Biomedical subjects

J J Struijk

Publications and source records attributed to J J Struijk.

At least 19 recordsLinked to original sources

PubMed2006

Classification of the long-QT syndrome based on discriminant analysis of T-wave morphology.

The long QT syndrome (LQTS) is a genetic disorder, typically characterized by a prolonged QT interval in the ECG due to abnormal cardiac repolarization. LQTS may lead to syncopal episodes and sudden cardiac death. Various parameters based on T-wave morphology, as well as the QT interval itself have been shown to be useful discriminators, but no single ECG parameter has been sufficient to solve the diagnostic problem. In this study we present a method for discrimination among persons with a normal genotype and those with mutations in the KCNQ1 (KvLQT1 or LQT1) and KCNH2 (HERG or LQT2) genes on the basis of parameters describing T-wave morphology in terms of duration, asymmetry, flatness and amplitude. Discriminant analyses based on 4 or 5 parameters both resulted in perfect discrimination in a learning set of 36 subjects. In both cases cross-validation of the resulting classifiers showed no misclassifications either.

Adolescent↗

PubMed2005

Influence of variable nerve fibre geometry on the excitation and blocking threshold. A simulation study.

The aim of the study was to investigate how variable fibre geometry influences the excitation and blocking threshold of an undulating peripheral nerve fibre. The sensitivity of the excitation and blocking thresholds of the nerve fibres to various geometric and stimulation parameters was examined. The nerve fibres had a spiral shape (defined by the undulation wavelength, undulation amplitude and phase), and the internodal length varied. Diameter-selective stimulation of nerve fibres was obtained using anodal block. Simulation was performed using a two-part simulation model: a volume conductor model to calculate the electrical potential distribution inside a tripolar cuff electrode and a model of a peripheral undulating human nerve fibre to simulate the fibre response to stimulation. The excitation threshold of the undulating fibres was up to 100% higher than the excitation threshold of the straight fibres. When a nerve was stimulated with long pulses, which are typically applied for anodal block (> 400 micros), the blocking threshold of the undulating fibres was up to four times higher than the blocking threshold of the straight fibres. Dependencies of the excitation threshold on geometric and stimulation parameters were the same as for a straight fibre. Dependencies of the blocking threshold on geometric and stimulation parameters were different compared with a straight fibre. Owing to the fibre undulation and variable internodal length, the blocking threshold and the minimum pulse duration to obtain anodal block were generally different in the proximal and distal directions. Owing to variable fibre geometry, the excitation threshold varied by up to +/- 40% of the mean value, and the blocking threshold varied by up to +/- 60 % of the mean value. Owing to undulation, the blocking threshold of large fibres could be higher than the blocking threshold of small-diameter fibres, even if they had the same geometry. The results indicate that, during skeletal muscle stretching and contracting or during variation in joint angle, the excitation and blocking thresholds of the nerve fibres change owing to variations in fibre geometry. A straight fibre model could be too simple for modelling the response of peripheral nerve fibres to electrical stimulation.

Electric Stimulation↗

PubMed2001

Calculation of electric fields in a multiple cylindrical volume conductor induced by magnetic coils.

A method is presented for calculating the electric field, that is induced in a cylindrical volume conductor by an alternating electrical current through a magnetic coil of arbitrary shape and position. The volume conductor is modeled as a set of concentric, infinitely long, homogeneous cylinders embedded in an outer space that extends to infinity. An analytic expression of the primary electric field induced by the magnetic coil, assuming quasi-static conditions, is combined with the analytic solution of the induced electric scalar potential due to the inhomogeneities of the volume conductor at the cylindrical interfaces. The latter is obtained by the method of separation of variables based on expansion with modified Bessel functions. Numerical results are presented for the case of two cylinders representing a nerve bundle with perineurium. An active cable model of a myelinated nerve fiber is included, and the effect of the nerve fiber's undulation is shown.

Electric Conductivity↗

PubMed2001

Evaluation of the cable model for electrical stimulation of unmyelinated nerve fibers.

The cable model, used to calculate the membrane potential of an unmyelinated nerve fiber due to electrical stimulation, is reexamined under passive steady-state conditions. The validity of two of the assumptions of the cable model are evaluated, namely that the membrane potential be a function of the axial coordinate only and that the extracellular potential due to the presence of the nerve fiber be negligible. The membrane potential calculated from the passive steady-state cable model is compared with the membrane potential obtained from an analytical three-dimensional (3-D) volume conductor model of a nerve fiber. It is shown that for very small electrode-fiber distances (of only a few fiber radii), both assumptions are violated and the two models give quite different results. Over a wide range of the electrode-fiber distance (about 0.1 mm to 1 cm), both assumptions are fulfilled and the two models give approximately the same results. For very large distances (more than 10 cm, independent of fiber diameter) only the second assumption is satisfied, but a modification of the activating function of the cable model allows to calculate the membrane potential in agreement with the 3-D model.

