The gases used in anesthesia practice deserve special attention. The existence of life without oxygen is known to all. Oxygen is the most frequently used gas by anesthetists in perioperative as well as critical care settings. In addition, nitrous oxide (N2O), medical air, Entonox, carbon dioxide, and heliox are other gases of importance. The gases in anesthesia can be categorized as follows:
Liquefied compressed gas: Liquefied compressed gas is one that becomes liquid to a large extent in containers at ambient temperature and pressures from 25 to 1500 psig. Examples include nitrous oxide and carbon dioxide.
Joseph Priestley first synthesized oxygen. It is colorless and odorless with a boiling point of –183°C and a critical temperature of –118°C. The critical temperature of oxygen above the room temperature makes its existence as a gas
at normal ambient temperature. The importance of it lies in the fact that reading on the pressure gauge correlates with the volume of oxygen remaining inside the cylinder.
Manufacturing of oxygen is a two-step process. First, atmospheric air is liquefied. Second, liquid air is separated into its components by fractional distillation. The commercial production of a large volume of oxygen also involves the fractional distillation of air.
Heliox comprises a variable proportion of oxygen and helium. The percentage of oxygen may vary from 21 to 50%. Heliox is useful in patients with upper airway obstruction. Theoretically, patients with airway obstruction offer more resistance to flow of gases, resulting in turbulent flow inside the airways. The low density (0.1669) of helium provides the advantage of low resistance to flow when breathed in as heliox. The decreased resistance lowers the work of breathing significantly.
Xenon is a rare gas, contributing no more than 0.0875 ppm of the atmosphere. It is manufactured by fractional distillation of air. Xenon has a good safety and efficacy profile, but its cost is the major barrier to its widespread clinical use.
The estimated minimum alveolar concentration (MAC) of xenon is 63%. Xenon has the lowest blood–gas partition coefficient (0.115) of all known inhaled anesthetics, making induction and emergence from anesthesia rapid. Other advantages are neuroprotection, less cardiovascular depression, and profound analgesia.