IEEE transactions on bio-medical engineering
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IEEE Trans Biomed Eng · Dec 2002
Rigorous Green's function formulation for transmembrane potential induced along a 3-D infinite cylindrical cell.
The quasi-static electromagnetic field interaction with three-dimensional infinite-cylindrical cell is investigated for both intracellular (IPS) and extracellular (EPS) current point-source excitation. The induced transmembrane potential (TMP), expressed conventionally via Green's function, may alternatively be expanded into a faster-converging representation using a complex contour integration, consisting of an infinite-discrete set of exponentially decaying oscillating modes (corresponding to complex eigenvalues) and a continuous source-mode convolution integral. The dominant contributions for both the IPS and EPS problems are obtained in simple closed-form expressions, including well documented special mathematical functions. ⋯ Only for shorter EPS distance should the cable equation solution (i.e., the order zero dominant mode) be included in addition to the modes of order one. For on-membrane EPS location, additional modes should be included as well. In view of our EPS result, we suggest that the cable equation modeling existing in the literature and related to functional electrical stimulation for EPS problems, should be critically reviewed and corrected.
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IEEE Trans Biomed Eng · Dec 2002
Comparative StudyExtracellular stimulation window explained by a geometry-based model of the neuron-electrode contact.
Extracellular stimulation of single cultured neurons which are completely sealing a microelectrode is usually performed using anodic or biphasic currents of at least 200 nA. However, recently obtained experimental data demonstrate the possibility to stimulate a neuron using cathodic current pulses with less amplitude. ⋯ Modulation of the voltage sensitive channels in the sealing part of the membrane appears to be the key to successful cathodic stimulation. Furthermore, the upper limit of the stimulation window can be explained as a normal consequence of the neuronal membrane electrophysiology.
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IEEE Trans Biomed Eng · Nov 2002
Comparative StudyEsophageal pulse oximetry utilizing reflectance photoplethysmography.
Peripheral perfusion is often poor and barely pulsatile in patients undergoing prolonged major surgery. Hence, the arterial blood oxygen saturation (SpO2) readings from commercial finger pulse oximeters can become unreliable or cease when they are most needed. To overcome this limitation, the esophagus has been investigated as an alternative measurement site, as perfusion may be preferentially preserved centrally. ⋯ The means (+/-SD) of the differences between the esophageal SpO2 and SaO2 results from blood gas analysis and CO-oximetry were 0.02 +/- 0.88% and -0.73 +/- 0.72%, respectively. In five (10.2%) of the patients, the finger pulse oximeter failed for at least 10 min while the esophageal SpO2 readings remained reliable. The results confirm that the esophagus may be used as an alternative monitoring site for pulse oximetry even in patients with compromised peripheral perfusion.
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IEEE Trans Biomed Eng · Nov 2002
Comparative StudyExploratory analysis of the spatio-temporal deformation of the myocardium during systole from tagged MRI.
Myocardial contractile function is, with perfusion, one of the main affected factors in ischemic heart diseases. In this paper, we propose an original framework based on functional data analysis for the quantitative study of spatio-temporal parameters related to the myocardial contraction mechanics. ⋯ We show that it is possible to detect abnormal strain patterns comparatively to this model, by generating distance maps at rest and under pharmacological stress. We demonstrate the consistency of the results for the circumferential deformation parameter on healthy and pathological data sets.
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IEEE Trans Biomed Eng · Oct 2002
Clinical TrialElevating pain thresholds in humans using depolarizing prepulses.
Electrocutaneous stimulation is a potentially useful communication tool for applications in virtual reality, sensory substitution, and sensory augmentation. Many of these applications require the use of arrays of small electrodes. Stimulation through small electrodes is often painful, however, limiting the practicality of such arrays. ⋯ The dose effect of PP amplitude was also highly significant (p < .0001), with larger PPs elevating pain thresholds more. To our knowledge, this is the first report of PPs being used to elevate electrical stimulation thresholds in humans. PPs may be useful for selective inactivation of neural subpopulations in many human neuroprosthetic applications.