive field. In addition, many neurons increased discharge rates (or rarely, decreased them) immediately prior to the expected occurrence of a reward, a visual task cue, or on hearing the approaching footsteps of a familiar person. None of these responses were correlated to eye movements, nor could they be attributed to any other body movement.

CNV, stretch reflex and reaction time correlates of preparation for movement direction and force

Ten human subjects were tested in a pre-cued choice reaction time (RT) paradigm in which the warning stimulus gave a varying amount of prior information regarding the direction (flexion or extension) or force level (weak or strong) for an impending right forearm movement. During the preparatory period (PP), either CNV was monitored from 8 scalp leads, or elbow stretch reflexes were tested at selected times using mechanical torque steps as stimuli. Mean RTs increased as the amount of prior information decreased. The locus of maximal rate of increase of scalp negativity migrated from the frontal lobe to the parietal lobe during the PP, under all conditions. Using laplacian derivations, it was found that the CNV at Cz did not distinguish among the different information conditions, whereas the CNV over the somatosensory arm area was greatest when direction information was given in advance. The CNV over the arm motor region was greatest when force information was available. The last 100 msec of the PP was characterized by the development of current sources over the premotor region for full information, and over the somatosensory region for the other conditions. This coincided with the appearance of very large late stretch reflexes or triggered reactions in the prepared agonist for full information, indicating that the intended movement had been fully processed by this time and awaited only a sensory trigger. The data support a parametric model of motor preparation, with direction and force being processed by at least partially independent networks.

Cerebellar nuclear activity in relation to simple movements.

SummarySingle unit activity in the fastigial, interpositus and dentate cerebellar nuclei was recorded in relation to simple elbow flexion and extension movements in two macaque monkeys. In common with proximal muscle activity, 94% of the task-related neurons had qualitatively similar discharge patterns for both directions of forearm movement. In many cases the flexion and extension discharge was virtually identical, but some cells had a distinct directional bias. The very few neurons which were directionally specific were located in the dentate and interpositus. Two had tonic activity well correlated to elbow angle. Task-related changes in discharge rate occurred earliest in dentate and latest in fastigial, but almost always during the period of concomitant proximal and elbow EMG changes. Correlations of phasic activity with movement velocity were uniformly weak. Many eye movement-related neurons were encountered in the fastigial, dentate and y-group nuclei. Fastigial eye cells, both bursting and tonic, tended to be highly direction specific, whereas dentate eye cells were usually omnidirectional and variable. For both arm and eye cerebellar cells, the directional preferences of phasic and tonic discharge, in the same neuron, could be opposed to one another.

Field potential oscillatory bursts in parietal cortex before and during reach.

Local field potentials were recorded in parietal cortex, areas 5, 7a and 7b, of a macaque monkey to determine if oscillatory bursts occurred in an observable relationship to behavioral events. The monkey performed a visually-guided reaching task to targets displayed on a touch-sensitive video monitor. The task was pre-cued with a 1.6 s preparatory period. Intracortical recordings were made with a microelectrode or epidural recordings with silver ball electrodes. Compared to the relaxed state, task performance was distinguished by a drop in power for frequencies below 20 Hz (most prominent in area 7), and an increase for frequencies above 20 Hz. For the beta frequency band 20-25 Hz, maximal power occurred during the preparatory periods, and minimal power during reach performance. Above 30 Hz, reach preparation and performance episodes did not differ significantly in spectral power, except in parts of area 5 where 40 Hz activity was observed to increase during movement. The spatial extent of the beta preparatory activity was monitored using an array of 15 epidural electrodes, positioned in 2 rows stretching from the arcuate sulcus to the lunate sulcus. During each preparation, premotor cortex was found to be the major focus of increased power at 20 Hz, whereas posterior parietal cortex was the dominant focus of increased power in the 21-29 Hz band. Although beta frequencies were most prevalent during early stages of motor preparation, the oscillatory bursts were not tightly time-locked to the visual signals. beta Frequencies may be associated with an internally-triggered process to prepare the upcoming movement.

