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Biomedical subjects

M P Mintchev

Publications and source records attributed to M P Mintchev.

33 records · Page 2Linked to original sources
PubMed1997

Comparative assessment of power dynamics of gastric electrical activity.

This study was undertaken to investigate the correlation between power dynamics of gastric electrical activity (GEA) assessed with different recording techniques. A total of seven eight-channel 1-hr combined recordings were obtained from three subjects in five consecutive postoperative days. Four channels were recorded from bipolar electrodes implanted into the gastric antral wall, and four channels were electrogastrographic (EGG). Six pairs of bipolar electrodes were inserted into the antral wall (three anterior; three posterior) of 16 anaesthetized dogs. Fourteen-channel (six internal GEA and eight EGG) 1/2-hr recordings were obtained from each dog. Sets of power values calculated from channel pairs (internal, EGG or mixed) were cross-correlated and the significance of the obtained correlation coefficients was examined (P < 0.05). The majority of power correlations of internal GEA channel pairs, and those of mixed (internal GEA-EGG) channel pairs were insignificant. These findings question the claims that EGG power dynamics mirrors the power dynamics of internal GEA.

Animals↗
PubMed1997

Electrogastrography can recognize gastric electrical uncoupling in dogs.

BACKGROUND & AIMS: Electrical uncoupling is the lack of electrical synchronization in different parts of the stomach. The ability of electrogastrography (EGG) to recognize gastric electrical uncoupling has not been adequately studied. The aim of this study was to determine the impact on EGG of surgically introduced gastric uncoupling in anesthesized dogs. METHODS: Six pairs of bipolar electrodes were inserted into the antral gastric wall of 16 anesthetized dogs at laparotomy. Eight-channel bipolar cutaneous EGGs were simultaneously recorded. Three separate half-hour recordings were made from each dog in the basal state and after each of two circumferential cuts of all gastric muscle. The stomach was divided into three equally sized areas, each with an electrode pair in its anterior and posterior walls. Gastric electrical activity was assessed visually. EGG was digitized and processed by computer. RESULTS: Criteria for EGG normality were defined from 9 dogs with normal gastric electrical activity. After the first antral cut, internal recordings from the area below the division were at a lower frequency and often irregular. The second cut produced three different frequencies. Suggested criteria for normality allowed correct recognition of 93% of all severely abnormal records. Records with mild gastric electrical abnormalities were recognized with a sensitivity of 74%. CONCLUSIONS: EGG can recognize severe electrical uncoupling.

Animals↗
PubMed1997

Reliability of percent distribution of power of the electrogastrogram in recognizing gastric electrical uncoupling.

The aim of this study was to examine the reliability of percent distribution of electrogastrographic (EGG) power in recognizing gastric electrical uncoupling. Sixteen anaesthetized dogs underwent laparotomy and implantation of six pairs of stainless-steel wire electrodes. Distal stomach was measured and three sections with approximately equal lengths were defined. Two pairs of electrodes were implanted in each section. Eight-channel EGG was also recorded. Three separate half-hour recordings were made: in the basal state; after a full circumferential separation of the distal antral section from the rest; after a second circumferential cut completely separating the middle from the proximal sections. EGG digital power spectra were split into three frequency ranges and dynamics of percent distribution of power was statistically examined. After the first cut, changes in the percent distribution of EGG power in the normal range were not significant (p = 0.2). Significant changes in the low range were noted (p < 0.05) and changes in the high range were borderline nonsignificant (p = 0.056). After the second cut, changes in percent distribution in the normal and the high range became significant (p < 0.01) while changes in the low range were insignificant (p = 0.075). Severe uncoupling was reflected in EGG by significant changes in the high-frequency range without internal tachygastria necessarily being present.

Animals↗
PubMed1996

Extracting quantitative information from digital electrogastrograms.

Cutaneous recordings of gastric electrical activity (electrogastrography (EGG)) could become a valuable non-invasive tool for recognising gastric electrical abnormalities. Although signals obtained with internally implanted electrodes deliver quantitative information, this technique cannot be used for diagnostic purposes because of its invasive nature. On the other hand, the objectivity of electrogastrography is still in question. The aims of this work are to develop computer techniques for extracting quantitative information from digital electrogastrograms, and to evaluate quantitatively EGG recordings from healthy volunteers. The dynamics of all four EGG parameters are studied: amplitude, frequency, time shift between different channels, and waveform. Four separate two-dimensional computer plots are developed using specially designed digital signal-processing procedures. Each parameter is evaluated in a study of 20 healthy volunteers. Frequency is found to be the only EGG parameter that shows quantitative consistency and merit.