Electric Stimulation↗

PubMed2000

Measurement of the performance of nerve cuff electrodes for recording.

New designs of cuff electrodes for the recording of signals from peripheral nerves are typically tested in acute animal experiments before long-term evaluation takes place. A reproducible, cost-effective and fast method is presented for evaluating cuff electrodes with respect to signal amplitude, noise rejection, and, in some cases, selectivity, as an alternative to acute in vivo experiments. Comparisons with a computer model and with signals obtained from rabbit tibial nerve give good agreement with the new method. It is shown that an imperfect closure of the cuff around the nerve can easily lead to more than 50% loss of the signal amplitude. Noise from sources external to the cuff is not significantly affected by the closing mechanism, but is strongly reduced by a tripolar cuff configuration as compared with a monopolar one (reduction factor 2.8 to 58, mean = 6.5, n = 6). In dual-channel cuffs, cross-talk is below 1.2% indicating a very high selectivity.

Action Potentials↗

PubMed1999

Magnetic and electrical stimulation of undulating nerve fibres: a simulation study.

Mathematical models of myelinated nerve fibres are highly stylized abstractions of real nerve fibres. For example, nerve fibres are usually assumed to be perfectly straight. Such idealizations can cause discrepancies between theoretical predictions and experimental results. One well-known discrepancy is that the currently used models predict (contradictory to experimental findings) that an activation of nerve fibres is not possible with a pure transverse electric field. This situation occurs when a magnetic coil is placed symmetrically above a straight nerve fibre for magnetic nerve stimulation, or when an anode and a cathode are placed equidistantly on a line perpendicular to the fibre in the case of electrical stimulation. It is shown that this discrepancy does not occur if the physiological undulation of peripheral nerve fibres is included in the models. Even for small undulation amplitudes (e.g. 0.02 mm), it is possible to activate the fibre in these positions. For physiological undulations, as found in the literature, and favourable (off-centre) positions, the typical reduction of the thresholds is in a range between one and five, compared with perfectly straight fibres.

Electric Stimulation↗

PubMed1998

Multigrid solution of the potential field in modeling electrical nerve stimulation.

In this paper, multilevel techniques are introduced as a fast numerical method to compute 3-D potential field in nerve stimulation configurations. It is shown that with these techniques the computing time is reduced significantly compared to conventional methods. Consequently, these techniques greatly enhance the possibilities for parameter studies and electrode design. Following a general description of the model of nerve stimulation configurations, the basic principles of multilevel solvers for the numerical solution of partial differential equations are briefly summarized. Subsequently, some essential elements for successful application are discussed. Finally, results are presented for the potential field in a nerve bundle induced by tripolar stimulation with a cuff electrode surrounding part of the nerve.

Biomedical Engineering↗

PubMed1998

Theoretical performance and clinical evaluation of transverse tripolar spinal cord stimulation.

A new type of spinal cord stimulation electrode, providing contact combinations with a transverse orientation, is presented. Electrodes were implanted in the cervical area (C4-C5) of two chronic pain patients and the stimulation results were subsequently simulated with a computer model consisting of a volume conductor model and active nerve fiber models. For various contact combinations a good match was obtained between the modeling results and the measurement data with respect to load resistance (less than 20% difference), perception thresholds (16% difference), asymmetry of paresthesia (significant correlation) and paresthesia distributions (weak correlation). The transversally oriented combinations provided the possibility to select either a preferential dorsal column stimulation, a preferential dorsal root stimulation or a mixed stimulation. The (a)symmetry of paresthesia could largely be affected in a predictable way by the selection of contact combinations as well. The transverse tripolar combination was shown to give a higher selectivity of paresthesia than monopolar and longitudinal dipolar combinations, at the cost of an increased current (more than twice).

Adult↗

PubMed1997

The extracellular potential of a myelinated nerve fiber in an unbounded medium and in nerve cuff models.

A model is presented for the calculation of single myelinated fiber action potentials in an unbounded homogeneous medium and in nerve cuff electrodes. The model consists of a fiber model, used to calculate the action currents at the nodes of Ranvier, and a cylindrically symmetrical volume conductor model in which the fiber's nodes are represented as point current sources. The extracellular action potentials were shown to remain unchanged if the fiber diameter and the volume conductor geometry are scaled by the same factor (principle of corresponding states), both in an unbounded homogeneous medium and in an inhomogeneous volume conductor. The influence of several cuff electrode parameters, among others, cuff length and cuff diameter, were studied, and the results were compared, where possible, with theoretical and experimental results as reported in the literature.

