Seeing Central African forests through their largest trees.

Large tropical trees and a few dominant species were recently identified as the main structuring elements of tropical forests. However, such result did not translate yet into quantitative approaches which are essential to understand, predict and monitor forest functions and composition over large, often poorly accessible territories. Here we show that the above-ground biomass (AGB) of the whole forest can be predicted from a few large trees and that the relationship is proved strikingly stable in 175 1-ha plots investigated across 8 sites spanning Central Africa. We designed a generic model predicting AGB with an error of 14% when based on only 5% of the stems, which points to universality in forest structural properties. For the first time in Africa, we identified some dominant species that disproportionally contribute to forest AGB with 1.5% of recorded species accounting for over 50% of the stock of AGB. Consequently, focusing on large trees and dominant species provides precise information on the whole forest stand. This offers new perspectives for understanding the functioning of tropical forests and opens new doors for the development of innovative monitoring strategies.

Genotyping of type b Haemophilus influenzae strains, comparison of strains collected before and during vaccine availability.

OBJECTIVE AND METHOD: Five hundred and seventy-eight strains of type b Haemophilus influenzae (521 isolated in children, and 57 in adults) were compared using pulsed-field gel electrophoresis (PFGE) to assess strain evolution and to study the impact of the generalization of anti-Haemophilus b (anti-Hib) vaccination in France. Among these strains, 398 (including 342 from meningitis) were isolated in 1985-1992 (pre-vaccination era), 39 (including 31 from meningitis) in 1993 (year of the generalization of anti-Hib vaccination), and 141 (including 50 from CSF) in 1994-2001 (vaccination era). RESULTS: A total of 102 PFGE patterns (patterns for 1-101 isolates) were obtained after SmaI restriction of the 578 strains. The strains isolated in children were distributed in 96 patterns, and those isolated in the adult in 34 patterns. The strains isolated during the pre-vaccination era presented 94 patterns. During the vaccination era, 50% of the patterns disappeared and 12 new patterns (11.7%) including 15 strains were observed. The strains belonging to the new patterns (including the two observed in 1993) were isolated in adults (n=7) from blood culture and bronchial secretions, and children (n=9) from CSF, blood culture, and bronchial secretions. In children, among the strains associated to vaccination failure, two presented with a new pulsotype. CONCLUSION: There is no evidence that the vaccination program brought about any drastic modifications in the type b strains causing meningitis or in the other type b strains in circulation whether in adults or children.

Experimental analysis of unsteady flows through a stenosis.

In this paper we present a study of the post-stenotic velocity flow field corresponding to oscillatory, pulsatile and physiological flow waveforms. Two-dimensional velocity measurements are performed in a 75% severity stenosis using a pulsed Doppler ultrasonic velocimeter. Emphasis is placed on the analysis of the experimental velocity-profile patterns. It is recognized that, beyond the influence of the flow parameters such as the Reynolds number and the frequency parameter, velocity profiles (hence wall-shear stresses) highly depend on the flow waveform. In addition to this analysis, a model of the stenosis influence length is proposed in the case of oscillatory flow.

[Mechanical properties of the arteries. Effects of cryopreservation]. / Propriétés mécaniques des artères. Effets de la cryopréservation.

The viscoelastic properties of the arterial wall are responsible for the blood pressure wave propagation throughout the arterial system. Arterial diseases may cause disorders in this propagation. We have developed a mock circulation system that allows assessment of viscoelastic properties in fresh or cryopreserved human arteries. It includes the following components:--a hydrodynamic generator that can simulate physiological pressure variations in fresh segments of human arteries;--a lightweight miniature flexible probe that can be placed around the artery to measure changes in the external diameter during systolo-diastolic cycles;--a computer program to analyse pressure and diameter data measured during a cardiac cycle. Using this system, it is possible to evaluate the main viscoelastic properties of the arterial wall (arterial compliance, arterial stiffness, midwall aortic stress, Young elastic modulus, incremental modulus). Human arterial samples were collected during organ harvesting in subjects from 18 to 35 years of age. Correlation between viscoelastic properties, arterial wall status, and histological aspect in nitrogen vapor (-140 degree C) were established. No statistically significant difference was observed in femoral arteries characteristics. Compliance decreased, while stiffness increased with statistically significant difference after cryopreservation in carotid arteries. No histological difference was observed in both arteries before and after cryopreservation.

