Martin Reichel

Implantable device for long-term electrical stimulation of denervated muscles in rabbits.

Although denervating injuries produce severe atrophic changes in mammalian skeletal muscle, a degree of functional restoration can be achieved through an intensive regime of electrical stimulation. An implantable stimulator was developed so that the long-term effects of different stimulation protocols could be compared in rabbits. The device, which is powered by two lithium thionyl chloride batteries, is small enough to be implanted in the peritoneal cavity. All stimulation parameters can be specified over a wide range, with a high degree of resolution; in addition, up to 16 periods of training (10–180 min) and rest (1–42 h) can be set in advance. The microcontroller-based device is programmed through a bidirectional radiofrequency link. Settings are entered via a user-friendly computer interface and annotated to create an individual study protocol for each animal. The stimulator has been reliable and stable in use. Proven technology and rigorous quality control has enabled 55 units to be implanted to date, for periods of up to 36 weeks, with only two device failures (at 15 and 29 weeks). Changes in the excitability of denervated skeletal muscles could be followed within individual animals. Chronaxie increased from 3.24±0.54 ms to 15.57±0.85 ms (n=55, p<0.0001) per phase in the 2 weeks following denervation.

Functional Electrical Stimulation of Long‐term Denervated, Degenerated Human Skeletal Muscle: Estimating Activation Using T2‐Parameter Magnetic Resonance Imaging Methods

Functional electrical stimulation (FES) of long-term denervated, degenerated human skeletal muscle has proven to be a suitable method for improving a number of physiological parameters. The underlying mechanisms of activation of a denervated muscle fiber can be described with suitably modified and extended Hodgin-Huxley type models, coupled with three-dimensional (3D) finite element models of the surrounding electrical field. Regions of activation within a muscle can be determined using a 3D computer model. However, simulation results have not yet been validated experimentally. During and immediately after exercise, muscle shows increased T2-relaxation times. It is thus possible to estimate muscle activation noninvasively and spatially resolved with the magnetic resonance imaging (MRI) method of T2 mapping, which was, therefore, chosen as a suitable validation approach. Six patients were scanned prior to FES training with a multislice multiecho MSME-sequence at 3 Tesla and then asked to perform one of their regular daily training-sessions (leg extensions). Subjects were then repositioned in the MR-scanner and two to five postexercise scans were recorded. Pre- and postexercise scans were coregistered and T2-parameter maps were calculated. Regions of interest (ROIs) were drawn manually around quadriceps femoris and its antagonists. Activation was detected in all patients. In well-trained patients, activation in the quadriceps was found to be considerably higher than in its antagonists. These experimental results will help further improve existing models of FES of denervated degenerated human skeletal muscle.

A novel approach to simulate Hodgkin-Huxley-like excitation with COMSOL Multiphysics.

A proof of concept for the evaluation of external nerve and muscle fiber excitation with the finite element software COMSOL Multiphysics, formerly known as FEMLAB, is presented. This software allows the simultaneous solution of fiber excitation by 1D models of the Hodgkin-Huxley type which are embedded in a volume conductor where the electric field is mainly dominated by the electrode currents. This way the presented bidomain model includes the interaction between electrode currents and transmembrane currents during the excitation process. Especially for direct muscle fiber stimulation (cardiac muscle, denervated muscle) the effects from secondary currents from large populations of excited fibers seem to be significant. The method has many applications, for example, the relation between stimulus parameters and fiber recruitment can be analyzed.

Effects of functional electrical stimulation in denervated thigh muscles of paraplegic patients mapped with T2 imaging

ObjectFunctional electrical stimulation (FES) for paraplegic patients, with the long-term goal of ultimately restoring muscle function, is associated with several positive effects: improvement of blood circulation, skin condition, peripheral trophism and metabolism, prophylaxis against decubitus ulcer and better physical fitness. Since fibres of denervated muscles (lacking a supplying nerve) need to be activated directly, the fraction of elicited muscle tissue follows the geometric distribution of the electrical field, which can be simulated using electrophysiological computer models. Experimental validation of these results, however, has not yet been established.Materials and methodsWe acquired T2 parameter images using a multislice multi-spin-echo MR sequence before and immediately after FES in nine denervated paraplegic patients and three healthy subjects in order to visualise the geometric distribution of activation by electrically induced muscle stimulation in denervated versus innervated (healthy) thigh muscle.Results and ConclusionAfter realigning and normalisation, maps of relative T2 increase were calculated. The results demonstrate that the spatial distribution of short-term effects of FES of denervated muscle tissue of paraplegic patients who regularly perform FES can be visualised by T2 parameter images. This may be used to refine models of the electrical field of FES in muscle and fibre activation in the future.

Estimating Activation of Denervated, Degenerated Muscle after Functional Electrical Stimulation with Magnetic Resonance Imaging

FES of long-term denervated, degenerated human skeletal muscle has been shown to be a suitable method for improving a number of physiological parameters. The underlying mechanisms of activation of a denervated muscle fiber can be described with suitably modified and extended Hodgin-Huxley type models, coupled with 3D finite element models of the surrounding electrical field. Regions of activation within a muscle can be determined using a 3D computer model. However, simulation results have not yet been validated experimentally. During and immediately after exercise, muscle shows increased T2-relaxation times. It is thus possible to estimate muscle activation non-invasively and spatially resolved with the MRI method of T2-mapping, which was, therefore, chosen as a suitable validation approach. Six patients were scanned prior to FES training with a MSME-sequence at 3 Tesla and then asked to perform one of their regular daily training-sessions (leg extensions). Subjects were then repositioned in the MR-scanner and two to five post-exercise scans were recorded. Pre- and post exercise scans were co-registered and T2-parameter maps were calculated. ROIs were drawn manually around quadriceps femoris and its antagonists. In well trained patients, activation in the quadriceps was found to be considerably higher than in its antagonists. These experimental results will help further improve existing models of FES of denervated degenerated human skeletal muscle.