Overview
∗ Portions of this chapter are reproduced by permission from Eisenkraft JB: The anesthesia delivery system, part I, vol 3. In Progress in Anesthesiology, San Antonio, TX, 1989, Cannemiller Memorial Education Foundation.
Anesthesia providers were once expected to know a great deal about the storage and supply of medical gases. In both large and small institutions, anesthesiologists often had to rely on their own knowledge and skill in this area to manage the many aspects of medical gases, from purchasing to troubleshooting.Changes in technology and institutional organization have relieved the anesthesiologist of the majority of these responsibilities. However, this should not excuse anesthesia providers from understanding the basic facts and safety principles associated with the use of medical gases for anesthesia. Invariably, other health care providers and administrators have little knowledge regarding these systems and look to anesthesia professionals for guidance in the use and handling of these gases in the hospital or clinic setting.
With few exceptions, the only medical gases encountered by practicing anesthesiologists today are oxygen, nitrous oxide, and medical air. For safety reasons, flammable agents are rarely, if ever, used in operating rooms (ORs) today. Nitrogen is used almost exclusively to power gas-driven equipment. Helium, carbon dioxide, and premixed combinations of oxygen and helium or carbon dioxide are generally no longer used. In certain uncommon clinical situations, other gases may be used. Helium is occasionally used as an adjunct in the ventilation of patients undergoing laryngeal surgery because of its low density and flow-enhancing characteristics. Carbon dioxide is infrequently used in the management of anesthesia for repair of selected congenital heart defects. Finally, nitric oxide is currently available for use as a pulmonary vasodilator. Anesthesiologists who use these gases must be fully versed in their characteristics and safe handling. For detailed information and numerous references relating to the handling and use of these and other unusual medical gases, along with a wealth of general information about medical gas cylinders, the reader is directed to publications from the Compressed Gas Association.
Medical gas manufacturers are subject to more stringent government and industry regulations and inspections than they have been in the past. This has helped markedly reduce the number of accidents related to medical gases. For these reasons, anesthesia training programs may not emphasize instruction in the various aspects of storing and using medical gases.
In addition, the recent increased concern regarding the safety of anesthetized patients has helped reduce the number of gas-related injuries. Inspired oxygen monitors with lower limit alarms provide the anesthesia practitioner with an early warning when the oxygen supply becomes inadequate or is contaminated with another gas. Mixed-gas monitoring and analysis is also becoming more common and provides the practitioner with an important way to quickly detect contaminants or unusual gas mixtures before the patient is injured. If the oxygen monitor fails, pulse oximetry can alert the anesthesiologist to problems with patient oxygenation related to inadequate oxygen supply.
Medical Gas Cylinders and their Use
Medical gases are stored either in metal cylinders or in the reservoirs of bulk gas storage and supply systems. The cylinders are almost always attached to the anesthesia gas machine. Bulk supply systems use pipelines and connections to transport medical gases from bulk storage to the anesthesia machine.
Virtually all facilities in which anesthesia is administered are equipped with central gas supply systems. Anesthesia practice is currently undergoing change in this regard, and many anesthetics are administered outside the OR, and even outside the hospital, where a central gas supply system may be unavailable. The current emphasis on providing care away from the hospital—such as in dental clinics, mobile lithotripsy units, and mobile magnetic resonance imaging facilities—will only increase the demands on the anesthesia provider to ensure a safe and continuous gas supply. E-cylinders are sometimes the only source of medical gas for anesthesia machines in these settings. If an anesthetic is being administered using only E-cylinders, then both the anesthesiologist and related support personnel must first ensure that an adequate supply of reserve cylinders is available. In addition, the amount of gas in the cylinders being used must be continually monitored, and the cylinders must be replaced before they are completely emptied. The importance of this cannot be overemphasized. Many anesthesia practitioners today have not been confronted with the possibility of running out of oxygen and having to change a tank while administering an anesthetic—but the evolving nature of anesthesia practice away from traditional facilities is likely to make this a more common occurrence. If an anesthesiologist anticipates this situation, it is imperative that the anesthesia machine be equipped with two oxygen cylinder yokes so that oxygen delivery can continue when the empty tank is changed.
Anesthesia practitioners should be familiar with two sizes of gas cylinders. The cylinder most often used by anesthesia providers is the E-cylinder, which is approximately 2 feet long and 4 inches in diameter. E-cylinders are also routinely used as portable oxygen sources, such as when a patient is transported between the OR and an intensive care unit (ICU). H-cylinders are larger, approximately 4 feet long and 9 inches in diameter, and are generally used as a source of gas for small or infrequently used pipeline systems. They may be used as an intermediate or long-term source of gas at the patient’s bedside. Almost all hospitals store H-cylinders of oxygen in bulk as a back-up source in case the pipeline oxygen fails or is depleted. H-cylinders of nitrogen are often used to power gas-driven medical equipment. H-cylinders that contain oxygen, nitrous oxide, or air have occasionally been used in ORs and are connected to the anesthesia machine via special reducing valves and hoses. Such uncommon configurations are not only potentially hazardous, they also defeat certain safeguards. Any practitioner who uses such a system must become thoroughly familiar with it and must be certain it complies with applicable regulations and guidelines.
