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Biomedical subjects

N de N Donaldson

Publications and source records attributed to N de N Donaldson.

10 recordsLinked to original sources
PubMed2003

Implantable telemeter for long-term electroneurographic recordings in animals and humans.

A system is described that amplifies an electroneurographic signal (ENG) from a tripolar electrode nerve cuff and transmits it from the implanted amplifier to an external drive box. The output was raw ENG, bandpass filtered from 800 to 8000 Hz. The implant was powered by radio-frequency induction and operated for coil-to-coil separations up to 30 mm. The testing and performance of the system is described. The input-referred noise was never more than 1 microV RMS, and, at some positions of the radio-frequency field, was 0.7 microV, close to the expected value for the amplifier used. The common-mode rejection ratio (CMRR) depended on the impedance imbalance from the cuff and the length of input cable. Devices with a short cable and low source impedance had CMRR of 84 dB, but, with 31 cm of cable and a real cuff, the CMRR fell to 66 dB. Recovery from a stimulus artifact took 5 ms. The responses of the cuff to external potential gradients and to common-mode signals are described theoretically or by simulation. The devices are available for use in neuroprosthetic or neurophysiological research.

Animals↗
PubMed2003

Recruitment by motor nerve root stimulators: significance for implant design.

Three paraplegics have been implanted with stimulators of the lumbar anterior roots. Twelve roots were trapped in slots, each with three electrodes, a central cathode and two anodes, but the anodes in all the slots were connected together to reduce the number of wires. Cross-talk between roots was observed at lower levels than expected. Cross-talk was assessed from the ratio of the root's threshold to the threshold of the contralateral response (expected ratio: 72). Two hypothetical reasons for this low ratio were: that the cathode current was not equally shared by the anodes; or that the contralateral responses were reflex. Experiments showed that neither explanation was valid. The ratio of the contralateral to ipsilateral threshold for individual slots (K(1)) was sometimes low because the ipsilateral threshold was high. By taking the ratio of the lowest contralateral response to lowest ipsilateral response, for all roots in each subject (K(2)), the ratio should approach the theoretical value. However, for the two subjects with small slots, it was 7.9 and 15.3, much less than 72, suggesting that the original theory was incorrect. Approximate calculations of the activation function suggest that the reason may be that roots which run close to a slot, but not through it, may pass through a virtual anode region outside the ends of the slots, and that anodal break stimulation in those regions causes the cross-talk. Our estimate is that this cross-talk would be expected to occur at intensities above 5.3 times the cathodal threshold. If the roots are stimulated in pairs, below the levels of cross-talk, experimental results show that the moments obtained in response are additive to within 5%.

Artifacts↗
PubMed2002

A reconnectable multiway implantable connector.

A well-tried plug-and-socket connector system designed for connecting multichannel implanted cables was adapted so as to allow disconnection and reconnection during surgery. Five different sealing techniques were tested in vitro, and it was found that only one of them had the required qualities of high leakage path impedance (taken as more than one megaohm for the worst sample) after three months of saline soak, together with demountability under surgical conditions. The system has subsequently been successfully implemented in a patient in whom reconnection was required two years after implantation.

Electric Impedance↗
PubMed1998

Selecting candidates for a lower limb stimulator implant programme: a patient-centred method.

OBJECTIVE: To develop an effective selection procedure for lower limb functional neurostimulation (LLFNS) for standing in paraplegia. DESIGN: The selection procedure and exclusion criteria were based on the previous experience for two clinical centres with experience of LLFNS. SETTING: Two Regional Spinal Injuries units in southern England. SUBJECTS: 254 fully rehabilitated paraplegics living in the community. INTERVENTION: Patients were invited to participate in the programme, and if suitable to subject themselves to a rigorous staged selection procedure from which they could withdraw at any time. OUTCOME MEASURE: Functionally successful home standing using closed-loop surface electrical stimulation. RESULTS: 57/254 patients were suitable on paper and were accessible. 19 of these (CI = 10-28) were interested in the project and attended one of the spinal centres for details. Twelve (CI = 5-19) of these fulfilled the selection criteria and started on the training programme; and 10 of them completed the muscle training programme successfully. Seven patients (CI = 2-12) achieved closed-loop standing in the laboratory and four patients (CI = 1-8) did so at home.

