The medical gases commonly used in operating rooms are oxygen, nitrous oxide, air, and nitrogen. Although technically not a gas, vacuum exhaust for disposal or scavenging of waste anesthetic gas and surgical suction must also be provided because these are considered integral parts of the medical gas system. Patients are endangered if medical gas systems, particularly oxygen, are misconfigured or malfunction. The anesthesia provider must understand the sources of the gases and the means of their delivery to the operating room to prevent or detect medical gas depletion or supply line misconnection.
A reliable supply of oxygen is a critical requirement in any surgical area. Oxygen is stored as a compressed gas at room temperature or refrigerated as a liquid. Most small hospitals store oxygen in two separate banks of high-pressure cylinders (H-cylinders) connected by a manifold. A liquid oxygen storage system is more economical for large hospitals. To guard against a hospital gas-system failure, the anesthesiologist must always have an emergency (E-cylinder) supply of oxygen available during anesthesia.
Most anesthesia machines accommodate E-cylinders of oxygen. As oxygen is expended, the cylinder’s pressure falls in proportion to its content. A pressure of 1000 psig indicates an E-cylinder that is approximately half full and represents 330 L of oxygen at atmospheric pressure and a temperature of 20°C. Compressed medical gases use a pin index safety system for these cylinders to prevent inadvertent crossover and connections for different gas types.
Nitrous oxide is almost always stored by hospitals in large H-cylinders connected by a manifold with an automatic crossover feature. Because the critical temperature of nitrous oxide (36.5°C) is above room temperature, it can be kept liquefied without an elaborate refrigeration system. If the liquefied nitrous oxide rises above its critical temperature, it will revert to its gaseous phase.
Because these smaller cylinders also contain nitrous oxide in its liquid state, the volume remaining in a cylinder is not proportional to cylinder pressure. By the time the liquid nitrous oxide is expended and the tank pressure begins to fall, only about 400 L of nitrous oxide remains. If liquid nitrous oxide is kept at a constant temperature (20°C), it will vaporize at the same rate at which it is consumed and will maintain a constant pressure (745 psig) until the liquid is exhausted. The only reliable way to determine the residual volume of nitrous oxide is to weigh the cylinder.
Air is becoming more frequently used in anesthesiology as the popularity of nitrous oxide and the use of unnecessarily high concentrations of oxygen have declined. Cylinder air is medical grade and is obtained by blending oxygen and nitrogen.