Ventricular flow dynamic past bileaflet prosthetic heart valves.

To characterise hydrodynamic properties of prosthetic heart valves in mitral position, ultrasonic velocity measurements were performed using a cardiovascular simulator. A Duromedics and a Saint-Jude Medical bileaflet heart valve were tested. The Saint-Jude valve was oriented first in an anatomical position, i.e. the tilt axis parallel to the septal wall, and then in an anti-anatomical position. In the anti-anatomical position, from mid diastole to mid systole, two contrarotative vortices are generated in the ventricle by the interaction between the flow directed by the leaflets downstream from the lateral orifices and the ventricular wall motions. In the anatomical position, the mitral flow penetrates the ventricle principally through the lateral orifice proximal to the septal wall, due to the vortex in the atrial chamber. The mitral inflow then circulates along the septal wall to the apex, and produces a large ventricular vortex during systole. In the anatomical position, the Saint-Jude thus provides a better ventricular washout during this phase. The mitral inflow through the Duromedics in the anti-anatomical position produces two contrarotative vortices in the ventricle, but in the opposite sense than downstream the Saint-Jude valve; the flows that penetrate through the lateral orifices are directed to the ventricular walls and then recirculate to the centre of the ventricle, providing a very fluctuating flow near the apex. Thus, a slight difference in valve design produces a significant difference in the ventricular flow fields.

Near velocity field downstream prosthetic valves in aortic position.

Using a cardiovascular simulator to duplicate in vitro the flow conditions through valves in aortic position, bidimensional velocity maps very near the valve are reconstructed, from an ultrasonic 8 Mhz doppler system, in an elastic model of the ascending aortic arch. Three mechanical heart valves representative of the different types of commercial models (a tilting disc, a ball in cage and a two-leaflet valve) and a new bileaflet prototype were investigated. From examination of the velocity field, it is possible to define the main characteristics of the valve wake and to observe the development of negative velocities associated with regurgitant flows. From a comparison with tests in rigid tubes, the role played by the arch elasticity is analysed.

In vitro study of a physiological type flow in a bifurcated vascular prosthesis.

Using a cardiovascular simulator allowing the production of physiological conditions, velocity field and pressure-diameter relationship were determined in a bifurcated arterial graft for two geometric configurations of implantation. Velocity measurements give the surgeon attractive and useful information for finding out the best way to implant the graft. The study of the pressure-diameter relationship showed a particular behaviour of the graft; the wall motion of the graft, compared with the pressure wave, being different both in the mother branch at the upstream anastomosis and in the daughter branch.

Accuracy of the simplified Bernoulli relationship in measuring pressure gradients across stenosis.

In order to test the validity of the modified Bernoulli equation in predicting pressure gradients across stenotic regions, we have constructed an in-vitro model and studied the influence of the length and of the severity of the stenosis. Under physiological conditions, simultaneous pressure gradients are estimated by both Doppler and direct pressure manometer techniques. Measurements of the pressure gradients (in the range 10-150 mmHg) by the two techniques show that the Doppler estimation using the modified Bernoulli equation underestimated the pressure transducer gradient measurements for every length of stenosis, this underestimation being greater than 45% for very severe stenosis.

[Reliability and perspectives in peroperative flowmetry. An original computer-assisted ultrasonic technic]. / Fiabilité et perspectives de la débimétrie per opératoire. Une technique ultrasonore informatisée originale.

To mitigate the lack of reliability of currently used flowmetry technics, an original method was developed based on a single window 15 MHz Doppler flowmeter and an Apple IIe computer with data acquisition system. Eleven successive measurements of instantaneous rate were made in an arterial section. A mean flow was determined by integration of mean rate in section. The reliability of the method was evaluated by an experimental study on a hydrodynamic bench reproducing physiologic flow conditions with an error always less than 8%. An in vivo study in 30 patients provided 100 measurements of arterial flow before and after surgical arterial reconstruction. The mean error value was less than 10%. Among the risks of error, those due to the probe was reduced by specially conceived probes. The most important factor was that of respiration, this latter factor imposing an integration time of rate of signal of 30 seconds to eliminate errors due to respiration. The conventional measurement time for other configurations is 6 seconds but this leads to a mean error of 25% which can attain 60%. The perspective of miniaturization of the apparatus in the near future with acceleration of measurements using a multi-window Doppler should allow flow measurement in 30 seconds and control of surgical reconstructions (on healthy arteries, vein grafts and prostheses but excluding PTFE). Similarly, it should be possible to improve evaluation of so-called vasoactive drugs.

