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Biomedical subjects

F A Roberge

Publications and source records attributed to F A Roberge.

At least 55 records · Page 3Linked to original sources
PubMed1978

Possible mechanisms underlying bursting pacemaker discharges in invertebrate neurons.

Certain invertebrate neurons generate endogenous bursts of action potentials due to an underlying slow membrane potential oscillation. An early hypothesis for the oscillations proposed a high resting sodium conductance that led to the depolarizing phase, followed by activation of an electrogenic sodium pump coupled to chloride ions, leading to the hyperpolarizing phase. Recent findings contradict this hypothesis. Current thought implicates two conductances in the generation of the oscillations. The depolarizing phase is due to an increase in a sodium or a calcium conductance; the hyperpolarizing phase is due to a subsequent increase in a potassium conductance, which may be either voltage dependent or triggered by an influx of calcium ions. The observation of a negative slope conductance region in the membrane I-V characteristic supports this hypothesis. Bursting cells also usually exhibit anomalous rectification, i.e., a decrease in slope conductance with depolarization, in the I-V characteristic. This decrease may result from a decrease in an outward current or an increase in an inward current. The more important bursting membrane characteristics have been incorporated into an electronic analog. The analog confirms the appropriateness of the two-conductance hypothesis. It also suggests that potassium ion accumulation outside the cell membrane may enhance bursting activity.

Action Potentials↗
PubMed1978

A new method for the precise and complete correction of distortion on cineangiographic image: its effect on left ventricular measurements.

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.

Cardiac Volume↗
PubMed1977

On the visualisation of nonstationarities in point processes.

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.

Computers↗
PubMed1977

[Simulation of the electrical activity of an excitable membrane with an electronic analog (author's transl)].

1. The sodium and potassium conductances of the HODGKIN-HUXLEY model are simulated by a field effect transistor with a series resistor. This arrangement leads to a simple analog model of the excitable membrane (fig. 1 and 2). 2. Normally, the model is silent (fig. 3), but it becomes automatic (fig. 4) when the decay time (de-activation) of the potassium conductance is at least twice the recovery from inactivation time of the sodium conductance (taud greater than 2 tauri). 3. The effects of changes in sodium (fig. 5 and 6) and potassium (fig. 7, 8 and 9) concentration gradients upon the membrane potential and the ionic currents are easily studied when the model is silent or automatic. 4. When automatic, an increase in the potassium concentration gradient induces a lengthening of the period and ultimately, when the gradient is very high, spontaneous activity is blocked (fig. 9). On the other hand, increases of sodium gradient over 30% of normal value do not modify the period (fig 6). 5. The potassium concentration gradient modifies the excitability solely through membrane polarization (fig. 8), while sodium concentration has no effect on it (fig. 5). 6. Results with the model strengthen the hypothesis that tetraethylammonium (TEA) acts on both the maximum potassium conductance (gK) and the mechanism of sodium conductance inactivation (Tauh) to lengthen the action potential as observed on the Ranvier node (fig. 10). Effects of TEA on potassium conductance activation are also discussed. 7. Because of its simplicity and accuracy, this model lends itself easily to many other simulations.

Action Potentials↗
PubMed1976

Computer characterization of sinus rhythm.

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.

Adult↗
PubMed1976

[Analogs and models of systemic arterial circulation].

This review article is primarily aimed at those researches using the experimental physiological approach and gives a survey of analogs and models of the systemic arterial blood flow. They are presented in increasing order of complexity in two sections: the "lumped" and the "distributed" parameter models. In the first section a brief description is given of the purely resistive, Windkessel and multi-element analogs with emphasis on their use in physiology for estimating several hemodynamic variables. The distributed parameter models, whose central idea is pulse wave propagation along the arterial tree, are then presented in five categories, using a new classification scheme. These categories are based on four criteria derived from the assumptions used in the formulation of the fluid and wall equations. The most recent non-linear models are treated in more detail with their common properteis as well as their particular contribution emphasized. In conclusion it is noted that the distributed parameter models also begin to be used to evaluate physiological variables in the experimental and clinical context and that this trend can only become more important in view of the advances made in the field of biomedical instrumentation.

Animals↗
PubMed1975

Roles of the AV junction in determining the ventricular response to atrial fibrillation.

The statistical properties of RR interval sequences during cholinergic atrial fibrillation were studied in anesthetized dogs both in control conditions and after the selective injection of dromotropic agents into the atrioventricular (AV) node artery. It was observed that RR interval histogram configurations depended mainly on the mean heart rate, regardless of whether it was a control or a post-injection sequence. The sequences were found to vary from almost regular at fast rates to highly irregular at slow rates, covering all intermediate possibilities. Since the injections of dromotropic agents into the AV node artery were carried out during sinus rhythm between the episodes of fibrillation, their influences on the AV junction, as reflected both on the length of the PR interval during sinus rhythm and on the RR interval dispersion during fibrillation, could be compared. The dispersion of RR intervals was found to increase as the PR interval duration became longer. In addition, it was observed that the generally random character of the RR interval sequences during fibrillation was not affected by the injection of dromotropic agents into the AV node artery. These results were interpreted as an indication that, for a well-established atrial fibrillation, the degree of ventricular irregularity (dispersion of RR intervals) is related to the conductivity within the AV junction and that the random character of RR interval sequences is related to the atrial fibrillatory activity itself.

Acetylcholine↗
PubMed1975

Computation of aortic pulse wave velocity and aortic pulse wave velocity and aortic extensibility from pressure gradient measurements.

This study is concerned with the computation of aortic pulse wave velocity based on simultaneous recordings of the aortic pressure gradient and first-time derivative of aortic pressure. These variables were recorded by means of a double-lumen catheter introduced in the aorta of four anesthetized closed chest dogs, and connected to critically damped manometer systems. Results of aortic pulse wave velocity were then compared: (i) to the true phase velocity obtained from spectra of apparent phase velocity, and (ii) to the pulse wave velocity computed from the time shift between maximum slopes of the pressure wave. From the aortic valves to 37 cm down the aortic trunk, pulse wave velocity increased from 410-460 cm/s to approximately 600-800 cm/s. Based on the wave propagation equation presented of Bramwell and Hill (Bramwell, J.C., and Hill, A. V. 1922. Proc. R. Soc. 93, 298-306), volumetric extensibility coefficients were computed from pulse wave velocity data. Results indicated that, from the aortic valves to 37 cm down to the aorta, the mean volumetric extensibility decreased from 0.43-0.56% deltaV/cm H2O to 0.16-0.25% deltaV/cm H2O (1 cm H2O = 94.1 N/m2).

Animals↗