Production

O₂ is produced through one of several methods. Chemical methods for producing small quantities of O₂ include electrolysis of water and decomposition of sodium chlorate (NaClO₃). Most large quantities of medical O₂ are produced by fractional distillation of atmospheric air.¹ Small quantities of concentrated O₂ are produced by physical separation of O₂ from air.

Air

Atmospheric air is a colorless, odorless, naturally occurring gas mixture that consists of 20.95% O₂, 78.1% N₂, and approximately 1% “trace” gases, mainly argon. At STPD, the density of air is 1.29 g/L, which is used as the standard for measuring specific gravity of other gases. O₂ and N₂ can be mixed to produce a gas with an O₂ concentration equivalent to that of air. Medical-grade air usually is produced by filtering and compressing atmospheric air.1,5

Figure 37-1 shows a typical large medical air compressor system. In these systems, an electrical motor is used to power a piston in a compression cylinder. On its downstroke, the piston draws air through a filter system with an inlet valve. On its upstroke, the piston compresses the air in the cylinder (closing the inlet valve) and delivers it through an outlet valve to a reservoir tank. Air from the reservoir tank is reduced to the desired working pressure by a pressure-reducing valve before being delivered to the piping system.

For medical gas use, air must be dry and free of oil or particulate contamination.⁵ The most common method used for drying air is cooling to produce condensation. For avoidance of oil or particulate contamination, medical air compressors have air inlet filters and polytetrafluoroethylene (Teflon) piston rings as opposed to oil lubrication. Large medical air compressors must provide high flow (at least 100 L/min) at the standard working pressure of 50 pounds per square inch gauge (psig) for all equipment in use.

Smaller compressors (Figure 37-2) are available for bedside or home use. These compressors have a diaphragm or turbine that compresses the air and generally do not have a reservoir. This design limits the pressure and flow capabilities of these devices. For this reason, small compressors must never be used to power equipment that needs unrestricted flow at 50 psig, such as pneumatically powered ventilators (see Chapter 42). However, small diaphragm or turbine compressors are ideal for powering devices such as small-volume medication nebulizers (see Chapter 36).

Carbon Dioxide

At STPD, CO₂ is a colorless and odorless gas with a specific gravity of 1.52 (approximately 1.5 times heavier than air).¹ CO₂ does not support combustion or maintain animal life. For medical use, CO₂ usually is produced by heating limestone in contact with water. The gas is recovered from this process and liquefied by compression and cooling. The FDA purity standard for CO₂ is 99%.³

Mixtures of O₂ and 5% to 10% CO₂ are occasionally used for therapeutic purposes as noted in Chapter 38. Therapeutic uses include the management of singultus (hiccups), prevention of the complete washout of CO₂ during cardiopulmonary bypass, and regulation of pulmonary vascular pressures in some congenital heart disorders. However, CO₂ mixtures are more commonly used for the calibration of blood gas analyzers (see Chapter 18) and for diagnostic purposes in the clinical laboratory.

Helium

Helium (He) is second only to hydrogen as the lightest of all gases; it has a density at STPD of 0.1785 g/L. He is odorless, tasteless, nonflammable, and chemically and physiologically inert. It is a good conductor of heat, sound, and electricity but is poorly soluble in water. Although He is present in small quantities in the atmosphere, it is commercially produced from natural gas through liquefaction to purity standards of at least 99%.³

He cannot support life, so breathing 100% He would cause suffocation and death. For therapeutic use, He must always be mixed with at least 20% O₂. Heliox (a gas mixture of O₂ and He) may be used clinically to manage severe cases of airway obstruction. Its low density decreases the work of breathing by making gas flow more laminar. He is discussed in more detail in Chapter 38.

Nitric Oxide

Nitric oxide (NO) is a colorless, nonflammable, toxic gas that supports combustion. It is produced by oxidation of ammonia at high temperatures in the presence of a catalyst. In combination with air, NO forms brown fumes of nitrogen dioxide (NO₂). Together, NO and NO₂ are strong respiratory irritants that can cause chemical pneumonitis and a fatal form of pulmonary edema. Exposure to high concentrations of NO alone can cause methemoglobinemia (see Chapter 11). High levels of methemoglobin can cause tissue hypoxia.

As discussed in Chapter 38, NO is approved by the FDA for use in the treatment of term and near-term infants for hypoxic respiratory failure. The American Academy of Pediatrics (AAP) has published a policy statement recommending the use of NO in the care of term and near-term infants when mechanical ventilation is failing because of hypoxic respiratory failure. The AAP suggests that NO be used before extracorporeal membrane oxygenation.⁶ A systemic review from the Cochrane database supports the recommendation that inhaled NO at 20 ppm may be beneficial in term and near-term infants who do not have a diaphragmatic hernia (see Chapter 31).⁷ The use of inhaled NO in the treatment of premature neonates with hypoxic respiratory failure does not improve outcomes and may increase the risk of intracranial hemorrhage.⁸

Nitrous Oxide

Nitrous oxide (N₂O) is a colorless gas with a slightly sweet odor and taste that is used clinically as an anesthetic agent. Similar to O₂, N₂O can support combustion. However, N₂O cannot support life and causes death if inhaled in pure form. For this reason, inhaled N₂O must always be mixed with at least 20% O₂. N₂O is produced by thermal decomposition of ammonium nitrate.¹

The use of N₂O as an anesthetic agent is based on its central nervous system depressant effect. However, only dangerously high levels of N₂O provide true anesthesia. N₂O/O₂ mixtures are almost always used in combination with other anesthetic agents.

