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P J Birkui

Publications and source records attributed to P J Birkui.

16 recordsLinked to original sources
PubMed1996

Modeling of cardiac electrophysiological mechanisms: from action potential genesis to its propagation in myocardium.

The aim of the present paper is to describe the different attempts at modeling cardiac electrophysiological mechanisms, mainly at the membrane and cellular level, from action potential genesis to its propagation in myocardium. The Hodgkin and Huxley model describing the nervous action potential's theoretical reconstruction is first recalled, for it represents the basic model for a large part of cardiac action potential models. These models (Beeler and Reuter, Van Capelle and Durrer, Luo and Rudy) are then successively studied as their main applications by diverse authors. Varied approaches, like the Fitzhugh-Nagumo model (derived from the Bonhoeffer-Van der Pol model of oscillatory systems) or cellular automata models applied to the study of ventricular activation wave propagation and diseases associated with its perturbation, are then presented and discussed. Other, different approaches, such as general studies of excitable media, are evoked. This paper concludes with a critical evaluation of these different methods of electrophysiological cardiac modeling and of the main domains in which they led to significant results and in which they appear able to generate future perspectives.

Action Potentials↗
PubMed1995

Effect of myocardial infarction and ischemia on induction of cardiac reentries and ventricular fibrillation.

The present work is aimed at investigating the effects of myocardial infarction and ischemia on induction of ventricular fibrillation. Electrophysiologic effects of global and local ischemia (variation of the dispersion of refractory periods as well as conduction velocity) on initiation of reentry mechanisms was studied by means of computer simulations based on a cellular automata model of propagation of activation wave through a ventricular surface element. A local area of ischemia where effects of the dispersion of refractory periods are investigated is then simulated. This is made using a Gaussian distribution characterized by its mean and standard deviation. These simulations show that ischemia is capable of initiating reentry phenomena which propagate through the whole ventricle; they are responsible for ventricular fibrillation which causes sudden cardiac death, even when ischemia only involves limited parts of the myocardium. Statistical study of the probability of reentries as a function of both of the size of ischemic zones and the rate of dispersion of refractory periods shows that the latter parameter is of primary importance in triggering cardiac reentries.

Cell Hypoxia↗
PubMed1995

Electrode polarity does not alter the initial ventricular activation sequence during pacing with extracardiac electrodes.

Ventricular epicardial mapping was performed in six closed-chest anesthetized dogs to investigate the cardiac electrical response to external pacing. A right thoracotomy was performed, complete AV block was produced by formaldehyde injected into the AV node and a sock electrode array, comprised of 127 unipolar electrodes, was placed over the ventricles. Isopotential and isochronal epicardial maps were generated by computer from the unipolar electrograms. Unipolar stimulation pulses were applied between pairs of different types of cutaneous (metal, carbon) and esophageal (metal) electrodes, and recordings were performed at maximum lung inflation. Isopotential maps recorded during the stimulation artifacts showed that the epicardial electrical field was little affected by the type of electrode but depended mostly on electrode position. A reproducible and relatively uniform apex-to-base potential gradient was regularly produced with anteroposterior and anteroesophageal electrode configurations. This uniform potential gradient induced ventricular activation sequences that showed interindividual differences. Thus, for each dog, the areas of initial activation observed on the isochronal maps during pacing tended to remain the same (i.e., apical, lateral, and anterior) despite changes in the stimulation protocol. Inverting the polarity of the electrodes did not appreciably change the site of origin of activation (81% remained the same) and activation never originated from the area showing the most negative potential during the stimulation artifact. In conclusion, since electrode polarity does not seem to alter the ventricular activation sequence during cardiac pacing with extracardiac electrodes, the standard nomenclature for cutaneous patches, which defines the negative electrode as the "active" electrode, may have to be revised.

Animals↗
PubMed1992

[Thoracic transcutaneous cardiac pacing].

Transcutaneous cardiac pacing was abandoned when endocardial pacing was developed, but it has now come back, with a new methodology, in cardiological intensive care. Its safety and efficacy have been improved by 50 to 100 sq. cm electrodes, impulses of 20 to 40 ms duration and programmed energy. A perfect electrocardiographic analysis can be obtained by specific treatment of the pacing signal. New research work has begun concerning the sequence of activation. The hypothesis of simultaneous atrial and ventricular pacing, which has been put forward after the first experimental studies, is seldom confirmed by clinical data: it has been demonstrated that pacing is exclusively ventricular and that its influence on haemodynamics is equivalent to that of endocardial ventricular pacing. Transcutaneous pacing is the only technique that can be used in non-hospital extensive care, and this leads to the concept of mobile units performing defibrillation, ensuring electrophysiological monitoring and capable to treat circulatory arrests due to pause or ventricular tachyarrhythmias. Transcutaneous pacing is being reconsidered in cardiology units, where it can be used to reduce ventricular tachycardias, since it has the advantage over endocardial pacing or being immediately applicable and completely safe.

