Storage and Delivery of Medical Gases



Storage and Delivery of Medical Gases


David L. Vines




The hospital “oxygen service” is the origin from which the current technology-laden field of respiratory care evolved. Although respiratory therapists (RTs) have assumed many more challenging duties, ensuring the safe and uninterrupted supply of medical gases is still a key responsibility.


There are many commercially produced gases, but only a few are used medically (Table 37-1). Medical gases are classified as laboratory gases, therapeutic gases, or anesthetic gases. Laboratory gases are used for equipment calibration and diagnostic testing. Therapeutic gases are used to relieve symptoms and improve oxygenation of patients with hypoxemia. Anesthetic gases are combined with oxygen (O2) to provide anesthesia during surgery. It is important for RTs to be familiar with all aspects of gases used in the clinical setting, especially the chemical symbols, physical characteristics, ability to support life, and fire risk. In regard to fire risk, medical compressed gases are classified as either nonflammable (do not burn), nonflammable but supportive of combustion (also termed oxidizing), or flammable (burns readily, potentially explosive).1 Of the gases listed in Table 37-1, the focus of this chapter is on the therapeutic gases.




Characteristics of Medical Gases


Oxygen


Characteristics


O2 is a colorless, odorless, transparent, and tasteless gas.1 It exists naturally as free molecular O2 and as a component of a host of chemical compounds. O2 constitutes almost 50% by weight of the earth’s crust and occurs in all living matter in combination with hydrogen as water. At standard temperature, pressure, and dry (STPD), O2 has a density of 1.429 g/L, being slightly heavier than air (1.29 g/L). O2 is not very soluble in water. At room temperature and 1 atm pressure, only 3.3 ml of O2 dissolves in 100 ml of water.


O2 is nonflammable, but it greatly accelerates combustion. Burning speed increases with either (1) an increase in O2 percentage at a fixed total pressure or (2) an increase in total pressure of O2 at a constant gas concentration. Both O2 concentration and partial pressure influence the rate of burning.2



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.



Fractional Distillation

Fractional distillation is the most common and least expensive method for producing O2. The process involves several related steps. First, atmospheric air is filtered to remove pollutants, water, and carbon dioxide (CO2). The purified air is liquefied by compression and cooled by rapid expansion (Joule-Thompson effect).


The resulting mixture of liquid O2 and nitrogen (N, N2) is heated slowly in a distillation tower. N2, with its boiling point of 195.8° C (320.5° F), escapes first, followed by the trace gases of argon, krypton, and xenon. The remaining liquid O2 is transferred to specially insulated cryogenic (low-temperature) storage cylinders. An alternative procedure is to convert O2 directly to gas for storage in high-pressure metal cylinders. These methods produce O2 that is approximately 99.5% pure. The remaining 0.5% is mostly N2 and trace argon. U.S. Food and Drug Administration (FDA) standards require an O2 purity of at least 99.0%.3



Physical Separation

Two methods are used to separate O2 from air.4 The first method entails use of molecular “sieves” composed of inorganic sodium aluminum silicate pellets. These pellets absorb N2, “trace” gases, and water vapor from the air, providing a concentrated mixture of more than 90% O2 for patient use. The second method entails use of a vacuum to pull ambient air through a semipermeable plastic membrane. The membrane allows O2 and water vapor to pass through at a faster rate than N2 from ambient air. This system can produce an O2 mixture of approximately 40%. These devices, called oxygen concentrators, are used primarily for supplying low-flow O2 in the home care setting. For this reason, details about the principles of operation and appropriate use are discussed in Chapter 51.



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



Storage of Medical Gases


Medical gases are stored either in portable high-pressure cylinders or in large bulk reservoirs. Bulk reservoirs require a separate distribution system to deliver the gas to the patient.



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





Cylinder Safety Relief Valves


In a closed cylinder, any increase in gas temperature increases gas pressure. Should the temperature increase too much (as in a fire), the high gas pressure could rupture and explode the cylinder. To prevent this type of accident, all cylinders have high-pressure relief valves. These relief valves are of three basic designs: frangible disk, fusible plug, and spring-loaded. The frangible metal disk ruptures at a specific pressure. The fusible plug melts at a specific temperature. The spring-loaded valve opens and vents gas at a set high pressure. In each case, the activated valve vents gas from the cylinder and prevents pressure from becoming too high.


Most small cylinders have a fusible plug relief valve. Most large cylinders have a spring-loaded relief valve. These safety relief valves are always located in the cylinder valve stems.



Filling (Charging) Cylinders


How a cylinder is filled depends on whether its contents will be gaseous or liquid. Some gases stored in liquid form can remain at room temperature, but others must be maintained in a cryogenic (low-temperature) state. Cryogenic storage is discussed later.




Liquefied Gases

Gases with critical temperatures greater than room temperature can be stored as liquids at room temperature (see Chapter 6). These gases include CO2 and N2O. Rather than being filled to filling pressure, cylinders of these gases are filled according to a specified filling density. The filling density is the ratio between the weight of liquid gas put into the cylinder and the weight of water the cylinder could contain if full. The filling density for CO2 is 68%. This system allows the manufacturer to fill a cylinder with liquid CO2 up to 68% of the weight of water that a full cylinder could hold. The filling density of N2O is 55%.


Cylinder pressures for gases stored in the liquid phase are much lower than for gases stored in the gas phase. Because the liquid does not fill the entire volume of a cylinder, the space above the liquid surface contains gas in equilibrium with the liquid. The pressure in a liquid-filled cylinder equals the pressure of the vapor at any given temperature.


Pressure in a cylinder depends on the state of its contents. In a gas-filled cylinder, the pressure represents the force required to compress the gas into its smaller volume. In contrast, the pressure in a liquid-filled cylinder is the vapor pressure needed to keep the gas liquefied at the current temperature.



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.




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.



Jun 12, 2016 | Posted by in RESPIRATORY | Comments Off on Storage and Delivery of Medical Gases
Premium Wordpress Themes by UFO Themes