Electrodiagnosis↗
PubMed1995

Conoidal dipole model of electrical field produced by the human stomach.

Spontaneous depolarisation and repolarisation due to ionic exchange are the main properties of smooth muscle cells in the human stomach. This change in the distribution of electrical charge results in the creation of an electric field. The field manifests itself as a potential difference (biovoltage), recorded both in vitro and in vivo and known as gastric electrical activity (GEA). The aim of the paper is to describe a computer model of this electric phenomenon, considering all anatomical and electrophysiological particularities of the stomach, and to simulate real in vivo experiments with a computer. In the proposed model, the depolarised smooth muscle cells are represented as organised electrical dipoles distributed with known density in an annular band that moves distally with increasing velocity. Computer simulations of in vivo experiments using this model not only give the waveform, duration, amplitude and frequency of GEA, but they also represent the phase lag between different channels, the difference in propagation velocity along greater and lesser curvatures, and the electric coupling between different parts of the stomach. The effects of changed electrode configuration, surface area and distance from the stomach are described. Mathematical modelling is done in spherical co-ordinates, and the simulations are performed in a specially designed user-friendly IBM PC environment. Some of the unsolved problems in cutaneous electrogastrography are also discussed.

Electrophysiology↗
PubMed1993

Accuracy of cutaneous recordings of gastric electrical activity.

BACKGROUND: This study was undertaken to determine the accuracy of cutaneous electrogastrography (EGG). METHODS: The ability of EGG to assess gastric contractions and electrical frequency, phase lag, and waveform was studied in 4 healthy volunteers with cutaneous electrodes before and after eating, 4 healthy fasting volunteers with intraluminal pressure recording tubes and cutaneous electrodes, 1 patient with surgically implanted gastric serosal electrodes, and 4 anesthetized dogs with serosal force transducers and implanted and cutaneous electrodes. In 2 of the dogs, an intragastric distending balloon was introduced. RESULTS: The use of long-distance serosal electrodes allowed direct comparison of internal and cutaneous signals. Cutaneous electrodes recognized 80%-85% of changes in frequency with spectral analysis. Only 30%-40% of increases in EGG amplitudes were associated with gastric contractions. Gradual distention of the canine stomach after blocking contractions with atropine and glucagon increased the EGG amplitudes. No appreciable phase lag could be discerned with EGG. The descending portion of the EGG waveform was predominant. CONCLUSIONS: Frequency dynamics is currently the only reliable cutaneous EGG parameter.

Adult↗
PubMedDate not supplied

Optimal placement of impedance epigastrography electrodes.

Impedance electrogastrography (IE) is a noninvasive method to assess gastric contractile activity. This indirect method uses a four-terminal impedance measuring method to determine impedance variations of the gastric region of the torso. These variations can be related to changes in the stomach shape and, therefore, to gastric contractions. Because the impedance variations are very small, the placement of the measuring electrodes on the skin is critical. In this study, a three-dimensional resistor model consisting of 4,466 resistors was used to simulate the human torso. The model was implemented on a personal computer, and the experimentally obtained optimal electrode locations were found to be superior to the commonly used ones. Measurements of volunteers confirmed these findings.

Body Mass Index↗
PubMedDate not supplied

Tuning non-linear adaptive compensators in non-ideal noise environment.

Adaptive compensation has been popular in a variety of biomedical signal processing applications. One of the major limitations of this method is the requirement for a primary signal + noise channel and a secondary noise channel, with the noises in the two channels being strongly correlated. In the majority of real-life biomedical applications this requirement can rarely be fulfilled since the secondary channel inevitably contains certain representation of the signal from the primary channel thus jeopardizing the adaptive compensation process. In the present work we describe a method for tuning an adaptive compensator containing a non-linear real-time multiplier in the reference channel in such non-ideal noise environment. The method is based on iterative adjustments of an exponential non-linear function which modifies in real-time the signal-to-noise balance in the reference channel in favor of the noise, preserving the correlation of the latter with the noise in the primary (signal + noise) channel but diminishing the signal content in the reference channel. Sets of model signals and noises are used to illustrate this methodology with a particular emphasis on biomedical signal applications. A quantitative description of the iterative tuning procedure is provided and the capabilities and limitations of this novel method for adaptive compensation are outlined.