Arrhythmias, Cardiac↗
PubMed1992

Noninvasive transcutaneous cardiac pacing: modern instrumentation and new perspectives.

Noninvasive transcutaneous cardiac pacing has evolved from a simple stand alone unit with no ventricular sensing to a complete cardiac arrest resuscitation system combining synchronous pacing and defibrillation capabilities and using a single set of multifunction electrodes. In current instrumentation, four configurations exist including stand alone unit, modular configuration, built-in monitor and recorder, and built-in monitor, recorder and defibrillator. In present day devices, ventricular sensing, extensive programmability, and large surface electrodes are general features. Capture monitoring requires specific integrated electrocardiographic capability. Future developments are expected to involve low threshold electrode technology, integrated mechanical monitoring, and interdevice electrode compatibility.

Cardiac Pacing, Artificial↗
PubMed1990

Directional variability of stimulation threshold measurements in isolated guinea pig cardiomyocytes: relationship with orthogonal sequential defibrillating pulses.

Reports on delivery of separated orthogonal pulses markedly improving cardiac defibrillation have suggested that the stimulation threshold of heart fibers varies in accordance with their orientation within the electric field. The present work was aimed at investigating the directional variability of stimulation thresholds in isolated guinea pig cardiomyocytes. This variability was measured in 48 single myocytes by rotating each one through a theta (theta) angle between two-fixed parallel electrodes 1.1 cm apart, thus making theta vary between the electric field and the myocyte axis. For theta = 0 degrees, the mean longitudinal current stimulation threshold was 16.92 +/- 4.20 mA (n = 48). When theta was increased by increments of 10 degrees up to 90 degrees, the stimulation threshold increased in an exponential way. For theta = 90 degrees, the mean transverse stimulation threshold was 63.13 +/- 13.30 mA. These results clearly demonstrate the dependence of isolated cardiomyocyte stimulation thresholds on their orientation within the electric field and may account for the improved efficacy of defibrillation previously observed after delivery of orthogonal pulses.

Animals↗
PubMed1988

Reduction of energy required for defibrillation by delivering shocks in orthogonal directions in the dog.

Reduction of energy required to defibrillate (ERD) seems to represent a necessary condition for intensive development of implantable defibrillator, so as for minimization of cardiac and pulmonary damages provoked by high energy transthoracic defibrillation electric shocks. The present work describes a defibrillation method using shocks delivered in orthogonal directions and separated by a 100 ms delay. Defibrillation threshold measured with classical unidirectional shocks on 30 dogs has been found to be 286.8 +/- 22.2 joules. In the same animals, defibrillation threshold measured by use of orthogonal shocks has been found to be 101.4 +/- 14.9 joules. We conclude that this crossed shocks method leads to a substantial reduction of ERD (64%).

Animals↗
PubMed1988

[Tobacco dependence: first risk factor of atherosclerosis. Survey among smokers and non-smokers in group dynamics].

Several epidemiological and experimental studies have demonstrated an increased risk of atherosclerosis in smokers. The secondary prevention of this risk factor is only possible, presently, through anti-tobacco consultations. Several approaches are possible but there are many recurrences within the year following the wean-off. In a group dynamics, including patients having tried another method, we have conducted an investigation among inveterate smokers (n = 106), former smokers (n = 50) and a group of non-smokers (n = 34). Group dynamics remains a suitable method for smokers willing to get rid of their addiction. The number of daily cigarettes consumed by inveterate smokers is considerably and lastingly reduced, and 27 p. cent of the patients quit smoking. Monitoring of laboratory tests (HbCo, Blood count, HDL-cholesterol, thiocyanate, etc.), electrocardiogram and chest X-Ray, completes this weekly consultation and permits a stronger motivation for the smoker to quit smoking. Sometimes associated with acupuncture or homeopathy, this method permits a progressive de-conditioning from tobacco addiction.

Arteriosclerosis↗
PubMed1986

[TM-mode echocardiography in the evaluation of ventricular function in convalescent patients after recent infarction. Correlations with hemodynamic parameters recorded at rest and after a cycloergometric test].

