Chest pulse-wave velocity: a novel approach to assess arterial stiffness.

Pulse-wave velocity (PWV) is considered as the gold-standard method to assess arterial stiffness, an independent predictor of cardiovascular morbidity and mortality. Current available devices that measure PWV need to be operated by skilled medical staff, thus, reducing the potential use of PWV in the ambulatory setting. In this paper, we present a new technique allowing continuous, unsupervised measurements of pulse transit times (PTT) in central arteries by means of a chest sensor. This technique relies on measuring the propagation time of pressure pulses from their genesis in the left ventricle to their later arrival at the cutaneous vasculature on the sternum. Combined thoracic impedance cardiography and phonocardiography are used to detect the opening of the aortic valve, from which a pre-ejection period (PEP) value is estimated. Multichannel reflective photoplethysmography at the sternum is used to detect the distal pulse-arrival time (PAT). A PTT value is then calculated as PTT = PAT - PEP. After optimizing the parameters of the chest PTT calculation algorithm on a nine-subject cohort, a prospective validation study involving 31 normo- and hypertensive subjects was performed. 1/chest PTT correlated very well with the COMPLIOR carotid to femoral PWV (r = 0.88, p < 10 (-9)). Finally, an empirical method to map chest PTT values onto chest PWV values is explored.

RF pulse concatenation for spatially selective inversion

It is shown that spatially selective inversion and saturation can be achieved by concatenation of RF pulses with lower flip angles. A concatenation rule which enables global doubling of the flip angle of any given excitation pulse applied to initial z magnetization is proposed. In this fashion, the selectivity of the single pulse is preserved, making the high selectivity achievable in the low flip-angle regime available for inversion and large flip-angle saturation purposes. The profile quality achievable with exemplary concatenated pulses is investigated in comparison with adiabatic inversion. It is verified that by using concatenated inversion in the transfer insensitive labeling technique (TILT), the MT artifact is suppressed. Copyright 2000 Academic Press.

Gravitational wave pulse and soliton wave collision

We study the collision of a gravitational wave pulse and a soliton wave on a spatially homogeneous background. This collision is described by an exact solution of Einsteins equations in a vacuum which is generated from a nondiagonal seed by means of a soliton transformation. The effect produced by the soliton on the amplitude and polarization of the wave is considered.

Pulse propagation sustained by noise in arrays of bistable electronic circuits

One-dimensional arrays of nonlinear electronic circuits are shown to support propagation of pulses when operating in a locally bistable regime, provided the circuits are under the influence of a global noise. These external random fluctuations are applied to the parameter that controls the transition between bistable and monostable dynamics in the individual circuits. As a result, propagating fronts become destabilized in the presence of noise, and the system self-organizes to allow the transmission of pulses. The phenomenon is also observed in weakly coupled arrays, when propagation failure arises in the absence of noise.

Heat pulse line-source method to determine thermal conductivity of consolidated rocks

An instrument designed to measure thermal conductivity of consolidated rocks, dry or saturated, using a transient method is presented. The instrument measures relative values of the thermal conductivity, and it needs calibration to obtain absolute values. The device can be used as heat pulse line source and as continuous heat line source. Two parameters to determine thermal conductivity are proposed: TMAX, in heat pulse line source, and SLOPE, in continuous heat line source. Its performance is better, and the operation simpler, in heat pulse line-source mode with a measuring time of 170 s and a reproducibility better than 2.5%. The sample preparation is very simple on both modes. The performance has been tested with a set of ten rocks with thermal conductivity values between 1.4 and 5.2 W m¿1 K¿1 which covers the usual range for consolidated rocks.

DNA topology: feeling the pulse of a topoisomerase.

The action of individual type II DNA topoisomerases has been followed in real time by observing the elastic response of single DNA molecules to sequential strand passage events. Micromanipulation methods provide a complementary approach to biochemical studies for investigating the mechanism of DNA topoisomerases.

Gravitational wave pulse and soliton wave collision

We study the collision of a gravitational wave pulse and a soliton wave on a spatially homogeneous background. This collision is described by an exact solution of Einsteins equations in a vacuum which is generated from a nondiagonal seed by means of a soliton transformation. The effect produced by the soliton on the amplitude and polarization of the wave is considered.

Synchronization in a lattice model of pulse-coupled oscillators

We analyze the collective behavior of a lattice model of pulse-coupled oscillators. By means of computer simulations we find the relation between the intrinsic dynamics of each member of the population and their mutual interactions that ensures, in a general context, the existence of a fully synchronized regime. This condition turns out to be the same as that obtained for the globally coupled population. When the condition is not completely satisfied we find different spatial structures. This also gives some hints about self-organized criticality.

