Biomedical subjects
Publications and source records attributed to A R LeBlanc.
Using wavelets, we computed the entropy of the signal at various frequency levels (wavelet time entropy) and, thus, find an optimal measure to differentiate normal states from ischemic ones. This new indicator is independent from the ST segment and yet provide a conclusive detection of the ischemic states.
In order to characterize ST-segment shifts during transient coronary artery occlusion, 24 patients with single-vessel disease were continuously monitored during percutaneous transluminal coronary angioplasty by use of a computerized orthogonal lead system. Changes of ST-segment (J + 60 ms) in leads X, Y, and Z and of the ST vector magnitude were analyzed by using 20 microV as a threshold for significant ST-segment shift. The sensitivity and magnitude of this shift were compared among the left anterior descending, right coronary, and circumflex artery groups (11, 8, and 5 patients, respectively) during balloon inflation. Significant ST-segment shifts were seen in 22 patients (92%) in ST-VM, Y, and Z leads and all patients in lead X (100%). There was no significant difference in sensitivity of either the ST vector magnitude or the most sensitive lead for occlusion detection among the three groups. There was a significantly greater magnitude of ST shift during left anterior descending artery occlusion than during right coronary artery and circumflex artery occlusions in ST-VM. Analysis of the direction of ST shifts in the X, Y, and Z leads showed a characteristic pattern, which could distinguish among the three coronary groups in 21 patients (88%). The presence of collaterals was significantly associated with ST-segment depression in leads oriented toward ischemia (3 of 6 patients) as compared with ST-segment elevation in the absence of collaterals (all of 15 patients), P > .01. It is concluded that ST-segment shift in the orthogonal leads is a reliable marker for myocardial ischemia. It is equally sensitive to occlusion of each of the three major coronary arteries and can thus identify the occluded coronary. An ST-segment depression instead of an elevation was related to the presence of collaterals, which may reflect a lesser degree of ischemia.
The main goal of this study was to simulate clinical body surface potential maps, recorded during percutaneous transluminal coronary angioplasty protocols, using a realistic geometry computer heart model. Other objectives were to address the question of reciprocal ST-segment changes observed in the 12-lead electrocardiogram during ischemia and to verify the hypothesis that the shortening of the QRS duration observed in left anterior descending (LAD) coronary artery occlusion may be explained by conduction delay in the septal His-Purkinje system. Simulation was achieved by first introducing into the heart model three transmural zones of mild, moderate, and severe ischemia for assumed occlusions in the LAD, left circumflex, and right coronary arteries. The heart model was then excited, in turn, with these three zones present for assumed occlusions in the LAD, left circumflex, and right coronary arteries. Myocardial conduction velocities in the regions of moderate and severe ischemia were assumed to be reduced to 75 and 50% of normal, respectively. Model action potentials in the mild, moderate, and severely ischemic zones were also altered to reflect known ischemic changes in these action potentials. Body surface potential maps and electrocardiograms were computed by placing the heart inside a numerical torso model. Simulated map patterns during both ST-segment and QRS were qualitatively similar to clinical maps. Reciprocal ST-segment depression was observed for all three occlusions in remote leads that did not overlie the ischemic zones. QRS shortening due to septal His-Purkinje conduction delay was verified. The simulation results attest to the model's ability to reproduce body surface potential distributions recorded following percutaneous transluminal coronary angioplasty protocols. The simulations also showed that reciprocal ST-segment changes occur as a natural consequence of the primary ischemic region and that there is no need to invoke a second region of ischemia. Finally, the model demonstrated that QRS shortening can occur in LAD occlusion despite a slowing of conduction down the septal His-Purkinje system.
Synchronized electrical stimulation was used to study the heart rate (HR) response to fluctuations in parasympathetic input to the sinus node in anesthetized dogs. This was obtained by varying the time interval (interpulse interval) between stimulatory vagal pulses. Spectral methods were used to estimate transfer functions between the excitatory signal and the resulting HR response for different intensities of vagal stimulation. The intensity of vagal stimulation was proportional to the number of pulses delivered in each cardiac cycle. From the estimated transfer functions, and based on a mathematical model of the time course of ACh concentration at the sinus node, filter models were derived by using a system identification approach. HR response was characterized by a combination of two different filter behaviors: a low-pass filter behavior of mean cut-off frequency of 0.065 Hz and an all-pass filter behavior. The magnitude of the low-pass filter gain decreased with increasing intensity of vagal stimulation. The magnitude of the all-pass filter gain increased and then decreased with increasing intensity of vagal stimulation. The all-pass filter characteristics of HR response during synchronized stimulation of the vagus nerves are specific to this mode of stimulation, because they were not observed in nonsynchronized modes of vagal stimulation. We can conclude that, during synchronized vagal stimulation, the HR response exhibits both a slow dynamic component and a fast component related to beat-to-beat variations.
