Forme des ressauts granulaires en canal incliné: implications possibles pour le dimensionnement des digues paravalanches

Quand un écoulement granulaire à surface libre en régime supercritique rencontre un mur normal à la direction de l'écoulement, il peut se former une discontinuité en hauteur et en vitesse, dit ressaut granulaire par analogie aux ressauts hydrauliques, se propageant vers l'amont. Pour cela, il faut que la hauteur de l'obstacle soit supérieure à une hauteur critique conduisant à la formation du ressaut qui marque la transition entre le régime supercritique et le régime subcritique. En dessous de cette hauteur critique, l'écoulement reste supercritique et déborde sans formation de ressaut. Au-dessus de la hauteur critique, si le ressaut formé est de hauteur inférieure à la hauteur critique alors le ressaut se propage vers l'amont sans débordement de l'obstacle. Si le ressaut est de hauteur supérieure à la hauteur critique du mur alors ce dernier est débordé par l'écoulement. Ces processus, associés à la hauteur critique de l'obstacle pour qu'il y ait formation d'un ressaut d'une part et à la hauteur du ressaut formé d'autre part, constituent aujourd'hui les bases des recommandations Européennes en matière de dimensionnement des digues paravalanches d'arrêt visant à stopper entièrement l'écoulement d'avalanche ainsi que des digues de déviation [1]. Les valeurs de hauteur critique de l'obstacle et de hauteur du ressaut sont calculées à partir des équations traditionnelles des ressauts hydrauliques et sont essentiellement fonction du nombre de Froude de l'écoulement incident. Plusieurs études (voir par exemple [2,3,4]) ont démontré que ces équations classiques de l'hydraulique offraient de bonnes prédictions pour les ressauts, normaux ou obliques, formés dans les écoulements granulaires, ces derniers étant de bons modèles pour les écoulements de neige. Cependant la plupart des expériences de laboratoire sont restées jusqu'à présent limitées à des pentes fortes où l'inertie est dominante (nombre de Froude de l'ordre de 5-15), ce qui reste des valeurs élevées par rapport aux valeurs du nombre de Froude généralement plus faibles (de l'ordre de l'unité ou de quelques unités) dans la zone d'arrêt des avalanches. Des questions viennent alors à l'esprit : que se passe-t-il aux faibles valeurs du nombre de Froude pour lesquelles les forces d'inertie dans le ressaut ne sont pas dominantes ? Quel sont les effets des fortes de gravité et de frottement dans le ressaut ? Les équations classiques de l'hydraulique sont-elles toujours valides ? Afin de progresser sur ces questions, nous avons étudié en détail les ressauts stationnaires formés dans les écoulements granulaires en pente. Nous avons développé un nouveau dispositif expérimental qui permet de générer des ressauts granulaires stationnaires. Une vanne placée à la sortie d'un canal incliné à fond lisse permet d'obstruer en partie les écoulements granulaires permanents formés dans le canal et de générer des ressauts se propageant vers l'amont. En ajustant le débit de sortie au débit entrant, il est possible de rendre le ressaut stationnaire dans une large gamme de pente et débit. Des mesures basées sur des techniques d'imagerie (détection de surface libre, analyse de diagrammes spatio-temporels obtenus à la base de l'écoulement, aux parois et à la surface libre à l'aide d'une caméra rapide) couplées à des mesures de débit massique nous ont permis de mesurer en détail la dynamique des écoulements incidents (hauteur, vitesse, masse volumique) et la forme des ressauts pour chaque type d'écoulement. Nous avons pu établir un diagramme de phase qui permet de mettre en évidence une riche variété de ressauts granulaires. A haut débit, les écoulements sont denses et la forme des ressauts granulaires varie largement avec la pente. Au-delà d'une certaine pente, les écoulements sont légèrement non uniformes (uniformément accélérés) et les ressauts sont très raides avec une zone de recirculation au pied du ressaut. Pour une large gamme de pente, les écoulements sont uniformes, la zone de recirculation dans le ressaut disparait et les ressauts sont de plus en plus diffus quand la pente diminue. A une pente critique égale à l'angle de frottement typique du matériau, il est impossible de former un ressaut. Cette première série de tests à haut débit met en évidence une transition entre ressauts raides avec recirculation et ressauts diffus. Les ressauts diffus ont des hauteurs plus grandes que la hauteur prédite par l'hydraulique alors que la hauteur des ressauts raides avec recirculation est bien prédite par l'hydraulique classique. Nous montrons qu'une équation de ressaut avec prise en compte du poids du ressaut dans la direction de l'écoulement diminué de la force de frottement dans le ressaut (comme initialement proposé par [5] et récemment revisité par [6]) permet d'expliquer l'écart observé entre l'hydraulique classique et les ressauts granulaires diffus, dits ressauts frictionnels, à bas nombre de Froude typiquement en dessous de 4 dans nos tests. A faible débit, les écoulements incidents sont plus dilués et génèrent des ressauts dits compressibles pour lesquels il existe, en plus de la discontinuité en hauteur et en vitesse, une variation non négligeable de la masse volumique à travers le ressaut. Nous proposons une équation qui prend en compte cette variation de masse volumique. Cette équation est ainsi capable de prédire la diminution de la hauteur des ressauts quand les écoulements incidents sont de plus en plus dilués du fait de la diminution du débit entrant (à pente constante). Notre étude expérimentale de laboratoire couplée à des solutions analytiques a permis de mettre en évidence (i) une première transition entre ressauts granulaires raides avec recirculation et ressauts granulaires diffus frictionnels et (ii) une seconde transition entre ressauts granulaires incompressibles et compressibles. Les effets des termes source dans l'équation de ressaut (poids, force de frottement) à faible nombre de Froude et de la variation de masse volumique pour les écoulements dilués mis en évidence pour les ressauts stationnaires sont susceptibles de se manifester aussi dans les ressauts instationnaires. Cela peut remettre en question les recommandations Européennes existantes en matière de dimensionnement des digues paravalanches, notamment quand le nombre de Froude est faible (typiquement inférieur à 4 dans nos tests). Notre étude montre que les ressauts frictionnels diffus ont une hauteur plus grande que la hauteur prédite par l'équation de l'hydraulique alors que les ressauts compressibles ont une hauteur plus faible. Si le dernier résultat va dans le sens de la sécurité, le premier résultat est lui plus problématique quant au dimensionnement actuel des digues et au risque de débordement en aval des digues. En effet, nos résultats suggèrent que la hauteur des ressauts, donc la hauteur de digue nécessaire, peut être sous-estimée dans les régimes d'écoulement à bas nombre de Froude si l'équation de l'hydraulique (sans les termes source) est utilisée