Oxygen Tanks
Oxygen (O 2 ) has a molecular weight of 32 and a boiling point of −183° C at an atmospheric pressure of 760 mm Hg (14.7 pounds per square inch in absolute pressure [psia]). The boiling point of a gas—that is, the temperature at which it changes from liquid to gas—is related to ambient pressure in such a way that as pressure increases, so does the boiling point. However, a certain critical temperature is reached, above which it boils into its gaseous form no matter how much pressure is applied in the liquid phase. The critical temperature for oxygen is −118° C, and the critical pressure , which must be applied at this temperature to keep oxygen liquid, is 737 psia. Because room temperature is usually 20° C and therefore in excess of the critical temperature, oxygen can exist only as a gas at room temperature.
E-cylinders of oxygen are filled to approximately 1900 pounds per square inch gauge pressure (psig) at room temperature: 1 atmosphere (atm) is 760 mm Hg, which equals 0 psig or 14.7 psia. At high pressures, psig and psia are virtually the same. When full, the cylinders contain a fixed number of gas molecules, the so-called fixed mass of that gas. These gas molecules obey Boyle’s law, which states that pressure times volume equals a constant (P 1 V 1 = P 2 V 2 ), provided temperature does not change. A full E-cylinder of oxygen with an internal volume of 5 L (V 1 ) and a pressure of 1900 psia (P 1 ) will therefore evolve approximately 660 L (V 2 ) of gaseous oxygen at atmospheric pressure (P 2 , or 14.7 psia). Thus Boyle’s law gives the approximate value:
V 2 = ( P 1 × V 1 ) / P 2 = ( 1900 × 5 ) / 14.7 = 660 L
If the oxygen tank’s pressure gauge reads 1000 psig, the tank is approximately 50% full (1000 ÷ 1900) and will evolve only 330 L (660 × 50%) of oxygen ( Fig. 1-1 ). If such a tank were to be used at an oxygen flow rate of 6 L/min, it would empty in just under an hour (330 ÷ 6 = 55 minutes). Likewise, a full (2200 psig) H-cylinder will evolve 6900 L of oxygen at atmospheric pressure. It is important to understand these principles when oxygen tanks are being used to supply the machine or a ventilator or to transport a patient. Because oxygen exists only as a gas at room temperature, the tank’s pressure gauge can be used to determine how much gas remains in the cylinder. Clearly, if a machine is equipped with two E-cylinders of oxygen, only one should ever be open at any time to ensure that both tanks are not emptied simultaneously.
Nitrous Oxide Tanks
Nitrous oxide (N 2 O) has a molecular weight of 44 and a boiling point of −88° C at 760 mm Hg. Because it has a critical temperature of 36.5° C and critical pressure of 1054 psig, nitrous oxide can exist as a liquid at room temperature (20° C). E-cylinders of nitrous oxide are filled to 90% to 95% of their capacity with liquid nitrous oxide. Above the liquid in the tank is nitrous oxide vapor, that is, gaseous nitrous oxide. Because the liquid nitrous oxide is in equilibrium with its vapor phase, the pressure exerted by the nitrous oxide vapor is its saturated vapor pressure (SVP) at the ambient temperature.
A full E-cylinder of nitrous oxide will evolve approximately 1590 L of gaseous nitrous oxide at 1 atm (14.7 psia). As long as some liquid nitrous oxide remains in the tank and temperature remains constant (20° C), the pressure in the tank will be 745 psig, or the SVP of nitrous oxide at 20° C ( Fig. 1-2 ). It should be clear that, unlike oxygen, the content of a tank of nitrous oxide cannot be determined from the pressure gauge. It can, however, be determined by removing the tank, weighing it, and subtracting the empty weight stamped on each tank (tare weight); the difference is the weight of the contained nitrous oxide. Avogadro’s formula for volume states that 1 g of molecular weight of any gas or vapor occupies 22.4 L at standard temperature and pressure. Thus, 44 g of nitrous oxide occupies 22.4 L at 0° C and 760 mm Hg pressure. At 20° C this volume increases to 24 L (22.4 × 293 ÷ 273); thus each gram of nitrous oxide is equivalent to 0.55 L of gas at 20° C.
Only when all the liquid nitrous oxide in the tank has been used up and the tank contains only gaseous nitrous oxide, can Boyle’s law be applied. In this instance, when the tank pressure (P 1 ) is 745 psig from gas only and the internal volume (V 1 ) of the E-cylinder is approximately 5 L, the volume (V 2 ) of nitrous oxide gas that will be evolved at atmospheric pressure (P 2 ) is represented by the following equation:
V 2 = ( P 1 × V 1 ) / P 2
( 745 × 5 ) / 14.7 = 253.4 L
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