133 patients (50 +/- 11 years), under observation at 1 to 2 months after the acute episode (38 +/- 10 days) and in the absence of medical treatment in most cases (90%), underwent the same day a maximal cycloergometric test, limited to symptoms of clinostatism with hemodynamic monitoring (Swan-Ganz 7F for pressure and thermodilution), and a good quality echocardiogram in TM mode (semi-automatic reading) in order to identify every valvular lesion. The ECHO-TM parameters of the left ventricular function (LV) taken into consideration (circumferential velocity of shortening of LV : CVSLV, percentage of systolic shortening of LV, telediastolic dimension of LV,E-septum distance, PR-AC, relation between the intervals Q-mitral block and aortic block-mitral point E,QC/A2E, left auricular dimension and mitral point B) have shown a low correlation (0.31) with the telediastolic pulmonary arterial pressure (TDPAP) at rest and after effort. The ECHO-TM parameters have not demonstrated significant differences when the patients are grouped according to the locus of the infarction (76 inferior, 50 anterior and 7 antero-inferior) and age (36 younger than 45 years, 87 aged between 45 and 64 years and 10 older than 64 years), whereas significant differences were found on segregating the patients according to the value of the last level at a threshold of 75 watts (44 patients did not attain 75 watts and 89 exceeded it).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1983

[Evaluation of physical fitness following uncomplicated myocardial infarct by bicycle ergometry in horizontal and vertical position].

For an increasingly early mobilisation after uncomplicated myocardial infarction, the stress tests appraised the individual physicalal aptitude in supine and upright positions without incident, in a simple and objective way. To complete this evaluation, the calculation of an energetic index EI was proposed as soon as the initial level of 25 watts during cycloergometric symptom-limited stress tests in 17 male patients (increment of 25 watts for each 6 minutes and interval of one hour between the two positions). The pulmonary wedge pressure (Swan Ganz) at 15 mmHg in upright position as soon as 25 watts separated the subjects into two groups GI (N = 5) and G II (N = 12). EI 25 W had a value of 33 +/- 4 for GI and of 48 +/- 7 for GII (p less than 0.001). In the following level, EI 50W had a value of 83 +/- 14 for GI and of 109 +/- 23 for GII (p less than 0.01). Although the heart rate HR reached at the last level (expressed in % of the maximal theoretical rate) and the systolic arterial tension SAT were no different between the two groups, the subjects of GI reached a mean load of 68 watts and these of GII of 109 watts (p less than 0.01). The index EI, calculated in a simple and immediate way with HR, SAT, W and morphometric data (height, area surface) would help to evaluate the physic aptitude. Because of its harmlessness, this stress test at low load in the two positions might be repeated to define an developmental profile for each person. The prognostic value of such an index remain to be specified.

Adult↗
PubMed1981

Study of relations between arterial oscillation period and heart rate.

This study was performed to determine the relationship between variations of arterial cross-section and cardiac period. Relative section variations were recorded by impedance rheography. In 176 anesthetized adult mongrel dogs, aortic and femoral rheograms, femoral pressure, and ECG were recorded. The cardiac period (Tc) and the interval between the systolic and the dicrotic wave, i.e., the arterial intrinsic period (Ta), were measured and related to each other (Tc/Ta ratio). In 125 dogs, the dicrotic wave appeared spontaneously. In these cases Tc/Ta 1.98 +/- 0.16. In 51 dogs, no dicrotic wave appeared spontaneously. Lengthening of cardiac period was obtained in 25 of these dogs by vagus stimulation. As related to the cardiac period before vagal stimulation Tc/Ta was then 0.98 +/- 0.08. In the remaining 26 dogs, such a lengthening was obtained by inducing a premature ventricular depolarization. In this group, Tc/Ta was 1.00 +/- 0.03. The results of the present study suggest that a constant adaptation exists between ventricular ejection and arterial dynamics. The physiologic implications are discussed.

Animals↗
PubMed1981

Closed-chest myocardial ischaemia in dog.

Myocardial ischaemia in dog was induced with releasable material in the distal segment of the anterior descending branch of the left coronary artery. Three releasable materials were tested: gel foam, wax microspheres (120-200 micron) and latex balloons, using different methods of introduction. The left carotid route was selected for introduction of a preformed catheter. The gel foam and wax microspheres caused transitory ischaemia, which was proximal for the foam and distal for the microspheres. The balloons made it possible to standardize the ischaemia as its localization and duration could be selected. This material therefore provided a model for stable chronic ischaemia. Nine dogs were observed by means of precordial mapping (36 electrodes) during the phase following ischaemia or for a period of 4 weeks. The results of these experiments are analysed and correlated with histological results for the post-mortem phase.

Animals↗
PubMed1980

Energetic expenditure during ergometric training after myocardial infarction.

The immediate effects of a single rehabilitation program were appraised in 55 patients with stabilized myocardial infarction (40 inferior and 15 anterior). Subjects were submitted to functional evaluation before and after 20 sessions of interval training on different types of apparatus (cyclorowing, treadmill, bicycle ergometer) and divided into three groups comprising 32, 15 and 8 patients respectively. In both inferior and anterior localization of coronary disease, the predictive value of the energetic indexes Ei1 and Ei2 indicated the improvement in physical fitness resulting from exercise training. For each type of infarct, 15% of the cases studied showed no such improvement after both short- and long-term training. Early changes in energetic indexes are positively correlated with the results of functional evaluation program. Where training is not beneficial, these indexes may therefore be used to help define session by session individualized programs.

Adult↗