Retinoic Acid Signalling and the Control of Meiotic Entry in the Human Fetal Gonad

The development of mammalian fetal germ cells along oogenic or spermatogenic fate trajectories is dictated by signals from the surrounding gonadal environment. Germ cells in the fetal testis enter mitotic arrest, whilst those in the fetal ovary undergo sex-specific entry into meiosis, the initiation of which is thought to be mediated by selective exposure of fetal ovarian germ cells to mesonephros-derived retinoic acid (RA). Aspects of this model are hard to reconcile with the spatiotemporal pattern of germ cell differentiation in the human fetal ovary, however. We have therefore examined the expression of components of the RA synthesis, metabolism and signalling pathways, and their downstream effectors and inhibitors in germ cells around the time of the initiation of meiosis in the human fetal gonad. Expression of the three RA-synthesising enzymes, ALDH1A1, 2 and 3 in the fetal ovary and testis was equal to or greater than that in the mesonephros at 8–9 weeks gestation, indicating an intrinsic capacity within the gonad to synthesise RA. Using immunohistochemistry to detect RA receptors RARα, β and RXRα, we find germ cells to be the predominant target of RA signalling in the fetal human ovary, but also reveal widespread receptor nuclear localization indicative of signalling in the testis, suggesting that human fetal testicular germ cells are not efficiently shielded from RA by the action of the RA-metabolising enzyme CYP26B1. Consistent with this, expression of CYP26B1 was greater in the human fetal ovary than testis, although the sexually-dimorphic expression patterns of the germ cell-intrinsic regulators of meiotic initiation, STRA8 and NANOS2, appear conserved. Finally, we demonstrate that RA induces a two-fold increase in STRA8 expression in cultures of human fetal testis, but is not sufficient to cause widespread meiosis-associated gene expression. Together, these data indicate that while local production of RA within the fetal ovary may be important in regulating the onset of meiosis in the human fetal ovary, mechanisms other than CYP26B1-mediated metabolism of RA may exist to inhibit the entry of germ cells into meiosis in the human fetal testis

Peri-operative red blood cell transfusion in neonates and infants: NEonate and Children audiT of Anaesthesia pRactice IN Europe: A prospective European multicentre observational study