A simplified bioelectric model of the atrioventricular (AV) junction is proposed to better understand the underlying biophysical generation of intracardiac His-bundle signal and to analyze the effects of electrocatheter displacements on waveform morphology and estimation of AV node conduction time (AVCT). Based on this model, an inverse problem approach has been developed to estimate electrode displacements occurring in real recordings. A measurement correction method is proposed to improve estimation of AVCT. Results illustrate autonomic influences on AVCT, a phenomenon hardly measured with common techniques.
Many integrated clinical information systems depend on large knowledge bases containing dictionary of terms as well as specific information about each term and the relationships between terms. We propose a knowledge base model called MD Concept which is based on a semantic network and uses an object-oriented paradigm and relational tables. A prototype has been developed which integrates the Unified Medical Language System (UMLS) with other databases including the Systematized Nomenclature of Medicine (SNOMED II), the Diagnostic and Statistical Manual of Mental Disorders (DSM-IIIR) and a pharmaceutical database. We demonstrate how a user can easily navigate in this knowledge world using a browser.
A model is proposed to explain mechanisms of propagation in the atrioventricular (AV) node of the heart. The model is based on a simplified two-dimensional anatomic description of the central node region and on a bidomain tissue model for the propagation. The central region is described as a three-tissue compartment model; the proximal tissue composed of ANL cells, the central tissue composed of N cells and the distal tissue composed of NH cells. The central N region is outlined as an unexcitable gap by forcing these cells to behave as depressed cells for which the fast ionic currents are inactivated. This model has allowed the authors to test the electrotonic gap hypothesis, which explains the time-delay properties of the AV node. Typical conduction curves have been obtained, as well as common patterns of Wenckebach blocks. Premature stimulus and resulting mapping of cell responses in the N region of the model show typical membrane potential dissociation in two components. The model has also been submitted to several programmed stimulation protocols in documented anterograde and retrograde conduction. The results show that the model is valid and covers several well known dynamic properties of the AV node.
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Quantitative analysis of cardiac arrhythmias has been the subject of intensive research during the last 10 years. Several systems have been designed to help in the processing of cardiac signals: single or multiple lead electrocardiograms (ECG), electrograms from intracardiac catheters, esophageal recordings, etc. The main objective of these developments was oriented toward positive identification of arrhythmias or rhythm-disturbance counts to imitate the cardiologist's interpretation in contexts such as routing ECG and ambulatory recordings. However, these systems were mainly measurement tools aimed at extracting auricular and ventricular depolarization timings plus gross morphology description. The domain of morphology analysis of beat-to-beat auricular depolarization on ECG has never been highly active due to poor signal conditions. For routine ECG, automatic interpretation was set as an objective to complement computer-assisted ECG interpretation of conduction problems (i.e., morphology analysis of a representative beat extracted or averaged from the dominant rhythm). The limitations of rhythm interpretation in this context are well known. In the ambulatory ECG context, the analysis procedures are relatively simple and are often summarized as trivial counts describing, most exclusively, the ventricular arrhythmic behavior of the heart over a relatively long duration. Waveform-detection and measurement have been the bottleneck of advancement in arrhythmia analysis since highly reliable detection of events on a beat-to-beat basis are necessary to perform a valid analysis. Rare approaches have proposed probabilistic definition of event detection. The present review puts emphasis on the potential of several methods which have been demonstrated as powerful in identifying short- or long-duration heartbeat patterns, mode of heartbeat initiation, mode of heartbeat coupling, etc. Globally, these methods are referred to as time series analysis, modeling of rhythm patterns, simulation, and pattern recognition. A delay in the advancement of the study of arrhythmogenesis and limiting the analysis of arrhythmias to textbook descriptions is not justified when put in perspective of the potential of implementing powerful techniques which have been more or less neglected or used in a narrow way.
