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L Hydrogène, Un Gaz Respirable

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L Hydrogène, Un Gaz Respirable V OCABULAIRE L’hydrogène, un gaz respirable ? Hydrogen, a breathing gas? ● B. Gardette* Résumé : Au-delà de 50 mètres, les interventions sous-marines par plongeur sont pratiquées en saturation au mélange hélium/oxy- gène (héliox). Quand ce mélange est utilisé au-delà de 200 mètres, plusieurs facteurs limitent l’efficacité du plongeur et peu- vent induire fatigue et insécurité : – effets neurologiques : syndrome nerveux des hautes pressions (SNHP) ; – problèmes dus à la densité du gaz : syndrome respiratoire des hautes pressions (SRHP) ; – douleurs articulaires : syndrome articulaire des hautes pressions (SAHP). Ces symptômes sont inclus dans le syndrome des hautes pressions (SHP). COMEX, de 1963 à 2000, a réalisé un programme de recherche très important sur la plongée à l’héliox et aux mélanges hydro- gène/oxygène (hydrox) et hydrogène/hélium/oxygène (hydréliox). Ces plongées expérimentales montrent une non-toxicité de l’hydro- gène pour des pressions inférieures à 2 MPa. Mais pour PH2 > 2 MPa, une “narcose hydrogène” peut survenir chez les plongeurs. Le mélange hydréliox diminue le SHP et la fatigue des plongeurs et augmente leurs performances et leur sécurité. Mots-clés : Hydrogène - Gaz respiratoire - Plongée - Pression. Summary: Below 50 msw most diver interventions have been carried out in saturation using helium/oxygen mixture (heliox). When heliox breathing mixture is used down to 200 msw, many factors limit the diver’s efficiency and may induce fatigue and insecurity: – neurological effects of the pressure: High Pressure Nervous Syndrome (HPNS); – ventilatory problems due to the density of the gas: High Pressure Respiratory Syndrome (HPRS); – articular pains: High Pressure Articular Syndrome (HPAS). These symptoms are included in the general term of High Pressure Syndrome (HPS). COMEX, from 1963 to 2000, undertook a major research program, the most extensive in the world, on deep experimental diving with heliox and hydrogen/oxygen (hydrox) or hydrogen/helium/oxygen (hydreliox) breathing gas mixtures. These deep experimental dives showed a no toxicity effect of hydrogen for hydrogen pressure lower than 2 MPa. But for PH2 >2MPa, “hydrogen narcosis” was able to appear on divers. Hydreliox gas mixture decreases HPS and fatigue of divers and increases per- formance and safety. Keywords: Hydrogen - Breathing gas - Diving - Pressure. a plongée à l’air comprimé comporte des limites. et à la pression absolue (Pabs) à laquelle il est respiré (2-3). L’essoufflement, la toxicité de l’oxygène (1) et les Dans les années 1930, des expériences ont été entreprises par L effets de la narcose à l’azote imposent au plongeur de la marine américaine pour alimenter le plongeur non plus en air ne pas dépasser les 50/60 mètres de profondeur (2-5) (décret du comprimé, mais en mélange respiratoire synthétique dans lequel ministère du Travail 90.277 du 28 mars 1990). Les effets bio- l’azote est remplacé par un autre gaz inerte, diluant de l’oxy- chimiques des gaz sur l’organisme sont directement corrélés à gène, l’hélium. Ce dernier, très inerte sur le plan biochimique, leur pression partielle (loi de Dalton : PP = Pabs x %). Ainsi, en d’une densité sept fois inférieure à celle de l’azote, permet de plongée, la toxicité d’un gaz comme l’oxygène ou l’azote meilleures performances en supprimant les effets de la narcose est proportionnelle à sa concentration (%) dans le mélange et la gêne respiratoire. La ventilation du plongeur à grande profondeur dépend essen- * Direction scientifique de la COMEX, Marseille. tiellement de l’augmentation des résistances respiratoires due à 160 La Lettre du Pneumologue - Volume VI - no 4 - juillet-août 2003 l’accroissement de la densité du mélange gazeux, qui est pro- 200 mètres de profondeur, mais deux facteurs limitent l’effica- portionnelle à la profondeur atteinte. En effet, suivant la loi de cité du plongeur : le SNHP et les limitations ventilatoires dues à Poiseuille, les résistances respiratoires augmentent avec la vis- la densité du mélange gazeux. cosité du gaz (écoulement laminaire) et sa densité (écoulement turbulent). En hyperbarie, c’est l’écoulement turbulent qui est L’HYDROGÈNE prépondérant, donc la densité du mélange gazeux (3). L’hydro- gène aura ainsi un avantage certain par rapport à l’hélium (deux L’idée, initiée par Lavoisier, d’utiliser l’hydrogène comme un gaz fois moins dense). respiratoire à faible densité remonte au XVIIIe siècle. Dès la fin du Sur le plan thermique, en plongée profonde, les pertes se font sur- XIXe siècle, l’hydrogène a été utilisé comme gaz traceur pour la tout par convection respiratoire, donc en relation avec la densité mesure du volume résiduel pulmonaire, puis abandonné pour des du mélange gazeux respiré. raisons de sécurité (6). Le premier homme qui a eu le courage Cet effet, exercé par la pression sur la déperdition calorique