Английский язык для медиков - Беликова Елена (книга читать онлайн бесплатно без регистрации .txt) 📗

56. Artificial oxygen carries

Artificial oxygen (O 2) carries aim at improving O 2 delivery. Artificial 0 2 carries thus may be used as alternative to allogeneic blood transfusions or to improve tissue oxy—genation and function of organs with marginal O ₂ supply. Artificial O ₂ carries can be grouped into modified hemoglobin (Hb) solutions and perfluorocarbon (PFC) emulsions. The native human Hg molecule needs to be modified in order to decrease O ₂ affinity and to prevent rapid dissociation of the native tetramer into dimers. The O ₂ transport characteristics of modified Hb solutions and PFC emulsions are fundamentally different. The Hb solutions exhibit a sigmoidal O ₂ dissociation curve similar to blood. In contrast, the PFC emulsions are characterised by a linear relationship between O ₂ partial pressure and O ₂ content. Hb solutions thus provide O ₂ transport and unloading capacity similar to blood. This means that already at a relatively low arterial O ₂ partial pressure substantial amounts of O ₂ are being transported. In contrast, relatively high arterial O ₂ partial pressures are necessary to maximize the O ₂ transport capacity of PFC emulsions.

Modified Hb solutions are very promising in improving O ₂ transport and tissure oxygenation to a physiologically relevant degree. Because cross—matching is unnecessary, these solutions hold great promise as alternative to allogeneic blood transfusions and as O ₂ therapeutics, which might be of great value also in the prehospital resuscitation of trauma victims or in specific situations in intensive care medicine. In patients with a reduced cardiac contractility and normal or elevated mean arterial pressure Hb infusion may increase systemic and pulmonary vascular resistances with consequent reduction in cardiac output. In contrast, in a previously healthy trauma victim, suffering from severe hypovolaemia due to massive haemorrhage, the combined effects of volume replacement, added O ₂ transport capacity, and mild vasoconstriction due to the infusion of a modified Hb solution may be beneficial.

PFC are carbonfluorine compounds characterised by a high gas—dissolving capacity, low viscosity, and chemical and biological inertness. Manufacturing an emulsion with very specific characteristics is a great technologic challenge. After intravenous application, the droplets of the emulsion are being taken up by the reticular—endothe—lial system, droplets are slowly broken down, the PFC molecules are being taken up in the blood again and transported to the lungs, where the unaltered PFC molecules are finally excreted via exhalation. The ability of PFC emulsions to transport and efficiently unload O ₂ is undisputed. With the application of perflubron emulsion, cardiac output tender to increase.

New words

saturation – насыщение гемоглобина кислородом

emulsion – эмульсия

oxygen – кислород

solution – раствор

O ₂ transport – транспорт кислорода

tissure oxygenation – оксигенация тканей

physiological – физиологический