Are extent and force independent movement parameters ? Preparation- and movement-related neuronal activity in the monkey cortex

Movement extent and movement force can be independently controlled in motor performance. Therefore, independent representations of extent and force should exist in the central nervous system (CNS). To test this hypothesis, microelectrode recordings were made in sensorimotor cortex of monkeys trained to perform visually cued wrist flexion movements of two extents, against two levels of frictional resistance. An initial preparatory signal (PS) provided complete, partial or no information about extent and/or force of the movement, which had to be performed in response to a second, response signal (RS). The activity of 511 neurons of the primary motor cortex (MI), the premotor cortex (PM), the postcentral cortex (PC), and the posterior parietal cortex (PA) was recorded in two monkeys. Both reaction time (RT) and neuronal data suggest that there exist independent, neuronal mechanisms responsible for the programming of either parameter. On the one hand, partial information about either movement parameter shortened RT when compared with the condition of no prior information. On the other hand, there were, among others, two discrete populations of neurons, one related only to extent, the other only to force. Preparatory changes in activity related to either movement parameter were mainly located in the frontal cortex, especially in the PM. After occurrence of the RS, the percentage of selective changes in activity increased and tended to extend to the parietal cortex. In particular during the movement, force-related changes in activity have been encountered in PA. Furthermore, we conducted trial-by-trial correlation analyses between RT and preparatory neuronal activity for all conditions of prior information. The mean correlation coefficient was significantly higher in the condition of information about movement extent than of information about movement force and it was significantly higher in MI/ PM than in PC/PA.

Stretch reflex modulation during a cyclic elbow movement

Small torque pulses were delivered to the forearm in order to test the stretch reflex of the brachialis and triceps arm muscles in 11 normal subjects performing a cyclic movement about the elbow in the horizontal plane. The flexion-extension movement was paced by a metronome and performed under various loading conditions. Reflexes for each muscle were tested either in each 50 msec segment of the 2 sec cycle period, or in a smaller number of selected phases. A late reflex, appearing at a latency of about 60 msec (measured from the onset of the torque increment), was modulated extensively during the movement cycle. The amplitude of the late reflex increased markedly at the onset of a muscle contraction. In many of the subjects reflex responsiveness began to increase as early as 200 msec prior to the onset of voluntary muscle activity. Peak reflex responses were elicited by stimuli delivered 100-150 msec prior to the peak rate of increase of dynamic load (composed of inertial, viscous and elastic forces). The increase in responsiveness was followed by a drop which was generally coincident in time with the peak rate of increase of the load opposing muscle contraction. The modulation of the late reflex is appropriately timed for reflex-generated tension to help counteract dynamic loads, intrinsic to the movement.

Measurements of human forearm viscoelasticity

In human subjects, stiffness of the relaxed elbow was measured by three methods, using a forearm manipulandum coupled to a.d.c. torque motor. Elbow stiffness calculated from frequency response characteristics increased as the driving amplitude decreased. Step displacements of the forearm produced restoring torques linearly related to the displacement. The stiffness was very similar to that calculated from natural frequencies at amplitudes above 0.1 rad. Thirdly, elbow stiffness was estimated from brief test pulses, 120 ms in duration, by mathematically simulating the torque-displacement functions. Stiffness values in the limited linear range (under +/- 0.1 rad) were higher than in the linear range of the first two methods. A major component of elbow stiffness appears to decay within 1 s. The coefficients of viscosity determined from the simulation were, however, very similar to those calculated from the frequency response. Test pulse simulation was then used to determine joint impedance for different, actively maintained elbow angles. Joint stiffness and viscosity increased with progressive elbow flexion.

Movement-induced gain modulation of somatosensory potentials and soleus H-reflexes evoked from the leg. I. Kinaesthetic task demands

Abstract Movement-related gating of cerebral somatosensory evoked potentials (SEPs) occurs during active and passive movements of both the upper and the lower limbs. The general hypothesis was tested that the brain participates in setting the gain of the ascending path from somatosensory receptors of the human leg to the somatosensory cortex. In experiment 1, SEPs from Cz’ and soleus H-reflexes were evoked by electrical stimulation of the tibial nerve in the popliteal fossa during passive movement about the right ankle. Early SEPs and H-reflexes sampled during simple passive movement were significantly attenuated when compared with stationary controls (P<0.05). The additional requirement of tracking the passive ankle movement with the other foot led to a significant relative facilitation of mean SEP, but not H-reflex amplitude, compared with means from passive movement alone (P<0.05). In experiment 2, SEPs were evoked in the active (tracking) leg during a forewarned reaction-time task. Subjects were required to move in a preferred direction or to track the passive movement of their right foot with their left. Significant attenuation of early SEP components occurred 100 ms prior to EMG onset (P<0.05), with no apparent effect due to tracking. In the 3rd experiment, SEPs and H-reflexes were evoked in the passively moved leg (the target for active movement of the left leg) during the same forewarned reaction-time task. During the warning period, SEPs were significantly attenuated compared with stationary controls for non-tracking movements, but not for movements involving tracking (P<0.05). It is concluded that centrifugal factors are important in modulating SEP gain required by the kinaesthetic demands of the task.