ven- d’expérimenter l’hydrogène sur lui-même est l’ingénieur Arne Zet- tilatoire, est accentué par le fait que la chaleur spécifique de terström lors de tests réalisés entre 1943 et 1945 par la marine l’hélium et sa conductibilité thermique sont cinq à six fois plus royale suédoise. En dehors de quelques expériences sur des ani- grandes que celles de l’azote. Cette différence est encore plus maux en laboratoire, aucun programme substantiel ne sera entre- marquée avec l’hydrogène (3). pris avant 1967. En 1968, lors de la mission HYDRA 1, la COMEX Moyennant la mise au point de tables de décompression spéciale- essaye de placer sous hydrogène l’un de ses plongeurs d’élite, René ment adaptées à ce nouveau mélange appelé héliox (hélium/oxy- Veyrunes, en pleine eau à la profondeur de 250 mètres. L’équipe- gène), les limites de profondeur imposées par l’air ont été considé- ment du plongeur est alors insuffisant pour le protéger du froid et, rablement repoussées et les plongeurs sous-marins ont pu se retrouver à peine sorti, il doit retourner à sa tourelle. Il faudra attendre 1982 aussi à l’aise à 150 mètres de profondeur qu’ils l’étaient auparavant pour que le programme de recherche sur l’hydrogène soit relancé, à 30 mètres avec de l’air comprimé. Mais, à partir de 150 mètres sur- profitant de l’évolution du matériel et des technologies de la plon- git très progressivement un nouvel obstacle. Le plongeur, fort heu- gée profonde mises au point sur les chantiers pétroliers. reusement, ne retrouve pas cette altération de la conscience propre à L’objectif est alors de définir les méthodes industrielles néces- l’ivresse des profondeurs entraînée par l’azote, mais éprouve sim- saires à l’emploi pratique des mélanges hydrogénés et d’en étu- plement quelques vertiges, un léger tremblement et une certaine mal- dier les limites chez l’homme. En 1983 (HYDRA 3), seize plon- adresse des gestes. Tout cela correspond à la phase initiale du syn- geurs, avec à leur tête Henri Delauze, président de la COMEX, drome d’excitabilité du système nerveux central décrit en 1968 par effectuent une série de plongées en mer, entre 70 et 91 mètres, X. Fructus, R. Naquet et R. Brauer, sous le nom de “syndrome ner- au large de Marseille. La même année, lors de HYDRA 4, six veux des hautes pressions” (SNHP) (tableau I) (2-5). plongeurs respirent pour la première fois de l’hydrogène en cais- son sous une pression de 31 bars, équivalente à une pression de Tableau I. Les limites de la plongée. 300 mètres. Deux années plus tard, en 1985, six plongeurs réali- sent la première saturation au mélange hydréliox (hydrogène/ Profondeur (m) Mélanges gazeux Facteurs limitants hélium/oxygène) à 450 mètres de profondeur (HYDRA 5). 0-6 O2 Toxicité oxygène Pour démontrer la faisabilité industrielle de la plongée à l’hydro- 0-60 Air (N2 – O2) Narcose azote Gênes respiratoires gène, la suite du programme de recherche chez l’homme devait comprendre une démonstration, en conditions réelles de travail 60-150 Héliox (He – O2)– 60-200 Hydrox (H – O ) Narcose hydrogène en mer, des capacités de travail de l’homme à une profondeur 2 2 encore jamais atteinte : plus de 500 mètres. Pour réaliser ce pro- 150-450 Héliox (He – O ) SNHP – SRHP – SAHP 2 jet ambitieux, deux plongées d’entraînement et de sélection ont Hydréliox (H2 – He – O2)– 450-700 Hydréliox (H – He – O ) SNHP – SRHP été réalisées dans les caissons de la COMEX à Marseille : HYDRA 2 2 6 (1986) à 520 mètres avec de l’hydréliox et HYDRA 7 (1987) à SNHP : Syndrome nerveux des hautes pressions. SRHP : Syndrome respiratoire des hautes pressions. 260 mètres avec de l’hydrox (hydrogène/oxygène). La confirma- SAHP : Syndrome articulaire des hautes pressions. tion “en réel” (HYDRA 8) s’est déroulée en Méditerranée en février-mars 1988 à partir du navire Orelia. Elle a démontré la remarquable efficacité des plongeurs à 520 et 534 mètres de pro- Cet obstacle présente des caractéristiques spécifiques, et des fondeur, semblable à celle qui est habituellement observée sur un études ont été menées dès 1963 au Centre d’essais hyperbares chantier à 200-250 mètres de profondeur au mélange héliox (7). (CEH) de la COMEX en vue de repousser les limites de l’inter- Puis ce fut HYDRA 9, en 1989, qui permit de préciser la plage vention humaine à grande profondeur avec une maîtrise calculée d’utilisation de l’hydrox et les effets chez l’homme d’une expo- du SNHP. Jusqu’en 1982, six grandes séries de plongées d’essai sition de longue durée. Dix ans après le début du programme au mélange héliox ou trimix (hélium/azote/oxygène) ont ainsi été HYDRA, tous les éléments étaient enfin réunis pour dépasser les réalisées dans les caissons du CEH à Marseille. frontières de l’intervention humaine, ce qui fut réalisé lors de la Aujourd’hui, plusieurs sociétés internationales de travaux sous- plongée d’essai HYDRA 10, qui se déroula d’octobre à décembre marins réalisent couramment des saturations héliox à 150- 1992 dans les caissons du CEH de la COMEX à Marseille.
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