Artifacts↗
PubMedDate not supplied

A quantitative study of the pixel-shifting, blurring and nonlinear distortions in MRI images caused by the presence of metal implants.

Magnetic resonance imaging has found an increasing number of medical applications in recent years due to its technical merits as well as its non-invasive nature. However, its full potential has been severely limited by magnetic susceptibility difference artefacts caused by the presence of ferromagnetic sources such as orthopedic implants, dental work or metallic needles used in neurosurgery. In this study, we propose a method to numerically quantify the distortions resulting from the magnetic susceptibility differences by investigating the phenomena from three perspectives: (1) pixel displacement, (2) blurring and (3) nonlinearity. For this purpose, phantom images obtained from a magnetic resonance scanner were studied. Attempts made to reconstruct an ideal image from its distorted version by appropriately compensating for the three types of distortions yielded encouraging results.

Artifacts↗
PubMedDate not supplied

Three-dimensional static parametric modelling of phasic colonic contractions for the purpose of microprocessor-controlled functional stimulation.

The study aimed at creating an integrated electromechanical model of invoked phasic contractions in canine colon during direct high frequency voltage stimulation. The model utilized data obtained from two large anaesthetized dogs that underwent laparotomy and serosal implantation of two circumferential electrode pairs into a distal segment of the left colon. The strength distribution of the stimulating electric field was analysed over a cylindrical mesh-surface grid modelling the interrogated colonic segment. Recordings of the stimulating current were utilized to model smooth muscle depolarization using linearized macroscopic tissue conductivity. The invoked contractile stress was related to the stimulating electric field strength using an exponential sigmoid function. Artificially produced occlusion of the lumen was derived for a pair of 5mm electrodes positioned on a cylindrical mesh-surface of 2 cm diameter and 15 cm length. The model simulated contractions invoked by stimuli of different amplitude (up to 12 V) with 98.6% accuracy of approximation. Macroscopic tissue conductivity was modelled as a combination of two first-order exponential terms involving a 3ms time constant. Real-time simulation of the current drawn by the smooth muscle during 10 V/50Hz bipolar voltage stimulation was performed. The integrated electromechanical model facilitates the quantification of microprocessor-controlled phasic colonic contractions.

Action Potentials↗
PubMedDate not supplied

Quantitative evaluation of the dynamics of external factors influencing canine gastric electrical activity before and after uncoupling.

Electrical uncoupling could be a major source of gastric motility abnormalities. Non-invasive recognition of the onsets of gastric electrical uncoupling could become an avenue for early detection of gastric motor abnormalities. Cutaneous recordings of gastric electrical activity (GEA), known as electrogastrograms (EGG), are dynamically influenced by a variety of external factors, jeopardizing their reliability. In the present study the dynamics of external factors influencing both internal GEA and EGG during surgically induced gastric electrical uncoupling are quantitatively explored. Sixteen anaesthesized dogs underwent implantation of six stainless steel pairs of bipolar electrodes, positioned among the sites for future divisions of the stomach into three equal sections. In the basal state and after each surgical cut, simultaneous 30-min recordings of six internal GEA and eight EGG channels were performed in each dog. The signals were filtered, conditioned and digitized to obtain their one-dimensional representations in the sampled time domain. These representations were regarded as particular projections of multidimensional dynamic vectors located in the phase space, which incorporated both the dynamics of the electrical processes taking place in the stomach and the dynamic interactions between the external factors influencing the recordings. Embedding dimensions in the phase space were considered quantitative expressions of all these factors. Comparative statistical evaluation of the dynamics of the embedding dimensions for both internal GEA and EGG channels was performed. Although the number of embedding dimensions did not change significantly with the introduction of gradual uncoupling both in the internal GEA and in the EGG channels, opposite tendencies towards increment in the former and decrement in the latter were noted. After the introduction of severe uncoupling, cutaneous embedding dimensions became very similar to the internal ones. A significant number of external factors influencing EGG recordings preclude reliable recognition of mild electrical uncoupling from EGG.

Animals↗
PubMedDate not supplied

Hardware-software co-design of portable functional gastrointestinal stimulator system.

