Medical Gases: Storage and Supply

Overview

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 most 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 (O ₂ ), nitrous oxide (N ₂ O), 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 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 when flow is turbulent. Carbon dioxide is infrequently used in the management of anesthesia for repair of selected congenital heart defects. Finally, nitric oxide (NO) is currently available for use as a pulmonary vasodilator. Anesthesiologists who use these gases should 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 (CGA). ^,

Medical gas manufacturers are subject to more stringent government and industry regulations and inspections than they have been in the past. This has helped to 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, increased concern regarding the safety of anesthetized patients has helped reduce the number of gas-related injuries. Inspired oxygen monitors with lower concentration 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 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 while 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 (61 cm) long and 4 inches (10 cm) 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 (122 cm) long and 9 inches (23 cm) 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 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 (far 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 (psig) pressure 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} \times V_{1} / P_{2} = 1900 \times 5 / 14.7 = 660 L

$V_{2} = P_{1} \times V_{1} / P_{2} = 1900 \times 5 / 14.7 = 660 L$

If the oxygen tank’s pressure gauge reads 1000 psig, the tank is approximately 50% full (1000 ÷ 1900) and contains 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.

Fig. 1.1, Oxygen remains a gas under high pressure. The pressure falls linearly as the gas flows from the cylinder; thus, in contrast to nitrous oxide, the pressure remaining always reflects the amount of gas remaining in the cylinder.

Nitrous Oxide Tanks

Nitrous oxide 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%–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 the 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 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.

Fig. 1.2, At ambient temperature (20°C), nitrous oxide liquefies under high pressure, and the pressure of the gas above the liquid remains constant independent of how much liquid remains in the cylinder. Only when all the liquid has evaporated does the pressure start to fall, and then it does so rapidly as the residual gas flows from the cylinder.

Only when all the liquid nitrous oxide in the tank has been used up and the tank contains only gaseous nitrous oxide, one might expect that Boyle’s law could be applied. Thus, 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 would be evolved at atmospheric pressure (P ₂ ) is represented by the following equation:

V_{2} = (P_{1} \times V_{1}) / P_{2}

$V_{2} = (P_{1} \times V_{1}) / P_{2}$

(745 \times 5) / 14.7 = 253.4 L

$(745 \times 5) / 14.7 = 253.4 L$

At this point the tank appears to be 16% full (253 ÷ 1590). A tank showing a pressure of 400 psig at 20°C should evolve 136 L [(400 ÷ 745) × 253] of nitrous oxide gas. However, nitrous oxide vapor does not behave like an ideal gas and does not obey Boyle’s law. In fact, the tank contains a greater fraction of nitrous oxide. This is because a vapor that is so close to its saturation point is more compressible than an ideal gas. In fact, almost one-quarter of the full tank remains when the last drop of liquid nitrous oxide has just evaporated. Therefore, when the pressure in the cylinder begins to fall, approximately 400 L are left to be evolved. Now, when the pressure in the tank decreases to 400 psig, there is about 215 L of nitrous oxide remaining (400 ÷ 745 × 400).

While anesthesia is being administered, it is not practical to remove the nitrous oxide cylinder from the anesthesia machine and weigh it accurately enough to determine how much nitrous oxide is left. When the nitrous oxide is being used rapidly, the latent heat of vaporization causes the cylinder itself to become cold. If humidity is sufficient in the surrounding atmosphere, some moisture (or even frost) may collect on the outside surface of the cylinder over the portion that is filled with liquid nitrous oxide. The moisture line, or frost line, which may drop as the gas is used, can provide an indication of when the nitrous oxide will run out. A number of tapes and devices are available to mark the cylinders for this purpose, but their reliability has not been tested. If nitrous oxide is to be used as an anesthetic, it is best to begin with a full cylinder because the length of time the cylinder will last can be calculated. For example, a full E-cylinder of nitrous oxide used at a flow rate of 3 L/min will last about 9 hours (3 × 60 × 9 = 1620 L).

AIR Tanks

Medical air is most commonly provided in E-size cylinders which are color coded yellow. A full cylinder has a pressure of 1900 psig and contains about 625 L ( Table 1.1 ). The air in the cylinder exists as a gas and will obey Boyle’s law (see previous discussion of oxygen tanks). The air tank has a unique pin index and can only be installed on the air hanger yoke of the anesthesia workstation.