Long-term human exposure to N₂O has been associated with a form of neuropathy. In addition, epidemiologic studies have linked chronic N₂O exposure with an increased risk of fetal disorders and spontaneous abortion.¹ On the basis of this knowledge, the National Institute for Occupational Safety and Health (a division of the Occupational Safety and Health Administration) has set an upper exposure limit for hospital operating rooms of 25 ppm N₂O.¹

Gas Cylinders

The containers used to store and ship compressed or liquid medical gases are high-pressure cylinders. The design, manufacture, transport, and use of these cylinders are carefully controlled by both industrial standards and federal regulations. Gas cylinders are made of seamless steel and are classified by the U.S. Department of Transportation (DOT) according to their fabrication method. DOT type 3A cylinders are made from carbon steel, and DOT type 3AA containers are manufactured with a steel alloy tempered for higher strength.¹

Markings and Identification

Medical gas cylinders are marked with metal stamping on the shoulders that supplies specific information (Figure 37-3).^1,9 Although the exact location and order of these markings vary, the practitioner should be able to identify several key items of information.

The letters DOT or ICC (Interstate Commerce Commission) are followed by the cylinder classification (3A or 3AA) and the normal filling pressure in pounds per square inch (psi). Below this information usually is the letter size of the cylinder (E, G, and so on) followed by the cylinder serial number. A third line provides a mark of ownership, often followed by the manufacturer’s stamp or a mark identifying the inspecting authority. An abbreviation indicating the method of cylinder manufacturer is usually on the opposite side of the cylinder. Also in this area is information about the original safety test and dates of all subsequent tests.

Safety tests are conducted on each cylinder every 5 or 10 years, as specified in DOT regulations.1,9 During these tests, cylinders are pressurized to five thirds of their service pressure. While the cylinder is under pressure, technicians measure cylinder leakage, expansion, and wall stress. The notation EE followed by a number indicates the elastic expansion of the cylinder in cubic centimeters under the test conditions. An asterisk (*) next to the test date indicates DOT approval for 10-year testing. A plus sign (+) means the cylinder is approved for filling to 10% greater than its service pressure. An approved cylinder with a service pressure of 2015 psi can be filled to approximately 2200 psi. After hydrostatic testing, cylinders are subjected to internal inspection and cleaning.

In addition to these permanent marks, all cylinders are color-coded and labeled for identification of their contents.1,10 Table 37-2 lists the color codes for medical gases as adopted by the Bureau of Standards of the U.S. Department of Commerce.¹¹ For comparison, the color codes adopted by the Canadian Standards Association also are included. Color codes are not standardized internationally. For this reason, cylinder color should be used only as a guide. As with any drug agent, the cylinder contents always must be identified through careful inspection of the label. To be absolutely sure about the O₂ concentration provided by a cylinder, the user must analyze the gas before administering it (see Chapter 18).¹²

TABLE 37-2

Color Codes for Medical Gas Cylinders

Gas	United States	Canada
O2	Green	White*
CO2	Gray	Gray
N2O	Blue	Blue
Cyclopropane	Orange	Orange
He	Brown	Brown
C2H4	Red	Red
CO2-O2	Gray/green	Gray/white
He-O2	Brown/green	Brown/white
N2	Black	Black
Air	Yellow*	Black/white
N2-O2	Black/green	Pink

C₂H₄, Ethylene.

^*Vacuum systems historically are identified as white in the United States and yellow in Canada. For this reason, the CGA recommends that white not be used for any cylinders in the United States and that yellow not be used in Canada.

Cylinder Sizes and Contents

Letter designations are used for different sizes of cylinders (Figure 37-4). Sizes E through AA are referred to as “small cylinders” and are used most often for transporting patients and anesthetic gases. These small cylinders are easily identified because of their unique valves and connecting mechanisms. Small cylinders have a post valve and yoke connector. Large cylinders (F through H and K) have a threaded valve outlet (Figure 37-5) (discussed later).

Measuring Cylinder Contents

Because of the previously described differences in the physical state of matter of compressed and liquid gases, different methods are needed to measure the contents of the cylinder.

Compressed Gas Cylinders

For gas-filled cylinders, the volume of gas in the cylinder is directly proportional to its pressure at a constant temperature. If a cylinder is full at 2200 psig, it will be half full when the pressure decreases to 1100 psig. To know how much gas is contained in a compressed gas cylinder, one needs only to measure its pressure.

Liquid Gas Cylinders

In a liquid gas cylinder or container, the measured pressure is the vapor pressure above the liquid. This pressure bears no relationship to the amount of liquid remaining in the cylinder. As long as some liquid remains (and the temperature remains constant), the vapor pressure and the gauge pressure remain constant. When all the liquid is gone and the cylinder contains only gas, the pressure decreases in proportion to a reduction in volume. Monitoring the gauge pressure of liquid gas cylinders is useful only after all the liquid vaporizes. Weighing a liquid-filled cylinder is the only accurate method for determining the contents.

Figure 37-6 compares the behavior of compressed gas and liquid gas cylinders during use. The vapor pressure of liquid gas cylinders varies with the temperature of the contents. The pressure in an N₂O cylinder at 21.1° C (70° F) is 745 psig; at 15.6° C (60° F), the pressure decreases to 660 psig. As the temperature increases toward the critical point, more liquid vaporizes, and the cylinder pressure increases. If a cylinder of N₂O warms to 36.4° C (97.5° F) (its critical temperature), all the contents convert to gas. Only at this temperature and higher does the cylinder gauge pressure accurately reflect cylinder contents.