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Biomedical subjects

F A Roberge

Publications and source records attributed to F A Roberge.

At least 37 records · Page 2Linked to original sources
PubMed1989

Using structural and visual information in physiological systems modeling.

This paper emphasizes the growing importance of precise models of biological structures to be used in conjunction with dynamic models of physiological events for improved physiological systems modeling. Medical imaging developments, represented mainly by computerized tomography, have been oriented principally towards medical diagnostic applications, that is towards the obtention of semi-quantitative information using sophisticated three-dimensional image display and manipulation capabilities. On the other hand, three-dimensional reconstruction of objects for modeling purposes presents rather more stringent requirements. The most important are the precise and independent representation and manipulation of the structures involved, the possibility of modifying the model parameters, the access to geometric data for specific measurements and transformation, and the compatibility with CAD/CAM software. Methods to obtain and manipulate such information are becoming available and a particular system used for the geometric modeling of the vertebral spine and knee is outlined.

Biomechanical Phenomena↗
PubMed1988

The inverse problem in electrocardiography: solutions in terms of equivalent sources.

This paper reviews those inverse electrocardiographic solutions that compute the electrical activity of the heart in terms of equivalent sources such as multipoles or multiple dipoles, as opposed to more realistic source formulations such as epicardial potentials. It treats, in succession, inverse solutions in terms of a single fixed-location dipole, a multipole series, moving dipoles, and, finally, multiple fixed-location dipoles. For each category of solution, simulation studies, animal experiments, and work involving human subjects are reviewed. Finally, more recent work that seeks to compute the cardiac activation isochrones, from the time integrals of the torso potentials during the QRS complex of the electrocardiogram, is described. The paper concludes with a discussion on the future of inverse electrocardiographic solutions in terms of equivalent sources.

Animals↗
PubMed1987

Revised formulation of the Hodgkin-Huxley representation of the sodium current in cardiac cells.

The purpose of this paper is to revise the parameters of the Hodgkin-Huxley formulation for the Na+ current in ventricular myocardial cells. To this end we have assembled much of the recent voltage clamp data on cardiac preparations obtained with modern voltage clamp and patch clamp techniques. The selected activation and inactivation characteristics of the Na+ channel and other membrane parameters represent a good compromise between available experimental measurements and lead to a reasonable average representation of the cardiac Na+ membrane current. The resulting Na+ conductance changes during the action potential upstroke are much larger than in earlier models, so that the upstroke is much faster and the peak depolarization is close to the Na+ equilibrium potential. The firing threshold level is nearly constant for resting potentials in the range of -70 and -90 mV. The maximum rate of rise of the action potential displayed by the new model is quite comparable to experimental observations.

Action Potentials↗
PubMed1987

Using Vmax to estimate changes in the sodium membrane conductance in cardiac cells.

Relative changes in the sodium conductance of the resting cardiac cell membrane are often estimated from relative changes in the maximum rate of rise of the action potential (Vmax). This approach has given rise to some controversy and it has not been possible so far to test it directly on an experimental basis. We have examined here the validity of this estimation using three different Hodgkin-Huxley representations of the cardiac membrane sodium current. The two basic requirements are a constant membrane capacitance and a negligible relative value of the nonsodium membrane currents at the time of Vmax. It is shown further that the approach leads to a satisfactory estimation only when the latency of Vmax is kept constant and a correction factor for the sodium driving force is applied to Vmax measurements. This conclusion applies either to a nonpropagated action or to an action potential propagated at constant velocity, provided that the membrane is not too strongly depolarized. It is valid for a wide range of sodium equilibrium potentials and a range of maximum sodium conductances limited to about 50% of the nominal value.

Action Potentials↗
PubMed1986

Reconstruction of propagated electrical activity with a two-dimensional model of anisotropic heart muscle.