Action Potentials↗

PubMed1997

Recruitment characteristics of nerve fascicles stimulated by a multigroove electrode.

The recruitment characteristics of fascicle-selective nerve stimulation by a multigroove electrode have been investigated both theoretically and in acute experiments. A three-dimensional (3-D) volume conductor model of fascicles in a multigroove device and a model of myelinated nerve fiber stimulation were used to calculate threshold stimuli of nerve fibers in these fascicles. After their exposition, fascicles from rat sciatic nerve were positioned in different grooves of appropriate sizes and stimulated separately. The device appeared to be suitable for fascicle-selective stimulation, because both computer simulations and acute animal experiments showed that crosstalk between neighboring fascicles is not a problem, even when monopolar stimulation was used. The threshold stimulus was lower for a small fascicle than for a large one. When the amount of (conducting) medium between contact and perineurium or its conductivity was reduced, threshold stimuli were lower. Moreover, simulations predict that the slopes of recruitment curves are smaller and inverse recruitment order is less pronounced. Simulations also showed that a small contact is preferable to a large on, because a small contact gives a slightly smaller slope of the recruitment curve. Both experimentally and theoretically a significantly smaller slope of recruitment curves was obtained by stimulation with a cathode and an anode at opposite sides of the fascicle, driven by two current sources giving simultaneous pulses with different, but linearly dependent amplitudes.

Animals↗

PubMed1996

Transverse tripolar spinal cord stimulation: theoretical performance of a dual channel system.

A new approach to spinal cord stimulation is presented, by which several serious problems of conventional methods can be solved. A transverse tripolar electrode with a dual-channel voltage stimulator is evaluated theoretically by means of a volume conductor model, combined with nerve fibre models. The simulations predict that a high degree of freedom in the control of activation of dorsal spinal pathways may be obtained with the described system. This implies an easier control of paraesthesia coverage of skin areas and the possibility to correct undesired paraesthesia patterns, caused by lead migration, tissue growth, or anatomical asymmetries, for example, without surgical intervention. It will also be possible to preferentially activate either dorsal column or dorsal root fibres, which has some important clinical advantages. Compared to conventional stimulation systems, the new system has a relatively high current drain.

Analgesia↗

PubMed1995

Significance of the spinal cord position in spinal cord stimulation.

The effects of the antero-posterior and medio-lateral positions of the spinal cord in the dural sac on the perception threshold and paresthesia coverage in spinal cord stimulation were analyzed. The distributions of the dorsal cerebrospinal fluid (CSF) layer thickness, measured from transverse MR scans of normal subjects at various spinal levels, were used to calculate the distributions of threshold voltages for the stimulation of spinal nerve fibers by a computer model. These theoretical threshold distributions were shown to fit well to the corresponding distributions of perception threshold measured in patients. It is concluded that the thickness of the dorsal csf layer is the main factor determining the perception threshold and paresthesia coverage in spinal cord stimulation: an increasing thickness raises the threshold and reduces the coverage, and vice versa. The effects of an asymmetrical electrode position with respect to the spinal cord midline were also analyzed by computer modeling. It is concluded that a lateral asymmetry of less than 1 mm gives a significant reduction of perception threshold and may result in unilateral paresthesiae.

Adult↗

PubMed1995

Effects of electrode geometry and combination on nerve fibre selectivity in spinal cord stimulation.

The differential effects of the geometry of a rostrocaudal array of electrode contacts on dorsal column fibre and dorsal root fibre activation in spinal cord stimulation are analysed theoretically. 3-D models of the mid-cervical and mid-thoracic vertebral areas are used for the computation of stimulation induced field potentials, whereas a cable model of myelinated nerve fibre is used for the calculation of the excitation thresholds of large dorsal column and dorsal root fibres. The size and spacing of 2-D rectangular electrode contacts are varied while mono-, bi- and tripolar stimulation are applied. The model predicts that the highest preferential stimulation of dorsal root fibres is obtained in monopolar stimulation with a large cathode, whereas dorsal column fibre preference is highest in tripolar stimulation with small contacts and small contact spacings. Fibre type preference is most sensitive to variations of rostrocaudal contact size and least sensitive to variations of lateral contact size. Dorsal root fibre preference is increased and sensitivity to lead geometry is reduced as the distance from contacts to spinal cord is increased.

Computer Simulation↗

PubMed1994

Selective stimulation of sacral nerve roots for bladder control: a study by computer modeling.