BACKGROUND: Little is known about current clinical practice concerning peri-operative red blood cell transfusion in neonates and small infants. Guidelines suggest transfusions based on haemoglobin thresholds ranging from 8.5 to 12 g dl-1, distinguishing between children from birth to day 7 (week 1), from day 8 to day 14 (week 2) or from day 15 (≥week 3) onwards. OBJECTIVE: To observe peri-operative red blood cell transfusion practice according to guidelines in relation to patient outcome. DESIGN: A multicentre observational study. SETTING: The NEonate-Children sTudy of Anaesthesia pRactice IN Europe (NECTARINE) trial recruited patients up to 60 weeks' postmenstrual age undergoing anaesthesia for surgical or diagnostic procedures from 165 centres in 31 European countries between March 2016 and January 2017. PATIENTS: The data included 5609 patients undergoing 6542 procedures. Inclusion criteria was a peri-operative red blood cell transfusion. MAIN OUTCOME MEASURES: The primary endpoint was the haemoglobin level triggering a transfusion for neonates in week 1, week 2 and week 3. Secondary endpoints were transfusion volumes, 'delta haemoglobin' (preprocedure - transfusion-triggering) and 30-day and 90-day morbidity and mortality. RESULTS: Peri-operative red blood cell transfusions were recorded during 447 procedures (6.9%). The median haemoglobin levels triggering a transfusion were 9.6 [IQR 8.7 to 10.9] g dl-1 for neonates in week 1, 9.6 [7.7 to 10.4] g dl-1 in week 2 and 8.0 [7.3 to 9.0] g dl-1 in week 3. The median transfusion volume was 17.1 [11.1 to 26.4] ml kg-1 with a median delta haemoglobin of 1.8 [0.0 to 3.6] g dl-1. Thirty-day morbidity was 47.8% with an overall mortality of 11.3%. CONCLUSIONS: Results indicate lower transfusion-triggering haemoglobin thresholds in clinical practice than suggested by current guidelines. The high morbidity and mortality of this NECTARINE sub-cohort calls for investigative action and evidence-based guidelines addressing peri-operative red blood cell transfusions strategies. TRIAL REGISTRATION: ClinicalTrials.gov, identifier: NCT02350348

Perioperative critical events and morbidity associated with anesthesia in early life: subgroup analysis of United Kingdom participation in the neonate and children audit of anesthesia practice in Europe (NECTARINE) prospective multicenter observational study

BACKGROUND: The NEonate and Children audiT of Anaesthesia pRactice IN Europe (NECTARINE) prospective observational study reported critical events requiring intervention during 35.2% of 6542 anesthetic episodes in 5609 infants up to 60 weeks postmenstrual age. The United Kingdom (UK) was one of 31 participating countries. METHODS: Subgroup analysis of UK NECTARINE cases (12.8% of cohort) to identify perioperative critical events that triggered medical interventions. Secondary aims were to describe UK practice, identify factors more commonly associated with critical events, and compare 30-day morbidity and mortality between participating UK and non-UK centers. RESULTS: Seventeen UK centers recruited 722 patients (68.7% male, 36.1% born preterm, 48.1% congenital anomalies) undergoing anesthesia for 876 surgical or diagnostic procedures at 25-60 weeks postmenstrual age. Repeat anesthesia/surgery was common: 17.6% patients prior to and 14.4% during the recruitment period. Perioperative critical events triggered interventions in 300/876 (34.3%) cases. Cardiovascular instability (16.9% of cases) and/or reduced oxygenation (11.4%) were more common in younger patients and those with co-morbidities or requiring preoperative intensive support. A higher proportion of UK than non-UK cases were graded as ASA-Physical Status scores >2 or requiring urgent or emergency procedures, and 39% required postoperative intensive care. Thirty-day morbidity (complications in 17.2%) and mortality (8/715, 1.1%) did not differ from non-UK participants. CONCLUSIONS: Perioperative critical events and co-morbidities are common in neonates and young infants. Thirty-day morbidity and mortality data did not demonstrate national differences in outcome. Identifying factors associated with increased risk informs preoperative assessment, resource allocation, and discussions between clinicians and families

Morbidity and mortality after anaesthesia in early life: results of the European prospective multicentre observational study, neonate and children audit of anaesthesia practice in Europe (NECTARINE)

Background: Neonates and infants requiring anaesthesia are at risk of physiological instability and complications, but triggers for peri-anaesthetic interventions and associations with subsequent outcome are unknown. Methods: This prospective, observational study recruited patients up to 60 weeks' postmenstrual age undergoing anaesthesia for surgical or diagnostic procedures from 165 centres in 31 European countries between March 2016 and January 2017. The primary aim was to identify thresholds of pre-determined physiological variables that triggered a medical intervention. The secondary aims were to evaluate morbidities, mortality at 30 and 90 days, or both, and associations with critical events. Results: Infants (n=5609) born at mean (standard deviation [sd]) 36.2 (4.4) weeks postmenstrual age (35.7% preterm) underwent 6542 procedures within 63 (48) days of birth. Critical event(s) requiring intervention occurred in 35.2% of cases, mainly hypotension (>30% decrease in blood pressure) or reduced oxygenation (SpO2 <85%). Postmenstrual age influenced the incidence and thresholds for intervention. Risk of critical events was increased by prior neonatal medical conditions, congenital anomalies, or both (relative risk [RR]=1.16; 95% confidence interval [CI], 1.04-1.28) and in those requiring preoperative intensive support (RR=1.27; 95% CI, 1.15-1.41). Additional complications occurred in 16.3% of patients by 30 days, and overall 90-day mortality was 3.2% (95% CI, 2.7-3.7%). Co-occurrence of intraoperative hypotension, hypoxaemia, and anaemia was associated with increased risk of morbidity (RR=3.56; 95% CI, 1.64-7.71) and mortality (RR=19.80; 95% CI, 5.87-66.7). Conclusions: Variability in physiological thresholds that triggered an intervention, and the impact of poor tissue oxygenation on patient's outcome, highlight the need for more standardised perioperative management guidelines for neonates and infants

