Medical Gases in the Hospital
Medical Gases in the Hospital
A pure grade of gas to be used for medical purposes. These gases are normally stored at 137 bar. The medical gases commonly used in operating rooms are oxygen, nitrous oxide, air, and nitrogen. Patients are endangered if medical gas systems, particularly oxygen, malfunction. The main features of such systems are the sources of the gases and the elements to prevent and detect medical gas depletion or supply line misconnection.
Types of Medical Gases
i) Oxygen: A reliable supply of oxygen is a critical requirement in any surgical area. Medical grade oxygen (99% or 99.5% pure) is manufactured by fractional distillation of liquefied air. 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. Only one bank is utilized at one time. The number of cylinders in each bank depends on anticipated daily demand.
A liquid oxygen storage system is more economical for large hospitals. Liquid oxygen must be stored well below its critical temperature of -119°C because gases can be liquefied by pressure only if stored below their critical temperature. A large hospital may have a smaller liquid oxygen supply or a bank of compressed gas cylinders that can provide one day's oxygen requirements as a reserve.
Most anesthesia machines accommodate one or two E-cylinders of oxygen. As oxygen is expended, the cylinder's pressure falls in proportion to its content. Oxygen cylinder pressure should be monitored before use and periodically during use. The method to calculate the amount of oxygen in a cylinder will be discussed in session II.
ii) Nitrous Oxide: Nitrous oxide, a most commonly used anesthetic gas, is manufactured by heating ammonium nitrate. It is almost always stored by hospitals in large H-cylinders Bulk liquid storage of nitrous oxide is economical only in very large institutions.
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.
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 residual volume of nitrous oxide is to weigh the cylinder.
iii) Air: The use of air is becoming more frequent in anesthesia as the potential hazards of nitrous oxide and high concentrations of oxygen receive increasing attention. Cylinder air is medical grade and is obtained by blending oxygen and nitrogen. Dehumidified but unsterile air is provided to the hospital pipeline system by compression pumps. The inlets of these pumps must be distant from vacuum exhaust vents to minimize contamination. Because the critical temperature of air is -140.6°c, it exists as a gas in cylinders whose pressures fall in proportion to their content.