The aim of this study was to investigate theoretically the conditions for the activation of the detrusor muscle without activation of the urethral sphincter and afferent fibers, when stimulating the related sacral roots. Therefore, the sensitivity of excitation and blocking thresholds of nerve fibers within a sacral root to geometric and electrical parameters in tripolar stimulation using a cuff electrode, have been stimulated by a computer model. A 3-D rotationally symmetrical model, representing the geometry and electrical conductivity of a nerve root surrounded by cerebrospinal fluid and a cuff was used, in combination with a model representing the electrical properties of a myelinated nerve fiber. The electric behavior of nerve fibers having different diameters and positions in a sacral root was analyzed and the optimal geometric and electrical parameters to be used for sacral root stimulation were determined. The model predicts that an asymmetrical tripolar cuff can generate unidirectional action potentials in small nerve fibers while blocking the large fibers bidirectionally. This result shows that selective activation of the detrusor may be possible without activation of the urethral sphincter and the afferent fibers.

Animals↗

PubMed1994

MR assessment of the normal position of the spinal cord in the spinal canal.

PURPOSE: To investigate intradural geometry, which strongly influences the effects of epidural spinal cord stimulation. METHODS: Axial MR images with turbo spin-echo were made of 26 healthy subjects at C-4 through C-6, T-5 and T-6, and T-11 and T-12, at T-11 and T-12 both in the supine and the prone position. Measurements were made of the dorsomedial and the ventromedial cerebrospinal fluid layer and the anteroposterior and transverse sizes of both the spinal cord and the dural sac. The samples of all variables were analyzed statistically. The distance between spinal and vertebral midline was also determined. RESULTS: The dorsal cerebrospinal fluid layer was 1.5 to 4.0 mm at C-4 through C-6 and 4.0 to 8.5 mm at T-5 and T-6. At T-11 it was 2.0 to 6.0 mm in the supine position and was increased by approximately 2.2 mm in the prone position. At T-12 these values were 1.5 to 4.5 mm and approximately 3.4 mm, respectively. Differences between the spinal and vertebral midline up to 1.5 to 2.0 mm occurred in approximately 40% of the images. CONCLUSIONS: Because there are variations of the dorsal cerebrospinal fluid layer among subjects by more than a factor of 2, and significant variations of the mediolateral position of the spinal cord, information on these parameters in patients will be essential for the optimal application of epidural spinal cord stimulation.

Adult↗

PubMed1993

Excitation of dorsal root fibers in spinal cord stimulation: a theoretical study.

In epidural spinal cord stimulation it is likely that not only dorsal column fibers are activated, but that dorsal root fibers will be involved as well. In this investigation a volume conductor model of the spinal cord was used and dorsal root fibers were modeled by an electrical network including fiber excitation. The effects of varying some geometrical fiber characteristics, as well as the influence of the dorsal cerebrospinal fluid layer and the electrode configuration on the threshold stimulus for their excitation, were assessed. The threshold values were compared with those of dorsal column fibers. The results of this modeling study predict that, besides the well known influence of fiber diameter, the curvature of the dorsal root fibers and the angle between these fibers and the spinal cord axis were of major influence on their threshold values. Because of these effects, threshold stimuli of dorsal root fibers were relatively low as compared to dorsal column fibers. Excitation of the dorsal root fibers occurred near the entry point of the fibers.

Electric Stimulation↗

PubMed1992

Recruitment of dorsal column fibers in spinal cord stimulation: influence of collateral branching.

An electrical network model of myelinated dorsal column nerve fibers is presented. The effect of electrical stimulation was investigated using both a homogeneous volume conductor and a more realistic model of the spinal cord. An important feature of dorsal column nerve fibers is the presence of myelinated collaterals perpendicular to the rostro-caudal fibers. It was found that transmembrane potentials, due to external monopolar stimulation, at the node at which a collateral is attached, is significantly influenced by the presence of the collateral. It is concluded that both excitation threshold and blocking threshold of dorsal column fibers are decreased up to 50% compared to unbranched fibers.

Action Potentials↗

PubMed1991

Epidural spinal cord stimulation: calculation of field potentials with special reference to dorsal column nerve fibers.

The effect of electrical stimulation with several electrode combinations on nerve fibers with different orientations in the spinal cord was investigated by computing the steady-state field potentials and activating functions. At first an infinite homogeneous model was used while secondly the spinal cord and its surrounding tissues were modeled as an inhomogeneous anisotropic volume conductor. The effect of mediodorsal epidural stimulation was calculated. It was concluded that with cathodal stimulation, mediodorsally in the epidural space, longitudinal fibers are depolarized, but dorsoventral ones are hyperpolarized. With anodal stimulation the opposite will occur. It was found that parameters substantially affecting the potential distribution in the dorsal columns are the conductivity of the white matter and the width and the conductivity of the csf layer.

Action Potentials↗