Adult↗
PubMed1997

Apparatus and methods for studying artificial feedback-control of the plantarflexors in paraplegics without interference from the brain.

Apparatus has been built to explore the practical feasibility of using automatic control with electrical stimulation of paralysed legs to restore function. The experiments are performed with paraplegics with the aim of achieving a realistic postural task: to see whether the body may be maintained upright by stimulation of the plantarflexors when the other joints are braced. Significantly, the intact upper body, under natural control of the brain, cannot interfere with the automatic control. The "Wobbler" apparatus allows measurement of the ankle muscle properties in isometric conditions or in sinusoidal motion. Using the biomechanical properties of the body, which are also measured, controllers for stabilising the body can be designed. Controllers can be dynamically tested, imitating anterior-posterior sway, while the body is held upright, before "actual standing" is attempted.

Biomechanical Phenomena↗
PubMed1997

Feedback control of unsupported standing in paraplegia--part I: optimal control approach.

This is the first of a pair of papers which describe an investigation into the feasibility of providing artificial balance to paraplegics using electrical stimulation of the paralyzed muscles. By bracing the body above the shanks, only stimulation of the plantarflexors is necessary. This arrangement prevents any influence from the intact neuromuscular system above the spinal cord lesion. In this paper, we extend the design of the controllers to a nested-loop LQG (linear quadratic Gaussian) stimulation controller which has ankle moment feedback (inner loops) and inverted pendulum angle feedback (outer loop). Each control loop is tuned by two parameters, the control weighting and an observer rise-time, which together determine the behavior. The nested structure was chosen because it is robust, despite changes in the muscle properties (fatigue) and interference from spasticity.

Biofeedback, Psychology↗
PubMed1997

Feedback control of unsupported standing in paraplegia--part II: experimental results.

This is the second of a pair of papers which describe an investigation into the feasibility of providing artificial balance to paraplegics using electrical stimulation of the paralyzed muscles. By bracing the body above the shanks, only stimulation of the plantar flexors is necessary. This arrangement prevents any influence from the intact neuromuscular system above the spinal cord lesion. In this paper, we present experimental results from intact and paraplegic subjects.

Adult↗
PubMed1996

FES standing: control by handle reactions of leg muscle stimulation (CHRELMS).

Many paraplegics can stand using functional electrical stimulation (FES) of their extensor muscles without any modulation of the stimulus intensity by feedback. This is possible because, using handles and the intact upper-body muscles, the position of the pelvis can be controlled and this determines the positions of the two legs. Nevertheless, it is highly desirable that some practical method of modulating the stimulus intensities is used to reduce muscle fatigue and improve posture. We propose that the goal of the controller should be the minimization of the handle reaction forces by referring these forces to the equivalent leg joint moments and, as far as possible, making corresponding changes to the stimulation of the leg muscles. If force sensors are used only at the handles, the joint moments in the two legs are indeterminate. We hypothesize that in some patients acceptable results may still be obtained if we treat the two legs as one. This method will control standing up, standing, and sitting down; the user will be able to "feel" that his leg muscles are tiring and will be able to "posture switch" without any explicit instruction to the controller to change mode.

Arm↗
PubMedDate not supplied

Technical note. Estimated electrode operating conditions of the first London Mk V implanted stimulator.

The electrodes of an implanted stimulator must be operated in such a way that the task of converting an electron current in metal to an ion current in tissue is achieved without release of noxious chemical species into the tissue, and without significant corrosion of the electrode metal. Both these effects depend on the charge density at which the electrodes are operated. In bringing a newly designed multichannel stimulator into service, the charge densities can only be known when the operating stimulus strengths have been determined. Even then, unless the stimulator is arranged to telemeter out what it is doing, any charge-density figures obtained have the character of estimates, rather than measurements. The first London Mk. V stimulator is used to enable a paraplegic woman to stand up. This note provides estimates of the average charge density and corrosion rate for the Pt--Ir electrodes used. These fall within limits which we believe to be generally accepted as safe.

Adult↗