Oxygen

• Oxygen is used in the management of all forms of hypoxia (other than histotoxic), as an adjunct in the management of shock and in the treatment of carbon monoxide poisoning, pneumatosis coli, decompression sickness and anaerobic infections.
• Respiration occurs within mitochondria to produce CO2 and water.
• Excretion is as exhaled CO2 and metabolic water.
• Oxygen is presented as a compressed gas in cylinders at a pressure of 137 bar (13 700 kPa) at 15°C; the cylinders are black with white shoulders and are available in several different sizes.
• Those cylinders commonly used in hospital are C–J containing 170–6800 l, respectively.
• The AZ cylinder is MRI-compatible and contains 170 l.
• Oxygen is also available commercially in liquid form, one volume of liquid oxygen yielding 840 volumes of gaseous oxygen at 15°C and 1013 mb.
• Liquid oxygen is stored in a vacuum-insulated evaporator (VIE) which ranges in liquid capacity from 1600 to 18 675 l, depending on its size.
• Oxygen is a colourless, odourless, tasteless gas which supports combustion and is explosive in the presence of grease.
• Oxygen has a molecular weight of 32, a specific gravity of 1.105, a critical temperature of −118.4°C, and a critical pressure of 50.8 atmospheres.
• Oxygen is supplied at 99.5% purity with maximum amounts of carbon monoxide and CO2 of 5.0 vpm and 300.0 vpm, respectively.
• Liquid oxygen appears pale blue.
• The essential role of oxygen is in the process of oxidative phosphorylation.
• Elemental oxygen is combined with hydrogen ions via mitochondrial cytochrome oxidase; the energy released is used for the synthesis of ATP.
• Oxygen is administered by inhalation via fixed-performance or variable-performance devices.
• Depending on the device used, inspired concentrations of up to 100% may be achieved.
• The following toxic effects are associated with the use of high concentrations of oxygen: CO2 retention in patients with respiratory failure who are predominantly dependent upon a hypoxic drive to respiration, retrolental fibroplasia in neonates, acute oxygen toxicity, chronic oxygen toxicity, and prolonged administration of 100% oxygen may interfere with red blood cell formation.
• If 100% oxygen is administered at 3 atmospheres, approximately 6 ml of dissolved oxygen is contained within each 100 ml of plasma.
• The administration of 100% oxygen causes cerebrovascular constriction due to an increased sensitivity to adrenergic agonists, resulting in a decrease in the cerebral blood flow.
• When 100% oxygen is administered at atmospheric pressure, each 100 ml of plasma contains approximately 1.7 ml of dissolved oxygen.
• Mild respiratory depression results from the administration of 100% oxygen due to a decrease in sensitivity of the respiratory centre to CO2.
• The coronary blood flow decreases, secondary to coronary arterial vasoconstriction.
• The binding of oxygen with haemoglobin tends to displace CO2 from the blood (the Haldane effect).
• In contrast, the PVR and mean arterial pressure decrease.
• The gas is freely permeable through normal alveolar tissue.
• Oxygen is transported in the blood predominantly combined to haemoglobin; in addition, each 100 ml of plasma contains 0.3 ml of dissolved oxygen at normal atmospheric pressure and an FiO2 of 0.21.
• The administration of 100% oxygen causes a slight decrease in the heart rate due to an effect on chemoreceptors, a slight increase in the diastolic blood pressure, and a decrease of 8–20% in the cardiac output due to myocardial depression.
• Nitrogen is eliminated from the lungs within 2–3 minutes, from the blood within 5 minutes, and from the body within 2 hours.
• Fixed-performance devices include anaesthetic breathing systems with a suitably large reservoir and Venturi-operated devices (also known as high airflow oxygen enrichment, or HAFOE, devices).
• Variable-performance devices include Hudson face masks, partial rebreathing masks, nasal cannulae, and nasal catheters.
• A number of factors determine the FiO2 delivered by a variable-performance device: gas flow rate, peak inspiratory flow rate, respiratory rate, and how tightly fitting the face mask is.