Influence of the haemodynamic parameters on the repartition of the flow between an artery and its graft.

The aim of this study was to investigate the repartition of the flow between an artery and its graft for several values of the geometric and dynamic parameters (length and severity of the stenosis, Reynolds number and frequency pulse value). The model, fabricated in silicone, was included in an hydrodynamic test bench, allowing to reproduce physiological conditions. Our study showed that the severity of the stenosis was the most influent parameter on the repartition of the flow; in particular, a 75% severity was necessary to obtain a repartition 70-30% between the graft and the artery.

Velocity field of a Björk-Shiley valve prosthesis: influence of the disc orientation.

An experimental investigation was carried out on the development of physiological flows downstream of a Björk-Shiley valve prosthesis. The post-valvular velocity field was determined by an ultrasonic method in an elastic model of the aortic arch. The flow development in the ascending aorta was strongly dependent on the orientation of the tilting disc. The rotating direction of the vortices and the site of the maximum velocity were influenced by the orientation.

Velocity profiles in the wake of two prosthetic heart valves using a new cardiovascular simulator.

In this paper we present a study of the post valvular flow field on a new cardiovascular simulator including an elastic model of the aortic arch. Transverse and vertical two-dimensional velocity measurements are performed with an ultrasonic velocimeter. Two prosthetic heart valves are tested in the aortic position. The behaviour of the velocity vectors patterns during one pulsatile cycle is one of the most striking features of the flow.

Velocity distribution along an elastic model of human arterial tree.

An experimental investigation of an elastic model of the human arterial tree, has been performed for physiological type flow by pulsed Doppler ultrasonic velocimetry. The arterial tree model, fabricated in clear polyurethane, includes the aortic arch, with a Starr-Edwards ball valve mounted in the root of the aorta, the descending aorta and the iliac bifurcation. Our study showed that the velocity profile, a few centimeters beyond the valve, is skewed, with higher velocities towards the top and the inner wall (anatomically the posterior and left lateral wall). An inward shift of the maximum velocity and reverse flow are denoted along the inner wall of the aortic arch. The velocity profiles in the descending aorta are blunted. Downstream from the vertex of the iliac bifurcation, there is vorticity creation, but the branching effect is quickly damped by the pulsatility of the flow and the elasticity of the wall.

The effect of unsteadiness on the flow through stenoses and bifurcations.

This paper is concerned with the influence of a stenosis or a bifurcation on the flow through a tube. In particular the effect of unsteadiness is investigated using simple pulsatile and physiological type flows (Fig. 1). The experimental investigations reported herein are concerned with velocity measurements and flow visualizations. (see formula in text) These measurements, performed in a 60 degrees bifurcation, have permitted the reconstruction of the three-dimensional velocity profiles. The importance of the secondary flow in the branching is analyzed for various values of the flow parameters. Results of tests show a strong influence of unsteadiness on flow characteristics and then on hemodynamic factors. One conclusion is the following: if hemodynamic factors play an important role in the problems of atherosclerosis, then, for macrocirculation studies, it is necessary to take into account unsteadiness and, in particular, the actual shape of the flow-time forcing function.

Simulation of blood flow by a hydrodynamic generator.

Physiological flows are produced by a volumetric gear pump. The motor-pump group is controlled by an amplifying comparator, and produces a flow whose rate follows the control signal. This signal is delivered by a synthetizing generator and its shape, amplitude, and frequency are adjustable. This hydrodynamic generator supplies a test bench on which the modifications of flow patterns induced by models of vascular stenoses are studied. The study of the velocity distribution has been effected with a pulsed-Doppler ultrasonic velocimeter.