To investigate ventricular tachycardias produced in healthy canine myocardium by stimulation of sympathetic ganglia or cardiac nerves, we simultaneously recorded a surface ECG and 63 ventricular electrograms in anesthetized open-chest dogs. Isochronal and isopotential maps were generated off-line by computer. Ventricular tachycardia with uniform beat-to-beat morphology was induced in 13 or 22 dogs by electrical stimulation of the left stellate ganglion (five experiments), the left middle cervical ganglion (four experiments), the left caudal pole cardiopulmonary nerve (two experiments), or the ventrolateral cardiac nerve (eight experiments). It was not inducible by stimulation of the right-sided major cardiopulmonary nerves or ganglia. In most instances the earliest measured electrical excitation occurred on the posterior aspect of the ventricles. Isochronal maps demonstrated a radial spread of the impulse away from the area of earliest excitation. Changes in the region of earliest excitation and (or) activation pattern were accompanied by changes in QRS morphology. The potential gradients measured between areas displaying positive and negative T waves on the anterior and left lateral aspects of the ventricles were significantly increased by ventrolateral cardiac nerve stimulation. However, the ventricular regions where these potential gradients existed differed from the regions of earliest excitation during ventricular tachycardia. These results demonstrate that the thoracic autonomic nervous system can induce repetitive ventricular excitation originating from consistent loci.
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S-2395 is a new, long-acting, non-selective beta-receptor blocking agent without apparent intrinsic cardiodepressive action and no receptor selectivity. Its electrophysiological actions were studied in intact and chemically sympathectomized dogs. In normal dogs, S-2395 very slightly increased atrial monophasic action potential (MAPa) duration, atrioventricular functional (FRP) and ventricular effective (ERP) refractory periods. The onset of the atrial supernormal conduction phenomenon (SNCP) was delayed and the ventricular SNCP was abolished. These results are consistent with the action of low concentrations of beta-blocking agents. In contrast S-2395 reduced the ventricular MAPv and increased the ERP/MAPv ratio of sympathectomized animals. Such modifications are usually seen with concentrations higher than necessary for simple beta-receptor blockade. In conclusion S-2395, like several others beta-blockers, had minor effects on the classical electrophysiological parameters of normal dogs. However, it suppressed ventricular SNCP and had a more pronounced action in sympathectomized dogs who are known to have higher levels of circulating catecholamines and who present post-synaptic supersensitivity. The SNCP has been linked with re-entrant arrhythmias. Considering that beta-blockers prevent arrhythmias specially in hyperadrenergic states, the suppression of the ventricular SNCP by S-2395 could thus be the mechanism by which this drug and possibly other beta-blockers might exert their antiarrhythmic action.
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Explore the source record for details and available documents.
Explore the source record for details and available documents.
Some of the problems associated with the precise evaluation of left ventricular volumes from angiographic images are due to the magnification and distortion of the pictures obtained from X-ray equipment. Magnification errors can be evaluated easily but the distortion (due to pincushion effect) are more difficult to measure. A mathematical method is proposed for a complete point-by-point correction of errors due to magnification and X-ray distortion. This method is applicable to most currently used angiographic equipment. Since more and more angiographic images are processed with local or remote digital computers, the applicability of the proposed technique becomes realistic. Results from this study show that a 5-30% overestimation of usual geometric ventricular parameters is encountered if no correction is made for distortion. End-diastolic measurements are more affected than end-systolic ones because of the greater degree of spherical distortion at the periphery of the picture area.
A method is described for graphically presenting interval data, such as neural interspike intervals or electrocardiographic R-R intervals, in a form that facilitates the identification of nonstationaries. The method is essentially a plot of isoprobability contours of the cumulative interval histogram, as functions of time. A sequential algorithm is used for updating the contour-line positions. This display is used in an interactive system for visually identifying nonstationarities, and for subsequently comparing selected segments of the data quantitatively using the Kolmogorov-Smirnov test.
Sinus rhythm tracings, including sinus tachycardia and bradycardia, are characterized quantitatively by means of an ECG measurement program which has been subjected to rigorous evaluation. The analysis is performed on tracings of short duration (10 sec). The features of regularity and stability are considered for the R wavetrain. Regularity is evaluated from the normalized differences between sucessive RR intervals. Stability is determined by the ratio of maximum and minimum RR interval durations. Due to the difficulities of automatic beat-to-beat detection and measurement of P waves, an estimate of the PR interval is obtained from a pseudo-PR interval determined from certain features of the P and R wavetrains. The constancy of this pseudo-PR interval is evaluated, and its absolute value is uded as a characteristic of the type of sinus rhythm.