The propagated electrical activity in normal anisotropic cardiac muscle is characterized by directionally dependent variations in the rising phase of the action potential. An important question concerns the relation between such variations and the propagation velocity and extracellular potentials. This problem was studied here in a sheet of cells, under conditions of uniform intracellular anisotropic resistivity and constant electrical membrane properties, through a numerical solution of the two-dimensional propagation equation. The numerical solution implies a lumping of the cytoplasmic and intercellular resistances into an equivalent junctional resistance to form a distributed resistive network representing the intracellular domain. The interstitial space is assumed isotropic and unbounded, with a resistivity of 100 omega X cm. The electrical properties of the cell membrane are represented by a Beeler-Reuter model. The stimulus current is applied to a small area of the sheet, and attention is focussed on the stable propagated events occurring some 5 or 6 length constants away from the stimulation site. The numerical solution is a good approximation of a continuous uniform structure when the cell size is less than 10% of the length constant along both major axes. Conditions of non-uniform propagation, with directionally dependent variations in the maximum rate of rise and time constant of the foot of the action potential were simulated by increasing the cell size to 30% of the length constant in the transverse direction of the sheet. Our results indicate that the directional changes in the maximum rate of rise correspond to small modifications of the extracellular potentials, while the directional changes in time constant of the foot are associated with the propagation velocity. The maximum effects are observed along the transverse direction as follows: a 19% increase in maximum rate of rise corresponds to a decrease of about 6% in the peak-to-peak amplitude of the extracellular potential, and a 24% increase in time constant of the foot is associated with a decrease of about 7% in the propagation velocity. Under the conditions of the present study, however, the simulated directional changes in maximum rate of rise are smaller than those experimentally observed so the corresponding changes in the extracellular potentials are probably underestimated.

Action Potentials↗
PubMed1985

Numerical integration in the reconstruction of cardiac action potentials using Hodgkin-Huxley-type models.

A comparison between traditional numerical integration methods and a new hybrid integration method for the reconstruction of action potential activity is presented, using a mathematical model of the cardiac Purkinje fiber (MNT model). It is shown that the hybrid integration method reduces importantly the overall computation time required for solving the Hodgkin-Huxley differential equations describing membrane electrical events. To accomplish this, the particular form of the gating variable equations is exploited to reformulate the step-by-step computation. In this way, the time increment can be made much larger compared with traditional methods when the membrane potential changes slowly. A mathematical analysis of the hybrid integration method is presented also, together with a numerical verification of its performance both for the propagated and nonpropagated membrane action potential. It is shown that the local error, that is the error arising at each integration step, and the cumulative integration error are strictly controlled by the membrane potential offset. Using the MNT model, the nonpropagated cardiac Purkinje action potential can be reconstructed in real time with an accuracy of 1% for the potential and 5% for the time of occurrence of its main features. In reconstructing propagated events, the hybrid integration method allows computation time savings by a factor of 10 or more compared to accurate Runge-Kutta schemes.

Action Potentials↗
PubMed1985

Localization of cardiac ectopic activity in man by a single moving dipole. Comparison of different computation techniques.

The accuracy of different computation techniques for the non-invasive localization of cardiac ectopic activity was evaluated. Body surface potentials were recorded from 63 leads in 14 patients with implanted pacemakers. The location, orientation and magnitude of a single moving dipole (SMD) were computed from the first eight terms of a truncated multipole expansion estimated from the body surface potentials. The SMD trajectories obtained during the QRS complex were plotted along with the heart outlines and pacing leads obtained independently from chest x-rays. The origin of the SMD trajectories was compared to the position of the pacing lead to evaluate the accuracy of the SMD. The optimum computation technique used a least-squares (LS) estimation of the multipole expansion truncated at 15 multipoles, in conjunction with a torso model that included regions of lower conductivity representing the lungs. With this method, the SMD trajectories originated near the pacing lead (25 +/- 12 mm) and adequately represented the progression of the ectopic wavefront across the entire heart silhouette. With the LS techniques using 8 or 24 multipoles, the spans of the trajectories were respectively too short, or too long to cover the heart, and the average distance between the SMD at QRS onset and the pacing lead was larger. With a surface integration technique, the SMD-pacing lead distances were similar, both for a finite homogeneous torso model with a fixed geometry, as well as for torso models adapted to the torso geometry of each patient. The SMD was found adequate to represent the progression of an ectopic wavefront, and to localize its origin in man.

Aged↗
PubMed1984

Application of the single moving dipole inverse solution to the study of the Wolff-Parkinson-White syndrome in man.