Table 1.1

Typical Volume and Weight of Available Contents of Medical Gas Cylinders ^a

Modified from Compressed Gas Association: Characteristics and safe handling of medical gases, publication P-2, ed 7. Arlington, VA, 1989, Compressed Gas Association.

Cylinder Style and Dimensions	Nominal Volume (in ³ /L)	Unit of Measure	Air	CO ₂	Cyclopropane	He	N ₂	N ₂ O	O ₂	Mixtures of Oxygen
Cylinder Style and Dimensions	Nominal Volume (in ³ /L)	Unit of Measure	Air	CO ₂	Cyclopropane	He	N ₂	N ₂ O	O ₂	He	CO ₂
B	87/1.43	psig		838	75				1900
3.5 × 13 in		L	370		375				200
8.89 × 33 cm		lb-oz	1–8	1–7.25					–
		kg		0.68	0.66				–
D	176/2.88	psig	1900	838	75	1600	1900	745	1900	^b	^b
4.25 × 17 in		L	375	940	870	300	370	940	400	300	400
10.8 × 43 cm		lb-oz	–	3–13	3–5.5	–	–	3–13	–	^b	^b
		kg		1.73	1.51	–	–	1.73	–	^b	^b
E	293/4.80	psig	1900	838		1600	1900	745	1900	^b	^b
4.25 × 26 in		L	625	1590		500	610	1590	660	500	660
10.8 × 66 cm		lb-oz	–	6–7		–	–	6–7	–	^b	^b
		kg		2.92		–	–	2.92	–	^b	^b
M	1337/21.9	psig	1900	838		1600	2200	7.45	2200	^b	^b
7 × 43 in		L	2850	7570		2260	3200	7570	3450	2260	3000
17.8 × 109 cm		lb-oz	–	30–10		–	–	30–10	122 cu ft	^b	^b
		kg		13.9		–	–	13.9	–	^b	^b
G	2370/38.8	psig	1900	838		1600		745		^b	^b
8.5 × 51 in		L	5050	12300		4000		13800		4000	5330
17.8 × 109 cm		lb-oz	–	50–0		–		56–0		^b	^b
		kg		22.7		–	–	25.4		^b	^b
H or K	2660/43.6	psig	2200			2200	2200	745	2200 ^c
		L	6550			6000	6400	15800	6900
		lb-oz	–			–	–	64	244 cu ft
		kg	–			–	–	29.1	–

a Computed contents are based on normal cylinder volumes at 70°F (21.1°C), rounded to no greater than 1% variance.

b The pressure and weight of mixed gases vary according to the composition of the mixture.

c 275 cu ft/7800 L cylinders at 2490 psig are available on request.

Characteristics of Gas Cylinders

Size

Table 1.1 gives a list of the sizes, weights, and volumes of the common cylinders that contain various medical gases. As noted, the anesthesia provider will most often encounter oxygen and nitrous oxide in E-cylinders and a variety of gases in H-cylinders. Although other gas cylinders are found in the OR—such as those used for gas-powered equipment, laparoscopy equipment, and lasers—these are not likely to be in the domain of anesthesia personnel.

Color Coding

Table 1.2 lists the color markings used to identify medical gas cylinders. Although the internationally accepted color for oxygen is white, green is used in the United States, primarily for reasons of tradition; in addition, yellow is used to identify compressed air, which represents another exception to international standards. Anesthesiologists working in countries other than the United States should be aware of these differences. Because nitric oxide cylinders are not standardized in color and are frequently supplied as bare aluminum, it is important to check the label and not solely rely on color coding to identify a compressed gas.

Table 1.2

Color Marking of Compressed Gas Containers Intended for Medical Use

Modified from Compressed Gas Association: Standard color marking of compressed gas containers intended for medical use, publication C-9, ed 3. Arlington, VA, 1988, Compressed Gas Association.

Gas	U.S. Color	Canadian Color
Oxygen	Green	White ^a
Carbon dioxide	Gray	Gray
Nitrous oxide	Blue	Blue
Cyclopropane	Orange	Orange
Helium	Brown	Brown
Nitrogen	Black	Black
Air	Yellow ^a	Black and white
Mixture other than oxygen and nitrogen	A combination of colors corresponding to each component gas
Mixture of Oxygen and Nitrogen
Oxygen 19.5%–23.5%	Yellow ^a	Black and white
All other oxygen concentrations	Black and green	Pink

psig, Pounds per square inch.

a Historically, vacuum systems have been identified by white in the United States and yellow in Canada. Therefore it is recommended that white not be used in the United States and yellow not be used in Canada as markings to identify containers for use with any medical gas.

