The carrier gas in anaesthesia: nitrous oxide/oxygen, medical air/oxygen and pure oxygen

The gas passing the module for the delivery of inhalation anaesthetics and carrying vapourized anaesthetics into the breathing system is called the carrier gas. Oxygen is the absolutely indispensable component of the carrier gas. Additive gaseous components can be medical air (nitrogen), nitrous oxide, cyclopropane, or xenon, the latter three being anaesthetic gases themselves. Cyclopropane is not used any more and xenon is not approved as a medical gas yet, leaving medical air and nitrous oxide as the only currently available adjuncts to oxygen.

As review articles on nitrous oxide recommend unequivocally not using this gas routinely, and many anaesthetists already consistently omit any use of this gas, the alternative carrier gases are either a mixture of medical air and oxygen or pure oxygen. A nitrogen/oxygen mixture is absolutely inert with respect to metabolism or environment; there are no contraindications or shortcomings at all. The use of pure oxygen will result in high oxygen concentrations, improving the patients' safety by increasing the pulmonary oxygen reservoir and rendering impossible the development of hypoxic gas mixtures within the breathing system. Whether the incidence of postoperative wound infections or postoperative nausea and vomiting is reduced by perioperative hyperoxia is still under scientific discussion. In general, the use of high oxygen concentrations over a period of 6-8 h is not harmful for the patient, assuming the performance of recruitment manoeuvres, consistent use of positive end-expiratory pressure, and thorough observance of the contraindications that can result from the increased vulnerability of tissues to hyperoxia.

Nitrous oxide should not be used routinely as a component of the carrier gas any more. A mixture of medical air and oxygen must be acknowledged to be the gold standard. Pure oxygen may be used as a carrier gas if medical air or properly performing flow controls for medical air are not available.