The single moving dipole (SMD) inverse solution was performed in 28 patients with the Wolff-Parkinson-White preexcitation syndrome to see if the calculated position of the SMD during the initial delta wave could indicate the site of the underlying accessory pathway. This site was first estimated to be at one of eight locations around the atrioventricular ring, from the patient's QRS and ST segment body surface potential maps, as has been described by others. Next, SMD parameters were calculated during the delta wave so as to approximate, on a numerical torso model, the patient's body surface potential map. Visualization of the calculated position of the SMD around the atrioventricular ring was done by projecting it on a plane parallel to this ring. This plane corresponded to the most basal transverse section of a heart model present in the torso model. One limitation was the use of non-varying heart and torso models for all patients. As a result, the SMD technique lacked the precision to separate accessory pathway sites into eight atrioventricular locations. However it was capable of distinguishing between patients belonging to the larger classes of right-sided, posterior, and left-sided preexcitation, formed by combining adjacent atrioventricular accessory pathway locations. With more accurate heart and torso models, it may be possible to increase SMD resolution so as to locate accessory pathway sites deep within the heart. This would represent an advantage over the surface potential map approach which only identifies the site of earliest epicardial breakthrough associated with the accessory pathway.

Adolescent↗
PubMed1982

Telemedicine in northern Quebec.

Television transmission of diagnostic and educational information can help to improve specialized medical care in remote and underserviced areas. This paper describes a pilot study in which the Canadian satellite Anik-B was used to link the James Bay area in northern Quebec with two large Montreal teaching hospitals. Broad-band real-time television was well suited for tele-education and teleconsultation activities. A much less costly method, using narrow-band slow-scan television, was also examined, but it requires improvements. The technology of telemedicine is in place, but its future use is impeded by the prohibitive costs of operating an efficient two-way broad-band television system for several remote health care sites. A solution to this problem may be an intermediate-band system combining some of the low-cost features of narrowband slow-scan television with the interactive high-resolution advantages of broad-band real-time television.

Canada↗
PubMed1982

Narrowband teleradiology.

A narrowband communication system using the Canadian satellite ANIK-B was used to transmit radiographic images from northern Quebec to Montreal. This slow scan television (SSTV) system was used to study the accuracy of the radiologist's interpretation, both with samples of pre-selected films and current patients. Several hours of training in the technique of radiographic interpretation from a static image displayed on a TV monitor was an important factor affecting performance. In selected patients, based on a sample of 124 error-free direct viewing interpretations by four radiologists, SSTV reading by the same fully trained radiologists was correct in 84.5%. With actual patients, and assuming the interpretation from direct viewing to be the correct one, SSTV reading of the radiographs by the same radiologist gave a corresponding figure of 84.1% in a sample of 518 patients containing 73.4% of negative findings, and 89.4% in another sample of 305 patients, including 82% of negative findings. The present error rate on SSTV reading is twice as great as radiographic interpretation using a broadband television system.

Canada↗
PubMed1981

A comparative evaluation of three different approaches for detecting body surface isopotential map abnormalities in patients with myocardial infarction.

Three approaches for detecting abnormalities in body surface potential maps recorded from patients with myocardial infarction were evaluated. The maps are generated from 26 simultaneously recorded unipolar electrocardiograms. All three approaches detect the deviations in certain parameters from control values determined from 50 normal subjects. The first approach emphasizes qualitative deviations in the trajectories of the surface potential map extrema during QRS and correctly classified all but one infarct in a test group comprising 30 normals and 30 cases of myocardial infarction. The second approach classifies a test subject as abnormal if any one of his 26 lead waveforms deviates appreciably at any instant during QRS from the mean waveform for the particular lead plus or minus two standard deviations, these being determined from the control group. This method, while correctly identifying all infarcts, resulted in a large number of false positives, misclassifying 22 of 30 normals. A final method was to obtain an instant by instant plot of the correlation coefficient between the mean surface potential map during QRS for the 50 normals and that of the subject being tested. Test cases were classified as abnormal if any correlation coefficient value fell below an envelope determined from the correlation coefficient plots obtained by correlating the maps of all 50 normals with their own mean. Twenty-nine normals and 26 infarcts were correctly classified. On the basis of these results, the first approach is superior to the other two for detecting surface potential map abnormalities in patients with myocardial infarction.

Electrocardiography↗
PubMed1981

Teleradiology in northern Quebec.

A two-way television network using the Canadian satellite ANIK-B was utilized to transmit radiographic images from Northern Quebec to Montreal. The accuracy of the radiologist's interpretation and his satisfaction with the TV system were studied using a series of 67 preselected cases and 425 current clinical cases. The four participating radiologists gave correct TV interpretations in 81% of the 39 selected cases presented at the beginning of the experiment. This value reached 94% for the other 28 selected cases presented after three months of regular use of the TV system. With current clinical cases, the agreement between TV and direct interpretations was 93%. Although magnification was available, correct identification of very small lesions proved to be the major source of error. On the whole, the radiologists were satisfied with the TV system.

Adult↗