Cylinder Markings

Certain codes are stamped near the neck on all medical gas cylinders. The U.S. Department of Transportation (DOT), which has extensive regulations concerning the marking and shipping of medical gas cylinders, requires a code to indicate that the cylinder was manufactured according to its specifications ( Fig. 1.3 ). The service pressure (in psig) is stamped on each cylinder and should never be exceeded. Each cylinder is also given its own serial number and commercial designation; the final code stamped on the cylinder is usually the date of the last inspection and the inspector’s mark. Medical gas cylinders must be inspected at least once every 10 years, at which time they should also be tested for structural integrity; this is done by filling the cylinder to 1.66 times the normal service pressure. The date of this inspection is often circled with a black marker to indicate that the cylinder has been checked by the supplier ( Fig. 1.4 ).

Fig. 1.4, An E-cylinder of oxygen. The inspection date, August 2008, has been painted white to indicate the cylinder was checked at the time it was delivered to the facility. All cylinders must be checked for leaks and structural integrity with an overpressure test at least once every 10 years.

All medical gas cylinders should come from the supplier accompanied by a tag with three perforated sections, each designating a different stage of use: empty, in use, and full. The portion of the tag marked “full” should be removed when a cylinder is put into service. This is not usually critical, however, because it is generally obvious when a cylinder is in use; making use of the tag marker becomes important when an empty cylinder is removed from the machine. If the tag is not used correctly at the outset, the problem is compounded with each successive stage of the cylinder’s use, and the final result is storage of an empty cylinder as a full one. Although a discrepancy in weight may alert a user to an incorrectly labeled cylinder, this error may be easily overlooked in an emergency situation.

Pressure Relief Valves

All medical gas cylinders must incorporate a mechanism to vent the cylinder’s contents before explosion from excessive pressure. Explosion can result from exposure to extreme heat, such as in the event of a fire, or from accidental overfilling. These mechanisms are of three basic types—the fusible plug, frangible disk assembly, and safety relief valve—and are incorporated into the cylinder; as such, they cannot be inspected by the user. The fusible plug, made of a metal alloy with a low melting point, will melt in a fire and allow the gas to escape. With certain gases, such as oxygen or nitrous oxide, this can aggravate the fire because oxygen and nitrous oxide are both strong oxidizers. The frangible disk assembly contains a metal disk designed to break when a certain pressure is exceeded and thereby allow the gas to escape through a discharge vent. Finally, the safety relief valve is a spring-loaded mechanism that closes a discharge vent. If the pressure increases, the valve opens and remains open until the pressure decreases below the valve’s opening threshold. Some cylinders have combination devices that incorporate a fusible metal plug with one of the other two mechanisms.

Connectors

Fig. 1.5 illustrates the tops of typical valves for both of the most common small (E) and large (H) cylinders. As previously mentioned, large cylinders have valve outlets that are coded and are unique to the gas content of the cylinder. The coding is based on the threads and diameter of the outlet port orifice. Regulators to reduce and control the pressure of the gas, also specific for each type of gas, are attached to these threaded valve ports. It is highly unsafe to use a regulator for one type of gas on a valve port of a cylinder of another type of gas.

Fig. 1.5, Typical cylinder valves. (A) A small cylinder packed valve, such as would be found on an E-cylinder. Note that the female-type port is not unique to the gas type. (B) A large cylinder packed valve, such as would be seen on an H-cylinder. Note that the male type of outlet port has a unique diameter and threads as a safety feature intended to help ensure correct connections.

Small cylinders have cylindrical ports or holes in their valves to receive the yoke, either on an anesthesia machine or free standing, from which the gas will flow. A washer (Bodok seal), usually made of Teflon, is necessary to make this connection gas tight. Care must be taken to ensure that the retaining screw that holds the cylinder in the yoke is not placed into the safety relief device instead of in its intended location in the conical depression opposite the valve port ( Fig. 1.5A ). The connection between cylinder valve and yoke is made gas specific by the pin index safety system for small cylinder connections.

Gas Cylinder Safety Issues

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