In the recent years, functional electrical stimulation has been applied to restore impaired motility in the gastrointestinal tract. Unlike other methods of electrical stimulation of the gut, microprocessor-controlled, sequential electrical stimulation has been shown to induce peristalsis and enhance emptying in acute canine gastric and colonic models. This study aims at completing the development of a portable microprocessor-based functional stimulator system consisting of a microelectronic stimulator, patient-specific computer-based real-time software and a programming interfacing device. The ultimate goals of the design are to ensure that (1) the portable stimulator can be efficiently utilized in chronic animal experiments; and (2) the device can be further miniaturized into an implantable version. The designed portable stimulator generates four channel sequential bipolar rectangular pulse trains with programmable parameters within the stimulation requirements obtained from a previously developed computer model. Real-time simulation of colonic peristalsis and a case-specific stimulation model were implemented using patient-specific computer-based software. A chronic canine case study confirmed the feasibility of this microprocessor-controlled stimulation method for future clinical applications in humans.

Algorithms↗
PubMedDate not supplied

Electrogastrographic impact of multi-site functional gastric electrical stimulation.

Microprocessor-controlled multi-site functional gastric electrical stimulation using synchronized higher frequency (above 25 Hz) bipolar voltages has been suggested as a possible new avenue towards efficient artificial control of gastric motility. However, the effect of this sequential stimulation on gastric electrical activity is unknown. Because of the substantial strength of the invoked sequential contractions, using implanted electrodes to assess gastric electrical activity (GEA) in experimental animals is difficult, if not impossible. Electrogastrography (EGG), the non-invasively obtained recording of GEA, provides an excellent opportunity to study the changes associated with the multi-site functional stimulation. In this study, we investigated 4 unconscious dogs with 4 circumferential sets of subserosally-implanted stimulating electrodes and 4 force transducers attached close to each stimulating electrode set at operation. The abdominal cavity of each dog was closed after the electrode implantations and 3 standard neonatal electrocardiographic electrodes were attached overlying the abdominal projection of the gastric axis. Two-channel EGG recordings were obtained for 1/2 h before any electrical stimulation and during 1/2 h of intensive sequential stimulation. Significant changes were observed in the EGG pattern during stimulation. These changes were quantified and compared to the basal recordings. Our findings indicate that the interrelation between spectral changes in cutaneous EGG and mechanical activity of the stomach could be far more complex than previously speculated.

Animals↗
PubMedDate not supplied

Do increased electrogastrographic frequencies always correspond to internal tachygastria?

This study was undertaken to investigate the possible origin of some cutaneously recorded higher frequency electrogastrographic signals. Computer modeling of gastric electrical uncoupling was performed using previously described conoidal dipole model. Cutaneous electrogastrograms were simulated after uncoupling was introduced. In separate, real-life experiments, 6 pairs of bipolar electrodes were inserted into the gastric wall (3 anterior, 3 posterior) of 15 anesthetized dogs at laparotomy to record 6 channels of internal gastric electrical activity (GEA). Eight-channel bipolar cutaneous electrogastrography (EGG) was simultaneously recorded. Three separate 1/2-hr recordings were made from each dog in the basal state and after each of two circumferential cuts of all gastric muscle. Distal stomach was surgically divided into three equal-sized areas, each with an electrode pair in its anterior and posterior walls. Gastric electrical activity and EGG were digitized, bandpass-filtered and analyzed in frequency domain using the fast Hartley transform. The phenomena of tachygastria and tachyarrhythmia were quantitatively compared in internal and cutaneous recordings. Computer modeling indicated that it is possible to record cutaneous "tachygastric" or "tachyarrhythmic" signals without them being present internally. Real experiments on dogs showed higher percentage of tachyarrhythmia in EGG than in the internal signal in the basal state. After the first circumferential cut, the periods of tachygastria and tachyarrhythmia increased, with EGG signals showing again a higher percentage. This tendency persisted after the second circumferential cut. A similar pattern was observed when monitoring the percentage of uncorresponding tachygastrias/tachyarrhythmias. Our findings indicate that gastric electrical uncoupling can be another possible reason for abnormal frequency characteristics of EGG. Not all cutaneously recognized tachygastrias and/or tachyarrhythmias could be related to objectively existing internal tachygastric events.

Animals↗