Pulse wave analysis aortic pressure . distole should also be considered

La rigidité anormalement haute des artères à grande conductance est un marqueur de l'augmentation du risque cardiovasculaire et est typiquement retrouvée chez les patients diabétiques ou hypertendus. Ces vaisseaux deviennent plus rigides avec l'âge, expliquant la haute prévalence d'hypertension systolique chez les personnes âgées. Cette rigidification agit sur la pression sanguine de plusieurs façons. Notamment la fonction windkessel est gênée, menant à l'augmentation de la pression systolique et de la pression puisée, la diminution de la pression diastolique, et ainsi à l'augmentation de la postcharge ventriculaire gauche associée à une probable diminution de la perfusion coronarienne. De plus, la propagation des ondes de pression le long de l'arbre vasculaire est accélérée, de sorte que les ondes réfléchies générées au site de décalage d'impédance atteignent l'aorte ascendante plus tôt par rapport au début de l'éjection ventriculaire, aboutissant à une augmentation de la pression systolique centrale, ce qui n'arriverait pas en présence de vaisseaux moins rigides. Dans ce cas, au contraire, les ondes de pression antérogrades et réfléchies voyages plus lentement, de sorte que les ondes de réflexion tendent à atteindre l'aorte centrale une fois l'éjection terminée, augmentant la pression diastolique et contribuant à la perfusion coronarienne. La tonométrie d'applanation est une méthode non invasive permettant l'évaluation de la forme de l'onde de pression au niveau l'aorte ascendante, basée sur l'enregistrement du pouls périphérique, au niveau radial dans notre étude. Nous pouvons dériver à partir de cette méthode un index d'augmentation systolique (sAIX) qui révèle quel pourcentage de la pression centrale est du aux ondes réfléchies. Plusieurs études ont montré que cet index est corrélé à d'autres mesures de la rigidité artérielle comme la vitesse de l'onde de pouls, qu'il augmente avec l'âge et avec les facteurs de risques cardiovasculaires, et qu'il est capable de préciser le pronostic cardiovasculaire. En revanche, peu d'attention a été portée à l'augmentation de la pression centrale diastolique due aux ondes réfléchies (dAIX). Nous proposons donc de mesurer cet index par un procédé d'analyse développé dans notre laboratoire, et ce dans la même unité que l'index systolique. Etant donné que les modifications de la paroi artérielle modulent d'une part la vitesse de l'onde de pouls (PWV) et d'autre part le temps de voyage aller-retour des ondes de pression réfléchies aux sites de réflexion, toute augmentation de la quantité d'énergie réfléchie atteignant l'aorte pendant la systole devrait être associée à une diminution de l'énergie arrivant au même point pendant la diastole. Notre étude propose de mesurer ces deux index, ainsi que d'étudier la relation de l'index d'augmentation diastolique (dAIX) avec la vitesse de propagation de l'onde de pouls (PWV) et avec le rythme cardiaque (HR), ce dernier étant connu pour influencer l'index d'augmentation systolique (sAIX) . L'influence de la position couchée et assise est aussi étudiée. Les mesures de la PWV et des sAIX et dAIX est réalisée chez 48 hommes et 45 femmes âgées de 18 à 70 ans, classés en 3 groupes d'âges. Les résultats montrent qu'en fonction de l'âge, le genre et la position du corps, il y a une relation inverse entre sAIX et dAIX. Lorsque PWV et HR sont ajoutés comme covariables à un modèle de prédiction comprenant l'âge, le genre et la position du corps comme facteurs principaux, sAIX est directement lié à PWV (p<0.0001) et inversement lié à HR (p<0.0001). Avec la même analyse, dAIX est inversement lié à PWV (p<0.0001) et indépendant du rythme cardiaque (p=0.52). En conclusion, l'index d'augmentation diastolique est lié à la rigidité vasculaire au même degré que l'index d'augmentation systolique, alors qu'il est affranchi de l'effet confondant du rythme cardiaque. La quantification de l'augmentation de la pression aortique diastolique due aux ondes réfléchies pourrait être une partie utile de l'analyse de l'onde de pouls.

Pattern selection in a lattice of pulse-coupled oscillators

We study spatio-temporal pattern formation in a ring of N oscillators with inhibitory unidirectional pulselike interactions. The attractors of the dynamics are limit cycles where each oscillator fires once and only once. Since some of these limit cycles lead to the same pattern, we introduce the concept of pattern degeneracy to take it into account. Moreover, we give a qualitative estimation of the volume of the basin of attraction of each pattern by means of some probabilistic arguments and pattern degeneracy, and show how they are modified as we change the value of the coupling strength. In the limit of small coupling, our estimative formula gives a pefect agreement with numerical simulations.

Mechanisms of synchronization and pattern formation in a lattice of pulse-coupled oscillators

We analyze the physical mechanisms leading either to synchronization or to the formation of spatiotemporal patterns in a lattice model of pulse-coupled oscillators. In order to make the system tractable from a mathematical point of view we study a one-dimensional ring with unidirectional coupling. In such a situation, exact results concerning the stability of the fixed of the dynamic evolution of the lattice can be obtained. Furthermore, we show that this stability is the responsible for the different behaviors.