Difficult tracheal intubation in neonates and infants. NEonate and Children audiT of Anaesthesia pRactice IN Europe (NECTARINE): a prospective European multicentre observational study

International audienceBackground: Neonates and infants are susceptible to hypoxaemia in the perioperative period. The aim of this study was to analyse interventions related to anaesthesia tracheal intubations in this European cohort and identify their clinical consequences.Methods: We performed a secondary analysis of tracheal intubations of the European multicentre observational trial (NEonate and Children audiT of Anaesthesia pRactice IN Europe [NECTARINE]) in neonates and small infants with difficult tracheal intubation. The primary endpoint was the incidence of difficult intubation and the related complications. The secondary endpoints were the risk factors for severe hypoxaemia attributed to difficult airway management, and 30 and 90 day outcomes.Results: Tracheal intubation was planned in 4683 procedures. Difficult tracheal intubation, defined as two failed attempts of direct laryngoscopy, occurred in 266 children (271 procedures) with an incidence (95% confidence interval [CI]) of 5.8% (95% CI, 5.1-6.5). Bradycardia occurred in 8% of the cases with difficult intubation, whereas a significant decrease in oxygen saturation (SpO2<90% for 60 s) was reported in 40%. No associated risk factors could be identified among co-morbidities, surgical, or anaesthesia management. Using propensity scoring to adjust for confounders, difficult anaesthesia tracheal intubation did not lead to an increase in 30 and 90 day morbidity or mortality.Conclusions: The results of the present study demonstrate a high incidence of difficult tracheal intubation in children less than 60 weeks post-conceptual age commonly resulting in severe hypoxaemia. Reassuringly, the morbidity and mortality at 30 and 90 days was not increased by the occurrence of a difficult intubation event

Incidence of severe critical events in paediatric anaesthesia (APRICOT): a prospective multicentre observational study in 261 hospitals in Europe

Background Little is known about the incidence of severe critical events in children undergoing general anaesthesia in Europe. We aimed to identify the incidence, nature, and outcome of severe critical events in children undergoing anaesthesia, and the associated potential risk factors. Methods The APRICOT study was a prospective observational multicentre cohort study of children from birth to 15 years of age undergoing elective or urgent anaesthesia for diagnostic or surgical procedures. Children were eligible for inclusion during a 2-week period determined prospectively by each centre. There were 261 participating centres across 33 European countries. The primary endpoint was the occurence of perioperative severe critical events requiring immediate intervention. A severe critical event was defined as the occurrence of respiratory, cardiac, allergic, or neurological complications requiring immediate intervention and that led (or could have led) to major disability or death. This study is registered with ClinicalTrials.gov, number NCT01878760. Findings Between April 1, 2014, and Jan 31, 2015, 31â127 anaesthetic procedures in 30â874 children with a mean age of 6·35 years (SD 4·50) were included. The incidence of perioperative severe critical events was 5·2% (95% CI 5·0â5·5) with an incidence of respiratory critical events of 3·1% (2·9â3·3). Cardiovascular instability occurred in 1·9% (1·7â2·1), with an immediate poor outcome in 5·4% (3·7â7·5) of these cases. The all-cause 30-day in-hospital mortality rate was 10 in 10â000. This was independent of type of anaesthesia. Age (relative risk 0·88, 95% CI 0·86â0·90; p<0·0001), medical history, and physical condition (1·60, 1·40â1·82; p<0·0001) were the major risk factors for a serious critical event. Multivariate analysis revealed evidence for the beneficial effect of years of experience of the most senior anaesthesia team member (0·99, 0·981â0·997; p<0·0048 for respiratory critical events, and 0·98, 0·97â0·99; p=0·0039 for cardiovascular critical events), rather than the type of health institution or providers. Interpretation This study highlights a relatively high rate of severe critical events during the anaesthesia management of children for surgical or diagnostic procedures in Europe, and a large variability in the practice of paediatric anaesthesia. These findings are substantial enough to warrant attention from national, regional, and specialist societies to target education of anaesthesiologists and their teams and implement strategies for quality improvement in paediatric anaesthesia. Funding European Society of Anaesthesiology